Le Pierres Nolwenn

Huaylla Fredy

Stutz Benoit

Perraud Julien

Long-term solar heat storage process by absorption with the KCOOH/H2O couple: Experimental investigation

Keywords: heat storage, KCOOH/H2O, absorption, solar heat, building

This paper presents experimental results obtained with a prototype of a solar heat storage absorption process using the KCOOH/H2O couple. The process principle and the prototype tested are described, and both charging and discharging results are presented and discussed. The parameters influencing the results the most are water and solution flow on the reactor heat exchangers as well as the heat source and sink temperatures. The system's main limitation is the wetting of the exchanger plates. However, the tests prove the interest of the KCOOH/H2O couple in this type of low-temperature heat storage process.

Introduction

About half of the world's final energy demand is used for heating and cooling purposes. The building sector consumes energy in large quantities, accounting for more than 30% of global final energy consumption (IEA, 2015). In this context, efficient and sustainable heat storage in buildings is one of the challenges to overcome in order to improve energy efficiency, take greater advantage of available renewable energies and avoid fossil fuel depletion and its environmental and social consequences. Sorption and thermochemical technologies present an advantageous option to store heat, both for high-temperature applications (Zhang et al., 2016;[START_REF] Wokon | Investigations on thermochemical energy storage based on technical grade manganese-iron oxide in a lab-scale packed bed reactor[END_REF] and low-temperature space heating in buildings, because it allows storing heat over long periods, for example excess solar heat available during the summer until the following winter [START_REF] N'tsoukpoe | Numerical dynamic simulation and analysis of a lithium bromide/water long term solar heat storage system[END_REF]. Sorption and thermochemical storage systems also have higher energy density compared to sensible and latent systems, as shown in Figure 1 (N' Tsoukpoe et al., 2009). Their operating conditions are compatible with the use of conventional solar heat collectors, thus contributing to more sustainable energies. Several research projects have been developed around this technology in recent years (Zhang et al., 2014;[START_REF] Ibrahim | Solar absorption systems with integrated absorption energy storage-A review, Renewable and Sustainable Energy Reviews[END_REF]. However, until now no absorption system has reached the commercial stage, because of the difficulty finding an ideal absorption couple for this technology. Different interseasonal solar heat storage prototypes for heating purposes using gas/liquid sorption processes have been built and tested in the past. Among the lowtemperature heat storage systems, all of them used water as sorbate. Four different sorbents have been tested: LiCl, CaCl2, NaOH and LiBr.

ClimateWell in Sweden [START_REF] Bales | TCA Evaluation -Lab measurements, modelling and system simulation[END_REF] appears to be the leading pioneer in absorption heat storage systems. A sorption heat storage system based on a three-phase process [START_REF] Yu | Evaluation of a three-phase sorption cycle for thermal energy storage[END_REF] was developed in 2005 by the Solar Energy Research Center (SERC). The system uses the LiCl/H2O working pair. The system heat storage capacity was 35 kWh, and the corresponding energy density for short-term storage about 85 kWh/m 3 [START_REF] Bales | Laboratory Tests of Chemical Reactions and Prototype Sorption Storage Units[END_REF]. Its main feature, however, was its ability to operate as a heat pump for cooling or heat production, and the storage function was not the main objective of the system.

Another prototype based on an absorption process using the LiCl/H2O couple was constructed (Zhao et al., 2016) at Shangai Jiao Tong University. The prototype was tested under conditions representative of transition or winter conditions. Although the system was designed for short-term heat storage applications, the working principle could be adapted for long-term purposes [START_REF] Yu | Evaluation of a three-phase sorption cycle for thermal energy storage[END_REF]. This process, first presented by [START_REF] Lourdudoss | Energy storing absorption heat pump process[END_REF], is known as the three-phase process. For typical conditions, the stored energy is around 8.5 kWh. Considering a 157-L prototype volume, the system's energy storage density is 54 kWh/m 3 . This value can be increased by 20% by raising the evaporation temperature using a heat recovery technology.

A study was conducted [START_REF] Liu | Inter-seasonal solar energy storage for buildings by absorption[END_REF] at the LOCIE laboratory at the University Savoie Mont Blanc, using the CaCl2/H2O couple. Calcium chloride's main advantage is that it is mass produced at very low cost. Although good power levels were obtained in the charging tests, the absorber power during the discharging tests was poor. The reason for this behavior was, according to the authors, due to the presence of noncondensable gases in the system as well as low wettability of the helical fin exchange surfaces where the solution and water film flowed. The absorber power was improved by increasing the evaporator heat transfer fluid (HTF) inlet temperature and mass flow. Therefore, using the system for building space heating applications is possible if technical improvements are made on the prototype. A closed absorption system involving water and calcium chloride (CaCl2) was also proposed by [START_REF] Quinnell | Mass transfer during sensible charging of a hybrid absorption/sensible storage tank[END_REF] at the University of Minnesota in the USA. This study was original in that the solution and water tanks were only one stratified reservoir, with low mixing of stored liquids, taking advantage of (1) natural density gradients between the different solutions and (2) the fact that diffusion is 10,000 times slower than thermal conduction. Experimental and simulation studies have been carried out to show the viability of the single-storage-tank design [START_REF] Quinnell | Mass transfer during sensible charging of a hybrid absorption/sensible storage tank[END_REF]2014). The results demonstrated that natural convection does not disrupt the stable mass fraction distribution and therefore that the concept holds promise for long-term storage. [START_REF] Fumey | Operation results of a closed sorption heat storage prototype[END_REF]2017) at the EMPA laboratory developed a closed sorption heat storage system based on the NaOH/H2O couple. The economic criterion also prevailed in the NaOH salt selection, because its cost is much lower than LiCl or LiBr. The prototype was constructed in a 7-m-long container with solar collectors as a heat source. The system's experimental results in the discharging mode were less than those theoretically calculated. The best measured values were around 1 kW for cases in which a very low temperature difference between the absorber and the evaporator were considered [START_REF] Daguenet-Frick | Seasonal thermochemical energy storage: Comparison of the experimental results with the modelling of the falling film tube bundle heat and mass exchanger unit[END_REF]. Higher temperature differences reduced the absorber power. Problems were encountered in the solution pumping at the absorber/desorber heat exchanger due to cavitation phenomena generating incomplete tube bundle external surface wetting. The influence of noncondensable gases on the system's performance was also highlighted by the authors. [START_REF] N'tsoukpoe | Experimentation of a LiBr-H2O absorption heat storage process: prototype design and first results[END_REF] constructed a prototype at the University Savoie Mont Blanc, to test the potential of a long-term absorption heat storage system using the LiBr/H2O pair. The prototype was designed to have an 8-kWh heat storage capacity. In general, the charging mode worked in a satisfactory manner with an average generation rate around 2 kW. On the other hand, the results obtained during the discharging tests showed no significant heat transfer between the absorbing falling film and the HTF, even if the absorbing falling film reached a satisfying temperature level. Different reasons were pointed out: the use of copper alloy for the tubes of the absorber/desorber heat exchangers, which did not prove to be corrosion-resistant to the LiBr solution; the presence of noncondensable gases in the reactor; and wettability problems.

Zhang et al. ( 2014) also designed and constructed an interseasonal absorption thermal energy storage prototype, using the LiBr/H2O couple. Experimental results associated with the discharging mode for space heating applications gave heating rates between 12 kW and 7 kW, with the higher values present for a short time at the beginning of the test. The energy storage density of the system associated with the space heating was 110 kWh/m 3 . However, LiBr is an expensive salt, which could limit its extensive use.

In the present study, the innovative KCOOH/H2O couple, which to the authors' knowledge has never been used in an absorption heat storage process, was selected and implemented in an absorption heat storage prototype, a first original aspect of the present experiment. As explained in the next section, this couple presents possible advantages in comparison to the other absorption couples presented above. This study also follows several research projects conducted at the University Savoie Mont Blanc in the absorption storage area, as highlighted in the previous section. In this new research stage, an original prototype was developed to overcome the technical difficulties and physical limitations observed on the previous experimental benches. Original heat and mass exchangers and gravitational fluid circulation are also presented. In the following, this absorption heat storage prototype is described and the experimental results obtained are presented and analyzed.

System description

Process description

The operating principle of the long-term solar heat storage process based on the absorption phenomenon is presented in Figure 2. The process is composed of four main components: a desorber, an absorber, a condenser and an evaporator, to which two solution storage tanks (for the diluted and concentrated solution) and an absorbate storage tank are added. The system operates in the charging mode when solar heat is available and in the discharging mode in heating demand periods. In summer, solar heat is received by a flat plate solar thermal collector and is supplied at around 60°C to the diluted solution that is transferred from its storage tank to the desorber. The solution releases the absorbate vapor and becomes concentrated (in absorbent). The absorbate vapor is condensed in the condenser using a heat sink (for example a ground-coupled heat exchanger) and flowed to the absorbate storage tank and stored in liquid form. The concentrated solution obtained flows from the desorber into the concentrated solution tank. After this charging period, the storage tanks are disconnected so that there is no mass exchange between the components: energy can therefore be stored in the form of a chemical potential as long as needed. During the following winter or heating period, the discharging phase can occur. The concentrated solution flows to the absorber, where it absorbs the absorbate that is evaporated by the heat transferred from a low-temperature heat source. The heat produced by this exothermic absorption phenomenon can then be used for building heating purposes, at temperatures over 25°C, for example, in a heating floor.

The solution can crystalize in the solution storage tank in this process. Indeed, at the end of the charging process, the mass fraction of absorbent in the solution is high. In the storage tank, the temperature of the hot solution coming from the desorber decreases due to heat loss to the surroundings. Therefore, if the solubility temperature of the solution is reached in the tank, crystals appear. Allowing crystal formation in the tank and thus increasing the solution mass fraction at the end of the charging process can multiply the system's energy storage density several times (N'Tsoukpoe, 2012). Indeed, in the discharging phase, the larger the amount of absorbate available for absorption, the greater the amount of sorption heat that can be recovered. Consequently, during desorption (charging phase), the more absorbate extracted from the diluted solution, the more heat stored. The storage density is then increased by this crystallization and therefore crystallization is relevant for the process's competitiveness, but the technical complexity of the process increases also, in particular pumping and circulating the solution (N'Tsoukpoe, 2013). Therefore, the system's components have to be carefully designed.

KCOOH/H2O couple selection

For the absorption storage process, one of the major issues is the choice of the absorbate/absorbent couple. In most of the absorption heat pumps available on the market, two possible couples are used: LiBr/H2O or H2O/NH3. The second choice is usually discarded in the case of storage processes, for security reasons: a large amount of absorbate (in this case NH3) has to be stored between the charging and the discharging phases, and NH3 should be avoided. Water is then the chosen absorbate, although its use imposes strong technical constraints on the system, because it must be operated under low pressure (typically 10-30 mbar, depending on the phase considered and the operating temperatures). As was shown in the literature review presented above, water was the choice of all the tested absorption storage prototypes presented.

The second step is the choice of the absorbent. Four of them have already been tested (LiCl, CaCl2, NaOH and LiBr). However, none of them has reached the commercial development stage. In the context of this long-term solar heat storage system, a multicriteria analysis on various possible absorbents [START_REF] Lefebvre | Procédé par absorption avec stockage d'énergie solaire intersaisonnier intensifié par la cristallisation de l'absorbant : Recherche & caractérisation thermodynamique de nouveaux couples; Conception de la cuve de stockage[END_REF] has led to the choice of KCOOH for the system. It absorbs water following the equilibrium :

KCOOH-nH2O + mH2O  KCOOH-(n+m)H2O + mΔhs
where Δhs is the sorption heat that is released during the absorption phenomenon.

The H2O/KCOOH solution is currently used mainly at low salt concentrations as brine or a biodegradable de-icing solution. It is also widely in use in the oil-drilling industry and as an ice-removing agent for planes (Aittomaki and Lahti, 1997). To the authors' knowledge, it has never been used in an absorption energy storage process. Mixtures of potassium formate and lithium bromide solutions were evaluated in an absorption chiller and showed promising performance [START_REF] Lucas | Performance evaluation and simulation of a new absorbent for an absorption refrigeration system[END_REF]. Moreover, potassium formate absorption and desorption characteristics have been tested: Riffat et al. (1996) showed that potassium formate has the potential to be a possible "drop-in" replacement for lithium bromide, because absorption rates of both working fluids were very similar and the heat of absorption only slightly lower when using potassium formate.

The H2O/KCOOH solution's thermophysical properties are still not precisely known for concentrations higher than 50% (Lowenstein, 2008;Melinder, 2007). This solution mass fraction range is useful for heat storage process applications. However, data from [START_REF] Balarew | IUPAC-NIST Solubility Data Series. 73. Metal and Ammonium Formate Systems[END_REF], [START_REF] Sidgwick | CCXXI.-The solubilities of the alkali formates and acetates in water[END_REF], [START_REF] Scatchard | The Freezing Points of Aqueous Solutions. VI. Potassium, Sodium and Lithium Formates and Acetates[END_REF], Aittomäki and Lahti (1997), [START_REF] Beyer | Vapor Pressure Measurements of NaHCOO + H2O and KHCOO + H2O from 278 to 308 K and Representation with an Ion Interaction (Pitzer) Model[END_REF] and Lefebvre (2010) make it possible to draw a Dühring diagram for the KCOOH/H2O solution up to a KCOOH mass fraction of 75%, as presented in Figure 3. This couple seems to present equilibrium conditions compatible with the use of solar collectors as a heat source during the summer and with a heating floor as a heat sink during the winter, as presented in Figure 3. Absorption temperatures during the winter phase can produce heat at temperatures between 25 and 36°C (usable by up-to-date heating floors) for KCOOH mass fractions between 62 and 76%, and the corresponding desorption temperatures during the summer are as low as 58°C, which can easily be reached by flat plate solar collectors.

The H2O/KCOOH solution is environmentally friendly, nontoxic and nonflammable; has an advantageous theoretical heat storage energy density; and costs less than half the price of the couples usually used in absorption processes (LiBr/H2O or LiCl/H2O) [START_REF] Lefebvre | Procédé par absorption avec stockage d'énergie solaire intersaisonnier intensifié par la cristallisation de l'absorbant : Recherche & caractérisation thermodynamique de nouveaux couples; Conception de la cuve de stockage[END_REF]. Its aqueous solution is also weakly alkaline and therefore noncorrosive and biodegradable [START_REF] Longo | Experimental measurement of thermophysical properties of H2O/KCOOH (potassium formate) desiccant[END_REF], and has a lower density and viscosity than absorbents such as lithium bromide [START_REF] Riffat | Experimental analysis of the absorption and desorption rates of HCOOK/H2O and LiBr/H2O[END_REF]. These advantages together with its relatively low cost make it a prime choice as the absorbent for the heat storage process.

Prototype description

Prototype components and dimensioning

A prototype was developed to demonstrate the feasibility of storing solar heat efficiently using this KCOOH/H2O absorption process. It was designed to store about 8 kWh of heat and to produce a heating power of approximately 1 kW.

In this storage process, the charging and discharging phases are successive, allowing the absorption and desorption phenomena to take place in the same component, as well as evaporation and condensation. For heat exchangers, a sandwich grooved vertical plate configuration was then chosen (Figure 4(b)). As mentioned by [START_REF] Do | A mathematical model for analyzing the thermal characteristics of a flat micro heat pipe with a grooved wick[END_REF] and [START_REF] Guo | Numerical modeling and analysis of grooved surface applied to film cooling[END_REF], a grooved surface allows better wetting of the falling films flowing along and within the grooves, which can ensure uniform distribution and wetting over the entire exchange surfaces. The plates are 50 cm high and 40 cm wide. Both exchangers are located in the same reactor, situated above the storage tank (Figure 4(a)) to allow the flows to return to these tanks by gravity.

A double-envelope configuration was chosen for the solution tank, where the 70-L internal cylinder (in glass) contains the KCOOH solution, and the annulus between the internal and external cylinders (in acrylic glass) contains water as the heat transfer fluid to control the tank's temperature. Only one solution tank is used, such that the solution mass fraction changes progressively over the charging and discharging phases. The prototype was designed to be able to test the solution's crystallization functionality. Thus, the solution is pumped out of the top of the solution tank using a float, and it flows back to the bottom of the tank. This avoids pumping crystals from the solution tank, as the crystals' density is higher than the solution's density. A double envelope was also designed to control the temperature of the stainless-steel 55-L water storage tank.

The picture and the scheme of this prototype are presented in Figures 5 and6, respectively. Three thermal modules are used in the prototype (external heaters/coolers in Figure 5). They control the solution tank temperature, the absorber/desorber heat transfer fluid temperature, and the water tank and the evaporator/condenser heat transfer fluid temperatures, respectively. W1 and W2 (Figure 6) are magnetic-driven volumetric pumps.

The capability of the reactor, the solution tank and the water tank to maintain vacuum conditions was tested prior to the experimental tests. The maximum leakage rate measured was 2.67.10 -4 (mbar.l)/s, which is satisfactory for this type of experiment. Consequently, a vacuum pump was used (W7 in Figure 6) to put the reactor container, solution tank and water tank in vacuum conditions before the experimental tests began, but the vacuum pump was off during the tests.

Instrumentation

The prototype was strongly instrumented for analysis of the functioning of this process (temperature, pressure, mass flow, volumetric flow, density, levels of the internal and external fluids were all measured). Probe positions are illustrated in Figure 6. The types and precision of the measurements are presented in Table 1.

Experimental conditions

Experiments were carried out in the conditions representing the system's functioning in the case of interseasonal solar heat storage for building heating in a French climate. The temperature of the desorber HTF should represent the temperature of the solar collectors during the summer. To ensure the highest possible solar collector efficiency, the temperature should be as low as possible, so the process was tested at desorber temperatures between 50 and 70°C. The corresponding condenser temperature should represent a heat release to an external heat sink. This sink could be the ambient air or a geothermal pipe, for example, so the process was tested for a condenser HTF entering at a temperature of 15°C. During the discharging phase, the HTF of the evaporator should represent the temperature of an external heat source during the winter period. Again, this source could be the ambient air or a geothermal pipe, so the process was tested for an evaporator HTF entering at a temperature of 15°C, assuming a constant ground temperature over the year. The corresponding absorber should provide heat for building heating. For this type of absorption process, similarly to the conventional heat pumps, the lower the absorber temperature, the higher the system performance. Thus, this process was assumed to be connected with a low-temperature heat floor in an efficient building; the absorber HTF temperature was then tested between 20 and 30°C.

The external fluid mass flows were chosen both sufficiently high to ensure both turbulence for an acceptable heat transfer rate in the exchangers, and sufficiently low for an acceptable temperature difference between the inlet and the outlet of the fluids to allow measurable heat flux in these components. The solution and the internal water flows over the grooved plates were optimized, as a compromise between different constraints:



The mass flow should be low enough for a measurable mass fraction difference between the solution inlet and outlet of the desorber/absorber or a measurable mass flow difference between the inlet and outlet of the evaporator.



It should also be low enough to have a thin film over the plates and consequently efficient heating or cooling of the film interface by the external HTF.  However, it should be high enough to correctly wet the exchanger plates.



It was also linked to technical constraints of the prototype: at excessively high mass flow rates, the solution overflowed the exchanger outlet funnel.

The tests were conducted to prove the feasibility of absorption storage using the KCOOH/H2O couple. Thus, tests were performed in stable conditions (except for the solution mass fraction, which changes through the absorption and desorption processes). The test lengths were limited due to laboratory security constraints (the system could not be left to function without someone in the room to check the machine). The values of the experimental conditions tested are presented in Tables 2 and3.

Experimental results

Charging tests

An example of charging test results is presented in Figure 7. The detailed test conditions are shown in Table 2, in the first column (June 23 rd ). During this 1.5-h test, the external parameters were kept constant. During the first hour of the test (before 10:00, Figure 7(a)), the internal parameters were also nearly constant. The temperature of the HTF decreased in the reactor (TK12 and TK14), providing heat to the solution for the desorption process. The heat provided to the desorber was about 2 kW during that period (Figure 7(b)). At the same time, the condenser HTF temperature increased in the reactor (TK2 and TK13), as the desorbed water from the solution condensed on the cooled condenser walls. The corresponding condensation heat was about 1.5 kW during that period (Figure 7(b): a negative value as the heat is extracted from the process). The sorption enthalpy is usually about 15% larger than the phase change enthalpy of water. The 500-W difference between the two heats involved is explained by the difference between the phenomena and by possible heat losses to the surroundings in the reactor: the reactor is not insulated and despite the low convective heat losses due to the low pressure in the system, radiative and conductive heat transfers in the connecting pipes and mechanical stands could transfer heat to the surroundings. During this first period, the temperature of the solution exiting the reactor was around 50°C (TK9 in Figure 7(a)). This sensible heat is later lost to the surroundings in the solution storage tank. However, a heat exchanger between the entering and exiting solution of the reactor is not a solution during that charging phase: when the hot solution at a high mass fraction of KCOOH exits the reactor, sudden cooling could lead to crystal formation in the heat exchanger, meaning the clogging of the exchanger.

After the first hour (after 10:00, Figure 7(a)), the temperature of the solution entering the reactor increased progressively. Indeed, the flow in the solution tank is of the plug-flow type, and the external cooling of the solution tank did not manage to keep the solution temperature constant at 23°C during the whole test. This temperature increase led to the small decrease of the absorbed heat in the desorber during the last half hour of the test (Figure 7(b)).

Other experimental results obtained during charging tests are presented in Table 2. Important criteria to weigh the performance of the system are the rate of desorbed water and the power exchanged at the desorber, i.e., the power that would be supplied by the solar collectors in a solar storage system. The rate of desorbed water increased when:

 The flow of solution increased within the range tested. For example, comparing the test of June 30 th with the test conducted on June 23 rd , the solution flow rate increased from 51 kg/h to 60 kg/h. This increase resulted in a moderate 5% increase of the desorbed water flow rate. This result was confirmed by the July 5 th test, when a low flow rate of 40 kg/h was tested, decreasing the rate of desorbed water by more than 30%. Increasing the flow rate of the solution improved the efficiency of the exchangers because their wetting rate was increased. This result is only valid for the range of flow rates studied (40-60 kg/h). Indeed, there exists an optimal solution flow rate beyond which a flow rate increase decreases the performance of the process, since the heat transfer from the exchanger wall to the liquid/vapor interface decreases with the thickness of the film [START_REF] Huaylla | Numerical and experimental analysis of an absorption reactor using LiBr/H2O for solar heat long-term storage[END_REF].  The desorption temperature increased. Indeed, comparing the June 23 rd test with the July 4 th test, the HTF temperature of the desorber increased from 57°C to 64°C, improving the rate of desorbed water by 7%. For the LiBr/H2O couple, N'Tsoukpoe et al. [START_REF] N'tsoukpoe | Numerical dynamic simulation and analysis of a lithium bromide/water long term solar heat storage system[END_REF] showed theoretically that a minimum desorption temperature must be chosen for this storage process. Below this minimum value, the water is not desorbed from the solution but only a sensible heat transfer is observed. This was also experimentally observed for the KCOOH-H2O couple on the July 12 th test, when a desorber temperature of 49°C divided the rate of water desorption by a factor of 3 compared to a temperature of 57°C. However, during the desorption tests conducted, the HTF temperature of the desorber could not be increased to temperatures higher than 70°C; otherwise boiling of the solution was observed on the heat exchanger plates, leading to droplet ejections from the film and thus a decrease in desorber performance.

 The mass fraction of the solution decreased. At high mass fractions, the driving force of the desorption process (linked to the difference between the equilibrium pressure of the solution interface and the actual pressure of the water vapor in the reactor) decreased, and therefore the rate of desorbed water decreased. The test conducted on July 20 th and 21 st desorbed an average of 1.13 kg/h and 1.05 kg/h of water, respectively, about 30% less than the 1.51 kg/h and 1.61 kg/h rate obtained on June 23 rd and July 4 th in similar conditions, but at lower mass fractions.

Discharging tests

An example of a charging test result is presented in Figure 8. The detailed test conditions are shown in Table 3, in the second column (July 1 st ). During this test, the air temperature in the room was between 29 and 32°C. Consequently, the absorber HTF temperature fluctuated slightly, due to the functioning of the thermal module heat pump (Figure 8(a)) that had to regulate the HTF temperature close to the ambient temperature. However, this fluctuation was limited to a ±0.5°C around 26°C. The HTF crossing the absorber was heated by the absorption phenomenon occurring in the solution. Moreover, the temperature pinch between the HTF and the solution was very low in this heat exchanger, lower than 2°C. The corresponding heat flux provided to the HTF was around 500 W (Figure 8(b)) throughout the test. During the same period, the HTF crossing the evaporator only provided about 130 W to the internal water for evaporation. The discrepancy cannot be explained by the difference between the phase change enthalpy and the sorption enthalpy. In this case again, heat gains from the surroundings provided heat to the reactor for the water evaporation: indeed, the evaporator temperature was set at 15°C to represent winter conditions, but the tests were conducted in a room at about 30°C, and therefore the external walls of the system could contribute to water evaporation.

Other experimental results obtained during discharging tests are presented in Table 3. For the absorption tests, the main target is the heating of the absorber HTF. The power delivered by the absorber to its HTF increased when:

 The solution flow rate increased. For tests taking place on June 27 th (solution average flow rate of 96 kg/h), July 7 th (69 kg/h) and July 13 th (38 kg/h), the average power outputs were 429 W, 309 W and 245 W, respectively. These results show the importance of the solution's flow rate, because cutting it in half also resulted in a twofold decrease of the power released. Funnels located at the bottom of the heat exchanger plates receive the falling films before the flows exit the reactor. Due to overflowing of the solution funnel at high flow rates, it was not possible to reach solution mass flows higher than 100 kg/h on this heat exchanger, but it is likely that a further increase of the solution flow rate would still lead to an increase in the heat provided because the wetting rates observed on the absorber plates were generally low in the conditions tested.  The water flow rate on the evaporator plates increased. Comparing the test of July 11 th with the test conducted on June 27 th , the water flow rate decreased from 103 kg/h to 64 kg/h, decreasing the heat release from 430 W to 340 W. However, this parameter had a smaller impact than the solution mass flow.  The absorber temperature decreased, as expected. For tests taking place on July 12 th , June 27 th and July 22 nd , the temperature HTF entering the reactor was 20°C, 25°C and 30°C, respectively. The average heating power outputs were then equal to 660, 430 W and 400 W, respectively, even though the mass fraction of the solution was higher during the latter test than for the previous one. This strong dependence on the absorber temperature eliminated the use of this couple in a heat storage process at high temperatures. It is therefore limited to heating floor diffusers.

The influence of the HTF mass flow rate in the evaporator was negligible within the range tested (300-400 kg/h). For higher mass fractions, it was expected that the heat produced would be greater than at lower mass fractions, but the tests did not show that this parameter had a strong influence. The results obtained on June 27 th and July 20 th were similar, although the mass fraction was much higher during the second test than the first one.

In our prototype, crystals appeared in the solution tank only at very high mass fractions, above 73%. This mass fraction, compared to the temperature of the tank during that period, does not correspond to the theoretical saturation values expected. Further tests seem to be needed to determine the precise saturation line of the KCOOH aqueous solution.

General conclusions on the experimental results

The heat storage process with the KCOOH/H2O couple was proven by this experiment.

Desorption tests showed the possible charge of this storage system at temperatures as low as 50-60°C, which can easily be reached by flat plate solar collectors. Discharge tests demonstrated heat production at a sufficient temperature level for building heating purposes using a heating floor (>25°C), with a solution at the absorber outlet at temperatures higher than 30°C. Absorption powers of about 400 W were often obtained in the tested conditions, and for the best conditions, average power of 660 W was provided by the absorber and transferred to the heat transfer fluid. Moreover, crystallization was reached in the solution storage tank (July 21 st and 22 nd ). The following dilution of the crystals during the absorption process was possible (July 22 nd test, Table 3). This shows that the system can be charged and discharged.

These tests were not conducted with the objective of weighing the storage density of the process, given that they were only feasibility demonstration tests. Consequently, the tests were only performed over short periods (about 1-4 h), and the whole mass fraction range of the solution was not exploited over any test (concentrations lower than 60% were not tested, for example, but theoretically this could be an option, as shown in Figure 4). It is therefore not possible to calculate the storage density using this couple: further tests with an improved prototype would be necessary.

Indeed, the absorption power obtained was limited, due to problems wetting the falling film exchangers, both for the solution on the absorber/desorber and the water on the evaporator. This low wettability stems from the low flow rates that are used in this storage process for a higher process efficiency, in contrast to the recirculation rates that are usually accepted in absorption chillers [START_REF] Huaylla | Numerical and experimental analysis of an absorption reactor using LiBr/H2O for solar heat long-term storage[END_REF]. Further investigation is needed to develop efficient falling film exchangers for this specific application. Moreover, the sensitivity of the process functioning to the desorber temperature was observed: at high desorption temperatures (in this case above 75°C), the solution boiled on the plates, leading to droplets of solution ejected from the film. This phenomenon has to be taken into account during the desorber design phase. An option would be to stack several plates in parallel, such that the ejected droplets from one plate could be received by the facing plate. However, stacks combining condenser and desorber plates facing each other, which could have been chosen for compactness of the reactor and to decrease the vapor pressure drop between these components has to be discarded, to avoid mixing risks between the solution and water flows.

Conclusion and outlooks

This study has proved that the KCOOH/H2O couple can be used in absorption processes. The performance of this couple is somewhat lower than that obtained by the LiBr/H2O couple. This confirms the choice of the former in the case of heat pumps, for example, for which the amount of absorbent is limited. However, in the storage process, the choice of this innovative couple could be an option, because it substantially decreases the investment cost of the process. Another major advantage of this sorbent is its low corrosivity, compared to the reference sorbent. However, the use of this innovative couple would be limited to building heating at low temperatures (lower than 30°C) and may not contribute to domestic hot water production, for example. Further work is still needed on the characterization of the couple: parameters such as the enthalpy of the solution as a function of its temperature and mass fraction as well as the diffusion coefficient of water into KCOOH solutions could not be found in the literature in the mass fraction range required for this storage process. This characterization work is still needed to model and optimize the system.

Zhang X., M. Li, W. Shi, B. Wang, X. Li, Experimental investigation on charging and discharging performance of absorption thermal energy storage system, En. Conv. and Manag. 85, p.425-434 (2014). Zhang H., Baeyens J., Cáceres G., Degrève J., Lv Y., Thermal energy storage: Recent developments and practical aspects, Prog. in Energy and Comb. Sc. 53, p. 1-40 (2016).Zhao Y. J., Wang R. Z., Li T. X., Nomura Y.; Investigation of a 10 kWh sorption heat storage device for effective utilization of low-grade thermal energy. Energy 113, p. 739-747 (2016).

Table 1. Technical characteristics of the system sensors ('Name' refers to Figure 6)

Figure 2 .Figure 3 :Figure 7 .

 237 Figure 2. Absorption storage system principle (CS: concentrated solution; DS: diluted solution) (N'Tsoukpoe et al., 2013)

Figure 8 :

 8 Figure 8: Charging test taking place on July 1 st , 2016: temperatures in the components (a) and corresponding heat powers (b)

Table 2 .

 2 If mass flow < 190 kg/h, 0.19 kg/h. If mass flow ≥ 190 kg/h, 0.1% of the measured value. Experimental results of the charging tests.

		Name	Precision
	Coriolis Flowmeter	F1 and F2	Mass flow: Density: 1 kg/m 3 .
	Coriolis Flowmeter	F3	Mass flow: 0.15% of the measured value + zero stability
			Density: 2 kg/m 3
	Coriolis Flowmeter	F4 and F5	Mass flow: 0.1% of the measured value + zero stability
			Density: 1 kg/m 3 .
	Rotameter	F6	1% of the measured value
	Rotameter	F7	Uncertainty: 4% of full scale (max: 500 kg/h)
	Temperature sensors	TK, TT	0.2 °C
	Pressure sensor	P1	0,25 mbar
	Pressure sensor	P2	0.5% of reading
	Pressure sensor	P3 and P4	0.3 mbar
	Level sensors	L1 and L2	1.5% of probe length

Table 3 .

 3 Experimental results of the discharging tests.

	Date	27/6/16 1/7/16 4/7/16 7/7/16 8/7/16 11/7/16 12/7/16 13/7/16 19/7/16 20/7/16 21/7/16 22/7/16
	length	1h53	2h57	3h19	2h40	4h00	3h00	3h18	3h41	2h56	2h46	2h46	1h28
	Evaporator
	HTF mass	300			400					300
	flow [kg/h]
	Absorber
	HTF mass							300
	flow [kg/h]
	Solution
	mass flow	95,8	98,7	98,6	69	70,6	97,1	96,3	38,4	93,8	89,9	52,8	93,8
	[kg/h]
	Water mass flow [kg/h]	103,6	92,5	102,1	94,3	115,9	64,2	108,6	107,9	98,7	102	105,4	100,3
	Absorber
	HTF inlet temperature	25		30		25		20		25			30
	[°C]
	Evaporator
	HTF inlet temperature							15
	[°C]
	initial mass
	fraction in the solution	72,9	74,72	75,9	73,9	76,2	75,9	76,6	74,4	75	77,8	79,2	75,2
	tank [%m]
	Final mass
	fraction in the solution	70,3	71,1	73,2	72	72,6	73,7	73,2	72,4	73,4	75,6	76,6	73,7
	tank [%m]
	average mass
	fraction variation	2,64	3,59	2,66	1,83	3,67	2,17	3,38	1,98	1,59	2,25	2,65	1,50
	[%/h]
	Absorber heat [W]	-429	-496	-190	-309	-367	-337	-661	-245	-223	-398	-401	-404
	Evaporator heat [W]	114	136	184	144	138	91	128	162	159	169	180

Acknowledgements

This work has been financed by the ANR (French National Research Agency) under the research project PROSSIS2 ANR-2011-SEED-0011-01.


A Moreau-Gaudry

O Chenu

M V Dang

J-L L Bosson

M Hommel

J Demongeot

F Cannard

B Diot

C Hughes

Y Payan

A Prince

N Vuillerme

Reduction of Prolonged Excessive Pressure in Seated Persons With Paraplegia Using Wireless Lingual Tactile Feedback: A Randomized Controlled Trial

Keywords: Paraplegia, Perceptive Supplementation, Pressure Ulcer Prevention, Sensory Substitution. I

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the onset of muscular tissue necrosis leading to deep PU [START_REF] Linder-Ganz | Pressuretime cell death threshold for albino rat skeletal muscles as related to pressure sore biomechanics[END_REF]. During long sitting periods, the two main factors that increase the risk of deep PU for persons with SCI are impaired mobility and a sensitivity deficit in the lower limbs that alters their capacity to feel excessive pressure in soft tissues under bony prominences. As a consequence, their ability to shift posture according to an alert signal perceived consciously or unconsciously, is reduced in comparison to healthy persons. Preventive strategies thus recommend that persons with SCI be trained to shift their weight every 15 to 30 minutes when seated [START_REF]Pressure ulcer prevention and treatment following spinal cord injury: a clinical practice guideline for health-care professionals[END_REF]. However, this trained behavior appears to fade away in most persons once they return home after a rehabilitation program. In order to palliate the deficient weight shifting behavior, it has been proposed to create pressure changes using innovative medical devices, such as alternating pressure cushions [START_REF] Stockton | Is dynamic seating a modality worth considering in the prevention of pressure ulcers?[END_REF], off-loading [START_REF] Makhsous | Measuring tissue perfusion during pressure relief maneuvers: insights into preventing pressure ulcers[END_REF] or dynamic sitting wheelchairs [START_REF] Reenalda | Effects of dynamic sitting interventions on tissue oxygenation in individuals with spinal cord disorders[END_REF].

In this work, we evaluate the principle of perceptive supplementation in the target population of persons with SCI. This principle is based on the supplementation of the sensitivity deficit, relative to the information concerning the prolonged pressure applied on the buttock area, by transmission through another functional sensory channel. The validity of the principle has been proven for blind persons in the approach designed by P. Bach-y-Rita [START_REF] Bach-Y-Rita | Sensory plasticity. Applications to a vision substitution system[END_REF]: the deficient vision of blind persons was supplemented by electrodes placed on their backs that produced tactile stimuli according to the visual scene in front of them. After an adaptation period, the blind person forgot the tactile stimuli and perceived objects to be at their actual place. Our hypothesis is that persons with SCI could also use a supplementary sensory input in order to augment the deficient sensitivity of their buttocks, and could thus adapt their weight shifting to the actual risk in their gluteal soft tissues. In this way, the person with SCI would maintain control of their dynamic sitting behavior rather than having a weight shifting pattern imposed upon them.

Although several modalities are possible for sensory input, we chose to use the tongue surface for this work. This choice was motivated by several reasons: firstly, Bach-y-Rita, a reference in the field of perceptive supplementation, obtained better perception results using electrical stimulation of the tongue than tactile systems [START_REF] Tyler | Closing an open-loop control system: vestibular substitution through the tongue[END_REF][START_REF] Bach-Y-Rita | Form perception with a 49-point electrotactile stimulus array on the tongue: a technical note[END_REF]. That the system is now commercialized as an FDA approved vision aid (BrainPort, Wicab, Inc.) and our own previous experience [START_REF] Robineau | Guiding the surgical gesture using an electro-tactile stimulus array on the tongue: A feasibility study[END_REF][START_REF] Vuillerme | Improving human ankle joint position sense using an artificial tongue-placed tactile biofeedback[END_REF][START_REF] Vuillerme | Tongue-placed tactile biofeedback suppresses the deleterious effects of muscle fatigue on joint position sense at the ankle[END_REF] demonstrate the feasibility of using the tongue for sensory input. Secondly, the tongue is the organ that possesses the greatest density of tactile receptors and its discriminative tactile capacity exceeds even that of the fingertips. Furthermore, the tongues moisture ensures good electrical contact and because of the protected environment of the mouth, the hydration and hence electrical properties of the tongue are more consistent than those of the skin, meaning that simpler voltage-control circuitry can be used [START_REF] Kaczmarek | The tongue display unit (TDU) for electrotactile spatiotemporal pattern presentation[END_REF]. Moreover, in persons with SCI, the tongue is one of the few organs to remain entirely free of deficiencies. A final important aspect concerns the stigmatization associated with the use of visible assistive devices. Sensory input via the tongue can be made such that it is not externally visible, therefore making the device more cosmetically acceptable.

To activate the lingual sensory channel, a person with SCI places a wireless lingual device (the Tongue Display Unit -TDU) in their mouth; the device delivers an electrostimulation to the tongue according to input from a pressure map sensor installed on the seat of the person's wheelchair. The underlying idea is to transfer via the tongue the "pain" that the person does not adequately perceive from his/her buttocks because of the SCI. Such a TDU device has already been described in previous studies performed on healthy subjects; these studies have shown that healthy subjects are able to successfully use the device in order to adequately shift their pressure load [START_REF] Chenu | A wireless lingual feedback device to reduce overpressures in seated posture: A feasibility study[END_REF]. Another study has shown that a person with paraplegia could benefit from the TDU as a supplementary sensory input [START_REF] Chenu | Pressure sores prevention for paraplegic people: Effects of visual, auditory and tactile supplementations on overpressures distribution in seated posture[END_REF].

Many studies have evaluated weight shifting behavior using subjects with no sensory deficiency or mobility limitation. We have chosen to evaluate the clinical relevance of our proposed approach using persons with SCI. To the best of our knowledge, this study is the first time that such an evaluation has been carried out.

Three elements were assessed by this clinical evaluation: (i) the effect of the TDU guided stimulations on the postural behavior of the subjects, (ii) the efficiency of the proposed strategy in terms of the reduction of prolonged excessive pressure at the seat/skin interface and (iii) the usability of the developed medical device. To evaluate these three complementary elements, a prospective, randomized, controlled clinical trial was designed in order to obtain a high level of clinical evidence in the target population (ClinicalTrials.gov identifier: NCT00429013 -Pressure Ulcer Formation Prevention in Paraplegics Using Computer and Sensory Substitution Via the Tongue).

II. MATERIALS AND METHODS

A. Study design

This was an open, monocentric, randomized, parallel-group study, conducted at the Clinical Investigation Centre -Innovative Technology (CIC-IT) of the Grenoble Alpes University Hospital.

B. Study subjects

Eligible subjects were persons aged 18 years or over with SCI. The exclusion criteria were as follows: pregnancy, lactation, known cognitive pathologies, persons under legal protection according to French regulation related to biomedical research, persons with pressure ulcers, persons with an intolerance to wearing a palate prosthesis, persons with buccal pathologies and persons unable to autonomously manipulate the palate prosthesis.

C. Materials

The medical device consisted of 3 parts (illustrated in Fig. 1): (i) the pressure map sensor is composed of 32x32 sensors, each of approximate dimension 1 cm², that can measure pressure from 0 to 200 mmHg, with a precision less than 1 mmHg. The data acquisition frequency was approximately 10 Hz. (ii) The TDU is a wireless electrotactile device that consists of an orthodontic palate including a battery, a radio chip for communication and a 6x6 electrotactile matrix. The electric signal conducted by the electrodes is similar to the one designed by Tyler et al [START_REF] Tyler | Closing an open-loop control system: vestibular substitution through the tongue[END_REF], except that the maximum current is 5V. The current is limited in order to improve safety and to simplify the design of such a small device. It was previously verified that 5V was sufficient for users to perceive the electrostimulation of the tongue. (iii) A laptop containing in-house written software that connects the pressure map sensor and the TDU.

D. Methods

Eligibility assessment, information delivery before the subject gave written informed consent and the inclusion of eligible subjects was performed by the principal investigator of the study. Subjects were randomly assigned to the intervention group (with the TDU) or the control group (without the TDU). The random assignment was carried out using a computer generated list of random numbers.

Each subject took part in two sessions, S1 and S2. S2 took place one week after S1. During each session, the subject was seated in their own wheelchair on a flexible pressure map sensor that recorded the pressure applied at their buttock area. Each session lasted for one hour during which the subjects watched a movie. Before S1, the subject chose a movie to watch from a selection. Before S2, the subject chose to either continue watching the same movie or chose a different movie to watch from the selection.

Before each session, a calibration was performed. For the calibration, the subject was asked to position himself in nine different postures (front-left, front, front-right, left, center, right, rear-left, rear and rear-right). For each posture, the system recorded the corresponding pressure map (referred to as the reference maps). During the following session, the current posture of the subject at any given time was determined by comparing the current pressure map with the nine reference maps. The most similar reference map was considered to be the current posture. The nine postures were therefore defined individually for each patient, depending on the ability of the subject to adopt each of the nine postures.

The sessions were carried out as follows: the session was started (t=0) and the pressure data was then read in real time. At t=t1 (60 +/-5 s), the current posture of the subject was estimated and the "cumulative excessive pressure map" (CEPM) from t=0 to t=t1 was calculated. The CEPM enables the area that has been the most exposed to excessive pressure during the previous 60 s to be identified. Considering the estimated current posture and the calculated CEPM, a target posture was determined that most decreases the risk arising from excessive pressure. Then, from t=t1 to t=t1+10 s, the algorithm evaluated if the target posture had been achieved: if so the weight shift was considered to be adequate. Fig. 2 illustrates the described process of adequate weight shifting. The described process was repeated for the duration of the session.

For S1 and S2 of the control group and for S1 of the intervention group, no directional instruction or any feedback of any kind was sent to the subject. For subjects assigned to the intervention group, the TDU was activated during the second session (S2). The previously described process was modified as follows: at t=60 s, a directional instruction on how to change the current posture to the target posture was sent to the subject via the TDU. The TDU signal was stopped as soon as the target posture was achieved or after 10 s. In the latter case (target posture not achieved after 10 s), the signal was stopped after 10 s as it was considered that either the signal was not perceived or the target posture was too difficult to achieve. The described process was repeated for the duration of the second session. The alert frequency was defined as 60 s as it has been estimated that able bodied office workers change seated position approximately once per minute [START_REF] Hedge | Dynamic sitting-how much do we move when working at a computer?[END_REF].

The CEPM was computed as follows: the mean of the pressure maps acquired since t=0 was calculated; from this mean pressure map, all values below an "unsafe" pressure threshold -Prisk -were disregarded and set to zero. The threshold, Prisk, was defined as 100 mmHg, which is in the range of physiological arterial blood pressure.

The target posture was calculated as follows: from the estimated current posture at t=t1, four easily achievable target postures were calculated (by a left, right, front or rear movement of the trunk). For each of these four achievable postures, the pressure decrease of the CEPM if the subject adopted the posture was computed. The posture that most decreased the pressure in the exposed area was considered the Fig. 1. Materials used in the study: while the subject is watching a movie, the pressure map sensor measures the pressure data [START_REF] Mckinley | Longterm medical complications after traumatic spinal cord injury: a regional model systems analysis[END_REF] and sends the data to the laptop (2), which analyses the pressure and -during the second session of the intervention group -initiates every 60 +/-5 s a wireless communication [START_REF] Linder-Ganz | Pressuretime cell death threshold for albino rat skeletal muscles as related to pressure sore biomechanics[END_REF] with the TDU (4) which is placed inside the subject's mouth, and which indicates to the subject to move in a given direction by means of an electrostimulation of the tongue. target posture.

The directional instruction, sent by the TDU to the tongue, was indicated according to the well-known principle of "avoidance", that is, by activating the electrodes on the opposite side of the target posture [START_REF] Elliot | Handbook of approach and avoidance motivation[END_REF]. The signal is an alert signal that enables the subject to react to reduce the "pain" and thus avoid a danger (i.e. the excessive pressure). For example, activating the left side of the TDU means "avoid this and go towards the right side". This concept is illustrated in Fig. 2.

E. Definition of R -the estimation of the risk of developing a PU

In order to quantify the estimation of the risk of developing a PU, a risk index, R, was defined. Though many factors have been identified that contribute to the risk of developing a PU when a pressure is applied on soft tissue, the two main factors are the level of applied pressure and the duration of the applied pressure. An "unsafe" level of pressure may lead to decreased tissue perfusion. If such a decreased tissue perfusion persists for an extended amount of time, it may create an ischemia that can lead to tissue necrosis if left untreated. The risk index R is therefore defined by considering the "interaction" of these two factors: the higher the "unsafe" pressure level is, the worse the tissue perfusion may be. Furthermore, the longer the "unsafe" pressure is continuously applied, the greater the risk of being in conditions that may promote the development of a PU.

The risk index R is therefore defined as the amount of time during which an inadequate tissue perfusion may be encountered, with the amount of time weighted by the level of inadequate perfusion. Inadequate perfusion is correlated with excessive pressure, i.e. the pressure difference between the current level of applied pressure and an "unsafe" pressure threshold (Prisk, previously defined at 100 mmHg).

The calculation of R for one sensor during a session is as follows (illustrated in Fig. 3): (A) the pressure P(t) applied to the sensor is measured with respect to time; (B) the indicator function 1[P(t)≥Prisk] is determined; (C) the instantaneous excessive pressure IEP(t), defined as (P(t)-Prisk)1[P(t) ≥Prisk], is then calculated; (D) the instantaneous time-cumulated excessive pressure IC(t), defined as IEP(t) cumulated over each period of time for which P(t)≥Prisk, is determined; (E) the instantaneous risk IR(t), defined as the instantaneous time-Fig. 2. This figure illustrates the process of adequate weight shifting, which consists of the following three main steps: [START_REF] Mckinley | Longterm medical complications after traumatic spinal cord injury: a regional model systems analysis[END_REF] determination of the current posture of the subject and the four easily achievable target postures: the current pressure map of the subject (a) is compared to the reference maps (b). The most similar reference map, here the posterior-left (red dashed line circle), is considered to be the current posture (c, red). From this posture, the four postures achievable in a single movement (in antero-posterior or medio-lateral direction) are identified (orange ellipses). [START_REF] Bennett | The cost of pressure ulcers in the UK[END_REF] determination of the target posture: the CEPM enables the most exposed area during the last 60 +/-5 s to be identified (d); the most exposed area is compared to the four achievable postures maps (e, orange ellipses) identified in step [START_REF] Mckinley | Longterm medical complications after traumatic spinal cord injury: a regional model systems analysis[END_REF]; the map that most decreases the pressure in the exposed area, here the posterior-right (green dashed line circle), is considered the target posture (f, green). [START_REF] Linder-Ganz | Pressuretime cell death threshold for albino rat skeletal muscles as related to pressure sore biomechanics[END_REF] electrostimulation and evaluation of the final posture: knowing the current and the target postures, a directional signal is sent by the TDU to the tongue (g) according to the principle of avoidance. In this example, the subject has to move from posterior-left to posterior right; as he must move to the right, the left part of the TDU is activated. The final posture of the subject is evaluated (h) in the same way as in step [START_REF] Mckinley | Longterm medical complications after traumatic spinal cord injury: a regional model systems analysis[END_REF]. In this example, the final posture is posteriorright (h, red), this corresponds to the target posture (f, green), we therefore consider the movement to have been correctly executed and the weight shift is considered adequate. Fig. 3. The calculation of the risk index R, for one sensor during a session, is as follows, using a 1.7 s example for illustration: (A) the instantaneous pressure P(t) measured by one sensor of the pressure map sensor; the red dashed line indicates the defined "unsafe" threshold, Prisk = 100 mmHg, above which pressures are considered to be potentially dangerous; (B) the indicator function 1[P≥Prisk] is computed that indicates the temporal ranges of potentially dangerous pressures; (C) the instantaneous excessive pressure, IEP(t) = 1[P≥Prisk]*(P(t)-Prisk), is then determined; (D) the instantaneous time-cumulated excessive pressure, IC(t), is calculated, which represents the accumulation of dangerous instantaneous excessive pressures; (E) the instantaneous risk, IR(t), is then calculated by dividing IC(t) by the mean excessive pressure. The risk index R of the sensor is defined as the mean of IR(t) over the session. The risk index of the entire pressure map sensor is defined as the sum of the risk indices calculated for each sensor. cumulated excessive pressure normalized by the mean excessive pressure, is then evaluated. The normalization is applied in order to take into account the variability of weight between subjects. Finally, the risk index R associated with the sensor, defined as the mean of IR(t) over the session, is calculated. The risk index of the entire pressure map sensor is defined as the sum of the risk indices calculated for each sensor.

F. Evaluation of the usability of the medical device

The acceptability of the medical device for use in daily life was evaluated by the subjects after the second session by means of a questionnaire. The pressure map sensor and TDU were evaluated separately, according to the facility of set-up, removal and usage and the comfort and psychological impact of usage. Each aspect was evaluated using a scale that varied from 0 (acceptable) to 10 (unacceptable) for use in daily life.

G. Outcomes

The main outcome was to evaluate the evolution in the proportion of adequate weight shifts between S1 and S2 and to compare the evolution between the intervention group and the control group.

Two secondary outcomes were evaluated: the first outcome was to evaluate the evolution of the estimation of the risk of developing a PU between S1 and S2 and to compare between the intervention group and the control group. The second outcome was to evaluate the acceptability of the medical device for use in daily life.

H. Statistical analysis

The statistical analysis was performed at the CIC-IT of the Grenoble-Alpes University Hospital. Continuous variables were expressed using medians and interquartile ranges [Q1; Q3] and compared using Wilcoxon tests (W). Data were analyzed according to both the worst-case scenario intention-to-treat (ITT) and per-protocol (PP) principles. The global significance threshold was set at 0.05 for the main and the secondary outcomes.

An interim analysis of the main outcome was planned in order to prematurely stop the study if possible to avoid unnecessarily recruiting subjects with SCI. To prevent the risk inflation of the alpha level, the significance threshold for this interim analysis was set at 0.01 (p<0.01).

The sample size, taking into account the interim analysis, was calculated from modelling the expected results and estimated to be 12 subjects per group. The interim analysis was therefore planned after five subjects per group had been recruited.

The statistical analysis was performed using GNU-R software, version 3.0.3 "Warm puppy" [START_REF]R: A language and environment for statistical computing[END_REF].

III. RESULTS

A. Study subjects

Twelve subjects were recruited from September 2006 to October 2008. However, although subject 10 was initially included, she was subsequently excluded due to an unexpected pregnancy. This subject did not take part in any session. Thus, after randomization, six subjects were assigned to the control group (without the TDU) and five subjects were assigned to the intervention group (with the TDU). Concerning the control group, the data of one subject (the first subject included in the study) was not analyzed because of a software failure: the nine calibration reference maps necessary to determine the posture of the subject during the subsequent session were not recorded. Concerning the intervention group, the data of one subject (the last subject included in the study) was not analyzed because of an irreversible material failure in the TDU (non-functioning wireless module, identified at the beginning of the first session). A flow chart summarizing the subjects included in the study is presented in Fig. 4.

An interim analysis had been planned after the inclusion of ten effectively analyzable subjects, in order to stop the study early if the main outcome had already been proved. However, because of the irreversible failure of the TDU and the excessive cost necessary to repair it, it was decided to conduct the statistical analysis on the nine PP subjects and the eleven ITT subjects that had been included. As the main outcome was proved (p<0.01), the steering committee decided to stop the study.

Table I resumes the clinical characteristics of the subjects on which the PP analysis was performed: age, gender, weight, height, body mass index (BMI), ASIA (American Spinal Injury Association) score used to characterize the level of neurological deficiency for persons with SCI [START_REF] El Masry | Validation of the American spinal injury association (ASIA) motor score and the national acute spinal cord injury study (NASCIS) motor score[END_REF]21] and the number of subjects with post-traumatic PU.

B. Main outcome: adequate weight shifting

The main outcome was to evaluate the evolution in the proportion of adequate weight shifts (a shift of the body in order Fig. 4. Flow chart that details the twelve subjects recruited for the study. After inclusion, subject 10 was excluded because of an unexpected pregnancy (exclusion criteria). Six subjects were assigned to the control group and took part in two identical sessions separated by one week. During each session, the pressure applied at the buttock area of the subject was recorded during one hour. Five subjects were assigned to the intervention group and followed the same experimental protocol, but were equipped with a TDU that was activated during the second session.

to relieve locally prolonged high pressure) and to compare between the intervention group and the control group. The results are presented in Fig. 5 and Table II. For S1 and S2 of the control group and S1 of the intervention group, no directional instruction or any feedback of any kind was sent to the subject. It was therefore expected that the proportion of adequate weight shifts would be low (control group: median p1 = 0.09, median p2 = 0.09; intervention group: median p1 = 0.04). These measurements constitute a reference value to which the proportion of adequate weight shifts in the intervention group using the activated TDU could be compared (intervention group: median p2 = 0.89). Between the first and second session, the proportion of adequate shifts was increased more by every subject in the intervention group than the subjects in the control group. The one-sided Wilcoxon test concludes that the intervention group had significantly greater values than the control group (ITT analysis: p=0.004, W=30; PP analysis: p=0.008, W=20). The magnitude of increase was greater than + 80 % for three of the four subjects in the intervention group, while in the control group it was less than + 11 %. Thus, once equipped with an activated TDU, a person with SCI shifted his or her weight in an adequate way more than 8 times out of 10.

C. Secondary outcome: estimation of the risk of developing a PU

The secondary outcome was to evaluate the evolution of an estimation of the risk of developing a PU and to compare between the intervention group and the control group. The originally defined significance threshold for the secondary outcome (p<0.05) was not adjusted for multiple analyses as only one analysis of the data was carried out. The secondary outcome results are presented in Fig. 6 and Table III. In Fig. 6, the initial distribution of risk index R1 (risk index R during ). This result was not due to differing initial risk index values (R1), as there was no significant difference in the R1 values of the two groups (Fig. 6, left panel, Wilcoxon test p=1).

D. Detailed analysis of individual subjects

It can be seen in the right panel of Fig. 6 and in Table III, that the risk index of subject 3 increased during the second session rather than decreased as was the case for each of the other subjects of the intervention group. For subject 3, the risk index almost doubled: R2/R1= 2.5/1.3 = 1.92. To better understand this "atypical" weight shifting behavior, Fig. 7 shows the timeline of instantaneous risk IR(t), not only for this subject, but also for two other subjects: one is representative of what is observed in the control group (subject 2, top row), and one is representative of what is observed in the intervention group (subject 7, middle row). The pressure map readings for the three subjects during S1 and S2, used to calculate the timelines of instantaneous risk IR(t) shown in Fig. 7, are shown in the video attached to this article.

During the first session, subject 2 from the control group appears to frequently shift weight until t=6 min (zone A: instantaneous risk IR(t) kept near zero). From t=6 min to t=11 min, the subject appears to not move and IR(t) increases linearly during the five minutes (zone B). Then, a partially effective weight shift occurs (point C: IR(t) drops from 6 min to 3 min); and then a completely effective weight shift is observed at t=14 min (point D: IR(t) drops to zero). During this first session, subject 2 appears to produce at least partial weight shifts every five minutes or less. During the second session, the same subject displays two long periods of 12 minutes without any weight shift. The risk index accordingly increases from R1=2.5 min for the first session to R2=3.5 min for the second session. Between the two sessions, a similar risk index increase is observed for every subject of the control group (R2/R1 > 1, cf. Fig. 6).

In contrast, it is apparent from the middle plots of Fig. 7 that subject 7 from the intervention group shifted weight much more often and effectively during the second session than during the first session. In fact, during the second session, this subject performed weight shifting according to the process defined using the activated TDU (that is, every minute and in accordance with the indicated direction) without any evidence of fatigue. For subject 7, the risk index thus decreased from R1=1.9 min to R2=0.7 min between the two sessions.

Subject 3 appeared to shift weight quite frequently during the first session: periods without perceptible weight shifts lasted mostly 1-2 min and were always less than 4 min. During the second session, however, the instantaneous risk IR(t) steadily increases during two long periods: from t=18 min to t=30 min, after which it then drops to zero, and then from t=30 min until the end of the session. When examining the contribution of each of the 32x32 sensors of the pressure map sensor, a single sensor u0contributed to 49% of the risk index measured during the second session. The sensor u0 was located at coordinates {23,16} of the pressure map, which corresponded to approximately the center of the left buttock of the subject. The sensor is indicated by a red circle in the attached video. The timeline IR(u0,t) of this sensor u0 (red line in the bottom right plot of Fig. 7) accounts for the majority of the cumulative excessive pressure measured during the longest steady increase of IR(t). The peaks could be interpreted as weight shifts that relieved the rest of the seat surface, except this spot.

E. Secondary outcome: usability of the medical device

The evaluation by the subject of the acceptability of the medical device for use in daily life was an important element to consider. The two parts of the device, the pressure map sensor and the TDU, were evaluated separately and the results are presented in Table IVa and Table IVb respectively.

The pressure map sensor was evaluated according to the facility of set-up (SetUp), the facility of usage (FacilityUse), the comfort of usage (ComfUse) and the psychological impact of usage (PsyImp) of the pressure map sensor in daily life. The TDU was evaluated according to the facility of set-up (SetUp), the facility of removal (Remove), the loss of tongue sensitivity (LossSens), the comfort of usage (ComfUse) and the psychological impact of usage (PsyImp) of the TDU in daily life. Each aspect was evaluated from 0 (acceptable) to 10 (unacceptable) for use in daily life.

The pressure map sensor was evaluated as being very easy to set-up and use, very comfortable to use and psychologically fairly convenient (median inconvenience 3). Conversely, the TDU was evaluated as being very inconvenient (median inconvenience 10), though very easy to set-up and remove (median difficulty 1).

IV. DISCUSSION

The objective of this study was to evaluate the feasibility and the benefit of using a system based on perceptive supplementation in order to reduce prolonged excessive pressure in the buttock area in persons with SCI. The long-term goal of this study is to help such persons prevent the formation of PU caused, in particular, by impaired mobility and a sensitivity deficit.

The innovative medical device used in this study was developed at TIMC-IMAG laboratory (Grenoble-Alpes University, France). As the device is not CE-marked, a risk analysis was conducted prior to the study in order to comply with French (European) regulation. The clinical protocol was then authorized by the national agency for the safety of medicine and health products (ANSM, previously AFSSAPS) and approved by the relevant ethics committee (CPP Sud-Est V -Grenoble). This translational research is, to the best of our knowledge, the first study designed with high-level methodology to evaluate this innovative medical device in a comparative way using persons with SCI. This study may therefore be the first contribution towards estimating the efficiency of the proposed strategy to reduce prolonged excessive pressure in the buttock area.

Over two years, only twelve persons with SCI were recruited for this study, despite advertisements being published in the local press. The limited number of subjects recruited illustrates the difficulty of conducting such a study with persons with SCI involving a non CE-marked innovative medical device.

An interim analysis was planned after ten effectively analyzable subjects had been included with the stopping criterion being that the main outcome was statistically significant (p<0.01). The inclusion of an interim analysis with a stopping criterion is a standard aspect of clinical trial methodology [START_REF] Moher | CONSORT 2010 explanation and elaboration: updated guidelines for reporting parallel group randomised trials[END_REF][START_REF] Kumar | Interim analysis: A rational approach of decision making in clinical trial[END_REF]. Indeed, it is encouraged to stop the trial if evidence of benefit has been demonstrated [START_REF] Whitehead | Stopping clinical trials by design[END_REF]. For this study, the interim analysis was particularly important considering the difficulty of recruiting persons with SCI and the considerable participation required from the included subjects. However, an irreversible failure of the TDU occurred after twelve subjects had been included, of which only nine were effectively analyzable. The data of subject 1 (control group) was not exploitable because of a software failure (calibration reference maps not recorded), subject 10 was excluded due to an unexpected pregnancy (exclusion criteria, subject excluded before randomization) and subject 12 (intervention group) could not take part in the second session because of the irreversible failure of the TDU. The steering committee decided to conduct the statistical analysis at this point to evaluate whether the study could be prematurely stopped before considering whether to replace the TDU (approximate cost 50,000€). The ITT and PP statistical analysis was performed on respectively eleven and nine subjects. As the main outcome was statistically significant for both analyses (ITT: p=0.004; PP: p=0.008), the stopping criterion had therefore been met and the steering committee decided to stop the study.

The main outcome of this study shows that subjects in the intervention group were able to adopt adequate weight shifting when receiving electrostimulations of the tongue by the TDU (session 2). Although the TDU had been previously tested by one subject with paraplegia (in only two directions) [START_REF] Chenu | Pressure sores prevention for paraplegic people: Effects of visual, auditory and tactile supplementations on overpressures distribution in seated posture[END_REF], the results of this study demonstrate statistically, for the first time, that persons with SCI (i) successfully perceived the TDU electrostimulations, and (ii) were able to modify their posture according to the electrostimulation of their tongue. In the control group, the improvement in adequate weight shifting between the two sessions was null as expected. On the contrary, in the intervention group, as the information was delivered through the TDU during the second session, the median improvement in adequate weight shifting was 86%. However, the median proportion of adequate weight shifts in the second session (p2) of the intervention group was 0.89, rather than the theoretical score of 1 that had been expected. This could be due to a number of different reasons: (i) possible algorithm failure arising from the identification of the postures from the pressure maps [START_REF] Chenu | Conception et validation d'un dispositif de suppléance perceptive dédié à la prévention des escarres[END_REF]; (ii) hardware failure (TDU); (iii) lack or difficulty of perception of the electrostimulation of the tongue; (iv) incorrect interpretation of the TDU message [START_REF] Robineau | Guiding the surgical gesture using an electro-tactile stimulus array on the tongue: A feasibility study[END_REF] and (v) impossibility for the subject to follow the given instruction, that is, to move their trunk in the indicated direction.

One secondary outcome measured the evolution of an estimation of the risk of developing PU. In this study, no statistical difference was observed between the initial distribution of the risk index (R1) in the control and intervention group, indicating a similar risk index in both groups during session 1. Though no evolution was expected, the risk index doubled between session 1 and session 2 in the control group. We have no immediate explanation for such a behavior, though one possible reason is less stress and thus less movement during the second session. Considering the statistically significant PP analysis results, the complete opposite result was obtained in the intervention group: the risk index was on average halved when the TDU was activated. Furthermore, a ratio of approximately 4 between the control and intervention group illustrates the efficiency of such a strategy.

The ITT analysis of the evolution of the risk index in the intervention group was not statistically significant. This is due to subject 3 who displayed a different behavior to the overall group behavior in terms of the main outcome Δp (subject 3 = 0.24 << median intervention group = 0.86) and the first secondary outcome R2/R1 (subject 3 = 1.92 >> median intervention group = 0.55). This was explained by the pressure measured at one particular sensor u0, located at coordinates {23,16} (approximate center of the left buttock), which contributed 49% of the risk index R2 measured during the second session. The highly localized nature of the excessive pressure (1 of 1024 sensors) makes it probable that it was caused by a very locally specific problem (temporary sensor dysfunction, a fold in the subjects clothing, a forgotten object in the rear pocket) rather than for example overall fatigue of the subject, which one would expect to impact a greater number of sensors.

Finally, as such a device would be used daily, a measure of its acceptability and usability was conducted. The results of the questionnaire indicated that the pressure map sensor of the device was positively accepted. However, the TDU of the device was very poorly rated. A different alerting device, acceptable for daily use by persons with SCI, therefore needs to be designed. Different options are currently being investigated such as, for example, the replacement of the TDU by a watch or a smartphone. Though these options are financially advantageous (much cheaper than the TDU), they could not be used by persons with tetraplegia, as the TDU can. A new design for the pressure map sensor part of the device is also currently being studied, in order to render it mobile with energetic autonomy.

Other improvements currently being studied involve work to optimize the alert threshold and the alert frequency. Both values were empirically defined for this study, as due to insufficient evidence, no definitive numerical values exist [START_REF] Coleman | A new pressure ulcer conceptual framework[END_REF]. The alert threshold (Prisk) was defined as 100 mmHg, in the physiological range of arterial pressure. Pressure -the force per unit area -is a function of both the mass and the morphology of the subject. Though pressure increases with increasing mass, studies have measured higher pressure under the bony prominences of subjects classified as thin than subjects classified as obese due to the reduced surface area [START_REF] Kernozek | The effects of body mass index on peak seat-interface pressure of institutionalized elderly[END_REF][START_REF] Garber | Body build and its relationship to pressure distribution in the seated wheelchair patient[END_REF]. The alert frequency was defined as 60 s for this study as it has been estimated that able bodied office workers change seated position approximately once per minute [START_REF] Hedge | Dynamic sitting-how much do we move when working at a computer?[END_REF]. An important perspective is to optimize both alerts such that the system is acceptable for daily life while minimizing the risk of developing PU (alert only in the case of real danger).

The optimization of the alert threshold and frequency would enable the definition of the risk index to be improved. For this study, it was necessary to have a quantifiable means of estimating the risk of developing PU. As no such risk index, to the best of our knowledge, exists in literature, an index was defined that combines the two main factors that contribute to the risk of developing PU: the level and the duration of applied pressure, that are recognized as being correlated with PU [START_REF] Gefen | Strain-time cell-death threshold for skeletal muscle in a tissue-engineered model system for deep tissue injury[END_REF]. Despite the evident limits of the defined risk index, consideration of the relative, rather than absolute, values of the index enabled the feasibility of using perceptive supplementation to aid subjects with SCI to reduce the risk of developing PU to be demonstrated.

A limit of this study is the small sample size which was caused by the irreparable failure of the device and the unjustifiably high cost to replace it considering that the device was evaluated as being very inconvenient for use in daily life. Nevertheless, this study has demonstrated with statistical significance (main outcome: p<0.01, PP and ITT) the feasibility of using perceptive supplementation to reduce prolonged excessive pressure in the buttock area and is a promising method to prevent PU. If the acceptability/usability issue can be resolved, such a device would be useful for (i) preventing PU in fragile persons with SCI, (ii) educating persons with recent SCI to adopt a safe behavior to prevent future PU, [START_REF] Linder-Ganz | Pressuretime cell death threshold for albino rat skeletal muscles as related to pressure sore biomechanics[END_REF] preventing PU in other at-risk groups (for instance elderly, diabetic).

V. CONCLUSION

We have reported here a study that evaluates the feasibility of an innovative medical device that aims to reduce prolonged excessive pressure, recognized as one of the main causes of pressure ulcers in persons with paraplegia. The device is composed of a pressure map sensor that measures the pressure at the buttock area in real time, combined with lingual tactile feedback that alerts and guides the user on how to adapt their posture in order to reduce prolonged excessive pressure.

The device was tested by eleven persons with paraplegia, separated in two groups: the control group and the intervention group. Global results show that the device enabled the subjects in the intervention group to reduce prolonged excessive pressure. Closer analysis of the results show that one subject in the intervention group did not benefit from the device, mainly due to the results of one particular sensor. Interestingly, our system could be adapted to detect this type of abnormality. In terms of acceptability, the subjects evaluated the TDU part of the device as being very inconvenient for use in daily life.

It is a perspective of this study to redesign the TDU part of the device such that it is acceptable for use in daily life (for example, using a watch or a smartphone). Further studies are also required to confirm these preliminary results.

Fig. 5 .

 5 Fig. 5. Evolution in the proportion of adequate weight shifts. Short lines indicate individual results. Long lines indicate the median within each group. The p-value is for a one sided Wilcoxon test between the intervention and control group. ** signifies a p-value < 0.01 and *** signifies a p-value < 0.005.

  of the risk of developing PU during S1; R2 = estimation of the risk of developing PU during S2; R2/R1 = evolution of the estimation of the risk between S1 and S2; the use of italics indicates that missing data has been coded according to the worst-case scenario.

  p1 = proportion of adequate weight shifts for S1; p2 = proportion of adequate weight shifts for S2; Δp = p2 -p1; the use of italics indicates that missing data has been coded according to the worst-case scenario. session 1) in the control and intervention group is shown in the left panel and the risk index evolution R2/R1 (where R2 is the risk index R during session 2) is shown in the right panel. According to the one-sided Wilcoxon test, although no statistical difference is observed between the control and intervention group during the first session (left panel), according to the PP analysis, the intervention group has a statistically significant (p<0.05) lower risk index ratio than the control group (right panel, ITT: p=0.06, W=6; PP: p=0.03, W=2). The absence of statistical significance in the ITT analysis results is considered in the discussion. Considering the PP analysis results, the risk index was decreased more for subjects equipped with lingual feedback than for subjects without lingual feedback. It may be noted that between the first and the second session, the risk index R was on average halved in the intervention group (PP: median [Q1; Q3], R2/R1 =0.55 [0.32; 1.03]), whereas it doubled in the control group (PP: R2/R1 =2.04[1.4; 2.08]

Fig. 6 .

 6 Fig. 6. Evolution of the estimation of the risk of developing a PU between the two sessions. Left panel: distribution of the risk index during session 1 (R1). Right panel: risk index evolution R2/R1 (where R2 is the risk index during session 2). Short lines indicate individual results. Long lines indicate the median within each group. The p-value is for a Wilcoxon test between the intervention and control group (two-sided test on the initial value, one-sided test on the evolution). * signifies a p-value < 0.05.

Fig. 7 .

 7 Fig. 7. Timeline of instantaneous risk IR(t) for subjects 2, 7 and 3 during S1 and S2. Subject 2 and 7 are representative of observations in, respectively, the control and intervention groups. Timeline reading for subject 2, first session: frequent and effective shifts (A), no movement (B), partially effective shift (C), completely effective shift (D). Subject 3 differs from the other participants of the intervention group in that the risk index increases from session 1 to session 2 (R1=1.3 min, R2=2.5 min) instead of decreases. For this subject, during S2, one single sensor at u0={23,16} contributed to 49% of the risk index measured over the 32x32 pressure map sensor; the timeline of cumulative excessive pressure R(u0,t) is plotted in red for this sensor.

  3.75]Each aspect of the pressure map sensor was evaluated from 0 (acceptable) to 10 (unacceptable) for use in daily life.

  aspect of the TDU was evaluated from 0 (acceptable) to 10 (unacceptable) for use in daily life.

TABLE III EVOLUTION

 III OF THE ESTIMATION OF THE RISK OF DEVELOPING A PU

		Subject	R1	R2	R2/R1	median [Q1; Q3]
		1 (ITT)	NA	NA	1.27
	Control group	2 6 8 9	2.5 8.2 9.2 1.2	3.5 16.7 11.7 2.5	1.40 2.04 1.27 2.08
		11	2.5	6.4	2.56
	Intervention group	3	1.3	2.5	1.92	R1: 2.9 [1.8; 5.2] R2: 2.2 [1.5; 2.6] R2/R1: 0.55 [0.32; 1.03] R2/R1: 0.74 [0.37; 1.92]

R1: 2.5 [2.5; 8.2] R2: 6.4 [3.5; 11.7] R2/R1: 2.04 [1.40; 2.08]

R2/R1: 1.72

[1.32; 2.56]

TABLE IVA ACCEPTABILITY

 IVA OF THE PRESSURE MAP SENSOR FOR USE IN DAILY LIFE

		Subject	SetUp	FacilityUse	ComfUse	PsyImp
		2	2	0	1	1
	Control group	6 8 9 11	0 1 1 1	0 0 1 1	0 5 2 1	0 7 8 3
		median	1	0	1
		[Q1; Q3]	[1; 1]	[0; 1]	[1; 2]

TABLE IVB ACCEPTABILITY

 IVB OF THE TDU FOR USE IN DAILY LIFE

	Subject	SetUp	Remove	LossSens	ComfUse	PsyImp

ACKNOWLEDGMENT

The authors gratefully acknowledge the contribution of each of the subjects included in the study. This study was made possible thanks to the TIMC-IMAG laboratory (Grenoble Alpes University, France) that made available the prototype of the evaluated medical device. The authors also acknowledge Guglielmi Technologies Dentaires who produced the wireless Tongue Display Unit for each of the study subjects and Vista Medical company who provided the pressure map sensor.

This research was supported by a grant from the Délégation Régionale à la Recherche Clinique (DRRC) of Grenoble Alpes University Hospital (reference PL/FM/BM DRRC 2006). A. Moreau-Gaudry is with the INSERM CIC 1406,