Pascal Lalanne
email: pascal@hydroclapeyron.com

Paul Byrne Hydroclapeyron

UNDERGROUND PUMPED-HYDRO ENERGY STORAGE USING CARBON DIOXIDE IN SHALLOW LINED ROCK CAVERNS; APPLICATION TO LONG-DURATION STORAGE AS THE ENERGY CONVERSION OF A PUMPED THERMAL ELECTRICITY STORAGE

Keywords: pumped hydro, storage, underground pumped-hydro, heat-pump, heat engine, carbon dioxide

A fast-paced energy transition needs a better penetration of renewables, of heating and of cooling in the worldwide energy mix. With three novelties 1of using shallow high-pressure LRC (Lined Rock Cavern) excavated close to storage needs, 2 -of using a slow-moving CO2 piston applying constant pressure on the hydro part of UPHES (Underground Pumped Hydro Energy Storage) and 3 -of relying on inexpensive thermal stores for the longduration storage of electricity, CO2 UPHES coupled with PTES (Pumped Thermal Electricity Storage) could become a game-changer by allowing the complete integration of intermittent renewable sources. The RTE of the latter system, devoid of the 4 gas machineries of conventional PTES, is targeted to 60-70% and its Capex cost for pure electricity storage is calculated at 2160 USD/kW + 10 USD/kWh (meaning a compounded Capex of 20 USD/kWh only). Thanks to an attractive COP of heat displacement, expected to reach 10 depending on required temperatures, the CO2 UPHES as well as UPHES-PTES hybrid storage could also allow a low cost, low investment (compounded Capex of 4 USD/kWh) and low-emission integration of intermittent renewables with future district heating and cooling networks.

INTRODUCTION

Due to the need of reducing emissions of carbon dioxide (CO2), which is the major greenhouse gas quantitatively in the atmosphere, most countries have decided to increase the share of solar and wind energy in their energy mix. Unfortunately, those two renewable energies are dependent on meteorological conditions and their production are highly intermittent. Besides energy storage of short and medium durations fulfilling several major functions on national power grids, usually by the means of capacitors, flywheels and mainly batteries, long-duration storage (hereby defined as ensuring continuity of power in a range from 5 to 500 hours) is expected to play an increasing role for allowing a higher penetration of intermittent renewables in the energy mix. In a 2018 report "Global Energy System based on 100% Renewable Energy -Energy Transition in Europe Across Power, Heat, Transport and Desalination Sectors", for the Lappeenranta University of Technology in Finland, Ram et al. come to the conclusions that, if the whole energy supply shall depend on renewables by year 2050, the associated worldwide needs for storage shall be in the range of 64,000 GWh of electricity storage of all durations plus 40,000 GWh in the form of thermal storage for pure thermal needs [START_REF] Ram | Global Energy System based on 100% Renewable Energy -Energy Transition in Europe Across Power, Heat, Transport and Desalination Sectors[END_REF]. Consequently, the availability of low-cost storage systems suitable for renewable energy should trigger the reduction of installed capacity of fossil fuel plants, not yet accomplished because of the discontinuous nature of the renewable energy resources. Regarding space cooling, a growing need where the novel storage has unique specific benefits, a 2018 study by the International Energy Agency projects a worrying increase of energy spent to produce cooling from 2000 TWh in year 2016 to 6250 TWh by year 2050 and a dramatic increase from 1.6 billion units of air-conditioners to 6.5 billion units during the same timeframe [START_REF]The future of cooling[END_REF].

PHES (Pumped Hydro Energy Storage) is presently the main worldwide technology for the long-duration storage of intermittent energy such as solar and wind. Cavazzini extents the scope of PHES in power grid beyond restoring the mere balance of supply and demand, to possibly provide much-needed frequency regulation services, requiring PHES to be operated over a wider range of power and with the shortest possible reaction time. The author concludes that an innovative design approach could overcome the remaining challenge where, under these operating conditions, reversible pump-turbine machines presently suffer from an unstable behavior [START_REF] Cavazzini | Solutions For Pumped Hydro Energy Storage Plants[END_REF]. Regarding long-duration as well as seasonal energy storage, Power to Hydrogen and Power to Methane are two field-proven technologies, however high Capex (Capital Expenditure) and low RTE (Round Trip Efficiency) of their present development stage presently preclude both of these technologies from being used for mainstream storage of electricity. An alternative technology, CAES (Compressed Air Energy Storage) is making progress by improving the value of its RTE through the development of AA-CAES (Advanced Adiabatic Compressed Air Energy Storage), however no grid-scale AA-CAES is yet available at this moment. A recent CAES concept for localized facilities such as hospitals, hotels, industrial centers, shopping malls, etc., especially when coupled with production of intermittent renewable energy, is micro-CAES which has the capability to complement a relatively low RTE on pure electricity storage by the supply of cogenerated heat which can be needed for the heating and/or for the hot water consumption of those facilities, resulting in a higher hybrid efficiency for electricity plus heat. Tallini et al. provides an economic analysis of such system in the framework of integration with photovoltaic energy [START_REF] Tallini | Applications of micro-CAES systems: energy and economic Analysis[END_REF].

Another technology for the long-duration storage of electricity, PTES (Pumped Thermal Electrical Storage), also called Power to Heat to Power, seems very promising due to the simplicity, the low Capex and the low embodied carbon of its energy storage within two liquid or solid thermal stores (such as water, vegetable oils, sand, boreholes in rock mass, etc.). Mercangoz et al. emphasize the benefits of using CO2 as the working fluid in a PTES, especially when water is the material for the thermal storages. Using detailed flow sheeting simulation, the authors achieves a RTE of 51% for a 36th European Photovoltaic Solar Energy Conference and Exhibition hypothetic pilot system of 1 MW using 4 machineries of efficiencies 80-86% and achieves a RTE of 65% for a hypothetic commercial system of 50 MW using 4 machineries of efficiencies 86-91 %, in both case for a CO2 based system operating between 122 °C and -3 °C [START_REF] Mercangöz | Electrothermal energy storage with transcritical CO2 cycles[END_REF]. Air or argon based power cycles operating over greater temperature ranges can obtain higher theoretical round-trip efficiencies (50-85%), depending on the design parameters. McTigue et al. study in details all sources of exergetic loss in an attempt to identify the maximum round-trip efficiency that can be achieved and conclude that with an 'optimistic' set of parameters that might be achievable with reciprocating devices, the thermodynamic round-trip efficiency could exceed 85% whilst the system simultaneously achieves an energy density almost an order of magnitude greater than for CAES [START_REF] Mctigue | Parametric studies and optimisation of pumped thermal electricity storage[END_REF]. Georgiou et al. perform interesting simulations on the compared performance of PTES versus another long-duration storage technology called LAES (Liquid Air Energy Storage) and state that roundtrip efficiencies of PTES at the higher end of pressure ratios reach values of ∼80%, adding that these predictions should be seen in light of component performance estimates that have not yet been proven, while for LAES, models show RTE of more than 55% with sufficient utilization of waste cold and heat, when a combination of these two factors is adapted [START_REF] Georgiou | A thermoeconomic analysis and comparison of pumped-thermal and liquid-air electricity storage systems[END_REF].

Unfortunately, as far as pilot systems are concerned, the RTEs of present PTES currently hover around 25-30% due to the limited efficiencies of two compressors and of two expanders working in gaseous or supercritical phase and due to other thermodynamic irreversibilities. For instance, Dietrich et al. realized in 2016 a model using butane as the working fluid and computed a maximum RTE of 27.3% [START_REF] Dietrich | Exergoeconomic Analysis of a Pumped Heat Electricity Storage System with Concrete Thermal Energy Storage[END_REF]. While Siemens Gamesa expects a full-scale deployed system to reach in the future round-trip efficiencies of 50%, a RTE of only 25% has been so-far obtained in June 2019 at Hamburg in Germany after several years of work on a pilot of 130 MWh per week [9]. Although a dozens of concepts, experimental pilot beds and small-scale demonstration plants have recently entered in development, no gridscale PTES has been proposed for construction so far. Taking in consideration the urgency of addressing climate change by increasing the percentage of solar and wind in the energy mix, it is a direction of this work to question whether large efficient gas compressors and gas turbines will be developed fast enough and to contribute by proposing a hybrid PTES devoid of gas machineries.

Electrochemical storages such as batteries are increasingly used by grid-scale operators and in a growing variety of functions, but will probably never be considered for long-duration storage because their shorter shelf-life and their heavy embodied-carbon consumed for the production of the battery and the mining of metals can only be compensated by a fast-cycling, meaning the operation of a high number of short storage cycles.

Due to the scarcity of suitable and readily approved new mountainous locations for PHES, several types of UPHES (Underground Pumped Hydro Energy Storage) have been considered for many years. For instance Pujades et al. have studied UPHES in deep mines and open pit quarries and their influence on groundwater flow [START_REF] Pujades | Underground pumped storage hydroelectricity using abandoned works (deep mines or open pits) and the impact on groundwater flow[END_REF]. Gravity UPHES, operating as PHES with the difference that the lower reservoir is located underground, are relatively costly in investment because, to offer a sufficient density of energy storage, they require to use deep caverns or deep disaffected mines located at depth of 300-1200 m as lower reservoirs. Salt domes are geographically scarce, must be of sufficient depth, and are not suitable to contain fresh water.

A few hydro-pneumatic UPHES were presented instead, using conventional hydroelectric pumps and turbines, where the difference of elevation is replaced by a piston of compressed air in the upper part of a cavern, each 0.1 MPa of pressure providing a 10-meter head. On year 1986, SHELL Internationale Research Maatschappij B.V of Netherlands filed a European Patent at EPO for such UPHES [START_REF]Storage and recovery[END_REF]. Bi et al. studied in 2013 a model of compressed-air pumped-hydro in adiabatic as well as in isothermal processes with the prospect of burying pressurized reservoirs underground and of overcoming some efficiency limitations of CAES by the use of hydro pumps and turbine [START_REF] Bi | Research on Storage Capacity of Compressed Air Pumped Hydro Energy Storage Equipment[END_REF]. Unfortunately, air warms-up with compression during electricity charge, and coolsdown with decompression during electricity discharge. RTE is therefore significantly reduced due to these irreversibilities. However, as reported by Odukomaiya et al., it must be added that at Oak Ridge National Laboratory a small-size aerial prototype of hydropneumatic energy storage was able to reach RTE ranging from 66% to 82% thanks to an innovative heating and cooling of air with water spray and thanks to utilization of waste heat [START_REF] Odukomaiya | Thermal analysis of near-isothermal compressed gas energy storage system[END_REF]. In all cases, unlined natural caverns or unlined mines must also be located deeply underground to maintain the tightness of compressed air through a hydrostatic containment by ground water (i.e.: a 1000meter depth to contain a pressure of 10 MPa). For both types of UPHES, gravity-type as well as hydropneumatic-type, the geographic availability of natural caverns, or disaffected mines, close to actual locations of electricity production, electricity consumption or close to power transmission networks, is also overly limited.

Several improvements are therefore needed so that UPHES can bring significant value for the storage of solar and wind energy. Based on three directions of innovation, the purpose of this early conceptual publication is to address the relevance of a novel type of UPHES for the bulk storage of intermittent electricity by disclosing its leading principles. A hybrid solution of low Capex cost (20 USD per kWh referring to the sole electricity capacity), targeting a medium-high RTE (60%-70%) is proposed, making use of the reliability and efficiency of pumped-hydro equipment, of the emerging technology of shallow LRC (Lined Rock Cavern), of the slow displacement of a CO2-water interface, and of the low Capex cost of large atmospheric thermal storages.

METHOD FOR REACHING THE NOVEL STORAGE DESIGN

The genuine process to reach the concept of this novel energy storage was actually incremental and dense in back and forth evaluations of numerous alternative solutions.

The first direction of improvement is related to the underground pumped-hydro reservoir. Because natural caverns or abandoned mines are too scarce and too remote, because salt domes are not suitable for fresh water, and because UPHESs of high depth are too costly to excavate and operate, the energy storage must be able to be easily excavated at shallow depth (50 -200 m under surface). Because shallow UPHES cannot be of the 36th European Photovoltaic Solar Energy Conference and Exhibition gravity-type, the energy storage must make use of the hydro-pneumatic type. Because shallow hydro-pneumatic UPHES cannot contain elevated gas pressure through the usual hydrostatic pressure of ground water, tightness toward gas and hydro-water must be achieved with the help of a liner, as in shallow LRCs used for natural gas storage in Sweden [START_REF] Johansson | Storage of highly compressed gases in underground Lined Rock Caverns -More than 10 years of experience[END_REF], and later experimented for shallow CAES pilot programs in Japan [START_REF] Rutqvist | Modeling of coupled thermodynamic and geomechanical performance of underground compressed air energy storage in lined rock caverns[END_REF] and in South Korea [START_REF] Kim | Characterizing Excavation Damaged Zone and Stability of Pressurized Lined Rock Caverns for Underground Compressed Air Energy Storage[END_REF]. As water appears under three different functions in this work, as the liquid for hydroelectric energy, as the mean of thermal storage, and as ground water creating hydrostatic pressure, we opt to name "hydro-water", the use of water as the liquid for hydroelectric energy.

Consequently, the novel underground PHES presented here uses artificial shallow LRC caverns for pressurized reservoirs. It is pointed-out that, at high operating pressures of 7-9.5 MPa, cylinder-shaped steel vessels in open-air locations not constrained by rock mass, cannot represent economical solutions for diameters larger than 5 m, since large aerial pressurized vessels benefit of no effect of scale in terms of mass of construction materials and since large thickness of metal are too costly to assemble related to the welding work and its certification. Carbon fiber vessels of large volume enjoy as well no effect of scale and remain uneconomical compared to pressurized caverns.

The second direction of improvement addresses the warming and cooling irreversibilities created by the compression and expansion of air as the hydro-pneumatic gas. To offer an acceptable RTE, the novel storage needs to replace air pushing on hydro-water by a fluid remaining at constant pressure despite the large variations of volume induced by inflows and outflows of hydrowater. Obviously, gas cannot achieve this constant pressure criterions, so the novel idea is to move hydropneumatic UPHES into the domain of liquid-vapor equilibrium (optionally supercritical). Consequently, due to the phase changes expansion and contraction of a fluid at constant pressure, during water inflows (electrical work supplied to the pump), as well as during water outflows (electrical work recovered by the turbine), most of the unwanted warming and of the unwanted cooling of the previous hydro-pneumatic gas solutions are eliminated.

Since the required volume of pressurized UPHES would grow too large, and therefore too costly for long duration storage (5 to 500 hours of operation at nominal power), the third direction of improvement is to realize that the storage of energy per-se must be diverted elsewhere for energy storage of long duration. We opt for large, low-cost, insulated, atmospheric, thermal pits. It will be explained below how a pumped-hydro LRC cavern, coupled with a proper fluid, can become an outstanding locus of conversion from electricity into heat transfer, as well as from heat transfer into electricity.

The following section reports on the data extracted from literature review. It defines the chosen options for the system and the associated economic evaluation by means of Capex (Capital expenditures) and RTE. Capex of storage is expressed in terms of USD/kW for the power of the conversion system and in USD/kWh for the available energy capacity. RTE is the expression of the ratio of recoverable electricity to the electricity consumed to fulfill storage and is expressed in percentage.

RESULTS OF LITERATURE REVIEW AND ECONOMIC ASSESSMENT

Novel use of existing LRC cavern for UPHES 3.1.1. History and characteristics of LRC caverns

Research and development on LRC cavern started in Sweden in the 1990's for the storage of natural gas under shallow depth and under high pressure till 28 MPa (including some tests at 52 MPa) [START_REF] Johansson | Storage of highly compressed gases in underground Lined Rock Caverns -More than 10 years of experience[END_REF]. The main distinctive features of LRC are the following: -the pressure of the gas is opposed by the surrounding rock mass, -a liner of steel or polymer is taking care of absolute tightness,a low ratio of cavern diameter to the thickness of rock-cover protects the cavern against the risk of ground lift-up thanks to the built-up of a near-cone induced by the coefficient of friction of the rock material, as illustrated by two of the three models of Figure 1. Kim et al. have extensively simulated from Sweden trials the conditions of this lift-up and performed sensitivity analyses on 20 variable factors [START_REF] Kim | Parametric sensitivity analysis of ground uplift above pressurized underground rock caverns[END_REF]. From their work, Figure 2 shows an assessment of the safety factor against lift-up depending on normalized storage pressure and on normalized storage depth. Perazzelli et al. studied in 2015 several design issues of LRCs, for their possible application to CAES, including ground uplift, buckling of liner, cycling and fatigue, and concrete plug [START_REF] Perazzelli | Design issues for compressed air energy storage in sealed underground cavities[END_REF]. To evaluate the integrity of cavern sides, Tunsakul et al. performed in 2016 a study of fracture patterns of rock mass around a pressurized storage cavern based on the element-free Galerkin (EFG) method with a cohesive crack model. Quite logically they mention that a value higher than 1.2 is preferred for the local stress coefficient K0, equal to the pressure horizontally borne by the rock mass divided by the vertical pressure induced by gravity [START_REF] Tunsakul | Evaluation of rock fracture patterns based on the element-free Galerkin method for stability assessment of a highly pressurized gas storage cavern[END_REF]. Fortunately, for shallow grounds below 200-meter depth, the condition of medium-high K0 (i.e.: value of from 0.8 to 1.5) is met for a high share of rock masses on Earth. The Swedish program, accomplished with the partnership of the French company Gaz de France (nowadays named ENGIE) started during 1988 by a pilot test of three shaftshaped caverns of 4.4-meter diameter and of 15-meter height at a shallow depth of only 50 m in the Grangesberg granite mountain. Tested liners were steel, thinner stainless steel, and polymer sheets. Later, a larger evaluation and commercial operation program was realized after 1999 at the Skallen mountain, using four large shaft-shaped caverns of 36-meter diameter, 50meter height, 40,000 m3 volume at a depth of 115 m with a large cupola and a curved form bottom [START_REF] Johansson | Storage of highly compressed gases in underground Lined Rock Caverns -More than 10 years of experience[END_REF]. One illustration is shown on Figure 3. The host rock in the area is crystalline gneiss of good quality, intersected by amphibolite dikes of poor quality. After a test period of one and a half years, the facility has been in commercial operation as a part of the Swedish gas grid since early 2004 [START_REF] Glamheden | Excavation of a cavern for high-pressure storage of natural gas[END_REF]. Despite tensile fracturing occurring in the unreinforced concrete layer placed between rock mass and liner, these caverns remain operational until this moment. One LRC cavern was developed at Hokkaido (Japan) in 1990 using a decommissioned coalmine at a 450-meter depth. Its function was the storage of compressed air for a pilot CAES at 8 MPa. This LRC takes the shape of a 57meter long tunnel, 6-meter diameter. The liner is composed of butyl synthetic rubber inside a thin shell made of prefabricated concrete blocks, using more synthetic rubber at the liaison of blocks [START_REF] Rutqvist | Modeling of coupled thermodynamic and geomechanical performance of underground compressed air energy storage in lined rock caverns[END_REF]. During 2010's, a pilot LRC CAES has been built in South Korea, 100-meter deep in a limestone ground. The pilot is composed of two tunnel-shaped LRC caverns of internal diameter 5 m. One cavern is using butyl sheets as liner, while the second cavern is using steel plates [START_REF] Kim | Characterizing Excavation Damaged Zone and Stability of Pressurized Lined Rock Caverns for Underground Compressed Air Energy Storage[END_REF].

3.1.2. Use of LRC cavern for hydro-pneumatic UPHES LRC represent an attractive solution for hydropneumatic UPHES because they can be excavated in shallow grounds and close to area of electric production, of electric consumption or close to power transmission networks. Although there is not yet any LRC actually used for hydro-pneumatic UPHES, we do not anticipate particular constraints related to this specific usage. Although it exceeds the scope of this work, we also do not anticipate particular limitations in the further use of LRCs for most other pressurized storage utilizations such as CO2 sequestration and storage, hydrogen storage, methane storage, etc. In all cases, to limit rock failures to unavoidable lowamplitude tensile fracturing in the vicinity of cavern periphery and to avoid stepping into the domain of shear fracturing, it is recommended that the maximum pressure operated in the cavern does not exceed the uniaxial compression strength of the rock mass, which is usually higher than 15 MPa for most rock formation. In the case of soft rock mass, usually of low Young's modulus (i.e.: lower than 10 GPa), large radial displacements of rock walls must be avoided to hinder a heavy tensile fracturing of the thin concrete shell protecting the liner, as well as to hinder a later buckling of the liner itself at depressurization [START_REF] Perazzelli | Design issues for compressed air energy storage in sealed underground cavities[END_REF]. This can be accomplished by a high-pressure cement-grouting of the rock mass to a radial distance of at least one half-diameter, creating a somehow synthetic rock mass of higher uniaxial compression strength and of higher Young's modulus.

Issues related to fatigue of the concrete shell and liner are of course linked with maximum and minimum pressures during operations, to the quantity of cycles along the cavern life time (40 years expected), to the quality of the rock mass and finally to the shell and liner themselves. Perazzelli et al. have conducted thorough simulations on this subject [START_REF] Perazzelli | Design issues for compressed air energy storage in sealed underground cavities[END_REF]. Another issue is the liner compatibility with fluids contained in the cavern. For instance, CO2 has a strong oxidizing effect on carbonsteel in presence of humidity. Moreover, CO2 shows a high permeability to elastomer membranes as well as to several other polymer materials.

Another consideration about LRC cavern, also not specific of use for hydro-pneumatic UPHES, is the high risk of damaging the liner by the hydrostatic pressure of ground water when the cavern gets depressurized for inspection or maintenance. For all caverns below ground water table, it is therefore compulsory to insert water drains between rock walls and the concrete shell. In that matter, taking benefits of available cliffs or hills, even of low-elevation, can be beneficial to excavate caverns above water tables.

36th European Photovoltaic Solar Energy Conference and Exhibition

In a different respect, untested for UPHES as well as for other storage use, it shall be possible to take benefit of the mass of high-rise buildings (25 -100 floors) to build a cluster of small-diameter shaft-shaped LRCs a few tens of meters under a thick concrete slab placed at the lowest level of underground parking lots. Subject to further geotechnical studies, this configuration of underground storage could become of interest whenever local or national administrations would enforce regulations for on-site Net Zero Emission requirements based on renewables + storage, in urban areas when land surface is highly constrained. To avoid thermodynamic irreversibilities due to warming and cooling of air during compression and expansion, the novel storage offers to replace air pushing on hydro-water by a constant-pressure fluid at liquidvapor equilibrium (optionally supercritical). Only one fluid, CO2, was found by this work to match the four criteria of being inexpensive, of having at ambient temperature a saturation vapor pressure compatible with the pressure range of the hydroelectricity industry (3-15 MPa), of being nonflammable and of being environmental-friendly in terms of depletion of the ozone layer. As this fluid happens to generate some greenhouseeffect in atmosphere, undertaking its sequestration underground is also considered beneficial.

Although CO2 implies the complex use of an impermeable membrane to avoid losses due to its high solvability in water at high pressure, it allows to recharge and discharge electricity at constant pressure, roughly 5.7 MPa (570-meter head) when ambient temperature is 20 °C (or roughly 420-meter head in configurations where the receiving pound is located at ground surface 150 m above the cavern) or at much higher pressure when heated. Some semi-crystalline polymers such as PET (Polyethylene Terephthalate), Polyamide 11 (Rilsan), Nylon 6, Nylon 66 and metals such as aluminum foil are promising candidates for the assembling of an impermeable multiple-layer membrane. The French national institute of research IFP Energies Nouvelles has extensive experience with permeation of CO2 and is interested to assist the definition, assembling and testing of a suitable membrane.

Although condensation and gasification of CO2 happens at nearly constant temperature and pressure in the UPHES LRC cavern, or preferably in a smaller dedicated LRC cavern located beside the main cavern, back and forth supply and removal of important amount of heat are compulsory to the phase changes of CO2. The system uses therefore a wide area of heat exchangers at the lower and upper parts of the smaller cavern. Spraying of droplets of CO2 pumped from the bottom of the smaller cavern toward its cupola shall facilitate phase change. Forced circulation of CO2 between the two caverns will probably be compulsory to reduce temperature gradients and to allow a complete phase change by convection of the CO2 mass.

By performing simple calculations on enthalpy of CO2 latent heat, we can assess that the amount of heat displaced to perform a full change of phase of the CO2 mass at work (optimal CO2 mass to maximize the mechanical work by displacement of the maximum volume of water) is very significant and roughly equal to 500% of the hydroelectric energy charged in the cavern. The Pressure-Enthalpy diagram related to CO2 is appended in Figure 4. 3.2.2. Operation as a storage of short or medium duration Although, the highest benefits of the novel storage refer to long-duration storage, it can sometimes be used as a short-duration storage (less than 1 hour of operation) or as medium duration storage (1 to 5 hours of operation), particularly in off-grid area, where the novel UPHES can replace the ubiquitous use of diesel generators.

In such use, when heat or cooling is not needed by users, one single contraction of CO2 corresponds to one complete energy charge and one single expansion of CO2 corresponds to one complete energy discharge. Depending on ambient temperature at the location, and on the location of the receiving pond, the pumped-hydro head goes to around 400-700 m. In such use, the large amount of hydro-water available internally gets used to buffer the large heat transfers, increasing and decreasing in temperature by around 10 K in each process.

Operation as short-duration or mediumduration storage, plus as heat-pump

Quite often, heat or cooling is highly valuable, i.e. for district heating and district cooling, supermarkets, hotels, hospitals, industries, call-centers, etc. and gets produced by the UPHES working in a slow and near-reversible way as heat-pump with an attractive COP (Coefficient Of Performance, equal to the ratio of useful thermal energy produced over electric energy consumed). We can read from Figure 4 that to export heating at temperature higher than ambient, we need to pump-in hydro-water at a pressure higher than 7 MPa. We can read as well that to export cooling at temperature lower than ambient, we need to let-out hydro-water at the turbine at pressure lower than 6 MPa. From simple computation of mechanical work, a COP of up to 10 can be expected, depending on required temperatures for heating or for cooling, related to the decrease of hydroelectric energy recovered.

For the particular case of cooling needs, it is important to stress-out that the cavern extracts heat at the same time it produces hydroelectricity at the turbine. Indeed, due to the CO2 mass acting as a PCM (Phase Change Material) in the quite uncommon pair liquid-gas for phase change, the cavern stores significant amount of cooling at the same time it stores electricity, and additionally, does not need power to restitute such cooling. This unique feature could prove very useful for complementing intermittent renewable energies such as solar and wind to buildings having high cooling needs, 36th European Photovoltaic Solar Energy Conference and Exhibition such buildings in tropical areas, on-grid as well as off-grid. By enabling the continuous supply of hybrid energy (electricity + cooling), this feature could bring a radical solution to the worrying trend of fast-increasing energy spent worldwide for cooling, especially in China, USA, India and Indonesia [START_REF]The future of cooling[END_REF][START_REF] Hannu | Meeting global cooling demand withphotovoltaics during the 21st century[END_REF].

The feature of CO2 acting as PCM for heating storage does not allow for the same benefit, since the production of heating only exists during charging of electricity and cannot happen without electricity supply. However, this issue can be fixed by the provision of an ancillary thermal storage (i.e.: an insulated tank of hot water) allowing to fulfill heating needs around the clock, including outside of timeframes of electricity charging. 3.2.4. Economic evaluation as a storage of short and medium duration, with or without the heat-pump function. Comparison with electrochemical storage.

Based on a system of power 100MW and of energy capacity 400MWh (4 hours), and to calculations based on a compounded excavation cost of 300 USD/m3 for pressurized LRC (main cavern + smaller CO2-dedicated cavern), of a cost of 30USD/m3 for the hydro-water reception pound, of a cost of 750 USD/kW for turbine + generator, of a cost of 350 USD/kW for pump + motor, a cost of 250 USD/kWh for heat exchanger and of a cost of 150 USD/kw for building and ancillary equipment , the two features of using LRC cavern and of replacing air by CO2 bring the Capex of this short and medium duration storage of pure electricity to 1500 USD/kW Elec. + 200 USD/kWh Elec.

The power component of Capex is simply the sum of the 4 items related to power (750 USD/kWh +350 USD/kWh + 250 USD/kWh +150 USD/kWh = 1500 kWh), while the energy component needs to be calculated in a table like Table 1 below. The limitation of this configuration appears immediately by realizing that stored energy is proportional to the high cost of excavation of cavern volume and that no significant effect of scale can be expected.

For value of energy density in this configuration, it is assumed that water takes 70% of the cavern volume and that recovered CO2 pressure gets limited to 90 bar, bringing a value of stored energy equal to 1.750 kWh per m3 (before loss at the hydro turbine).

Volume in m 3

Cost in USD/ m 3 Because of light thermal irreversibilities due to the temperature pinches at heat exchangers (i.e. 5 K at each side) occurring during the back and forth changes of phases of CO2, the RTE of this constant pressure UPHES shall be slightly lower than the RTE of conventional gravity PHES or of gravity UPHES. In this respect, it is specified that although Figure 4 shows several thermodynamic cycles close to the critical point of CO2, it is likely that simulation and experimentation will encourage the authors to operate alternative cycles farther apart from this point, to ensure a better stability of the volumetric mass of the fluid as well as a better heatexchange coefficient. RTE shall however be significantly higher than the RTE of hydro-pneumatic UPHES using air or any other gas in the sole gaseous phase. Our preliminary target for RTE in the simplest application for storage of short and medium duration, without the heatpump function, is 73%, computed from typical efficiency of 90% each for the hydro pump set and the hydro turbine set, and from 5% as a rough valuation of the loss of work due to the temperature pinch at each of the two heat exchanges. When the system is requested to produce heating of mid-high temperature (50-80 °C) and/or cooling of mid-low temperature (5-15 °C), the absolute figure of recovered electricity in kWh needs to be lowered by a value approximately equal to 1/10th of the thermal energy exported or brought-in. Validation by simulation and experimentation are planned in a later stage and are absolutely needed to ascertain a more reliable value of RTE in each storage mode. A cooperation agreement has been signed with the French national institute of research IFP Energies Nouvelles to assist in simulation of heat exchanges, in assessment of RTE and in the dimensioning of heat exchangers for a 2 m3 aerial prototype.

Cost

As a crude estimate of the pure Capex for the supply of heating and cooling, we can simply divide the Capex for electricity by 500%, which brings a Capex of heat and cooling to roughly equal 300 USD/kW Thermal + 40 USD/kWh Thermal.

Related to this use as storage of short and medium duration, and compared to electrochemical storages such as batteries, whose Capex is expected to reach similar values of 200 USD/kWh by years 2025-2030s, this novel UPHES offers the distinctive benefits of a longer lifetime (40 years expected), of not relying on foreign-sourced metals nor on high embodied-carbon metals, of not requiring challenging disposal nor recycling, plus of displacing a large amount of heating and cooling at each contraction or expansion of the CO2 mass. Another challenge specific of long-duration storage is to ensure a very low discharge rate (such as allowing energy losses of only 0.10 -0.25% per day).

Long duration storage shall also show a very low embodied-carbon. In such storage most of the energy capacity will be put at work only a limited number of times in the shelf-life of the storage, consequently any longduration storage with embodied-carbon above a low threshold (in grams of embodied-carbon per kWh of energy capacity) would never be able to repay for its 36th European Photovoltaic Solar Energy Conference and Exhibition manufacturing footprint. This single much as Capex costs, is the main reason why most analysts conclude that electrochemical storages will remain restricted to the applications of short-duration (less than 1 hour) and medium duration (1 to 5 hours) timeframes. According from a year 2013 report from Stanford University, the Energy Intensity of batteries over their lifetime is only 2 to 10 despite the assumption of fastcycling, while the Energy Intensity of pumped-hydro is around 210 [START_REF]Stanford scientists calculate the carbon footprint of grid-scale battery technologies[END_REF].

In addition to the three constraints above, longduration storages must show a reasonably high RTE like every other storage (i.e.: higher than 60%), failing which the operational cost of stored energy would be heavily penalized by the cost of having consumed an amount of energy significantly higher than the amount of delivered energy. A high RTE is a major factor to charge a low LCOS (Levelized Cost Of Storage) to end-users.

To this date, only conventional PHES fulfills this set of constraints, albeit with strong geographical limitations for additional projects, regarding new suitable locations as well as reasonably fast approval process.

Moving the storage capacity outside of UPHES, toward thermal stores

In response to the medium-high Capex for pure UPHES, a further innovative leap in technology and cost structure is needed as the third improvement of UPHES to achieve a Capex compatible with long-duration storage (5 to 500 hours), one order of magnitude lower than the previously computed value of 200 USD/kWh for storages of short and medium duration.

Since the required volume of such pressurized UPHES would prove too large, and therefore too costly for long duration storage, the storage of energy must be diverted elsewhere. We opt for large, low-cost, insulated, atmospheric, thermal pits or thermal tanks as described in Figure 5. One pit, called Hot Store contains hot water at 95 °C, similar to the 40,000 m3 -200,000 m3 pits used in Denmark for seasonal thermal storage, losing only 0.1 K per day [START_REF] Energinet | Technology Data for Energy storage[END_REF]. A second pit, called Cold Store contains a salted ice slurry at -20 °C. The exergy from a thermal energy source is defined by the amount of mechanical work recoverable by a Carnot machine. Very interestingly, one shall realize that exergy of hot water is very high, much higher than energy of water subject to the sole earth gravity. For instance, exergy of water at 100 °C related to ambient temperature is equal to 12 kWh/m3. This is one order of magnitude higher than the mechanical energy of the same water in pumped-hydro, equal to 1.2 kWh/ m3 for a hydro plant of 360-meter head.

Figure 6 illustrates the principles of energy storage for a PTES using a UPHES for electricity conversion, while Figure 7 describes its main components. The radical innovation is that those two stores are thermally charged and recharged by the heat supplied and extracted during the time hydroelectricity is charging the UPHES, the slow moving CO2-water interface acting as a novel type of heat-pump. Figure 8 describes the system operation on the Pressure -Enthalpy diagram related to CO2.

Figure 6: principles of energy storage for a PTES using a UPHES for electricity conversion

For the Hot Store to work in the domain of 95 °C temperature, it is pointed-out that pressure and temperature of pure CO2 in the cavern need to exceed the critical pressure and temperature of CO2 at the critical point (7.39 MPa, 31.1 °C). Such charging and discharging operations in trans-critical phases do not create specific irreversibilities of their own, except for the added complexity that overall heat exchanges for the expansion and contraction of supercritical CO2 at the upper constantpressure ceiling needs to be performed by a glide of several heat exchanges at increasing and decreasing temperatures. It is specified than despite incursions of the working fluid in supercritical phase, the hydro-pump and the hydroturbine remain the only two machineries needed by the novel storage. The French public institute of research IFP Energies Nouvelles is presently assisting to determine whether it could be valuable, instead of using pure CO2 at every occasion, to consider using binary/ternary mixtures of CO2 with selected molecules in low molar fraction, in order to displace the critical point of the CO2 mixture. If results of simulations and experiments were to prove positive, the novel storage could benefit of several types of CO2 mixtures, stored in several smaller dedicated caverns, to fulfill different situations (production of hydro mechanical energy only, co-generation of cooling, co-generation of heating, operation as hybrid UPHES-PTES).

It is pointed out as well that, compared to most other PTES, thermal stores temperatures selected by this novel storage are classified as low. Several other PTES are using temperatures of 500-750 °C for the Hot Store in pursuit of a higher storage density, at the cost of higher Capex of energy and of sophisticated operating procedures. Other PTES using CO2 as working fluid for energy conversion, albeit in continuous high-speed circuits of vapor compression differing greatly from the discontinuous expansion/contraction of CO2 in closed caverns, use temperatures of 120-140 °C for the Hot Store in order to gain in energy [START_REF] Kim | Isothermal transcritical CO2 cycles with TES (thermal energy storage) for electricity storage[END_REF]. Our analysis is that the challenge of lowering CO2 emission worldwide can only be achieved with the simultaneous higher penetration of low-cost and low-emission district heating and district cooling to residential buildings, office buildings and to some industrial developments, such as pulp and paper, pharmaceuticals, agro-food industry, textile, etc. In particular, our vision objects to the growing installation of stand-alone electric heaters and stand-alone airconditioners because in most countries those lowefficiency devices will remain powered by electricity of high carbon intensity, often from coal power plants, for many decades to come. Therefore, we foresee that the low temperatures selected for our thermal stores shall allow utility operators to readily integrate those stores, as well as the UPHES conversion caverns, for the production of lowcost and low-emission district heating and district cooling.

An additional benefit for this UPHES-PTES hybrid storage to work in the "low temperatures" domain is that this feature provides to the system a larger occurrence to increase its RTE, when very-low-grade geothermal energy or very-low-grade industrial heat gets available from the nearby environment (very-low-grade being hereby defined from being in the 50-65 °C temperature range). Because very-low-grade heat is considered "end-energy", it comes "for free" and goes unaccounted-for in the definition of system efficiency. Consequently, RTE of the novel storage could exceed 100% in conditions where very-low-grade heat is available at sufficient power and temperature.

When an external source of low-grade energy is freely available (low-grade being hereby defined from being in the 65-95 °C temperature range), the same "low temperatures" feature of the working fluid shall allow the UPHES to work as an autonomous thermal engine, producing net-work without depending on thermal stores nor on external electricity source. Examples of such net production of electricity are the availability of low-grade geothermal energy or low-grade industrial heat. When the low-grade source of energy is of suitable temperature however of low power (i.e. of weak flow), the procedure is to include this inexpensive energy in the heating process of the CO2 mass or to store it in water tanks to operate only a few hours per day in net-work mode. Operational modes taking benefit of very-low-grade or low-grade energy usually require the use of the ambient environment as a secondary thermal source.

With or without the availability of external thermal sources supplementing the two thermal stores, in discharge mode, the effect of applying a hot source, and subsequently a cold source, to a stand-alone UPHES cavern creates a near-reversible thermal engine, slowly expanding/contracting CO2, pushing-out hydro-water to the hydroelectric turbine, and subsequently sucking-in hydro-water back. The favorite configuration to maintain steady power is to rig two caverns A and B asynchronously in parallel to a single turbine set and to use two receiving ponds, or one larger receiving pond, located at the ground surface. While cavern A pushes-out hydro-water at 9.5 MPa pressure thanks to the expansion of CO2 heated by 95 °C water at its evaporator heat exchanger, cavern B gets its condenser heat exchanger connected to a -20 °C coolant, reducing gradually its inner pressure down to 2 MPa and allowing hydro-water to be sucked-in from the surface with close to no pumping work. Before cavern A gets entirely emptied, cold CO2 of cavern B had been pre-heated, or had been swapped with an external mass of pre-heated CO2, and get ready to take-over the pushing-out of hydro-water to the same turbine set thanks to the expansion of CO2 heated by 95 °C water, while cavern A gets prepared to suck-in the fresh water needed for the continuous operation of the turbine set.

In charge mode, for transferring heat from Cold Store toward Hot Store in a steady way for the power network while the storage is in heat-pump mode, the favorite configuration is also to rig two UPHES caverns asynchronously in parallel to a single pump set and to use one or two receiving ponds located at ground surface.

Whenever the Cold Store, due to design or to temporary dysfunction, is of temperature significantly higher than -20 °C, or when the Cold Store is just replaced by the ambient environment as it often happens in PTES, the favorite operative configuration would be to rig two caverns A and B in opposition, communicating by the mean of hydro-water, and by the mean of either one single pump set when charging in heat-pump mode, or of one single turbine set when discharging in thermal engine mode. Thus, the operation of heat-pump and of thermal engine by two caverns rigged in opposition avoids the detrimental back-work ratio of having to repeatedly spend mechanical work to reintroduce hydro-water at mid pressure inside the colder cavern A or B due to the insufficiently low temperature of the Cold Store. In such a configuration of caverns in opposition, illustrated in Figure 9, receiving ponds are left unused/idle.

Economic targets of the UPHES-PTES hybrid long-duration storage

Target for RTE of our novel storage, at 60% -70% may seem an ambitious figure compared with results of only 25-30% currently achieved for pilot systems of other PTES. Besides similar thermal irreversibilities due to imperfect heat exchanges, other existing PTES use highspeed circuits of a working fluid (steam or refrigerant) and need 4 complex turbine or piston machineries working only in gaseous and supercritical phases (1 compressor and 1 expander for the heat-pump mode, 1 compressor and 1 expander for the thermal machine mode). Kim et al. show that, for a traditional vapor compression process bearing a typical back-work-ratio of 36.5% and assuming realistic grid-scale efficiencies of 80% for each machinery, the best RTE which can be attained in theory by high-speed transcritical circuit PTES is 45% [START_REF] Kim | Isothermal transcritical CO2 cycles with TES (thermal energy storage) for electricity storage[END_REF]. Dietrich et al. realized in 2016 a model using butane as the working fluid and computed a maximum RTE of 27.3% only [START_REF] Dietrich | Exergoeconomic Analysis of a Pumped Heat Electricity Storage System with Concrete Thermal Energy Storage[END_REF]. While Siemens Gamesa expects a full-scale deployed system to reach in the future round-trip efficiencies of 50%, a RTE of only 25% has been so-far obtained in June 2019 at Hamburg in Germany, after several years of work on a pilot of 130 MWh per week [9]. The present lack of highrange RTE in pilot and demonstrator systems is probably the cause impeding the worldwide unfolding of grid-scale PTES. In the favorite configuration of the novel hybrid UPHES-PTES, only 1 operation of a hydro pump and 1 operation of a hydro turbine are needed for a conversion round trip. Thermal irreversibilities of the hybrid UPHES-PTES differs slightly from those of the simpler UPHES as described in paragraph 3.2.4. by the minor fact that hot water and cold water get stored and loose some energy to the ambient environment during storage time (i.e. 3% loss per month). To ascertain the value of RTE in the hybrid mode relatively to the 60-70% target, is planned for early 2020 the construction of an aerial prototype made out of two steel vessels, of 35 mm metal thickness, for a total pressurized volume of 2 -3 m3.

Thanks to the low cost of atmospheric insulated thermal storages, assessed at 40 USD/ m3 for the Hot Store pit [START_REF] Hannu | Meeting global cooling demand withphotovoltaics during the 21st century[END_REF] and evaluated at 100 USD/ m3 for the Cold Store slurry pit, the expected Capex of a hybrid UPHES-PTES assisted by two large thermal stores can be evaluated, using the same other cost components than in paragraph 3.2.4. above, to a Capex of electricity equal to 2160 USD/kW + 10 USD/kWh (meaning a compounded Capex of 20 USD/kWh only, for a 216-hour storage).

Calculations can be counterintuitive because in this configuration the cavern is not any longer a component of energy capacity Capex cost but, as the mere locus of energy conversion, a component of power Capex cost. Based on the same required power of 100 MW but with an energy capacity increased to 100 hours and to 10 GWh, based on two caverns in asynchronous parallel mode achieving a complete cycle in 2.5 hours, we calculated that we need 200 000 m3 of caverns, 200 000 m3 of water receiving pond, 987 000 m3 of Hot Store and 525 000 m3 of Cold Store to achieve the required power and energy capacity.

The power component of Capex cost is therefore increased, still for 100 MW, of an extra cost of 200 000 m3 X (300 USD + 30 USD), thus 660 USD/kW, raising this power Capex cost to 2160 USD/kW. The energy component of Capex needs to be calculated in a table like Table 2

DISCUSSION

A hybrid solution of low Capex (20 USD per kWh of electricity) and of medium-high targeted RTE (60 -70 %) is proposed, making use of the reliability and efficiency of pumped-hydro equipment, of the emerging technology of shallow LRC (Lined Rock Cavern) and of the low Capex of large atmospheric thermal stores. More thermodynamic work is needed, by software simulation and small-scale experimentations to precisely evaluate the values of thermal irreversibility at heat exchangers, the possible gradients of temperature of CO2 within the caverns, and the Round Trip Efficiency of the system in different storage modes. More work is needed to define the components of the membrane ensuring tightness of CO2 related to hydro-water. Some chemistry work is needed on the possible use of binary/trinary mixtures of CO2 with selected molecules in low molar fraction, in order to optionally displace the critical point of CO2 mixtures according to the different storage modes.

CONCLUSIONS

Innovative ways to enlarge the contribution of Pumped-Hydro is desirable for the energy transition. This paper has at first studied a CO2 UPHES using a LRC cavern as the place of storage of mechanical energy, concluding to a Capex of pure electricity equaling 1500 USD/kW Elec. + 200 USD/kWh Elec.. Despite being simpler for storages of short and medium term durations, this configuration suffers, for long durations and heavy energy capacity, from the high cost of cavern excavation and from a weak effect of scale. A second configuration is therefore proposed for long-duration storage, where the CO2 UPHES is actually the conversion subsystem of a larger PTES, leading to a Capex of electricity equaling 2160 USD/kW + 10 USD/kWh (meaning a compounded Capex of 20 USD/kWh only, for a 216-hour storage). The RTE of the latter system, devoid of the 4 gas machineries of conventional PTES is targeted to 60-70%S. With the three innovations of using high-pressure and low-depth LRCs caverns excavated close to storage needs, of using a CO2 piston slowly applying constant pressure on the hydro part of UPHES and of relying on inexpensive Hot Store and Cold Store for the long-duration storage of electrical energy, CO2 UPHES coupled with PTES could become a game-changer by allowing a complete integration of intermittent renewable sources in the energy-mix. Moreover, the paper has showed that, due to an attractive COP of heat displacement, expected to reach 10 depending on required temperatures, the CO2 UPHES as well as the UPHES-PTES hybrid storage could also grant a low LCOS, a low investment (compounded Capex of only 4 USD/kWh thermal, for a 216-hour storage) and low-emissions for the integration of intermittent renewables deemed to displace fossil fuels in future district heating and cooling networks, as required by cities and by some industrial developments, accelerating further the energy transition.

PATENTS

Pending Patent: Dispositif hybride de stockage ou de conversion énergétiques, à fluide propulseur liquide, gazeux ou supercritique, filed at INPI, by Pascal Lalanne holder of patent rights, on the 19th of June 2019, with priority date 16th of February 2019 from previous pending patent.

Figure 1 :

 1 Figure 1: three models for action of ground against liftup due to gravity. W is weight, alpha is the angle of friction of the rock mass, usually 30-45 degrees [19].

Figure 2 :

 2 Figure 2: computerized value of the safety factor against lift-up related to normalized storage pressure (dimensionless value of pressure in cavern divided by rock weight applied to cavern ceiling) and related to normalized storage depth (dimensionless value of cavern ceiling depth divided by cavern inner radius) [17].

Figure 3 :

 3 Figure 3: vertical plane section of the Skallen gas storage plant, showing one of the 4 LRCs [19].

3. 2

 2 Replacing air by CO2 at liquid-vapor equilibrium in a hydro-pneumatic UPHES 3.2.1. Reaching constant pressure for a hydropneumatic UPHES

Figure 4 :

 4 Figure 4: Different applications of the UPHES for storage of short and medium durations, illustrated on the Enthalpy -Pressure diagram related to CO2.3.2.2. Operation as a storage of short or medium duration Although, the highest benefits of the novel storage refer to long-duration storage, it can sometimes be used as a short-duration storage (less than 1 hour of operation) or as medium duration storage (1 to 5 hours of operation), particularly in off-grid area, where the novel UPHES can replace the ubiquitous use of diesel generators.In such use, when heat or cooling is not needed by users, one single contraction of CO2 corresponds to one complete energy charge and one single expansion of CO2 corresponds to one complete energy discharge. Depending on ambient temperature at the location, and on the location of the receiving pond, the pumped-hydro head goes to around 400-700 m. In such use, the large amount of hydro-water available internally gets used to buffer the large heat transfers, increasing and decreasing in temperature by around 10 K in each process.3.2.3. Operation as short-duration or mediumduration storage, plus as heat-pump Quite often, heat or cooling is highly valuable, i.e. for district heating and district cooling, supermarkets, hotels, hospitals, industries, call-centers, etc. and gets produced by the UPHES working in a slow and near-reversible way as heat-pump with an attractive COP (Coefficient Of Performance, equal to the ratio of useful thermal energy produced over electric energy consumed). We can read from Figure4that to export heating at temperature higher than ambient, we need to pump-in hydro-water at a pressure higher than 7 MPa. We can read as well that to export cooling at temperature lower than ambient, we need to let-out hydro-water at the turbine at pressure lower than 6 MPa. From simple computation of mechanical work, a COP of up to 10 can be expected, depending on required temperatures for heating or for cooling, related to the decrease of hydroelectric energy recovered.For the particular case of cooling needs, it is important to stress-out that the cavern extracts heat at the same time it produces hydroelectricity at the turbine. Indeed, due to the CO2 mass acting as a PCM (Phase Change Material) in the quite uncommon pair liquid-gas for phase change, the cavern stores significant amount of cooling at the same time it stores electricity, and additionally, does not need power to restitute such cooling. This unique feature could prove very useful for complementing intermittent renewable energies such as solar and wind to buildings having high cooling needs,

3. 3 A

 3 novel long-duration storage, hybrid of hydropneumatic UPHES with PTES Layout of the text 3.3.1. Overall considerations for long duration storage Long duration storage faces the tough challenge that it needs to require a very low Capex for energy capacity (i.e.: below 50 USD/kWh) because in such storage, most of the available energy is left idle over long periods of time and delivers actual work only a couple of times per day, per week or even per month.

Figure 5 :

 5 Figure 5: insulated, atmospheric, thermal pits and thermal tanks, designed for thermocline layers.

Figure 7 :

 7 Figure 7: Main components of the hybrid UPHES-PTES for long-duration storage (here in stand-alone configuration). UPHES for storage of short and medium duration uses the same components, except for the absence of the two large thermal stores.

36thFigure 9 :

 9 Figure 9: UPHES-PTES for rigged with two caverns in opposition.

Figure 10 :

 10 Figure 10: illustrations of scale and of energy density for the UPHES-PTES storage.

Table 1 :

 1 Capex cost of Energy Capacity for the UPHES system

					in
					USD
	LRC	cavern	230 000	300	69 000 000
	(400MWh	@
	1.750kWh	per
	m3)
	Receiving pond	230 000	30	6 900 000
	for Hydro water
	Total Capex cost			75 900 000
	Total with 5%			80 000 000
	provision for CO2
	cost, etc.
	Total Capex cost			200
	per KWh of
	energy

Table 2 :

 2 below. Capex cost of Energy Capacity for the hybrid UPHES-PTES system Using the same components of Capex cost, as a crude estimate of the Capex for the pure supply of heating and cooling, we can simply divide the Capex for electricity by 500%, which bring a Capex of heat and cooling equal to roughly 432 USD/kW Thermal + 2 USD/kWh Thermal 36th European Photovoltaic Solar Energy Conference and Exhibition (meaning 4 USD/kWh only, for a 216-hour storage).

			Volume	Cost	in	Cost in USD
			in m3	USD/m3
	Hot Store for 10	987 000	40	39 480 000
	GWh Electrical
	(atmosph.)
	Cold Store for	525 000	100	52 500 000
	10	GWh
	Electrical
	(atmosph.)
	Total cost			91 980 000
	Total cost with			100 000 000
	9%	ancillary
	provision
	Total Capex cost			10
	per KWh of
	energy


Malgorzata Kloc
email: mkloc@houstonmethodist.org

Ahmed Uosef

Jarek Wosik

Jacek Z Kubiak

Rafik Mark Ghobrial

RhoA pathway and actin regulation of the Golgi/centriole complex

In vertebrate cells, the Golgi apparatus is located in close proximity to the centriole. The architecture of the Golgi/centriole complex depends on a multitude of factors, including the actin filament cytoskeleton. In turn, both the Golgi and centriole act as the actin nucleation centers. Actin organization and polymerization also depends on the small GTPase RhoA pathway. In this chapter we summarize the most current knowledge on how the genetic, magnetic or pharmacologic interference with RhoA pathway and actin cytoskeleton directly or indirectly affects architecture, structure and function of Golgi/centriole complex.

Pericentriolar location of the Golgi apparatus

In vertebrate cells, the Golgi apparatus consists of stacks of flattened membranous cisternae interconnected by tubular cisternae into a continuous Golgi ribbon. The main function of the Golgi is the posttranslational modification of the proteins delivered from the endoplasmic reticulum (ER) and their subsequent sorting and delivery, within the Golgi-derived secretory vesicles, to the cell surface or to the endosomal-lysosomal system. During interphase, the Golgi apparatus is located near the nucleus, in close proximity to or surrounding the centrioles and their pericentriolar material (PCM; collectively called the centrosome). In preparation for cell division, during the G2 phase of the cell cycle, the Golgi ribbon separates into isolated stacks that migrate to the cell periphery and away from the centrioles that migrate to the opposite poles of the cell to nucleate the microtubules of the spindle [START_REF] Colanzi | Cell-cycle-specific Golgi fragmentation: how and why?[END_REF][START_REF] Cervigni | JNK2 controls fragmentation of the Golgi complex and the G2/M transition through phosphorylation of GRASP65[END_REF] Sütterlin and [START_REF] Suẗterlin | The Golgi and the centrosome: building a functional partnership. The Golgi and the centrosome: building a function[END_REF][START_REF] Rios | The centrosome-Golgi apparatus nexus[END_REF]. In dividing cells, the centrioles and /or centrosomes function as the main microtubule organizing centers (MTOCs) for asters and the spindles. Additionally, in ciliated cells, the centrioles are converted into basal bodies and organize cilia (Bettencourt-Dias and Glover, 2007).

Although this subject is beyond the scope of this review, the Golgi itself is also able to nucleate microtubules [START_REF] Miller | Golgi-derived CLASP dependent microtubules control Golgi organization and polarized trafficking in motile cells[END_REF][START_REF] Maia | Modulation of Golgi-associated microtubule nucleation throughout the cell cycle[END_REF][START_REF] Rios | The centrosome-Golgi apparatus nexus[END_REF]. Interestingly, the Golgi and the centrosome share the molecules needed for microtubule assembly.

One such molecule is the scaffolding protein kinase N-associated protein AKAP450, which recruits other proteins required for microtubule formation [START_REF] El | The Scaffolding Protein CG-NAP/AKAP450 Is a Critical Integrating Component of the LFA-1-Induced Signaling Complex in Migratory T Cells[END_REF].

Since Golgi-derived and centrosome-derived microtubule arrays differ in their geometry, it is believed that they may play complementary and/or different functions [START_REF] Rios | The centrosome-Golgi apparatus nexus[END_REF].

For example, it was shown that Golgi-derived microtubules controlled the speed of cell migration during wound closure (Hutardo et al., 2011;[START_REF] Rios | The centrosome-Golgi apparatus nexus[END_REF].

Physical contact between the Golgi and centriole is also required for cell polarization and movement when the Golgi and centriole move in unison to the leading edge of the cell. A striking example of the polarization of the Golgi/centriole complex is the formation of the immunological synapse between interacting immune cells during the immune response [START_REF] Kloc | The newly found functions of MTOC in immunological response[END_REF]). Although we still do not fully understand the role of the association between the Golgi and centriole, it is believed that it facilitates polar/directional movement of the Golgi-derived secretory vesicles on the microtubules nucleated at the centrioles. Thus, it defines a polarity of secretion of various molecules [START_REF] Rios | The centrosome-Golgi apparatus nexus[END_REF] and the recycling and directional positioning of the receptors in the cell membrane.

In contrast to vertebrate cells, in yeast, invertebrates, and plants, the Golgi stacks are dispersed in the cytoplasm, do not form ribbon and are not located in the vicinity of the centrioles (daSilva et al., 2004; [START_REF] Kondylis | The Golgi apparatus: lessons from Drosophila[END_REF][START_REF] Preuss | Characterization of the Saccharomyces Golgi complex through the cell cycle by immunoelectron microscopy[END_REF]. This suggests that pericentriolar localization of the Golgi is not a prerequisite for its most basic functions.

The question that arises is how the peripherally located Golgi stacks are repositioned at the centriole and centrosome after cell division? It has been shown that the movement of the Golgi stacks from the cell periphery toward the cell center, and the maintenance of the pericentrosomal location of Golgi, depend on the microtubules and the motor protein dynein [START_REF] Cole | Golgi dispersal during microtubule disruption: regeneration of Golgi stacks at peripheral endoplasmic reticulum exit sites[END_REF][START_REF] Corthesy-Theulaz | Cytoplasmic dynein participates in the centrosomal localization of the Golgi complex[END_REF][START_REF] Harada | Golgi vesiculation and lysosome dispersion in cells lacking cytoplasmic dynein[END_REF][START_REF] Rios | The centrosome-Golgi apparatus nexus[END_REF][START_REF] Thyberg | Role of microtubules in the organization of the Golgi complex[END_REF], which is recruited to the Golgi by its resident membrane protein Golgin 160 [START_REF] Hicks | The NH2-terminal Domain of Golgin-160 Contains Both Golgi and Nuclear Targeting Information[END_REF][START_REF] Yadav | A primary role for Golgi positioning in directed secretion, cell polarity, wound healing[END_REF][START_REF] Yadav | Golgin160 recruits the dynein motor to position the Golgi apparatus[END_REF][START_REF] Rios | The centrosome-Golgi apparatus nexus[END_REF]. The anchoring of Golgi stacks in the vicinity of centriole/centrosome depends on a cis-Golgi network-associated protein GMAP-210 (TRIP11) [START_REF] Infante | GMAP-210, A cis-Golgi network associated protein, is a minus end microtubule binding protein[END_REF][START_REF] Rios | GMAP-210 recruits gamma-tubulin complexes to cis-Golgi membranes and is required for Golgi ribbon formation[END_REF]). It has been shown that depletion of GMAP-210 causes dispersion of Golgi stacks, while the experimental targeting of GAMP-210 to the mitochondria causes their clustering around the centrioles [START_REF] Yadav | A primary role for Golgi positioning in directed secretion, cell polarity, wound healing[END_REF][START_REF] Yadav | Golgin160 recruits the dynein motor to position the Golgi apparatus[END_REF][START_REF] Rios | The centrosome-Golgi apparatus nexus[END_REF].

Recently, it was shown that not only does the centriole affect the positioning and function of the Golgi but also that the Golgi regulates positioning of the centriole and centrosome [START_REF] Bisel | ERK regulates Golgi and centrosome orientation towards the leading edge through GRASP65[END_REF]Sütterlin and Colanzi, 2010). [START_REF] Bisel | ERK regulates Golgi and centrosome orientation towards the leading edge through GRASP65[END_REF] showed that in the rat kidney (NRK) cell wound healing assay, the reorientation of the centrosome and accompanying Golgi toward the leading edge of the migratory cells depends on the phosphorylation of the Golgi structural protein GRASP65 by the extracellular signal-regulated kinase (ERK). This study showed that phosphorylation of GRASP65 or treatment with brefeldin A, which both cause disassembly of the Golgi ribbon is necessary for centrosome orientation and subsequent polarization of migrating cells.

Golgi/centriole complex and actin

It is well established that the structure of the Golgi depends on centriole-and Golgiderived microtubules [START_REF] Kreis | Role of microtubules in the organization of the Golgi apparatus[END_REF]

Golgi and actin

The Golgi cisternae are stacked parallel to each other and have a very distinct morphology; they are flattened in the center and swollen at the rims, which bud off to form transporting vesicles. The actin filaments associated with the Golgi apparatus maintain the structural integrity of Golgi stacks, provide mechanical rigidity and flatness of the cisternae, form a physical barrier preventing the formation of transport vesicles at the center of the cisternae, and facilitate scission of the transport vesicles at the rims of the cisternae (Fig. 1). In addition, actin, together with the actin motor proteins, such as myosin II and VI, translocates the vesicles away from the Golgi to their intracellular or extracellular destinations (Fig. 1 decreased the rigidity of Golgi membranes. They also showed that the mechanical force applied to the Golgi membranes disrupts the dynamics of Golgi-associated actin and decreases the formation of Golgi-derived vesicles [START_REF] Guet | Mechanical Role of Actin Dynamics in the Rheology of the Golgi Complex and in Golgi-Associated Trafficking Events[END_REF]. In addition, [START_REF] Makhoul | Intersectin-1 interacts with the golgin GCC88 to couple the actin network and Golgi architecture[END_REF] showed that actin regulation of the Golgi dynamics depends on the interaction between the golgin (trans-Golgi network (TGN), GCC88, protein) and its binding partner intersectin-1 (ITSN-1).

Another fascinating phenomenon dependent on actin/myosin is the stop-and-go motion of the Golgi described in plant cells [START_REF] Nebenfuhr | Mobile factories: Golgi dynamics in plant cells[END_REF]. Although the movement of Golgi through the cytoplasm is probably restricted to plant cells, where the Golgi stacks are dispersed in the cytoplasm, [START_REF] Breuer | System-wide organization of actin cytoskeleton determines organelle transport in hypocotyl plant cells[END_REF] showed that the Golgi apparatus oscillates (wiggles) in the hypocotyl cells of Arabidopsis. The authors believe that because "the wiggling resembles the searching behavior of foraging animals or microbes that has been suggested to optimize food search efficiency", the Golgi may use this motion to more efficiently transport various compounds from the ER or/and distribute Golgi-derived material within the cell [START_REF] Breuer | System-wide organization of actin cytoskeleton determines organelle transport in hypocotyl plant cells[END_REF]. Further studies are needed to establish if the fascinating phenomenon of the Golgi oscillation is limited to the plant cells or is a universal phenomenon of all eukaryotic cells.

Centriole and actin

It has been known for years that the centriole and centrosome interact with actin filaments in interphase and dividing invertebrate and vertebrate cells, eggs and embryos. For example, it has been shown that actin is associated with the cnidarian sperm centrioles (Kleve and Cark, 1980) and that cortical actin plays a role in centrosome separation at the onset of mitosis in Drosophila embryos and 3T3-L1 adipocytes [START_REF] Cao | Cortical actin dynamics facilitate early-stage centrosome separation[END_REF]Wang et. al., 2008). In Drosophila syncytial blastoderm embryos, centrosomes direct the assembly and position of cortical actin caps and cytokinetic furrows [START_REF] Stevenson | Centrosomes and the Scrambled protein coordinate microtubule-independent actin reorganization[END_REF]. Ciliated cells in the neural tube have centriole-derived basal bodies that are anchored by actin filaments [START_REF] Antoniades | Making the connection: ciliary adhesion complexes anchor basal bodies to the actin cytoskeleton[END_REF]. However, although the results of proteomic analyses showed the presence of actin and actin-associated proteins at mammalian and human centrosomes [START_REF] Andersen | Proteomic characterization of the human centrosome by protein correlation profiling[END_REF][START_REF] Firat-Karalar | Proteomic analysis of mammalian sperm cells identifies new components of the centrosome[END_REF]Jacobsen et al., 2011), until recently, there was no direct proof that the centriole and centrosome could nucleate actin filaments. In 2016, Farina and coinvestigators, used isolated centrosomes from human Jurkat and HeLa cells to provide evidence that, indeed, the centrosome nucleates actin filaments through the action of the centrosomal Wiskott-Aldrich syndrome protein (WASP) family member, WASH [START_REF] Zigmond | How Wasp Regulates Actin Polymerization[END_REF], which stimulates the Arp2/3 complex [START_REF] Farina | The centrosome is an actin-organizing centre[END_REF]. The Arp2/3 protein complex contains seven subunits. Two of these subunits, the actin-related proteins ARP2 and ARP3, serve as nucleation sites for actin filaments [START_REF] Veltman | WASP family proteins: their evolution and its physiological implications[END_REF]. A question that arises concerns the roles of centrosomederived actin filaments. Although further studies are needed to discover all potential functions, a recent study by [START_REF] Obino | Actin nucleation at the centrosome controls lymphocyte polarity[END_REF] showed that in the resting lymphocytes, the actin filaments nucleated by centrosomal Arp2/3 anchor the centrosome to the nucleus. Upon lymphocyte activation, which leads to its polarization and formation of the immunological synapse, the level of ARP2/3 at the centrosome decreases. This, in turn, reduces actin nucleation at the centrosome and allows centrosome detachment from the nucleus and its movement together with the accompanying Golgi complex to the immunological synapse [START_REF] Obino | Actin nucleation at the centrosome controls lymphocyte polarity[END_REF]. Ultimately, the positioning of the Golgi at the immunological synapse allows for the polar delivery of cytokines, enzymes, and receptors toward the lymphocyte target [START_REF] Kloc | The newly found functions of MTOC in immunological response[END_REF]. Recent studies by [START_REF] Inoue | Actin filaments regulate microtubule growth at the centrosome[END_REF] on the actin present in the lymphocyte centrosomes showed that centrosomal actin physically blocks elongation of the centrosome-derived microtubules.

The role of RhoA pathway in Golgi and centriole architecture and functions

In all eukaryotic cells, the actin cytoskeleton is regulated by the small GTPase RhoA and its downstream effector ROCK1 p160 kinase. RhoA is activated by RhoA specific guanine exchange factors, GEFs that activate GTPases by stimulating the release of guanosine diphosphate (GDP) and binding of guanosine triphosphate (GTP; Cherfils and Zeghouf, 2013; Fig. 2A). RhoA is also reciprocally regulated by the Rho GTPase Rac1 and mTOR pathways (Fig. 2A; [START_REF] Byrne | Bistability in the Rac1, PAK, and RhoA Signaling Network Drives Actin Cytoskeleton Dynamics and Cell Motility Switches[END_REF][START_REF] Laplante | mTOR signaling at glance[END_REF].

Recent studies indicate that, surprisingly, ROCK1 kinase is not only the effector of RhoA but also an effector of Rac1 (Fig. 2A; Soriano-Castell et al., 2017). As previously discussed, the actin cytoskeleton plays a major role in the structure and function of the Golgi/centriole complex. Thus, it should be expected that interference with the RhoA pathway would disrupt actin cytoskeleton and indirectly affect the Golgi/centriole complex. Indeed, studies from our laboratories showed that the inhibition of RhoA GEFs using Rhosin or Y16 inhibitors, the inhibition of ROCK1 using the Y2762 inhibitor, or the genetic deletion of RhoA reorganizes the actin cytoskeleton in mouse macrophages and disrupts the structure of the Golgi, which fragments, disperses and/or migrates from its perinuclear position to the macrophage tail (Fig. 2B, Figure 3; Chen et al., 2017, 2018; Liu et al. 2016a[START_REF] Liu | Macrophage/monocyte-specific deletion of RhoA down-regulates fractalkine receptor and inhibits chronic rejection of mouse cardiac allografts[END_REF]. Interestingly, actin disorganization does not cause repositioning of the centriole, which remains in the perinuclear position. Thus, the RhoA interference causes uncoupling of the Golgi from the centriole (Fig. 3). We also showed that the macrophage-specific deletion of RhoA caused defective recycling, which relies on the Golgi-derived vesicular pathway, of macrophage fractalkine receptor CX3CR1 [START_REF] Liu | Macrophage/monocyte-specific deletion of RhoA down-regulates fractalkine receptor and inhibits chronic rejection of mouse cardiac allografts[END_REF]. Interestingly, very similar effects on macrophage phenotype, the Golgi and receptors were achieved when the macrophages were exposed to the magnetic field gradient interference [START_REF] Wosik | Magnetic field changes macrophage phenotype[END_REF]. In these studies, we have designed a permanent rare-earth magnet setup with defined magnetic field-gradient patterns and investigated the effect of these fields on mouse peritoneal macrophages grown in in vitro culture. We observed that the magnetic force elongated macrophages and arranged them in distinctive rows/waves. The location and alignment of magnetic-fieldelongated macrophages correlated very well with the simulated distribution and orientation of magnetic-force lines [START_REF] Wosik | Magnetic field changes macrophage phenotype[END_REF]. Also, the exposure of cultured RAW 264 cells (Abelson murine leukemia virus-transformed mouse macrophage) to the magnetic force had a similar effect (Uosef et al. unpublished). We also showed that similar to RhoA interference, the magnetic field exposure elongated macrophages, changed Golgi complex and cation channel receptor TRPM2 distribution, and modified the expression of macrophage molecular markers [START_REF] Wosik | Magnetic field changes macrophage phenotype[END_REF].

All these studies suggested that the effect of RhoA pathway interference on the Golgi/receptor recycling was indirect, resulting from the changes to the actin cytoskeleton. However, there are studies indicating that RhoA and/or its binding partners are associated with the Golgi apparatus and thus are directly involved in the regulation of some aspects of the Golgi structure and functions [START_REF] Chi | Roles of rho GTPases in intracellular transport and cellular transformation[END_REF][START_REF] Long | Rho GTPases operating at the Golgi complex: Implications for membrane traffic and cancer biology[END_REF]. [START_REF] Quassollo | A RhoA signaling pathway regulates dendritic Golgi outpost formation[END_REF] showed that in the neuronal cells, RhoA, ROCK, and serine-threonine kinases, LIMK1 and PKD1, are necessary for the scission and fission of the dendritic Golgi outposts (GOPs) needed for the trafficking of synaptic receptors (Fig. 1). In another study, Zilberman et al. (2011) showed that the effects of the overexpression of RhoA, which causes dispersion of the Golgi ribbon and defects in the fission of the transporting vesicles, depend on a member of the formin protein, mDia1, localized at the Golgi membranes. [START_REF] Camera | Citron-N is a neuronal Rho-associated protein involved in Golgi organization through actin cytoskeleton regulation[END_REF][START_REF] Camera | The RhoA-associated protein Citron-N controls dendritic spine maintenance by interacting with spine-associated Golgi compartments[END_REF] showed that the Golgi apparatus of neural cells contains RhoA binding protein Citron-N, which controls actin filament polymerization through the local assembly of ROCK kinase and actinbinding protein, Profilin IIa. Another study showed a centriolar function of RhoA. [START_REF] Aoki | Regulation of mitotic spindle formation by the RhoA guanine nucleotide exchange factor ARHGEF10[END_REF] showed that the RhoA GEF ARHGEF10, is localized at the centrosome in G1/S and M phases of the cell cycle. The ARHGEF10-regulates RhoA and controls centrosome duplication through its binding partner-a motor protein KIF3B, which is colocalized with ARHGEF10 at the centrosome.

In summary, these data indicate that the RhoA pathway is both indirectly, through the actin cytoskeleton, and directly, through the Golgi/centriole associated factors, involved in the regulation of Golgi/centriole structure and functions. 71. Zilberman, Alieva, N.O., Miserey-Lenkei, S., Lichtenstein, A., Kam, Z., Sabanay, H.,Bershadsky, A. (2011). Involvement of the Rho/mDia1 pathway in the regulationof Golgi complex architecture and dynamics. Mol. Biol. Cell 22, 2900-2911.

  ; Thyberg and Moskalewski,1999; Miller et al., 2009; Maia et al., 2013, Rios, 2014). However, in the last decades, numerous studies have shown that actin filaments are also very important for the establishment and maintenance of the Golgi architecture (Colon-Franco et al., 2011; Egea et al. 2006; Guet et al., 2014; Lowe, 2011). Surprisingly, it has been shown that both the Golgi and the centriole are able to nucleate actin filaments (Chen et al., 2004; Dubois et al., 2005, Carreno et al., 2004; Guet et al, 2014; Obino et al., 2016).

  ; Brownhill et al. 2009; Buss et al., 2002; diCampli et al.,1999; Egea et al., 2006; Fath, 2005; Guet et al., 2014; Miserey-Lenkei et al., 2010). Recently, Campany et al. (2019) showed that the Golgi-associated myosin MYO1C, which co-localizes with the Golgi-associated actin, is necessary for actin stabilization, Golgi compaction, and anterograde and retrograde vesicular transport. Depolymerization of actin filaments with cytochalasin D, latrunculin B or mycalolide B induces swelling of the cisternae, while stabilization of actin filaments with jasplakinolide causes fragmentation of the cisternae (Egea et al. 2006). Recently, Guet et al. (2014) used internalized microspheres trapped in optical tweezers to deform Golgi membranes and showed that the alteration in actin dynamics and inhibition of actomyosin (myosin II)

Fig. 1 .

 1 Fig. 1. Diagram of actin filament and RhoA role in the scission from the Golgi cisternae and in the transport of the cargo-containing vesicles. The actin filaments associated with the Golgi provide mechanical rigidity and flatness of the cisternae, form a physical barrier preventing formation of transport vesicles at the center of the cisternae, and facilitate scission of the transport vesicles at the rims of the cisternae. Actin polymerization is regulated by RhoA pathway. The vesicles transport their cargo, such as various receptors, to the cell membrane using actin filament and actin motors, or alternatively, the microtubules and microtubule motors.

Fig. 2 .Fig. 3 .

 23 Fig. 2. Diagram of the RhoA pathway, actin, centriole and Golgi. A) In normal, untreated cells, RhoA is activated by RhoA specific GEFs and is reciprocally regulated by mTOR and the Rac 1 pathway. RhoA regulates actin filament polymerization and organization through its downstream effector ROCK kinase. Recently, it has been shown that ROCK is also a direct downstream effector of Rac1 (Soriano-Castell et al., 2017). A properly organized actin cytoskeleton and fully functioning RhoA pathway support perinuclear organization of Golgi and centrioles, the Golgi-derived vesicular transport of various molecules and receptors, and the recycling of the receptors from the membrane back to the Golgi. B) Pharmacologic, genetic or magnetic interference with the components of the RhoA pathway such as: GEFs inhibition with Rhosin or Y16, Rock inhibition with Y27632, RhoA deletion or the exposure of macrophages to the magnetic field gradient, elongate macrophage, disrupt actin

Acknowledgments

The authors gratefully acknowledge support from the William and Ella Owens Medical Research Foundation to JW and MK, the William Stamps Farish Fund to MK and RMG, the State of Texas through the Texas Center for Superconductivity at the University of Houston to JW, Polish Ministry of National Defense project "Kościuszko" # 571/2016/DA to JZK and Polish Ministry of National Defense project "Kościuszko" # 5508/2017/DA to JZK. We also thank Kenneth Dunner Jr. for the electron microscopy work and acknowledge CCSG grant NIH P30CA016672 to MDACC High Resolution Electron Microscopy Facility.