Ugo Pelay

Lingai Luo
email: lingai.luo@univ-nantes.fr

Yilin Fan

Driss Stitou

Mark Rood

Thermal Energy Storage Systems for Concentrated Solar Power Plants

Keywords: Concentrated solar power (CSP), Thermal energy storage (TES), Integration, Thermochemical, Energy density CLFR Compact Linear Fresnel collector CRS Central Receiver System CSP Concentrated

Solar thermal energy, especially concentrated solar power (CSP), represents an increasingly attractive renewable energy source. However, one of the key factors that determine the development of this technology is the integration of efficient and cost effective thermal energy storage (TES) systems, so as to overcome CSP's intermittent character and to be more economically competitive. This paper presents a review on thermal energy storage systems installed in CSP plants. Various aspects are discussed including the state-of-the-art on CSP plants all over the world and the trend of development, different technologies of TES systems for high temperature applications (200 °C -1000 °C) with a focus on thermochemical heat storage, and storage concepts for their integration in CSP plants.

TES systems are necessary options for more than 70% of new CSP plants. Sensible heat storage technology is the most used in CSP plants in operation, for their reliability, low cost, easy to implement and large experimental feedback available. Latent and thermochemical storage technologies have much higher energy density thus may have a bright foreground. New concepts for TES integration are also proposed, especially coupled technology for higher operating temperature and cascade TES of modularized storage units for intelligent temperature control.

The key contributions of this review paper consist of a comprehensive survey of CSP plants, their TES systems, the ways to enhance the heat and/or mass transfers and different new concepts for the integration of TES systems.

Introduction

The use of renewable energy is essential today to decrease both the consumption of fossil resources and the production of carbon dioxide partly responsible for the greenhouse gas effect [START_REF] Ervin | Solar heat storage using chemical reactions[END_REF][2]. Among every renewable resources (e.g., wind, ocean, geothermal and solar), solar energy is showing encouraging promises due to the great quantities of solar irradiation flux arriving on earth.

Among various solar energy technologies, concentrated solar power (CSP) is particularly attractive due to its advantages in terms of high efficiency, low operating cost and good scale-up potential [START_REF] Zhang | Concentrated solar power plants: Review and design methodology[END_REF][START_REF] Desideri | Analysis and comparison between a concentrating solar and a photovoltaic power plant[END_REF]. Solar energy is converted into electricity by means of a CSP plant composed of four main elements: a concentrator, a high temperature solar receiver, a fluid transport system and a power generation bloc (e.g., Rankine cycle, Stirling cycle). It is estimated by IEA that the CSP will contribute up to 11% of the global electricity production in year 2050 [5].

CSP technologies

A wide range of concentrating technologies exist; the most developed are parabolic trough collectors (PTC), linear Fresnel reflectors (LFR), solar power towers (SPT) and parabolic dish collectors (PDC), as summarized in Table 1. PTC plants use parabolic reflectors to focus sunlights on an absorber tube located on the focal line of the parabola. Reflectors and the absorber tube can move together to follow the sun from sunrise to sunset [5] [START_REF] García | Performance model for parabolic trough solar thermal power plants with thermal storage: Comparison to operating plant data[END_REF]. LFRs consist of curved reflectors on each side of an absorber tube. A recent design called compact linear Fresnel reflector (CLFR) uses two parallel reflectors for each mirror's row, needing less area than a PTC to reach a given power output [START_REF] Barlev | Innovation in concentrated solar power[END_REF]. SPTs use heliostat field collectors (HFCs) to reflect and focus sunlights onto a central solar receiver located on the top of the tower. It is a relatively flexible technology because a variety of heliostat fields, solar receiver designs and heat transfer fluids (HTFs) could be used. PDCs concentrate sunlights on a focus point above a parabolic reflector. The reflector and receptor, tracks the sun. Besides these conventional types, CSP technology can also be combined with thermoelectric systems (i.e., concentrated solar thermoelectric) for direct electricity production without using a power cycle [START_REF] Barlev | Innovation in concentrated solar power[END_REF]. [START_REF] Zhang | Concentrated solar power plants: Review and design methodology[END_REF]9,[START_REF] Agalit | Numerical investigations of high temperature packed bed TES systems used in hybrid solar tower power plants[END_REF]

Table 1. Comparison between different CSP technologies

Thermal energy storage

A major drawback of solar energy is its temporal intermittency. To overcome this problem, one solution is to use a backup system (energy hybridization) that burns fossil fuel or biomass. A second solution is to use a thermal energy storage (TES) system to store heat during sunshine periods and release it during the periods of weak or no solar irradiation (Figure 1).

Figure 1. CSP plant with a TES system

The development of an efficient and cost-effective TES system is crucial for the future of CSP technologies [START_REF] Pardo | A review on high temperature thermochemical heat energy storage[END_REF]. Economically, TES allows an increase in the duration of electricity production. Moreover, integrating a TES system in specific CSP configurations permits optimization of electricity resale and the CSP electricity production [START_REF] Guédez | Reducing the Number of Turbine Starts in Concentrating Solar Power Plants Through the Integration of Thermal Energy Storage[END_REF]. Indeed, as shown in Figure 2 [START_REF] Sioshansi | The Value of Concentrating Solar Power and Thermal Energy Storage[END_REF], electricity prices vary during a day depending on demand. The solar intensity's largest periods do not correspond to the electricity's most expensive periods. Adding a TES allows heat storage during high solar intensity periods and provides productions during high electricity cost periods. [START_REF] Sioshansi | The Value of Concentrating Solar Power and Thermal Energy Storage[END_REF] There are currently three kinds of TES systems available: sensible heat storage, latent heat storage and thermo-chemical heat storage [START_REF] Gil | State of the art on high temperature thermal energy storage for power generation. Part 1-Concepts, materials and modellization[END_REF][START_REF] Medrano | State of the art on high-temperature thermal energy storage for power generation. Part 2-Case studies[END_REF]. Sensible heat storage systems are most mature. They are widely used in industrial plants, most notably in Spain within the "PS10" and "PS20" projects (2007 and 2009), the "Andasol 1" and "Andasol 2" plants (2008) and also in the USA (e.g., within "Solar One", 1982) [START_REF] Medrano | State of the art on high-temperature thermal energy storage for power generation. Part 2-Case studies[END_REF]. Latent heat storage allows large amounts of energy to be stored in relatively small volumes (high energy density) and is cost competitive. Among these techniques, thermochemical heat storage receives increasing attention. Two recent reviews were published, focusing on low to medium temperature (0 -300 °C) thermochemical reactions pertaining to long-term sorption solar energy storage [START_REF] N'tsoukpoe | A review on long-term sorption solar energy storage[END_REF] and to chemical heat pump technologies [START_REF] Wongsuwan | A review of chemical heat pump technology and applications[END_REF]. Cot-Gores et al. [START_REF] Cot-Gores | Thermochemical energy storage and conversion: A-state-of-the-art review of the experimental research under practical conditions[END_REF] and Prieto et al. [START_REF] Prieto | Review of technology: thermochemical energy storage for concentrated solar power plants[END_REF] also presented reviews on sorption and chemical reaction processes for TES application. Reaction candidates for medium or high temperature applications (250 -800 °C) were listed by Felderhoff et al. [START_REF] Felderhoff | Thermochemical heat storage for high temperature applications-A review[END_REF].

Figure 2. Example of peak energy production and cost of energy for a CSP plant

Objectives of this paper

Based on the existing literature, we observe CSP plants worldwide and their characteristics (e.g., power, presence of a TES system, storage capacity) are partially (e.g., in [START_REF]CSP World Map[END_REF]) presented, but a full description would be beneficial. In parallel, TES materials for CSP application need to be updated, especially those for thermochemical heat storage. Moreover, the issue of appropriate and new concepts for TES integration in CSP plants which is very important, is discussed here in more detail.

The present paper has therefore the following objectives:

• The most complete survey on CSP plants in operation, under construction and in project all over the world and on the trends of development in order to highlight the economic necessity of the integration of TES systems in CSP plants to compete with current energy production technologies. • A review on different technologies of TES systems and potential materials for CSP application, with a special focus on thermochemical heat storage. • A summary of storage concepts and integration configurations proposed in the literature These contributions are important because the literature survey clearly shows the important role of TES systems in current and in future CSP plants. Moreover, if the selection of an adapted TES technology is the first step, its proper integration in a CSP plant is the next important step. Reviewing different storage concepts allows an overview on conventional TES integration configurations as well as innovative concepts recently proposed.

TES systems in CSP plants: the state-of-the-art

This section presents a literature survey on almost all CSP plants worldwide, including those already in operation, under construction or planned project. First, a large part of the data have been collected from the websites "cspworld.org" and "globalenergyobservatory.org". Then a detailed research has also been made for each identified CSP plant on its proper website or related documents so as to find additional information. A statistical analysis of these data will show the general trends of CSP technologies such as the increasing plants' average power or the necessity of TES systems for economic competitiveness.

To better illustrate the historical development, the surveyed CSP plants are divided into five groups: (i) 19 plants in operation with production began before year 2000 (named "before 2000" hereafter); (ii) 24 plants in operation with production started between year 2000 and 2010 (named "2000 to 2010"); (iii) 85 plants in operation with production began after 2010 (named "after 2010"); (iv) 35 plants now under construction and (v) 74 planned plants. The total number of samples thus reaches 237 CSP plants. Detailed data for every plant are provided in Table 1, 2 and 3 of the Data-in-Brief article [START_REF] Pelay | Technical Data of concentrated solar power plants in operation, under construction and planned_submitted[END_REF].

Statistical analyses are provided about the purpose of plant, the average power and the storage/hybridization (Figure 345). It should be noted that despite all the efforts made by the authors, not all values are available for every CSP plant. In the current study, if a plant's value was not available (n.a.) for a certain item, then that plant was eliminated from the analysis of that item. Table 2 counts the number of plants with known values used for each item of analysis compared to the total number of plants of each group.

Table 2. Number of samples for each item of analysis

Purpose of plants

Types of CSP technology

Figure 4 shows the historical evolution of used CSP technologies. PTC and SPT are the most used technologies for plants in operation. Meanwhile, trends also show that there will be a higher ratio of SPT for CSP plants in project. LFR represents the third most frequently used CSP technology. PDC is a relatively young technology with no plants in activity. Nevertheless, it represents 7-9% of plants under construction and planned, making it a non-negligible technology in the future.

Average power of CSP plants

Figure 5.a shows the historical evolution of CSP plant's average power. The average power has increased by a factor of 6 from about 21 MWelect for power plants in operation before 2000 to more than 120 MWelect for planned power plants. One reason for this increasing average power is the change of the purpose of the plants since more than a quarter of plants in operation is related to a demonstration or a R&D purpose (Figure 3). These plants were not built for electricity sale thus produced generally a few megawatts. Plants under construction and planned are more commercial plants of higher power output, including the 2000 MWelect facility for the planned "TuNur" plant in Tunisia.

Hybridization

Sometimes CSP plants are hybridized with a backup system that provides supplementary energy source when solar intensity is not adequate and when a quick response is needed [START_REF] Suresh | Methodology for sizing the solar field for parabolic trough technology with thermal storage and hybridization[END_REF]. Fossil fuels (i.e., coal, natural gas or diesel) are the most commonly used backup energy sources while biomass (e.g., "Borges solar", Spain, in operation) or solar photovoltaic (e.g., "Ashalim csp plant", Israel, planned) are occasionally used. Natural gas remains as the preferential fuel for hybridization [START_REF] El Gharbi | A comparative study between parabolic trough collector and linear Fresnel reflector technologies[END_REF]. With the hybridization of the backup system, the continuous electricity production of the CSP plant can generally be guaranteed which increases the plant global efficiency [START_REF] Suresh | Methodology for sizing the solar field for parabolic trough technology with thermal storage and hybridization[END_REF].

The percentage of hybridization for each group of CSP plants is shown in Figure 5.b. The use of hybridization systems is globally decreasing. This is mainly due to the increasing price of fossil fuels. Another possible reason is the increasing use of TES systems that will be discussed later. In fact, very few plants operate only with solar energy. A TES system and/or a hybridization system are always necessary to be added into a CSP plant to overcome the temporal variability of solar energy. However, care must be taken with these results, particularly for plants under construction and planned because of missing data.

Thermal storage

Figure 5.c indicates the percentage of plants implementing a TES system. About half of the plants (47%) currently in operation integrate a TES system. There is a notable increase in the use of TES systems for plants under construction (72%) and planned (77%). This increase can be partially explained by the technological progress and advancements achieved in storage systems and the necessity to install such systems to be economically competitive.

Sensible storage is the dominant TES technology used in CSP plants. It is the most developed technology that has been studied, tested and installed. Ruths tank (also called steam accumulator) is the only alternative for limited storage capacity installed in a CSP plant (e.g., "Augustin Fresnel", France, in operation, 0.25 hour of storage; "Alba Nova", France, under construction, 1 hour of storage). In the meantime, it is advisable to put in perspective these data and particularly those relating to plants under construction and planned, because in a certain number of cases the planned storage mode is not known. Figure 5.d shows the evolution of the average storage capacity, represented by the time during which the plant is able to work with the energy provided by the TES system. The average storage capacity has steadily increased, from 3 hours to more than 7 hours for plants currently in project. As it will be possible to see in next parts, research on TES have developed more efficient and less expensive technologies. Today, plant stakeholders would benefit to install a TES to increase the profitability of their facilities. Many plants under construction or planned intend to do mass production of electricity and require a TES system large enough for continuous production during periods of low solar intensity. In addition, a number of plants built before 2010 have low storage capacity TES systems because their goal is to investigate the heat and mass transfer characteristics and validate their analytical/numerical models but not to produce electricity at a lower cost.

Short summary

Table 3 summarizes the state of the art of CSP plants in operation and future trends (plants under construction and planned). CSP plants have a great future, indicated by the increases in the number, the average power output per plant and proportion of commercial plants. To allow plants to operate and to be economically competitive, a hybridization system and/or a TES system is necessary. Hybridization systems used to be a standard option to increase the dispachability of CSP plants. However, they usually consume fossil fuels with increasing costs and high CO 2 emissions. In the meantime, TES technologies have no such problems and could become an alternative or even a replacement. A quick geographical analysis of the planned CSP plants shows that USA, Italy and Chile are the leading countries for CSP installation. These countries are not traditional oil producers and tend to use TES systems instead of fossil source backups.

Another trend is that even if several storage technologies are available, sensible storage is almost the only choice. However, we will see in the next section that other types of storage have great advantages and deserve to be studied.

Table 3. Summary for TES systems in CSP plants

TES technologies for CSP application

As we saw in the above section, the integration of a TES system is necessary, which allows a plant to operate more stably and to meet the demands of the power grid. Therefore, selecting appropriate TES technologies and designing highly efficient TES systems are becoming the research focus. In a recent review [START_REF] Tian | A review of solar collectors and thermal energy storage in solar thermal applications[END_REF], Tian and Zhao summarized main criteria to be considered in the design of a solar TES system. Aspects were proposed, including technical properties, economic costs and environmental impacts. Among influencing factors, the most expected features of a TES system are a high energy density to reduce the necessary amount of materials and the occupancy space, reliable mechanical and chemical stability and enhanced heat transfer during charges or discharges. Meanwhile, the cost of TES system (including storage materials, reservoirs, heat exchangers) is also an important criterion which directly determines the payback period of the investment. Finally, storage materials should be environmental friendly, low corrosive, complete reversible and non-or low-flammable.

As previously mentioned, TES technologies available for CSP application are classified into sensible, latent and thermochemical categories. Sensible heat storage is currently the most commonly used technology (Table 3), with a large number of low-cost materials available [START_REF]Survey of Thermal Storage for Parabolic Trough Power Plants[END_REF][START_REF] Zhao | Thermal property characterization of a low melting-temperature ternary nitrate salt mixture for thermal energy storage systems[END_REF][START_REF] Zhao | Heat transfer enhancement of phase change materials (PCMs) in low and high temperature thermal storage by using porous materials[END_REF]. However, the lowest energy density of this technology results in large size of TES systems. Latent heat storage technology has a higher energy density, but a poor heat transfer performance due to very low thermal conductivity of the materials. Thermochemical storage has the highest storage energy density, thus seems to be the most promising technology for the future. Nevertheless, problems such as more complex design of reactors, low chemical stability, poor heat and mass transfers and degraded performance over charge/discharge cycles hinder its real application in CSP plants [START_REF] Prieto | Review of technology: thermochemical energy storage for concentrated solar power plants[END_REF].

Different technologies to store thermal energy for CSP application (between 200°C and 1000°C) are described below. Emphasis is put on recent advances in thermochemical heat storage technology, which is under-developed but has a great potential.

Sensible heat storage

In sensible heat storage, thermal energy is stored/released by raising/decreasing the temperature of a storage material. It is a pure physical process without any phase change during charge or discharge. Therefore, the amount of heat stored depends on the product of the mass, specific heat, and temperature variation of the storage material. In addition to the density and the specific heat of the storage material, other properties are also important for sensible heat storage: operating temperature, thermal conductivity and diffusivity, chemical and thermo-chemical stability of the materials and the cost [START_REF] Gil | State of the art on high temperature thermal energy storage for power generation. Part 1-Concepts, materials and modellization[END_REF]. The major drawback of sensible heat storages is their limited energy density (between 60 kWh t.m -3 for sand (200-300°C), rock, mineral oil and 150 kWht.m -3 for cast iron (200-400°C) [START_REF] Fernandes | Thermal energy storage: "How previous findings determine current research priorities[END_REF]) which makes storage units particularly large. Moreover, the storage units should be designed in a proper manner for heat discharge at relatively constant temperatures.

Sensible heat storage materials in solid and liquid form are the most studied materials and are at an advanced stage of development. Solid materials generally have a wide range for possible temperature variation (200 -1200 °C), with high thermal conductivity (from 1 to 40 W.m -1 .K -1 ) and relatively low cost (0.05 -5 $.kg -1 ). Concrete and castable ceramics are intensively studied which have good characteristics (0.05 $.kg - 1 and good thermal conductivity: 1.5 W.m -1 .K -1 [START_REF] Fernandes | Thermal energy storage: "How previous findings determine current research priorities[END_REF]) to be used as a solid heat storage materials [START_REF] Lovegrove | A solar-driven ammonia-based thermochemical energy storage system[END_REF]. In order to improve the stability of concrete at high temperature, a new concrete with polypropylene fibers was specially developed [START_REF]Survey of Thermal Storage for Parabolic Trough Power Plants[END_REF] [START_REF] Zhao | Thermal property characterization of a low melting-temperature ternary nitrate salt mixture for thermal energy storage systems[END_REF]. Other solid materials such as graphite (e.g., "Lake Cargelligo" in Australia) and pebble stone (e.g., "Airlight Energy Baha Plant" in Morocco) are also occasionally used, as listed in Table 1 of [START_REF] Pelay | Technical Data of concentrated solar power plants in operation, under construction and planned_submitted[END_REF]. A comprehensive list of solid sensible heat storage materials for CSP applications and their main characteristics are provided by Gil et al. [START_REF] Gil | State of the art on high temperature thermal energy storage for power generation. Part 1-Concepts, materials and modellization[END_REF].

Instead of using new manufactured materials, industrial waste or or by-product materials (i.e., asbestos containing wastes, bischofites, slags, etc.) [START_REF] Gutierrez | Advances in the valorization of waste and by-product materials as thermal energy storage (TES) materials[END_REF][START_REF] Ortega | Thermophysical characterization of a by-product from the steel industry to be used as a sustainable and low-cost thermal energy storage material[END_REF] and low cost materials (i.e., sand, rocks) are also available [START_REF] Grirate | Experimental and numerical investigation of potential filler materials for thermal oil thermocline storage[END_REF][START_REF] Jemmal | Thermophysical and chemical analysis of gneiss rock as low cost candidate material for thermal energy storage in concentrated solar power plants[END_REF]. Their adapted thermal properties, low cost and strong environmental and societal benefits (reduced landfilled waste amounts and the greenhouse emissions) make them good candidates for TES systems at high temperature [START_REF] Gutierrez | Advances in the valorization of waste and by-product materials as thermal energy storage (TES) materials[END_REF].

Liquid materials are also widely used in CSP plants. Molten salts are considered as one of the best materials for CSP application [START_REF] Gil | State of the art on high temperature thermal energy storage for power generation. Part 1-Concepts, materials and modellization[END_REF][START_REF]Survey of Thermal Storage for Parabolic Trough Power Plants[END_REF][START_REF] Zhao | Thermal property characterization of a low melting-temperature ternary nitrate salt mixture for thermal energy storage systems[END_REF][START_REF] Zhao | Heat transfer enhancement of phase change materials (PCMs) in low and high temperature thermal storage by using porous materials[END_REF] owning to their features including excellent thermal stability at high temperatures, low vapor pressure, low viscosity, high thermal conductivity, non-flammability and nontoxicity. The two leading candidates are "solar salt" (60% NaNO3 and 40% KNO3) used in "Archimede", Italia, in activity and "MSEE/CatB", USA, in activity, and "HitecXL" (48% Na(NO3)2, 7% NaNO3 and 45% KNO3 [START_REF] Forster | Theoretical investigation of the system SnOx/Sn for the thermochemical storage of solar energy[END_REF]). HitecXL was tested in "PSA plant", Spain and "Themis plant", France. It was developed as a second option for Hitec (40% NaNO2, 7% NaNO3 and 53% KNO3, melting point at 142 °C) [START_REF]Survey of Thermal Storage for Parabolic Trough Power Plants[END_REF] [START_REF] Forster | Theoretical investigation of the system SnOx/Sn for the thermochemical storage of solar energy[END_REF]. A new series of ternary salts mixtures was proposed with ultra-low melting temperatures at 76 °C, 78 °C or 80 °C, preventing the solidification of salt and making the TES system much easier to manage [START_REF] Zhao | Thermal property characterization of a low melting-temperature ternary nitrate salt mixture for thermal energy storage systems[END_REF][START_REF] Ren | Preparation and thermal properties of quaternary mixed nitrate with low melting point[END_REF]. Mixtures of salts consisting of KNO3, LiNO3 and Ca(NO3)2 showed reduced viscosity by a factor of 5 compared to that of commercial synthetic oils or molten salts. Such eutectic salts whose melting temperatures points are also below 100 °C have also been reported by Wang et al. [START_REF] Wang | Novel low melting point quaternary eutectic system for solar thermal energy storage[END_REF].

Effective solutions to increase the energy density of sensible heat storage materials like molten salts consist in adding encapsulated PCMs [START_REF] Cingarapu | Use of encapsulated zinc particles in a eutectic chloride salt to enhance thermal energy storage capacity for concentrated solar[END_REF][START_REF] Bellan | Numerical analysis of charging and discharging performance of a thermal energy storage system with encapsulated phase change material[END_REF][START_REF] Nithyanandam | Analysis of a latent thermocline storage system with encapsulated phase change materials for concentrating solar power[END_REF] or nanoparticles [START_REF] Seo | Enhancement of specific heat of ternary nitrate (LiNO3-NaNO3-KNO3) salt by doping with SiO2 nanoparticles for solar thermal energy storage[END_REF][START_REF] Andreu-Cabedo | Increment of specific heat capacity of solar salt with SiO2 nanoparticles[END_REF][START_REF] Zhang L Di | Effect of nanoparticle dispersion on enhancing the specific heat capacity of quaternary nitrate for solar thermal energy storage application[END_REF][START_REF] Madathil | Preparation and characterization of molten salt based nanothermic fluids with enhanced thermal properties for solar thermal applications[END_REF]. Encapsulation means the coating of PCM particles with a protecting shell to improve the chemical stability and to prevent individual particles from coalescing with one another during melt/freezing cycles. Adding encapsulated PCMs in sensible materials can be considered as a new coupled technology, as will be detailed in the later section of this paper. Adding nano-particles in sensible storage materials will increase its specific heat. Andreu-Cabedo et al. [START_REF] Andreu-Cabedo | Increment of specific heat capacity of solar salt with SiO2 nanoparticles[END_REF] showed that the specific heat of a solar salt can be increased by 25.03% by adding 1 wt.% of silica nanoparticles (SiO2).

Other liquid materials are also used in CSP plants. For example, oils have been used in "Dahan Power Plant" in China. Liquid sodium having a very high thermal conductivity (71 W.m -1 .K -1 )) is used in "PSA SSPS-CRS plant" in Spain. However, additional safety measures are necessary due to their unstable nature caused by their high vapor pressure [START_REF] Zhao | Thermal property characterization of a low melting-temperature ternary nitrate salt mixture for thermal energy storage systems[END_REF] which may cause serious security problems. Liquid sensible heat storage materials for CSP applications and their main characteristics are described by Gil et al. [START_REF] Gil | State of the art on high temperature thermal energy storage for power generation. Part 1-Concepts, materials and modellization[END_REF].

Gaseous materials have been used in CSP plants in operation such as compressed air or steam as storage media (e.g., "Exresol-1" "Planta Solar 20" and "Planta Solar 10" in Spain), as shown in Table 1 of [START_REF] Pelay | Technical Data of concentrated solar power plants in operation, under construction and planned_submitted[END_REF]. Their main physical properties are shown in Table 4. The advantages of using gaseous materials are their availability, low cost, non-toxicity and wide range of operating temperature. The disadvantages of using gaseous materials are their low thermal conductivity and very low energy density. Even if the gas is stored compressed, the large volume of the reservoirs makes this storage option very challenging for CSP plants with high power output.

Latent heat storage

In latent heat storage, thermal energy is stored/released by a material while changing its phase at a constant temperature. It is also a pure physical process without any chemical reaction during charge or discharge. The amount of heat stored is generally the latent heat of phase change (latent heat of fusion for a solid-liquid transition and latent heat of vaporization for a liquid-vapor transition).

Phase change materials (PCM) for latent heat storage have the advantage to charge or discharge in a narrow temperature range, close to their phase change temperature. They also have higher energy densities owing to their high phase change latent heat (between 27 kJ.kg -1 for TRIS ((NH2)C(CH2OH)3) and 790 kJ.kg -1 for the eutectic salt (80.5%LiF-19.5%CaF2) compared to sensible heat storages. The phase change concerning thermal storage could be either solid-liquid or solid-solid, because volume and pressure are roughly constant.

The main drawback of PCMs is their low thermal conductivity (usually 0.2 -0.8 W.m -1 .K -1 [START_REF] Wongsuwan | A review of chemical heat pump technology and applications[END_REF][START_REF] Tammer | Energy Storage for Direct Steam Solar Power Plants[END_REF]), resulting in very slow charge and discharge processes [START_REF] Do Couto Aktay | Thermal Conductivity of High-Temperature Multicomponent Materials with Phase Change[END_REF]. Several approaches may be considered to enhance their heat transfer, including using transportable PCMs [START_REF] Tay | Review on transportable phase change material in thermal energy storage systems[END_REF] (detailed in section 4.2 of this paper), adding thermal conductivity promoters into pure PCMs and using high conductivity PCMs made of metal alloys [START_REF] Risueno | Experimental investigation of Mg-Zn-Al metal alloys for latent heat storage application[END_REF][START_REF] Blanco-Rodríguez | Thermophysical characterization of Mg-51%Zn eutectic metal alloy: A phase change material for thermal energy storage in direct steam generation applications[END_REF][START_REF] Andraka | Metallic Phase Change Material Thermal Storage for Dish Stirling[END_REF]. One of the common additives for PCMs is the graphite. The thermal conductivity of PCM/graphite composites could reach 5 to 10 W.m -1 .K -1 depending on the amount of graphite added [START_REF] Singh | Development of graphite foam infiltrated with MgCl2 for a latent heat based thermal energy storage ( LHTES ) system[END_REF]. Other thermal conductivity promoters are also investigated, such as insertion of a metal matrix or foam [START_REF] Trelles | Numerical simulation of porous latent heat thermal energy storage for thermoelectric cooling[END_REF][START_REF] Hoogendoorn | Performance and modelling of latent heat stores[END_REF][START_REF] Abujas | Performance comparison of a group of thermal conductivity enhancement methodology in phase change material for thermal storage application[END_REF][START_REF] Martinelli | Experimental study of a phase change thermal energy storage with copper foam[END_REF], fins [START_REF] Jung | Thermal analytical model of latent thermal storage with heat pipe heat exchanger for concentrated solar power[END_REF], dispersion of high conductivity particles [START_REF] Mettawee | Thermal conductivity enhancement in a latent heat storage system[END_REF], micro-encapsulation of the PCMs [START_REF] Griffiths | Performance of chilled ceiling panels using phase change material slurries as the heat transport medium[END_REF][START_REF] Hawlader | Microencapsulated PCM thermal-energy storage system[END_REF][START_REF] Sharma | Developments in organic solid -liquid phase change materials and their applications in thermal energy storage[END_REF][START_REF] Elmozughi | Encapsulated phase change material for high temperature thermal energy storage -Heat transfer analysis[END_REF][START_REF] Xu | Application of phase change materials for thermal energy storage in concentrated solar thermal power plants: A review to recent developments[END_REF] or special designs of TES system with embedded heat pipes [START_REF] Nithyanandam | Cost and performance analysis of concentrating solar power systems with integrated latent thermal energy storage[END_REF] or finned heat pipes [START_REF] Khalifa | Performance of suspended finned heat pipes in high-temperature latent heat thermal energy storage[END_REF]. Recent advances on the geometrical configurations of TES systems using PCMs are reviewed by Mao [START_REF] Mao | Recent developments in geometrical configurations of thermal energy storage for concentrating solar power plant[END_REF] and the shell materials used in the encapsulation of PCMs could be found in the review of Jabob and Bruno [START_REF] Jacob | Review on shell materials used in the encapsulation of phase change materials for high temperature thermal energy storage[END_REF]. PCMs made of metal alloys (i.e., Mg-Zn-Al) have the advantages such as high thermal conductivity (around two orders of magnitude higher than that of molten salts) and good thermo-chemical stability. However, their high price (2-3 $/kg) is an obstacle to be used in large scale TES [START_REF] Blanco-Rodríguez | Thermophysical characterization of Mg-51%Zn eutectic metal alloy: A phase change material for thermal energy storage in direct steam generation applications[END_REF].

Inorganic and organic substances with potential use as thermal storage material at high temperature and their physical-thermal properties are listed by Gil et al. [START_REF] Guédez | Reducing the Number of Turbine Starts in Concentrating Solar Power Plants Through the Integration of Thermal Energy Storage[END_REF] and Sharma et al. [START_REF] Sharma | Developments in organic solid -liquid phase change materials and their applications in thermal energy storage[END_REF]. Melting temperature of these PCMs generally varies between 100 and 900 °C. Materials whose phase change temperatures are the lowest (100 -300 °C) can be used as TES systems for PTC, LFR and PDC. Materials with higher phase change temperatures (> 400 °C) are suitable to be used in SPT or PDC whose operating temperature goes up to 600 °C or 1500 °C, respectively [START_REF] Zhang | Concentrated solar power plants: Review and design methodology[END_REF][8].

Thermochemical heat storage

Different from sensible or latent heat storages, thermochemical TES technology is based on reversible chemical reactions, which are characterized by a change in the molecular configuration of the reactants (combination or decomposition). Solar heat is used to drive an endothermic chemical reaction, and then stored in the form of chemical potential. During the discharge, the stored heat can be recovered by the reversed exothermic reaction, sometimes by adding a catalyst. The advantages of thermochemical storage relies on their high energy density (up to 10 times greater than latent storage [START_REF] Pardo | A review on high temperature thermochemical heat energy storage[END_REF]) and the indefinitely long storage duration at ambient temperature. As a result, it is a very attractive option and fairly economic competitive [START_REF]Survey of Thermal Storage for Parabolic Trough Power Plants[END_REF].

Metallic hydrides, carbonates system, hydroxides system, redox system, ammonia system and organic system can be used for thermochemical heat storage at medium or high temperatures (300 -1000 °C) [START_REF] Pardo | A review on high temperature thermochemical heat energy storage[END_REF]. Table 5 groups these reactions with their mains characteristics, advantages/disadvantages and experimental feedback with corresponding references. Some reactions (e.g., PbCO3/PbO; CaCO3/CaO; (CH4+CO2)/(CO+H2); C6H12/C6H6; NH3/N2; BaO2/BaO; (CH4+H2O)/(CO+H2)) suffer from the problem of incomplete reversibility, which is a key drawback because their storage capability gradually decreases after cycles of operation.

Table 5. Main reactions used in high temperature thermochemical heat storage [START_REF] Pardo | A review on high temperature thermochemical heat energy storage[END_REF], [START_REF] Gil | State of the art on high temperature thermal energy storage for power generation. Part 1-Concepts, materials and modellization[END_REF], [START_REF] Lovegrove | A solar-driven ammonia-based thermochemical energy storage system[END_REF], [START_REF] Shiizaki | Development of plate fin reactor for heat recovery system using methanol decomposition[END_REF][START_REF] Hahne | Thermal energy storage: some views on some problems[END_REF][START_REF] Kubota | Heat releasing characteristics of CaO/CaCO3 reaction in a packed bed for high temperature heat storage and temperature up-grading[END_REF][START_REF] Steinfeld | Design aspects of solar thermochemical engineering-A case study: Two-step water-splitting cycle using the Fe3O4/FeO ReDox system[END_REF][START_REF] Murray | Solar production of aluminum by direct reduction : Preliminary results for two processes[END_REF] Candidate reactions possessing good reversibility include Co3O4/CoO, SO3/SO2 and particularly TiH2/Ti. They have good potential for CSP application but more experimental feedback is needed [START_REF] André | Screening of thermochemical systems based on solid-gas reversible reactions for high temperature solar thermal energy storage[END_REF]. Mg/MgH2 and NH4HSO4/(NH3+SO3) have shown to have good reversibility by intensive experimental tests but how to store the reaction products remains as a technical barrier. Indeed, these products in gaseous form (H2, SO3 and NH3) need to be compressed and stored in large tanks, implying higher installation and operation costs. Some alternative solutions were proposed and tested, such as the storage of H2 with metals hydrides at low temperatures [START_REF] Rönnebro | Metal Hydrides for High-Temperature Power Generation[END_REF] or by adsorption [START_REF] Krieger | Thermodynamic analysis of reversible hydrogenation for heat storage in concentrated solar power plants[END_REF] but additional costs seem inevitable. Hydration/dehydration of CaO/Ca(OH)2 is another promising candidate with numerous expected features (e.g., good reversibility, low operating pressure, low price and no-toxicity of the product and large experimental feedback). Meanwhile, technical problems such as low thermal conductivity, agglomeration and sintering still remain to be solved.

Common drawbacks of materials used for thermochemical storage are low thermal conductivity which slows the heat transfer and low permeability which reduces the mass transfer. Many studies are devoted to the material properties and reactor designs so as to enhance the mass transfer during combination and also to enhance the heat transfer during decomposition [START_REF] Michel | Thermochemical process for seasonal storage of solar energy : characterization and modeling of a high-density reactive bed To cite this version : Thermochemical process for seasonal storage of solar energy : characterization and modeling of a high-density[END_REF][START_REF] Stitou | Conversion, Stockage, Transport de l'énergie thermique par procédés thermochimiques et thermo-hydrauliques[END_REF]. The latest advancements and proposed methods are reviewed by Aydin et al. [START_REF] Aydin | The latest advancements on thermochemical heat storage systems[END_REF]. The available solutions are the construction of matrix [START_REF] Willers | The two-stage metal hydride heat transformer[END_REF] in the reactant part or the melting of the reactant with additives (e.g., expended graphite matrix [START_REF] Pierrès | Design of a thermochemical process for deep freezing using solar low-grade heat[END_REF][START_REF] Stitou | Performance of a high temperature hydrate solid/gas sorption heat pump used as topping cycle for cascaded sorption chillers[END_REF], aluminum foam [START_REF] Klein | Development of a two-stage metal hydride system as topping cycle in cascading sorption systems for cold generation[END_REF], cobalt/cobaltous oxide based honeycombs [START_REF] Karagiannakis | Cobalt/cobaltous oxide based honeycombs for thermochemical heat storage in future concentrated solar power installations: Multi-cyclic assessment and semi-quantitative heat effects estimations[END_REF]). Roßkopf et al. [START_REF] Roßkopf | Improving powder bed properties for thermochemical storage by adding nanoparticles[END_REF] also reported that power bed properties and thermal stability of thermochemical materials can be improved by adding nanoparticles (i.e., SiO2).

Short summary

Table 6 summarizes main characteristics of different TES technologies and key features.

Concepts for TES integration

Beyond choosing the suitable TES technology for CSP application, the TES system must be coupled in a proper way with the power generating cycle (e.g., Rankine cycle). Pertinent concepts that integrate TES system and the power generating cycle remain as one of the key issues for the actual application of TES technology. This section will review the commonly proposed concepts for TES integration and compare their advantages and disadvantages. Special attention is given to new trends of integration concepts, especially when more than one TES technologies are envisaged.

Classification

Based on the motion state of storage materials during charge or discharge, the concepts for TES integration can be broadly classified into two concepts: active concept and passive concept. In active storage systems, the storage medium itself flows to absorb (charge) or release (discharge) heat by forced convection. Contrarily in passive storage systems, the storage medium (often solids) is kept motionless and heated or cooled by the circulation of another heat transfer fluid (HTF).

Active systems can be subdivided into direct and indirect systems. In an active direct system, the storage medium itself also serves as the HTF used for the solar field (Figure 6a). During charge phase, the HTF is directly stored in a hot tank located at the exit of the solar field. During discharge phase, the hot HTF in the hot tank is pumped through the power cycle and then stored in a cold tank at the entrance of the solar field for the next charge/discharge cycle [START_REF] Herrmann | Two-tank molten salt storage for parabolic trough solar power plants[END_REF]. This concept needs no heat exchanger between HTF and storage materials. But suitable storage materials (molten salts or steam) are required because they should meet the requirements to be a good HTF and a good storage material at the same time.

In active indirect systems, the HTF and the storage medium are different. A typical active indirect concept is shown in Figure 6b, with two separate tanks for the storage. During the charge phase, the storage material in the cold tank is pumped through a heat exchanger where it is heated by the HTF, and then stored in the hot tank. During the discharge phase, the flow direction of storage material is reversed to release the heat to the HTF. In this manner, hot and cold materials are separately stored. An alternative to two tanks concept is using a single tank (Figure 6c) where hot materials are stored on the top while the cold materials on the bottom. They are separated because of the temperature stratification. The zone between the hot and cold fluids is called the thermocline. Usually a filler material (e.g., quartzite rock, sand, concrete, industrial waste) is added in the tank to enhance the thermocline effect and to reduce the needed quantity of storage materials. Therefore, it is about 35% cheaper than two tanks systems [START_REF] Gil | State of the art on high temperature thermal energy storage for power generation. Part 1-Concepts, materials and modellization[END_REF]. Some studies showed that filled thermocline with encapsulated PCM was also appropriate [START_REF] Nithyanandam | Analysis of a latent thermocline storage system with encapsulated phase change materials for concentrating solar power[END_REF]. One of the key factors for highly efficient thermocline storage is maintaining the temperature stratification by a controlled procedure of charge and discharge and by appropriate methods or devices to avoid mixing. The filling configuration of filler materials in the storage tank is certainly a key point to be considered. Wu et al. [START_REF] Wu | The impact of concrete structure on the thermal performance of the dual-media thermocline thermal storage tank using concrete as the solid medium[END_REF] studied the thermocline behaviors of four typical concrete structures during the discharging process, including the channel-embedded structure, the parallel-plate structure, the rod-buddle structure and the packed-bed structure. They found that the packed-bed structure seemed to have better thermal performance [START_REF] Wu | The impact of concrete structure on the thermal performance of the dual-media thermocline thermal storage tank using concrete as the solid medium[END_REF].

In a passive storage system, HTF circulates through the TES system to heat up or cool down the storage materials kept still inside. In the case of sensible storage using concrete, the TES system usually contains a tubular heat exchanger (Figure 6d) integrated into storage materials, forming a compact heat storage-exchanging unit. The high thermal conductivity of materials and the good contact between the concrete and the tubes enable high heat transfer rates. However, the discharge temperature may vary when concrete is cooled down. This problem may be overcome by using PCMs and thermochemical materials, but with lower heat transfer rates.

Table 7 summarizes the concepts for TES integration in CSP plants, principles, storage materials, examples of system integration, as well as their advantages and disadvantages.

New trends for TES integration

Based on recent literature, several new trends for TES integration may be observed: (i) coupled TES technologies for possible higher operating temperature; (ii) cascade concept with modularized units; and (iii) mechanical circulation of granular particles.

Coupled systems using two types of storage technologies (e.g., sensible-latent, sensiblethermochemical, sensible-sensible) allow increased storage capacity, reduced delay between charge and discharge and increased power bloc working temperature (till 1400°C [START_REF] Erickson | Coupled chemical-thermal solar power system and method[END_REF]). High operating temperature for CSP will increase the CSP's efficiency for cheaper electricity production. However, the TES design, operation and control become more complex with higher associated costs.

A good example is the coupled chemical-thermal solar power system [START_REF] Erickson | Coupled chemical-thermal solar power system and method[END_REF], as shown in Figure 7. The CSP system couples a thermal and a chemical energy pathway. The thermal pathway utilizes a HTF to collect concentrated sunlights as thermal energy at medium or high temperature (<700°C) and to transfer this energy to a thermal-to-electric power cycle. In parallel, the chemical pathway uses a redox material (e.g., Co3O4/Co3O, BaO2/BaO (Table 5)) which undergoes direct reduction in the receiver to store the solar energy as chemical potential. This redox material is then oxidized at very high temperatures (1400°C) in the power cycle in series with the thermal pathway heat exchanger. This thermal-chemical coupling allows the power cycle to perform at very high temperature, increasing its global solar-to-electric conversion efficiency. However, the receiver design seems complex as it also serves as a reactor for chemical reaction. [START_REF] Erickson | Coupled chemical-thermal solar power system and method[END_REF] Another example of a coupled TES system is the three-part thermal energy storage system for direct steam generation (Figure 8) [START_REF] Laing | Thermal energy storage for direct steam generation[END_REF]. The storage system is composed of three blocks, two are filled with sensible materials (concrete) for preheating and superheating and an intermediate one with implementing a PCM material (NaNO3) to evaporate/condensate the HTF. The compact design is particularly adapted for direct steam generation.

Figure 8. Example of coupled sensible-latent three-part thermal energy storage system for direct steam generation [START_REF] Laing | Thermal energy storage for direct steam generation[END_REF] Azanganeh et al. [START_REF] Zanganeh | Stabilization of the outflow temperature of a packed-bed thermal energy storage by combining rocks with phase change materials[END_REF] proposed a coupled latent-sensible thermocline TES system, as shown in Figure 9. It combines rocks in the main body and encapsulated PCMs placed on the top of the tank, with the purpose of maintaining constant outflow temperature. Their investigation indicated that only a small amount of PCM (1.33% of the total volume) was sufficient to achieve outflow temperature stabilization around the PCM's melting point [START_REF] Zanganeh | Stabilization of the outflow temperature of a packed-bed thermal energy storage by combining rocks with phase change materials[END_REF].

Adding encapsulated PCMs in sensible materials as a new trend is intensively investigated [START_REF] Cingarapu | Use of encapsulated zinc particles in a eutectic chloride salt to enhance thermal energy storage capacity for concentrated solar[END_REF][START_REF] Bellan | Numerical analysis of charging and discharging performance of a thermal energy storage system with encapsulated phase change material[END_REF][START_REF] Nithyanandam | Analysis of a latent thermocline storage system with encapsulated phase change materials for concentrating solar power[END_REF]. Cingarapu et al. [START_REF] Cingarapu | Use of encapsulated zinc particles in a eutectic chloride salt to enhance thermal energy storage capacity for concentrated solar[END_REF] showed that adding 10 vol% of coated Zn particles over the base of chloride salt eutectic augments from 15% to 34% the volumetric storage capacity depending on the cycling temperature range (ΔT= 50°C -100°C) .

Figure 9. Scheme of the combined sensible and latent heat concept for thermal energy storage [START_REF] Zanganeh | Stabilization of the outflow temperature of a packed-bed thermal energy storage by combining rocks with phase change materials[END_REF] The second trend of TES integration is the cascade of modularized TES units. Conventional TES systems used to be made in one block. Novel TES systems now consist of several blocks to reduce thermal inertia of the systems and easy control of the HTF's temperature passing through. The main disadvantages of the cascade concept are their more complicated design than a single unit and their higher costs due to the use of several heat exchangers, tanks and regulation systems.

One example of this trend is the cascade latent heat storage system [START_REF] Michels | Cascaded latent heat storage for parabolic trough solar power plants[END_REF], using different phase change materials with different temperatures of phase change (306 -380 °C) (Figure 10). Therefore, it is possible to store energy within a wide temperature range or to discharge at a constant temperature. However, the final design is complicated and the low thermal conductivity of PCMs needs to be improved.

Figure 10. Example of a cascade latent heat storage system [START_REF] Michels | Cascaded latent heat storage for parabolic trough solar power plants[END_REF] Another example of cascade TES system for Direct Steam Generation (DSG) is shown in Figure 11 [START_REF] Oguz | A solar energy system[END_REF]. The storage unit is composed of several blocks of sensible material (e.g., concrete, molten salt, sand). The steam can pass through one or several blocks depending of its temperature. The main advantage of this system is the reduced thermal inertia of the TES system by dividing one storage block into three. However, the costs for construction and for the associated control system are inevitably higher than those for a single block TES system. [START_REF] Oguz | A solar energy system[END_REF] Other new concepts proposed involve mechanical displacement of solid PCM or chemicals reactants [START_REF] Muren | Concentrating solar power methods and systems with liquid-solid phase change mateial for heat tranfer[END_REF] [START_REF] Zhiwen | Chemical looping fluidized-bed concentrating solar power system and method[END_REF]. A good example of this type of systems is a CSP system using fluidized-bed technology for thermal energy conversion and solid particles for thermal energy storage [START_REF] Ma | Development of a Concentrating Solar Power System Using Fluidized-bed Technology for Thermal Energy Conversion and Solid Particles for Thermal Energy Storage[END_REF][START_REF] Iniesta | Gravity-fed Combined Solar Receiver/Storage System Using Sand Particles as Heat Collector, Heat Transfer and Thermal Energy Storage Media[END_REF], as shown in Figure 12. This system uses granular particles as both HTF and storage materials. Solid granular particles are moved from the cold tank to the solid-particle receiver by a bucket lifter system. After absorbing the solar heat and reaching a very high temperature (> 1000°C), they then fall into the hot silo used as TES system by gravity. The stored heat could be discharged to the working fluid of the power generation cycle by circulating the granular particles through the fixed bed heat exchangerThe mains advantages of this system are the high stability of particles at high temperature and their low cost. However, its application in large-scale is still challenging and needs to be further tested. [START_REF] Ma | Development of a Concentrating Solar Power System Using Fluidized-bed Technology for Thermal Energy Conversion and Solid Particles for Thermal Energy Storage[END_REF] Besides all these new concepts, researchers [START_REF]Centro de Investigaciones Energeticas-Medioambientales y Tecnologicas[END_REF] also proposed to use an intermediate HTF to reduce the HTF pressure in the heat exchanger. But the number of heat exchangers will increase, leading to increased costs. Table 8 summarizes these new concepts proposed in the literature for TES integration with their advantages and disadvantages.

Figure 11. Example of a solar energy system

Table 8. New concepts for TES integration in CSP plants

Conclusion

This work provides an extensive review on the thermal energy storage systems (TES) in CSP plants worldwide. The state-of-the-art for CSP plants, different TES technologies and the concepts for their integration in CSP plants are discussed. The major conclusions may be summarized as follows.

• CSP technology is already mature and readily available for commercialization, indicated by the increasing number of commercial CSP plants that are currently under construction or planned and by their increasing average power output. • TES systems seem indispensable for the future more powerful CSP plants, for the latter to be more economically competitive and more dispatchable. Progress in TES technologies permit the increased storage capacity of their TES systems. • A vast majority of CSP plants in operation use sensible heat storage, for their reliability, low cost and large experimental results obtained. The low energy density makes them more suitable for small or moderate power plants but less competitive for large-scale powerful CSP plants. • Technologies of latent storage and thermochemical storage have much higher energy density, thus will have a bright foreground.

Some technological barriers remain to be overcome for latent and thermochemical storage, which are also the key issues of current research and development:

• To enhance the thermal conductivity of PCMs and thermochemical materials;

• To remove solid deposits that form at the exchange surfaces between PCM and the HTF;

• To enhance the heat and mass transfers during thermochemical reactions;

• To reduce the transition time during the switch from charge to discharge (and vice versa);

• To propose suitable and adaptive concepts for TES integration in CSP plants; and • To design modularized TES units and their appropriate assembling and management. -High thermodynamic efficiency due to high operating temperature.

-Large space area occupied.

-Relatively high installation cost.

-High heat losses.

PDC 120-1500 100-1000 Difficult -Relatively small area occupied.

-High thermodynamic efficiency due to high operating temperature.

-Relatively high installation cost.

-Little experimental feedback. Coupled chemicalthermal solar power system and method [START_REF] Erickson | Coupled chemical-thermal solar power system and method[END_REF] Sensiblethermochemical Active direct 1400 °C (in the power bloc) n.a.

-Two pathway (thermal and thermochemical) to transfer energy from the receiver to the power cycle (boiler)(Figure 7)

-Very high temperature in the power cycle, higher than with classical HTF (e.g., oil, molten salt, steam)

-Complex design with a solar receiver which is at the same time a thermochemical reactor and a heater for the thermal pathway HTF -4 tanks needed to store Hot HTF, cold HTF, reduced reactants and oxidized reactions

Figure 3

 3 Figure 3 shows different purposes of CSP plants (commercial, demonstration or Research & Development) and their respective percentage for each group of plants. CSP plants with a commercial purpose always represent the vast majority. Research and development (R&D) used to be the second biggest purpose for plants in operation. However, their installation decreases gradually in recent years. No plant is planned in the future specially for R&D purpose, indicating that CSP technologies have become relatively mature and readily available for commercialization. Meanwhile, the proportion of plants built for a demonstration purpose remains almost constant since 2010.

Figure 3 .

 3 Figure 3. Purposes of CSP plants and their respective percentage

Figure 4 .

 4 Figure 4. Types of technology and their respective percentage

Figure 5 .

 5 Figure 5.a. Evolution of average power of CSP plants Figure 5.b. Percentage of hybridization for CSP plants

Figure 6 .Figure 6 .Figure 6 .Figure 6 .

 6666 Figure 6.a. Active direct concept for TES integration

Figure 7 .

 7 Figure 7. Example of coupled chemical-thermal solar power system[START_REF] Erickson | Coupled chemical-thermal solar power system and method[END_REF]

Figure 12 .

 12 Figure 12. Example of a pressurized fluidized bed for an air-Brayton gas-turbine combined CSP system[START_REF] Ma | Development of a Concentrating Solar Power System Using Fluidized-bed Technology for Thermal Energy Conversion and Solid Particles for Thermal Energy Storage[END_REF]

Figure 6 .Figure 7 .Figure 8 .

 678 Figure 6. CSP plant with a TES system

Figure 10 .Figure 7 .Figure 8 .Figure 9 .Figure 10 .Figure 11 .Figure 12 .

 10789101112 Figure 10.a. Evolution of average power of CSP plants

-

  tanks system (Figure6a): the HTF is directly stored in a hot tank at the exit of the solar receiver or in a cold tank at the entrance of solar receiver -No need of heat exchanger between the HTF and the storage material Single tank (Figure6c) to store hot and cold fluids, separated by temperature stratification.-Relatively low cost due to the use of only one tank and low cost filler materials (e.g. quartzite rock and sand) composed of a HTF, a block of PCM with crossing channels, one heat exchanger at the bottom (charge) and one heat exchanger at the top (

Table 4 : Gaseous materials for sensible heat storage

 4

Table 6 . Summary for different TES technologies

 6

Table 9 .

 9 Comparison between different CSP technologies

	CSP	Operating	Solar	Storage	Advantages	Disadvantages
	technology	temperature	concentration	integration
		(°C)	ratio	possibility
	PTC	20-400	15-45	Possible	-Relatively low installation cost	-Relatively large area occupied.
					[11].	-Low thermodynamic efficiency
					-Large experimental feedback.	due to low operating temperature.
	LFR	50-300	10-40	Possible	-Relatively low installation cost.	-Low thermodynamic efficiency
						due to low operating temperature.
	SPT	300-1000	150-1500	Highly possible
				with low storage
				cost [12,13]

Table 2 .

 2 Number of samples for each item of analysis

	Group
	Item of	In operation, before 2000		In operation, 2000 to 2010		In operation, after 2010
	analys
	is
	Purpo se of plants	Com merci al 9	Demonstration 0	R&D 4	n .a . 6	Com merci al 13	Demonstratio n 6	R&D 3	n .a . 2	Com merci al 63	Demonstration 8	R&D 13	n .a . 1
	Type					n					n					n
	of	PTC	SPT	LFR	PDC	.a	PTC	SPT	LFR	PDC	.a	PTC	SPT	LFR	PDC	.a
	techn					.					.					.
	ology	9	4	0	0	6	12	8	3	0	1	63	14	8	0	0
	Avera	Available Value		n.a.		Available Value	n.a.		Available Value	n.a.
	ge							22		2			83		2
	power		19		0
	of CSP
	Hybrid	Yes		No	n.a.	Yes		No	n.a.	Yes		No		n.a.
	ization	8		2	9		11		6	7		47		11		27
	Storag	Yes		No	n.a.	Yes		No	n.a.	Yes		No		n.a.
	e	9		7	3		10		10	4		30		38		17
	Distrib ution of TES techn ology	Sensi ble 6	Late nt 0	Thermoche mical 0	othe r 0	n .a . 7	Sensi ble 10	Laten t 0	Thermoche mical 0	othe r 0	n .a . 5	Sensi ble 27	Laten t 0	Thermoch emical 0	othe r 1	n .a . 2 1
	Storag	Available Value	n.a.		Available Value		n.a.		Available Value	n.a.
	e							9		15			27		58
	capaci		1		18
	ty
	Total
	numb er of			19					24					85
	plants

Group Item of analysis Under construction Planned Purpose of plants

		Commercial	Demonstration	R&D	n.a.	Commercial	Demonstration	R&D	n.a.
		29	4		1	1	66	8		0	0
	Type of	PTC	SPT	LFR	PDC	n.a.	PTC	SPT	LFR	PDC	n.a.
	technology	21	5	6	3	0	26	29	6	5	8
	Average power of	Available Value		n.a.			Available Value		n.a.
	CSP		31		4			74		0
	Hybridization	Yes		No		n.a.	Yes		No		n.a.
		7		11		17	13		10		51
	Storage	Yes		No		n.a.	Yes		No		n.a.
		16		6		13	24		7		43
	Distribution of TES	Sensible	Latent	Thermochemical	other	n.a.	Sensible	Latent	Thermochemical	other	n.a.
	technology	14	0	0	1	1	19	0	0	0	5
	Storage capacity	Available Value		n.a.		Available Value		n.a.
			13		22			13		61
	Total number of plants			35					74

Table 3 .

 3 Summary for TES systems in CSP plants

			Current situation	Future trends
	Purpose of plants	-69 % for commercial production	-Increased proportion of commercial plants;
			-13 % for demonstration	-Almost stable proportion of demonstrative plants;
			-18 % for R&D	-No plant specifically for R&D
	CSP technology	-SPT and particularly PTC are the most used	-SPT is going to be the most used technology
			technologies	-Increased proportion of PDC
			-PDC is hardly used
	Average	power	-33 MWelect	-Increased average power, 126 MWelect for plants in
	output			project
	Hybridization		-Significant presence of hybridization system	-Reduced use of hybridization systems
			(77%)
	Storage		-47% of plants have a TES system	-Increased presence (>70%) of TES system
	Average storage	-3 h for the plants built before 2010	-Increased storage capacity till 7.65 hours for planned
	capacity		-7 h for the plants built after 2010	plants
	Type of TES		-Vast majority of sensible storage	-Vast majority of sensible storage

Table 4 :

 4 Gaseous materials for sensible heat storage

	Materials Operating temperature	Density (kg.m -	Thermal conductivity (W.m -1 .K -	Energy density (kJ.kg -1 .K -	Cost of
		(°C)	3 )	1 )	1 )	material
	Steam	100-700	0.28 (500°C)	0.067 (500°C)	2.13 (500°C)	low
	Air	100-1000	0.441 (500°C)	0.0578 (500°C)	1.098 (500°C)	low

Table 5 .

 5 Main reactions used in high temperature thermochemical heat storage

	Reaction	Charge/discharge	Reaction	Advantages	Disadvantages	Experimental
	& Phase	temperature &	enthalpy &			feedback
		pressure	Energy
			density
	2BaO2(s) +	1236-1326 °C	77 kJ.mol -1	-Low operating pressure (0-10	-Low reversibility	-Only
	ΔH ↔			bars)	-Little experimental feedback	equilibrium or
	2BaO(s) +	0-10 bar	328 kWh.m -3	-No catalyst needed	-O2 storage	kinetic study
	O2(g)			-No side reactions		[122,135]
	[122,135]
	Solid-gas
	Organic
	system	1496/1076 °C	250 kJ.mol -1	-High reaction enthalpy	-Side reaction	-Studies for
	CH4(g) +			-Large experimental and	-Low reversibility	the transport
	H2O(g) + ΔH	20-150 bar	7.8 kWh.m -3	industrial feedback (for the	-Cost of CH4	of thermal
	↔ 3H2(g) +			transport of thermal energy)	-Catalyst needed	energy [93-94]
	CO(g)				-H2 storage
	[136-139]
	Gas-Gas
	CH4(g) +	1496/1076 °C	247 kJ.mol -1	-High reaction enthalpy	-Side reaction	-Technical-
	CO2(g) + ΔH			-Large experimental and	-Low reversibility	economic
	↔ 2H2(g) +	3.5 bar	7.7 kWh.m -3	industrial feedback	-Cost of CH4	survey realized
	2CO(g)				-Catalyst needed (Ni-based or	for the
	[136,138,140-				Ru-based)	production of
	142]				-H2 storage	100 MWe
	Gas-Gas					[141]
						-A solar
						reactor of 300
						kW developed
						by DLR;
						problems of
						catalyst
						deactivation
						observed [142]
	C6H12(l) +	863/943 °C	206 kJ.mol -1	-High reaction enthalpy	-Low reversibility	-Only
	ΔH ↔				-Side reaction	simulation
	C6H6(g) +	1-70 bar	530 kWh.m -3		-Catalyst needed	work [96-98]
	3H2(g)				-Toxicity
	[106,139,143]				-H2 storage
	Liquid-Gas
	SO3(l) + ΔH	1346-1546/1046-	98 kJ.mol -1	-High energy density	-Corrosive product	-Only
	↔ SO2(g) +	1146 °C		-Industrial feedback	-Toxicity	simulation
	½.O2(g)		646 kWh.m -3		-Catalyst needed	work [144]
	[106,144]	1-5 bar			-O2 storage
	Liquid-Gas

Table 6 .

 6 Summary for different TES technologies

	Technology	Cost	Energ	Occupi	Temperat	Technolo	Flexibility	Heat	Advantages	Disadvanta
			y	ed	ure for	gy	charge/disch	transfer		ges
			densit	area	material	feedback	arge
			y		storage
	Sensible	Low cost	Low	High	High	-Large	Switch within	Quite	-Large	-Heat
		material				experime	a short time	good	experimental	losses
		s				ntal			and	during
						feedback			commercial	storage
						-Majority			feedback	-Low
						of CSP			-Easy	energy
						plants in			implementati	density
						operation			on

Table 7 .

 7 Different concepts for TES integration, their advantages and disadvantages

	Concept	Principle	Storage materials	Example of integration	Advantages	Disadvantages
	Active	HTF used in the
	direct	solar field is also
		used as storage
		material

Acknowledgement

Authors would like to thank the financial support of the French "Agence National de la Recherche" within the project In-STORES (ANR-12-SEED-0008).

M&O Maintenance and Operation n.a. not applicable R&D Research and Development SPT Solar Power Tower STC Solar Tower Collector TES Thermal Energy Storage

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