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Ultra High Temperature Latent Heat Energy Storage and Thermophotovoltaic Energy Conversion

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Ultra High Temperature Latent Heat Energy Storage and Thermophotovoltaic Energy Conversion Ultra high temperature latent heat energy storage and thermophotovoltaic energy conversion Alejandro Datas(*), Alba Ramos, Antonio Martí, Carlos del Cañizo and Antonio Luque Instituto de Energía Solar – Universidad Politécnica de Madrid, Madrid, 28040, Spain (*) corresponding autor: [email protected] Keywords: LHTES (latent heat thermal energy storage), high temperature, thermophotovoltaics, silicon, boron, PCM (phase change materials), CSP (concentrated solar power). Abstract A conceptual energy storage system design that utilizes ultra high temperature phase change materials is presented. In this system, the energy is stored in the form of latent heat and converted to electricity upon demand by TPV (thermophotovoltaic) cells. Silicon is considered in this study as PCM (phase change material) due to its extremely high latent heat (1800 J/g), melting point (1410ºC), thermal conductivity (~25 W/m-K), low cost (less than $2/kg or $4/kWh) and abundance on earth. The proposed system enables an enormous thermal energy storage density of ~ 1 MWh/m3, which is 10-20 times higher than that of lead-acid batteries, 2-6 times than that of Li-ion batteries and 5-10 times than that of the current state of the art TES systems utilized in CSP (concentrated solar power) applications. The discharge efficiency of the system is ultimately determined by the TPV converter, which theoretically can exceed 50%. However, realistic discharge efficiencies utilizing single junction TPV cells are in the range of 20-45%, depending on the semiconductor bandgap and quality, and the photon recycling efficiency. This concept has the 3 potential to achieve output electric energy densities in the range of 200-450 kWhe/m , which is comparable to the best performing state of the art Lithium-ion batteries. Nomenclature list Ac TPV cells total area (m2) cps heat capacity of solid PCM (J/g-K) c pl heat capacity of liquid PCM (J/g-K) c speed of light (m/s) E radiative energy flux (W/m2-sr) Etot energy stored instantaneously in the PCM in the form of specific and latent heat (J) h Plank constant (Js) k Boltzmann constant (J/K) kl thermal conductivity of liquid PCM (W/m-K) ks thermal conductivity of solid PCM (W/m-K) L container length (m) L f latent heat (J/g) N photon flux (#photons/m2-sr-sec) ns refraction index of the semiconductor Pel TPV output electrical power (W) Ql heat flux in the liquid PCM (W) Qs heat flux in the solid PCM (W) R1 distance from center to the emitter (m) R2 distance from center to the outer container wall (m) rm distance from center to the solid-liquid front (m) t time (s) T1 temperature at the emitter (K) T2 temperature at PCM-wall boundary (K) Tm melting temperature of PCM (K) Tc TPV cells temperature (K) V TPV cell voltage (V) G TPV cell semiconductor bandgap (eV) photon energy (eV) density (g/m3) BR reflectivity of the back-surface reflector in TPV cell int internal photoluminescence quantum efficiency of the TPV cell ext external photoluminescence quantum efficiency of the TPV cell 1 Introduction LHTES (latent heat thermal energy storage) employs energy to cause the phase change transition in a material that subsequently stores energy in the form of latent heat. That material is referred to as PCM (phase change material) and is the key element determining the overall performance of the storage system. PCMs promises one of the highest energy densities and lowest costs of existing TES materials [1], [2]. However, current LHTES solutions are subject to a very low heat extraction rate from the storage medium, which is attributed to the low solid-phase thermal conductivity and moderate latent heat of PCMs. Current research efforts focus on developing relatively sophisticated PCM encapsulation so that thermal conductivity is notably enhanced [1], [3]–[6]. However, these strategies inherently have a lower energy density potential, since part of the volume is dedicated to the PCM host. An alternative solution consists of directly using PCMs with higher thermal conductivity and latent heat. As a general rule, the heat of fusion of materials increases with melting temperature [1], [7]; thus, there is an interest on moving towards higher melting point PCMs. However, in TES for power generation there is a maximum temperature imposed by the heat transfer fluid (HTF) that is used to carry the heat from the PCM to the heat engine, which degrades at high temperatures. Maximum temperatures are typically below 500 ºC [8]. Other technological options exist though; such as thermophotovoltaics (TPV) [9], [10], thermionic [11] or hybrid thermionic-photovoltaic [12] devices that do not require HTFs and consequently have the potential to operate at extraordinary high temperatures. Previous works have proposed conceptual system designs for solar thermal energy storage based on very high melting point PCMs, such as pure silicon and boron (melting points of 1410ºC and 2076ºC, respectively) and TPV converters [13]–[19]. The first experiments on molten silicon for TES applications have been recently carried out at the University of South California with the aim of developing a solar thermal propulsion system for microsatellites [20]. The container damage due to freezing expansion of pure silicon was the most relevant engineering concern. Besides, a highly asymmetric freezing profile was observed due to the use of non-adiabatic container walls, which produced regions of molten silicon encased in solid silicon that ultimately resulted in high stress and container damage. It is worth noting that these issues could be solved in future designs by several means, such as using quasi-adiabatic container walls, i.e. improved container thermal insulation, or reducing the container fill factor [20]. In the opinion of the authors, an especially interesting solution consists on using silicon alloys instead of pure silicon, in order to reduce the freezing expansion coefficient of the PCM. We believe that, among all the possibilities, the silicon- boron system is particularly interesting due to the extremely high latent heat of boron (4650 J/g) and the moderately low melting temperature (1385ºC) for the eutectic Si0.92B0.08 [21], [22]. Besides, the silicon lattice parameter contracts upon alloying with boron [21], which suggests that freezing expansion issues could be eliminated. Other practical concern is the thermo-chemical compatibility between the container and the PCM at those high temperatures. A variety of refractory materials have been extensively used for casting solidification of all kind of metals at high temperatures, including silicon and boron. Some examples are BN (used in [20]) SiC and Si3N4. In these cases, important selection criteria are the wetability, solubility and reactivity of the container with the PCM, along with evaporation and oxidation of the PCM in oxygen-rich atmospheres. Thus, it is still needed an extensive research on the compatibility of these materials for the particular application of LHTES, especially concerning cycling and long term reliability. In this work we present a conceptual LHTES system design for both S2H2P (solar-to-heat-to- power), commonly referred to as CSP (concentrated solar power), and P2H2P (power-to-heat-to- power) applications. Notice that other P2H2P concepts have been previously proposed to store the excess of electricity in the grid and co-generate heat and electricity [23]. The concept presented in this paper is based on the same operation principles than previously proposed systems in [13]–[19], i.e. high temperature PCM and TPV energy conversion. The proposed system differentiates from the previous designs in its geometrical configuration, which facilitates the thermal insulation of the PCM and the integration of an independent (mobile) TPV generator, providing a tunable power discharge rate (from zero to full-discharge mode). We assess this concept theoretically to predict its performance under several assumptions, which range from idealistic (to provide the upper bounds of this concept) to more realistic, to provide its actual short-term potential. 2 System description Figure 1 shows two possible configurations of the LHTES system presented in this paper for P2H2P [24] (left-hand side) and S2H2P or CSP (right-hand side) applications. In the P2H2P case, an electric heating system is used for melting the PCM. Among all the possible options, an inductive electric heater could be used if the PCM is magnetic or electrically conductive (e.g. iron or metallurgical silicon). Other options include resistive or microwave heating. In any case, electrical energy is stored in the form of the latent heat within the PCM. In the S2H2P case (right-hand side in Figure 1); concentrated solar power heats the inner walls of the vessel containing the PCM. If the sunlight concentration factor is high enough [13], [14] (above 1000 suns) the solar heat will produce the melting of the PCM and consequently, solar energy will be stored in the form of latent heat. Other arrangements not illustrated in this paper may use the waste heat from high temperature industrial processes or other kinds of electric heating. In both cases of Figure 1 the stored heat is released in the form of electricity by using a TPV converter, which comprises a number of infrared sensitive photovoltaic cells that directly produce electricity from radiant heat. In contrast to conventional heat engines, the contact-less nature of TPV converters enable extremely high temperature operation, which is essential for this kind of systems. Besides, TPV can provide extremely high power densities (power-to-weight and power-to- volume ratio) at low maintenance costs (neither moving parts nor working fluids within the converter) along with silent operation, which is important for decentralized ES applications. Furthermore, the TPV conversion efficiency is very high, potentially exceeding 50% due to the possibility of sub-bandgap photon recycling, which can be accomplished, for instance, by using reflectors in the back side of the TPV cells [25].
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