A Hybrid Electric and Thermal Solar Receiver

Abstract

•Hybrid solar receiver delivering both electricity and thermal energy proposed•Receiver enabled by transparent aerogel and spectrally selective light pipe•Modeling predicts >22% efficiency with current subcomponent properties•Efficiency greater than 35% predicted for improved subcomponents PV and solar thermal systems are the main methods for solar energy conversion. PV cannot utilize the entire solar spectrum, and it is expensive to store the generated electricity. Solar thermal systems convert sunlight to electricity using thermal energy as an intermediary, allowing the use of inexpensive thermal storage, but are more expensive than PV overall. Hybrid systems that convert some of the solar spectrum to electricity directly using PV and the rest to thermal energy can achieve higher efficiency than PV or solar thermal systems while allowing the use of thermal storage. Here, we introduce a stacked hybrid receiver design enabled by transparent aerogel and a spectrally selective light pipe. Modeling predicts a 35% solar-to-electricity conversion efficiency with further subcomponent improvement and >26% efficiency with the best subcomponent properties reported to date. Further development of this receiver could thus yield a high-efficiency option for solar energy conversion. Solar energy offers a promising renewable energy source; however, it is expensive to store electricity from photovoltaics (PV), the most widely deployed solar electricity technology. Solar thermal energy technologies can be paired with inexpensive thermal storage, but are more expensive overall. We have developed a solar receiver that combines PV and solar thermal systems to efficiently convert solar radiation to electricity (to be used immediately) and thermal energy (to be stored and converted to electricity on demand). This paper describes the Hybrid Electric And Thermal Solar (HEATS) receiver and models its performance. An idealized model predicts high solar-to-electricity efficiency (35.2%) with high dispatchability (44.2% of electricity from thermal energy) at an operating temperature of 775 K. Modeling using measured performance values for HEATS subcomponents predicts 26.8% efficiency and 81% dispatchability with silicon PV and 28.5% efficiency and 76% dispatchability with gallium arsenide PV, both operating at 700 K. Solar energy offers a promising renewable energy source; however, it is expensive to store electricity from photovoltaics (PV), the most widely deployed solar electricity technology. Solar thermal energy technologies can be paired with inexpensive thermal storage, but are more expensive overall. We have developed a solar receiver that combines PV and solar thermal systems to efficiently convert solar radiation to electricity (to be used immediately) and thermal energy (to be stored and converted to electricity on demand). This paper describes the Hybrid Electric And Thermal Solar (HEATS) receiver and models its performance. An idealized model predicts high solar-to-electricity efficiency (35.2%) with high dispatchability (44.2% of electricity from thermal energy) at an operating temperature of 775 K. Modeling using measured performance values for HEATS subcomponents predicts 26.8% efficiency and 81% dispatchability with silicon PV and 28.5% efficiency and 76% dispatchability with gallium arsenide PV, both operating at 700 K. The amount of solar energy that reaches Earth is four orders of magnitude greater than human demand.1Crabtree G.W. Lewis N.S. Solar energy conversion.Phys. Today. 2007; 60: 37-42Google Scholar Due to this abundance, solar energy is a promising renewable energy source that should play an important role in reducing further global climate change. The most heavily deployed solar electric conversion technology is single-junction photovoltaics (PV), the great majority of which uses silicon.2REN21 Steering Committee Renewables 2016 Global Status Report. REN21, 2016Google Scholar Single-junction PV cells made of a single bandgap material are unable to convert a large portion of the energy in the broad solar spectrum to electricity: photons with lower energy than the bandgap are unusable and photons with energy much higher than the bandgap lose most of their energy to thermalization. These spectral losses, combined with radiative recombination losses, lead to the Shockley-Queisser efficiency limit of 32.2% for silicon at room temperature and under unconcentrated solar illumination.3Shockley W. Queisser H.J. 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Ilinca A. Perron J. Energy storage systems—characteristics and comparisons.Renew. Sust. Energ. Rev. 2008; 12: 1221-1250Google Scholar Solar thermal systems are not widely deployed because they are more expensive than PV overall.11Hernández-Moro J. Martínez-Duart J.M. Analytical model for solar PV and CSP electricity costs: present LCOE values and their future evolution.Renew. Sust. Energ. Rev. 2013; 20: 119-132Google Scholar A third type of system takes advantage of the best of both PV and solar thermal: a “hybrid” system. A hybrid system contains both a PV cell and a thermal absorber. In such a system, photons converted most efficiently by the PV cell (photons in the “PV band”) are directed to the PV cell. Low-energy (long-wavelength) photons, which cannot be converted by the PV cell, and high-energy (short-wavelength) photons, which would be converted inefficiently, are directed to the thermal absorber instead. This approach improves overall system efficiency and leads to the collection of some thermal energy, which is cheaper to store than electricity.12Branz H.M. Regan W. Gerst K.J. Borak J.B. Santori E.A. Hybrid solar converters for maximum exergy and inexpensive dispatchable electricity.Energy Environ. Sci. 2015; 8: 3083-3091Google Scholar, 13Bermel P. Yazawa K. Gray J.L. Xu X. Shakouri A. Hybrid strategies and technologies for full spectrum solar conversion.Energy Environ. Sci. 2016; 3: 2123Google Scholar For these hybrid systems, “dispatchability” refers to the portion of electricity generated from the heat engine divided by the total electricity generated (from both the heat engine and PV), since heat is cheaper to store than electricity. Two main schemes have been proposed in the past for hybridized systems: dichroic mirror systems and hot PV cell systems. In a receiver using a dichroic mirror arrangement, the thermal collector is physically separate from the PV cell, and a dichroic mirror is used to split the solar spectrum between the PV cell and the thermal collector.14Imenes A.G. Buie D. McKenzie D. The design of broadband, wide-angle interference filters for solar concentrating systems.Sol. Energy Mater. Sol. Cells. 2006; 90: 1579-1606Google Scholar, 15Crisostomo F. Taylor R.A. Surjadi D. Mojiri A. Rosengarten G. Hawkes E.R. Spectral splitting strategy and optical model for the development of a concentrating hybrid PV/T collector.Appl. Energy. 2015; 141: 238-246Google Scholar, 16Otanicar T.P. Theisen S. Norman T. Tyagi H. Taylor R.A. Envisioning advanced solar electricity generation: parametric studies of CPV/T systems with spectral filtering and high temperature PV.Appl. Energy. 2015; 140: 224-233Google Scholar In a hot PV cell arrangement the thermal collector is in contact with the PV cell, so the PV cell must operate at the temperature of the collected thermal energy.17Luque A. Martı́ A. Limiting efficiency of coupled thermal and photovoltaic converters.Sol. Energy Mater. Sol. Cells. 1999; 58: 147-165Google Scholar, 18Vorobiev Y. González-Hernández J. Vorobiev P. Bulat L. Thermal-photovoltaic solar hybrid system for efficient solar energy conversion.Sol. Energy. 2006; 80: 170-176Google Scholar, 19Sharaf O.Z. Orhan M.F. Concentrated photovoltaic thermal (CPVT) solar collector systems: Part I—fundamentals, design considerations and current technologies.Renew. Sust. Energ. Rev. 2015; 50: 1500-1565Google Scholar Unfortunately, both dichroic mirror and hot PV systems face challenges. Dichroic mirror receivers are optically complicated due to the multiple paths the incident solar light must traverse.20Mojiri A. Taylor R. Thomsen E. Rosengarten G. Spectral beam splitting for efficient conversion of solar energy—a review.Renew. Sust. Energ. Rev. 2013; 28: 654-663Google Scholar While this difficulty can be addressed with relative ease in spectrally splitting PV systems,21McCambridge J.D. Steiner M.A. Unger B.L. Emery K.A. Christensen E.L. Wanlass M.W. Gray A.L. Takacs L. Buelow R. McCollum T.A. et al.Compact spectrum splitting photovoltaic module with high efficiency.Prog. Photovoltaics Res. Appl. 2011; 19: 352-360Google Scholar, 22Zhao Y. Sheng M.Y. Zhou W.X. Shen Y. Hu E.T. Chen J.B. Xu M. Zheng Y.X. Lee Y.P. Lynch D.W. et al.A solar photovoltaic system with ideal efficiency close to the theoretical limit.Opt. Express. 2012; 20: A28Google Scholar, 23Eisler C.N. Kosten E.D. Warmann E.C. Atwater H.A. Polyhedral specular reflector design for ultra high spectrum splitting solar module efficiencies (>50%).Proc. SPIE Int. Soc. Opt. Eng. 2013; (88210B): 8821Google Scholar, 24Lloyd, J.V., Kosten, E.D., Warmann, E.C., Flowers, C.A., and Atwater, H.A. (2014). Ray trace optimization of a light trapping filtered concentrator for spectrum splitting photovoltaics. 2014 IEEE 40th Photovoltaic Specialist Conference (PVSC) (IEEE), 2249–2252.Google Scholar hybrid systems have the added challenge of collecting thermal energy efficiently at high temperatures.25Jiang S. Hu P. Mo S. Chen Z. Optical modeling for a two-stage parabolic trough concentrating photovoltaic/thermal system using spectral beam splitting technology.Sol. Energy Mater. Sol. Cells. 2010; 94: 1686-1696Google Scholar, 26Saroha S. Mittal T. Modi P.J. Bhalla V. Khullar V. Tyagi H. Taylor R.A. Otanicar T.P. Theoretical analysis and testing of nanofluids-based solar photovoltaic/thermal hybrid collector.J. Heat Transfer. 2015; 137: 91015Google Scholar, 27Yu Z.J. Fisher K.C. Wheelwright B.M. Angel R.P. Holman Z.C. PVMirror: a new concept for tandem solar cells and hybrid solar converters.IEEE J. Photovolt. 2015; 5: 1791-1799Google Scholar, 28Cygan D. Abbasi H. Kozlov A. Pondo J. Winston R. Widyolar B. Jiang L. Abdelhamid M. Kirk A.P. Drees M. et al.Full spectrum solar system: hybrid concentrated photovoltaic/concentrated solar power (CPV-CSP).MRS Adv. 2016; : 1-6Google Scholar Hot PV cell systems face challenges in cell stability, as few materials can maintain efficient operation at the high temperatures required for solar thermal electricity generation.29Landis G. Merritt D. Raffaelle R. Scheiman D. High-Temperature Solar Cell Development. NASA, 2005Google Scholar, 30Singh P. Ravindra N.M. Temperature dependence of solar cell performance—an analysis.Sol. Energy Mater. Sol. Cells. 2012; 101: 36-45Google Scholar Some hot PV cell systems circumvent the stability challenge by operating at relatively low temperatures (∼100°C), which allow for co-generation of thermal energy but are not sufficient to generate electricity from the thermal energy at high efficiency.31Kribus A. Kaftori D. Mittelman G. Hirshfeld A. Flitsanov Y. Dayan A. A miniature concentrating photovoltaic and thermal system.Energy Convers. Manag. 2006; 47: 3582-3590Google Scholar, 32Chow T.T. A review on photovoltaic/thermal hybrid solar technology.Appl. Energy. 2010; 87: 365-379Google Scholar, 33Paredes, S., Burg, B.R., Ruch, P., Lortscher, E., Malnati, F., Cucinelli, M., Bonfrate, D., Mocker, A., Bernard, A., Ambrosetti, G., et al. (2015). Receiver-module-integrated thermal management of high-concentration photovoltaic thermal systems. 2015 IEEE 42nd Photovoltaic Specialist Conference (PVSC) (IEEE), 1–6.Google Scholar, 34Wang N. Han L. He H. Park N.-H. Koumoto K. Nazeeruddin M.K. Grätzel M. Wang P. Zakeeruddin S.M. Grätzel M. A novel high-performance photovoltaic–thermoelectric hybrid device.Energy Environ. Sci. 2011; 4: 3676Google Scholar Hybrid PV/solar thermal systems are therefore advantageous in theory, but existing hybrid schemes face practical challenges. The ideal hybrid receiver would have a single optical path but would also allow the PV cell to operate at low temperature. Here we report a novel receiver structure called the Hybrid Electric And Thermal Solar (HEATS) receiver, which meets both of the desired characteristics. This synergistic combination is achieved by using a spectrally selective light pipe (SSLP) structure to absorb non-PV band photons (as thermal energy) while directing PV-band photons to a cold PV cell on the other side, as shown in Figure 1. The light pipe does not need to be thermally in contact with the PV cell, so the PV can operate at low temperature. Thus, the HEATS receiver offers an alternative approach to hot PV cell receivers and dichroic mirror receivers for hybridizing PV and solar thermal systems. Using this “light pipe” configuration offers new opportunities and challenges in achieving efficient operation. The light pipe absorber offers more flexibility in receiver geometry than the traditional systems, which essentially always use tubes as absorbers. The challenge in achieving efficient operation in a light pipe configuration arises from minimizing thermal losses, as the light pipe must be maintained at the elevated heat delivery temperature. The traditional solution for reducing thermal losses in solar thermal receivers is to evacuate the receiver to eliminate convection losses and use a spectrally selective coating that has low emittance at wavelengths longer than the solar spectrum (mid to far infrared) in order to reduce radiation losses.35Bermel P. Lee J. Joannopoulos J.D. Celanovic I. Soljačić M. Selective solar absorbers.in: Chen G. Prasad V. Jaluria Y. Annual Review of Heat Transfer. 15. Begell House, 2012: 231-254Google Scholar Unfortunately, the traditional strategy would not effectively reduce radiation losses for this arrangement because the light pipe's walls have much greater surface area than its aperture. This configuration makes the light pipe look like a cavity for emitted radiation, and leads to high radiative losses even if the light pipe walls have low emittance in the mid to far infrared wavelengths.36Bedford R.E. Ma C.K. Emissivities of diffuse cavities: isothermal and nonisothermal cones and cylinders.J. Opt. Soc. Am. 1974; 64: 339Google Scholar An alternative approach for the HEATS receiver is to cover the light pipe apertures with a transparent thermal insulator. If the chosen material is transparent in the solar spectrum it will not affect the path of solar photons, and its low thermal conductivity can lead to a significantly reduced surface temperature. As thermal losses originate from the exposed receiver surface, lowering the surface temperature will significantly reduce convection and radiation losses. Thus there are two key subcomponents of the HEATS receiver not already used in solar energy conversion systems: the SSLP and the transparent thermal insulator. The SSLP can be fabricated by applying a thin-film multi-layer coating on a thermally conductive (e.g., copper) substrate. The desired properties of the transparent thermal insulator, that is, high solar transmittance (>90%) and low thermal conductivity (<0.1 W/m/K), are present in silica aerogel. Aerogels are porous materials that naturally exhibit low thermal conductivity due to a low volume fraction of solid.37Soleimani Dorcheh A. Abbasi M.H. Silica aerogel; synthesis, properties and characterization.J. Mater. Process. Technol. 2008; 199: 10-26Google Scholar Silica aerogels in particular can be made transparent in the solar spectrum.38Tillotson T.M. Hrubesh L.W. Transparent ultralow-density silica aerogels prepared by a two-step sol-gel process.J. Non-Cryst. Solids. 1992; 145: 44-50Google Scholar In practice the HEATS receiver could take the stacked structure shown in Figure 1A. The HEATS receiver would be used in conjunction with a solar concentrator, such as a parabolic trough or a linear Fresnel reflector, to increase the intensity of incident solar radiation. Concentrated solar radiation incident on a glass cover transmits through glass and aerogel to the SSLP. The SSLP absorbs low- and high-energy photons as thermal energy, conducting it to pipes carrying a heat transfer fluid. It should be noted that these pipes would lie outside the optically active region of the receiver, so they do not intercept sunlight directly. Sunlight would be contained with thin, specular reflectors on either side of the optically active region. The heat transfer fluid collects the high-temperature thermal energy to deliver it to a heat engine for conversion to electricity or to be stored for later use. The mid-energy photons converted most efficiently by the PV cell are transmitted through the SSLP and through another layer of aerogel to the PV cell. The aerogel layers serve to thermally insulate the SSLP from the PV cell (to keep the PV cell cool) and from the environment (to minimize thermal losses). It should be noted that in a real implementation, the sides of the receiver would be covered by conventional insulation to reduce thermal losses and protect the internal receiver components from environmental factors. As silica aerogel is mechanically fragile, the other internal components would be supported by rigid connections to the receiver external frame, while the aerogel slabs would sit on the layer below them. Due to aerogel's low density, the stresses from being supported by the thin SSLP fin edges would be low enough so as not to damage the aerogel. Thermal stresses due to the temperature gradient across the aerogel layers can be accommodated by the highly porous structure, and temperature gradients applied to aerogels in the process of characterizing thermal properties have not led to damage. To give a quantitative idea of the mechanical properties of silica aerogel, our fabricated samples have an approximate compressive strength of 3 MPa and an approximate flexural strength of 0.2 MPa, measured at room temperature using a uniaxial compression and 3-point bending test, respectively.39Alaoui A.H. Woignier T. Scherer G.W. Phalippou J. Comparison between flexural and uniaxial compression tests to measure the elastic modulus of silica aerogel.J. Non-Cryst. Solids. 2008; 354: 4556-4561Google Scholar While these values reflect the fragility of aerogel, we have found that with careful handling, aerogel samples can be placed in various device configurations (e.g., in order to measure their thermal and optical properties) without damaging them. In addition to the prospect of higher efficiency than traditional vacuum tube receivers, the HEATS receiver also offers potential cost and reliability benefits. Since the HEATS receiver does not require vacuum operation, there is no need for high-precision bellows that maintain a vacuum seal throughout daily thermal expansion cycles. Additionally, the performance of vacuum tube receivers degrades over time as gas permeates into the vacuum annulus, but this degradation mechanism would not affect the HEATS receiver. The primary figure of merit for the HEATS receiver is its efficiency: how much solar radiation incident on the receiver is converted to useful energy. Defining efficiency for hybrid systems is not as simple as in purely PV or thermal systems, since the hybrid receiver delivers both electricity and heat. One metric which has been proposed for hybrid systems is exergetic efficiency, which is the exergy collected by the receiver divided by the incident solar radiation, where exergy is calculated as the sum of the electrical output of the receiver and the work potential of the thermal output of the receiver.12Branz H.M. Regan W. Gerst K.J. Borak J.B. Santori E.A. Hybrid solar converters for maximum exergy and inexpensive dispatchable electricity.Energy Environ. Sci. 2015; 8: 3083-3091Google Scholar This definition of efficiency puts a premium on thermal energy, which can never be converted at the efficiency of a Carnot engine in practice (valuing the full work potential implies conversion at Carnot efficiency), as an intentional and quantitative way to assign value to the higher dispatchability of electricity generated from thermal energy. Exergetic efficiency therefore intrinsically increases the value of dispatchable energy by the ratio of the Carnot efficiency to the real efficiency of the heat engine, and yields higher efficiency values than the efficiencies at which hybrid receivers are actually able to convert sunlight to electricity. Due to this shortcoming, in this paper we will use total electrical efficiency ηelec, which is the electricity delivered by the receiver divided by solar radiation incident on the receiver, assuming that the heat from the receiver can be converted to electricity at the endoreversible limit:ηelec=P+Qh(1−TcTh)Qsolar,(Equation 1) where P is electrical power from the PV cell, Qh is heat delivered by the receiver, Th and Tc are the temperature of heat delivered to the heat engine (it is assumed that there is no temperature drop between the receiver and heat engine) and the cold-side reservoir that the heat engine can reject heat to, respectively, and Qsolar is solar radiation incident on the receiver. Here the endoreversible limit is used in place of the Carnot limit since it is much closer to the efficiency of heat engines achieved in practice.40Bejan A. Entropy generation minimization: the new thermodynamics of finite-size devices and finite-time processes.J. Appl. Phys. 1996; 79: 1191Google Scholar The endoreversible limit is a slight overestimate for dispatchable electricity due to losses associated with thermal storage, but very high (≥99%) round-trip storage efficiencies can be achieved, so we will ignore those losses here.41Mehos M. Turchi C. Jorgenson J. Denholm P. Ho C. Armijo K. On the Path to SunShot. Advancing Concentrating Solar Power Technology, Performance, and Dispatchability. National Renewable Energy Laboratory, 2016Google Scholar It should be noted that this total electrical efficiency includes both electricity generated directly via PV and electricity generated using the intermediate thermal step. It should also be made clear that this is a receiver efficiency (as opposed to a full system efficiency), denoting electrical power delivered divided by incident solar radiation on the receiver. Electrical efficiency does not include any information about the fraction of dispatchable electricity (that is, the portion which comes from heat rather than PV), so we will report this fraction explicitly alongside mentions of efficiency. In this case, the dispatchability ratio γ is given byγ=Qh(1−TcTh)P+Qh(1−TcTh),(Equation 2) which corresponds to the portion of electricity from the receiver that comes from thermal energy divided by the total electricity generated. While splitting exergetic efficiency into total electrical efficiency and dispatchability requires more numbers to describe receiver performance, we choose to use these metrics because total electrical efficiency is more easily compared with other systems, and in this approach the added value of dispatchability is not prescribed. From Equations 1 and 2, in order to determine the total electrical efficiency and dispatchability of the HEATS receiver, the fraction of solar radiation incident on the receiver that is converted to electricity by the PV cell as well as how much thermal energy can be collected from the SSLP for a given Th (where thermal energy collected is solar radiation absorbed minus thermal losses) must be determined. Efficiency also depends on Tc, which is assumed to be 37°C for this paper, a representative value for power plant cooling systems.12Branz H.M. Regan W. Gerst K.J. Borak J.B. Santori E.A. Hybrid solar converters for maximum exergy and inexpensive dispatchable electricity.Energy Environ. Sci. 2015; 8: 3083-3091Google Scholar To estimate the performance that could be achieved with the HEATS receiver, it is useful to consult a simple, idealized model. In our simplified model, the PV cell is assumed to have an external radiative efficiency of 1%, edge effects in the receiver are ignored (i.e., transport through the layers is treated as one dimensional), and thermal properties of the receiver do not vary with temperature. For this model, properties of the HEATS receiver subcomponents, such as aerogel transmittance, are prescribed directly and treated as constant within each spectral band. The details of how this idealized model is used to calculate the electrical and thermal output from the HEATS receiver are described in Supplemental Information Section S1. The properties used for the idealized model are summarized in Table 1, and the definitions of these properties and justifications of these values are discussed in Supplemental Information Section S1.Table 1Fixed HEATS Receiver Properties Used in Idealized ModelPropertySymbolValueIncident solar radiationQsolar30 kW/m2Glass transmittanceτglass0.96Effective transmittance of PV band through SSLPηLP,PV0.95Effective absorbance of thermal bands in SSLPηLP,h0.95Transmittance of thermal bands through one aerogel layerτag,h0.97Transmittance of PV band through both aerogel layersτag,PV20.97Effective heat transfer coefficient through aerogel layerhag3 W/m2/KPV cell external radiative efficiencyN/A1%Heat engine cold-side reservoir and ambient temperatureTc310 K Open table in a new tab This model treats the PV properties and the temperature at which thermal energy is collected Th as inputs. Here the PV efficiency is given for a PV cell with an imposed bandgap energy, an external radiative efficiency of 1% (an achievable value for high-quality PV cells), and assuming the PV cell operates at Tc and is exposed to 30× solar concentration.42Miller O.D. Yablonovitch E. Kurtz S.R. Strong internal and external luminescence as solar cells approach the Shockley-Queisser limit.IEEE J. Photovolt. 2012; 2: 303-311Google Scholar, 43Green M.A. Radiative efficiency of state-of-the-art photovoltaic cells.Prog. Photovoltaics Res. Appl. 2012; 20: 472-476Google Scholar To determine the fraction of the solar spectrum which is directed to the PV cell (fPV), the spectral efficiency of the PV cell is compared with the endoreversible efficiency of a heat engine at Th (as in Equation 1), where spectral efficiency is given by44Yu Z.(Jason) Leilaeioun M. Holman Z. Selecting tandem partners for silicon solar cells.Nat. Energy. 2016; 1: 16137Google ScholarηPV,λ(λ)=VOCFF JSC(λ)I(λ),(Equation 3) where VOC is open circuit voltage of the cell, FF is the fill fraction of the cell, JSC(λ) is short-circuit current density per unit wavelength at the given wavelength, and I(λ) is spectral irradiance at the given wavelength. Wavelengths for which spectral efficiency ηPV,λ is greater than the endoreversible heat engine efficiency 1−Tc/Th are directed to the PV cell, while the rest of the solar spectrum is intended to be collected as thermal energy. Thus fPV is chosen to maximize receiver total electrical efficiency, given in Equation 1. For high-performing PV cells, spectral efficiency increases approximately linearly with wavelength, until dropping sharply near the bandgap. This means that wavelengths for which ηPV,λ>1−Tc/Th will be in a single continuous band, as in Figure 1B, and it is feasible to design an interference filter that reflects this single band. Performance of the HEATS receiver is a strong function of operating temperature and PV cell bandgap, and with this simple model many different receiver configurations can be explored. Figure 2 shows receiver total electrical efficiency and dispatchability fraction for varying temperatures and PV cell bandgaps. The optimal bandgap for maximizing efficiency is around 0.7–0.9 μm, as shown in Figure 2. This optimum arises because for large bandgaps only a small portion of the solar spectrum can be