High Concentration Photovoltaic System Research Articles

Numerical Simulation and Experimental Analysis of Electrical Properties of InGaP/InGaAs/Ge Triple-Junction Solar Cell with Reflective High-Concentration Photovoltaic System

Photovoltaic power systems is one of the most important ways of utilizing solar energy, and high-concentration photovoltaic systems have received much attention due to its high conversion efficiency. This study developed a mathematical model of an InGaP/InGaAs/Ge triple-junction solar cell (TJSC) according to a single-diode equivalent circuit. Moreover, a correlation of the series resistance of TJSC considering the effects of temperature and incident energy density was obtained and further verified by experiments. Based on a reflective high-concentration photovoltaic system, the electrical properties, including the short-circuit current, open-circuit voltage, peak power, and efficiency, were investigated. The results showed that the series resistance increased from 0.0095 Ω to 0.0123 Ω with temperature increased from 298 K to 358 K at the incident of 2×105 W /m2, and it increased from 0.0094 Ω to 0.0196 Ω as the incident energy density increased from 2.0×105 W/m2 to 1.0×106 W/m2 at temperature of 298K. In addition, the relationships between other electrical properties and the incident energy density or temperature were investigated. The simulations were remarkably consistent with the experimental results. The experimental values were slightly less than the theoretical values, which was caused mainly by the temperature instead of the incident energy density.

Impact of microchannel heat sink configuration on the performance of high concentrator photovoltaic solar module

Concentrating photovoltaic technology is growing rapidly among other solar energy technologies, as it reaches an electrical conversion efficiency up to 46%. The solar cell temperature increases during the conversion process, which in turn negatively affects the system’s performance by decreasing its efficiency. Thus, for safe and efficient operation, low and uniform temperature should be achieved. In this work, numerical simulations were completed to study and compare two microchannel heat sink designs as cooling devices for a high concentration photovoltaic (HCPV) system. ANSYS Fluent software was employed to solve three-dimensional conjugate heat transfer equations. The comparison was conducted under uniform solar radiation with a concentration ratio of 1000 suns (1 sun = 1000 W/m2) and a wide range of coolant inlet flowrate. The impact of heat sink design and coolant inlet flowrate variation were studied and highlighted regarding average cell temperature, electrical efficiency, temperature non-uniformity, and local temperature distribution. The results showed that heat sink design has a remarkable impact on the cooling performance of the HCPV/Thermal system. Whereas, configuration 1 achieved a lower average cell temperature by 11% than configuration 2, while the electrical efficiency was increased by 1.4%, at the minimum studied flowrate.

Development of high concentration photovoltaics (HCPV) power plants in the US Southwest: Economic assessment and sensitivity analysis

High concentration photovoltaic (HCPV) technologies offer several advantages over typical PV systems. In this study, a detailed economic assessment and sensitivity analysis of deploying HCPV plants in the US southwest is performed. To do this, a 20 MWdc HCPV power plant is designed using the System Advisor Model (SAM) and is evaluated from both technical and financial aspects. The sensitivity analysis shows that reducing the installed cost per capacity of the HCPV system can significantly improve the economics and reduce the levelized cost of electricity (LCOE) and levelized power purchase agreement (LPPA). Also, providing proper incentives including both investment tax credits (ITC) and production tax credit (PTC), loan debt ratio, and interest rate are among the most influential parameters. The analysis shows that with the 30% ITC, the LCOE, and LPPA of the designed HCPV projects are between $65.4/MWh and $69.3/MWh and between $71.0/MWh and $75.2/MWh, respectively. The estimated LCOE and LPPA values are almost 50% higher than the utility-scale PV projects. If proper funding is provided to continue the support of the HCPV system, they would become more economical and competitive with PV plants. Federal and state government policies must also be established to advocate HCPV systems.

Numerical analysis of passive cooled ultra-high concentrator photovoltaic cell using optimal heat spreader design

In the high concentrator photovoltaic (HCPV) systems with solar concentration ratios up to 2000 Suns, significant heat is generated in the used solar cell layer. This high generated heat requires an efficient and smart cooling technique to keep it operating at a safe operating temperature. In this paper, another ultra-high concentrator photovoltaic (UHCPV) system with a smaller cell area of 1 mm2 operating at a high solar concentration ratio (CR) up to 10,000 Suns is proposed. This smaller area requires a simple passive cooling technique even at high CR. The optimal dimensions of a passive cooling method using heat spreader are defined. A 3D thermal model for the multijunction solar cell with the heat spreader coupled with the multi-objective genetic optimization algorithm is used to define the optimal heat spreader dimensions . The model is validated with the results in the literature. The model is used to estimate the cell temperature generated electric power, and cell efficiency at different wind speed, ambient temperature, solar radiation, heat spreader length, thickness, and CR. The heat spreader dimensions were optimized for CR = 6000 suns, the optimal thickness and length were 2 mm and a of 47.5 mm, respectively. These dimensions are enough for the safe operation of the UHCPV at CR of 6000 Suns. As a case study, for a UHCPV module with a total number of cells of 10 by 10, the generated power is around 319 W at CR of 10,000 Suns. At the same condition, the monocrystalline silicon solar cell in the PERSEID SOLAR company can generate a maximum power of 144.9 W/m2. For the same area, for the UHCPV module, the generated electric power is around 319 W for 1 m2 of the module. Therefore, around 120% increase in the power can be accomplished with the use of the UHCPV module. In the UHCPV module, the total area of the cell is around 1 cm by 1 cm. Therefore, the module cost could be very low.

Optical modeling and Performance Design Of A Fresnel Lens For CPV Units

In CPV technology, a focus on keeping low costs has led most recent designs to use concentration ratios of 1000X or higher. For that, many high-concentration photovoltaic systems use refractive Fresnel lenses because it is one of the main transmission-type concentrators. In this work, a solar concentrator system composed from a typical Fresnel lens made of PMMA, with an effective diameter of 350mm and a squared solar cell of 10x10mm is proposed. The design of a Fresnel lens is introduced; we studied and analysed a squared design configurations with multiple tuning of focal lengths, facet spacing and an extended range of wavelengths. The results show that the Fresnel lens is performant in the visible spectrum with facet spacing of 1mm and the focal length between 265 to 350mm. The acceptance angle of the whole system was between 0.6 to 0.8° and the optical efficiency obtained by simulations was between 80 to 90%.

Development of two-phase flow microchannel heat sink applied to solar-tracking high-concentration photovoltaic thermal hybrid system

To enhance the energy conversion efficiency of a concentration photovoltaic (CPV) system, a high-concentration photovoltaic thermal (HCPVT) hybrid system is proposed in this study. This system extends the functionality of the CPV from simply generating electricity to simultaneously providing electricity and heat. Thus, the utilization of the exhaust heat, which would otherwise be lost to the environment, could enhance the overall system efficiency and boost the economic value of the generated power output. However, in view of the significant influence of the operating temperature on the photovoltaic efficiency and longevity of the solar cells, an efficient cooling of the photovoltaic module during the solar-tracking process has become a great challenge to the development of HCPVT hybrid systems. In the present work, a radially expanding microchannel heat sink (REMHS) employing two-phase flow boiling is proposed to achieve the twin objectives of cooling the solar cells and generating steam from the recovered heat. The radially expanding microchannels are designed to facilitate a spontaneous vapor removal while alleviating flow maldistribution arising due to the changes in orientation of the device. By conducting flow boiling tests of deionized water, the heat transfer characteristics of the proposed REMHS under various orientation angles were investigated. The local heat transfer coefficient of the REMHS increased by 32%, as the orientation angle of the microchannels increased from 0° to 45°, and then remained steady with any further increase in the orientation angle. In addition, the heat transfer capability of the REMHS exhibited a strong dependence on the heat flux and was weakly correlated with the flow rate, which is typical of an evaporation liquid film heat transfer. Additionally, by conducting outdoor real-time sun tracking tests under a concentration ratio of 1070 suns, it was found that the proposed REMHS maintained a superior flow boiling performance under a high-concentration solar energy. The highest surface temperature on the cell surface remained below 110 °C and the observed maximum temperature difference was below 5 °C.

Theoretical Investigation of the Temperature Limits of an Actively Cooled High Concentration Photovoltaic System

Concentrator photovoltaics have several advantages over flat plate systems. However, the increase in solar concentration usually leads to an increase in the solar cell temperature, which decreases the performance of the system. Therefore, in this paper, we investigate the performance and temperature limits of a high concentration photovoltaic Thermal system (HCPVT) based on a 1 cm2 multi-junction solar cell subjected to a concentration ratio from 500× to 2000× by using three different types of cooling fluids (water, ethylene glycol and water mixture (60:40), and syltherm oil 800). The results show that, for this configuration, the maximum volumetric temperature of the solar cell did not exceed the manufacturer’s recommended limit for the tested fluids. At 2000× the lowest solar cell temperature obtained by using water was 93.5 °C, while it reached as high as 109 °C by using syltherm oil 800, which is almost equal to the maximum operating limit provided by the manufacturer (110 °C). Overall, the best performance in terms of temperature distribution, thermal, and electrical efficiency was achieved by using water, while the highest outlet temperature was obtained by using syltherm oil 800.

Knowledge-Based Sensors for Controlling A High-Concentration Photovoltaic Tracker.

To reduce the cost of generated electrical energy, high-concentration photovoltaic systems have been proposed to reduce the amount of semiconductor material needed by concentrating sunlight using lenses and mirrors. Due to the concentration of energy, the use of tracker or pointing systems is necessary in order to obtain the desired amount of electrical energy. However, a high degree of inaccuracy and imprecision is observed in the real installation of concentration photovoltaic systems. The main objective of this work is to design a knowledge-based controller for a high-concentration photovoltaic system (HCPV) tracker. The methodology proposed consists of using fuzzy rule-based systems (FRBS) and to implement the controller in a real system by means of Internet of Things (IoT) technologies. FRBS have demonstrated correct adaptation to problems having a high degree of inaccuracy and uncertainty, and IoT technology allows use of constrained resource devices, cloud computer architecture, and a platform to store and monitor the data obtained. As a result, two knowledge-based controllers are presented in this paper: the first based on a pointing device and the second based on the measure of the electrical current generated, which showed the best performance in the experiments carried out. New factors that increase imprecision and uncertainty in HCPV solar tracker installations are presented in the experiments carried out in the real installation.

Thermal management of high concentrator solar cell using new designs of stepwise varying width microchannel cooling scheme

High concentrator photovoltaic (HCPV) system are generally exposed to high solar concentration ratios and reach high temperatures. An advanced cooling technique is compulsory to attain the highest net output power along with the safe operation of the system components. Different designs of stepwise varying width microchannel heat sink are investigated in this study. The main purpose of this study is to investigate the influence of the channel geometry of longitudinal rectangular internal fins and different water inlet mass flow rates on the performance of an HCPV system. A three-dimensional thermal model is developed and used to compare the performance of four different designs of stepwise varying width microchannel heat sinks. These designs are compared with the conventional multichannel heat sink design. The results show that the heat sink design and the coolant mass flow rate have a significant impact on the cell temperature, electrical cell efficiency, system thermal efficiency, electrical exergy efficiency, thermal exergy efficiency, total exergy efficiency and thermal resistance of the heat sinks. For instance, using one of the proposed stepwise varying width microchannel heat sink at solar concentration ratio of 1000 suns and increasing the coolant flowrate from 25 to 1000 g/min decreased the solar cell temperature from around 71.7° C to 40° C with solar cell temperature non-uniformity decreased from 15.5 °C to 9 °C respectively.

Numerical study on the uniformity of reflective high concentration photovoltaic system with two-stage reflective concentrator

In a reflective high concentration photovoltaic (RHCPV) system, density-uniformity of solar energy flux irradiates upon receiver surface can directly affect solar cell’s photoelectric conversion efficiency. Uneven high intensity irradiation will also introduce “hot spots” which cause damages to GaAs solar cells. This paper presents a two-stage reflective mirror-type concentrator to achieve the uniformity of energy flux on receiver. A computation software, TRACEPRO, is used to model and numerically simulate the two-stage reflective mirror-type concentrator. Simulation results show that a flux uniformity of greater than 99.5% and effective area ratio for the photoelectric conversion of 77.0% can be achieved. The impacts of theoretical concentration ratio to the effective area for photoelectric conversion, maximum irradiance, average irradiance, and focal length are evaluated. At constant concentration ratio, the effective area for photoelectric conversion, the maximum irradiance, and the average irradiance are not affected by the focal length of PR. When the ratio of flux (light beams collected by the receiver) and emitted flux (incident light beams) is constant, the relationships of the maximum and average irradiance and the theoretical concentration ratio are linear, and the effective area for photoelectric conversion is also not affected by the theoretical concentration ratio. Meanwhile, a microchannel radiator is designed and studied, and GaAs solar cell surface temperature decrease with the increase of flow rate in microchannel.

Environmental Impact of the High Concentrator Photovoltaic Thermal 2000x System

High Concentrator Photovoltaic Thermal (HCPV/T) systems produce both electrical and thermal energy and they are efficient in areas with high Direct Normal Irradiance (DNI). This paper estimates the lifecycle environmental impact of the HCPV/T 2000x system for both electrical and thermal functionalities. Process-based attributional method following the guidelines and framework of ISO 14044/40 was used to conduct the Life Cycle Assessment (LCA). The midpoint and endpoint impact categories were studied. It was found that the main hotspots are the production of the thermal energy system contributing with 50% and 55%, respectively, followed by the production of the tracking system with 29% and 32% and the operation and maintenance with 13% and 7%. The main contributor to the lifecycle environmental impact category indicators was found to be the raw materials acquisition/production and manufacturing of the thermal energy and tracking systems. The results indicate that the lifecycle environmental impact of the HCPV/T 2000x system is lower compared to fuel-based Combined Heat and Power (CHP) and non-Renewable Energy Sources (non-RES) systems.

Numerical and experimental research of the characteristics of concentration solar cells

The development of automatic tracking solar concentrator photovoltaic systems is currently attracting growing interest. High concentration photovoltaic systems (HCPVs) combining triple-junction InGaP/lnGaAs/Ge solar cells with a concentrator provide high conversion efficiencies. The mathematical model for triple-junction solar cells, having a higher efficiency and superior temperature characteristics, was established based on the one-diode equivalent circuit cell model. A paraboloidal concentrator with a secondary optic system and a concentration ratio in the range of 100X–150X along with a sun tracking system was developed in this study. The GaInP/GalnAs/Ge triple-junction solar cell, produced by AZUR SPACE Solar Power, was also used in this study. The solar cells produced by Shanghai Solar Youth Energy (SY) and Shenzhen Yinshengsheng Technology Co. Ltd. (YXS) were used as comparison samples in a further comparative study at different concentration ratios (200X–1000X). A detailed analysis on the factors that influence the electrical output characteristics of the InGaP/lnGaAs/Ge solar cell was conducted with a dish-style concentrating photovoltaic system. The results show that the short-circuit current (Isc) and the open-circuit voltage (Voc) of multijunction solar cells increases with the increasing concentration ratio, while the cell efficiency (ηc) of the solar cells increases first and then decreases with increasing concentration ratio. With increasing solar cell temperature, Isc increases, while Voc and ηc decrease. A comparison of the experimental and simulation results indicate that the maximum root mean square error is less than 10%, which provides a certain theoretical basis for the study of the characteristics of triple-junction solar cell that can be applied in the analysis and discussion regarding the influence of the relevant parameters on the performance of high concentration photovoltaic systems.

Life Cycle Assessment of New High Concentration Photovoltaic (HCPV) Modules and Multi-Junction Cells

Worldwide electricity consumption increases by 2.6% each year. Greenhouse gas emissions due to electricity production raise by 2.1% per year on average. The development of efficient low-carbon-footprint renewable energy systems is urgently needed. CPVMatch investigates the feasibility of mirror or lens-based High Concentration Photovoltaic (HCPV) systems. Thanks to innovative four junction solar cells, new glass coatings, Position Sensitive Detectors (PSD), and DC/DC converters, it is possible to reach concentration levels higher than 800× and a module efficiency between 36.7% and 41.6%. From a circular economy’s standpoint, the use of concentration technologies lowers the need in active material, increases recyclability, and reduces the risk of material contamination. By using the Life Cycle Assessment method, it is demonstrated that HCPV presents a carbon footprint ranking between 16.4 and 18.4 g CO2-eq/kWh. A comparison with other energy means for 16 impact categories including primary energy demand and particle emissions points out that the environmental footprint of HCPV is typically 50 to 100 times lower than fossil fuels footprint. HCPV’s footprint is also three times lower than that of crystalline photovoltaic solutions and is close to the environmental performance of wind power and hydropower.

High Concentration Photovoltaics (HCPV) with Diffractive Secondary Optical Elements

Multi-junction solar cells can be economically viable for terrestrial applications when operated under concentrated illuminations. The optimal design of concentrator optics in high concentration photovoltaics (HCPV) systems is crucial for achieving high energy conversion. At a high geometric concentration, chromatic aberration of the primary lens can restrict the optical efficiency and acceptance angle. In order to correct chromatic aberration, multi-material, multi-element refractive elements, hybrid refractive/diffractive elements, or multi-element refractive and diffractive systems can be designed. In this paper, the effect of introducing a diffractive surface in the optical path is analyzed. An example two-stage refractive and diffractive optical system is shown to have an optical efficiency of up to 0.87, and an acceptance angle of up to ±0.55° with a 1600× geometric concentration ratio, which is a significant improvement compared to a single-stage concentrator system with a single material. This optical design can be mass-produced with conventional fabrication methods, thus providing a low-cost alternative to other approaches, and the design approach can be generalized to many other solar concentrator systems with different cell sizes and geometric concentration ratios.

Comparison of operational performance and analytical model of high concentrator photovoltaic thermal system at 2000 concentration ratio

This paper presents the development of a model based on efficiency equations to evaluate the performance of an HCPV/T system and compares its outputs with data of an operational case-study system installed in Palermo, Italy. The model is validated with data of the operational system to show real performance. The model can evaluate (a) the electric efficiency of the InGaP/InGaAs/Ge TJ solar cell and (b) electrical and thermal power/energy production potential of one module. The model predictions are compared with experimental electric and thermal data by obtaining linear regression plots of experimental results vs. analytical results; the R2 for experimental electrical and thermal results are 0.91 and 0.87 respectively. Using the model, the evaluated average daily analytical and experimental InGaP/InGaAs/Ge TJ solar cell efficiencies are 33 % and 25 % respectively; with a maximum daily experimental value of 30 %. It was found that the annual analytical and potential (based on derived equations from experimental data) electric energy produced by one module are 158 kWh/m2/year and 144 kWh/m2/year respectively, while the annual analytical and potential thermal energy are 375 kWh/m2/year and 390 kWh/m2/year respectively.

On the Thermal Performance of a Microparallel Channels Heat Exchanger

Abstract This paper presents the experimental and theoretical analysis of a micro heat exchanger designed for the waste heat recovery from a high concentration photovoltaic (HCPV) system. A test bench was built to analyze the thermal behavior of a heat exchanger targeted to work in a similar condition of an existing HCPV panel. A high power heater was encapsulated inside a copper cartridge, covered by thermal insulation, leading to dissipated heat fluxes around 0.6 MW/m2, representative of the heat flux over the solar cell within the HCPV module. The experimental campaign employed water as the coolant fluid and was performed for three different mass flow rates. An infrared camera was used to nonintrusively measure the temperature field over the micro heat exchanger external surface, while thermocouples were placed at the contact between the heat exchanger and the heater, and at the water inlet and outlet ports. In the theoretical analysis, a hybrid numerical–analytical treatment is implemented, combining the numerical simulation through the comsolmultiphysics finite elements code for the micro heat exchanger, and the analytical solution of a lumped-differential formulation for the electrical heater cartridge, offering a substantial computational cost reduction. Such computational simulations of the three-dimensional conjugated heat transfer problem were critically compared to the experimental results and also permitted to inspect the adequacy of a theoretical correlation based on a simplified prescribed heat flux model without conjugation effects. It has been concluded that the conjugated heat transfer problem modeling should be adopted in future design and optimization tasks. The analysis demonstrates the enhanced heat transfer achieved by the microthermal system and confirms the potential in reusing the recovered heat from HCPV systems in a secondary process.

Theoretical and experimental study on the uniformity of reflective high concentration photovoltaic system with light funnel

Abstract The efficiency of GaAs solar cell was significantly reduced resulting from non-uniformity of energy flux-density on receiver surface, and damage of solar cells was made by “hot spots”. In this paper, a light funnel is added between the concentrator and the receiver to improve the uniformity of energy flux-density on receiver surface, which is designed according to the ray tracing method. The simulation result shows that the uniformity of the energy flux-density on the receiver surface is 92.26%, which agrees with the experimental result (94.64%). According to the maximum output efficiency of GaAs solar cell, the layout range of the cell is determined. The simulation results shown that the cell is arranged in the radius of 58 mm, which is similar to the experimental value (60 mm), and the effective area rate of the photoelectric conversion is 69.4%. At normal operation temperature (20 °C), the photoelectric conversion efficiency is 29.64%, and the total efficiency is about 83%.

Optimum sizing of the inverter for maximizing the energy yield in state-of-the-art high-concentrator photovoltaic systems

The sizing of the inverter in comparison to the rated capacity of the photovoltaic generator is investigated for high-concentrator photovoltaic (HCPV) systems. An HCPV module of typical characteristics is modelled and parameterized, taking into account direct normal irradiance (DNI), ambient temperature, air mass and aerosol optical depth as atmospheric inputs, while the DC losses of the HCPV generator are allowed to vary in the ranges reported in the literature. A set of 80 commercial inverters are analysed to obtain the typical efficiency curves of state-of-the-art low-, medium-, and high-efficiency inverters. Four locations worldwide with high annual DNI levels and different average values of the weather variables influencing HCPV performance are studied. Results show that the inverter can be sized between 84% and 112% of the rated capacity of the HCPV generator at Concentrator Standard Test Conditions depending on the scenario considered for maximizing the final energy yield of the system. The proposed methodology uses analytical equations, all the model parameters are provided and justified and atmospheric inputs are obtained from meteorological databases in order to make the application easy regarding its use in other locations where the climate data is available.

Enhancement of the cooling capability of a high concentration photovoltaic system using microchannels with forward triangular ribs on sidewalls

Numerical simulations were performed to investigate a microchannel heat sink device as cooling option for a high concentration photovoltaic system. COMSOL Multiphysics 5.1 software is used to solve three-dimensional equations which consider conjugate heat transfer, viscous dissipations, and temperature-dependent-properties. This study investigates the integration of microchannels with complex geometric features on its inner walls into the solar cell structure, to enhance the heat transfer performance of a microchannel heat sink-based active cooling system. Inner sidewall mounted forward triangular ribs are considered in aligned and offset distributions along the microchannel walls. In addition, numerical analysis is developed for a conventional flat plate heat sink integrated to a high concentration photovoltaic system to stablish a baseline solar cell temperature. The numerical results show that a micro-channel heat sink device can control and keep in very low range the solar cell temperature (<301 K). Compared to a smooth microchannel, forward triangular ribs installed on the sidewalls enhance the heat transfer capability. Microchannels with aligned and offset rib distributions increase the Nusselt number between 1.8 and 1.6 times, respectively, and increase the average friction factors between 3.9 and 2.3 times, respectively. The microchannel heat sink device with forward triangular ribs is more efficient and effective at Re ≤ 200, since the pumping power reaches a high percentage of the total power generated by solar cell when Re > 200. At Re = 400, the pumping power reaches 41% and 23% of the total power generated by a multi-junction solar cell in the aligned and offset rib distribution, respectively. The pumping power is greatly reduced while using smooth microchannel, because the maximum pumping power is only 9.5% of the solar cell power at Re = 400, however, the resulting solar cell temperature is slightly higher compared to microchannels with aligned and offset rib configurations. A microchannel heat sink provides a more effective cooling solution compared to a passive flat plate heat sink for a high concentration photovoltaic system. In addition, the possibility of direct integration of a microchannel heat sink into a solar cell structure as proposed in this study, represents an interesting option to feasibly increase thermal performance to a considerable level by maintaining the solar cell temperature in a very low range.

A Hybrid Electric and Thermal Solar Receiver

Summary 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.