THERMOPHOTOVOLTAIC GENERATION OF ELECTRICITY: Sixth Conference on Thermophotovoltaic Generation of Electricity: TPV6
738(2004); http://dx.doi.org/10.1063/1.1841874View Description Hide Description
Following a short discussion on the origin of Thermophotovoltaic (TPV), this presentation offers a retrospective of the progress and results of the recurrent efforts in TPV conducted in the United States by the Military during the last 40 years. The US Army’s interest in TPV, for the development of portable power sources, started a few years after the energy conversion approach was conceived. TPV technology was seen to offer a solution for the Army’s need for power in the 10 to 1500 Watt range. The technology offered the means to overcome the limitation of size and weight found in existing commercial power sources, with the additional advantage of silent and multifuel operation. Hence, the Army invested research and development (R&D) funding to investigate TPV feasibility for tactical field application. After an initial decade of continuous research studies by the Army, the support for this technology has experienced cycles of significant efforts interrupted by temporary waiting periods to allow this technology to further mature. Over the last four decades, several TPV proof of concept systems were developed. The results of their testing and evaluation have demonstrated the feasibility of the technology for development of power sources with output of several watts to a few hundreds watts. To date, the results have not been found to adequately demonstrate the applicability of TPV to the development of military power generators with output above 500 watts. TPV power sources have not been developed yet for Army field use or troop testing. The development risk is still considered to be moderate‐to‐high since practical‐size systems that go beyond the laboratory test units have not been designed, constructed, tested. The greatest need is for system development, along with concurrent continued component development and improvement. The Defense Advanced Research Project Agency (DARPA) support for TPV R&D effort has been drastically reduced. The Army is still pursuing a 500watt TPV unit demonstrator. No further collaboration among DARPA, Army, NASA is contemplated, which seems indicative of the beginning of a new period of waiting for additional maturing of this technology. The Army’s assessment about the viability of TPV for integrated systems indicates that the technology will require a few more years of development. However, at this time, for the completion of component and system development, a strong effort is needed in the private sector. The achievement of the necessary ruggedness for some critical components, acceptable overall efficiency, and system thermal management, is essential for a new, strong restart of TPV effort by the Military.
738(2004); http://dx.doi.org/10.1063/1.1841875View Description Hide Description
The thermophotovoltaic (TPV) research activity in Japan has prospered from the second half of the 90s. In this paper, we will present an overview of TPV research activities in Japan. TPV technologies have been surveyed by research committees in NEDO as a part of the research activity of the New Sunshine Project. The TPV is considered as a new application of non‐conventional solar cells, and the situation of TPV technologies, especially TPV cells, in USA and EU is surveyed. Systematic investigative research on TPV systems was performed by ENAA on FY1997 and 1998. In this investigative research on potential market for a TPV power source in Japan has been focused on how TPV can contribute to energy conservation and environmental protection and harmony, compared with conventional engine or turbine generators and underdeveloped power generation technologies such as fuel cells or chemical batteries, etc. In addition to the investigative research, the technical research activities are introduced in this paper.
738(2004); http://dx.doi.org/10.1063/1.1841876View Description Hide Description
Thermoelectric energy conversion is a well known and established technology, especially if compared with thermophotovoltaic generators (TPVGs). Key properties of thermoelectric generators (TEGs), typical thermoelectric materials, common and new TEG applications will be described and related to TPVG properties.
738(2004); http://dx.doi.org/10.1063/1.1841877View Description Hide Description
An overview on the development of thermophotovoltaic (TPV) systems heated with concentrated sunlight (STPV) and with a combustion flame (CTPV) is given. Only a few experimental works on STPV are reported. Lifetime investigations of TPV emitters in solar dish concentrators were carried out. A complete STPV system reported in the literature achieved a system efficiency below 0.1 %. In contrast to these experiments we present simulations, which show that an optimised STPV system with Yb2O3 emitter and high efficient Si solar cells is able to achieve a system efficiency in the order of 30 %. Quite more experimental results are reported for CTPV systems, but a series production and commercialisation of CTPV was not achieved, so far. An application for a CTPV system is a portable electrical power supply. The highest system efficiency reported so far is 6 %. Whether this efficiency is sufficient to successfully compete with generators driven by a gas engine or a diesel motor, remains unclear. Another application for CTPV that was reported is a gas fired stove, which produces electricity in addition to heat. The application of a CTPV system for electrically autonomous domestic central heating systems could probably result in a first commercialisation of TPV. For this application, a system efficiency in the order of 1 % is sufficient. A gas fired TPV system is presented, which uses a novel foam ceramic emitter made from Yb2O3 and commercially available Si solar cells. The proof‐of‐concept of this prototype CTPV system could successfully be furnished. Possible applications for this CTPV system are: electrically autonomous domestic heating systems, parking heating systems for vehicles, heaters for caravans and boats or large industrial burner systems.
738(2004); http://dx.doi.org/10.1063/1.1841878View Description Hide Description
This paper discusses advances made in the field of Micron‐gap ThermoPhotoVoltaics (MTPV). Initial modeling has shown that MTPV may enable significant performance improvements relative to conventional far field TPV. These performance improvements include up to a 10× increase in power density, 30% to 35% fractional increase in conversion efficiency, or alternatively, reduced radiator temperature requirements to as low as 550°C. Recent experimental efforts aimed at supporting these predictions have successfully demonstrated that early current and voltage enhancements could be done repeatedly and at higher temperatures. More importantly, these efforts indicated that no unknown energy transfer process occurs reducing the potential utility of MTPV. Progress has been made by running tests with at least one of the following characteristics relative to the MTPV results reported in 2001:
• Tests at over twice the temperature (900°C).
• Tests at 50% smaller gaps (0.12 μm)
• Tests with emitter areas from 4 to 100 times larger (16 mm2 to 4 cm2).
• Tests with over 20× reduction in parasitic spacer heat flow.
Remaining fundamental challenges to realizing these improvements relative to the recent breakthroughs in conventional far field TPV include reengineering the photovoltaic (PV) diode, filter, and emitter system for MTPV and engineering devices and systems that can achieve submicron vacuum gaps between surfaces with large temperature differences.
738(2004); http://dx.doi.org/10.1063/1.1841879View Description Hide Description
Over the last several years, JX Crystals has invented and systematically developed the key components for thermophotovoltaic systems. These key components include GaSb infrared sensitive cells, high power density shingle circuits, dielectric filters, and hydrocarbon‐fueled radiant tube burners. Most recently, we invented and demonstrated an antireflection (AR)‐coated tungsten IR emitter which when integrated with the other key components should make TPV systems with efficiencies over 10% practical. However, the use of the AR tungsten emitter requires an oxygen‐free hermetic seal enclosure. During a 2003 Small Business Innovative Research (SBIR) Phase I contract, we integrated a tungsten emitter foil and a commercial SiC radiant tube burner within an emitter thermos and successfully demonstrated its operation at high temperature. We also designed a complete stand alone 500 W TPV generator. During the upcoming SBIR Phase II, we plan to implement this design in hardware.
738(2004); http://dx.doi.org/10.1063/1.1841880View Description Hide Description
TPV technology has advanced rapidly in the last five years, with diode conversion efficiency approaching >30%, and filter efficiency of ∼80%. These achievements have enabled repeatable testing of 20% efficient small systems, demonstrating the potential of TPV energy conversion. Near term technology gains support a 25% efficient technology demonstration in the two year timeframe. However, testing of full size systems, which includes efficiency degradation mechanisms, such as: non‐uniform diode illumination, diode and filter variability, temperature non‐uniformities, conduction/convection losses, and lifetime reliability processes needs to be performed. A preliminary analysis of these differential effects has been completed, and indicates a near term integrated system efficiency of ∼15% is possible using current technology, with long term growth to 18–20%. This report addresses the system performance issues.
738(2004); http://dx.doi.org/10.1063/1.1841881View Description Hide Description
A TPV generator has been designed, constructed, and operated at Fraunhofer ISE. The work comprises the development of selective emitters, the GaSb TPV cell modules, a micro burner and the over‐all system. In this paper the system design details are presented and first experimental results are given. The measured over‐all system efficiency (electric energy output/total chemical energy input) is 1.7%. This number was achieved with an unstructured tungsten emitter. It is in good agreement with a model that was developed independently and applied to the system geometry. The model predicts system efficiency to increase significantly by using a micro structured tungsten emitter.
738(2004); http://dx.doi.org/10.1063/1.1841882View Description Hide Description
Developed solar TPV system consists of sunlight tracker, sunlight concentrator, absorber of concentrated sunlight, selective emitter of radiation, internal reflectors of radiation from the emitter, and PV cells cooled by water or forced air. The concentration ratio exceeding 8000 suns is ensured by the developed 300W dish mirror with secondary compound parabolic concentrator. The emitter is made of tungsten evacuated in a vacuum bulb. To decrease the losses of the photons emitted back to outside of TPV system, the area of the emitter surface exceeds up to 10 times the absorber aperture area. The developed PV cells based on Ge and GaSb have a back‐surface mirror, which reflects the sub‐bandgap photons to the emitter increasing its temperature and overall system efficiency.
738(2004); http://dx.doi.org/10.1063/1.1841883View Description Hide Description
Electric power was obtained using the super‐adiabatic combustion TPV power generation system. In the system, a porous emitter made of alumina (Ceramic Foam) is installed at the middle. At both sides of the emitter, porous quartz glass plates with many pores are set up. A mixture of air and methane is introduced into the porous media, and flows uniformly across the cross section, where the flow direction changes regularly. Combustion occurs around the surface of the porous emitter. Through energy recirculation, the temperature of the emitter reaches about 1500K under the condition of the equivalence ratio of 0.2. Furthermore, the long‐wavelength components of the radiation emitted from the porous emitter are partially absorbed by the porous quartz glass, then, the short wavelength components are introduced into the TPV cell. Though the view factor was very small and there was multiple‐surface reflection by the quartz glass, we obtained the output power, which was small, using the super‐adiabatic combustion TPV system.
738(2004); http://dx.doi.org/10.1063/1.1841884View Description Hide Description
Solar thermophotovoltaic efficiency is theoretically estimated using the following optimisation parameters: sunlight concentration ratio, absorber/emitter temperature/efficiency, photon recirculation efficiency and TPV cell parameters. It has been found that emitter temperature exceeding 2000 K, absorber/emitter efficiency of 90% and TPV systems efficiency exceeding 30% can be obtained at sunlight concentration ratio exceeding 8⋅103 suns with using GaSb cells with back surface reflector and grey‐body emitter in vacuum. Utilization of the selective emitter allows to increase the efficiency: calculated efficiency of TPV system with tungsten emitter increases from 30% to 36%.
738(2004); http://dx.doi.org/10.1063/1.1841885View Description Hide Description
A cascaded radiant burner has been developed and based on this burner, a novel integrated TPV system has been built. In this system, low bandgap GaSb cells and silicon concentrator solar cells are employed integratedly. The unique cascaded radiant burner consists of two different radiators which cascade‐emit two streams of radiation with different spectra. The two different radiators are arranged in tandem, with their surface temperatures being different as well. Two streams of radiation output are matched, respectively, to the bandgaps of silicon cells and GaSb cells. Thus, one stream of radiation output illuminates silicon concentrator solar cells while the other drives low bandgap GaSb cells in the integrated system. In this work, the combustion performance of the cascaded radiant burner was investigated at varying degrees of exhaust heat recuperation. The electrical output characteristics of both silicon concentrator solar cells and GaSb cells in the gas combustion‐driven TPV system were measured under various operating conditions. It is shown that this innovative design considerably increases the TPV system efficiency, due to the cascaded utilization of heat released during natural gas combustion and the optimized thermal management.
Animation Tool and Q.E. Measurements for Estimating the Optimum System Efficiency of a TPV Generator738(2004); http://dx.doi.org/10.1063/1.1841886View Description Hide Description
In order to achieve high efficiency in a thermophotovoltaic (TPV) generator, it is important that a high fraction of emitted photons with energies below the TPV cell band gap are reflected back to the emitter. This can be accomplished in several ways, and one possibility is to place an edge filter between the emitter and the TPV cell array in an elliptical optic design. By this arrangement the cooling demand on the TPV cells is reduced and the reflected radiation is reused in the emitter, the fuel demand is decreased. An animation tool, developed in the Excel® program, for determining the optical availability in such a system has been presented previously. The animation components were a blackbody emitter, an adjustable edge filter, and an array of TPV cells. The tool was used to demonstrate the importance of an efficient filter and the usefulness of optics that makes the edge of the filter as sharp as possible.
In such a system the TPV cells are the dominant cost. It is possible to tailor the selective filter in order to fit the bandgap of a certain TPV cell. For this purpose it is a great advantage to be able to measure the Quantum Efficiency Q.E. of the cell and how Q.E. changes at different temperatures. When this behaviour is known it is possible to tailor the selective edge filter in order to get higher efficiency. A simple measurement setup has been developed utilizing a lamp with IR reflector, a monochromator, a radiometer, and a heating system based on hot water.
The animation tool has been further developed for estimating the optical efficiency, and more factors may be varied. The tool can now handle also the emittance of the emitter, absorption losses in mirrors and filter, as well as TPV cell characteristics such as adjustable Q.E., fill factor FF, and Uoc/(Eg/q).
738(2004); http://dx.doi.org/10.1063/1.1841887View Description Hide Description
The development of lightweight, efficient power for emerging NASA missions and recent advances in thermophotovoltaic (TPV) conversion technology have renewed interest in the possibility of combining radioisotope heat sources with photovoltaic energy conversion. Thermophotovoltaic power conversion uses advanced materials able to utilize a broader, spectrally tuned range of wavelengths. Spectral control, including the combination of emitter, TPV module, and filter, is key to high‐efficiency operation. This paper summarizes the performance characteristics of monolithic integrated module (MIM) PV cells and arrays, tandem filters, and tungsten emitters fabricated for the present studies. The current, voltage, quantum efficiency, and diode efficiency of multi‐junction 0.60 eV bandgap devices are presented for individual PV cells and strings of several cells. This paper discusses the design considerations for mechanical layout of PV cell arrays and integration with filters. The vacuum facility to be used to test these PV cell arrays is also described.
738(2004); http://dx.doi.org/10.1063/1.1841888View Description Hide Description
Stand‐alone PV applications that supply a constant load can benefit from a small reliable back‐up generator. It allows to reduce the size of the PV array and the battery significantly with only a very small contribution from the back‐up generator in the range of 5 to 10% of the total energy demand. In addition, a significant reduction of the investment cost and improvements of operational safety of remote PV applications can be achieved. In the power range from some W to some kW, a TPV generator can be competitive to other established electric generator technologies. TPV offers a compact, reliable, quiet and safe technology with the potential for low cost and versatile fuel usage, including bio fuels.
Starting in 1994, a TPV‐system has been developed for grid independent operation of gas heating systems. With improving efficiency, the focus was shifted towards a CHP development based on natural gas for households. The realised system concept can theoretically achieve 7% efficiency based on a Kanthal emitter operating at 1300°C and GaSb cells. In the framework of the research and training network TPVCell the system will be used to realise a TPV generator with a minimum efficiency of 2%. In the next step it is planned to improve the existing recuperative burner concept by software based design methods and to realise a new prototype. For the long term, the overall system efficiency target is 10%.
In 1st part, the paper will briefly explain the system concept and show the achieved results. In the 2nd part, the authors will present simulation results for the application of such a TPV system in stand‐alone photovoltaic systems.
PowerSphere: A Novel Photovoltaic Cavity Converter Using Low Bandgap TPV Cells for Efficient Conversion of High Power Laser Beams to Electricity738(2004); http://dx.doi.org/10.1063/1.1841889View Description Hide Description
PowerSphere is a variant of the recently developed Photovoltaic Cavity Converter (PVCC) for High Concentration Photovoltaic applications. Both systems share the benefit of photon recycling in a cavity and have therefore the potential to convert high‐density radiant power to electricity at unprecedented efficiencies. Unlike PVCC that is optimized for the full content of the solar spectrum, the PowerSphere is designed to convert the monochromatic beam of a laser efficiently to electricity. The PowerSphere concept introduced here greatly benefits from recent advances in the area of high power, near‐IR lasers and advanced bandgap engineering involving ‘tunable’ III–V cells operating in the same range of the spectrum. Given this new scenario the spectral response of such cells can be perfectly tuned to the frequency of a high power laser to achieve conversion efficiencies in excess of 60%. Other key phenomena that allow achieving such high conversion rates are described in the main text of this paper. PowerSphere concept when fully developed can greatly contribute to the advancement of Laser Power Beaming technology for terrestrial, near‐space and space applications. The paper explores the performance potential of Laser Power Beaming (LPB) systems using a PowerSphere as a receiver and discusses some of the critical issues that require further studies in this promising area of ‘Wireless Power Transmission’.
738(2004); http://dx.doi.org/10.1063/1.1841890View Description Hide Description
Understanding and optimisation of heat transfer, and in particular radiative heat transfer in terms of spectral, angular and spatial radiation distributions is important to achieve high system efficiencies and high electrical power densities for thermophtovoltaics (TPV). This work reviews heat transfer models and uses the Discrete Ordinates method. Firstly one‐dimensional heat transfer in fused silica (quartz glass) shields was examined for the common arrangement, radiator‐air‐glass‐air‐PV cell. It has been concluded that an alternative arrangement radiator‐glass‐air‐PV cell with increased thickness of fused silica should have advantages in terms of improved transmission of convertible radiation and enhanced suppression of non‐convertible radiation.
738(2004); http://dx.doi.org/10.1063/1.1841891View Description Hide Description
A Monte Carlo Method (MCM) based model has been developed to estimate the radiative energy transfer rate in the optical cavity comprising a TPV power system. The MCM model is augmented by back‐end calculations for the conversion of the photon flux to electrical power based on the TPV cell characteristics. The model has been developed using Mathematica™ and allows for semi‐transparent media, as well as non‐gray spectral and directional characteristics of surfaces in the optical cavity to be included. The overall model was used to corroborate the performance of a real TPV power system that was built at Fraunhofer ISE, and to conduct a preliminary parametric study of the role of emitter type and temperature.
738(2004); http://dx.doi.org/10.1063/1.1841892View Description Hide Description
Spectral control is a key technology for Thermophotovoltaic (TPV) direct energy conversion systems because only a fraction (typically less than 30%) of the incident thermal radiation has energy exceeding the TPV cell band gap energy, Eg, and can thus be converted to electricity. The goal for TPV spectral control in most applications is twofold: (1) Maximize TPV efficiency by minimizing transfer of low energy, below band gap photons from the radiator to the TPV cell, (2) Maximize TPV surface power density by maximizing transfer of high energy, above band gap photons from the radiator to the TPV cell. Spectral efficiencies of ∼83% for 0.52eV TPV cells and ∼76% for 0.60eV TPV cells have been achieved using an interference filter in series with a plasma filter to form a tandem filter with a high emissivity (∼0.8) radiator at 950°C. In contrast, inherent absorption in frequency selective surface (FSS) filters due to ohmic losses results in lower spectral efficiency as compared to tandem filter performance.
738(2004); http://dx.doi.org/10.1063/1.1841893View Description Hide Description
Front surface spectral control filters significantly improve the efficiency of thermophotovoltaic (TPV) converters. Tandem filter designs for 0.52 and 0.60 eV cells were fabricated. Energy and angle weighted spectral efficiencies of ∼83% for the 0.52 eV application and ∼76% for the 0.60 eV applications were achieved with ∼78% angle weighted above bandgap transmission. Manufacturing demonstrations of both designs were completed with good yield. Design improvements were made using angle weighted spectral utilization and above bandgap transmission as refinement goals. Current development work addresses elimination of the plasma filter and alternate substrates.