Sliver cells in thermophotovoltaic systems

Previous modelling has indicated that silicon solar cells should be thinner than 100 μm to be optimal for use in thermophotovoltaic (TPV) systems. Sliver cells are a novel type of thin photovoltaic cells fabricated from single crystal semiconductor wafers, with their contacts at the edges of the cell. A computational model was constructed to examine and compare the performance of silicon sliver cells with silicon conventional back-contact cells in TPV systems. Within the range of parameters investigated it was found that the lateral carrier transport resistance of sliver cell geometries limits their power output relative to conventional cells in TPV systems. In practical systems, the efficiencies are comparable.     

Design and analysis of solar thermophotovoltaic systems

Solar thermophotovoltaic (STPV) is a light-electricity conversion system consisting of a solar concentrator, an emitter, a filter, PV cells and the cooling system. This paper is aimed at establishing a method for both purposes of design and performance analysis of the STPV device, which consists of a set of governing equations integrating all these components. The effects of several structural and operational parameters on the performance of the STPV system are investigated. The concentrator performance corresponding to the non-parallelism of solar rays, different aperture ratios and tracking errors is analyzed. A 3-D model is constructed to investigate heat transfer process in the thermo-electric conversion system. The performance of the spectral filter is studied. In addition, the effect of the cooling subsystem on the STPV is analyzed. Numerical simulation suggests that the emitter temperature seriously affects the system performance. An optimized non-periodic filter will remarkably improve the spectral efficiency of the STPV. As an example, an optimized design and analysis of the whole system is carried out in this article.     

微热光电系统中的微燃烧研究

描述了一种新颖的MEMS动力源概念，即微热光电(TPV)系统。该系统将使用氢气作为燃料，每立方厘米体积能够发出1～10W的电力。微燃烧室是该系统中最重要的元件之一，为了获得较高的电能输出，燃烧室壁面的温度分布要求高而且均匀。由于微燃烧室面容比大，热损失显增加；着火困难并使火焰窒熄。为了测试微燃烧室内燃烧的可行性和确定影响燃烧的有关因素，进行了实验和数值模拟。结果表明燃烧室壁面能够得到要求的高温，且温度分布均匀。     

Heat transfer and thermophotovoltaic power generation in oil-fired heating systems

The focus of this study is the production of electric power in an oil-fired, residential heating system using thermophotovoltaic (TPV) conversion devices. This work uses experimental, computational, and analytical methods to investigate thermal mechanisms that drive electric power production in the TPV systems. An objective of this work is to produce results that will lead to the development of systems that generate enough electricity such that the boiler is self-powering. An important design constraint employed in this investigation is the use of conventional, yellow-flame oil burners, integrated with a typical boiler. The power production target for the systems developed here is 100 W – the power requirement for a boiler that uses low-power auxiliary components. The important heat transfer coupling mechanisms that drive power production in the systems studied are discussed. The results of this work may lead to the development of systems that export power to the home electric system.     

Thermal modeling for thermophotovoltaic systems adopting the radiation network method

The radiation heat transfer model for thermophotovoltaic (TPV) system is constructed by adopting a total-spectrum radiation network. However, no consideration of the wavelength shift in the optical filter will lead to the discrepancy between the evaluated spectral emissive power of the optical filter and the Planck law result. Therefore, a modified spectral radiation network in which the wavelength shift effect is expressed as the form of spectral heat source is developed. The modified spectral radiation network is applied in the thermal analysis of a realistic TPV system in which a quartz envelope takes the place of the optical filter, while the optical filter is combined with the cell array. Several important design parameters are figured out, including spectral radiation power density and spectral efficiency of the system, as well as the equivalent radiative temperature of the quartz envelope.     

Photothermoelectric cell for thermophotovoltaic systems and solar power plants with concentrators

We have attempted to justify the appropriateness of the photoelectric and thermoelectric (photothermoelectric) cells in the development of efficient thermophotovoltaic systems and facilities with photoelectric converters and concentrators of solar radiation. These cells are p-n structures based on narrow-band semiconductors capable of the simultaneous thermo- and photogeneration of electron-hole pairs under the effects of irradiation. We discuss the reduction of losses in the course energy conversion carried out with a photothermoelectric cell instead of separate thermo- and photoelectric cells. In addition, we describe the characteristic features and estimate the efficiency of photothermoelectric cells in the thermophotoelectric systems and facilities for the photoelectric conversion of the concentrated solar radiation.     

微热光电系统带双环形翅片燃烧室的数值模拟

由于能量密度的极大优势,应用燃烧释放碳氢燃料的化学能并转化为电能或者机械能的微动力系统有巨大的发展潜力和广阔的应用前景。微热光电系统是其中的一种,而微燃烧室是该系统的核心部件。在实验验证的基础上,对带有双环形翅片燃烧室内的燃烧过程进行了数值模拟。结果表明,双环形翅片能增强微燃烧室内混合气体的湍流扰动,改善燃烧状况,有效地提高燃烧效率。     

Theoretical estimations of the efficiency of thermophotovoltaic systems using high-intensity silicon solar cells

Efficiencies have been calculated for (a) heat-mirror filter and (b) rear-mirror thermophotovoltaic systems using actual materials values for source emissivity ((λ)), mirror-filter transmissivity (τ_M(λ)) and intensity-dependent cell parameters V_oc and FF. The ratios of heat-mirror and rear-mirror efficiencies to direct sunlight impingement efficiency are estimated to be 1.6 and 1.9, respectively (2500°K thermal source), if mirror and other extraneous absorption are taken to be zero in both cases. The more realistic inclusion, for the first case, of 2 per cent light absorption in the filter/mirror and, in the second case, of 2 per cent absorption of low energy light in the cell and 8 per cent absorption at the rear mirror, plus a 2 per cent thermal loss directly from the hot source (for both), reduces the advantage of both to about a factor of 1.2.     

A solar thermophotovoltaic converter

A model of a thermophotovoltaic (TPV) converter is presented. Sunlight was focused by an optical system into a spherical cavity made of tungsten or of ytterbium oxide, thereby heating the cavity.The spectral region of the incandescent radiation emitted by the cavity in the range 0.6 – 1.1 μm (corresponding to the maximum efficiency of silicon cells) was directed onto a distribution of cells facing the radiator. The part of the spectrum not in the range 0.6 – 1.1 μm was sent back to the radiator and recycled. Conversion efficiencies of about 24% are possible in a TPV converter operating with a 2000 K radiator.     

Upper bounds for solar thermophotovoltaic efficiency

The main components of thermophotovoltaic (TPV) devices are the primary lens (or mirror), the absorber, the PV cell, and a photon recuperator system. A theory integrating all these components is used in this paper to analyse a particular type of TPV device (plane disk absorber and PV cell). The TPV efficiency is maximized by using three optimization parameters, namely absorber, PV cell temperatures, and cell voltage. Almost ideal operation conditions are envisaged and upper bounds are obtained for the TPV efficiency. They are strongly dependent on PV cell bandgap and radiation concentration. Preliminary results suggest the existence of an optimum solar radiation concentration ratio. The improvement in thermal design quality allows the usage of PV cells based on wide bandgap semiconductors.     

A prototype design model for deep low-enthalpy hydrothermal systems

This paper introduces a prototype design model for deep low-enthalpy hydrothermal systems. The model predicts, empirically, the lifetime of a hydrothermal system as a function of reservoir porosity, discharge rate, well spacing, average initial temperature of the reservoir, and injection temperature. The finite element method is utilized for this purpose. An extensive parametric analysis on a wide range of physical parameters and operational scenarios, for a typical geometry, has been conducted to derive the model. The proposed model can provide geothermal engineers and decision makers with a preliminary conjecture about the lifetime of a deep low-enthalpy hydrothermal system. The proposed modelling technique can be utilized as a base to derive elaborate models that include more parameters and operational scenarios.     

Low bandgap GaInAsSbP pentanary thermophotovoltaic diodes

The liquid phase epitaxial growth of pentanary GaInAsSbP lattice matched onto InAs substrates is reported for use in narrow bandgap thermophotovoltaic cells. The epitaxial layers were characterised and exhibited bright photoluminescence up to room temperature. Prototype thermophotovoltaic cells were fabricated, which were sensitive in the mid-infrared spectral range having cut-off wavelengths in the range 4.0-4.5 μm at room temperature.     

Fabrication and simulation of GaSb thermophotovoltaic cells

In this paper we report on the recent progress in fabrication and simulation of GaSb photovoltaic (PV) cells with a Zn diffused emitter. The form of Zn profiles in the emitter has an essential impact on the power output of a PV cell. Different types of Zn profiles were realized and used for simulation of PV device parameters. Calculations based on PV cell measurements show that efficiencies up to 30% can be achieved assuming a blackbody temperature of 1300–1500 K and a perfect band edge filter.     

Cost-efficient thermophotovoltaic cells based on germanium substrates

This paper describes the realisation of cost-efficient thermophotovoltaic (TPV) cells based on germanium substrates. Because the majority of the photons incident on the TPV cell in a typical TPV system will have a long wavelength it is important to apply optical confinement in the TPV cell. In this paper this has been done by using a highly reflective rear contact. Electrical contact at the rear has been created with a laser (laser fired contact, LFC) such that the metal is locally heated and contact is formed.The optimal TPV cell is based on a low doped p-type germanium substrate having a 500 nm thick n-type emitter , where front and rear are both passivated with hydrogen rich PECVD amorphous silicon. An AM1.5G record efficiency has been measured for a germanium TPV cell with an LFC rear contact. The application of optical confinement leads to a clear improvement of the spectral response in the wavelength region which is dominant in TPV systems and results in a potential 20% increase of current density compared to the use of a classical germanium photovoltaic cell.     

Power generation performance of hydrogen-fueled micro thermophotovoltaic reactor

A tubular platinum reactor with a perforated annular array enables fuel/air mixtures to exchange sides, thus sustaining flames and preventing heat loss. Consequently, the combustion efficiency and operational range can be enhanced. A hydrogen/air mixture was introduced into inner and outer chambers at different equivalence ratios and flow velocities to chemically and physically investigate the interplay between the chambers. The benefits of hydrogen include a high gravimetric heating value, flame speed, and diffusion capacity and short chemical reaction time. The coexistence of heterogeneous (surface) and homogeneous (gas) reactions in the micro TPV reactor was examined and elucidated in terms of aerodynamics, mass and heat transfer, and chemical reactivity. Furthermore, a TPV reactor with TPV cell arrays was assembled, and the corresponding radiant efficiency of the emitter and the overall efficiency of the proposed micro TPV system were determined in this study.     

A solar thermophotovoltaic converter using Pbs photovoltaic cells

A solar thermophotovoltaic converter using PbS photovoltaic cells is proposed. The converter is in the form of a flat plate consisting of a heat mirror, a black absorber, a cell filter and PbS photovoltaic cells. Theoretical analysis shows that, ideally, the efficiency of such a system is about 30%.     

Modeling and performance of microscale thermophotovoltaic energyconversion devices

We analyze the feasibility of energy conversion devices that exploit microscale radiative transfer of thermal energy in thermophotovoltaic devices. By bringing a hot source of thermal energy very close to a receiver fashioned as a pn-junction, the near-field effect of radiation tunneling can enhance the net power flux. We use the fluctuational electrodynamic approach to microscale radiative transfer to account for the spacing effect, which provides the net transfer of photons to the receiver as a function of the separation between the emitter and receiver. We calculate the power output from the microscale device using standard thermophotovoltaic device relations. The results for the performance of a device based on indium gallium arsenide indicate that a ten-fold increase in power throughput may be realized with little loss in efficiency. Furthermore, we develop a model of the microscale device itself that indicates the influence of semiconductor band-gap, energy, carrier lifetime and doping     

Porous media combustion for micro thermophotovoltaic system applications

The micro combustor is the key component of the micro TPV power generator. To obtain high power density and performance efficiency, it is important for a micro combustor to achieve a high and uniform temperature distribution along the wall. In this paper, we compare the performance of a micro cylindrical combustor with and without employing porous media. Results indicate that packing the combustor with porous media can significantly enhance the heat transfer between the high temperature combustion products and the emitter wall. The use of porous media increases the contact area thereby increasing the temperature along the wall of the micro combustor resulting in an increase in its radiation energy. The effects of some parameters on radiation energy of the micro combustor are also highlighted.     

Heat transfer modelling in thermophotovoltaic cavities using glass media

Optimisation of heat transfer, and in particular radiative heat transfer in terms of the spectral, angular and spatial radiation distributions, is required to achieve high efficiencies and high electrical power densities for thermophotovoltaic (TPV) conversion. This work examines heat transfer from the radiator to the PV cell in an infinite plate arrangement using three different arrangements of participating dielectric media. The modelling applies the Discrete Ordinates method and assumes fused silica (quartz glass) as the dielectric medium. The arrangement radiator–glass–PV cell (also termed dielectric photon concentration) was found to be superior in terms of efficiency and power density.     

Characterisation of rare earth selective emitters for thermophotovoltaic applications

We investigate selective radiation emitters made from rare earth oxides suitable for thermophotovoltaic (TPV) systems. Yb₂O₃ and Er₂O₃ emitters were fabricated and their radiation power, temperature and emissivity were measured in the entire relevant spectral range. We found temperatures of 1735 K for the Yb₂O₃ emitter and of 1680 K for the Er₂O₃ emitter both heated with a 1.35 kW butane burner. The maximum emissivities of the selective peaks were 0.85 at 1.27 eV, and 0.82 at 0.80 eV for the Yb₂O₃ and the Er₂O₃ emitter, respectively. The emission spectra show gas emission lines originating from the combustion process in addition to the selective emission bands. An estimation based on a simplified combustion model show that a TPV system with a system efficiency of about 10% can be realised using an Yb₂O₃ emitter, silicon photocells and a perfect selective filter.