摘要:

A new thermophotovoltaic (TPV) power generation system has been proposed on the basis of reciprocating‐flow super‐adiabatic combustion in porous media and has been investigated through numerical simulation. The porous media consists of a thin foam‐ceramic plate sandwitched by a pair of porous quartz glass plates packed perpendicular to the flow direction in an insulated flowing‐duct. By energy recirculation through the reciprocating‐flow combustion system, the foam‐ceramic temperature becomes about two times higher than the theoretical (adiabatic) flame temperature, using only a small amount of combustion heat. Strong radiant energy is emitted from the foam‐ceramic plate. Of those, long wavelength components are absorbed by the porous quartz glass and are regenerated to preheat working‐gases. On the other hand, short wavelength components pass through the porous quartz glass. About 48&percent; of the combustion heat is converted into the radiant energy in a short wavelength range which are useful for (GaSb) TPV power generation. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1539369

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Radiating surfaces at typical temperatures in the range of 1200 – 2000 K are the source of energy which is incident on the PV cells of TPV systems. The radiators or emitters may have very pronounced or weak spectrally selective properties on which TPV system efficiencies depend very much. This paper gives an overview of existing emitter technologies. Two different approaches to realize spectrally selective emitters are discussed in detail: rare‐earth oxides and engineered tungsten emitters. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1539370

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Optimising the spectral emissivity of the IR radiation source in a TPV generator is one of the crucial steps towards high efficiency TPV conversion. In this paper we present different approaches to the preparation of selective emitters to be coupled to high efficiency photovoltaic cells. The emitters are designed to work at a temperature of about 1500K and they have been prepared to be used either as external coatings for the burner or as a structural material for the burner itself. Composite ceramics containing rare earth cations, prepared by slip‐casting, with various concentration of rare earths were prepared by Slip Casting and Slurry Coating. Rare earth oxides have been incorporated into different oxide matrices, namely Silica, Alumina, Zirconia and their combination. The final aim was to find the material that exhibits the best performance in terms of both high selective power emission, good efficiency along with acceptable thermo‐structural properties (high temperature thermal shock resistance, good strength, no creep). The power density emitted by samples as function of the temperature has been tested in the range 1000nm–5000nm. The high temperature emission measurements and the structural tests indicate that a good compromise between the functional and the thermo‐structural properties may be reached. The results of the tests on the emitter coatings carried out in a TPV generator at the operating conditions are also presented in this paper. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1539371

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Spectral control through the use of selective emitters is an important means of improving the efficiency of thermophotovoltaic (TPV) systems. The availability of PV cells having a wide range of bandgap energies, along with the development of selective emitters having a range of emission band energies, gives the developer of TPV power systems a wide range of options. The rare earth aluminum garnet selective emitters developed in our laboratory offer a number of advantages when used in such systems [1–3]. In this paper, we present results of a detailed computational study of the effects of a number of design and operating parameters, including emitter film thickness, scattering within the film, and the temperature gradient across the film thickness, on the performance of three rare‐earth aluminum garnet selective emitters. Specifically, we present emitter efficiency, net emittance, wavelength‐dependent emittance, total output power, and the useful output power, for Er3Al5O12, Ho3Al5O12, and Tm3Al5O12emitters, as functions of the above parameters. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1539372

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Spectral emittance and thermal stability of two‐dimensional W surface gratings are investigated to obtain high‐temperature resistive selective emitters for TPV applications. Numerical calculations based on rigorous coupled‐wave analysis are performed to determine the structural profile of surface gratings. According to the determined parameters, W gratings composed of rectangular microcavities with the period of 1.0 &mgr;m are fabricated on single crystalline and polycrystalline W substrates by means of fast atom beam etching technique. The grating shows a strong emission peak which can be explained by the confined modes inside the cavities. The grating made from a single crystalline W shows very high thermal stability up to 1400K, while the polycrystalline grating is deformed at a high temperature because of the grain growth. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1539373

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The microstructuring of a tungsten emitter allows a remarkable increase of the emittance in a defined spectral range with low mid IR emission at the same time. The optical properties of the selective emitter are determined by the grating period and the shape of the grating. Furthermore for the spectral angle dependancy the structure depth plays a significant role. As theoretical investigations using Rigorous Coupled Wave Analysis (RCWA) have shown the spectral angle dependency decreases in case of deep or high spatial‐frequency structures. At usual TPV emitter temperatures (typically above 1400K) microstructured selective tungsten emitters show both surface diffusion and the so‐called recrystallisation effect. In some cases a modification and destruction of the surface structure due to this phenomena have been observed. At first, the technique of producing selective tungsten emitters is described. In addition, the result of our investigations concerning thermal stability of both shallow and deep gratings is presented and some attempts to handle thermal deterioration are discussed. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1539374

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Silicon operating in a vacuum is a good candidate thermal emitter since it has a high melting point (1680 K). The semiconductor bandgap, which can provide selective emission, adds to the potential for high operating temperature and, therefore, high radiated power. We present the detailed emitter theory, along with both theoretical and experimental results for spectral emittance of thin (∼1 &mgr;m) silicon films on sapphire substrates with a platinum backing. These results show the importance of temperature and film thickness in determining the selective spectral emittance and, with the proper material parameters, can be readily extended to other materials and systems. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1539375

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Multi‐quantum well photovoltaic cells offer a number of advantages over conventional “single‐gap”cells for thermophotovoltaic applications, first of all because they can reach a higher open circuit voltage under the same radiation source and with the same absorption edge. Material quality issues and the constraints imposed by the commercial available substrates indicate that InxGa1−xAs/InyGa1−yAs/InP strain‐balanced heterostructures are suitable to obtain good quality multi‐quantum wells with an absorption edge just below 2.0 &mgr;m. Structural stability in the presence of a high density of elastic energy such as in the case of a strain‐balanced multi‐layer is a very important issue to be addressed by optimising key parameters like composition, thickness of wells and barriers and number of periods. In this paper we present and discuss the mechanisms of plastic relaxation of these structures with a particular attention to the impact of the extended defects generated by the local breakdown of the crystal lattice to the electrical properties of the devices. Then, after the presentation of the optimum structure with an absorption edge at 1.96 &mgr;m, we discuss the issue of a further extension of the absorption edge through the use of a so‐called virtual substrate, that is a buffer structure between the substrate and the device designed to relax to a given extent with a minimum number of dislocations propagating towards the active region. On the basis of a recipe based on the experimental results on InGaAs single and multi‐layers grown on GaAs, we have designed a series of step‐graded buffer structures providing good virtual substrates with a lattice parameter larger than GaAs. Strain‐balanced multi‐quantum wells have been grown on InxGa1−xAs virtual substrates with 0.14 < × < 0.35 with a residual density of threading dislocations of about 105cm−2. Work is in progress to remove the residual morphological undulation (cross hatch) induced by the misfit dislocations confined in the buffer structure and to extend this approach to InP. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1539376

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Thermophotovoltaic (TPV) generators can reduce pollution by lowering their operating temperature, but the choice of semiconductor materials for this purpose is limited. We present results on an InGaAs p‐n cell lattice‐matched to InP which is optimised for the Erbia emission spectrum peak at a wavelength of 1.5&mgr;m. However, for lower temperature TPV applications at longer wavelengths one is constrained by the lack of lattice‐matched materials. In order to extend the absorption towards 1900 nm for a selective emitter based on Thulium strain‐compensated InGaAs/InGaAs quantum well cells (QWCs) on InP have been designed and characterised. We present data showing that strain‐compensated QWCs extend the spectral response (SR) to longer wavelengths and can show a lower dark current density than the bulk InGaAs p‐n cell despite the QWC having a lower band‐gap. We have developed a model for the SR of strained multi‐quantum well (MQW) systems in InGaAsP, including quantum effects as well as strain‐induced changes. SR modelling of strain‐compensated structures is compared with experimental data, and efficiencies, for a Thulia spectrum, are predicted. Our study also shows that back surface reflection must be taken into account in these devices. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1539377

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

In order to achieve high efficiency in a TPV generator, it is important that a high fraction of emitted photons with energies below the TPV cell bandgap are reflected back to the emitter. This can be accomplished in several ways. We present the idea of an internally reflecting egg‐shaped double cone with the emitter at one end, an edge filter at the wide center, and the TPV array at the other end. So far, we have studied this geometry by means of both ray tracing analysis in a computer program and by means of measurements with an emulated emitter. A sharp switchover from transmission to reflection in a multiple layer edge filter can be achieved only if the angles of incident rays are confined to a fairly narrow angular interval. The two used methods both show that the studied optics can lower the angular spread of rays incident onto the filter and that some 96&percent; of the emitted rays (in the ideal case) reach their goal without passing the filter or being reflected by the filter more than once. A suggestion of a whole wood powder fuelled TPV system with this egg‐shaped double cone and edge filter implemented is also given, as well as an animation tool for modelling the optical part of the TPV system. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1539378

出版商:AIP    

年代:1903

数据来源: AIP