Wilhelm Durisch

Characterisation of photovoltaic generators

Reliable knowledge on the performance of different photovoltaic generators (as single cells, modules, laminates, shingles, car roofs, etc.) under actual operating conditions is essential for correct product selection and accurate prediction of their electricity production. For this purpose, an outdoor test facility was erected at the Paul Scherrer Institute, PSI. It consists of a sun-tracked sample holder, electronic loads and a PC-based measuring system. Insolation is measured with pyranometers, pyrheliometers and reference cells. Characterisation of a generator under given test conditions means the precise acquisition of its electrical behaviour under varying load. The generators efficiency and all the relevant electrical parameters are derived on-line from a series of measured current/voltage (I/V) values. I/V-scans at constant insolation and at different generator temperatures enable the temperature coefficients of the efficiency and the electrical parameters to be determined. Thereafter I/V-scans at different insolations (10-1200 W/m2) and air masses (1.1-5) yield (via temperature correction) the insolation dependence of the efficiency at constant temperature. A complete scan takes about 5-15 s. Samples of size varying from 1 by 1 mm up to 1.5 by 1 m can be tested at currents up to 32 A and at voltages up to 120 V. For modelling purposes, the results are represented in the form of correlations, e.g. the efficiency as a function of the operating parameters temperature, insolation and air mass. Results obtained in PSIs test facility were confirmed by the Fraunhofer-Insitut fur Solare Energiesysteme, D-79100 Freiburg, Germany. Measurements are presented from some modules and single cells as well as some efficiency correlations. Results are also presented on lamination losses, on PSIs high efficiency cell, on Gratzel cells and watch modules as well as on shading effects and of a small thermophotovoltaic generator.

Small self-powered grid-connected thermophotovoltaic prototype system

In an earlier paper, we reported on a small grid-connected thermophotovoltaic (TPV) system consisting of an ytterbia mantle emitter and silicon solar cells with a 16% efficiency (under solar irradiance at standard test conditions, STC). The emitter was heated using a butane burner with a rated thermal power of 1.35 kW (referring to the lower heating value). This system produced an electrical output of 15 W, which corresponds to a thermal to electric (direct current) conversion efficiency of 1.1%. In the interim, further progress has been made, and significantly higher efficiencies have been achieved. The most important developments are:- (1) The infrared radiation-absorbing water filter between the emitter and silicon cells (to protect the cells against overheating) has been replaced by a suitable glass tube. By doing this, it has been possible to prevent losses of convertible radiation in the water, and to protect the cells against the flue gasses. (2) Cell cooling has been significantly improved, in order to reduce the cell temperature, and therefore increase the conversion efficiency. (3) The shape of the emitter has been changed from spherical to a quasi-cylindrical geometry, in order to obtain a more homogeneous irradiation of the cells. (4) The metallic burner-tube, on which the ytterbia emitter was fixed in the initial prototypes, has been replaced by a heat-resistant metallic rod, carrying ceramic discs as emitter holders. This has prevented the oxidation and clogging of the perforated burner tube. (5) Larger reflectors have been used to reduce losses of useful infrared radiation. (6) Smaller cells have been used, to reduce the electrical series-resistance losses. A system efficiency of 1.5% was attained by applying all these improvements to the basic 1.35 kW prototype. By using preheated air for combustion (at approximately 370 °C), 1.8% was achieved. In a subsequent step, a photocell generator was constructed, consisting of high-efficiency silicon cells (21% STC efficiency). In this generator, the spaces between the cells were minimized, in order to achieve as high an active cell area as possible, while simultaneously reducing radiation losses. This new system has produced an electrical output of 48 W, corresponding to a system efficiency of 2.4%. This is the highest-ever-reported value in a silicon-cell-based TPV system using ytterbia mantle emitters. An efficiency of 2.8% was achieved by using preheated air (at approximately 350 °C). An electronic control unit (fabricated of components with low power consumptions, and including a battery store) was developed, in order to make the TPV system self-powered. This unit controls the magnetic gas-supply valve between the gas-supply cylinder and burner as well as the high-voltage ignition electrodes. Both the control units own power consumption and the battery-charging power are supplied directly by the TPV generator. A small commercial inverter is used to transfer excess power to the 230 V grid. In future systems, the effect of preheating the combustion air will be studied in more detail. Finally, this system will be scaled up to provide self-powered domestic boilers.

Cost Estimates of Electricity from a TPV Residential Heating System

A thermophotovoltaic (TPV) system was built using a 12 to 20 kWth methane burner which should be integrated into a conventional residential heating system. The TPV system is cylindrical in shape and consists of a selective Yb2O3 emitter, a quartz glass tube to prevent the exhaust gases from heating the cells and a 0.2 m2 monocrystalline silicon solar cell module which is water cooled. The maximum system efficiency of 1.0 % was obtained at a thermal input power of 12 kWth. The electrical power suffices to run a residential heating system in the full power range (12 to 20 kWth) independently of the grid. The end user costs of the TPV components ‐ emitter, glass tube, photocells and cell cooling circuit ‐ were estimated considering 4 different TPV scenarios. The existing technique was compared with an improved system currently under development, which consists of a flexible photocell module that can be glued into the boiler housing and with systems with improved system efficiency (1.5 to 5 %) and geometry. P...

Solar irradiation measurements in Jordan and comparisons with Californian and Alpine data

In order to obtain reliable irradiation data for the design, operation and economic assessment of solar power stations, a measurement campaign has been performed in Jordan. As promising sites the desert near Quwairah in the South of Jordan, the stony desert South East of Amman and the elevated plateau near Naqb and also in the South of Jordan were chosen. Measurements were performed during the period of June 1989 to July 1992. The data were evaluated and compared with data of Barstow, California and the Swiss Alps. From the yearly sums of the direct normal irradiation in 1990 and 1991 at Quwairah (2701 and 2436 kWh/m2 respectively) it is concluded that this site is comparably as good for solar thermal power stations as the Barstow site. The global normal irradiation (usable with sun-tracked photovoltaic panels) had the surprisingly high values of 3551 and 3373 kWh/m2 in 1990 and 1991 respectively. Occasionally peak values of the global normal irradiation greater than the solar constant were measured (up to 1500 W/m2). Missing global normal data from other arid sites do not permit comparison. As known before, the corresponding values in the Swiss Alps are considerably lower (1100-1700 kWh/m2 for the direct normal irradiation and 2000-2500 kWh/m2 for the global normal irradiation respectively, depending on the year and site). In addition to the direct normal and global normal irradiation, the global horizontal and global inclined (30° South) irradiation were measured, amounting to 2353 and 2499 kWh/m2 respectively in 1990. Data have also been collected on wind, rainfall, ambient temperature, dew point and surface pressure. All data are available in a computer accessible form, in particular as a yearly set of 5 min mean values of the direct normal irradiation for 1990. Combining the ground measured data with METEOSAT-data resulted in a unique map of the direct normal irradiation for Jordan and surrounding countries, indicating attractive sites for solar power stations. The measurement campaign was made possible by active support from the Ministry of Energy and Mineral Resources, MEMR and the Jordan Electricity Authority, JEA, both at Amman, as well as by generous financial support of the Swiss Committee for Scientific Research, KWF, Berne.

Record electricity-to-gas-power efficiency of a silicon solar cell based TPV system

We report on the fabrication and detailed optical characterisation of a selective incandescent mantle Yb/sub 2/O/sub 3/ emitter. A TPV prototype system was built by arranging commercially available monocrystalline silicon solar cells around the emitter. This system reached, without preheating of the combustion air, a record electricity-to-gas power efficiency of 2.4%. A model is presented which allows an accurate simulation of the system properties. First experiments with a demonstration system based on a 20 kW methane burner and a 500 cm/sup 2/ emitter gives evidence that this TPV design can be up-scaled. An optimisation of this system and the integration of a highly selective filter should permit the realisation of a high power (> 1 kW) cost effective silicon TPV system for decentralised generation of electricity in residential heating systems.

Practical thermophotovoltaic generators

For use in gas-fired thermophotovoltaic systems, a selective emitter made from Yb2O3 foam ceramic has been developed. This foam ceramic is mechanically stable, and FTIR spectroscopy showed that 10% of the radiation power emitted by the foam can be converted by Si photocells. The thermal and thermal-shock stability of Yb2O3 foam ceramic was analyzed. The foam passed 200 heating/cooling cycles without major damage. Tubes were manufactured from this material and tested in a thermophotovoltaic demonstration system. An electrical power of 86 W was achieved at a thermal power of 16 kW. Using a simulation model, the potential efficiency of a thermophotovoltaic system based on our technology applied for the conversion of concentrated solar radiation was estimated.

Cost estimate of electricity produced by TPV

A crucial parameter for the market penetration of TPV is its electricity production cost. In this work a detailed cost estimate is performed for a Si photocell based TPV system, which was developed for electrically self-powered operation of a domestic heating system. The results are compared to a rough estimate of cost of electricity for a projected GaSb based system. For the calculation of the price of electricity, a lifetime of 20 years, an interest rate of 4.25% per year and maintenance costs of 1% of the investment are presumed. To determine the production cost of TPV systems with a power of 12–20 kW, the costs of the TPV components and 100 EUR kW−1el,peak for assembly and miscellaneous were estimated. Alternatively, the system cost for the GaSb system was derived from the cost of the photocells and from the assumption that they account for 35% of the total system cost. The calculation was done for four different TPV scenarios which include a Si based prototype system with existing technology (ηsys = 1.0%), leading to 3000 EUR kW−1el,peak, an optimized Si based system using conventional, available technology (ηsys = 1.5%), leading to 900 EUR kW−1el,peak, a further improved system with future technology (ηsys = 5%), leading to 340 EUR kW−1el,peak and a GaSb based system (ηsys = 12.3% with recuperator), leading to 1900 EUR kW−1el,peak. Thus, prices of electricity from 6 to 25 EURcents kWh−1el (including gas of about 3.5 EURcents kWh−1) were calculated and compared with those of fuel cells (31 EURcents kWh−1) and gas engines (23 EURcents kWh−1).

Efficiency of selected photovoltaic modules under varying climatic conditions

It is essential that test results should be available for photovoltaic modules under actual operating conditions if products are to be selected correctly, and if electricity generation predictions are to be reliable. For this reason, an outdoor test facility was erected at Paul Scherrer Institute, PSI, allowing fast current/voltage-characteristic acquisition for photovoltaic modules under all climatic conditions. Module performance is mainly dependent on cell temperature, insolation intensity, air mass and diffuse fraction in the global irradiance. Efficiency and its temperature coefficient are determined by evaluating current/voltage-scans at constant insolation, air mass and diffuse irradiance, but different cell temperatures. Additional scans at different insolation intensities and air masses but constant diffuse irradiance allow their effect on the efficiency to be studied and modelled, leading to the Standard Test Condition (STC) data as well as to the behaviour of the modules in part-load operation and at different air masses. Ultimately, the most important result is a correlation representing the modules efficiency as a function of the insolation intensity, cell temperature, air mass and diffuse irradiance. This paper presents results obtained from two modules, i.e. from a monocrystalline module (SIEMENS Solars SM110) and from an amorphous module (UNISOLARs triple junction module US-30). The module (cell) efficiencies under Standard Test Conditions were 12.4 (14.9) and 6.26 (7.43) % respectively. The corresponding temperature coefficients of the efficiency were found to be minus 0.062 and minus 0.010 %/°C. The STC powers were found to be 107.8 and 29.8 W, e. g. 2 and 0.7 % below the rated power specified by the suppliers. Compared to results of other modules [3], these two modules perform very well. Regarding the amorphous module, however, it is still uncertain whether the degradation process is complete. Additional tests will be required after an extended period of outdoor exposure to answer this question. The monocrystalline module demonstrates an attractive part load behaviour, and its efficiency shows a maximum value at an insolation intensity of approximately 600 Wm-2. No significant AM-effect on the efficiency was observed with this module. A strong decrease in efficiency with increasing air mass was found with the US-30 triple junction module. However, its sensitivity against varying diffuse irradiance seems to be negligible and its part-load behaviour is fairly good. The relatively low absolute value of the temperature coefficient of efficiency makes it a good candidate for hot climate applications. In contrast with crystalline silicon cells, the amorphous triple junction cell exhibits a non-linear efficiency vs. temperature relation. Highest efficiencies, up to 8 %, were measured with this module at low air masses (1.2 - 1.5) under the blue-filtering effect of clouds in front of the sun. Measurements from PSIs test facility were well confirmed by the Fraunhofer Institute for Solar Energy Systems, FhG-ISE, D-79100, Germany.

A TPV system with silicon photocells and a selective emitter

We have realised and investigated a thermophotovoltaic generator which uses silicon photocells and an Yb/sub 2/O/sub 3/ selective emitter. Theoretical calculations show that in principle an electrical output power of several hundred watts can be achieved with such a system using 20 kW thermal input power. We have built a small TPV system base on a camping gas lamp which reached an efficiency of 1.9% and an electrical output power of 25 W. Commercial silicon solar cells which are water cooled are used in this system. The efficiency improvements when using a TCO filter instead of the water filter and optimised silicon photocells are discussed. We show measurements of the optical properties of an optimised ZnO filter and the electrical characteristics of PSI high efficiency photocells. To demonstrate the use of the TPV in a residential heating system, a TPV demonstration system with a thermal input power of 20 kW and 161 W electrical output power was developed.

Advances in Gas-Fired Thermophotovoltaic Systems

In a first and completely new approach, a vacuum plasma-spray coating technique was used to deposit selective emitting rare-earth oxide films of ytterbia (Yb 2 O 3 ) on porous silicon-infiltrated silicon carbide foams (Si-SiC). The plasma-spray coating technique offers a new and promising way to produce selective emitting coatings on different refractory substrates with complex geometries. The adhesion and thermal shock stability were tested until a film thickness of 130 μm was achieved; the selective emittance of the oxide coating has been found to be dependent on the film thickness. The material combination Si-SiC and Yb 2 O 3 , however, needs some major improvement regarding high-temperature stability and high thermal cycling loads. In a different approach, the advantage of low emitting Al 2 O 3 fibers and good thermal matching was combined with Yb 2 O 3 slurry coating of flexible alumina (Al 2 O 3 ) fiber bundles, formed into a cylindrical shape. The thin fiber structure tried to imitate the famous incandescent mantle emitters of Auer von Welsbach, but with a more rugged structure. Even though the fibers of the woven emitter were thin, the low thermal conductivity of Al 2 O 3 led to a distinct reduction of the surface temperature and emittance, and a shielding effect of the radiation emanating from the hot inner walls by the cooler outer grid structure was inevitable. Optical filters consisting of a water film and of transparent conducting oxides (TCO) have been developed and tested to protect the photocells against overheating and to reflect nonconvertible off-band radiation back to the emitter. The water film led to a significant reduction of the cell temperature and increased cell performance, whereas with the TCO filters only a reduction of the cell temperature was observed.