Small self-powered grid-connected thermophotovoltaic prototype system

Abstract

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radiation-absorbing water filter between the emitter and silicon cells (to protect the cells against overheating) has been replaced by a suitable glass tube.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 (4) The metallic burner-tube, on which the ytterbia emitter was fixed in the initial prototypes, has been replaced by a heat-resistant metallic rod, carrying ceramic discs as emitter holders.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 unit's 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.