In an earlier paper we reported on a small grid-connected thermophotovoltaic system consisting of an ytterbia mantle emitter and silicon solar cells with 16 % efficiency (under Standard Test Conditions, STC). The emitter was heated up with a butane burner with a rated thermal power of 1.35 kW. 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, progress has been made and significantly higher efficiencies were achieved. The most important development steps are: 1) Water filter between emitter and cells replaced by a suitable glass tube to prevent absorption losses in water and protecting the cells against the flue gasses. 2) Improved cooling of the cells to reduce their temperature and increase their conversion efficiency, 3) Geometry of spherical emitter changed to quasi-cylindrical, in order to obtain a more homogeneous irradiation of the cells. 4) Upgrade of burner tube, on which the ytterbia emitter is fixed, to a heat-resistant metallic rod to avoid scaling of tube. 5) Larger reflectors used to reduce radiation losses, 6) Smaller cells used to reduce series resistance losses. Applying all these improvements measures to the 1.35 kW-system, a system efficiency of 1,5 % was attained With preheated combustion air (370°C), 1.8 % was achieved. A new, somewhat larger photocell generator consisting of high efficiency silicon cells led to a system efficiency of 2.4 %. With preheated combustion air (370°C) 2,8 % was achieved. With these generators the spaces between the cells are minimised to increase the active cell area and to reduce radiation losses. In order to realize self-powered operation with one of the prototype systems, an electronic control (made of components with low power consumption) was developed. It controls the magnetic gas supply valve of the burner and also the high-voltage ignition electrodes, A small commercial solar inverter is used to place excess power on the 230 V grid.
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