Effect of Heat Exchanger on the Operation of a Directly Coupled Photovoltaic-Electrolyser

Abstract

Direct coupling of photovoltaic (PV) modules to electrolyser(s) (ECs) can benefit from reduced component costs by omitting power electronics. Thermal integration of the PV to the EC could potentially enhance performance by cooling the PV and heating up the EC. However, conventional PV and EC constructions need an additional heat exchanger to make this possible. Without concentrating optics, the temperature of the PV remains comparatively low, which could be an added challenge for effective use of the waste heat. Considering this, the question is, how much thermal integration can benefit the PV-EC operation, and how complex a sufficiently efficient heat exchanger would be. To study the effect of heat exchange on the device operation, we compared the simultaneous operation of two identical sets of PV modules directly electrically coupled to EC stacks. One of the devices was also thermally coupled using a heat exchanger at the back of the PV module to heat up the electrolyte (1.0 M KOH) before it enters the EC (Figure 1.a, the thermally coupled device is on the left in Figure 1.b). The heat exchanger prevented contact between the corrosive KOH and the PV module but enabled heat transfer from the PV module to the electrolyte. The PV modules consisted of nine series-connected 6-inch wafer silicon heterojunction solar cells, and the total collection area, of each, was ca. 2600 cm2 (51 cm × 51 cm), of which ca. 2480 cm2 was active. The devices were operated outdoors in Berlin, Germany (52° 25ʹ 53.3ʺ N, 13° 31ʹ 25.9ʺ E) for a total of about 700 hours, of which the last about 500 hours were continuous, except for few short maintenance breaks. During testing, the solar to hydrogen efficiency of both devices was typically in the 8 – 12 % range, reducing with increasing irradiance. Typical peak hydrogen production rate on a sunny day (800 – 850 W/m2) was about 120 ml/min with the heat exchanger and about 110 ml/min without, the highest measured values being about 10 ml/min higher. The heat exchanger improved the performance under irradiance over about 500 W/m2, and at over 800 W/m2 irradiance, the enhancement corresponded to about 10 % increase in the hydrogen production rate. On the other hand, interestingly, the heat exchanger also reduced the hydrogen production rate at low irradiance conditions. This, together with the fact that most of our testing days were comparatively cloudy, probably explains why the total hydrogen yields over the 700-hour period were very similar for both devices, with a slight 10 litre advantage for thermal integration (ca. 770 litres vs ca. 760 litres). Nevertheless, based on our results, even a moderately efficient heat exchanger enhances the PV-EC operation in sunny conditions. Since much of the annual hydrogen yield would be produced in such conditions, the concept of transferring heat from PV to EC shows definite promise, but further development and optimization is needed to extract the full benefits of our approach. The authors acknowledge support from the German Federal Ministry of Education and Research in the framework of the project CatLab (03EW0015A). The present study benefits from work started under the PECSYS project (ended December 2020) funded by the FUEL CELLS AND HYDROGEN 2 JOINT UNDERTAKING under grant agreement No. 735218. This Joint Undertaking receives support from the EUROPEAN UNION’S HORIZON 2020 RESEARCH AND INNOVATION programme and Hydrogen Europe and N.ERGHY. Figure 1