Improvement of efficiency of PV panels for water heating by changing heater resistance
Engineering for Rural Development
The main use of solar energy in Latvia is for water heating in solar collectors. But solar energy can be used for heating of water or air not only with solar collectors, but also with photovoltaic (PV) panels. Here electrical heaters have been used; from the electro-technical viewpoint they are of active resistance with power factor cosϕ = 1. Such kind of water heating has some advantages in comparison with solar collectors. In case of solar collectors heating goes on only when the temperature
... en the temperature of the heat carrier exceeds that of the heated water, therefore, the efficiency of heating strongly depends on the temperature of the heated water. Power of the electric heater does not depend on the temperature of the water. In case of solar collectors heated water (or heat carrier) must be transported at rather a large distance from the collector to the place where the heated water is used, leading to large heat loss. In case of PV panels the electric energy has been transported with much less loss. Because of non-linear load curves of PV panels and dependence of the shape of the curve on solar irradiance and the air temperature, the energy received from the panel depends on the resistance of the heater. Traditionally voltage converters with the function of searching of point of maximal power have been used. Such converters are very expensive. Therefore, we propose to use constant resistance with the value appropriate to the used PV panel and to the mean weather conditions and solar irradiance. Measurements of heating of water have been done. It is found that the lower the solar irradiance, the larger resistance gives better results. In our situation the load resistance 4.6 Ω gives the best results, while from the producer's information at high solar irradiances 3.8 Ω load resistance is the best.