Defining an Annual Energy Output Ratio between Solar Thermal Collectors and Photovoltaic Modules

Photovoltaics (PV) and Solar Thermal (ST) collectors are sometimes competitors, as investment capacity, energy demand, and roof space are limited. Therefore, a ratio that quantifies the difference in annual energy output between ST and PV for different locations is useful. A market survey assessing the average price and performance both in 2013 and 2021 was conducted, showing a factor of 3 cell price decrease combined with a 20% efficiency increase, while ST showed negligible variation. Winsun simulations were conducted, and the results were plotted on the world map. Despite variations due to local climate, the ratio of energy production (ST/PV) increases at lower latitudes mainly due to (a) higher air temperature increasing ST output but decreasing the PV output; (b) solar radiation reducing ST efficiency to zero while having a minor impact on PV efficiency. The ratio was calculated for several ST operating temperatures. For latitudes lower than 66°, the ratio of a flat plate at 50 °C to a PV module ranges from 1.85 to 4.46, while the ratio between a vacuum tube at 50 °C and a PV module ranges from 3.05 to 4.76. This ratio can support the decision between installing ST or PV while combining different factors such as energy value, system complexity, and installation cost.