Defining an annual energy output ratio between PV and solar thermal

Photovoltaics (PV) and solar thermal (ST) collectors are often competing between themselves not only because the investment capacity is limited but also because the energy demand and roof space is limited and both types of panel provide energy which can be converted to a different type of energy under a certain efficiency. Therefore, it makes sense to develop a ratio that quantifies the difference in annual energy output between standard ST and PV for different locations. This ratio is useful, for example, to support the decision between installing ST or PV, when combined with other local specific information such as the value of heat and electricity for a specific location and application, the system complexity and efficiency, and others. A market survey was conducted for assessing the average performance specifications of the panels. Simulations were conducted and several ratios were plotted in the world map. Despite the large variations occurring due to local climate, the ratio increases at lower latitudes due to two factors: a) the efficiency of a PV panel is reduced with the increase of air temperature while, in solar thermal, the effect is the opposite; b) Under low intensity solar irradiance, the efficiency of a PV panel is maintained while a solar thermal collector can have its efficiency reduced to zero. For latitudes lower than 66o, the ratio flat plate at 50oC to PV is ranging from 1,85 to 4,46 while in the ration between vacuum tube at 50oC and PV from 3,05 to 4,76.