A hybrid solar photovoltaic-thermal collector is the combination of a solar thermal unit and a photovoltaic panel for the simultaneous generation of heat and electricity. In these systems, a fluid is used to cool photovoltaic panels and, thus, prevent their reduction of electrical efficiency. The hot fluid leaving the system can also be used in various kinds of engineering applications, from agriculture to heating, ventilation and air conditioning units, and process heat in utilities. Coolants used in photovoltaic-thermal units include air, water and nanofluids, among which air is less efficient than water and nanofluids due to its low specific heat capacity. Although extensive research has been done on the exergy performance of photovoltaic-thermal units, the number of published review articles in this field is very limited. This paper presents a critical review with some recommendations for future research on the topic of exergy examination of water-based and nanofluid-based photovoltaic- thermal units. As a first step, the concept and mathematical exergy relations are introduced. Then, waterbased and nanofluid-based photovoltaic-thermal units are exergetically discussed in detail, followed by the description of novel units. At the end of each section, some suggestions are presented for future exergy examination of those types of photovoltaic-thermal units.
