Experimental assessment of short cycling in a hybrid photovoltaic-thermal heat pump system

An experiment is conducted in this paper to analyse the effect of short cycling phenomenon on the performance of a hybrid photovoltaic-thermal heat pump system. The system utilises indirect expansion to cool a photovoltaic-thermal panel through a photovoltaic-thermal water buffer tank that supplies a heat pump, providing space heating using a convection radiator. A photovoltaic-thermal module rated at 200 W electrically and 650 W thermally is generates electricity and absorbs heat to be utilized by a water-to-water heat pump with heating capacity of 7.77 kW. The system was monitored over one-hour operating times for a range of solar irradiances [250–650 W/m2], water flow rates through the photovoltaic-thermal module [7.3 l/min–17.3 l/min] and photovoltaic-thermal buffer tank water volumes [45–75 litres]. Results show the system successfully cools the photovoltaic surface by up to 33 °C. This leads to an electrical efficiency increase of up to 0.7%, correlating to a 17.5% increase in generated electricity. The heat pump experiences coefficient of performance values up to 7. Short cycling frequency increased as the solar irradiance increased. Varying water flow rates through the photovoltaic-thermal module have less influence on system performance than solar irradiance, however, increasing the flow rate from 7.3 l/min to 17.3 l/min results in the photovoltaic surface temperature reducing by 1 °C and short cycling frequency increasing. Variation in the photovoltaic-thermal buffer tank water volume from 45 to 75 litres decreased the photovoltaic surface temperature by 5 °C and increased the short cycling frequency from 6 cycles/hour to 3 cycles/hour.