Cost-effectiveness of multi-purpose solar thermal systems and comparison with PV-based heat pumps

• A criterion is proposed for sizing of Multi-Purpose Solar Thermal Systems (MPSTS).
• Optimal sizing was based on maximization of Net Present Value.
• The analysis was made for places with different climatic and economic conditions.
• Optimal collector areas are expressed in function of Peak Heating to Cooling Ratio.
• MPSTS are always less advantageous than multi-purpose PV-based systems.

Even under the powerful thrust of 20–20–20 measures, solar thermal systems are experiencing a slow-down in their development in most EU countries. One reason is their traditional confinement to sanitary hot water (SHW) production; another one is the growing competition with photovoltaic (PV) systems. In order to widen the use of solar thermal collectors, they should also be able to contribute to space heating and cooling and become Multi-Purpose Solar Thermal Systems (MPSTS). This paper addresses the issue of optimal sizing of MPSTS. The criterion adopted is based on maximization of Net Present Value and has been applied to some cities in Italy and in Pakistan with diverse climate conditions. Results show that optimal thermal collector areas per peak cooling demand (Ac/Pc) can be conveniently expressed as a function of Peak Heating to Cooling Ratio (Ph/Pc). Optimum Ac/Pc varies between 3 and 5 m2/kWc and decreases with increasing Ph/Pc. The paper also analyses and compares a MPSTS with a multi-purpose PV-based heat pump system using a traditional one (boiler and compression chiller) as reference. Results show that steadily decreasing prices have made PV systems more favorable, even without consideration of public subsidies.