Eventually, a sequence of consecutive cloudy days in winter will require discharging the TES without additional heat supply from the BIPV/T roof. A backup system is therefore necessary. One of the most important design revisions has been the change of the backup system from a large pellet boiler to a ground source loop using the same HPs as the BIPV/T system. The pellet boiler presented several shortcomings: cost, higher operating temperatures (implying more complex piping), and the need for a stock of wood pellets. In contrast, using a ground source for the heat pumps has the advantage of simplicity: a single 3-way valve allows for easy selection between the BIPV/T air and the ground to provide thermal energy to the HPs.

2.2. Solar collector and domestic hot water (DHW) system

Two 20-tube vacuum (Apricus 20) tube solar collectors will be the main source of thermal energy for the DHW needs. A heat exchanger wrapped around the grey water drain (“power-pipe”) contributes to increasing the temperature of the water entering the DHW tank. The evacuated tubes will be located on the south facade of the house overhangs, at a 45° inclination angle. A heat dissipater will release excess thermal energy gathered by the collector during the summer. A controllable rolling canopy will extend over the collector and over the edge of the overhangs to improve shading during the summer months. The DHW tank will have a volume of 400 L. An internal coil, having a piping system with a set of controllable valves will allow heat transfer between the tank, the solar collectors and the TES reservoir. A desuperheater coil installed in one of the heat pumps can also transfer heat to the DHW tank via a coaxial pipe linked to the tank’s drain valve. To guarantee the supply of hot water, an instant electric water heater can raise the