Two ducts bring the hot air from the roof to an air-to-water heat exchanger (HX). Due to its size (cross-section of approximately 1.2 m x 1 m) this HX is located in the ceiling of garage. The HX allows the use of the BIPV/T to heat the water in a large reservoir in one of two ways: (a) directly, via a piping system connecting it to a coil inside the TES tank; or (b) by using the hot air to heat the water of the source side of two twin heat pumps (HPs). System (a) can be used when the BIPV/T air temperature is considerably higher than that of the bottom of the tank. During the winter, however, the BIPV/T air temperatures will often be between 10 and 30 °C, which is an optimal source temperature range for the operation of the HPs. The details of the selection of these HPs have been described elsewhere [3]. Each heat pump is rated at 10.6 kW. They can operate together or one at a time, extending the range of air flow rates that can be useful to provide thermal energy; it has been rated from about 400 to 1000 L/s.

The aforementioned large water reservoir represents the main active thermal energy storage (TES) of the ANZEH. Its volume (about 4000 L) permits storing about one day’s worth of heating for a heating load of 6 kW. Apart from the coil making the direct link to the HX, and the pipes connecting it to the HPs, the TES tank has an additional coil connecting it to the domestic hot water (DHW) tank. The TES tank will deliver the thermal energy to a plate heat exchanger, which in turn transfers heat to a low temperature radiant floor heating system installed in the concrete slabs in the basement, main and upper floors. A vertical division, and two adjustable horizontal baffles with a perforation in the centre, will enhance the thermal stratification of the tank.

However, the main factor limiting this stratification is the high flow rates required by the HPs; the temperature difference between the maximum and minimum temperature in the tank will be about 5-8 °C.