3.1. Passive and active thermal storage in the house

Simulations indicate that during a clear sunny or even a partially sunny day, no heating will be required [1]. The main challenge of the control strategy thus consists of gathering as much thermal energy as possible during sunny days, and storing it so that it can be used over a sequence of cloudy days (at least 2 days), thus minimising the use of the backup (i. e., the ground source). The many systems of this house make controlling it a challenging task. However, despite the inherent complexity of the problem, the key issue is the rational balance of the two thermal storage media for stockpiling solar thermal energy: the passive thermal storage in the building’s structure thermal capacity (mainly in the floor slab and the masonry wall) and the active thermal storage (TES tank).

The passive storage of the ANZEH is charged by the solar heat gains obtained through the windows (which are practically an essential component of the heating system) and by the radiant floor heating pipes. The passive storage is discharged when it gives heat to the indoor space. Naturally, this thermal energy is eventually released to the surroundings of the house. The TES reservoir (active storage) can be charged in four ways: (a) through the direct recovery of thermal energy from the BIPV/T air, via the HX; (b) with one or both HPs using the BIPV/T air; (c) with the HPs, but using the ground as the heat source; and (d) with excess energy from the solar collector loop. The TES is discharged mainly by its use as a source for the radiant floor heating system; it is also discharged by delivering thermal energy to the DHW tank and through natural heat losses to the surroundings.

The following considerations should be addressed in the design of the control strategy:

• Although temperature fluctuations are needed to take advantage of the thermal mass potential for storing thermal energy, comfortable indoor conditions must be maintained at all times.

• These temperature fluctuations occur at time scales of several hours, much longer than the time constants of the sensors and of the HVAC system.

• The COP of the HP(s), and thus the energy delivered to the TES, depends mainly on the temperatures of the BIPV/T air and the bottom of the TES tank. The COP also depends on the water flow rates on both sides of the HP(s), and the air flow rate through the HX.

• The thermal energy stored in the tank increases with its temperature. Since the COP of the HP(s) decreases as the temperature of the tank increases, the decision to charge the tank should be made based on the availability of thermal energy at the present time and in the future.