Because this idea didn’t work out, we describe it only briefly.

A photovoltaic panel warms up as the sun shines on it, reducing its efficiency. To combat this problem, hybrid PV-thermal building facades pass air behind a PV facade, which not only keeps the facade cool, but also provides a supply of warm air that could be used in the building.

While this idea is attractive in principle, the supply of air is only a few degrees above ambient temperature and therefore of rather limited use. We wondered whether it might be feasible to use a heat pump to upgrade the energy in the warm air to a more useful temperature. Particularly attractive was the idea that we could use the electrical output of the facade to drive the heat pump.

Analysis

To investigate this system we did a simple steady-state analysis comparing the double-hybrid system to a hypothetical system optimised for thermal output alone (eg by replacing the PV panel with a black absorber plate and installing a glass cover).

We asked the question: how much of the electrical output of the PV panel needs to be used by the heat pump in order to make the thermal performance of the system identical to that of the dedicated air heater? By “identical” we meant that both the temperature and quantity of thermal energy delivered should be the same in each case.

The calculations showed that, except under very poor solar conditions where the power output is tiny, we would have to devote all or nearly all of the electrical output of the PV to the heat pump in order to achieve the performance of the dedicated solar thermal collector. This being the case, the double-hybrid system starts to look like a very expensive way of building a solar thermal collector, and we took it no further.