Should thermal or photovoltaic solar installations be preferred?

Today, thermal solar panels are the best solution for meeting hot water needs. But in a few years, with the expected advances in technology, it will become increasingly interesting from an economic and energy point of view to cover all the energy needs of a building with photovoltaic solar panels, coupled with a heat pump.

According to the principles of energy efficiency, to produce hot water from the sun, the conversion between solar radiation and hot water should be as direct as possible. It therefore seems much more logical to use solar thermal panels directly (with a theoretical efficiency of more than 60%, but in practice 25-40%), rather than powering a water heater with electricity produced by solar panels with a much lower efficiency (of the order of 16%). A priori, there is no comparison!

However, the situation is not as simple as it seems.

Solar thermal systems consist of solar collectors in which water circulates (in a closed circuit), heated directly by solar radiation and then stored in a boiler. For aesthetic reasons, but also because of the risk of frost (despite the addition of antifreeze to the circuit water), most installations of this type in Switzerland do not place the boiler on the roof, immediately above the solar collectors, as is the case in some southern European countries. As a general rule, in Switzerland the boiler is usually located in the basement, and the water is circulated over the entire height of the building by means of pumps, with control valves and collectors for adjusting.

In addition, the highest production of hot water takes place during the summer. This season often coincides with a holiday period, during which there is a risk of overheating. To prevent this risk, additional devices are needed to dissipate some of the energy and to avoid overpressure in the circuit with degradation of the antifreeze fluid.

In addition, auxiliary heating should be introduced to produce hot water during rainy periods. In the end, all of this equipment makes solar thermal installations relatively expensive, between 2,000 and 3,000 CHF/m² of panels when installed in existing buildings (much less in a new building). Their efficiency depends heavily on their use; if the hot water produced is consumed, the efficiency of solar thermal installations can exceed 60%. But if it is not used, the efficiency is obviously zero! It is therefore important to correctly size the installation according to actual needs.

A competing approach is to use photovoltaic solar panels and use some of the electricity produced to power a heat pump to produce domestic hot water and to meet space heating needs [→ Q27]. When the heat pump is off (especially during holiday periods), this strategy has the advantage of being able to inject electricity from photovoltaic panels into the grid, rather than having to dissipate energy from thermal solar collectors.

Due to the constant progress in photovoltaic panel and heat pump technology, the efficiency of this type of combined system is now over 60%, thus exceeding the effective efficiency of thermal installations, although there is still room for improvement. The costs of these installations are still high, but they are falling steadily while the price of solar thermal installations is tending to stagnate.