Should I Use a Solar Thermal or Solar PV System?

By: Kathryn Senior PhD - Updated: 27 Nov 2015 | *Discuss

 

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Q.

Is the payback better on solar thermal or solar PV systems?

A.

For most of Europe, solar thermal systems can give relatively quick savings for people wanting solar power in their already-built homes, where a lot of energy is spent on water heating.

The fundamental different between a solar PV system and a solar thermal system is that the former converts light from sunlight into electricity that can be used to power different systems, including those used for heating water or the home. A solar thermal system takes the heat from the Sun and collects it and then shares it out again to specifically heat water or air.

Deciding between the two systems is quite tricky. Solar thermal systems are much more efficient in terms of the percentage of the Sun's energy that they can make use of. Solar thermal systems are about 70% efficient – meaning they can use nearly three-quarters of the energy from the Sun that falls on them. A PV converts sunlight to electric power at an efficiency of just over 10%. You therefore need a greater area of solar panels in a PV system to generate the same amount of heat.

Solar photovoltaic cells will deliver around 750-900 kWh of electricity per year per kW of installed capacity. Solar thermal collectors will deliver around 1,500-2,500 kWh of heat energy for a typical domestic installation. Solar thermal will almost always give a far shorter payback.

Storing the heat energy from solar thermal is done by means of an accumulator. If the space is there, this is not a problem. A solar thermal system would not do much in winter, and you would never install a solar thermal system without a primary heat source. Solar PV systems generate a trickle of electricity in the winter. The electricity gained is a product of the intensity of the sun's rays that hit the solar cell. On a cloudy day, this is seriously reduced.

Solar PV systems currently give typical paybacks of 40-80 years, solar thermal 20-40 years (depending on what fuel is being replaced). PV is one of our best hopes for a sustainable future), but it is incredibly expensive and should be done for the environment, not for your pockets.

Generally, solar thermal systems also tend to be cheaper but there are problems which makes these systems less than ideal. The main one is that the system transfers heat rather than electricity and heat is difficult to store efficiently. A home that uses a solar thermal system that is being newly built can be designed to incorporate thermal mass walls that contain large masses of concrete, or even thermal mass floors. It is difficult to add these to a house that was built 15 or 150 years ago. Solar thermal systems used in existing houses need a larger water storage tank and the heat is transferred through water.

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