The difference between solar thermal and solar PV

I was in a meeting this week and found myself talking to someone who confusing solar water heaters with solar photovoltaic panels.  This took me a bit by surprise as I thought people were quite familiar with the two different technologies, as they’ve been around for a while, but it seems not.

Solar Thermal

Solar water heaters or solar geysers use the heat from the sun to warm up water, most often in flat panels on the roof, or water contained in tubes surrounded by a vacuum within another tube.  The warmed water is stored in a geyser, which often has back up heater like a normal geyser, to ensure there is hot water even on overcast days.  Flat panels are often used in temperate climates, where there is a lot of direct sunlight and where it is not typical to have overnight sudden frost.  Evacuated tubes are good for colder climates, or where there is sudden frost overnight, as sudden freezing of water causes it to expand and can crack flat plates, whereas the vacuum around the inner tube provides a measure of insulation from frost.

Solar water heaters help to reduce the electricity consumed for heating water, and can be an excellent investment for households, where a large chunk of electricity is used on heating water.

Solar photovoltaic (PV)

Solar PV systems are able to convert sunlight to electricity.  They are typically made up of individual silicon based cells that are connected together in a module or panel.  They can be connected to a building’s distribution boards to supplement the electricity supplied by the grid, or the electricity can be exported directly from the system to the grid.  They are able to convert direct sunlight, or, depending on the type of module used, they can generate electricity even on a muggy day.  Unlike solar geysers, they perform better when the module temperature is kept fairly low, and the light energy that is most important; not the thermal energy.

Both systems can be installed together, as they serve very different purposes.  It doesn’t make sense to install a PV system on its own if your biggest electrical load is from your geyser, and if you don’t have any hot water needs, it clearly doesn’t make sense to install a solar water heater.

Other solar thermal applications

Solar thermal installations can also be used to generate electricity, but this makes use of the thermal energy to heat water to turn a turbine, to generate electricity.  Thermal -> mechanical -> electrical energy.  These are often large scale installations, called Concentrated Solar Power (CSP) but there are some small scale systems that have been installed.

Lastly, solar thermal can also be used to run a building’s cooling system, as strange as that sounds.  The water is heated up, the steam is used to power the chilling machinery, which runs the airconditioning system.  Thermal -> mechanical -> cooling.

Generally though, most solar thermal installations are your run of the mill solar water heaters.  How to tell which you have:

  • do they have a series of glass tubes? – solar water heater
  • if it’s a glass plate, do they have lots of individual squares that are distinguishable? – solar PV
  • if it’s a glass plate, which looks like a solid black panel and/or there’s a tank installed above it? – solar water heater

EDIT: I have been reminded that I forgot to write about absorption chillers.  Read up on them here: http://www.gasairconditioning.org/absorption_how_it_works.htm