Solar Thermal

While solar photovoltic panels (PV) receive most of the attention because of their ability to make electricity and enable their owners to sell power back to the grid, the PV panels typically convert about 15% of the sunlight energy that falls upon them into electricity.  This means they achieve 15% efficiency.

Solar thermal panels achieve much higher rates of energy conversion.  Typically 80% or higher.  So why don’t these systems receive more attention?

Solar thermal can be used to heat hot water for showers and other home use, can be used to heat the home or office in the colder months instead of burning gas or fuel oil and can be used to heat a swimming pool.  All of these are valuable savings in terms of carbon emissions and fuel.

The first reason these systems do not receive so much attention is that most of the time these systems replace natural gas as the energy source and as a heat source natural gas is very cost effective based on today’s costs.  Further as we convert to more solar PV, to wind and geothermal for electricity generation, we will often be increasing the supply of natural gas and so that will tend to keep its price low.  First point – it is politically difficult to raise the price of natural gas to the consumer but unless we do that we will not create living  and work structures that use less fossil fuels and emit less carbon.

I now have experience with 3 such solar thermal systems and the are wonderful but they are more complex than solar PV.  The control systems for solar thermal are not as well designed and as thoughtful in providing energy savings and the systems generally require storage of the heat to be useful.  Moreover, in the most progressive parts of the USA – like California and Texas – you need less heat to comfortably live in winter.  So the folks that tend to drive change are not as motivated for solar thermal as they are for solar PV.

A little more about thermal storage.  Solar thermal collects the sun’s energy and moves this energy away from panels on a rooftop and into storage tanks where it is typically stored as hot water.  A tank can store enough for 3-5 showers or enough to heat a normal house for several hours but not all night.  Storage typically can cost as much as the solar panels.  In solar PV there is typically no storage as when you produce extra electricity you sell it back to the grid.

Looking a little closer into the 3 cases you can see how the storage works –

  • Using solar to heat a swimming pool it is the pool itself that provides the storage.  So the question arises where do you set the pool temperature.  Suppose you like the pool at 82F (28C).  You can use the sun to heat the pool to that temperature.  But on sunny days sometimes you will have to stop heating even though there is more to collect.  An alternative strategy is to continue heating the pool to say 84F (29C) which is a bit warmer than you might find optimal.  But then the pool can coast down over night and even a cloudy next day having stored more thermal energy.  Using natural gas or electricity to heat a pool necessarily has no value in heating more than the exact temperature you like.  But solar has times when energy is essentially free and other times when it is simply not available.  So it is better to have a controller with a soft set point that allows for some over heating/over storage to compensate for times when no heat may be available.
  • solar heating of a home is the same situation.  The tank that stores thermal energy in the basement of your home may be able to store 100,000 BTU but your home with all of its furniture, appliances and such can store much more.  So again supposing you like the home at 72F (22F) but can handle living in the home down to 68F (20C) and up to 76F (25C).  In this case you can adopt a control strategy where when the sun is out the home continues warming until the higher temperate is reached and then stop or then start to store the extra heat in the storage tank.  As the sun sets the home coasts down in temperature from 76 to 68 before any stored or fossil energy will be called into play.

Solar thermal will receive more attention in some future posts.  it is a critical part of our energy future because it can operate at so much higher levels of collection efficiency and that means it is all the more cost effective.

Posted in Essays, Green Perspectives, StartUp Ideas

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