Tuesday 24 July 2012

Bathing in Sunshine - Solar Heating for Swimming Pools

If there’s a better application for solar heating, it’s hard to think of it

Among the running costs for swimming pools, energy ranks pretty high.  For an indoor municipal swimming pool, water heating makes up around 25% of the energy use, for an outdoor pool without space heating or lighting requirements, water heating rises to 65% of the total (see reference here).  So it will come as no surprise that more and more pool owners and facility managers are considering investing in solar water heating as a way of reducing their environmental impact and cutting energy bills at the same time.

How do you Heat a Swimming Pool with Solar Panels?

The integration of a solar heating system with a swimming pool is relatively straightforward.
The simplest and lowest-cost form of solar heating for a swimming pool is to divert the pool water filtration circuit through a series of black plastic or rubber pipes which are placed in direct sunlight.  Unglazed solar panels like these will raise the temperature of a swimming pool, but only in good weather.

A more sophisticated solar heating system would use glazed solar panels such as flat plate or evacuated tube solar panels and will add heat to the swimming pool on more cloudy days or on days that are sunny but cold. For an outdoor pool, this can mean a much extended swimming season. For an indoor pool, this means making a contribution to lowering running costs all year round.

How a solar swimming pool heating system works
Click for larger version
Copyright Viridian Solar

A more sophisticated solar heating system would use glazed solar panels such as flat plate or evacuated tube solar panels and will add heat to the swimming pool on more cloudy days or on days that are sunny but cold.  For an outdoor pool, this can mean a much extended swimming season.  For an indoor pool, this means making a contribution to lowering running costs all year round. 

Since these types of panel are used all year round, they often have a solution of antifreeze running in them rather than the pool water.  In sub-zero temperatures this prevents freezing water from damaging pipes and solar panels.  A heat exchanger is added to the pool filtration/heating circuit, so that as the pool water circulates through the filtration circuit it picks up heat from the solar circuit.  (See the diagram above).

As an added bonus, glazed solar panel systems can be configured to produce water at temperatures suitable for domestic hot water.  In a municipal pool the solar system can then provide heat for pre-swim and post-swim showers.  In a domestic swimming pool, the solar system can also provide hot water for use in the house.  In this case, the solar circuit has a motorised valve which can divert the solar heat to a heat exchanger coil in a hot water cylinder.  (See the shaded part of the diagram).

The solar controller decides whether the temperature rise in the solar panels is high enough to produce the hotter temperatures required for showers and will divert the heat to the cylinder as a priority.  Once the cylinder is completely hot, or if the light levels are so low that only lower temperature output from the solar panels is possible, the controller switches the motorised valve to circulate the solar fluid to the pool heat exchanger.

So Why is a Swimming Pool Such a Good Application for Solar Heating?

The efficiency with which a solar panel works is strongly influenced by the temperature that it is being asked to work at.  If the temperature is above that of its surroundings , then some of the energy collected from the light is lost from the panel by conduction, convection and radiation.  The greater the temperature difference, the greater the losses, the less heat that ends up being transferred into the pool or hot water store and the lower the efficiency with which the solar panel works.

A solar heating system heating a cylinder for domestic hot water might start the day at 10°C, and operating at an efficiency of more than 80%, but finish the day operating at 70°C and an efficiency of less than 30%.

By contrast, swimming pool water is usually in the range of 25-27°C for fitness swimming up to 29-31°C for children’s swimming.  These much lower temperatures mean that the solar panel is operating at towards the highest efficiency levels all day long, so the annual energy yield of each solar panel is much greater for a swimming pool application than for domestic water heating.

Solar energy is also a variable source of energy, where the available energy on one day could be ten times more than the next.  When using solar energy to heat domestic hot water, the volume of water available to be heated in the hot water cylinder will often limit the amount of energy that can be collected.  Once the cylinder is all hot, the solar system has to switch off – there’s nowhere to put the sunlight for the rest of the day.

By contrast, a swimming pool is a very large body of water.  It takes large quantities of energy to change the temperature by only a fraction of a degree.  This large volume can absorb heat from the very sunny days and hold it over for days with lower solar energy.  The Pool Water Treatment Advisory Group advises that swimming pools can operate very satisfactorily at temperatures 1-2 degrees lower than their advised maximum.  It is therefore possible to allow the pool temperature to fluctuate within a small range to make the most of the sunny days when they come.

Chart showing solar energy addition to an indoor
swimming pool with year-round usage

Finally, solar energy availability is seasonal.  Indoor pools that are used all year have a fairly constant demand for heating energy, so the solar system can be economically sized to meet heating demand in summer, requiring back-up heating in other seasons.

Chart showing solar energy addition to an outdoor
swimming pool with May to October useage

An outdoor pool is likely to have a use profile that almost perfectly matches the availability of solar energy – with the most hours spent without a cover being during the best weather.  The solar can even contribute towards reducing the heat-up spike at the start of the season if the pool can be heated for a few weeks before opening.

Practical Considerations for Solar-Heated  Swimming Pools

When considering whether your pool is suitable for solar water heating the following issues should be considered:
·         Available Area.  The area of solar panels required can be chosen to emphasise energy saving or most rapid payback of the cost, and will depend on factors such as insulation levels of the pool, hours spent uncovered and will differ for indoor or outdoor pools.  Your solar specialist should be able to predict energy savings with specialised software.  As a rule of thumb to check whether you have enought room for solar panels, the solar panel area being equal to between a quarter and a half of the surface area of the pool is a good starting point.

·         Over-shading.  Avoid locations where the solar panels would be shaded by other buildings or trees for large parts of the day during the seasons where the pool is in use, as this can seriously diminish the performance.  Bear in mind that trees and hedges can grow.

·         Orientation.  Solar panels do not have to face directly south to work well, anywhere between east and west on the south side is good.

·         Plant room space.  For heating the pool, very little additional space is required.  For a system that is also heating water for showers, a solar hot water cylinder will be required.

Eight Point Nine More Good Reasons to Go Solar

Since September 2011, a government scheme, the Renewable Heat Incentive is rewarding owners of solar heating installations with cash back for every unit of energy the solar system generates (currently 8.9 pence per kWh for 20 years).  The scheme is currently suitable for indoor pools only and non-domestic situations (that is, not for those attached to single houses).

From summer 2013, the scheme will be extended to domestic houses, and systems installed before then can still qualify for the payments once the scheme starts.

More information on this incentive scheme can be found here.