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.

Tuesday 10 July 2012

Solar Heating and Solar PV – Which is the Best?

There’s only one way to find out.....ask the solarblogger!

Solar Heating Panels (left) and Solar PV Panels (right), but which do I like more...?
(Images, Viridian Solar)

Two Technologies Separated by a Common Language

In solar heating systems, solar panels (sometimes called solar collectors) absorb light and use its energy to heat up a fluid flowing through the panel.  The fluid circulates around a loop of pipe-work and drops off the heat by warming a hot water store (cylinder) for later use.  (A more detailed description can be found here).
By contrast, solar electric (often called photovoltaic, or PV) systems convert light directly into an electric current.  Power electronics equipment (called an inverter) conditions the electricity so that it matches that used in the building.  If the building is not using sufficient electricity at that moment in time, electricity flows out into the power grid and is used somewhere else.  (For more details click here).

Both technologies work well, even under the sometimes cloudy skies of the UK.

OK, so which Solar Technology is better for me?

Roof Area Required

For a typical family home, an area of three or four square metres of panel area is needed for a solar water heating system.  By contrast, the average solar PV installation for a domestic property is 2.8kWp (source STA), requiring around 20 square metres of panel area.

Carbon Emissions Avoided

Solar heating systems are most commonly generating energy to replace the use of a gas heating system, whereas solar PV systems offset the use of grid electricity.  Taking the gas boiler efficiency into account, generating clean electricity saves twice the amount of carbon dioxide emissions per unit generated than does heating water to prevent a gas boiler firing.
As the carbon intensity of grid electricity is brought down by wind turbines, PV farms and nuclear power stations, the difference between the two technologies will reduce.
Of course, in an electrically heated home a unit of solar heat has the same carbon value as a unit of solar electricity.


Both technologies will give good results when facing generally towards the South and anywhere between East and West.  Domestic scale solar heating systems are normally less sensitive to orientation than PV.


Obviously, since both systems require light to generate energy, over-shading is a Bad Thing.  However, solar PV systems are more sensitive to shade than solar heating systems.
A patch of shade over a small part of one PV panel can have a multiplier effect - significantly reducing the output of the whole panel, and other panels in the wiring group.  Designing the layout, positioning and wiring scheme for a PV system to avoid shading effects can be a very important part of ensuring a good annual energy output.
Solar heating systems do not suffer from this multiplier effect, so are much less sensitive to shading.

Integration with Building Services

A solar PV system is pretty much stand-alone.  The only integration with the systems in the building is to be connected to the wiring at the main consumer unit (fuse board).
A solar heating system requires a hot water cylinder to store the heat during the day for later use, so space needs to be found for this if there is not already a cylinder cupboard.  Connections need to be made to the existing heating system.  Solar heating raises more integration considerations than solar PV.

Servicing and Maintenance

Both types of solar system require minimal maintenance.  Unless installed in an unusual location, solar panels are adequately cleaned by the action of rain.
The inverter in a solar PV system generally has a lifetime less than that of the solar panels, and may need to be changed every 10-12 years, at a cost of £500 - £1800 for the inverter, depending on the size of the system.
Solar heating systems need only routine inspection to check for problems.  However most use a heat transfer fluid with a limited lifetime.  This may need to be replaced every 5 years or so at a cost of around £50 for the fluid.  The circulating pump will also have a lifetime of around 10 years.

Energy Savings

Studies have shown that a solar heating system for a family home will produce between 900 and 1,500 kWh of heat energy each year.  Taking into account the boiler efficiency, this might give fuel savings of between 1,300 and 2,100kWh per year.  The savings are limited by the hot water use in the building.
With gas at 5p/kWh, savings from household energy bills would be in the range of £65 - £105 per year,  or £90 - £150/year (electrical heating, assuming a mix of daytime and overnight electricity tariffs).
A well-located and un-shaded solar PV system will produce around 850kWh per year per kWp under UK weather conditions, so our average domestic system of 2.8kWp will produce 2,380kWh per year.
However, unlike solar heating which stores the energy for later use, most grid-connected PV systems export any excess generation to the power grid.  This means that the generation is not limited by the demand of the building, but the energy saved in the building depends greatly on the electricity usage patterns.  If the occupants are at home and using electricity during the daytime (and especially during summer), then a large proportion of the energy generated will displace electricity bought at 15p/kWh.  If not, then the energy saving accruing to the householder might be a small proportion of that generated.

Households with PV systems can make changes to make the most of solar electricity generation, for example using timers to cause washing machines or dishwashers to come on during the middle of the day.

Yes, Yes, but which type of Solar Panel is the best?

Inevitably, it depends:

If your household uses lots of hot water, but does not use much electricity during daylight hours, then solar heating may make the greater savings on energy bills. If, however, you have high electricity use during the daytime and low hot water use then solar PV would definitely be the better choice.
If your goal is to reduce carbon emissions, you don’t care where the energy is saved, and you’re on the gas grid, then PV is likely to be the better choice.  The energy generated by PV has higher carbon intensity and solar heating output is limited by the household demand for hot water.
If you have a large, clear roof area available then this is suitable for PV.  If the roof has a smaller area available or you want a more discreet-looking installation then solar heating may be the better choice due to the smaller panel area.
Of course, if you still can’t make up your mind, there are systems available that include both solar technologies together with matched solar panels.

A Word about Government Incentives

In an effort to cut through to the fundamentals, this discussion has deliberately steered clear of government incentive schemes for renewable energy such as the Feed in Tariff and forthcoming Renewable Heat Incentive. 
Such schemes obviously greatly influence the relative attractiveness of renewable technologies and are likely to be just as influential in decision making as the factors discussed above.  Up-to date information on these two incentive schemes can be found here.

Tuesday 3 July 2012

Who Pays for Greener Homes?

Why does the cost of building a new home matter to house builders?

This may seem like a stupid question, but I’m not talking about the cost of a pallet of bricks, ton of cement or roll of glass wool.  I’m talking about additional costs from meeting tighter environmental regulations. 
Specifically, why do developers of new homes care about the higher building costs from building to ever higher standards of energy efficiency?  Still seem like a stupid question?  Let me explain further.
Let us first accept the argument that “people won’t pay for this stuff”, that a more energy efficient home will command the same price as a less energy efficient home (an accepted wisdom that some in the construction industry are now starting to question). 
Let’s also assume that house builders are price-takers.  The price of a house is largely set by the “second-hand” market because buyers always have the choice to buy an existing rather than a new home.  So if building costs go up and prices stay the same the house builder’s profits are squeezed, right?

Well, not necessarily.  If changes to regulations are published well in advance of coming into force, and if regulations are applied consistently to all developers, then the price that any developer is prepared to pay for land should fall to preserve their profit margin.  Since all buyers of land build to the same regulations, they are all affected equally.  All the “pain” of the new regulations will fall onto the landowner selling to the developer. 

The value of an acre of agricultural land is around £5,000 per acre.  The same land with planning permission to build houses would sell for millions of pounds per acre.  The windfall that accrues to landowners when planning permission is granted is so massive that a squeeze on their margins is of little consequence to their decision to sell and certainly would not win them any sympathy from anyone else.

So can anyone answer the question – why do house builders care about costs?

Please use the comments section below.

You may also be interested in the following posts:

A Million Missing Low Energy Homes
Zero Carbon Homes – Carefully Check the Small Print