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.