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