Shining examples: PV and solar thermal examined

Even in the UK, solar energy is a plentiful resource. Over the course of a year, the sun beams down between 900 and 1,200kWh per square metre. And whereas wind energy is intermittent and ineffective in built-up areas, solar energy is as effective in a street as a field, and there is a simple linear relationship between panel area and energy produced.

It is thought that by the middle of the century the sun could provide 10% of the UK’s energy needs. But do you know your photovoltaics from your solar thermals? Read on, and you soon will …

PV panels

How do they work?

PV panels take advantage of the photovoltaic effect, whereby photons of light transfer their energy to electrons in a silicon crystal. Sunlight falling on the crystals generates a direct current with an equivalent voltage of 12V. When enough of them are joined together this can be put through an inverter to give an alternating current, suitable for use in homes and offices with 240V circuits. In all varieties of panel, the crystals are encapsulated with ethylene vinyl acetate or transparent resin and sandwiched between low-iron glass and a backing layer of metal, plastic or glass. Three types of system are available.

• Monocrystalline panels use cells sliced from whole crystals and are the most efficient type (about 15% of full potential energy is harvested – the rest being lost in the conversion process).
• Polycrystalline panels use cells sliced from a cast block of many crystals and are less efficient (about 13% of potential energy is harvested).
• With thin-film amorphous silicon, panels are sprayed with a crystalline solution, creating panels much like those of solar-powered pocket calculators. These are cheap but inefficient: only 7% of the potential energy is turned into electricity. Only buildings with a huge expanse of roof would really benefit from this option.

What sort of buildings is it most suitable for?

Buildings that use a lot of electricity in the daytime, such as offices. PV should be used as part of an integrated approach because the electricity it generates requires expensive batteries to store.

How much power do they produce?

The average PV panel has an output potential of about 1.5-2kWp. In the UK, this could provide about a fifth of a home’s yearly electrical requirements. Panel areas are between 8m2 and 20m2, depending on the technology used. However, energy production does vary seasonally. At the height of summer, full output might provide all of a home’s needs, but anything over this amount of electricity would be wasted or sold back to the grid at an unattractive rate (about 2p/kWh).

What are the capital costs of installation?

The basic cost of installation, kit and wiring is between £4,000 and £9,000 per kWp. Systems that can be integrated as an intrinsic part of a roof are more expensive. But as panels for these take the form of roof tiles, which would have to be installed anyway, some of the costs are offset.

What are the maintenance and running costs?

PV panels have no running costs. And as they have no moving parts, they also require no maintenance, save for occasional cleaning. But users generally have to top up their electricity by other means, such as from the grid (at about 6p/kWh).

What is the break-even point?

There are two very different concepts to grapple with here: carbon and cash. The break-even point for carbon – when the amount of carbon saved equals the amount used in the manufacture of the panels – comes after about three years in the UK.

Cash payback periods, however, can range from 60 to 120 years. Installation, which accounts for about half the total cost, could get cheaper as Britain’s workforce becomes more acquainted with these technologies. But manufacturing costs are not likely to reduce in the near future.

What is the average lifespan of panels?

PV panels are expected to last for between 15 and 30 years, depending on the technology used. Monocrystalline silicon lasts longest, followed by polycrystalline silicon, while thin-film amorphous silicon has the shortest lifespan. Towards the end of their life, the efficiency of all types of panel decreases.

Is planning permission needed?

If the panel is set at the same pitch as the roof and is not too far proud of its profile, it should be acceptable.

What incentives or grants are available?

Our energy industry is set up to allow surplus electricity to be sold back to the grid. But as the rate at which it is set is so low, there is no real incentive for widespread PV adoption. In Germany, for example, it is a statutory requirement that electricity is bought back at three times the rate at which it is sold, so the break-even for cost comes much sooner, and uptake is far greater.

Grants are available through two bodies:

• The Carbon Trust provides tax breaks and interest-free loans to businesses
• The Energy Saving Trust makes grants available to householders through the DTI. The maximum payment is £2,500.

Solar thermal panels

How do they work?

Two types of system are commonly available:

• Flat plates are the simple, cheap but less efficient, option. Usually rectangular, they comprise a metal absorber plate – which is in contact with water pipes – and a glazed panel, often with surface coatings that can reduce reflection. Acting like greenhouses, these panels work best on hot days.
• Evacuated tubes are more expensive but more efficient, as their energy is derived from light, which is converted into heat. This is important for a climate like Britain’s, where lots of sunlight does not necessarily mean lots of heat. The principle of these evacuated tubes is similar to that of a thermos flask but whereas flasks use silvery glass, these are transparent. The glass forms an outer vacuum tube, which light can penetrate but from which heat cannot escape. On reaching the inner pipework element, the light converts into heat in the water through contact with an absorber, such as black paint.

Both systems use a mixture of water and antifreeze in the heated pipes and this is pumped through heat exchangers, which are connected to hot water tanks in the traditional manner.

What sort of buildings is it suitable for?

Like PV, the long-term storage of solar thermal energy is difficult. For this reason, these systems are not generally used for space heating, which is needed most when there is least sun. Hot water provision is therefore its main application, so it is less useful in offices, where hot water requirements are minimal. Its main use is in domestic developments.

How much hot water do they produce?

Panels range in size from about 0.7m2 to 8m2. Being more efficient, evacuated tubes usually take up less space than flat plates for a typical heat output of 3kW. Most manufacturers say one panel will provide the average home with about two-thirds of its annual hot water needs, although some experts believe this to be a conservative estimate.

What are the costs of installation?

The basic cost of installation, kit and plumbing is £2,000-3,000 for flat plates and £3,500-4,500 for evacuated tubes. Integrated roof systems, like those for PV, are available. The slate-like panels of these can also be mixed with PV slate panels.

What are the maintenance and running costs?

Running costs are quite low but a small amount of electricity is needed for the pumps. These systems also need to be tied into top-up systems, which make up the shortfall in the winter and on cloudy days. The glazing on flat plates needs replacing periodically. The powder used in evacuated tubes needs changing every 20-30 years.

What is the break-even point?

Solar thermal panels (of both types) save about 0.2kg of carbon dioxide per kWh per year, so to save as much carbon as PV, a 1kW system would have to run for 1,500 hours. In terms of cost, their payback is much quicker than PV, at about 10 years.

What is the average lifespan of panels?

Service lives of between 10 and 25 years can be expected for flat plates, while 20 to 35 years is quoted for evacuated tubes. Because capital costs are recovered more quickly than with PV, lifespan is less of an issue.

Is planning permission needed?

The same considerations apply as for PV.

What incentives or grants are available?

Our infrastructure is not set up to allow surplus heat to be sold back to any sort of grid, although it could conceivably be sold to a neighbour. Like PV, the Carbon Trust and the Energy Saving Trust exist to help businesses and householders respectively.

In the case of te latter, £400 is available per household.