Solar thermal (the panels on the right in the image above) is a close cousin to solar photovoltaic (PV) technology (shown above, left). Both convert sunlight into useful energy. PV converts it to electricity while solar thermal converts it to heat.

Not so long ago solar thermal was the leading solar technology. It was affordable, readily available and cut the heating bill.

The introduction of Feed-in Tariffs (FiTs) in 2008 incentivised solar PV with generous tariffs. That made PV more attractive and the massive take-up of that technology served to increase efficiency and drive down the purchase price.

But solar thermal remains and still does a useful job.

What does a solar thermal system look like?

There are two main types:

  • evacuated tube
  • flat plate

A flat plate collector is an insulated box with pipes running through it, covered by a flat glass or polycarbide plate. These can be used for roof integration (where they are mounted in the roof in place of the roof covering) as well as roof mounting, where they are mounted on top of the roof covering.

An evacuated tube is a glass tube carrying a vacuum, similar to a Thermos flask. They come in various diameters, from 65mm to 100mm, the larger ones generally being more efficient. They also have a pipe running down the middle with fluid in it to collect the heat and carry it to the hot water store. The tube system can only be mounted above the roof and is more fragile than a flat plate.

Which type of solar thermal system is best?

Manufacturers of each can prove conclusively that their product is more efficient than the alternative.

  • Evacuated tube products are a bit more expensive and in some situations, notably when the roof is not directly south-facing, more efficient.
  • Flat plate collectors are a bit cheaper but not so efficient when they are not south-facing.
  • There are in-roof mounted systems that look very similar to rooflights and others, from the likes of Sonnenkraft, that can be mounted horizontally on a flat roof or vertically on a wall.

How does a solar thermal system work?

Light passing through glass is refracted, changing its wave length and essentially trapping it and producing heat. The heat is captured in a fluid (generally a 50/50 water/glycol mix) and conveyed to a hot water cylinder.

The cylinder needs to be bigger than the normal 80-litre copper cylinder – typically 200 to 350 litres – as the system needs to store as much heat as possible while the sun shines.

In fact, solar panels don’t actually need ‘sunshine’, as in a hot sunny day. As long as there is light they will produce heat, but obviously more light means more heat.

As well as the collector there are valves, a pump and a control system that detects when heat is needed and is available. It then switches the pump on and controls how much heat is produced. This is generally mounted in the attic or close to the collectors.

Are there any incentives?

Solar thermal qualifies for the Renewable Heat Incentive (RHI) with a rate of 20.06p/kWh.

This means that a typical system producing around 2,000kWh per year will produce around £400 per year for seven years.

This is close to meeting the cost of the system, which is what it is intended to do.

But to qualify for the RHI, the solar thermal system can only be used to meet domestic hot water demands, not space heating or for heating swimming pools.

Do panels need planning consent?

Legislation around solar panels on domestic properties changed in 2008 so that, in most cases, installation now falls under Permitted Development and does not require planning consent. There are exceptions to this, notably in conservation areas and on, or near, listed buildings. If there is any doubt then speak to the local authority.

How much does a system cost?

Solar panel technology has not changed much in 35 years and so there has not been much change in price. Prices have dropped by maybe 20% in the past five years (solar PV, on the other hand, has more than halved in price).

A typical 4m2 flat panel installed on-roof, including a 200-litre tank, all labour, peripheral parts, control systems and VAT at 5% will cost from £2,500 to £4,000, depending on the quality of the system and ease of installation. On a new build, scaffold is already in place and roof tiles are not yet on so installation will be easy and prices could dip below £2,500.

In terms of value, a system of that size will produce around 2,000kWh of hot water each year. The life of the system will be in excess of 30 years, a minimum of 60,000kWh total production.

Running and maintenance costs of solar panels are less than £10 per year. Assuming an installed cost of £3,500, the unit cost of production will be around 5.8p/kWh, before RHI.

What do I need to install a solar thermal system?

A 4m2 flat plate (being two panels) or 18 to 24 tube (varying with the type of tubes) array on a south-facing roof elevation with an inclination of 22° would be ideal for a solar thermal array for a typical house with four people living in it, but there is a degree of flexibility:

  • An evacuated tube array could be as much as 45° east or west of south and still be reasonably efficient.
  • A flat plate array can be mounted east-west, with one panel on the east elevation and two on the west to achieve the same output as a south-facing array.

What is actually needed will vary with what it is to be used for:

  • A house built, or renovated, to a bit better thermal efficiency than current Building Regulations requirements could have its whole space heating and domestic hot water demands met with just a solar thermal system, for eight or nine months of the year.
  • A house built to Passivhaus standard could do the same for 12 months, but it would not qualify for RHI.

For a swimming pool, a solar thermal array covering 50% of the surface area of the pool will heat an outdoor pool for six months of the year and an indoor pool for more than nine months (how much more depends of the construction of the pool room). But this does not qualify for RHI.

Is a solar thermal system worth the investment?

Some say that solar thermal is a dead technology. The drop in PV prices, brought about by government incentives, means that it is cheaper to produce hot water using PV than solar thermal. This could be true if RHI did not exist.

But it does, and right now it virtually pays for the system in seven years, meaning that the actual unit cost of energy from solar thermal is below 2p/kWh. RHI will be dropped, probably within the next two years, which may then kill the technology.

Until then it remains a robust, simple, reliable technology that will provide useful energy for a very long time. Certainly, if the system is used in a way that qualifies for RHI, it is still worth considering.

How do I find a supplier?

Your first port of call should be the Solar Trades Association. There will be many solar thermal installers in every location to which, again, due diligence must be applied.

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