Any farms that use lots of warm water could benefit from a solar thermal system. This could either be for washing down (food processing, food production, dairies, on-farm abattoir facilities), or farms with camping facilities and holiday lets, B&Bs, etc.

Dairy and other water heaters used for wash-down can use significantly more energy than a cylinder in your house, because the stored water temperature is so high, 85 degrees rather than 60 degrees. You need to add lots more energy the higher you take the water temperature, in much the same way that driving fast uses more fuel than cruising.

Dairies and food processing

Solar thermal systems are in some ways a much better match in these circumstances than PV (solar electric), for a few reasons:

• A smaller panel area needed to gain enough energy to heat the store

• If heating water on Economy 7 then a PV system won’t do any good at night, whereas a solar thermal system can store energy from the daytime, reducing load on the water heater

• PV systems rarely put out the full 13amps required to power a water heater, unless you have a system much bigger than 4kW, a common installation size

• For every 1kWh of electrical energy spent running the pump, you could harvest 10-15kWh of heat energy.

The best way of operating a solar system in a dairy environment is to pre-heat the water going into the existing dairy heaters. This eliminates a number of potential problems that might be faced if one tried to completely replace the existing heaters, and ensures that if there was ever a problem with the solar system the dairy heaters would be able to cope with the required demand.

Holiday lets and camping

If you run holiday facilities of any sort you will be aware people can be free and easy with the hot tap when on holiday, partly because they believe it is included in the price.

But you may be able to charge them per shower and get paid through the Renewable Heat Incentive for producing this energy as well.

The amount of energy it takes to heat 100 litres of water to the 60-65 degrees at which your hot water cylinder operates is roughly 6kWh. A couple of 10 minute showers could easily use that amount of water, especially if they are power showers, so multiply that by the number of showers taken using your facilities over the year to get an idea of how much energy you are using. Baths use roughly 100 litres of water for each fill, which contains roughly 60 litres of hot water at 60 degrees.

In either case, a solar thermal system is going to reduce your heating costs significantly and produce a guaranteed income stream for 20 years.

Making it pay

The crux of whether or not solar thermal makes a sound business decision hinges on how the tariff system works and what the costs are.

The Renewable Heat Incentive complements the PV Feed-in Tariffs, for production of heat rather than electricity. There are a lot of similarities between the two schemes, with the main difference being that for heat there is no grid to push surplus energy into, and therefore no export tariff element to the RHI.

The RHI is currently set at 8.9p/kWh, and as for PV, the tariff is index-linked and will last for 20 years from the time of installation – not 25 years as for PV.

Currently, the RHI only applies to commercial systems, so to obtain it the hot water use must be on the farm, not the farmhouse. The domestic RHI has been delayed, so you can install on the farmhouse as well, but no payments will be made until at least the early part of next year.

A factor which simplifies payback calculations for dairy systems is that generally a fixed amount of hot water is used every day for the wash down.

How it works

Solar diagram

Solar thermal is a very simple idea: flat sheets of metal get hot in the sun, then heat is extracted by taking cool fluid past the metal and pumping the heated fluid into the water store via a coil, just like a boiler might do in the house. This hot water is circulated using a low-power pump (generally around 40 to 50 watts).

There is a myriad of panels available and all the normal caveats for getting a good-quality system installed apply. Ignore adverts for systems that claim to be “the most efficient”. Efficiency with solar thermal systems relies far more on design of the system as a whole than it does on the panels. Worry about the quality of your installer more than the perceived efficiency of the product.

A solar thermal system will need some maintenance over the years (more than PV), so choose your installer wisely.

Panels or evacuated tubes?

This is a debate that has been artificially stoked up for years by manufacturers and installers with their own allegiances. In truth it is possible to use either equally effectively, and each has their own merits.

Farm environments should use very robust equipment, especially if there is any chance of the collectors getting mounted at ground level. For that reason the argument lies in favour of panels, if possible. However, there may well be situations where evacuated tubes prove to be a more sensible choice.

You will often find tubes referred to as being more efficient. What people mean is that a tube system taking up 1.5sq m of roof might produce the same amount of energy as a 2sq m flat panel. However, solar thermal system efficiency is dictated by far more variables than just how efficient the collectors are.

Successful design means understanding how to get the right number of panels on to heat the water to the operating temperature safely without overheating the system which increases the likelihood of maintenance. Whether you choose tubes or panels is neither here nor there, but in my experience panel systems tend to be more robust and need a little less maintenance because they are not as efficient.

• A well-designed solar system should provide around 50-60% of the energy required to lift the water temperature to around 60 degrees

• The lifespan of a well-maintained solar thermal system should exceed 20 years

• Payback should be eight to 10 years, given a straightforward installation for larger systems.

Is my farm suitable?

If you have a south-facing roof not too far from your stored hot water cylinder/ heaters then there is probably a way to install a successful solar thermal system.

Avoid shading from trees and buildings, but solar thermal is much better able to deal with shading than PV. It is possible to mount thermal panels on all roof types, and the weights are not significantly higher than PV panels. A panel system would weigh in the region of 20kg/sq m, compared with PV at around 15-18kg/sq m. Tube systems are lighter and may be best used on problematic roofs where weight may be an issue.

Performance is always best on south-facing roofs and you can expect a drop in performance of around 15-20% by taking the system to face east or west, and 7-10% for either south-east or south-west. However this can be compensated for by increasing the surface area of panels (or number of tubes). Optimal tilt angle is the same as for PV (around 35-40 degrees), but it’s possible to install systems at any angle if you use the correct equipment.

When solar thermal might not be appropriate

• There is nowhere appropriate to site the panels due to shading: the panels should be shaded less than 20% of the time

• The panels are too far away from the stored hot water. Pipe runs of up to 100m are possible, but this would not be economical for a very small system. Run pipes underground if necessary

• Showers are heating water electrically rather than taking from a hot water cylinder. If your showers heat (rather than just pump) electrically then the savings you make will be limited to just washing up, hand washing and baths.

Cost and payback

Obviously this is a “how long is a piece of string” question. The economics change based on scale and what fuel you displace. If you use a small amount of water, have a small solar thermal system (eg holiday lets) and your water is normally heated by gas, then the payback is going to be drastically different to someone heating large volumes off oil, LPG or electricity (as is the case for dairy or other water heaters).

Example: Commercial dairy scenario

• For a straightforward system heating around 800 litres and a system price of around £13,000 you could expect performance at around 6000 kWhours

• Payback allowing for fuel inflation at 8.7% would be at year eight

• Payback allowing for fuel inflation at 12% would be at year seven

• Obviously there are a lot of variables to consider, such as scaffolding, whether the roof needs strengthening, etc. This example assumes everything would be quite straightforward. Other factors which affect payback are whether you are heating mainly on Economy 7 and how many wash downs you do each day. The more you use, the better value for money you will get. Fuel inflation has an impact, but maybe not as much as you might think.

Example 2 domestic:

This example assumes that when the RHI domestic stream starts it will be the same as the commercial rate of 8.9p/kWh. Only time will tell exactly what system they use to incentivise at the domestic level.

This example also assumes two different fuel inflation rates: Heating oil inflation in the last quarter of 2011 was a staggering 23%, but the average across all energy types was 8.7%. Obviously it varies wildly across the different fuel types.

• 200 litre cylinder, solar produces 1,500kWh a year costing £5,000 to install: normally heated on oil

• Benefits are: 1,500kWh saved fuel cost + RHI payments = £120 + £135 for the first year

• Payback allowing for fuel inflation at 8.7% would be at year 13

• Payback allowing for fuel inflation at 15% would be at year 11

Below is a table of domestic fuel costs from November last year, showing prices once the standard boiler inefficiencies are taken into account.

Energy cost comparison – fuel type price (p/kWh) 

 
 Electricity (standard rate)  15
 Electricity (online rate)  14
 Mains gas (standard rate)  5
 Heating oil  8
LPG  8
Butane  15
Propane  10
Seasoned wood  6
Wood pellets  7
Smokeless fuel  9
(Source, NottEnergy)  

Ben Whittle is technical manager at solar thermal and PV installers Southern Solar. For more information, go to http://www.southernsolar.co.uk/