Generating electricity from solar PV panels may be in vogue right now, but heating water with solar thermal panels is showing encouraging results too.

At Greenmount Campus, part of the College of Agriculture Food and Rural Enterprise (CAFRE) in Northern Ireland, they have been using solar thermal panels to heat parlour wash water since October 2009.

A 20sq m array of solar tubes, fitted to a frame and angled at 40deg, was installed next to the dairy. Water passes through the tubes, where the sun raises its temperature from a typical ambient figure of 10C to 20-80C (depending on the time of the year). This means the electric heater in the 1200-litre (260gal) water tank doesn’t have to work as hard to get it up to the 85C required.

The dairy needs 360 litres (80gal) of water to be heated up to 85C twice a day. Before the solar tubes were installed, that meant a daily electricity consumption of 48kWh. Over the course of a year that adds up to 135kWh (costing £15) for each cow.

David Trimble, CAFRE’s renewable energy technologist, is pleased with the way the system has performed. Solar thermal panels obviously work best in summer, he points out, but they’ll function well even on a sunny day in February.

Instead of the 48kWh of electricity used each day before the new system was installed, power consumption with the solar panels in use was just 6.5kWh in May, 7.2kWh in June, 11.4kWh in July, 10.7kWh in August and 9.7kWh in September.

Averaged over the five-month period, electricity consumption was a fifth of what it would previously have been.

The fact that the performance of the panels dropped as the summer progressed looks a bit strange, until you remember that 2010 involved lots of sunshine early in the season and then a return to cloudier skies in late July and August.

And even though Northern Ireland isn’t noted for its Riviera-type climate, Mr Trimble reckons that the solar panels should cut the annual electricity bill for heating parlour wash water by half.

The Greenmount set-up cost £10,000 including installation. That’s more than a farm would pay, mainly because the college wanted more temperature-measurement equipment. The college’s daily requirement for 360 litres of hot water is also higher than the average dairy farm.

Mr Trimble reckons that a 5-6sq m set-up for a typical farm would cost £4000-5000 and is best installed when a new parlour is built. Savings should be at least £5 a year for each cow, giving a payback period of 10-12 years.

However when the government’s long-awaited Renewable Heat Incentive finally arrives, it could cut that payback period in half – much as the feed-in tariffs have cut the payback period for solar PV panels.

Depending on where you are, there may be a grant too. In NI, for instance, farmers can get a one-off grant of £900-£1100/farm from the local electricity supplier NIE.

• The solar heating system at Greenmount will be demonstrated at the Practical on-farm Renewable Energy event at the college on Tuesday 2 November.

How do they work?

The Greenmount set-up used evacuated solar tubes (also called Sydney tubes). These consist of a double-glazing-style outer glass tube and then an inner one, with a vacuum between the two. The inner one is black to absorb light and a reflector behind it helps maximise light capture.

They should last 25 years and have low maintenance and operating costs.

The build was done by Bluesky Renewables using Calpak solar tubes.