Solar hot water systems have the most application in the high energy use agricultural sectors. For example, a dairy cow uses 350 kiloWatt hours (kWh) of electricity per year, of which around 40 per cent is used for heating water. Northern Ireland has less direct sunlight than many parts of Europe. The combination of direct and diffuse sunlight which can be harnessed for use is between 1,000 and 1,100kWh per square meter each year.

What is a Solar Hot Water system?

A solar water hot water system has three main components.

Solar collectors

These 'collect' as much heat as possible from the sun’s radiation, retain the heat against loss to the surrounding environment and transmit the heat as efficiently as possible to the hot water storage system.

The collector ideally should face due south, although the annual energy collection will only vary by a maximum of 10 per cent when the surface is anywhere between 30º to the south east or south west.

There are two main types of collector:

Flat plates

Flat plate collectors have three components:

  • a transparent collector cover; this needs to have a high transmission of visible and near infra - red radiation, to capture the maximum amount of solar radiation, and a minimum transmission of infra-red radiation, to minimise heat radiation back from the absorber to the atmosphere
  • an absorber plate from which heat can be removed by a heat transfer fluid
  • substantial insulation to minimise heat loss back into the atmosphere.

Evacuated tubes

These are glass tubes evacuated similarily to thermos flasks, to reduce the heat losses from convection and thermal conduction. The absorber can be a thin strip of coated metal running the length of the tube or a coating on the outer surface of an inner glass tube. In this case, a reflector is necessary to make use of the absorber area away from the sun. These reflectors are frequently used as concentrators and the entire unit of tubes and reflectors is referred to as a Compound Parabolic Concentrator (CPC).

Evacuated tube systems are more expensive per square metre of collector area than flat plate systems. However, they would be expected to perform better in cold, changeable and windy conditions. This is particularly the case when the system operates at higher temperatures. Typically, a dairy plant cleaning system requires water at 85ºC, whereas domestic heating systems use water at 60ºC.

Heat transfer system

This consists of insulated pipe-work containing the heat transfer fluid, a water pump, a heat exchange system, measurement equipment and a control system. The heat transfer fluid has a high proportion of antifreeze for frost protection.

Hot water storage

The water heated by the solar system during the day needs to be stored for later use. The storage vessel can contain the electric water heater where the pre-heated water has the temperature raised further, as necessary.

Is there a range of possible systems for farm use?

The most common system is a pressurised indirect primary circuit. In this system the hot water from the solar collectors is pumped through a heat exchanger in the boiler to pre-heat the water. It is a sealed system i.e. the fluid in the system does not come in direct contact with the hot water supply.

Lower cost options are available and include thermo-siphon systems. In these, the collector is integrated with the storage tank in a prefabricated unit and the heat transfer fluid circulates without pumping, due to the differing densities of hot and cold water.

What is the annual output of a system?

Systems are designed to deliver a high proportion of the requirements in the summer months, make a significant contribution in the spring and autumn and some input during the winter. The system is designed to give the best economic balance between the initial set-up cost and the year round output. Well designed systems normally provide no more than 50 per cent of the annual hot water requirements.

Annual distribution pattern of solar radiation in kWh per m2.

What about running costs and maintenance?

Solar heating systems have a long life with low maintenance.

A recent survey by the DTI (UK) of 700 systems installed over the past 30 years, showed that the majority of the systems had no significant problems and work reliably for 20 - 25 years or more.

A study carried out by Action Renewables in 2007 reported that it took £12 per year to run the pump for a domestic system. This will be greater for a farm scale system but overall will not be a significant cost.

What is the payback time on the investment?

Paybacks times on domestic systems are around ten years.  This is a modest return on capital, however, there are a number of factors that will affect the payback period:

  • A farm system used 365 days a year will be more efficient than a domestic system
  • A farm business can get tax write-off on the initial investment
  • Changes in electricity prices will alter the payback time

Is planning necessary?

The Carbon Trust recommends that a business should contact their planning authority for advice before proceeding with a solar installation.

Should I install solar?

For any process requiring energy, the first step is to cut costs by improving the efficiency of the existing system. Use of solar should only be considered after achieving the maximum benefit from energy efficiency measures, such as the timing of use, insulation and monitoring and control systems.

Providing hot plant wash water for the Greenmount dairy unit prior to the solar system being installed required 48 kWh per day. During the summer months of 2010, the average daily electricity usage was:

Month Average daily electricity usage (kWh)
May 6.5
June 7.2
July 11.4
August 10.7
September 12.9

This was a daily average use of 9.7 kWh, just 20 per cent of that prior to the solar system being installed.

Checklist for solar installations

  • orientation of collectors to the sun - between south-east and south-west.
  • Angle of the collectors - between 30º and 40º to the horizontal
  • no shading
  • space for a hot water storage tank
  • area of collectors - one sq metre will heat 45 litres to 60ºC

For further information, contact David Trimble by telephone on 028 9442 6682, or by email to

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