Solar energy for the farm
Solar Water Heating on the Farm
David Trimble, Renewable Energy Technologist, CAFRE
In the UK solar systems are primarily used for heating water rather than for generating electricity. They have 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 percent 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.
1. What is a Solar Hot Water system?
A solar water hot water system has three main components:
a. 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 percent when the surface is anywhere between 30o to the south east or south west.
There are two main types of collector:
i. Flat plates
Flat plate collectors have three components:
- (i) Atransparentcollectorcover.Thisneedstohaveahightransmissionofvisibleandnear 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.
- (ii) An absorber plate from which heat can be removed by a heat transfer fluid.
- (iii) Substantial insulation to minimise heat loss back into the atmosphere.
Cross section of a plate collector showing the pipe work attached to the absorber plate
ii. Evacuated tubes
These are glass tubes evacuated similarly 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).
An array of evacuated tubes
The operation of a parabolic concentrator in a CPC system
Evacuated tube of the type in the Greenmount dairy unit
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.
b. A 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.
c. 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.
2. Are 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.
3. 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 percent of the annual hot water requirements.
Annual distribution pattern of solar radiation in kWh per m2
4. 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.
Action Renewables in a study in 2007 reported that it took £12 a 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.
5. What is the payback time on the investment?
Payback periods on domestic systems are around twelve years. This is a modest return on capital, however there are a number of factors that will shorten the payback period:
- A farm system used 365 days a year will be more efficient than a domestic system
- Increasing energy prices
- Grant assistance such as the Farm Modernisation Grant
Tax write-off on the investment
6. Is Planning necessary?
The Carbon Trust recommends that a business should contact their planning authority for advice before proceeding with a solar installation.
7. 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:
(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 percent of that prior to the solar system being installed.
8. Checklist for solar installations
- Orientation of collectors to the sun - between south-east and south-west.
- Angle of collectors to the horizontal - between 40¡ and 45¡ to the horizontal
- No shading
- Space for a hot water storage tank
- Area of collectors - 1 sq meter will heat 45 litres to 60¡C
For further information contact David Trimble at (028) 9442 6682 or david.trimble@dardni.gov.uk
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