Different types of collectors
The collectors are one of the most important components of a solar system. Their main role is to capture the energy which the sun radiates to heat water for use. The collectors are generally mounted on a roof, but can also be mounted on walls. The optimal position for a collector is at an incline of 35ºc to horizontal and facing south, as shown below:
As seen a house facing south gains the most exposure annually, however, collectors can be mounted on both the east and west facing roof space if your house does not face South, which is a duo-pitched roof system. Collector sizing must be considered in all applications and over sizing may be necessary where optimal levels of irradiation are not expected.
There are three main types of solar collector;
Evacuated Tubes
Evacuated tube collectors have two categories:
- Direct flow evacuation tubes
- Heat pipe evacuation tubes
Both of these collectors are similar in appearance, but work in different ways. Both contain evacuated glass tubes which are mounted on a frame and connected by a manifold. Heat pipe tubes can only be installed vertically, but direct flow tubes are more flexible and can be installed both horizontally and vertically. Due to vacuum within the tube as well as other design factors, the Evacuated Tube collector tends to run at an efficiency some 10-15% greater than the flat plate collector.
Flat Plate Collectors
These collectors are based on a thin absorber sheet, which is unusually copper, backed by a fluid tubing system all placed in an insulated casing with a glass covering. Fluid is then circulated through the tubing which absorbs heat from the collector and transports it to the heat exchanger. The insulated casing ensures minimal heat loss from the collector, which can reach efficiencies of 75- 80%. The transparent cover protects against weather damage and prevents heat from being carried away in the wind.
Unglazed Plastic Collectors
As these collectors are unglazed then absorb a high amount of the suns energy, however, they are not insulated so a high portion of the heat is lost, particularly on windy days. They are primarily used for heating swimming pools, where the pool water circulates directly through them.
Both the evacuated tube and flat plate collectors are used for Active Solar Heating systems for houses and other building types. The unglazed plastic collector is exclusively used for outdoor purposes, such as for heating swimming pools where lower temperatures are required.
Evacuated tube and flat plate collectors used for heating systems should have selective surfaces, which are good absorbers of the sun’s radiation, absorbing 90-95% of the solar radiation falling on them, but are also poor radiators meaning they lose less than 15% of the radiation falling on them. Non selective surface collectors absorb a similar 90-95% of the sun’s energy but are much more effective radiators, releasing up to 90% of the maximum heat efficiency.
There is not one optimal size for a collector, as it depends on a number of factors, such as:
- Estimated hot water usage
- The percentage of solar contribution to the annual hot water load.
- Geographical location
- Inclination of roof
- Type of collector.
For domestic solar hot water systems it has been found that installing ¾ of a m² of flat plate collector or evacuated tubes for each person using hot water will give satisfactory results. Heat loss through the system will be larger the smaller the collector it is. This generally results in the average 3/4 bed household installing a collector of 3-4m² of flat plate collectors and 2-3m² of evacuated tubes, with a hot water cylinder volume of between 25-75 litres per m² of collector area. To meet he domestic hot water requirement, one needs roughly 1m² of collector area per person living in the property
In the UK there are manufacturers and suppliers of all types of solar collector. The products available from the members of the Solar Trade Association (STA) are all high quality and well proven.