The Collectors are the Panels and they convert the suns light into heat. There are two main types of collectors on the market today:
a. (Vacuum) Flat Plate Collectors:
Sizes of flat plate collectors range between 1meter squared and 12 meters squared. They can weigh between 22kg/m2 and 32kg/m2. A Flat Plat panel is made up of an Absorber, which is a heat conducting metal sheet (made of copper or aluminium) and is coated in a dark coloured “paint”. This “paint” is actually a “spectral-selective coating” i.e. it is a specially designed paint that has a layered structure which optimises the conversion of short wave solar radiation into heat while keeping the thermal radiation as low as possible. The heat transfer medium (usually a water/glycol mixture) is then pumped through small tubes that are fixed to the Absorber. When the sun hits the Absorber it transfers the heat to the transfer medium, which is then heated and passed back to the Thermal Store. On top of the absorber is a glass cover to protect the Absorber. This glass is specifically chosen to be low reflective and have a high transmittance, thereby achieving the greenhouse effect within the panel itself (i.e. it allows the heat in and then traps it, in much the same way the Greenhouse effect works for the planet). Finally, the back and the edges of the panel are heat insulated so as to reduce thermal loss from the panel. This insulation, along with all other parts of the panel, is equipped to work even at very high temperatures as when the panels are idle temperatures of the panel can reach 150-200 degrees Celsius.
The average annual efficiency of a flat plate collector is around 40% meaning that a 1 meter squared collector will generate around 400kWh of energy per annum (in the UK each meter squared area receives around 1,000kWh of radiation per annum).
An example of a Flat Plate installation where the panel is built into the roof:
Advantages of Flat Plate Collectors:
- They are cheaper than Evacuated Tube Collectors
- It offers multiple roof fixing options such as built into the roof
- Aesthetically it is viewed as more pleasing than evacuated tubes
- It has a very good price/performance ratio
Disadvantages of Flat Plate Collectors:
- They are less efficient than Evacuated Tube Collectors
- It is hot suitable for generating higher temperatures
- It requires more roof space than Evacuated Tube Collectors
b. Evacuated Tube Collectors
An absorber is run through the centre of a glass tube as a flat metal strip. The tubes are evacuated in order to suppress thermal losses through convection. Even with an absorber temperature of 120 degrees Celsius, the outer glass can remain cold to touch. In this manner, the Greenhouse effect used in Flat Plate Collectors is increased, thereby decreasing the thermal losses. There are two types of evacuated tube collectors.
1. Direct Flow-Through:
The heat transfer medium is fed through the centre of the glass tube in a tube-in-tube system, alongside the Absorber. This is then heated and fed through to the base of the panel where it flows to the Thermal Store.
2. Heat Pipe:
In this system a Heat Pipe is inserted into an Evacuated tube (i.e. a tube inside a tube). The Heat Pipe is the Absorber and is filled with a liquid of either water or alcohol. This liquid boils at temperatures as low as 25 degrees Celsius, and as it boils it turns into a Vapour. This vapour rises upwards and into a heat exchanger to transfer the heat to the heat transfer medium as it flows through the top of the panel. The vapour then turns into condensation as it cools and flows back down to the bottom of the heat pipe to start again. For the panels to work they must be tilted at least at an angle of 25 degrees.
The average annual efficiency of an Evacuated Tube collector is around 50% meaning that a 1 meter squared collector will generate around 500kWh of energy per annum (in the UK each meter squared area receives around 1,000kWh of radiation per annum).
Advantages of Evacuated Tube Collectors:
- It is more efficient as there is no loss of energy from the vacuum tube
- It works better all year round
- It works better in low radiation
- It achieves high output temperatures
- Each tube can be replaced individually if they break
Disadvantages of Evacuated Tube Collectors:
- They are more expensive
- Can only be mounted on roof systems
Home Pages of interest:
- Visit the Solar Thermal home page
- Visit the Energie Solar home page
- Visit the Solar Thermal and Swimming Pools home page
Other Pages of interest in this section:
- What is Solar Heat Power?
- How Does Solar Thermal Work?
- Types of Solar Thermal Systems
- Flat Plate versus Evacuated Tube Collectors
- Annual Energy Production
- Components of a Solar Thermal System
- Costs of a Solar Thermal System
- Servicing and Warranty
- Solar Thermal Financials and Paybacks
- Our Products
- Solar Thermal and Swimming Pools
