Flat Plate Collector
Solar Flat Plate Collectors for Solar Hot Water
A Flat Plate Collector is a heat exchanger that converts the radiant solar energy from the sun into heat energy using the well known greenhouse effect. It collects, or captures, solar energy and uses that energy to heat water in the home for bathing, washing and heating, and can even be used to heat outdoor swimming pools and hot tubs.
For most residential and small commercial hot water applications, the solar flat plate collector tends to be more cost effective due to their simple design, low cost, and relatively easier installation compared to other forms of hot water heating systems. Also, solar flat plate collectors are more than capable of delivering the necessary quantity of hot water at the required temperature.
A solar flat plate collector typically consists of a large heat absorbing plate, usually a large sheet of copper or aluminium as they are both good conductors of heat, which is painted or chemically etched black to absorb as much solar radiation as possible for maximum efficiency.
This blackened heat absorbing surface has several parallel copper pipes or tubes called risers, running length ways across the plate which contain the heat transfer fluid, typically water.
These copper pipes are bonded, soldered or brazed directly to the absorber plate to ensure maximum surface contact and heat transfer. Sunlight heats the absorbing surface which increases in temperature. As the plate gets hotter this heat is conducted through the risers and absorbed by the fluid flowing inside the copper pipes which is then used by the household.
The pipes and absorber plate are enclosed in an insulated metal or wooden box with a sheet of glazing material, either glass or plastic on the front to protect the enclosed absorber plate and create an insulating air space. This glazing material does not absorb the suns thermal energy to any significant extent and therefore most of the incoming radiation is received by the blackened absorber.
The air gap between the plate and glazing material traps this heat preventing it from escaping back into the atmosphere. As the absorber plate warms up, it transfers heat to the fluid within the collector but it also loses heat to its surroundings. To minimize this loss of heat, the bottom and sides of a flat plate collector are insulated with high temperature rigid foam or aluminium foil insulation as shown.
Typical Flat Plate Collector
Flat plate collectors can heat the fluid inside using either direct or indirect sunlight from a wide range of different angles. They also function in diffused light, which is dominant on cloudy days as it is the surrounding heat that is being absorbed and not the light, unlike photovoltaic cells. How hot the circulating water gets will depend mostly on the time of the year, how clear the skies are and how slowly the water flows through the collectors pipes.
Direct and Indirect Solar Thermal Systems
There are several different ways to heat water for use in the home. Solar water heating systems that use flat plate solar collectors to capture the suns energy can be classed as either direct or indirect systems by the way in which they transfer the heat around the system. In order to heat your water successfully and use it during both the day and the night, you will need to have both a solar collector to capture the heat and transfer it to the water and also a hot water tank to store this hot water for use as needed.
Direct Solar Thermal System
A direct solar water heating system, also known as an active open-loop system, uses a pump to circulate the water around the system. The cooler water is pumped directly from the home to a central water storage or immersion tank and passes through the solar collector for heating. The hot water leaves the flat plate collector and returns back to the tank flowing in a continuous loop. From there, the water is pumped back into the house as hot usable water.
A low voltage 12 volt pump can be used which can be powered by a small photovoltaic cell or electronic controller making the system more green. Direct systems are usually used in warmer climates with few cold days or is drained in winter to stop the water in the pipes from freezing. Chemicals can not be added to the water for protection as the same water that circulates through the flat plate collector is used in the home.
In a passive direct hot water system, the system does not use pumps or control mechanisms to transfer the heat created to the storage tank. Instead, passive systems are what are called “open-loop systems” which use the natural force of gravity to help circulate the water around the system. This type of system uses a solar flat plate collector combined with a horizontally mounted storage tank of some kind located immediately above the collector.
The water heated by the sun rises naturally using convection through the solar collectors pipes and enters the storage tank situated above. As the heated water enters the storage tank above, the cooler water is forced out and flows down to the bottom of the collectors aided by gravity as cold water is more dense than hot water. This cycle of hot water rising and cooler water falling is known as a “thermosyphon flow” and continuously repeats unaided while the sun is shining.
Thermosyphon Hot Water System
The thermosyphon system is the most common type of solar heated hot water system on the market and most commercially available passive direct solar hot water systems use this type of roof-mounted flat plate collector and storage tank combination.
However, care must be taken when installing such a system as the combined weight of the solar collector, storage tank and the water itself my be too much for the design of the supporting roof.
When passive solar hot water systems are used for bigger buildings than for houses, businesses or offices, there is often more than one storage tank for the heated water.
So called remote thermosyphon system works on the same principle as the previous passive direct thermosyphon system, except that the storage tank is located away within the roof space or void dissipating the weight over a larger area and also protecting the storage tank from cold weather and temperatures. However, for the thermosyphoning process to work correctly, the base of the water storage tank must be situated at least 1 to 2 feet (300 to 500mm) above the top of the flat plate collectors. This dist