The purpose of a solar collector is to transform the solar radiation into heat. This solar heat can be used for heating water or air. There are three main types of solar collectors that are used for domestic applications. All these types of solar collectors are key components of active solar heating systems.
- Flat-plate or flat panel solar collector
Flat panel collector is a weatherproof insulated box. This box contains a dark absorber plate under one or more glass or translucent polymer covers. These collectors are very common choice for domestic purposes.
- Evacuated tube collectors
Evacuated tube collectors have parallel rows of transparent glass tubes. Each tube has a metal absorber tube inside the outer glass tube. These collectors are used more commonly for bigger applications.
- Concentrated collector-storage systems
Integral collector or Concentrated Collector storage system feature black tank or tubes in an insulated box. When cold water passes through this solar collector system, it heats the water.
The Flat-plate or flat panel collectors are most commonly used solar collectors for domestic space heating and water heating systems. A flat panel collector is basically an insulated metal box which has a glass or plastic/Polymer cover called the glazing and a dark absorber plate. The glazing can be transparent or translucent.
The glazing allows the light to pass through but reduces the amount of heat that can escape. Since the bottom and the sides of the collector are usually insulated, it further minimizes the heat loss. The absorber plate of the flat panels is usually black because dark colors help absorb more solar energy as compared to the light colors.
Absorber plate absorbs the solar radiation and transfers the heat to a fluid that circulates in tubes through the collector. Generally metals such as copper or aluminum are used for absorber plates because they both are good heat conductors. Copper is a better conductor but more expensive, it is also less prone to corrosion in compare to aluminum.
There are two types of Flat panel or flat plate collectors: liquid and air collectors.
- Liquid Collectors In a liquid collector, solar energy heats a liquid as it flows through tubes. The circulating fluid in flat panel collectors heats the fluid to a temperature which is less than the boiling point of water. These types of collectors are very useful for the applications requiring heat throughout the winter months. The flow tubes can be routed in a parallel or a serpentine pattern.
A serpentine pattern helps eliminate the possibility of leaks and also ensures a uniform flow. But this serpentine pattern will not be appropriate for the systems that require drainage for freeze protection. The curved passages of serpentine pattern will not allow draining it completely. Liquid flat panel collectors may be glazed or unglazed.
Glazed liquid collectors are the most common type of solar collectors. Glazed collectors are used for providing domestic and commercial hot water and also for heating indoor swimming pools. Unglazed collectors are commonly used to heat outdoor pools… when both the outdoor temperature and solar radiation level is high.
The most common type of Liquid flat panel collectors uses potable household water as liquid or fluid. This fluid is heated when it passes directly through the collector. This heated water then flows to the domestic use like bathing, laundry, etc.
The design of these types of flat panel collectors is called a “Direct” or “Openloop” system. In the low or freezing temperature locations the liquid collectors require freeze protection…the collectors use an antifreeze type of heat-transfer fluid. In the systems using fluids, the fluid absorbs the heat from the collector and then it passes through a heat exchanger. Generally this heat exchanger exists in the water storage tank which transfers the heat to the water. These types of designs are called “Indirect” or “Closed loop” systems.
- Air Collectors
In an air based Flat panel collector, there is no circulating fluid but air. These Air based collectors are ideally used for space heating. The absorber plates in the air collectors are made of metal or non metallic materials.
The air flows thru the absorber by the help of a fan or thru natural convection, heating the air. In some Air collectors heating systems, fins or corrugations on the absorber are used to improve the heat transfer. But this can increase the cost of operations since lot more power would be needed to power the fans.
Overall, air systems are economical since they use less expensive materials such as plastic glazing…due to low operating temperature than that of liquid collectors. Flat panel collectors can be mounted in different ways like mounting the system on a roof or free-standing.… depending upon the size of collector, type of building and application.
- Evacuated Tube Collectors
Evacuated Tube Collectors are the collectors with the highest efficiency. These collectors consist of evacuated glass tubes placed in parallel rows. Inside each tube, one more glass tube is located, which is enclosed in a strongly absorbing material.
As the tubes are evacuated, there is vacuum inside these tubes, there is no space for both convection and radiation, therefore absorbed heat has little means of escape. As air transfers heat from the hot surface of the tube to the cooler surface of the glass and hence resulted in heat loss by convection.
By eliminating the air in Evacuated Tube Collectors, you have eliminated convective heat loss. Because of this the temperature within the tube itself can arrive at extreme values, with temperatures of 170 °F to 350 °F frequently achieved.
To curtail radiant heat loss, the tube is covered with a selective surface. Evacuated tube collectors are best suited for high temperature applications and also for low temperature applications too, like domestic water and space heating. Evacuated Tube Collectors are very efficient and are showing very good results but the problem with these collectors is this that they are very costly.
Because of this they are well-matched to commercial and industrial heating applications and also for cooling applications (by regenerating refrigeration cycles). In the places where it is often cloudy, they can also be an effective alternative to flat-plate collectors. Evacuated-tube collectors have two main groups.