Solar
Collectors
Comparing
Solar Collectors
Every day vast amounts of energy are send to the Earth from the Sun in the form of electro-magnetic radiation. The best known of this radiation is visible light. We call this energy Insolation. Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter (W/m2 ). The average energy available in South Africa is 80 kWh/m2 per day.
Solar Collectors are designed and built with the following 4 principles in mind:
- Heat Absorption: Any object left in the sun will heat up, but dark surfaces absorb more heat – thus all collectors are dark in colour. Special coatings increase absorption.
- Heat Transfer: Heat is lost during transfer from one component to the next. The tighter the connection , the lower the heat loss
- Heat Radiation: The heat radiation from the collector must be minimized. This is accomplished by special coatings on the glass and collectors
- Insulation: Heat loss must be minimized by insulation the collectors
The two most common types of Solar Water Heating Collectors are Flat Plate collectors and Vacuum Tube collectors.
Flat Plate Collectors represents the conventional technology that has been available commercially for more than 100 years. One of the most important efficiency factors of Flat Plat collectors are the connection between the absorber plate, which collects heat, and the fluid channels which contain heat transfer liquid. The better this connection, the better the heat transfer and higher the overall system efficiency.
Vacuum Tube Collectors represents the latest solar technology. The two basic designs of Vacuum Tubes are Heat Pipe and U Tube. Heat loss is prevented by enclosing the collectors in a vacuum. The better the connection between the heat absorber area and the fluid channel, the higher the efficiency of the collector.
Summary:
Due to single heat transfer process, heating is far more efficient with Vacuum Tube Heat Pipe, as compared to Vacuum U-Tube.
| Heat Pipe | U-Tube | Flat Plate | |
| Absorption | Absorber Plate with selective coating collects Solar Radiation | Coating on Inner Glass collects the Solar Radiation | Absorber Plate with selective coating collects Solar Radiation |
| Heat Transfer Method | As the Absorber Plate heats up, the small amount of liquid contained in the copper tube evaporates. The vapor rises quickly to the element, taking with it the heat collected. At the element, the heat is transferred to the water, the vapor condensates and the process is repeated | As the Absorber on the Internal glass layer heats up, the heat is transferred to the Heat Exchange Fins. The fins then heat up the Fluid Channel. The liquid in the fluid channel evaporates and heat the element. At the element, the heat is transferred to the water via the manifold. | Heat Collected by the Absorber Plate is transferred to the liquid flowing through the fluid channels. Absorber plate to Fluid Channel is one component resulting in high percentage heat transfer to the fluid in the channels |
| Heat Transfer Points | 2 X heat transfer points:
| 4 X heat transfer points:
| 1 X heat transfer point:
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| Insulation: against heat loss | Vacuum prevents the heat collected by the Absorber Plate to escape | Vacuum between two glass layers - reduce heat loss. The internal components are not under vacuum. Majority of heat is retained | Glass cover & Insulated back reduce heat loss |
| Heat Radiation: Prevention |
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Vacuum Tube Collectors
- The radiation from the sun is absorbed by the collector of the Vacuum Tube
- The collector heats the liquid inside the heat pipe
- The hot liquid in the pipe rises to the top where the heat is transferred to the water flowing through the manifold
Because the heat pipe is housed inside a vacuum, no heat can escape from the vacuum tube. It is the same principle as a vacuum coffee flask, where hot water is inside, but the outside stays cold to the touch. Even after hours the hot liquid inside remains warm
There are two kinds of vacuum tube collectors namely Heat pipe and U-Tube collectors. The details of the Home Comfort Vacuum Tube collector is shown in the table below:
