Direct-flow evacuated-tube collectors contains a group of glass tubes and in the interior of each glass tube there is a flat or curved aluminum fin connected to a metal (generally copper) or glass absorber pipe. The aluminum fin is covered with some coating which helps in increasing its absorption of solar radiation however reduces its radiative heat loss.
The heat transfer fluid is water which circulates all the way through the pipes, one for inlet fluid and the other for outlet fluid.
There are several types of Direct-flow evacuated tube collectors which are distinguished by the arrangement of these pipes.
Glass-Metal Type: These tubes consist of a single glass tube. Within this tube there is a copper heat pipe or water flow pipe with attached fin.
These tubes are very efficient but can have problems relating to loss of vacuum. As the rate of expansion is different for the glass and metal tubes, it may result in weakening of the seal between them.
This in turn will result in the loss of vacuum. With the loss of a vacuum, the efficiency of an evacuated-tube collector will suffer a lot Glass-Glass Type: In this type the two glass tubes are merged together and the inner tube has the coating of some selective surface which increases its tendency to absorb solar energy well and reduces the radiative heat loss.
Vacuum is created in the space between the two glass tubes, and as both the tubes are made of glass, the problem of breaking of seal is greatly reduced and hence there is no air and hence there is no loss of vacuum. Therefore glass-glass tubes are much more reliable than glass-metal tubes.
Although the Glass-glass tubes are not usually as efficient as glass-metal tubes or in other words output is better in case of glass-metal tubes but are cheaper and have longer life.
- Heat pipe evacuated-tube collectors
Heat pipe evacuated-tube collectors works on the principle of a heat transfer mechanism. In this large amount of heat with a very little difference in temperature are moved between the hot and cold interfaces. In a heat pipe evacuated-tube collector, at the hot interface a fluid turns to vapor and the gas flows and
condenses on the cold interface. The liquid drops back to the hot interface to evaporate again and the cycle is repeated. Heat pipe evacuated tube collectors consist of a sealed metal tube with the metal having high thermal conductivity usually copper tube. This tube is linked with a black copper absorber plate and is kept within a vacuum-sealed solar tube. The heat pipe is hollow and the vacuum is created in the space inside. Within the heat pipe there is some fluid like alcohol or purified water or a mixture of water and alcohol along with special additives in little quantity. When solar light falls on the surface of the absorber, the liquid inside turns in to vapor. Because of the absence of air liquid boils at much lower temperature, as there is no heat loss.
As the heat pipe evacuated-tube collectors have no mechanical moving parts, therefore they require very low maintenance.
This type of solar collector makes use of direct beam radiation. Reflective surfaces are used to focus sunlight onto a small area, where it is absorbed and transformed to heat and in some cases electricity. Working of concentrator collector can be easily explained by making use of magnifying glass to burn paper. Take a magnifying glass and then focus the glass until the sunlight comes to a very fine point, you will find that the paper will start burning. This is how concentrating collectors work. But in concentrator collector mirrors, polished metal, or even shiny plastic are used to focus the rays of the sun onto a comparatively smaller area to produce heat. In this case in place of concentrating sunlight to burn paper, they use the concentrated sunlight to heat some other thing. It may be air, water, or a liquid salt solution.
Concentrators can enormously boost the power flux of sunlight. Concentrator collectors are usually used for high-temperature applications like generating electricity via steam production and thermal detoxification. Concentrator collectors show their best performance while pointed directly at the sun.
For getting the direct access to sunlight, these systems make use of tracking mechanisms to move the collectors. Single-axis trackers track the sun by moving east to west whereas dual-axis trackers follow the sun all through the year and move east to west and north to south. There are passive trackers as well which use Freon to provide the movement. Passive trackers present a low-maintenance substitute to mechanical systems.
Concentrator collectors being very expensive and require frequent maintenance, are used typically in commercial applications for electrical utilities.
For residential purpose parabolic-trough concentrating systems are used. In this, parabolic trough concentrates the sun’s energy on an absorber tube that contains a heat-transfer fluid which transports the heat to the water tank or to the generator. In the majority of residential systems single-axis trackers are used as they are less expensive and simpler than dual-axis trackers. These set ups can provide hot water, space heating, and water purification. Concentrating collectors are ideal for the climates having high percentage of direct sunlight and few clouds.
That’s why large concentrations of these types of collectors are more common in hot arid locations. Advantages of Concentrator collectors • Value for money package and is cheaper than flat panels or evacuated tubes
- If there is clear sky, it absorbs maximum energy with its dual axis tracking system
- It requires less space and collects more energy per square foot
- Accumulate energy at peak efficiency during the entire day • Faster payback in comparison to other solar technologies
- As this is installed on its own post therefore there is no interference with roof maintenance Limitations of Concentrator collectors: Concentrators are extremely sensitive to weather. Since it can merely focus direct solar radiation, because of this their performance is poor on hazy or cloudy days.
A decrease in incident solar energy will result in remarkable decreases in the system’s yield. On the whole, if there is no direct sunshine implies next to no power. Concentrators are ideal for the areas of high insulation, for example the areas close to the equator and in the deserts. The backup system: Backup system is required to have an uninterrupted hot water supply, even in the cloudy days.
A usual backup system is a normal water heater which makes use of mains electricity or gas to heat the water. In this system the tank have two coils through which the heat transfer fluid from the solar collector is
dispersed to heat the water. In proper sunny days, the water leaving the tank will be at or above the temperature required for household hot water so in this case the conventional water heater will not turn on.
On the other hand, when the water leaving the tank is not up to the desired temperature, the conventional heater will turn on to bring the water up to the desired temperature.