20 Questions And Answer On Domestic Hot Water Solar Systems – part 1 Of 4

1. Why tempered glass 4 mm with low content of iron oxide is used in the production of collectors?

The 4 mm ticked, tempered glass is much stronger than the normal one. That eliminates the possibility of any eventual break ups. In addition, in rare cases when there is a break up, the glass split on many small pieces, which are not harmful or dangerous (like the car glasses).

The tempered glass with iron oxide has better penetration coefficient for the solar radiation in comparison with the normal glass. In this way the technical characteristics of the collectors are improved.

It is noticed that the green color of the normal glass (seen from aside) is from the iron oxide in it.

2. What does the selective coating of the absorbers give?

The main components of the solar collector are the frame, insulation, transparent cover and absorber. The absorber is a part inside the collector, which consist of black absorbing disk (gills) and copper tubes, which are very strong connected to the gills.

When the solar collector is exposed to sun, the solar radiation which falls on the collector, penetrate through the glass and it is absorbed from the black disk if the gill. The latter warms up. After that the heat exchange liquid (water, or water and antifreeze, etc.), which is in the absorbing tubes also warms up and lead the energy to the water into the reservoir.

When there is sunshine and the solar collector works, the black absorbing disk (gills) is hot. With each black hot surface, the absorber emits radiation into the environment. This means heat energy waste from the front transparent coating of the solar collector and therefore the effectiveness is decreased.

The selective coating on the absorber lessens such wastes and improves the effectiveness of the collector. The above mentioned coating absorbs the solar radiation the same way as the black coating does.

The solar collectors with selective coating for the absorber function more effectively in areas with low sun radiation (Northern Europe). They are the better choice for heating water with temperature under 60-70 В°C, usually used in different industries.

The selective coating is laid on the absorber surface through chemical process (that is not paint). The price of the absorber with selective coating is higher than the one of absorber with black paint.

3. Why there is two types heat insulation in the collectors (rock wool and polyurethane)?

The solar collectors are part of the solar systems for domestic hot water. When the water in the system has not been used (during vacations for example), the water in the reservoirs gets really hot. The temperature of the absorber during the day is higher than the one of the water into the reservoir. On these circumstances, the absorbers with black paint can reach temperatures at around 150 В°C (In the absorber with selective coating the temperature can reach 200 В°C). The temperature (the stand still temperature), which the collector can reach is important element when an appropriate insulation is chosen.

Using polyurethane and rock wool is considered to be the best combination for insulation.

- The polyurethane is a strong insulation material and suggests thin, smooth insulation. It is not influenced from moisture, it is light. The temperature that it can stand depends on the type, but in any case is below 150 В°C.

- The rock wool is an insulation material which stands high temperatures. It is laid between the absorber and the polyurethane. The purpose is to decrease the temperature on which the polyurethane is exposed.

The result of this combination is that there is good thermal insulation. The latter improves the effectiveness of the collector and prolongs its life expectancy.

4. Why copper is used for the solar collector’s absorber?

The absorber can be made of different materials. The type sandwich absorbers are made from steel sheets. In other cases we see copper or steel tubes with gills, made from copper, aluminum or steel.

The copper is more diathermanous than the other mentioned above. Therefore little copper is needed for the production of one absorber. That will lead to less weight of the absorber and lower price.

The copper provides long life expectancy of the product, because it resists the corrosion of the hot water (copper tubes) and other corrosive substance (gills).

Steel Type sandwich collector is heavy. That can cause problems during the transportation. Also decreases the thermal effectiveness of the solar system for domestic hot water.

This is because the way the solar system works. In the morning the solar radiation heats the absorber at first and the water in the reservoir begins to warm up. At the end of the day, the mass of the absorber cools and the solar radiation emit and waste in the environment. Therefore when the mass of the absorber is large, we need more solar radiation to heat, and at the end the waste are bigger. Bigger mass – bigger waste.

The same process occurs during days with sun and clouds.

5. Why polyurethane is used for insulation of the hot water reservoirs?

Polyurethane is strong insulation material and it gives smooth, thin insulation. It is not influenced by moisture, it is light and hard. The ordinary types of polyurethane can stand high temperature on the outer surface of the hot water reservoir.

Lots of the reservoirs have double jacket, which works as heat exchanger. The temperature on the outer surface of the reservoir in this case can be more than 150 В°C (when the solar system works).

All manufactures use polyurethane for the insulation of the hot water reservoir. Its thickness vary between 50 mm and 70 mm. The first is considered to be the normal one. The round shape of the reservoir is suitable for constructing the needed polyurethane forms on comparative low prices.

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