Solar hot water panels: what they are and how they work:
Solar hot water panels are also called solar collectors. They are where the sun’s heat is collected for the solar hot water system. They are usually painted with either a selective coating, or matt black. This is in order to absorb as much heat as possible. Light usually enters through glass. This reduces heat loss.
The glazed container works like a greenhouse. The glass lets in the light, and prevents most of the heat from escaping. The black collectors can heat the water to higher temperatures in the hotter environment inside the glazed chamber than they would be able to without it. Solar collectors are essential to a solar hot water system. You can’t have a solar hot water system without them.
Types of solar hot water panels:
There are three primary types of solar hot water panels: batch, flat plate, and evacuated tube.
In batch solar collectors the water tank is inside the solar collector. The tank is either painted black or with a selective coating, while the solar collector is deep, with reflective material on the back and a glass panel on the front.
Flat plate solar collectors are the most common type of solar collector. They are defined by including a heat-absorbing solid plate of material. Fluid flows across this and soaks up the heat. A glass plate in front of the heat absorbing plate and fluid acts to trap heat.
Evacuated tube is the most mechanically complicated of the three main collector types. It is therefore more expensive. It is also the most efficient at producing hot water, which is why they are used. They are especially good for heating water to the higher temperatures needed in industrial applications.This high efficiency means that it is possible for the systems to overheat if they are not used.
Evacuated tube collectors consist of a series of glass tubes containing either one or two pipes. These pipes contain the fluid that is being heated. Inside the glass tube but outside the pipe is a vacuum. There are two types of evacuated tube solar collectors. They are direct-flow evacuated tube collectors and indirect-flow evacuated tube collectors.
Direct flow evacuated tubes contain two pipes. One pipe contains the liquid flowing into each tube, and the other pipe carries the liquid out again. The drawback of a direct flow evacuated tube is that if one of the tubes and the pipes within it break, the system stops working. It is also difficult to replace the tubes.
With indirect-flow evacuated tube collectors, there is one pipe per tube. If a collector breaks, you only lose the heat from the broken collector. The rest of the tubes keep working and you continue to get hot water, if somewhat less hot than before the tube broke.
Integrated tank solar collectors are a bit of a different topic, as the collectors are the same as the evacuated tube collectors described above, but the water tank is placed directly above them and integrated with the collectors. This reduces the complexity and expense of the solar hot water system. Unfortunately, it doesn’t work well in below-freezing temperatures without special modifications.
Solar pool heaters are often made of black rubber with water inside them, without glass. This makes them cheaper, and since the water doesn’t need to be as warm as domestic hot water the fact that it can’t heat the water to high temperatures isn’t a problem.
Pros and cons of each type of collector:
Pro: easiest collector to make
low cost of total system
Con: long time taken to heat water in the morning
batch systems are not frost-resistant
heavy collectors make them harder to roof-mount
Pro: less expensive than evacuated tube
Con: loses more heat to environment than evacuated tube. This is mainly important in cold climates and in commercial applications.
More expensive than batch
Pro: most efficient at absorbing heat
best variety for cloudy climates
water reaches highest temperatures
best variety for industrial applications
Con: most expensive collector type
direct flow systems are hard to repair if tube is broken and one break renders system unuseable
can overheat if hot water isn’t used