Solar collectors part 2.

Solar heat
Solar heat

Solar Collectors

by Rich Schroeher

There are basically two types of collectors, flat plate and evacuated tube.

Within flat plate collectors most are usually glazed with a copper absorber plate. There are some collectors that are unglazed plastic collectors designed to heat swimming pools that have been repackaged to heat domestic hot water. Although the unglazed collectors can under the right conditions be more efficient than glazed collectors this happens in the very low temperature ranges of output.

Applications such as heating pools are a perfect example. They are usually used in conditions that are above freezing and are capable of heating large quantities of water to relative low temperatures ( under 90 degrees). However during freezing conditions the efficiency falls off dramatically as the absorber is exposed to the ambient conditions. These systems work well in tropical locations such as southern California, Mexico, Florida etc where the outdoor temperatures rarely fall below 50 degrees. In northern climates they become ineffective during the winter months as the exposed absorber has to be warmer than the water in the storage tank to start and transfer heat. Systems such as these would be appropriate in a northern climate for a summer home application or summer camp where the system is only used during the warm months.

The typical flat plate collector for northern climates is a glazed collector. Typically in an insulated box with a glass cover. Most manufacturers use a copper absorber plate and tubes due to copper’s efficiency in transferring heat. These are then coated with a selective coating to maximize the heat absorbed by the collector. There is little difference in the coatings as far as efficiency goes, most manufacturers are within one or two percent in the efficiency of the coatings. The thing to judge when making your decision is the size of the absorber. Collectors come in various sizes and the amount of heat you will be able to capture is directly proportional to the area of the absorber plate exposed to the sun. for most applications the more the merrier up to a point.

Another type of collector is evacuated tube collectors. These are absorber tubes housed within a vacuum bottle. Because they are within a high vacuum there is little heat that can re radiate out of the collector. They work by absorbing heat at the absorber plate and heating a few drops of water within a tube. As this water turns to steam it rises and is transferred to a fluid within a manifold and pumped to heat the tank. These types of collectors are typically used in applications that have limited space for collectors or an application where high temperatures are needed such as in solar air conditioning. The advantages to these types of collectors is they will produce a lot of BTU’s with a relatively small surface area. The disadvantages are they can overheat in summer months if there is not enough water being used such as if you are on vacation. There should be a way built into the system to dissipate heat to protect the system from overheating. Typically called a heat dump it is basically a radiator to vent excess heat to the outdoors. Flat plats on the other hand do not reach as high a temperature and can be circulated at night effectively cooling the tank. Another disadvantage is in areas where there is a high amount of snow or frost the tubes are so well insulated that the snow will not melt off. Flat plates on the other hand once some of the glass is exposed will radiate some heat to melt the snow or frost.



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