Solar Thermal

ther-mal [ thur-muh l] adjective
1. pertaining to, or caused by heat or temperature: thermal capacity
solar thermal system on roof

Solar Thermal by definition is anything pertaining to the heating of any mass by means of solar radiation. Typical Solar Thermal applications in a home primarily involve solar water heating. The sun provides an endless stream of energy during the day, which can be captured through various methods and transferred to liquids, using either glycol as an intermediary or heating water directly.

Heating water by way of the sun is an idea that has existed all the way back to the beginning of man kind and recorded history.

Today's solar thermal products exist in three different categories, though only two are relevant to the average California home owner. These categories are broken down by temperature, with low-temperature collectors, medium-temperature collectors, and high-temperature collectors.

For the purposes of this discussion, we'll simply skip over high-temperature collectors, as these are more power-plant scale designs that involve parabolic focusing mirrors to collect tremendous energy from the sun from a very large area, focuses to a very small window.

Low-Temperature Collectors

Low-Temperature Collectors are the typical solar water heaters that are used in conjunction with swimming pools. They tend to be large fabric-like collectors that are rolled out over large portions of the southern and western facing roofs on homes and utilize the pool's existing pump to simply pump the water from the pool up and through these collectors on the roof, gaining the thermal and radiant energy from the sun, then falling back down into the pool, much warmer than it left, thus heating the pool.

These collectors have existed for decades now, and can typically add up to a full month to each end of the swimming season in the valley.

Medium-Temperature Collectors

These collectors have started to gain in popularity recently with the shift in energy efficient building standards and a general improvement in people's conscious decision to make energy efficient upgrades to their homes in hopes of being more "Green".

Medium-Temperature Collectors can be deployed in many styles on the roof, and they vary greatly in individual designs, as well. Some may be either glazed or unglazed glass panels that resemble electric solar panels, some resemble the low-temperature collectors one might use with a swimming pool, while still others can appear almost space-aged with a series of cylindrical vacuum tubes attached by a heat exchanger on the roof.

Another key difference in medium-temperature collectors is that they often use a thermal conductor instead of using the water directly. Solar Thermal systems exist in both open-loop and closed-loop designs.

An open-loop design is one where the water you hope to heat is actually moved through the thermal collector, much like in a low-temperature collector design. This method is adequate when outdoor temperatures do not get so cold that they may cause the plumbing in the collectors to explode from expansion when the liquid freezes.

It is this freeze problem that led to the development of closed-loop systems. Closed-loop systems use a thermal conductor, in most cases a liquid called glycol, due to its tolerance to much colder temperatures before freezing. Glycol systems can withstand outdoor temperatures far below the freezing point of water, without risks of freezing.

Glycol also provides a much more viable heating element. Since glycol has less resistance to energy change than water, it makes for a more ideal heat exchanger. It is able to heat up faster on the roof in the thermal collector than water would, and likewise, can shed its heat much more rapidly in the heat exchanger down in the water storage tank.

There are many different manufacturers out there that provide excellent ways to heat water, whether it be low-temperature collectors for your pool, or medium-temperature collectors for your home's hot water needs. In the end, the only deciding factors would be serviceability, quality of workmanship, aesthetics, and of course, price.

In any event, the amount of reheating of the water is always going to be the ultimate guide to how efficient a solar thermal system will be to your home's current economic system. Since solar thermal systems only work when the sun is out, it is important to have a large enough storage tank so that you have that hot water when you want it, and you take advantage of the energy provided by the sun!

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