Solar Hot Water
The production of solar hot water is one of the most cost-effective solar power house applications; typically reducing conventional water heating cost by about two-thirds. However, a solar heating system can save as much as 85% on hot water cost in some climates, which can be as much as 25% of total home energy costs. All solar applications provide the dual benefit of reducing fossil power expenses and reducing the associated fossil fuel environmental impacts of conventional energy generation. These are essential solar energy facts to understand for decision making.
The two main components of virtually all solar water-heating systems are solar panels which serve as the heat collectors, and a storage tank. A fluid of some type (which is often water) is used to move the heat from the collector to its point of heat transfer, storage or usage. The type of fluid and way it is handled will depend on how the hot water is used and the system employed to transport the fluid and transfer the heat.
There are two types of solar hot water systems; active and passive, but active systems are the most common. The three main home uses of active solar hot water systems are to heat water for consumption, space heating, and heating swimming pools. There are three types of solar panels (collectors) used in solar hot water systems, but the most common is the flat plate collector.
Active solar hot water systems
Active solar hot water systems use electric pumps, and controllers to circulate water (or other fluids) through the collectors. There are two categories of active solar hot water systems:
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Direct-circulation systems which employ pumps to circulate pressurized potable water through the panels (the heat collectors). These systems are suitable only for areas that do not have extended periods of hard freezing.
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Indirect-circulation systems pump a non-potable heat-transfer fluid through collectors. Heat exchangers transfer the heat from the fluid to the potable water. The two most common indirect systems are:
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The antifreeze system which employs a heat transfer fluid that is usually a mixture water and a non-toxic food-grade propylene glycol.
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The drainback system, which is a good choice in colder climates, pumps water through the collectors. The water in the collector drains back into a weather-protected reservoir tank when the pumps stop.
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Passive solar hot water systems
Passive solar hot water systems use thermosyphon and gravity to naturally circulate water as it is heated. Due to the absence of electrical components, passive systems require less maintenance, and have a longer useful life span. The two main types of passive systems are:
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Integral collector storage systems: also known as ICS or “batch” systems, are good for areas that rarely experience freezing, in homes that have significant daytime and evening hot-water needs; but not predominantly morning needs. They have one or more black storage tanks inside a glazed insulated box. Cold water is first preheated in the solar collector tanks and then continues to a conventional water heater.
- Thermosyphon systems: use the natural convection of warm water rising to circulate water from the tank through the collectors as water in the collector heats and rises naturally into the tank above. To conceal it from view, the storage tank is often placed in the house’s attic of the solar power house. These systems are moderately priced, cost effective and reliable. Below is a skematic of a thermosyphon system:
Flat-plate solar hot water panels (collectors)
The solar power home hot water applications of consumption, space heating and pool heating only require temperatures at or below 180°F which is in the heating range of flat-plate panels. Although evacuated-tube collectors can reach temperatures higher than 200°F, their cost is approximately double that of flat-plate panels. Therefore, flat-plat are almost exclusively used in these applications.
As depicted below,a flat-plate collector is typically an insulated box with a glass or transparent plastic cover and a black absorber plate.
