Domestic Hot Water
Solar hot water systems are most commonly used to heat water for basic household needs such as laundry, bathing, dishwashing and cooking. These systems are commonly referred to as "domestic hot water systems".
Domestic hot water systems typically use solar energy to pre-heat the water that is incoming to a conventionally fueled heating tank. The warmer the water from the solar heater, the less conventional fuel will be needed to provide the household's hot water needs. During the summer months in the Midwest, a properly sized solar hot water system will provide almost 100% of a household's needs. In the winter, or during extended cloudy periods, the amount of hot water provided by the sun may be 30% or less. Midwest Wind and Solar's hot water professionals tend to install systems that meet between 60% and 70% of the annual load.
The size of the solar collectors suitable for your site will depend upon the manufacturer and your hot water usage, but typically they will require between 50 and 100 square feet of mounting area. The most common mounting technique used today is install the collectors flush with the roof on a south facing exposure. Alternatives, such as ground mounting, or rack mounting on gable ends are also sometimes possible.
Solar water heaters can also be used to provide space heating. The same set of solar collectors can be used to provide hot water for both space heating and space heating needs, although space heating will generally require a much greater collector area and storage capacity. Additional controls and heat exchangers are also needed. Due to these extra costs, and because sunshine is relatively scarce when heating loads are highest (for example at night and during the winter) solar energy is more often used to heat domestic water than it is for space heating.
Midwest Wind and Solar's past experience in installing systems sized to provide space heat can help you consider the pros and cons of various options. You should also make sure to carefully consider passive solar and other building efficiency measures that will reduce your heating loads to help you take maximum advantage of the available solar resource.
Solar heaters are often the most economical way to heat a swimming pool. Compared to conventional pool heaters using propane or oil, solar pool heating systems can pay for themselves in four years or less. If you currently don't heat your pool, a solar heating system can provide an economical way to extend your pool season, starting earlier in the spring and extending later into the fall.
Solar pool heaters work by circulating pool water directly through collectors and then rerouting the warmed water to the pool. System controllers sense when collectors are warmer than the pool water, and open valves diverting water from the pool circulator through the collectors and then back into the pool. The controller can be set to automatically keep the pool temperature anywhere between 65 and 100 degree Fahrenheit. The collectors used for pool heating systems are often less expensive than those used for domestic hot water systems, providing significant economic advantages.
Commercial facilities with high hot water demands and access to a good southern exposure can be great candidates for solar hot water. Restaurants, bakeries, beauty salons, health clubs, and hotels are all potentially good sites. A commercial installation generally makes use of the same system design and components as residential systems, including a conventional back-up for hot water heating during high load and low sun periods. Midwest Wind and Solar's hot water professionals can help you determine the applicability of solar for your site.
Solar Hot Water System Types
Closed Loop - Glycol System
Closed loop systems use a heat-transfer fluid to collect heat and a heat exchanger to transfer the heat to household water. Active closed loop systems use electric pumps, valves, and controllers to circulate the heat-transfer fluid, usually a glycol-water antifreeze mixture, through the collectors. This glycol-water antifreeze mixture makes closed-loop glycol systems effective in areas subject to freezing weather. For this reason, closed loop systems are preferred for year round use in the Midwest region.
Closed Loop - Drainback System
Drainback systems use water as the heat-transfer fluid within the collector loop. The water is forced through the collectors by a pump and then is drained by gravity to the storage tank and heat exchanger. These systems have no valves to fail and when the pumps are off, the collectors are empty, thereby assuring freeze-protection and auto shut-off if the water in the storage tank becomes too hot. A drainback system is not suggested in areas prone to freezing tempratures.
Solar hot water systems are made up of collectors, storage tanks, piping, controls, and in some cases pumps. Active systems use pumps to circulate water or other heat transfer fluid through the system. Passive systems have no pumps and rely on gravity or natural convection to circulate water depending on the system.
There are two types of collectors; the flat-plate and the evacuated tube collector. The flat plate collector is an insulated, weatherproofed box, made of metal or plastic, containing a dark absorber plate beneath a translucent cover (typically tempered, low-iron glass). Copper piping carrying heat exchange fluid travels in an S-shaped pattern between the absorber plate and translucent cover. A evacuated tube collector.
Most solar hot water systems require a well-insulated storage tank. We will plumb the solar storage tank in series with a conventional water heater therefore pre-heating the water before it gets to your domestic hot water heater.
A differential temperature controller monitors the temperatures at the solar collector outlet and at the storage tank. When the collectors are warmer than the tank, the control turns on a circulator which circulates a heat-transfer fluid, usually water or a water-glycol mixture, through the collectors and back to the heat exchanger located in or adjacent to the storage tank.
A heat exchanger transfers heat from the heat-transfer fluid (usually either water or a water-glycol antifreeze mixture) to the household water supply. The heat exchanger itself is a series of copper coils submerged in the water storage tank or a compartment outside of the storage tank.
How a Home Solar Water Heating System Works
A home solar water heating system is a very simple and maintenance free way to immediately reduce your monthly energy costs. Both evacuated tube and flat plate solar hot water systems work in a similar manner.
In most residential solar hot water systems that use evacuated tubes, cold water flows into the bottom of the solar storage tank (1).
The solar loop’s heat transfer fluid (usually a water & glycol mixture) is pumped up to the solar collector (2).
Inside the solar collector, it is heated with the sun’s energy (3). The evacuated tubes are very efficient at trapping heat from the sun and transferring it into this fluid.
As the fluid moves through the solar evacuated tube collector and heated up, it is then pumped back down inside the heat exchanger in the solar tank, heating the water inside the solar tank (4).
This is just one of the more common designs used in a home solar hot water system. Other designs are available, and can be used depending on your specific application.