Solar Heat Basics

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 New Mexico, 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 50%. Valverde Energy sizes domestic hot water systems to provide 75% plus of annual domestic hot water needs.

The size of the solar collectors suitable for your site will depend upon your hot water usage, but typically they will require between 32 and 64 square feet of mounting area. Collectors are usually mounted on the roof. Alternatives, such as ground mounting, or rack mounting on a wall 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 domestic needs, although space heating will generally require a much greater collector area. Additional controls and heat exchangers are also needed. New Mexico is a great environment for solar space heating because of our solar resource.

Through Valverde Energy’s years of experience in installing solar systems we 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. This is evidenced through historical data. Some customers will see a reduction of energy usage of as much as 90% in a very well insulated home with high energy consciousness, others may only see a 30% offset of traditional fuels in a poorly insulated structure. Remember that in New Mexico we have well over 300 solar days, so the object is to keep the energy gained daily in the home or business overnight.

Solar heaters are often the most economical way to heat a swimming pool. Compared to conventional pool heaters using propane or electric, solar pool heating systems can pay for themselves in four years or less. A solar pool 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 degrees Fahrenheit. Systems used for this type of solar pool heating are less expensive than those used for a year round pool heating system. A year round pool heating system incorporates heat exchanges and freeze protection and are often used for indoor pool applications.

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. Valverde Energy can help you determine the applicability of solar for your site.

This may sound contradictory, but in larger commercial applications solar cooling utilizes solar collectors connected to an absorption chiller as manufactured by Yasaki. Yasaki chillers are very reliable and have one moving part and have a long history of reliable operation (over 25 years). Valverde Energy staff has been certified and trained by Yasaki to install this type of 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 New Mexico.

Drain-back 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. Drain-back systems rely on full drainage of collectors to assure freeze-protection. These types of systems in New Mexico are very challenging to install and maintain due to the variations in architecture and extreme weather conditions. 

Open loop systems heat and circulate household (potable) water directly in collectors prior to distribution in the household. This is the lower cost system but cannot be used in New Mexico climates due to freezing. Another type of open loop system is a batch heater that is simply a black tank filled with water and placed inside a south-facing, insulated, glazed box, or in the same envelope of the house where the batch heater absorbs solar energy. The tank may incorporate a selective surface that that absorbs sun well but inhibits radiant loss. In climates where freezing occurs, batch heaters must either be protected from freezing or drained for the winter. Batch heaters are inexpensive and have few components; therefore they require less maintenance and experience fewer failures. Open loop systems are good economical choices for seasonal applications such as summer cabins or in applications where they are designed into the building thus preventing any freeze or water damage issues.

The most commonly used collector is the flat-plate collector. It 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 from the absorber plate returns the heated fluid to where the heat is needed. Typically, metal collectors are more sturdy, fire resistant, and expensive to manufacture and ship than plastic collectors. Valverde Energy recommends the higher quality collectors due to a much lower life cycle cost.

Another type of collector used is evacuated tube collectors. This type of collector is often touted as a more efficient collector but this is generally not true in home applications (see collector output ratings done by the SRCC). For the same square foot of roof space flat plate collectors are more efficient, more reliable and cheaper until higher operating temperatures (above 180F) are needed for process heat or solar cooling systems often found in commercial solar applications.

Most solar hot water systems require a well-insulated storage tank. Some systems use converted water heater tanks for storage or plumb the solar storage tank in series with a conventional water heater. Valverde Energy uses a solar storage tank that can use a built in back up system as this type of system will maximize the solar resource available and uses the least space. A separate solar tank fully maximizes the solar resource but does take another footprint next to the existing water heater.

More information to come

A differential temperature controller monitors the temperatures at both 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 a water-glycol mixture, through the collectors and back to the heat exchanger located in or adjacent to the storage tank. More complicated controls are used for integration into a space heating system. Valverde Energy uses ThermSun solar systems for these applications because of the repeatability and simplicity of the design. This is critical for low cost of ownership of the solar energy asset and enhances easy (set and forget) customer operation.

A heat exchanger transfers heat from the heat-transfer 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. Valverde Energy uses submerged heat exchangers for domestic hot water systems as this systems uses less moving parts and is not as susceptible to our hard water conditions in New Mexico. Space heating systems are not as exposed to hard water conditions if at all, depending on what type of heating system Valverde Energy is integrating the solar system to.