SOLAR HOT WATER pre-heat systems fall into two basic categories — active and passive types — with numerous subcategories.
A passive solar DHW pre-heater is also known as a breadbox heater or batch heater. It is essentially a large (30- to 40-gal.), glass-lined storage tank that is built into a super-insulated, triple-glazed box mounted directly on the roof of the building being served.
The advantage is that it's simple in design with only one moving part, the water.
The disadvantages are that these systems usually require a beefed-up roof structure due to the compounded loading. They are not aesthetically pleasing. They must have supply and return piping protected from freezing when and where the potential exists. And they can create a major water damage claim if not properly installed.
While simple in design, these systems should not be applied in areas where extremely cold temperatures may exist for long periods of time.
As of this writing, numerous plastic film solar panels are being tested for potential applications in equatorial climates. They are essentially 4-by-8-ft. plastic bags, 6- to 12-in. deep that are held in frames laid flat on flat roofs. Because they are thin, they must be operated at a pressure just over that of atmospheric. Their use is intended for Third World countries that have never had the benefit of hot water for sanitation purposes. I felt compelled to include them in the passive category even though they are not readily available for the North American market.
Active DHW pre-heat systems comprise three basic subcategories of mechanical systems: closed-loop, drain-down and drain-back. The five subcategories of solar collectors are: flat plate water, flat plate air, evacuated tube, focusing concentrator and parabolic trough.
Drain-down systems were given the appropriate title of ' freezedown' systems.
Drain-down systems have pretty much left the North American market. Their concept was simple. The water that was kept in the storage tank was also the water that was circulated through the solar collector. It eliminated the need for heat exchangers and glycol needed for the typical closed-loop pressurized system.
The major problem with this design was its potential for being exposed to freezing temperatures at the solar panel. The system was dependent upon a drain-down valve and vacuum breakers to drain the solar collectors in the event that a freezing condition was detected. Unfortunately, either the drain valve or the vacuum breaker would fail and the collectors would not drain, and the collectors and supply and return piping would be destroyed by the freezing conditions. The systems were given the appropriate title of "freeze-down" systems. They should also have been confined to equatorial applications.
Their advantage was their high efficiency, no heat exchanger was required and they had a lower installed cost.
The disadvantage was that they required good pitch to allow water to completely drain out of the system to a drain in the dwelling. And they depended upon mechanical components for freeze protection that usually failed and destroyed the solar collectors.
A closed-loop pressurized system is similar in design to the closed-loop hydronic heating systems that most heating contractors know. The solar panels were mounted on the roof. A heat exchanger and storage system were mounted in the mechanical room. The collector system was charged with a non-toxic antifreeze solution. The system had a minimum of two pumps in most cases, with one pump to move the fluid between the solar collectors and the heat exchanger in the mechanical room, and another to move the thermal storage fluid between the heat exchanger and the storage tank.
Some systems had their heat exchanger immersed in the storage tank and, consequently, required only one pump. The solar loop was generally treated as a closed loop system and required an expansion tank, air separator and pressure-relief valve.
Their advantage was that the collectors could be mounted lower than the heat storage tank. If mounted on the roof, drain back provisions were not required. The pump required to move solar fluid between the collectors and heat exchanger was relatively small due to circulation needs only.
The disadvantage was that the fluid required annual maintenance. If the system saw frequent, long-term exposure to stagnating conditions, the fluid would turn acidic and attack metallic components. If expansion tanks were undersized or had failed, the pressure in the solar loop would exceed the pressure-relief valve setting and the solar fluid would be lost. Without this fluid, no heat was transferred from the
solar collectors to the storage tank. This can cause a lot of free energy to go uncollected.
Tune in next month as we continue to rediscover the most efficient means of heating water known to mankind, solar energy. Until then, Happy Solar-Heated Hydronicing!
Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 303/778-7772.