An Active, or direct pump system circulates tap water through the Solar collectors(heating system). These systems are called open loop and are simple, efficient and reliable. A pump (3'- 12') circulates water stored in the tank from the lowest (coldest) part of the tank to the solar collectors and then back to the tank.
In alternating current (AC) (regular electricity supply), the pump is switched on and off by a temperature sensitive controller that constantly monitors the tank and collector temperatures and turns the pump on when the water temperature in the collectors is 8 to 10 degrees Fare height hotter than the water temperature in the tank. When this temperature difference falls between 2 to 4 degrees Fare height the controller turns the pump off providing complete automatic operation.
In direct current DC systems the power supply is usually provided by Photovoltaic cells. This type of system does not normally need a differential controller once the pump, PV module and collectors are properly sized. If there is enough sunlight for the PY module to generate DC current then the pump will operate and circulate the water. If on the other hand there is not enough sunlight for the PV module to generate sufficient power then the pump will not operate.
Periodic maintenance is required if hard water with high mineral content is being heated. This maintenance removes the build up of lime scale that chokes the collector and tank recirculating pipes over a period of time. The use of a lime removing dispenser such as those supplied by sunpower - where phosphate crystals are slowly dissolved into the water ~ will eliminate the need for periodic maintenance to remove lime scale build up, providing the phosphate crystals are replenished when required.
This system does not require you to have the tanks on the roof as a pump is utalised to circulate the water between the tank and the collectors. The tank may be located hidden away for a more aesthetically pleasing appearance, depending on the roof design, may be completely hidden from view. The tank wish is the most vulnerable part of the system can be located indoors out of sight, protected from the elements, which leads to a longer tank service life. During hurricanes or strong winds the indoor tank will not be a threat as a flying object.
One should remember that pumps whether AC or DC require maintenance and replacement from time to time additionally the controller and PV module also require replacement after a period of time.
Passive Solar water heaters are the most reliable of all solar heaters, as they do not require maintenance of mechanical pumps. The circulation of water between the tank and the solar collectors is done by thermal convection. These systems are known as thermo siphon units and require that the tank be located higher than the collectors for the thermal convection to function.
When the sun warms the water in the collectors it changes density and becomes lighter than the colder water in the bottom of the tank. This change in density causes the water heated in the collectors to float to in the top of the tank, while being replaced by colder heavier water that falls to the bottom, where it is heated. This heating process continues during daylight hours.
At night there is no circulation since there is no sunlight to beat the water and make the thermo siphoning work. The hot water is therefore stored in the tank until required.
Periodic maintenance is required if hard water with high mineral content is being heated. This maintenance removes the build up of lime scale that chokes the collector and tank recirculating pipes over a period of time. The use of a lime removing dispenser such as those supplied by sunpower - where phosphate crystals are slowly dissolved into the water ~ will eliminate the need for periodic maintenance to remove lime scale build up, providing the phosphate crystals are replenished when required.
Passive waterheaters do not contain moving parts therefore maintenance on pumps and controls is eliminated. This also means the system is less expensive as there is less to install.
In deciding which system is appropriate, you should consider that in passive systems:
1. The tank needs to be higher than the collectors are. This usually means it must be located on the roof higher than the collectors. Some property owners and architects prefer not to have the tanks on the roof for aesthetics Additionally, the tank on the roof means the weakest part of the system is subject to the elements, which leads to a shorter tank life.
In making your choice between active and passive systems one should consider that the heaviest part of the system is the tank full of water. Modifications may have to be made to the roof to support the heavy weight of the tank.
Finally, in Caribbean countries, hurricanes are a reality and tanks on the roof may become unsecured during a particularly strong storm.