Solar Water Heaters

Active Systems

Open Loop and Closed Loop Systems

Solar Water Heaters—also called solar domestic hot water or solar thermal systems—can be a cost-effective way to generate hot water for your home. They can be used in any climate and the fuel, sunshine, is free!

Unlike the Passive systems which have no moving parts, the Active systems use electric pumps, valves, and controllers to circulate water or other heat-transfer fluids through the collectors. They can be more expensive than passive systems but can also be more efficient in certain situations. Active systems are usually easier to retrofit than passive systems because their storage tanks do not need to be installed above or close to the collectors. But because they use electricity, they will not function in a power outage. There are two types of Active Systems: Open-Loop (Direct) and Closed-Loop (Indirect).

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Open-Loop Active Systems - Direct Circulation

Open Loop Systems are suitable for single application domestic hot water (DHW) systems, in mild and moderate climates where there is no risk of freezing.

Pumps circulate household water through the collectors and into the home. They work well in climates where it rarely freezes.

Recirculation systems are a specific type of open-loop system that provide freeze protection. They use the system pump to circulate warm water from storage tanks through collectors and exposed piping when temperatures approach freezing.

Open Loop Systems:
Helio-Flo
Alternate Energy Technologies
SunEarth

Advantages of Open Loop

It is the simplest and typically the least expensive active system to install. There is no heat exchanger, which allows efficient heat transfer directly to the water. The system operates at standard line pressure. It is simple to add capacity to the system if demand changes. The system integrates easily with existing systems.

Disadvantages Open Loop

Some stored heat can be lost when the system re-irculates. It does, however recirculate from the colder bottom of the tank and supply pipes, and there are minimal losses in the loop and none through the collector. The primary freeze protection needs electricity or battery back-up.

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Closed-Loop Active Systems - Indirect Circulation

Suitable for single and multiple solar heating application systems, e.g. domestic solar water heating (single), solar water heating hot tub, solar swimming pool heating or solar space heating systems. Suitable for areas with questionable water quality and all climate conditions. Preferred option for extremely cold areas.

Pumps circulate a non-freezing, heat-transfer fluid through the collectors and a heat exchanger. They are popular in climates prone to freezing temperatures.

These systems pump heat-transfer fluids (usually a glycol-water antifreeze mixture) through collectors. Heat exchangers transfer the heat from the fluid to the household water stored in the tanks. Closed-loop glycol systems offer good freeze protection. However, the glycol must be checked each year and changed every 3 to 10 years, depending on glycol quality and system temperatures.

Drainback systems use water as the heat-transfer fluid in the collector loop. A pump circulates the water through the collectors. The water drains by gravity to the storage tank and heat exchanger; there are no valves to fail. When the pumps are off, the collectors are empty, which assures freeze protection and also allows the system to turn off if the water in the storage tank becomes too hot.

Closed Loop Systems:
Helio-Pak - Flat Plate Collector
TrendSetter - Evacuated Tube Collector

Advantages of a Closed Loop

Closed Loop system allows simple switching between heat exchangers for multiple applications, as well as excellent freeze protection. Special solar domestic hot water tanks are available with internal heat exchangers (coils), with or without electric backup. Twin coil tanks are also available that allow surplus solar energy to be directed to spa, pool, floor etc. The top coil can also be connected to the house boiler as a DHW backup. No freeze sensors or drain down valves are necessary.

Disadvantages of a Closed Loop

It is generally more complicated than an open loop system, because either a tank with a heat exchange coil or an external heat exchanger is required. As a heat exchanger is required the collector loop will run at slightly higher temperatures than an open loop system. The collector loop needs to be pressurized. (8 -12 psi) The antifreeze may need to be recharged on a 3 to 5 year basis.

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Collector Types

Both Closed-Loop and Open-Loop systems may use Flat Plate Collectors or Evacuated Tube Collectors:

Flat-plate collector
Glazed flat-plate collectors are insulated, weatherproofed boxes that contain a dark absorber plate under one or more glass or plastic (polymer) covers. Unglazed flat-plate collectors—typically used for solar pool heating—have a dark absorber plate, made of metal or polymer, without a cover or enclosure.

Gobi Solar Collector
Alternate Energy Technologies
Sun Earth Solar Collector

Evacuated-tube solar collectors
They feature parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin's coating absorbs solar energy but inhibits radiative heat loss. These collectors are used more frequently for commercial applications.

Apricus Collector

Which collector is right for me?

The desired temperature range of the material to be heated is the most important factor in choosing the correct type of collector. An uncovered absorber is certainly not suitable for producing process heat. The amount of radiation on that spot, exposure to storms, and the amount of space must all be carefully considered when planning a solar array.

A good collector alone does not guarantee a good solar system. All components should be of high quality and similar capacity and strength. Please contact our solar consultants for advice on the best system for your particular needs.