Heating systems: types of solar water heaters

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Types of Solar Water Heaters

This page involves types of Domestic Solar Hot Water Systems.

For other systems, see:
Solar Electricity: Photovoltaic PV Systems
Solar Space Heating basics
Solar Space Heating in radiators, radiant floor and air forced systems
Solar Cooling

Passive and active solar water heaters

Solar water heating systems may have circulation pumps or not. If the system uses a pump to transfer the heat to the storage tank then it is called a active system.

If the water (or the anti-freeze liquid) in these systems circulates without the aid of pumps or controls, the system is called a passive system.

Active and Indirect Solar Water Heater
Image Credit: EERE

Thermosyphon systems

Passive systems – those without pumps - use thermodynamic forces as the thermosyphon, based in cold water heavier nature and higher density (relatively to cold water).

Thermosyphon systems are very popular in hot and sunny temperate climates, but they are rarely used in countries as the United States, except in sunnier states as California or Florida.

Direct Solar water heating systems

Direct solar water heating systems (also known as “open loop” system) do not use antifreeze solutions: the hot water is directly heated into the collector. The system circulates pure, potable water (which is commonly stored in a hot water tank)

Due to the fact that they do not use anti-freeze, these system shouldn’t be used in cold climates, namely in its simplest configuration

Batch or bulk storage system

These solar water heater systems are a good example of direct (they do not use anti-freezing solutions) and also passive systems (they do not use pumps).

They are very simple systems combining the solar panel and the storage into one single unit (hence its technical designation: ICS – Integrated Collector Storage system).

Since this type of system do not use anti-freeze solutions and is very simple in its essence, it should only be used in hot or temperate-hot climates. To avoid strong heat loses during the night, the hot water generated at the collectors level should be consumed quickly or transferred into a storage tank before evening.

Passive Solar Water Heater
Image Credit: EERE

Indirect solar water heaters

Indirect (or ‘closed loop’) solar water heaters use an antifreeze solution and a pump to move it through the solar collector and out of it. After being heated, the antifreeze solution is driven into a heat-transfer unit where it warms the water… (the antifreeze solution is continually flowing into and out of the solar collector without ever mixing with the house’s water).

This system is the most appropriate in climates with extended periods of below-zero temperatures. Most of the U.S., except parts of southern states, need indirect systems.

Which system to choose?

If you need freeze protection or if the water in your region is an hard type one, choose an active closed loop system with antifreeze and, obviously, a heat exchanger.

In other climates, you may choose either a simple system (as the thermosyphon one) or, in case of larger needs, a open loop direct system with an heat pump and a storage tank.

Solar water heater and non sunny days

What about the days or periods when there isn’t enough solar energy to heat the water up to the needed temperatures?

The solution in this case is in an auxiliary heating system to solve the solar water heater insufficiencies during worst weather-days. Auxiliary electricity system are easy to implement, while standard gas water heater may be more difficult: they will demand a second solar storage tank:

Note:
For a full environmental solution, you may want to implement an auxiliary PV (photovoltaic solar electric) system. It would be a simple and cost-effective application of solar electric energy.