Solar Water Heating Systems

Understanding Solar Water Heating Systems

When it comes to domestic solar water heating systems, the saying that there is more than one way to skin a cat is particularly relevant.

They are all designed to heat water using the sun, but they go about reaching that objective in a many different ways. Factors such as your geographic location, weather, roof layout, water quality and budget all determine which one will be best for your specific situation.

Types of Solar Heating Systems

To better understand the different types, they can be divided into categories according to these factors...

  • The manner in which the water is heated either direct (open loop), indirect (closed loop) or heat pump.
  • The way in which the water or heat transfer fluid is circulated, known as active or passive.
  • The solar collector used to heat the water namely Batch (Integrated Collector Storage), Flat Plate or Evacuated Tube.

Direct Systems

In direct solar water heating systems, also known as open loop, the water is heated directly by the sun as it moves through the collector and back into the storage tank.

This exposes the water to low temperatures so direct systems are only suitable for locations where freezing temperatures or frost don't occur (higher than 4° C / 39.2° F) as frozen water will expand and damage the collector piping.

Due to their relative simplicity these solar water heater systems are reliable and efficient but they require maintenance to keep the pipes clear of mineral deposits (scaling).

Indirect Systems

Indirect solar hot water systems, also known as closed loop, make use of a heat transfer fluid such as glycol, freon or distilled water that is heated by the sun as it moves through the collector.

This fluid then flows through a heat exchanger located in the storage tank, indirectly heating the water up.

The advantage of using a heat transfer fluid is that it's freeze resistant so indirect solar thermal systems operate well in areas prone to frost and freezing but the fluid may evaporate over time so it needs to be topped up.

Heat Pump Water Heater

A heat pump transfers energy from ambient air heated by the sun into a storage tank containing water to raise its temperature. It uses electricity very efficiently as the power source so it can work at night too.

Find out which are the top three hot water heat pumps in various areas of the world, ranked by efficiency results.

Active Systems

This category is defined by water heaters that make use of pumps and controllers to circulate the water or heat transfer fluid. They are more efficient than passive ones but they need more equipment and maintenance.

They tend to be more expensive but can work in cold climates and they rely on mains or photovoltaic electricity to power the circulation pump and controller.

A good example of an active configuration is the drain back solar hot water heating system which provides guaranteed freeze protection by allowing the water or heat tranfer fluid to drain out of the collector when the pump turns off.

Passive Solar Hot Water Systems

These water heaters don't have pumps to circulate the heating fluid but make use of natural processes such as gravity and the natural circulation of water as it heats up.

Cheaper than their active counterparts, they also tend to be more reliable and last longer but they don't work on every roof design and freezing weather can prevent the circulation effect.

The passive Thermosiphon system works when the sun heats water in the collector causing it to become lighter than the cooler water in the elevated storage tank. Gravity then pulls the heavier, colder water down into the collector inlet and forces the warmer water back into the storage tank. This process repeats itself until the sun sets.

Batch (ICS) Collector

Unique amongst solar water heating systems, the tank acts as both the collector and the water storage receptacle at the same time.

Flat Plate Collector

Typically takes the form of a network of pipes contained within a dark, flat box that has a transparent top made of a tough material to withstand the elements.

Evacuated Tube Collector

Parallel rows of clear glass tubes that contain a vacuum which helps prevent heat from being lost back into the environment. Very popular for use in solar water heating systems in cold climates.

Top Three List

With so many solar heater designs around it's a very competetive market so here's a list of the top three solar water heating systems to help you cut through the confusion.