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How Solar Hot Water Systems Work

How a solar hot water system works

A solar hot water system works by using the suns energy to heat the water in your hot water cylinder. Typically a system will provide you with up to 75% of your hot water needs. Gas or electricity is used to generate the last 25% of your hot water requirements.

The great advantages of choosing a solar hot water system to reduce your electricity consumption is that these systems have the ability to store solar energy. Over a period of 24 hours very little heat is lost from a well-insulated cylinder so you don't need to use the water straight away when it is created.

There are two types of solar collectors for hot water systems. There are evacuated tube collectors and flat plate collectors.

Flat Plate Collectorsflat plate collectorFlat plate collector

These collectors are thin water proofed boxes that have a glass cover with a dark absorber plate underneath. The absorber plate collects the suns solar energy in the form of heat. The heat is then transferred to pipes which run through the absorber plate. The heat is then passed onto the water or heating fluid that flows through the pipes.

The positives and negatives of flat plate collectors:

  • They are high solar gain collectors.
  • They are known to be very cost effective.
  • The perform better in the summer.
  • They do not perform well in colder climates, so they are better suited to areas of upper North Island.
  • They are often not suitable in environments with high corrosion (e.g. close to the beach).

Evacuated Tube Collectorsevacuated tube collectorEvacuated Tubes

These collectors are a series of transparent glass tubes, each one containing a metal absorber tube attached to a metal fin. The fin absorbs the solar energy and inhibits heat loss. The fin heats the water or heat transfer fluid in the manifold at the top of the tubes.

The positives and negatives of evacuated tube collectors:

  • These systems are suitable in all areas of New Zealand.
  • They are known to perform better in overcast weather conditions than other types of collectors.
  • They can be used in areas with high corrosion.
  • They have a lower heat loss.

There are also two main types of hot water systems. Open-Loop and closed-loop systems.

Closed-loop system

A closed-loop solar water heating system works by circulating a heat-conducting fluid from the solar collector to the hot water cylinder. The heated fluid from the collector goes through a heat exchange system that transfers the heat to the water in the cylinder. The fluid is then re-circulated back to the solar collector.

Both flat plate and evacuated tube collectors can use this closed loop system.


Open-loop systemopen loop solar hot water system

An open-loop system works by circulating the household's water to the collector on the roof, where it is heated before it goes back to the cylinder inside the house.

A controller is a device that is used with both types of systems. It manages the use of a boost system, which is either gas or electric. The controller is set with a minimum temperature, so whenever the water in the cylinder goes below a certain temperature it will automatically turn the boost system on. The controller can also be set to turn on the boost system at certain times of the day, like when there is a high use of hot water.
A hot water cylinder to match

Retro fitting a solar system with a standard cylinder is very common and can be very economical. But to maximize the performance of the solar hot water system it can be best to match the collector with an optimal size cylinder, which tends to be larger than conventional cylinders and slightly different in design.

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