March, 2008 Archive

Solar Collectors

Solar Collector is the term generally used to describe a device that converts the suns radiation into something more useful, such as heat or electricity. This article concentrates on the solar collectors designed for converting the suns energy into heat which in turn heats your domestic and commercial hot water. Also for this article we are going to ignore flat plate solar panels which you can read about in my other posts. If you are initially researching solar panels you should start by reading the earlier posts in this blog about solar water heating panels.

There are two types of solar collector commonly used in the UK.

- Direct Flow Solar Collectors
- Heat Pipe Solar Collectors

Both systems usually utilise an evacuated tube. This works in a similar way to your double glazing at home. It forms a vacuum barrier that stops heat transferring away from the collector and it stops the cold air cooling the heated liquid down. While this barrier is an effective heat insulator it allows most of the suns radiation to pass through into the absorber surface of the collector.

The difference between direct flow and heat pipe collectors comes when we look at how the heat is transfered from the collector to your hot water cylinder.

Direct Flow Solar Collector

These systems have a loop of pipe that runs through the evacuated tubes. The water that heats your cylinder comes into direct contact with the absorbing surface in each evacuated tube. As this water is heated by the sun it is pumped around the collector tubes and eventually passes through the secondary coil in your hot water cylinder. This secondary coil heats your hot water cylinder.

Heat Pipe Solar Collector

In this system the water does not pass through the evacuated tube, it runs in a manifold at the top of the panel. The evacuated tube contains a rod with a liquid that evaporates at a low temperature. As the liquid evaporates from the suns energy the heat rises to the top of the tube. The top of the pipe connects to a heat exchanger that heats the water in the manifold. The water in the manifold is then pumped through a coil in your storage tank and heats your hot water.

Direct Flow vs Heat Pipe Solar Collectors

Direct Flow is a very simple system that works in a similar fashion to a conventional flat plate collector but with the benefit of vacuum insulation. Some maintenance difficulties may arise if there is a problem with one of the tubes. Removing a single tube can be more demanding as the water pipes are running directly through the tube. However some of the new systems do allow the tube to be slid over the water pipes and removed fairly easily.
Heat pipes on the other hand can be easily disconnected from the manifold and replaced or maintained without fear of damaging water pipes. Heat pipe systems do tend to cost a little more than direct flow systems. Another disadvantage of heat pipes is that they rely on gravity to move the heat to the top of the pipe. This means that they always have to be installed at an angle and cannot be positioned flat against a flat roof. Putting aside the practical installation and maintenance issues the heat pipe collector will function better at lower levels of solar radiation. This is because the heat transfer fluid conducts heat much fast than the directly flowing water system.

For further information contact Intelligent Energy Solutions.