Solar Heating Panels

Sometimes confused with solar panels (photovoltaic cells), solar heating panels are primarily designed to heat water and not to produce electricity. The basic workings are very simple in that a panel is set to face the sun. The suns rays strike the panel and water filled tubes gather the heat, warming up the water as it passes through. The heated water is then transferred to an insulated hot water tank to be used when required.

The solar panel is normally mounted on the roof of a house (but could be built on the ground) facing in a southerly direction so as to gain as much of the passing sunshine as possible. Pipes pass from the solar panel to a hot water storage tank where it is held until required.

Your existing hot water system will have a bearing on the cost of installing solar. Combi boilers are not always compatible with solar panels but this is something you would have to take up with your solar panel installer. Ease of gaining access to your existing system will also have a cost bearing. Depending on how much work is required to connect your solar panel to your plumbing could mean a lot of extra work if it is hard to access.

It is difficult to give an estimation of how much of your hot water a solar panel may generate. Where you live and the local weather conditions are the biggest factor. If you happen to live in the middle of the Sahara it is fair to say your problem is more to do with obtaining the water than enough heat. Still, you should generally expect to get at least half to all of your hot water requirements from a well installed solar heating panel. And do not overlook the possibilities of heating the home too. Currently, most solar heating systems look to provide hot water but if the conditions are available, why not investigate heating the home. Used in conjunction with an under floor heating system would make for a very effective way to keep your house comfortable.  It would require larger storage of heated water and that may be cost prohibitive. This comes down a bit to what is the true cost of green energy saving and that can be found elsewhere on this website. When you realise we spend over half of our home energy budget on heating the water and house, anything that can help reduce it is going to have a sizeable effect on not just environmental issues but a saving in our pocket.

There are two principal systems that solar heating panels use, flat plate and evacuated tube.

Flat Plate Solar Heating Panels

The flat plate panel works on a very simple premise. The panel is a slim box, usually no more than 50mm to 100mm thick (2" to 4"). The top, or front of the panel is glass or Perspex and faces the sun. The underside, or base is, painted black with tubes mounted on it going up and down the panel. Water is pumped through the tubes, entering as cold water. As the water passes through the tube which is also painted black, sunlight entering through the glass panel comes into contact with the black surfaces which is then turned into heat. This heat  then transfers through the wall of the pipes to heat up the flowing water. By the time the water exits the panel it has warmed up where on it is pumped to a storage tank for later use.

Flat panels are reasonably efficient in taking heat from the sun and turning it into hot water but the design does allow for a lot of losses. The biggest problem is that the panel itself leaks out a lot of the heat. If the weather is sunny but the temperature is cold, the cold will bring down the heat within the panel and thus lower the amount of hot water being generated. This makes this kind of panel less useful in colder climates. 

Where it does score is in warmer climates. It is cheap to construct and with the addition of a solar photovoltaic panel added to drive a water pump, can produce plenty of hot water for little cost. The materials used in flat panels can also be much cheaper. The heat build up within the panel is normally within the limits to allow the use of plastic tubes rather than copper. As copper is becoming such an expensive commodity at this time,  plastic tubing can make a significant saving.

Overall, the flat panel is an affordable and easy to construct device which is simple to install but really only suited to certain climates.

Evacuated Tube Solar Heating Panels

How Evacuated tubes work is slightly more complex design. They still work on the basic principle of the sun shining on them but they take the available heat from the suns rays and convert it to heat without the losses that occur in a flat panel. This more complex design makes them more costly but they are currently coming down in price all the time. A good evacuated tube panel claims to capture over 90% of the available energy which compared to the 40% or so from a flat panel is quite impressive. Efficiency figures should be treated with a degree of scepticism as most are provided by those selling the product. Still, it is recognised that evacuated tubes are far better than flat panels.

Evacuated tubes are made of glass and though there are a few variations, they work on the same principle as a thermos flask (with the outer plastic or metal wrapping removed). There are two glass tubes, one slipping inside the other with a small space between the two of them. This space is sealed at each end of the tube and the air is sucked out of that space to create a vacuum. As the space is now a vacuum, heat can not transfer across it and thus acts as an insulator.

A U-shaped copper tube is then fed into one end of the tube which is about the same length of the glass tube. The 'U' sits at the lower end of the glass tube resulting in two ends of the copper tube sticking out of the upper end of the glass tube. These two ends serve as an inlet for cold water and an outlet for hot water. The end of the tube where the copper pipes stick out is then sealed.

The inner glass tube of our 'thermos' is coated with a paint like material. This coating contains elements such as aluminium that allows the heat from sunrays to pass into the inner portion of the glass tube, where the copper pipe is situated, but not let the heat back out. There is then a build up of heat in this inner part and the copper tube has water pumped through it. The water heats up and is then piped away to a storage tank. It is not that a complex system but the engineering required is quite involved. The seals where the copper pipe enters and exits can deteriorate in time but we are talking years if not decades before such seals fail. With the continuous advance of material technology, these seals should continue to improve.

The interesting thing with evacuated tubes is the build up of heat in the inner portion. Temperatures of over 200C can be reached in the warmest part of an evacuated tube system on a very sunny day.

The SPF website is a Swiss based research and  evaluating organisation that carry out independent testing of solar panel products and publish those results for free. Click Here to go to their website though be aware it does take a bit of figuring out how to use it. Still, persevere as it has a wealth of good info on all things solar.