A couple of weekends ago a group of us went to see a very interesting solar thermal installation in Stoke Newington. Generally, solar thermal systems supply heat for hot water only, but this system did a lot more than that as it was also plumbed into the heating system.

I’ll go into more detail on this particular system in a second post later this week, but first up we have a little overview of solar thermal technology…

Solar water heating systems are the most common form of microgeneration (small scale renewable) technology in the UK and Europe. This technology has been around for quite a long time (since the 1970s in this country), quietly generating heat in sunny weather to make a significant contribution to a home’s hot water energy consumption.

Naturally they perform best in the sunniest summer months, when virtually all of your hot water needs can be supplied by a good quality installation. Even during the winter months, a sunny day can result in a substantial amount of hot water being generated.

It is worth noting at this point that hot water energy consumption comprises around a quarter of a typical home’s total energy needs. Space heating uses the lion’s share, making up nearly 60% of total consumption. So a solar thermal system that only supplies hot water won’t cut your total energy consumption drastically, but it does make a good contribution.

Harnessing heat from the sun is, in essence, very simple: a collector is mounted on a south facing roof, absorbing the heat from the sun’s rays. The collector has a network of pipes through which a fluid passes, warms up and then takes the heat to a water-filled cylinder for storage and later use. There are two types of collector: flat plate and evacuated tube.

A flat plate collector is the simpler of the two, consisting of a glass covered heat absorber with pipework passing through it. This type of collector is the cheaper of the two, but it is also less efficient so harnesses less heat for a given area of collector.

Evacuated tube collectors consist of an array of glass tubes with copper strips running along the middle. The glass tubes have had the air removed from them which reduces losses due to convection and conduction and allows them to reach higher temperatures than flat plate collectors. This means that they offer better performance, absorbing more heat for a given area of panel,  but they do cost a bit more.

The fluid passing through the collector is usually water mixed with a form of antifreeze, along with a chemical that reduces corrosion. This fluid is kept completely separate from the hot water that comes out of your taps.

Apart from the collector, you’ll also need somewhere to store all of this lovely heat and a conventional hot water cylinder won’t be enough.

The most common solution is  to fit a twin-coil cylinder which has one coil in the lower part of the cylinder fed from the solar water heating and a second coil in the upper half of the cylinder which is fed from the boiler. The solar thermal system therefore supplies as much heat to the water as it can when the sun is out and then the boiler tops it up with additional heat as necessary.

This need to store heat means that combining solar water heating with an combination boiler can be tricky (although it is possible) because the space for a decent size cylinder will most likely have been given up for other purposes. If you are ever considering removing your hot water cylinder and opting for a combination boiler, consider whether you might want to fit solar thermal at a later date.

As ever, when considering microgeneration technologies make sure that you’ve gone as far as reasonably possible in terms of reducing your energy demand first. Ideally you should reduce demand as far as possible before considering generating because for the most part it is much more cost effective to reduce energy consumption than it is to generate.

The Energy Saving Trust has a good publication on solar water heating which gives a more detailed overview of the technology, but it only goes as far as using solar thermal for water heating. The system we saw was also plumbed into the heating system, allowing the user to make use of much more of the heat falling on the roof.

A system that just heats the hot water can only go so far because you only use so much hot water (unless of course you are particularly partial to baths). If you have lots of sunny weather the panel will keep generating heat and that heat has to go somewhere. Being able to deliver it to radiators or an underfloor heating system is a real bonus.

I’ll post more details about this fully integrated system later this week.