Ways to Design a Solar Thermal Hot Water System

Depending on how much hot water you need, the hot water system you already have, your budget and where you are located there are a few ways you can put a system together. Below we outline some of the system designs that you could implement

Active "Direct" Open Loop
In an active Direct open Loop System the water you use in your bath and taps would be the same water that at one time would have been heated by the sun in your solar panel (or other collector).

Colder water is taken from your hot water cylinder and pumped (the active part) to the solar collector to be heated. Hot water is then returned to the cylinder. In the UK to protect from freezing we install a controller that will shut off the supply to to the collector once the outside temperature drops to a certain level. You can click on the picture to see a bigger picture of how this system fits together.

We often recommend Active "Direct" Open Loop systems when customers are looking for a simple or low cost solution. In many cases an Active "Direct" Open Loop system can be installed using existing hot water stores and cylinders, most of the existing pipework remains the same and these systems integrate nicely with existing boilers.

Active Drain-Back System (with Water)
With an active Drain Back system using water the temperature of the water in the cylinder is monitored, once the temperature drops below a certain level water is pumped up to the Solar Collectors to be warmed. When the water is not being used it drains back into the system to avoid freezing.

Drain Back systems are also popular due to their ease to install and simple design.

Active "Indirect" Closed Loop (With Antifreeze/Glycol)
Active "Indirect" systems use a dedicated loop to transfer a fluid such as Glycol between the Solar Collector and the hot water storage cylinder. The fluid is heated up in the collector, this heat is then transfered to the water in the cylinder via a heat exchanger (often a heating coil similar to a heating element in a kettle).

The second coil in this kind of system is used to transer heat from other heating methods such as boilers and immersions when the Solar Collectors aren't being used.

Active "Indirect" Closed Loop systems are very popular in the UK but slightly more expensive then the direct systems as a twin coil cylinder will need to be installed if you don't already have one and there is more pipework.

Passive Thermal Siphon
This is where a water storage tank is placed higher then the solar collectors to allow the heat captured by the collectors to rise into the tank and warm the water. This is not a very efficient system but as it doesn't expend any energy it may be useful in some situations.

Batch Heaters
This is where the storage tank or cylinder is the collector, the water is warmed by the sun as the cylinder warms up. With this type of system thought needs to be given about what happens to the stored heat at night.

In Summary
We hope this article has helped outline some of your options, has explained some of the systems that are available and what could be implemented in your Solar Thermal system.

Glen Freeman is a renewable energy systems designer and co-owner of In Balance Energy Ltd. In Balance are specialist designers, suppliers and installers of bespoke energy systems for homeowners, businesses and communities in the UK. Their website can be found at http:// www.inbalance-energy.co.uk