Solar Water Heating – How It Works

The Basic Idea…

Solar Thermal systems create hot water by absorbing heat from the sun which is then stored in a specially designed hot water cylinder in the home.

This hot water is used in baths, showers and basins throughout the home.

The systems can be variously named as Solar Water Heating, Solar Hot Water or Solar Thermal. These are all different names for the same systems.

A Bit More Detail…

The sun’s energy is absorbed by a solar collector, usually mounted on the roof of a house, but alternatively attached to an ‘A’ frame in the garden.

The solar collectors come in one of two forms: evacuated tube collectors, or flat plate panels.

1. Evacuated Tubes

2. Flat Plate panels

A heat transfer fluid (usually a water/glycol mixture) passes through the collector (or panels), absorbing the sun’s heat. This fluid then flows into the hot water cylinder where heat is transferred to the water in the cylinder via a heat exchanger (or ‘coil’). This is the same way your boiler heats the water in your current cylinder.

The cylinder itself is ‘special’ in the sense that it has two heat exchangers (‘coils’) instead of the usual one. The one nearer the top of the cylinder is linked to your existing boiler, the one nearer the bottom to your solar collectors.

In this way, the sun’s energy is used to pre-heat the water in the cylinder. In winter months this means that your existing boiler only starts operating when the temperature of the water in the cylinder needs to be higher than already achieved by the solar water heating system. This means your boiler ‘fires-up’ less often and for a shorter period, meaning lower energy bills (gas, oil or electricity depending on how your boiler works) and of course, reduced CO₂ production as a consequence.

In the summer months there may be enough solar energy absorbed to provide all your hot water – the boiler doesn’t need to ‘fire-up’ at all. When these solar absorption levels are averaged out over the year, a typical Solar Water Heating installation in the UK will reduce the costs of hot water production by 60%-70%.

In order to successfully fit a Solar Water Heating system you will need the following basic requirements:

• Somewhere to locate the collectors. Ideally this is a roof oriented within 90⁰ of South, but systems will work (albeit less efficiently) facing in other directions. However, whatever the orientation it is important that the collector location is not in the shade from trees or other buildings for a significant part of the day (eg more than 4 hours approximately).
• Room to replace your existing hot water cylinder – usually in your airing cupboard. The new ‘solar cylinder’ (with the two heat exchangers – remember?) will be slightly bigger than the old one, perhaps 10%-15% taller and 2”-3” greater in diameter. Will it fit in?
• Ideally, you will not have a Combination (‘Combi’) boiler. Whilst it is possible to install a Solar Thermal system to work with a ‘Combi’ there is a fundamental problem in that Solar Hot Water systems require stored water to store the suns energy – something that a ‘Combi’ doesn’t do. To circumvent this, a thermal store can sometimes be added to the system depending on the specification of the ‘Combi’ in question. From a practical point of view however, this means quite a lot of extra work, complexity and, of course, cost – in most cases more than doubling the cost of the overall installation.

If You’re Still With Us, Here’s Some More Detail..

There are of course some other components that we need to mention so you have the complete picture. Please refer to the diagram below in order to understand more clearly.

Main System Components

1. Collector
2. Heat Transfer Fluid
3. Hot Water Cylinder (with Solar Coil & Boiler Coil)
4. Pump
5. Expansion Vessel (not shown)
6. Air Vent (not shown)
7. Controller

The Process

• The collector(s) absorb the sun’s heat, transmitting it through copper tubes to the heat transfer fluid within.
• The heat transfer fluid flows down into the solar hot water cylinder located in your airing cupboard.
• The heat transfer fluid flows through the solar coil at the bottom of the cylinder and as it does so it heats the surrounding water in the cylinder. As it heats the surrounding water, the heat transfer fluid starts to cool and flows out of the bottom of the cylinder to the pump. The pump will usually be located in the airing cupboard too, but doesn’t have to be (it could be sited in your loft for example).
• The pump is the ‘heart’ of the system. It pumps the heat transfer fluid around the system, ensures there is no flow in the wrong direction and releases pressure in the event of any build-up. At this point the cooled Heat Transfer Fluid is pumped back towards the collectors
• Between the pump and the collectors there are two further components, the Air Vent and an Expansion Vessel.
  • The Air Vent – used to remove any air bubbles during installation. Our system is distinct from most other systems in that the Air Vent can be installed in any part of the system (instead of at the highest point – which can be unsightly on top of the roof).
  • The Expansion Vessel – this comes into action when there is an excess volume of heat transfer fluid in the system; for example, if the cylinder requires no more hot water but the sun is still shining.
  • Finally, there is a controller, which is the system’s ‘brain’. The controller determines whether the pump should run or not. It does this by monitoring the temperature difference between the heat transfer fluid at the collectors and the water in the lower section of the hot water cylinder. When the difference in temperature between these two points is high enough (usually it is set at a 6⁰C difference), the controller switches the pump on. For example:
    • The temperature of the water at the bottom of the cylinder is 55⁰C
    • The temperature of the heat transfer fluid at the collector is 61⁰C

…then the controller will start the pump which will pump the heat transfer fluid around the system to the cylinder in order to raise the water temperature.