Solar Water Heating (solar thermal)


How It Works

A solar thermal panel (also called solar collector), is a dark coloured material that absorbs light and warms up, with a heat transfer fluid flowing through to transport the heat energy somewhere useful. In its simplest form this could be a black rubber bag laid out on the ground with swimming pool water flowing through it.

To reach useful temperatures for domestic hot water in the UK, the dark coloured surface needs to be thermally insulated to limit heat losses to the environment. A familiar example of collecting light energy in this way is the way a car dashboard heats up when the car is left parked in the sun.


The solar panel is located where it will get good light levels, often on the roof of the building, and a pump circulates heat transfer fluid (water, or water mixed with antifreeze) through the panel. The heat from the panel is transferred to a storage cylinder, most commonly using an indirect coil. By comparing temperatures at the storage cylinder with temperatures at the solar panel, the control system decides when the pump should run.

An auxiliary heat source is required for days when light levels don't raise the cylinder to the required temperature.



The upshot of this is that a proportion of the annual hot water requirement of the building is provided for free by the solar system, and a proportion is provided by the auxiliary heating system. The proportion of the total supplied by the solar heating system is termed the Solar Fraction, and this will depend upon the area of panels installed, the efficiency of the panels, and the demand pattern from the household.




The Different Types of Solar Panel (Solar Thermal)

There are two types of solar panel in general use, the flat and evacuated tube solar panel.


1. Flat Plate Solar Panels

The flat plate solar collector consists of a black surface arranged beneath a transparent cover,  with insulation to the rear and sides. A heat transfer fluid (often water or glycol solution, sometimes air) is circulated through the panel to transport the heat collected away to where it can be used.

 Within this generic format, many, many variations are possible. For example, the cover can be glass or plastic, single or double glazed, with or without anti-reflective coatings. Suppliers have a tendency to wax lyrical about how their particular way of joining the pipes to the black surface is better than all the others.


One technical feature of real note was invented in the 1980s. It is called a "spectrally selective surface", and is generally an engineered film of oxide applied to a metallic surface. Such a surface looks black to visible wavelengths of radiation (sunlight), but is like a silver surface to infra-red wavelengths. A material with a selective surface will absorb light just as well as a surface painted black, but will hold onto its heat much better by avoiding radiating heat away.
2. Evacuated Tube Solar Panels
Evacuated-Tube.pngLike in a thermos flask, a vacuum can be excellent thermal insulation. It reduces heat transfer by convection to zero. Conduction is not significantly altered under the level of vacuum typically used, but air is already a poor conductor.
The vacuum is held in a glass container (to let the light through). Since glass is very strong under compression but tends to fail in tension, a tube is a good geometry to use.


In a common implementation, the tube contains a fin of light absorbing material with a U-shaped pipe for the heat exchange fluid to pass through. A number of these tubes are combined into a manifold to create a larger area or 'panel'.
Vacuum tubes are popular as a retrofit option because the array can be taken up to the roof one tube at a time. The additional insulation of the vacuum means that the efficiency at elevated temperatures (>60C) is higher than for flat plate solar collectors, making them more suitable for industrial process heat applications.

Installation of Solar Panels on the roof

Solar heating panels can be installed roof integrated (in-roof) or fixed to brackets that reach behind the roof covering (above-roof). Both formats involve disturbing the roof covering, and it is recommended that an installer with roofing knowledge is used to ensure that the roof is left in a waterproof state.

Panels fixed above the roof covering add to the loads on the roof structure, whereas roof integrated systems are often of lower weight than the tiles or slates they replace. If the roof loadings change as a result of the installation then a structural engineer should be consulted. In any case, all timbers to which solar panels are attached should be assessed to ensure soundness. 

Care should be taken to ensure that penetrations where pipes pass through the roof covering and underlay are weatherproof and durable.



The Seasons and Solar Heating

Ave-Daily-Light-Energy-(1).pngOne consideration for the implementation of solar hot water systems is the seasonal nature of the solar energy. Changes in the weather and to the hours of darkness throughout the year act to limit the availability of solar energy in winter compared to summer.

The graph shows the average daily light energy for each month of a typical year. The total light energy is made up of two component parts - direct irradiation (straight from the sun and shown in orange) and diffuse irradiation (light that has reflected off clouds, the sky and the surroundings, shown in blue).

It can be seen that the daily average in the summer months is six to seven times higher than in the winter months.



Domestic hot water demand is relatively constant throughout the year, so the designer of a solar hot water system faces a choice - should the system be specified to meet hot water demand in winter, spring or summer?
A system that is sized to meet a household's requirements in winter will be six to seven times larger than it needs to be in summer. The solar panels are sitting idle for large parts of the year. The utilization of the system is poor and the cost-effectiveness of the over-sized system reduced compared to a system with higher utilization.
On the other hand, a system sized to match demand in summer will use more auxiliary heating in winter.
The compromise that is usually chosen maximises the ratio of useful energy collected to the capital investment. Normally this works out to be a system that just meets demand in the summer, and provides around 50-60% of the hot water throughout the year.




The effect of Tilt Angle and Orientation

The position that maximises the energy collected by a solar heating panel in the UK is facing south and tilted at an angle of 35 degrees from the horizontal.
What is somewhat counter-intuitive, however, is that the total energy collected in a year is not a strong function of the direction in which the panel faces. This is especially true for panels on a roof - tilted at around 35 degrees, where the difference between facing east or west and facing due south is only around 12%. Any duo-pitch roof has one side facing in this region.
The reason for this is that there are diminishing returns in the energy collected because there is a limit to how much energy can be stored for later use.


Integration with Heating Systems

The inclusion of solar water heating in a heating system need not be complicated. In fact, the solar system can co-exist happily as an addition to a conventional heating system without interfering with it at all.

The way to keep it simple is to arrange the plumbing so that the solar energy is used to feed the conventional system with pre-heated water. The conventional system is under thermostatic control, so only fires up when the solar has not raised temperatures sufficiently.

Two-Cylinders.pngThere are three plumbing arrangements that can achieve this trick:


The simplest to understand is a two cylinder arrangement, where the cold feed to the conventional hot water cylinder is first taken through a cylinder that is heated by the solar panel. On good solar days, the water coming into the second cylinder will already be hot enough, and the thermostat will not call for heat - the boiler will not fire. On less good solar days, the water coming into the second cylinder will not be warm enough to use, and the thermostat will call for the boiler to heat the second cylinder.
The most common, and most cost effective is to use a cylinder with two heat exchange coils. The coils are arranged one above the other, with the solar coil at the bottom.

Because the coils heat by convection, the boiler can only heat the top part oTwin-Coil.pngf the cylinder, where a zone of hot water will float on top of the cooler water at the bottom, a phenomenon called stratification. This means that irrespective of the boiler controls, the solar system always has a volume that it can heat.

On good solar energy days, the solar panel will heat the whole tank up to a useable temperature, and the cylinder thermostat will not call for heat from the boiler. On less good solar days, the cylinder thermostat will call for heat and the boiler will top up solar energy.


The boiler controls can be completely independent of the solar system.


Combi.pngA combi boiler heats water "on demand". A solar heated cylinder can be installed upstream of the combi boiler so that the boiler receives warm water rather than cold as an input, and has to use less fuel to heat up the water to the required temperature. A tempering valve is normally required to ensure that the temperature of the in-feed water to the boiler is not too hot, as many boilers cannot regulate their flame sufficient to accept water above a certain temperature without over-heating.


Solar Links


Sandtoft Roof Tiles

PV48 Solar System

T: 0844 9395 900




Viridian Solar

Solar Thermal

T: 01480 831501



The Carbon Trust

T: 0800 085 2005




The Energy Saving Trust

T: 0800 512 012




Feed In Tariff Information




Energy Solutions - PV Calculator




 Renewable Energy Association


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