How do solar thermal panels heat water for your home?
Solar water heating systems are sophisticated, engineered thermal systems which deliver heat energy for many different purposes. The most common use is to deliver carbon free hot water for homes, but they can also be used to provide solar swimming pool heating, solar underfloor heating and for industrial processes.
Thermal solar is the use of light to generate energy in the form of heat. It starts with the sun. The sun constantly emits energy by radiation in the form of light. Light can travel through the vacuum of space, but only limited amounts of heat can. When light reaches our planet, it is received in two forms. Beam radiation is radiation received from the sun without being scattered by the earth’s atmosphere. Other energy is received in a form that is scattered by the atmosphere and is referred to as diffuse radiation. The scattering occurs when the radiation passes through air molecules, water and vapour droplets and dust. The two forms together are known as total solar radiation.
A simple way to think of these two types of light is to think of beam radiation as the light that you receive directly from the sun and diffuse radiation as the light that exists on a very cloudy day. Each year we get about 4380 hours of light. Only some of this light is in the form of direct sunshine. Our solar thermal panels are designed to work not only in direct sunshine but also in the light levels experienced on a very cloudy day.
The black surface inside the glazed solar panel system is “selectively coated” with a special material made from aluminium oxide. It is called selective coating because the surface is coated in a way that some parts receive marginally more coating than others creating a surface which under a microscope looks like ranges of mountains. This enables many surfaces to be presented to the light.
The reason for the special selective coating is as follows: when light) strikes matter in the atmosphere of our planet it causes the molecules of the matter to vibrate. That vibration causes friction and friction is heat. Thus, when you are standing out doors in a cloudy day you can still get badly sunburnt, because the light is causing your skin to burn. The selective coating specially designed and used in our solar panels enables the molecules to vibrate and create heat in all types of light conditions as well as in below freezing weather conditions.
Once the heat is captured by the solar panels on the absorber plate it conducts into the pipe work running through the panels. This piping is filled with an edible safe glycol to prevent freezing. The glycol is thus heated by light.
The pipe work (which is highly insulated) forms a closed heating pipe which connects to a coil in your domestic hot water cylinder. If you already have a traditional gas central and water heating system, the solar heat circuit will connect to the lower coil in your cylinder and the fossil fuel heating system will connect to the upper coil in the cylinder. You will usually need a new cylinder because solar cylinders are larger than normal cylinders, taking advantage of the ability to store energy in the form of heat for days when there is little light energy available for the solar panels.
The solar heat circuit is pressurised usually to around 2 – 4 bar. This enables the solar system to work in very hot conditions (because pressuring the system increasing boiling point significantly) and the glycol enables the system to work even in ten degrees of frost.
Finally, in systems there are sensors in the panels and in the solar hot water cylinder. These sensors feed information about the state of the system to a digital controller, which is connected to a pumping station. When the digital controller recognises that there is energy available in the panels and energy is needed in the cylinder it switches on the pump; that pushes the hot glycol in the panels around the heat circuit and into the cylinder, making the solar coil in the cylinder very hot.
The solar coil conducts its heat to the water stored in the cylinder. When the cylinder reaches a pre-set temperature level the controller switches off the pump and the system returns to rest. The heat circuit is connected to a pressure vessel to enable the glycol to expand and contract without damaging the system. Some small amount of electricity is used to drive the pump; in typical installations, the electrical energy used for the pump and the controller will be less than 1½ % of the energy produced by the solar panels. It is sometimes thought by a layman that solar collectors are designed to collect as much light as possible and convert it to heat. This is actually not the case. When designing domestic water systems engineers know that there is no point in overheating the system because the hot water usage and storage capacity is finite and the way in which people use hot water and the times at which they use also have to be taken into account. If you make the solar collectors too hot they will have a very short life time and overheating will cause major problems.
We use specially designed and manufactured selective coating, which is designed not to get too hot. There are other coatings that we could use, at lower costs, which get hotter but we have to bear in mind that we are not making collectors to create more heat than can be usefully used because the higher the stagnation temperature and the more frequently the system reaches stagnation temperature the quicker is the aging process of the whole system so our panels for domestic hot water are designed so that users never experience overheating problems; you can safely go on holiday during the hottest times of the year knowing that your system will not overheat and will be safe.