How Solar Thermal Works
Global irradiation - The "fuel" of a solar thermal system
Solar thermal collectors make use of the energy contained within radiation that the Earth constantly receives from the sun. This radiation takes the form of both visible and invisible light across a wide spectrum. The radiation that is received at the outer surface of our atmosphere is termed solar insolation, and once it has passed through the atmosphere the radiation that we receive at the Earth's surface is called global irradiation. It is this global irradiation that "fuels" the solar thermal collector.
Global irradiation can be broken into two main components; direct and diffuse radiation. Direct radiation is received on the earth's surface with no interference from the atmosphere or clouds. Diffuse radiation is dispersed by the atmosphere or clouds before reaching the earth's surface. In Northern Europe the average percentage of diffuse radiation as a proportion of global irradiation is about 50% over a year, this figure is higher in the winter and lower in the summer.
Nearer the Equator this diffuse proportion can be nearer 20% of the total. The high level of diffuse radiation in Northern Latitudes has had an impact on collector design and in particular the proliferation of Evacuated Tube Collectors which are optimised to work in diffuse radiation.
Another factor that affects how much radiation that the earth's surface receives is how far it has had to travel through the earth's atmosphere, this reducing effect is known as air mass. If radiation hits the earth's atmosphere perpendicularly (Air Mass Index = 1), then it travels less distance through the atmosphere than if it hits at a more acute angle (Air Mass Index = 1.5). This effect will be seasonal and latitude dependent.
Solar Collector - Absorbing the radiation and generating heat
A solar collector works by transferring the energy contained in the solar irradiation into heat energy. There are two principle designs of solar collector; the Evacuated Tube Collector and the Flat-Plate Collector. Both these types of collector integrate in a solar circuit in the same way but their designs are radically different.
Evacuated Tube Collector
An Evacuated Tube Heat Pipe Collector is made up of a series of glass tubes containing heat pipes, that "plug-in" to a manifold. Solar irradiation is received by the tube and absorbed via a coating on the inside of the glass, this coating becomes hot and transfers heat energy through an aluminium foil to the copper heat pipe. Within the copper pipe there is an aqueous solution held in a vacuum that is allowed to rapidly boil and condense in the heat tip. The energy generated in the heat tip is then transferred to the solar circuit via the collector manifold.
Flat Plate Collector
A Flat Plate Collector has a large glass cover, an aluminium absorber plate and a copper pipe register. The higher performance and quality flat plates, such as the Barilla F22 AR, use anti-reflective glass to allow higher levels of irradiation to be absorbed. The absorber plate is heated and transfers energy to the copper pipe register attached to the back of it, fluid from the solar circuit is heated by circulating it through the copper pipes.
Solar Circuit - Utilising the heat that's been generated
Whether you chose a flat-plate or evacuated tube collector you will require a method of utilising the heat that the solar panels have generated, the diagram above shows a typical circuit that is capable of doing this. The main components in the circuit are a solar collector, a pump station and controller, an expansion vessel and a twin coil cylinder. The solar circuit is sealed and contains a solution of water / Propylene Glycol which is used as the heat transfer medium and for freeze protection. The controller is used to monitor both the collector and cylinder temperatures, using a sensor probe mounted within both items, as the temperature in the collector rises above the temperature in the base of the cylinder a pump is activated and the heated fluid in the collector is circulated through the solar coil in the cylinder. This process is repeated in steps until the cylinder reaches its maximum temperature, typically 60°C.
The purpose of the expansion vessel is to allow for the expansion of the fluid in the sealed system as it heats up. The discharge container, shown alongside the expansion vessel in the diagram above, is designed to capture any fluid that may released from the system via the Pressure Relief Valve attached to the pump station.
The Pressure Relief Valve is rated at 6 bar and is a safety device to ensure that the sealed circuit is protected from over pressurisation, if the system has been designed and installed correctly it is very rare that this valve would activate and discharge fluid.
The twin coil cylinder, as its name might suggest, contains two heat exchange coils one is located at the bottom of the tank and is dedicated to the solar circuit and the other coil is for an auxiliary heating device such as a gas boiler or heat pump. There are many other variations of cylinder and storage arrangement but the twin coil cylinder is the most common in both domestic and smaller non-domestic installations.
For more in-depth information please refer to our Frequently Asked Questions or ask your Solar Thermal Installer who will be familiar with the design and operation of this type of system.