Solar thermal belongs to active solar systems and is the technology that utilizes solar energy to produce heat. This energy is used to meet our needs in space heating and domestic hot water (DHW).
In domestic applications heating the heat transfer medium is water, which is heated by burning oil or gas and circulates to radiators or floor heating. In a large percentage and depending on the facility, the water can be heated by solar energy system resulting in the reduction of the fuel consumption. In Greece, where there is strong sunlight even during the winter months, the application of solar thermal systems is very efficient. These installations are already highly developed even in countries of northern Europe, where solar radiation is significantly lower than in Greece with a great success.
The basic principle of a solar thermal system is that it is installed and operates for contributing to heating with simultaneous use of other fuels (of the existing facilities). The fraction of contribution and hence the reduction of fuel consumption depends on the size of the installation concerned, its site topography and other factors. For example, a solar system with under floor heating, located in Crete, can cover 100% of needs for heating, discarding the oil use, while the same solar system located in North Greece, where solar radiation is reduced, the contribution rate will be significantly lower, but still, it certainly is very important in terms of reducing the fuel consumption and cost savings.
If there is no sun and there is need for heating, the supply is from an inactive container, called buffer tank. When the hot water in the buffer is below the setpoint temperature, an auxiliary power source is getting into operation, usually the one from the conventional central heating. The solar reservoir, connected to the water network has two heat exchangers, one fed from the heating medium coming from the collector and the second is connected to the heating boiler. If there is not requirement for hot water to the facility, the solar reservoir is able to keep it warm for several days. A well dimensioned solar system can provide enough energy to cover the period between May and September up to 100% of the needs for domestic hot water and from 50% till 70% annually. However in winter time, the conventional heating boiler must work additionally to the solar system. In buildings with proper insulation, solar energy can do more: during spring and autumn can provide cooling, while during winter can relieve the oil boiler or gas boiler from the total heat load.
The main parts from which a solar heating system consists of are:
A solar heating system can be installed in any heating system, either at the beginning of its construction or subsequently. The installation does not require any modification to the existing heating system if it is already installed. Mostly is practicable in under floor heating because water as heating medium circulates at low temperatures, so even with little sunshine is easily achievable according to the solar energy. The reduction in fuel consumption can reach up to 80%, which is very important for the household economy and has an enormous impact on the environment. In case where the heating system operates with (calorifier) heaters, it is wise to reduce the operating temperature using compensation kit and to increase the operation time period in order to succed more advantageous solar application.
For collecting the solar energy and heating the water, solar panels are used. The collectors are ensuring high absorption of solar energy and consequently a large contribution to the heating system. A typical collector is called flat plate solar collector and usually is placed on the roof. This collector is a rectangular plate with a permeable cover facing the sun. Small tubes are located inside the plate which carry water or another fluid such as antifreeze. The tubes are mounted on a metal absorber plate, which is painted black to absorb solar heat. The back and sides of the collector are insulated so as to to retain heat. Heat is accumulated inside the collector and as the fluid passes through the tubes is getting hot.
Similarly to the designing of a solar building, heating solar systems can be categorized in active and passive systems. The most common systems are active and that means that they use pumps to circulate the heated medium from the collector to the storage tank (buffer). While solar water heating system works well, it does not produce enough heating energy when the sun is not shining. That is why, always its operation is combined with a conventional system that uses fossil fuels, gas or a heat pump or a biomass boiler as the primary heating source.
The cost of installing a solar thermal system varies depending on the plant size and the prerequisite reduction in fuel consumption. The great advantage of such a system is that the size (and therefore the cost) can be adapted to the user’s requirements and be easily varied. For example, firstly it can be installed a system that covers 30% of the total heat load and after a time of one year the system can be extended by placing extra solar collectors so as to achieve 60% covering in the total heating need.