Solar hot water and heating systems
Payment for the consumption of energy resources used to provide heat supply, including heating and hot water supply, is one of the main items of consumption of the consumer budget. This is especially true for the owner of a private house, who decides the issue of water and heat supply to the building independently.
The organization of an energy efficient heat supply system for a summer house, a cottage using energy from renewable energy sources (RES) not only creates the effect of significant savings in non-renewable energy resources and consumer spending, but also contributes to the preservation of the environment.
Prototypes of modern hot water supply systems using solar energy (solar systems) have long been familiar to summer residents – who has not installed a tank on the roof of a summer shower on their site, regularly supplying “almost hot” water on sunny days? Modern solar systems, having inherited the main design principle, work much more efficiently. In theory, such systems, when properly installed and reasonably operated, are able to provide up to 90% of hot water needs. In practice, this directly depends not only on the choice of equipment and competent installation, but, first of all, on the region in which the house equipped with such a system is located.
The initial installation of a solar DHW system and even more so heating is a very expensive business, especially if you use advanced technical solutions for this. However, investments begin to pay off almost from the moment the system is put into operation.
In principle, such a solar system (Fig. 1, 2), designed to provide heating and hot water supply, includes the following main components:
– solar collectors;
– pumping module with safety group;
– water storage tank;
– a backup source of energy.
When installing a solar system, it is necessary to correctly take into account the movement of the sun in the sky and, as a result, the inclination and orientation of the roof, walls and solar collectors installed on it to the cardinal points.
The solar collector heats water or another type of heat carrier. The circulation of the liquid in the collector is provided by a pump; in some systems, natural circulation is carried out. The heated coolant circulates along the primary circuit, giving off thermal energy through the heat exchanger to the coolant of the secondary circuit (water) in the storage tank. The heat exchanger can be built into the tank in the form of a coil, or it can be made as a separate device. The heat accumulation process is automatically controlled by the controller that controls the operation of the pump in the solar system. If necessary, the automation starts a backup energy source.
Solar systems differ both in the type of coolant used in them (liquid – water, antifreeze and air), and in the duration of operation – year-round or seasonal, which is especially important in our country.
Seasonal solar hot water systems are usually single-circuit. They are actively used in the summer and transitional months, when the ambient temperature is positive.
If a solar system is used to heat a building, then it is usually performed in a double-circuit, and most often multi-circuit. At the same time, different coolants are used in different circuits, for example, in the solar circuit – aqueous solutions of non-freezing liquids, in the intermediate – water, and in the final, “consumer”, – air. Most often this applies not to seasonal, but to year-round heating systems for buildings; in such systems, as a rule, a powerful heat generator is also provided, operating, for example, on fossil fuel.
As noted above, the solar collector is the main element of any solar system; the efficiency of the system and the economic effect of its use depend on its choice.
The simplest type of solar collector is tubular (Fig. 4), installed at an angle to the horizon, for example, on the roof of a building or in a courtyard, in which the coolant is heated from the energy of solar radiation, passing through a battery of thin tubes. The circulation is provided by natural convection, which saves the owner from the energy costs for the pump.
Flat solar collectors
Flat solar collectors are the most common type of solar collectors in domestic hot water and heating systems (Fig. 5). The main element of a flat collector is an absorbing plate, which traps sunlight, converts it into heat and transfers it to the coolant. The surface of the heat sink is usually painted black; to reduce heat loss from the plate surface, a transparent coating is installed above it, and to reduce heat loss from the back side of the collector plate is covered with thermal insulation.
In normal operating mode, the heat accumulated in the collector is spent on heating the coolant circulating through it. Therefore, the main characteristic of a solar collector is the volume of heat
a heater heated to a predetermined temperature during the daylight hours by a square meter of the collector. In central Europe in the summer, the productivity of such collectors makes it possible to obtain 50-60 liters of water from 1 m2, heated to 60-70 ° С. The efficiency of such a collector is about 70%, which directly depends on the weather conditions and the region where the building is located. Flat collectors collect both direct and scattered radiation and therefore can work also in cloudy weather. In this regard, as well as in view of their relatively low cost, they are preferable when heating liquids to temperatures below 100 ° C.
Vacuum or vacuum collectors are capable of receiving water at a higher temperature; they are noticeably more efficient than flat solar collectors, but at the same time they are heavier, more expensive and, moreover, require competent operation.
By using the best possible heat insulator – vacuum – the overall heat loss in the collector is minimal. The efficiency of the evacuated collector remains consistently high even under adverse weather conditions. At an air temperature of -45 ° C and diffused sunlight, the productivity of the vacuum collector is 40% higher than that of other types of such equipment.
The main element of such collectors is a vacuum tube, the design of which consists of two tubes – an external and an internal one. Between the inner surface of the outer tube and the outer surface of the inner there is a sealed space from which air is evacuated to create a vacuum. As you know, vacuum, in which there is no medium for convective energy transfer, is one of the most effective types of thermal insulation. The inner tube is usually made of copper and has a selective coating that absorbs solar radiation, and the vacuum space prevents convective heat loss (Fig. 6). Solar radiation passes through the outer glass tube, enters the absorber tube and turns into heat. This heat is transferred to the liquid flowing through the inner tube.