Whether geothermal energy, solar thermal systems, electrical heating, condensing technology or gas heating, even the old-timer among heating systems today has the most modern and economical technology …
Warm air heating
The warm air heating uses the room air as a heat carrier. The warm air generated in an automatic heating system is fed into the rooms via air ducts.
Hypocaust air heating is also designed as warm air heating, but has a different structure in the details. It was made in the 1st century BC. Invented and can be seen today in excavation sites.
The profitability of a heating system depends on the acquisition costs and the operating costs. The latter are strongly influenced by the usage behavior or the comfort needs of the residents. The public sector grants purchase grants for some heating systems; these reduce the acquisition costs. Taxpayers in Germany can claim the costs for the craftsman’s services at a reduced rate (more here).
For the evaluation of the overall efficiency, the annual degree of utilization is more important than the degree of efficiency.
The efficiency only states the losses when the burner is running.
The annual degree of utilization describes the relationship between the useful heat provided and the amount of fuel used. The indication of the annual degree of utilization or the standard degree of utilization also takes into account (in addition to the losses that occur while the burner is running) all losses that occur during the burner shutdown.
Since burner runtimes of around 1,800 hours can only be achieved in one year and the burner is stationary for the rest of the time, an efficiency figure is always only a snapshot. In contrast, the degree of utilization considers the energetic efficiency over a certain period of time, e.g. B. a year. The efficiency can be improved by installing a condensing boiler - provided the return temperature in the heating circuit is relatively low. They also use the heat of condensation from the water vapor produced during combustion.
To heat buildings with geothermal energy, heat pumps use the stable temperature level below the earth’s surface to warm up the temperature-transferring medium in the heating circuit. For a single-family house, one or two holes close to the surface are necessary. The use of low-temperature heating such as B. the underfloor heating. One advantage of geothermal heating is that some of these systems can also be used to cool buildings in summer.
Only in some regions is it possible to use geothermal energy in geothermal hot water heating systems, in which the heating water is heated to the consumption temperature (up to over 40 ° C) directly via geothermal energy.
Electric power as an energy source in electric heating systems is often used for short-term use in fan heaters. Night-time storage heaters are sometimes used to heat apartments, which use the energy of the night-time electricity at the so-called low tariff (colloquially night tariff) at certain times - usually at night and in the afternoon - to heat up a heat-insulated storage tank and through convection - and additionally at Need at any time of the day via an additional blower - deliver. Oil-filled, fanless radiators with built-in heating rods and temperature controllers are also used for room heating.
The thermal energy required to heat a building can also be generated in a block-type thermal power station. This is based on the principle of combined heat and power; In addition to heat, it also generates electricity. The energy conversion can take place in very different ways (e.g. by an internal combustion engine, a steam turbine, a gas turbine, a Stirling engine or a fuel cell). The waste heat from the system can then be used to heat rooms, among other things. In addition to the combination with a heat accumulator, it is also common to use a peak load heater to cover the maximum heat requirement.
Thermal solar system
Solar thermal systems are solar systems that make heat from solar radiation usable (solar thermal). The heat is made usable in process technology or building technology or used in thermal solar power plants to generate electricity.
The direct conversion of sunlight into electricity - e.g. B. by means of solar cells - is called photovoltaics, the corresponding systems as photovoltaic systems.
Areas of application
Thermal solar systems are predominantly used in building services. The heat gained is mostly used to heat drinking water (dishwashing water, shower and bath water) and to warm up living spaces.
In the industrial sector, systems with mostly more than 20 m² collector surface are operated for the production of process heat in the temperature range up to 100 ° C or a little above, for example to accelerate biological and chemical processes in biomass processing or in the chemical industry or to heat / preheat air.
Solar thermal systems also include systems for solar air conditioning. Due to the high temperatures, they are comparable to the process systems.
On the other hand, they are used on an industrial scale in thermal solar power plants such as in Andasol. Most of these systems use concentrating collectors to focus the sun’s rays on an absorber point or an absorber line in which temperatures from 390 ° C to over 1000 ° C can be reached. This heat is then either used as industrial process heat or converted into electricity using generators (solar thermal electricity generation). Since concentrating systems are dependent on direct sunlight, they are only used in sunny and dry regions (in Europe for example in southern Spain).
In the following, this article concentrates on the use of solar thermal energy for domestic hot water heating and heating support, as this is (still) the most common and most widespread area of application in Central Europe.
The thermal solar system consists of a collector, which converts the solar radiation into heat, a solar thermal store that stores the heat that is not immediately used, and the connecting solar circuit through which the heat is transported from the collector to the store. This consists of pipes, fittings and drive units, which ensure the proper operation of the system, as well as a controller, which switches the heat transport on and off (except for gravity systems).
The solar collector is the part of the solar system that absorbs a large part of the energy of the sunlight (absorption), but at the same time - despite its own heating - gives off only a little of it as thermal radiation (emission). It transfers the absorbed heat with as little loss as possible to the so-called solar fluid in the solar circuit.
The most important structural distinction in collectors is between
“Air-filled” collectors that are protected against heat loss with conventional insulation materials (thermal insulation). They pioneered the efficient use of solar energy. Experience shows that they have a very long service life; there are supposed to be manufacturers who give a functional guarantee of 20 years.
evacuated tube collectors; the most common variants work according to the thermos flask principle: A second, outer (glass) tube is placed around the inner absorber tube containing the transport medium. For better insulation, the air is withdrawn from the space (vacuum). They are more efficient than other types of construction, especially when there are high temperature differences between the outside air and the absorber. They are therefore also used in the industrial sector, where process heat with a constant over 80 ° C is required.
Air-filled flat-plate collectors are much more common in Europe and are mainly used in building services. Vacuum collectors have a higher yield per square meter of absorber surface. However, when converting to the total area of the collector instead of the pure absorber area, the difference melts away significantly, since with air-filled collectors the absorber takes up a significantly larger proportion of the total area required for installation. In relation to the gross area, the yield of vacuum collectors is theoretically approx. 20% higher than that of flat-plate collectors. In the most common application of flat and tube collectors - in private family houses - a vacuum tube collector only enables marginal profit advantages to be achieved. The additional usable thermal energy of a vacuum tube collector is then 2-5% based on the total energy consumption of the house. There are type-related differences in performance with flat-plate collectors and vacuum tube collectors. It is impossible to compare the performance data that can be found in the Keymark certificates. Evacuated tube collectors bring greater yields, especially in the transition period and in winter, since better insulation comes into play at low outside temperatures. Even with large temperature differences between the outside temperature and the storage medium temperature (more than 40 °), the efficiency of the evacuated tube collectors is better. The flat-plate collector has an advantage when the temperature difference is low. As a result of the better insulation, evacuated tube collectors defrost somewhat more slowly. This can be a disadvantage in regions with a lot of snow.
So-called vacuum flat collectors are a mixed form. These represent an attempt to use the better insulation properties of the vacuum in “normal” flat-plate collectors. Due to their design, however, they tend to leak, so that penetrating air reduces the thermal insulation and has to be extracted regularly with the help of a vacuum pump.
In the case of register-shaped absorber pipes or if several solar absorbers / collectors are operated in parallel in a common hydraulic system (for example with a common circulation pump), they must be piped together according to Tichelmann, so that a fairly even flow of all absorber / collector segments is ensured.
Is the temperature that the collector reaches with standard irradiation of 1000 W / m2 when idling without solar fluid. The level of the stagnation temperature of the collector depends on its quality. Most of the time, in the certificates of collectors you will find temperatures between 170 and 230 degrees Celsius, with some collectors this temperature is given as over 300 ° C. The better a collector is insulated, the higher this temperature is. Every collector must be designed in such a way that it can withstand these extreme temperatures without damage. However, accelerated aging occurs more or less, depending on the design and make. Copper manifolds scale with repeated stagnation. There are also collectors with stainless steel collecting pipes. The thermal insulation material can age prematurely depending on the material used. In the vicinity of the collector, the pipes must withstand this temperature without damage. However, if a stagnant collector is refilled with solar fluid, this can lead to damage as the temperature shock may be too high. The collectors should therefore only be filled when the collector is covered or in the early morning hours or in the evening after the collector has cooled down.
A gas heater is a heating system that is operated with combustible gases. Most often this is natural gas, alongside so-called liquefied gases, which consist of a mixture of propane or butane. Town gas or biogas are less common.
The heat generated during combustion is transferred to a heat transfer medium in central heating. Depending on the version, this is water or air. A circulation device transports the heat transfer medium into the rooms to be heated.
In addition, warm drinking water can be produced.
In older buildings, individual rooms can also be equipped with gas heaters (gas heating stoves), which there release the heat generated by burning the gas directly into the room air. Today it is common to use gas-powered heating systems with heated water (heating water) as a heat carrier, the radiators of mostly entire buildings, but at least of an entire apartment, as gas heating for each floor.
A general distinction is made between calorific value gas heating and condensing gas heating. The calorific value is the measure of energy. It indicates the energy contained in the gas, which arises during combustion and subsequent cooling. The calorific value relates to the amount of heat. The calorific value is always lower than the calorific value. Condensing gas heating systems achieve a higher level of efficiency. Condensing boilers for natural gas also use the warm water vapor to generate thermal energy and do not simply emit it as water vapor.
Local or district heating
If the heat is generated centrally in a combined heat and power plant (principle of combined heat and power) or the process waste heat from industrial plants is used and distributed to several spatially distant heat consumers via pipe networks, one speaks of local heating or heating, depending on the spatial size of the heat network District heating supply. Such composite heating networks are used to supply heat to city districts and / or in industrial plants. To date, primarily crude oil, natural gas, coal, waste (→ waste-to-energy plant) and, in individual cases, nuclear energy have been used to generate heat. In the case of smaller heating networks in particular, waste heat from block-type thermal power stations (e.g. conversion of biogas, pellets, wood chips) or heat from wood chip heating plants is increasingly used.
Heating systems that use several heat sources are called polyvalent heating (bivalent = two-valued). Examples:
All-burner (classic wood or coke boiler) are polyvalent
combined solar burner heating (solar thermal and oil / wood / gas etc.), log wood combination systems (wood gasifier with pellet module), and others.
There are also systems for more than two forms of energy in power plant technology.
Heating systems that use three heat sources are called trivalent heating (trivalent = trivalent) or hybrid heating. Example:
- Condensing technology (e.g. oil / gas) and solar thermal and water-based fireplace
Hot water heating
Hot water heating consists of a central heat generator (boiler, combi boiler) that heats the heat transfer medium water and, with the help of a circulating pump or (rarely) through the density difference of the differently warm water (gravity heating; thermosiphon principle), via pipes to the radiators (radiators, Heating strips) pumps. These give off part of the thermal energy to the room air through convection. The cooled water flows back to the heat generator via the return lines. With a one-pipe heating system, which has a poorer thermal efficiency, there is no separate return - the radiators are arranged hydraulically in series.
The hot water heating works with flow temperatures between 30 ° C (low temperature heating system) and 90 ° C. Larger dimensions of the radiators or the use of floor or wall heating means that the energy from the flue gas can also be used with a low flow temperature. One then speaks of condensing technology (calorific value), in contrast to the previously common heating value technology (calorific value). Since acidic water condenses from the exhaust air, the chimney must be of a suitable design.
To compensate for pressure fluctuations due to the heating and cooling of the water in the system, a membrane expansion tank (MAG) is essential. In older heating systems there are occasional open reservoirs at the highest point of the heating system.
To fill the heating system, tap water is usually used, which is fed into the heating circuit via a backflow preventer (filling valve that prevents backflow from the heating pipe network into the drinking water network).
Air in the pipeline network must be removed from the water circuit via ventilators on the individual radiators and, in larger systems, on air bubble separators (automatic ventilators) so that all radiators can be supplied with hot water over the entire surface and there is no noise (flow noise) and corrosion in the network.
Hydraulic balancing is required for optimal operation of hot water heating. For this purpose, a pipe network calculation is carried out before the system is built. Hydraulic balancing is required both in VOB Part C and in the Energy Saving Ordinance and is carried out by heating engineers or (since 2003) system mechanics for sanitary, heating and air conditioning technology. Without hydraulic balancing, the heating elements may get differently warm and the circulation pump needs more electrical work (kWh) than necessary.