Heat pump
A heat pump is a machine that a firm amount of technical work thermal energy from a reservoir at a lower temperature (usually this is the environment ) receives and - together with the driving energy - as useful heat to a facility to heat the system at a higher temperature ( heating ) transfers. The process used is in principle the reverse of a heat power process, in which thermal energy is absorbed at a high temperature and partially converted into useful work and mechanical removed the residual energy at a lower temperature than heat, usually to the ambient. The principle of the heat pump is used for cooling ( as in a refrigerator), while the term "heat pump" is used only for the heating unit. In the cooling process is the useful energy the absorbed from the space to be cooled heat that is dissipated together with the input energy as heat to the surroundings.
- 3.1 Example of a ground water heat pump
- 3.2 Data Sheets
- 5.1 Electrically driven compression heat pump
- 5.2 heat pump with oil - or gas engine drive
- 5.3 Detailed description of heat pumps for building heating
Technical Realization
Heat pumps are generally operated with fluids which evaporate at low pressure, the supply of heat and condenses back to the compression ratio to a higher pressure with release of heat. The pressure is chosen so that the temperatures of the phase transition have a sufficient distance for heat transfer to the temperature of the heat source and heat sink. Depending on the fluid used is the pressure in different areas. Figure 1 shows the circuit diagram with the four required for the process components: evaporator, compressor (compressor), capacitor and inductor, Figure 2 the process in the Ts diagram. Theoretically it would be possible to make use of the working capacity of the condensate while relaxing on the lower pressure by a combustion engine, such as a turbine. However, the onset of this partial evaporation would cause major technical difficulties with only a small energy gain, so that one for the sake of simplicity here a choke used (relaxation constant total enthalpy ).
Particulars
Ten meters below the surface, the temperature - even in the cold season - about 10 ° C.
If a located below pressure liquid fluid (for example, propane, bp 56 ° C at 20 bar, -25 ° C at 2 bar) spent by pressure relief through the expansion valve through thin metal tubes into the ground, it desorbs heat and evaporates. It is then compressed and can now be liquefied by heat to the heating system of the dwelling house again in the capacitor, and the circuit is closed.
The energy required to drive the heat pump decreases - that is, the operation is more economical - the smaller the temperature difference between the ground temperature and the flow temperature of the heating system. This condition is met by low-temperature heating at best, so the heat in the living room is often delivered through a floor heating.
Depending on the design of the system of heating costs can be reduced by about 30 to 50%. By coupling with solar power, household electricity or natural gas to power the heat pump the carbon dioxide emissions compared to fuel oil can be significantly reduced.
The selection of the correct heat source is of particular importance because it determines the efficiency of a heat pump. Since the heat source often has a useful life of more than 50 years, their construction is an investment for generations.
" A measure of the efficiency of a heat pump is the seasonal performance factor. It describes the ratio of the useful energy in the form of heat to the compressor energy expended in the form of electricity. " With good facilities, this value is greater than 5.0 ( direct evaporation systems ). However, it must be noted that in the calculation of seasonal performance factor neither addition nor memory consumption losses are taken into account. In still operating older coal-fired power plants may consist of three parts heat energy only a partial flow are obtained. For electrically powered heat pumps, it is therefore advantageous to improve the use of electricity from renewable energy sources.
In the direct electrical heating, for instance with heating elements, the thermal energy generated accurately corresponds to the electrical power used. The electrical energy is considerably more valuable than heat energy at a low temperature, because, by using a heat engine, only part of the heat output to be again converted into electrical power.
The exhaust air to the outside air, the ground, the waste water or ground water can be removed by using heat of a heat pump. A multiple of the electrical power used for the heat pump, the heat source ( air, soil ) are removed and pumped to a higher temperature level. In the current account of the heat pump is supplied electrical power for the compressor drive and the heat removed from the environment. At the outlet of the heat pump, a part of the supplied power is at a higher level than the heat available. In the overall current account are still the losses of the process considered.
The ratio of the output to the heating circuit heat output to supplied electric compressor capacity is referred to as COP. The figure of merit has an upper value that can not be exceeded, and derived from the Carnot cycle. The coefficient of performance is determined on a test rig according to the standard EN 14511 (formerly EN 255 ) and is valid only at the specified test. According to EN 14511, the power number is also called COP ( Coefficient Of Performance). The COP is a quality criterion for heat pumps, it is not an energy assessment of the overall system.
In order to obtain as high a coefficient of performance, and therefore a high energy efficiency, the temperature difference between the temperature of the heat source and the use temperature should be as low as possible. The heat exchanger should be designed for the lowest possible temperature differences between the primary and secondary side.
The term heat pump based on the fact that heat from the environment raised to a higher usable temperature level ( pumped ) is. The heat pump has a compressor, driven electrically or by internal combustion engine. The compressor compresses refrigerant to a higher pressure where it warms up. Released during subsequent cooling and liquefying the refrigerant energy is transferred in a heat exchanger on the heat transfer medium of the heating circuit, usually water or brine. The refrigerant is then expanded in an expansion valve and it cools down. The cold coolant is fed into the evaporator ( boreholes, air evaporator) and proceeds by absorption of ambient heat ( anergy ) in the gaseous state.
A disadvantage of the heat pump is the substantially higher expenditure on equipment. Particularly expensive are effective evaporator ( geothermal probes, buried surface evaporator). The investment compared to a conventional gas or oil burners are significantly higher. For the regular maintenance time and maintenance is significantly lower, for example, are no cleaning and chimney sweep cost.
The heat pump process, named after Rudolf Plank Plank process is also referred to as combined heat and power engine. The limiting case of a reversible heat engine operating force is the left-handed Carnot process.
Refrigerant ( working gases )
From 1930 until the early 1990s, the chlorofluorocarbon substances were ( CFCs), the preferred refrigerant. They condense at room temperature under light pressure handleable. They are non-toxic, non-flammable and do not react with the usual materials. When CFCs are released, but they damage the ozone layer in the atmosphere and contribute to ozone hole at. In Germany, therefore, the use of chlorofluorocarbons materials in 1995 was banned. The fluorocarbons used as substitutes ( HFCs) not damage the ozone layer, however, contribute to the greenhouse effect and the Kyoto Protocol recognized as dangerous for the environment. As a natural refrigerants are pure hydrocarbons such as propane or propylene, whose flammability makes special safety measures are required. Inorganic, non-combustible alternatives such as ammonia, carbon dioxide or water were also used for heat pumps. Due to specific disadvantages, these refrigerants have not been successful in the larger scale. Ammonia (NH3 ) and carbon dioxide (CO2) are generally used in industrial cooling systems such as cooling houses and breweries.
Coefficient of performance and grade
The coefficient of performance ε a heat pump, English Coefficient Of Performance (COP ) is the ratio of the heat that is released into the heating circuit, and the energy used:
In typical performance figures 4-5 four to five times the performance employed as useful heat power is available, the gain comes from the extracted ambient heat.
The coefficient of performance depends strongly on the lower and upper temperature level. The theoretical maximum coefficient of performance of a heat pump is in accordance with the second law of thermodynamics bounded by the reciprocal of the Carnot efficiency
For temperatures, the absolute values should be used.
The grade of a heat pump is the actual coefficient of performance relative to the ideal figure of merit at the temperature levels used. It is calculated as:
Practically heat pumps grades are achieved in the range 0.45 to 0.55.
Example of a groundwater heat pump
The lower temperature level of a heat pump is 10 ° C (= 283.15 K), and the useful heat is transferred at 50 ° C (= 323.15 K). For an ideal reversible heat pump process, the inverse of the Carnot process, the performance figure would be 8.1. Real reachable is a figure of merit of 4.5 at this temperature level. With a unit of energy, exergy, which is introduced as a technical work or electrical power, 3.5 units anergy from the environment may be pumped to the high temperature level, so that 4.5 units of energy can be used as heat at 50 ° C heating water flow temperature. (1 unit exergy anergy 3.5 units = 4.5 units of heat energy).
In the overall analysis but the exergetic power plant efficiency and the power transmission losses must be taken into account, which reach an overall efficiency of about 35 %. The required 1 kWh exergy requires a primary energy input of 100/ 35 × 1 kWh = 2.86 kWh. If the primary energy is not used in the power plant but used locally for heating, is obtained at a combustion efficiency of 95 % - therefore 2.86 kWh × 95 % = 2.71 kWh of thermal energy.
With reference to the example above can ideally ( COP = 4.5 ) and 1.6 times in a conventional heating 0.95 times the fuel enthalpy used is converted as heat energy at a heating pump. Under very favorable conditions, a 1.65 -fold higher amount of heat over direct combustion can be achieved as in the detour power plant → current → heat pump.
On the test bench a coefficient of performance is achieved up to COP = 6.8 at a groundwater temperature of 10 ° C and a temperature of the useful heat of 35 ° C. In practice, however, the actual achievable performance over the year value, the seasonal performance factor (SPF ) incl losses and auxiliary drives, achieved by only 4.2. When air / water heat pumps, the values are much lower, so the actual reduction of primary energy demand is not as high fails and under unfavorable conditions ( eg power from fossil fuels), even more energy is consumed than with a conventional heating system. Then actually a complicated power heater is operated, which makes economic sense either in terms of climate protection yet. A heat pump with a JAZ > 3 is considered to be energy efficient. However, already saved from a SPF of 2 carbon dioxide emissions, according to a study already in the mix in 2008, with further expansion of renewable energies as well as the replacement of older power plants with more modern and efficient increases the savings potential, and existing heat pumps, continues to grow.
Datasheets
In the data sheets for the various heat pump products, the performance parameters are calculated based on the medium and the source and target temperature; for example:
- W10/W50 COP = 4.5,
- A10/W35: heating power 8.8 kW; COP = 4.3,
- A2/W50: heating power 6.8 kW; COP = 2.7,
- B0/W35: 10.35 kW heating capacity; COP = 4.8,
- B0/W50: 9 kW heating capacity; COP = 3.6,
- B10/W35: Heat output 13.8 kW; COP = 6.1
After several measured COP values at the WPT book. Information as W10/W50 denote the input and output temperatures of the two media. W is for Water, A for air (german air) and B for Sole (English brine ), the number behind it for the temperature in ° C. B0/W35 is, for example, an operating point of the heat pump with a brine inlet temperature of 0 ° C and a water outlet temperature of 35 ° C.
Classification of heat pump according to various criteria
- Compression electric / internal combustion engine
- Absorption (for example, ammonia absorption chiller, absorption of water into concentrated acid such as sulfuric acid )
- Adsorption (eg, adsorption and desorption of a substance to a surface such as activated carbon or zeolite, while the heat of adsorption is released or the heat of desorption recorded)
- Peltier effect
- Magnetocaloric effect
- Outside air
- Exhaust
- Groundwater ( with injection wells )
- Surface water
- Geothermal geothermal probe
- CO2 probe
- Spiral collector
- Filled area laid heat exchanger with liquid brine
- Filling flat -laid heat exchanger with refrigerant
- Thermally activated foundations
- Cool
- Freeze
- Hot water
- Heating with underfloor heating
- With radiators / radiators
- With air - convectors
There are several physical effects which can be used in a heat pump. The most important are:
- The heat of vaporization at Change of phase (liquid / gas);
- The heat of reaction mixture of two different substances;
- The temperature reduction during expansion of a ( non-ideal ) gas ( the Joule- Thomson effect );
- The thermoelectric effect;
- The Thermotunneling method;
- And the magnetocaloric effect.
Types of heat pump
Electrically driven compression heat pump
The electrically driven compression heat pump is the main application of heat pumps Represents the refrigerant is circulated in a closed circuit. It is sucked in by a compressor, compressed and fed to the condenser. The condenser is a heat exchanger in which the condensation heat to a fluid - is given - such as in a hot water circuit or into the ambient air. The liquefied refrigerant is then led to an expansion means ( capillary tube, thermal or electronic expansion valve). The adiabatic expansion, the refrigerant is cooled. The suction pressure is adjusted by the expansion device in combination with the capacity of the compressor in the heat pump such that the saturation temperature of the refrigerant is below the ambient temperature. In the evaporator, heat is therefore transferred from the environment to the refrigerant and leads to the evaporation of the refrigerant. As a heat source, the ambient air or a brine circuit can be used, which absorbs the heat from the ground. The evaporated refrigerant is then sucked by the compressor. In the example described above it can be seen that no substantially higher thermal efficiency compared with the conventional direct heating is possible by use of electrically driven heat pump with the assumed temperature level. The ratio improved for the benefit of electrically driven heat pump, if waste heat at a high temperature level as the lower heat source can be used or geothermal energy can be used at high temperatures using a suitable geothermal collector.
Heat pump with oil - or gas engine drive
A significantly higher thermal efficiency can be achieved if the primary energy can be used as gas or oil in a motor for generating mechanical power for the direct drive of the heat pump compressor. In an exergetic efficiency of the motor by 35 % and use of the engine waste heat to 90% of a total thermal efficiency of 1.8 can be achieved. However, the considerable extra effort must be considered in relation to the direct heating is due to the much higher investment and maintenance. However, there are already gas heat pumps on the market ( from 20 kW upwards Heiz-/Kühlleistung ), which make do with service intervals of 10,000 hours (usual maintenance for motor) and every 30,000 operating hours for the oil change and so provisions have longer service than boiler systems. In addition, it should be noted that certain manufacturers of motor-driven pumps gas to produce them in mass production, which are used in Europe lives of over 80,000 operating hours. This is the case because of the sophisticated engine management system, the low speeds and the optimized device processes.