Energy conversion efficiency
The efficiency is a measure of the efficiency of energy and changes the energy transfers. It is a dimensionless quantity and is the ratio of useful power to the power input or the ratio obtained in a given time useful energy supplied at the same time energy. Usually, the efficiency is indicated by the Greek letter.
Or
For example, the mechanical power delivered by an electric motor to the shaft and the electric power is supplied to the motor. The ratio can take values between 0 and 1 and can be specified as a percentage.
In addition to the general definition, also other concepts such as efficiency or coefficient of performance established that take into account depending on the department certain boundary conditions and characteristics of energy flow in the considered systems. Efficiencies and work numbers are always based on a consideration period ( usually one year ), for which the energies are summed.
The current output or input power or energy can be very different, regardless of the efficiency when power or energy absorption and release occur with a time delay, such as when charging and discharging of a battery, or when recording of solar energy by plants and subsequent energy release by burning.
The grade describes, in contrast, only the internal losses of a machine and falls mostly from significantly better. The difference between the reference and actual performance is called, or heat capacity.
- 8.1 Notes and references
- 8.2 sources
Range of values
The theoretically possible range of values is from 0 to 1 or from 0 to 100 %. The highest value ( 1 or 100 %) can not be achieved in practice in machines because all operations energy is converted by heat or friction into thermal energy. In heat engines, the efficiency is also limited by the exhaust loss.
An efficiency greater than 1 correspond to a perpetual motion machine of the first kind, which is against the principle of energy conservation. Devices that emit more energy than they receive or have saved are not possible.
In heat engines, the efficiency can never exceed the ideal efficiency of the Carnot cycle.
Mechanical efficiency
The mechanical efficiency is given for example in gears or bearings and is part of the overall efficiency of a system (eg drive train ). It takes into account the conversion of part of the mechanical input power to the heating of the components by the heat.
Thermal efficiencies
Thermal efficiency and process efficiency
The thermal efficiency and process efficiency is the ratio of the recovered mechanical power supplied to the heat flow in a heat engine, such as a steam turbine:
As with the thermal efficiency of the recovered mechanical power and the supplied thermal power.
The upper limit for each thermal efficiency is the Carnot efficiency:
With the lowest and the highest temperature occurring in the process in Kelvin.
Combustion efficiency
The firing efficiency ( FTW ) indicates the recovery of the heat from the combustion of a fuel at nominal power. It only takes into account the loss of heat by cooling the exhaust gases to the ambient temperature. An evaluation of the energetic efficiency of a heat source alone with the help of this flue gas loss is possible if all other losses are negligible. By the end of the 20th century, this approximate calculation was common for heating systems, today the plant efficiency or annual efficiency is considered.
The FTW is the difference of 1 (100% ) and the flue gas loss
Modern systems increase efficiency by reducing the exhaust gas temperatures and by recovering the heat of condensation of water vapor and hydrocarbons. They use the calorific value of a fuel, while in old plants only the calorific value could be used. There are placed on the chimney system demanding. The exhaust gases must be removed (eg fan ) partially active because they are no longer warm enough to ascend themselves. The chimney is exposed to corrosive attack by the combustion residue dissolved in the condensed water ( sooting ). Under certain conditions, also forms tar, which must be collected and returned to the combustion.
Full - condensing boiler, the air exhaust system or the heating of adjoining rooms use in boilers, the latent residual heat of the exhaust gas below the return temperature of the normal heating system, it is to be noted however that gases have a low heat capacity and sometimes with better thermal insulation of the house or other energy -saving measures " for the same amount of money ", if appropriate, a higher monetary benefits could be achieved.
The heat loss due to the reaction enthalpies in the formation of nitrogen oxides and the reduction thereof by reduction of the firing temperatures using porous burners or catalytic burner is not taken into account in ( the state of the art no longer relevant and thus outdated) method of calculating the combustion efficiency.
Boiler efficiency
The boiler efficiency hK (% ) is the ratio of nominal heat output in percent of rated load when measured at constant power at rated output. It takes into account how the FTW also the exhaust loss but in addition also the heat loss to the environment of the installation room.
Isentropic efficiency
The isentropic efficiency is mostly used for the description of heat engines.
Since thermal energy can not be completely converted into other forms of energy ( eg, electrical energy, mechanical energy ), the terms anergy and exergy have developed that identify which part of the thermal energy can be converted into work ( exergy ) and which must remain as thermal energy ( anergy ). It is thus
The efficiency of the actual thermal engine is always less than or equal to that of an ideal heat engine, the Carnot efficiency
The isentropic efficiency used this comparison process to compare it with the real process.
Gross and net efficiency
In particular, in thermal power plants a distinction is made between gross and net efficiency. The gross efficiency refers to the gross output, ie the electrical power without reference to own consumers, such as Feedwater pump:
( In this case, the mass flow of the supplied fuel and the calorific value of the fuel. )
The net efficiency on the other hand refers to the net output, ie the electrical performance minus the power consumption of own consumers:
In the German -speaking area is specified for power plants in the net efficiency, unless it is explicitly mentioned as another.
System efficiency and overall efficiency
If several machines and transformers after the other, their individual efficiencies are multiplied to the overall efficiency of the plant, the plant efficiency.
Example:
- ELECTRIC MOTOR 90% ( 0.90)
- Power plant 40 % (0.40 )
- Transformer at the power plant 99% ( 0.99)
- Transformer in the vicinity of the load 95% ( 0.95)
Overall efficiency: or 34%.
In this example it is assumed that the transfer of energy between the individual machines happens lossless. If this is not the case, additional efficiencies of energy transfer must be taken into account.
If the energy released in a thermal conversion process waste continue to be used, for example for air preheating, oil pre-heating or district heating, as is the case with combined heat and power ( see table below), increases the efficiency of the system, as a part of the real for the process lost heat can be used anyway.
Annual efficiency
The annual utilization rate is the average annual plant efficiency throughout all operating cycles of a heat generator.
It allows a more realistic cost -benefit analysis for energy saving measures than is possible with the approximate calculation of the FTW. Since average homes by improving the insulation consume less and less energy, the consideration of other losses is becoming increasingly important. Among the heat loss of the heat source due to radiation, the loss fall by the condensation of water in the fuel, heat required by frequent starts heating with poor efficiency in the starting phase, low burner running time by about large-sized boiler.
Even though modern individual units of a heating system usually have an efficiency at rated output of more than 90 %, the annual utilization rate amounts to only 60-80 %, which are emitted from the radiator.
Standard efficiency
The standard efficiency refers to the new technique of condensing boiler with modulating output control ( partial load ) by stepped part load operating points of 12.8 %, 30.3%, 38.8 %, 47.6 % and 62.6 % of the rated output, with.
The calculation is in accordance with DIN 4702 Part 8 set for
Efficiencies of greater than 100 %
Machines with efficiencies greater than 100% as " perpetual motion machine of the first kind ". Such machines may not even exist in theory due to the energy conservation law. If nevertheless efficiencies are given more than 100 % in practice, the cause lies in the imposition of an incomplete energy balance equation.
An example are condensing boilers, in which some of calorific related efficiencies of over 100% will be given. Here, the heating value of the fuel is recognized under " energy expended ". However, the calorific value is calculated from the total heat released minus the heat of vaporization of the water produced during combustion. Thus, the heating value includes only a portion of the total fuel energy. In contrast to the "conventional" boiler exhaust gas is the boiler cool down to prevent the vaporized during combustion condenses water. The released heat of condensation will benefit the useful energy, but was initially not recognized to the input energy.
If the efficiency is not calculated on the basis of the heating value but on the basis of gross calorific value of the fuel, in the ideal case, an efficiency of 100 % is achieved.
Heat pumps and refrigeration systems - for example, air conditioners and refrigerators - Works as reverse heat engine. In the technical literature is in these devices in addition to the term " efficiency " coefficient of performance ( ) is used as a measure of efficiency. The manufacturer's instructions indicate the coefficient of performance for refrigeration systems, however, often called " efficiency ". The heat pump promotes heat energy from the environment and bring them to the desired temperature level. The total heat provided this power is greater than the thermal power produced during the compaction process. Therefore, for this process " efficiencies " in excess of 100 % can be achieved. Typical values are between 300% and 800 %, which corresponds to an efficiency ( COP = ) 3-8. To avoid confusion, the thermal efficiency of heat pumps and refrigerating machines and COP ( Coefficient of Performance Data Sheet ) is referred to, which is less than the reciprocal of the Carnot efficiency.
Examples
Notes:
- Examples of the efficiency of light sources see: light yield.
**) The specification of efficiency with different " target energy types ", in this case electrically and thermally, is not meaningful, since these types of energy have a different " value" (see also entropy). So can be converted into heat electrical and mechanical energy to 100 %, the other way that is only possible within the limits mentioned above. Example: a cogeneration unit with conversion to 30 % electrical and 60 % thermal energy would result after this observation a (fake ) "efficiency " of 30% 60 % = 90%. With a combined cycle power plant with 60 % electrical efficiency I can provide 30 % electrical energy available and operate with the remaining 30 % of electrical energy a heat pump. With a plot ratio of 5 I get so 150% heat (eg heating ) - ie 2.5 times the amount of the cogeneration plant.
Indication of the efficiency with speaker data
Acoustic efficiency η ( eta ) of a speaker:
Pak = output acoustic power
Pe = electric power supplied
The definition of the acoustic efficiency is consistent with the conversion of the acoustic level.
In the speaker data never the very low efficiency is given in percent, but the sound pressure level in dB / W / m (or dB / (W * m)), which is incorrect labeled " efficiency ". The efficiency of 0.002 to 0.02 - that only between 0.2 and 2 percent. It can be converted into the sound pressure: