What percentage of the powered aircraft is consuming

Efficiency

Lexicon> Letter W> Efficiency

Definition: Share of the energy used that can be converted into the desired usable form of energy

English: efficiency, power efficiency factor

Categories: energy efficiency, basic terms, physical fundamentals

Formula symbol: η

Unit: (dimensionless or in%)

Author: Dr. Rüdiger Paschotta

How to quote; suggest additional literature

Original creation: 03/06/2010; last change: 04/25/2021

URL: https://www.energie-lexikon.info/ffektungsgrad.html

Be careful with the efficiency data: This variable often depends on the respective conditions.

The energetic one Efficiency of a machine is a quantitative indication of its energy efficiency. It is the ratio of the usable energy generated to the energy used. For example, an electric motor has an efficiency of 90% if it generates a mechanical drive power of 0.9 kW from 1 kW of electrical power. (The remaining 0.1 kW are referred to as power loss and are released as heat.) The efficiency often depends heavily on the operating conditions, for example the load and speed of a motor or the pressure conditions in a pump. It is usually of interest not to optimize the maximum efficiency of a device, but rather the efficiency averaged over various operating conditions.

Efficiencies can also be given for systems composed of several components; one often speaks of System efficiency.

The term efficiency is similar, but not completely synonymous with efficiency.

A low level of efficiency naturally means that a larger amount of energy has to be used for a certain benefit - with an impact on energy costs and, under certain circumstances, the climate impact from CO2Emissions. That is why people usually try to achieve the highest possible levels of efficiency. However, as shown in a section below, the relationship between efficiency and energy efficiency is not quite as close as is often thought.

Typical efficiencies

When it comes to generating electrical energy, different technologies have very different degrees of efficiency:

  • Modern coal-fired power plants achieve a little over 40%, old ones only z. B. 30% or sometimes even below 25%.
  • Nuclear power plants are typically somewhat lower (e.g. 35–40%), since the steam temperatures that can be achieved are lower.
  • Modern combined cycle gas power plants (with one gas and one steam turbine) achieve values ​​well above 50%, sometimes even around 60%.
  • Larger electric motors and electric generators can have efficiencies well over 95%, and this over a relatively wide load range.
  • A car engine (Otto engine or diesel engine) can achieve efficiencies in the range of 25 to 40% at medium to high loads. In contrast, when the workload is low (e.g. in city traffic), the efficiency can easily drop below 5%. This problem can be solved with hybrid drives, in which low power is generated by an electric motor.
  • Hydropower plants can sometimes harvest over 90% of the energy in the water, depending on the conditions (amount of water, height of fall, etc.).
  • Modern wind turbines can harvest around 40–50% of the energy in the wind; the theoretical maximum value is approx. 59%. It results from the fact that the air behind the turbine inevitably retains part of its kinetic energy because it has to be removed. In contrast, the losses in the generator are relatively low.
  • In the case of solar cells, typical efficiencies in practical use are 15%, although specially optimized solar cells achieve over 40% in the laboratory.

The question of the efficiency of an aircraft's jet engine is somewhat more difficult. No directly measurable mechanical power is output here, but it can be calculated as the product of thrust and speed. For the engines of typical commercial aircraft, this is around 30%.

The generation of heat is usually possible with a higher degree of efficiency:

  • Efficient boilers such as B. Gas condensing boilers can, under favorable circumstances, achieve efficiencies of over 95% in relation to the calorific value or over 105% in relation to the (slightly lower) calorific value.
  • Electric heating achieves exactly 100% efficiency based on the electrical power supply on site, and typically a few percentage points less, taking into account the line losses between the power plant and house. However, if the efficiency of power generation is taken into account, the system efficiencies are very low, possibly even below 30%.

Overall energetic efficiency with multiple use

Overall efficiencies in which quantities of electrical energy and heat are simply added are not very meaningful.

In thermal power stations and other systems with combined heat and power, two different types of usable energy are emitted: electrical energy and low-temperature heat. There is one accordingly electrical efficiency from Z. B. 30% and one thermal efficiency from Z. B. 50%. The Overall efficiency (the sum of the two numbers) is often around 80–90%. However, it makes more sense to specify an effective overall degree of utilization with a weighted consideration of electricity and heat generation.

Efficiency and energy efficiency

For several reasons, high energy efficiency is not necessarily to be equated with high energy efficiency:

Why is high efficiency not necessarily synonymous with high energy efficiency?
  • The efficiency does not take into account how valuable the energy used is. For example, the use of otherwise lost waste heat or environmental heat, even with a modest degree of efficiency, is more energetically advantageous than the “efficient” extraction of heat from valuable fossil raw materials or electrical energy.
  • It is often important to consider system efficiency and not just the efficiency of individual components. For example, direct electric heating is inefficient despite the 100% efficiency of the electric radiator if one takes into account the energy losses in the generation and distribution of electricity. In the case of power plants, it may also be necessary to consider which energy is required to provide the fuel; this is e.g. B. significant in lignite and nuclear power plants. Another aspect is the gray energy used to create the systems, which often plays a major role in renewable energies.
  • Relatively high efficiencies are z. B. of engines and boilers are often only achieved at full load or at least at high load, while the efficiency drops sharply at low load (in partial load operation). For example, the efficiency of a gasoline car engine can be over 25% at full load, but fall well below 10% in city traffic. On the other hand, well-designed electric motors can remain very efficient even when the workload is low. Therefore, especially in city traffic, the energy savings through the electric drive of cars (instead of internal combustion engines) (→ electric cars) are far greater than would be expected based on the comparison of the full-load efficiencies.
  • Also at some power plants, e.g. B. thermal power plants, the load following operation (with variable power) usually causes significant efficiency losses.
  • All efficiency considerations ignore the fact that efficiency can also depend very much on the type of use. For example, it is extremely inefficient to drive 10 km in a two-ton vehicle to buy bread - even if the drive unit is very efficient. The same applies to the heating and lighting of unused or only poorly used living spaces.

Another example is the comparison of solar collectors with photovoltaic modules: The efficiency of the latter drops much less when the sun is weak, and the electrical energy generated has a higher value.

Efficiency and costs

Technical improvements that increase the efficiency z. B. of electrical appliances or machines, are often associated with higher manufacturing costs. For example, the cost of materials for improved electric motors is often higher. In some cases, however, the increase in efficiency even serves to reduce costs. This applies in particular to solar cells for photovoltaic modules. Indeed, if a given output can be achieved with fewer cells, or if a module of the same size generates more energy, the cost per kilowatt hour generated can decrease even if the improved solar cells are more expensive to manufacture.

Differential efficiency

The differential The efficiency of a machine indicates the extent to which a small increase in the input power affects the output power. Is he z. B. 30%, this means that 0.3 W can be emitted for each additional watt consumed.

The differential efficiency is often higher than the actual efficiency. This means that the latter increases when the power consumed is increased: the higher its utilization, the more efficient the machine. For example, this is the case with an incandescent lamp: a slightly higher operating voltage increases the power consumption, while the light output increases more strongly. (Unfortunately this is at the expense of the service life.) Conversely, the power consumption when dimming an incandescent lamp decreases less than one might expect due to the reduced brightness.

Exergetic efficiency

The exergetic efficiency also takes into account the value of energy. This is often revealing.

The deficits of pure considerations of the energetic efficiency can partly be avoided, if additionally also the exergetic efficiency is taken into account. This indicates the proportion of exergy, which z. B. is retained in a process step. Large exergy losses in one step are often an indication that (possibly also at another point in the process chain) the energetic efficiency can also suffer.

Temperature efficiency of heat exchangers

The Temperature efficiency of a heat exchanger indicates how much of the theoretically possible heat transfer is achieved. With countercurrent heat exchangers, almost 100% can ideally be achieved. Such values, however, depend on the operating conditions, in particular on the size of the material flows. The article on heat recovery discusses in detail that the quantification of the energy efficiency of heat recovery systems contains a number of non-trivial aspects, and that various terms with significantly different meanings are used accordingly.

Questions and comments from readers

23.07.2019

As I read here (https://www.forschungsinformationssystem.de/servlet/is/342234/), the power consumption of a tram is on average 12.5 kWh / 100 pkm (figures from the VDV). Now that is the power consumption. In order to arrive at the primary energy demand in relation to the German electricity mix, one would have to factor in the average efficiency of German electricity production. How high is this average efficiency?

In rush hour traffic, the average occupancy rate of the cars is only around 1.2 people per vehicle. The average fuel consumption of passenger cars for cars with gasoline engines is 7.8 liters of gasoline per 100 kilometers. Vehicles with a diesel engine require an average of seven liters of fuel for the same route (see here: https://de.statista.com/statistik/daten/studie/484054/umfrage/ Averagesbedarf-pkw-in-privaten-haushalten-in-deutschland/ ). The energy content of gasoline (calorific value) is 8.5 kWh / l, that of diesel is around 13% higher and is 9.6 kWh / l. If we calculate for the unfavorable gasoline engine, then the gross energy consumption in commuter traffic is 7 liters / 100 km times 8.5 kWh / liter1 / 1.2 people = 49.6 kWh / 100 pkm, let's say a generous 50 kWh / 100 pkm. Can you say how the tram compares to a typical local means of transport?

Answer from the author:

Such comparisons are difficult because it is usually almost impossible to avoid comparing apples with pears - for example, quantities of primary energy from very different energy sources.

You can make additional assumptions that allow a direct comparison for certain fictitious cases. For example, one could assume that the electricity for the tram comes from an oil-fired power station, with an efficiency of around 40%. Then, with the figures you have given, we would effectively have a tram consumption of 12.5 kWh / 0.40 = 31.25 kWh per 100 passenger-kilometers; that would be around 37% less than the car you assumed. But this comparison is rather weird, because only the smallest part of the electricity is generated in oil-fired power plants. The tram scores much better considering that most of the electricity it produces is associated with lower emissions (and getting “greener” over time). In addition, the situation with regard to air pollutants is of course much more favorable.

In my opinion, the indication of an average efficiency of the various types of power plants would not really make any sense, since completely different types of primary energy occur, which would have to be assessed differently, e.g. B. in relation to harmful effects.

Here you can suggest questions and comments for publication and answering. The author of the RP-Energie-Lexikon will decide on the acceptance according to certain criteria. In essence, the point is that the matter is of broad interest.

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See also: efficiency, energy, exergy, energy efficiency, performance figure, energy efficiency ratio, power loss, annual performance factor, heat recovery, expenditure figure, radiation efficiency
as well as other articles in the categories of energy efficiency, basic concepts, physical principles

Understand everything?


Question: The efficiency of the drive in an electric car is around 90%, with a petrol engine it is a maximum of almost 40%. Does this mean that an electric car uses much less primary energy?

Correct answers: (b) and (c)
Most of the electricity comes from low-efficiency power plants; however, the overall efficiency of the electric car is considerably better when driving very slowly, because in this case the gasoline engine becomes extremely inefficient.


See also our energy quiz!