Energy returned on energy invested

The crop factor (english Energy Returned on Energy Invested, EROEI, sometimes EROI ) is an indicator used to describe the efficiency of a power plant or in the exploitation of energy sources.

• 5.1 Wind turbines
• 5.2 Photovoltaic Systems

Mathematical Description

The crop factor describes the ratio of the energy used for energy invested. In the case of power plants, electricity is often ( generally exergy ), while the hours spent in the plant life cycle " embodied energy " describes that should be specified as exergy in the ideal case. is also referred to as a cumulative energy expenditure

The higher the value, the more efficient is the energy source. So he answers the question: "How often do you get into the ambitious energy back out? " Values ​​above unity mean doing an overall positive energy balance.

Cumulative energy demand is made up of a fixed component ( plant construction, mining, and others) and a variable part (maintenance, fuel procurement ), which increases with time:

The energy used after a period is calculated from the average net power output to

The crop factor for a plant with the lifetime would therefore

The lifetime is therefore a crucial component for the yield factor.

Energy payback time

The energy payback time is the time at which the cumulative energy demand is equal to the energy used, ie. It follows

In contrast to the yield factor the energy payback time little of the overall efficiency of a power plant states, because they do not include the service life. For example, the energy required for fuel procurement can be very high, or the life of the plant is not much greater than the amortization period.

PRIMARY ENERGY Rate (r ) crop factor / payback period

In a different definition of the energy used is converted into that of primary energy that would require a hypothetical power plant to provide the same energy. This process starts from a fixed efficiency of this hypothetical power plant, which is usually estimated at = 34 %. The energy used is thus simply replaced by. To distinguish from the crop factor is the " primary energy rated " crop factor here referred to. The link with the crop factor is then simply

So he answers the question " How much more electricity is obtained when the primary fuel in the construction, operation, use, and fuel procurement of this power station is plugged in, instead of being converted into an existing power plant with 34 % efficiency in electricity".

The energy payback time is here the " primary energy rated payback time ". The relationship between two variables is:

To convert the energy payback time so you need to specify the relative usage of effort.

Note that is called simple in some German publications as " crop factor " and the " amortization period ". However, this does not correspond to the usual definition in the literature and the international definition of the Anglo Energy returned on energy invested ( EROEI ). Again, not the output ( "Harvest " ) with input ( "seed" ) is compared, but a hypothetical input with an actual input. So it is a " replacement factor ".

Approximation for small maintenance and fuel procurement costs

Is the maintenance and fuel procurement cost is small compared to the fixed costs, and small compared to the energy provided, so the crop factor simplifies to and the payback time. Both quantities are then the simple relationship

Linked.

Harvest factors and payback times of some types of power plants

The following table is a compilation of sources of different quality. Minimum requirement is a breakdown of the cumulative energy input to the material data. Frequently you will find collections of crop factors not prove the origin of the values ​​transparent. These are not included in this table.

The bold numbers are indicated in the respective literature source, the printed normally derived therefrom (see Mathematical description).

Review of oil fields

The EROEI is responsible for assessing the oil resources of great importance. While in the 1970s with the crude oil still high values ​​of an average of 40 could be achieved, they are now fallen significantly because of the difficult development. In particular, oil sands and oil shale have very low yield factors.

Determination of crop factor for power plants

Partly also the fuel used is included with in the bill, as it is burned to generate electricity irrevocably for fossil power plants, by definition, in addition to the energy expenditure for the establishment and operation of the power plant. This fossil power plants always have a crop factor of less than one. Renewable energy can as the only types of power plants harvest factors have greater one, as their energy sources such as wind, water or sun humanly not finite or regenerate when used sustainably (about of forest stands ). However, a comparison between fossil and non - fossil fuel power plants is but according to this definition no longer possible, as it is different for the two types of power plants.

Usually in the literature, the fuel is not included in the calculation of the crop factor, and only compared to the required construction and maintenance of energy with the energy produced. This allows different types of investment irrespective of the fuel are whether nuclear or solar, compared with each other.

The crop factor with consideration of the fuel use is obtained for fossil power plants over a long system operating time (20 years or more ) may be approximated from the system efficiency because the energy required for the construction and dismantling of the plant in relation to the over the entire duration of a total reacted very large amount of energy (fuel ) is very small. The calculation of the total required for the preparation of a product of energy is very complex in general. Depending on the source, and optionally the interests of the author as well as the harvest factors indicated can vary considerably. Even the life of the plant lifetime adopted affects the height of the crop factor and should therefore be specified with.

Energy payback time

The Energy payback time is closely linked with the term crop factor. It is also known by the terms energy payback time or just energy payback.

The Energy payback time is the time that must be operated an energy recovery plant, to the time spent for the production of energy is recovered when the crop factor that is equal to one. Systems that are powered by renewable energy, have energy payback times of a few months or years.

The energy payback time, although strictly speaking not a measure of economic efficiency, but it is relevant in the evaluation of technologies in terms of cost increase potential. Furthermore, it may be for the external representation of company's advantage when they seek short energy payback times.

Wind turbines

In the public debate about the use of wind energy is often the Energy payback time of wind turbines a bone of contention between proponents ( " a few months " ) and opponents ( " no energy payback "). While initial studies from the early days of wind power utilization ( 1970s and early 1980s), based on immature test facilities, well it could be concluded that an energy payback is hardly possible, numerous studies since the late 1980s that the modern, sophisticated series systems pay for energy in a few months.

In the results of the various studies, however, there are some differences. This is due firstly to the widely varying, location-dependent energy yield of wind turbines, on the other hand the observed life cycle (LCA = Life Cycle Assessment = life cycle analysis). In addition, often differ and the accounting methods. To some extent, only the production of the plant considered (old survey), some of the energy required for raw material extraction, production, transportation, installation, maintenance over the lifetime (usually 20 years) and dismantling and disposal of materials with added ( recent studies = CO2 footprint ). The thus calculated cumulative energy demand for Enercon E -82 wind turbine at 98 m concrete tower including 20 years of operation of the plant is the manufacturer who has to publish any further numbers, 2,880,000 kWh of primary energy consumption. This figure was confirmed by TÜV Rheinland as part of an assessment. If you set the primary energy consumption in relation to the electricity produced ( for the aforementioned 20 years) so you can reap the harvest factor. He is depending on the local wind conditions between 30 ( moderate location, eg German coast ) and 50 ( favorable location, eg selected locations in the German beach ).

Hybrid analyzes on the basis of process data and an input-output approach also capture the energy investment at the manufacturer and at the suppliers in machinery. This results in an energy payback time of less than one year.

Photovoltaic systems

For the production, transportation, maintenance, etc. energy is needed - including in the form of electricity and heat. These can be calculated - for example, based on the electricity bill involved the factories, the fuel consumption of the truck etc. If the plant is built, it produces electricity. The crop factor now indicates how much more ( electrical) power in the system in the course of their lives as total energy produced for their preparation and requires assembly and disassembly at end of life.

The Energy payback time of photovoltaic systems depends mainly on the following factors:

For southern European locations, the energy payback time (2011 ) is between 0.8 to 1.5 years for thin-film technologies and about 1.7 years for plants based on mono- and multicrystalline solar cells.

Carbon dioxide payback

The carbon dioxide payback, or greenhouse gas payback called, describes the duration until the greenhouse gases resulting in production are compensated by the energy again.

Energy intensity of nuclear energy

The reciprocal of the factor is in terms of harvesting energy consumption per produced unit value seen as energy intensity. Considering the nuclear fuel chain from uranium mining to decommissioning a nuclear power plant, this means an energy intensity of about 100 %, that the energy balance is negative and the energy production from energy point of view no longer makes sense ( sustainable ) is.

The energy intensity of the nuclear fuel chain, is seen very differently in different studies at intermediate uranium contents with 2 to 150 %: a study by the Centre for Integrated Sustainability Analysis 2006 identified an average of 18% in the range of 10 to 30%; the value of 150 % was found, the study of Storm / Smith for a uranium content of 0.013 %.

Falls below the content of uranium in the ore obtained the mark of approximately 0.01 % in the energy balance is the treatment of the extracted ore to the process step with the highest energy demand ( about 40%); from here is also the energy balance of nuclear energy negative: for the same installed nuclear capacity is expected to approximately 1/3 of excess demand, the ore grade of the pumped uranium rocks in the year 2078 reach because of the offer this limit, with a capacity increase of 2 % already 2059 year.

As part of the elaborate nascent uranium mining climb and the greenhouse effect of the nuclear power generation, the CO2 balance of the process is getting worse: at a head grade of approximately 0.01 % in turn he is mentioned with 288g / kWh, the ISA is an average value 60 g / kWh. Here, the uranium content is also a decisive factor in the amount of emitted CO2 in the process. It is assumed that all the necessary heat comes from burning fossil fuels and not by nuclear power plants.