Harvest factor

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

Mathematical description

The harvest factor describes the ratio of the energy used to the energy invested . In the case of power plants, electricity is mostly electricity (generally exergy ), while the “ gray energy ” used in the system life cycle describes which, ideally, should also be specified as exergy. is also known as cumulative energy expenditure referred

The higher this value, the more efficient the energy source. So it answers the question: "How often do you get the energy put in again?" Values ​​above one mean a positive overall energy balance .

The cumulative energy consumption is made up of a fixed part (plant construction, dismantling, etc.) and a variable part (maintenance, fuel procurement), which increases over time:

The energy used by a time calculated from the average net output to

The harvest factor for a plant with the service life would be

The lifespan is therefore a crucial component for the harvest factor.

Energetic amortization time

The energy payback time is the time at which the cumulative energy input is equal to the energy used, ie . This results in

In contrast to the harvest factor, the energetic amortization period says little about the overall efficiency of a power plant, as it does not include the service life. For example, the energy required to procure fuel can be very high or the service life of the system can not be much longer than the payback period.

Primary energy assessed harvest factor / amortization time

In a different definition, the energy used is converted into the primary energy that a hypothetical power plant would need to provide the same electrical energy. A fixed efficiency of this hypothetical power plant is assumed, which is usually estimated at = 34%. The energy used is thus replaced by . In order to distinguish it from the harvest factor, this “primary energy evaluated” harvest factor is referred to here . The relationship with the harvest factor is then

.

So he answers the question "How much more electricity does one get if the primary fuel is put into the construction, operation, use and fuel procurement of this power plant instead of being converted into electricity in an already existing power plant with 34% efficiency".

The energetic amortization period corresponds to the "primary energy- assessed amortization period" . The relationship between the two quantities is:

.

To convert this into the energetic amortization period, you need to state the relative cost of use .

Note that in some German-language publications it is simply referred to as “harvest factor” and “payback period”. However, this does not correspond to the usual definition in the specialist literature and the international definition of the English energy returned on energy invested (ERoEI). Here, too, the output (“harvest”) is no longer compared with input (“seed”), but a hypothetical input with an actual input. So it is a "replacement factor".

Approximation for small maintenance and fuel procurement costs

If the maintenance and fuel procurement costs are small compared to the fixed costs, and small compared to the energy provided , the harvest factor is simplified and the amortization time reduced . Both sizes are then about the simple relationship

.

linked together.

Harvest factors and payback times for some types of power plants

Cliff of usable net energy with decreasing harvest factor ( Energy Return on Energy Invested )

The table below is a compilation of sources of varying quality. The minimum requirement is a breakdown of the accumulated energy consumption according to material data. Often one finds collections of harvest factors that do not transparently prove the origin of the values. These are not included in this table.

The numbers in bold are those given in the respective literature source, those in normal print are those derived from it (see mathematical description ).

Type Harvest factor Payback period Primary energy rated
Harvest factor Payback period
Nuclear energy a)
Pressurized water reactor , 100% centrifuge enrichment 106 2 months 315 17 days
Pressurized water reactor , 83% centrifuge enrichment 75 2 months 220 17 days
Fossil energy a)
Brown coal , open pit mining 31 2 months 90 23 days
Hard coal , underground mining without coal transport 29 2 months 84 19 days
Gas power plant (GuD) , natural gas 28 9 days 81 3 days
Gas power plant (GuD) , biogas 3.5 12 days 10 3 days
Hydropower
Run-of-river power plant 50 1 year 150 8 months
Solar thermal b)
Desert, parabolic troughs + phenyl compounds medium 21st 1.1 years 62 4 months
Wind energy b)
1.5 MW ( E-66 ), 2000 VLh (German coast) 16 1.2 years 48 5 months
1.5 MW ( E-66 ), 2700 VLh (German coast, beach) 21st 0.9 years 63 3.7 months
2.3 MW ( E-82 ), 3200 VLh (German coast, beach) c) 51 4.7 months 150 1.6 months
200 MW park (5 MW systems), 4400 VLh (offshore) 16 1.2 years 48 5 months
Photovoltaics b)
Poly-silicon, roof installation, 1000 VLh (southern Germany) 4.0 6 years 12 2.0 years
Poly-silicon, roof installation, 1800 VLh (Southern Europe) 7.0 3.3 years 21st 1.1 years
a) The cost of procuring fuel was taken into account
b)The values ​​refer to the total energy output. The effort for storage power plants, seasonal reserves or conventional power plants for load balancing is not taken into account.
c)The information for the E-82 comes from the manufacturer, but has been confirmed by TÜV Rheinland.

Assessment of oil fields

The harvest factor is of great importance for assessing the oil reserves . While in the 1970s high values ​​of an average of 40 could still be achieved in oil production , these have fallen significantly today due to the more difficult development . Oil sands and oil shale in particular have very low harvest factors. Since the harvest factor only considers the relationship between the energy used and the energy obtained, the ecological consequences of oil production, for example through the flaring of the associated gases, are not considered.

Determination of the harvest factor in power plants

For fossil power plants, by definition, in addition to the energy expenditure for the construction and operation of the power plant, the fuel used is also included in the calculation, since this is irrevocably burned to generate electricity. This means that fossil power plants always have a harvest factor less than one. Renewable energies are the only types of power plant that can have harvest factors greater than one, as their energy sources such as wind, water or sun are not finite according to human judgment or regenerate if they are used sustainably (e.g. from forests). However, according to this definition, a comparison between fossil and non-fossil power plants is no longer possible, as it is different for both types of power plant.

Normally, the technical literature does not take fuel into account when calculating the harvest factor and only compares the energy required for construction and maintenance with the energy produced. This means that different types of systems can be compared with one another, regardless of the fuel, whether nuclear or solar.

The harvest factor, taking into account the fuel consumption, results for fossil power plants over a long period of system operation (20 years and longer) approximately from the system efficiency, since the energy expenditure for the construction and dismantling of the system in relation to the very large amount of energy converted over the entire period (fuel ) becomes very small. The calculation of the total energy required to manufacture a product is generally very complex. Depending on the source and, if applicable, the interests of the author, the specified harvest factors can fluctuate significantly. The duration of the assumed system life also has an influence on the level of the harvest factor and should therefore be specified.

Energetic amortization time

The energetic amortization time is closely related to the term harvest factor . It is also known under the terms energy payback time or simply energetic amortization .

The energetic amortization time describes the time that an energy generation system must be operated until the energy used for production has been recovered, if the harvest factor is therefore equal to one. Systems that are operated with renewable energies have energetic payback periods of a few months or years.

Strictly speaking, the energetic amortization time is not a key figure for profitability, but it is still relevant when evaluating technologies with regard to the potential for cost increases. Furthermore, it can be advantageous for the external presentation of companies if they strive for short energetic amortization times.

Wind turbines

In the public discussion about the use of wind energy , the energetic amortization time of wind turbines is often a controversial topic between supporters (“only a few months”) and opponents (“no energetic amortization”). While initial studies from the pioneering days of wind energy use (1970s and early 1980s), based on immature test systems, certainly led to the conclusion that an energetic amortization is hardly possible, numerous studies since the late 1980s have shown that today's mature series systems are amortize energetically in a few months.

However, there are certain differences in the results of the various studies. This is due on the one hand to the widely differing, location-dependent energy yields of wind turbines, and on the other to the life cycle under consideration (LCA = Life Cycle Assessment). In addition, the accounting methods often differ. Sometimes only the production of the system is considered (old studies), sometimes the energy consumption for raw material extraction, production, transport, assembly, maintenance over the lifetime (usually 20 years) and dismantling and disposal of the materials are added (more recent studies = CO 2 -Footprint). The cumulative energy expenditure calculated in this way for an Enercon E-82 wind turbine on a 98 m concrete tower including 20 years of operation of the system is 2,880,000 kWh of primary energy consumption, according to the manufacturer, who has not published any further figures. This number was confirmed by TÜV Rheinland as part of an assessment. If you put this primary energy consumption in relation to the amount of electricity generated (for the 20 years mentioned), this results in the harvest factor. Depending on the local wind conditions, it is between 30 (moderate location, e.g. German coast) and 50 (favorable location, e.g. selected spots on the German beach).

Hybrid analyzes based on process data and an input-output approach also record the energetic investment in the machinery at the manufacturer and the supplier. This results in an energetic amortization period of less than a year.

Photovoltaic systems

Energy is required for production, transport, maintenance, etc. - in the form of electricity and heat, among other things. This can be calculated - for example using the electricity bill of the factories involved, the fuel consumption of the trucks, etc. When the system is completely built, it produces electricity. The harvest factor now indicates how much more (electrical) energy the system produces in the course of its life than the total amount of energy required for its production, assembly and dismantling at the end of its life.

The energetic amortization time of photovoltaic systems essentially depends on the following factors:

1. Efficiency of a photovoltaic cell
2. Energy expenditure for the production of a single photovoltaic cell and for the production of the silicon required for it
3. Energy expenditure for the production of a module (frame, glass) from several photovoltaic cells
4. Energy expenditure for the transport (raw materials to the production site as well as module or system parts to the respective place of use)
5. Energy expenditure for the installation of a photovoltaic system consisting of several modules, for example on roofs
6. Electrical integration of the photovoltaic system into a power grid including an inverter
7. Energy expenditure for the dismantling of a photovoltaic system from several modules, for example on roofs
8. Energy expenditure for disposal or recycling into reusable raw materials.

For locations in southern Europe, the energy payback time (with production processes from 2011) was between 0.8 and 1.5 years for thin-film technologies and around 1.7 and 1.2 years for systems based on monocrystalline and multicrystalline solar cells.

Construction time of the plants

The usual definition of the energetic amortization period does not take into account the period between the use of energy for the production of a system and the start of energy production or energy conversion. Strictly speaking, this could be added to the payback period. While there are a few weeks to months between the energy-intensive production of essential raw materials and commissioning in wind turbines and solar parks, this time delay can be several years in large thermal or hydropower plants. With a similar period of energy use, a solar park or wind park has often already amortized energetically while a conventional power plant is still under construction.

Carbon dioxide amortization

Carbon dioxide amortization, also known as greenhouse gas amortization, describes the time it takes for the greenhouse gases generated during production to be balanced out again through energy generation.

Energy intensity of nuclear power generation

The reciprocal value of the harvest factor is understood as energy intensity in the sense of energy consumption per unit of value generated . If you consider the nuclear fuel chain from the mining of uranium to the decommissioning of a nuclear power plant , an energy intensity of over 100% means that the energy balance becomes negative and energy production is no longer sensible ( sustainable ) from an energetic point of view .

The energy intensity of the nuclear fuel chain is assessed very differently in various studies with average uranium ore contents of 2 to 150 percent: a study by the Center for Integrated Sustainability Analysis from 2006 determined an average of 18 percent in a range of 10 to 30 percent; The study by Storm / Smith determines the value of 150 percent for a uranium ore content of 0.013 percent.

If the uranium content in the extracted ore falls below the mark of approx. 0.01 percent, the processing of the extracted ore becomes the process step with the highest energy expenditure (over 40 percent) in the energy balance; From here on, the energy balance of nuclear energy generation will also be negative: with the installed nuclear capacity remaining the same, the ore content of the uranium rocks to be extracted should reach this limit value in 2078, also due to the demand exceeding supply by approx. 1/3, with a capacity increase of 2 Percent annually as early as 2059.

In the context of the increasingly complex uranium extraction, the greenhouse effect of nuclear energy generation is also increasing , the CO 2 balance of the process is getting worse: with an ore content of again approx. 0.01%, it is mentioned with 288 g / kWh, the ISA comes to one average value of 60 g / kWh. The uranium ore content also becomes a decisive factor in the amount of CO 2 emitted in the process . It is assumed that all the necessary heat comes from burning fossil fuels and not from nuclear power plants.

See also

Web links

Individual evidence

  1. ^ Karl-Heinrich Grote, Jörg Feldhusen (Ed.): Dubbel - paperback for mechanical engineering . 22nd edition. Springer , Berlin 2007, ISBN 978-3-540-49714-1 , chapter L2.
  2. a b B. Diekmann, K. Heinloth: Energy . 2nd Edition. Teubner, Stuttgart 1997, ISBN 3-519-13057-2 .
  3. a b c d e f g h i j D. Weißbach et al. (2013): Energy intensities, EROIs (energy returned on invested), and energy payback times of electricity generating power plants. Energy, Volume 52, pp. 210 ff. Doi : 10.1016 / j.energy.2013.01.029
  4. ^ E. Pick, Hermann-Josef Wagner : Contribution to the cumulative energy consumption of selected wind energy converters . Working report of the Institute for Ecologically Compatible Energy Economics, University of Essen, 1998.
  5. More wind power on land brings ecology into focus . In: vdi news. September 2, 2011. Retrieved September 17, 2011.
  6. Enercon Windblatt 4/2011 (PDF; 1.2 MB). Enercon website. Retrieved January 10, 2012.
  7. Rodoula Tryfonidou, Hermann-Josef Wagner: Offshore wind power - technology selection and aggregated presentation of results. ( Short version , PDF file, 109 kB) Chair for Energy Systems and Energy Economics, Ruhr University, Bochum 2004.
  8. a b Mariska de Wild-Scholten: Environmental profile of PV mass production: globalization. (PDF; 1.8 MB) 2011.
  9. ^ RH Crawford: Life-cycle energy analysis of wind turbines - an assessment of the effect of size on energy yield. (PDF file, 187 kB) 2007, accessed on August 30, 2018 (English).
  10. Johannes Kals: Operational Energy Management - An Introduction. Kohlhammer, Stuttgart 2010, ISBN 978-3-17-021133-9 , p. 172.
  11. ^ A b Jan Willem Storm van Leeuwen: Nuclear power - the energy balance. (PDF) Ceedata Consultancy, October 2007, archived from the original on February 4, 2012 ; accessed on March 12, 2012 .
  12. a b c d e A. Wallner, A. Wenisch, M. Baumann, S. Renner: Energy balance of the nuclear industry. (PDF 4.7 MB) Analysis of the energy balance and CO 2 emissions of the nuclear industry over the life cycle. Austrian Ecology Institute and Austrian Energy Agency, 2011, accessed on March 12, 2012 (German, 1MB summary ).
  13. Manfred Lenzen: Life cycle energy and greenhouse gas emissions of nuclear energy: A review. (PDF for a fee) ISA, Center for Integrated Sustainability Analysis, The University of Sydney, January 2008, accessed on March 12, 2012 (English).