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====== Function, boundary conditions and potentials of renewable energy generators ======

This article is based on a paper published in the 56th volume of the Research Group - Energy efficiency and renewable energy: Conflict of goals or synergy?. The original article by //Sven Huneke//, is availabe in German in the proceedings, which can be accessed [[https://passipedia.de/medien/medien/veroeffentlichungen/uebersicht_protokollbaende/protokollband_56|here]]. \\
More English articles about 56th volume of Research Group are [[https://passipedia.org/phi_publications#nr56_energy_efficiency_and_renewable_energy|here]]

===== Definition of a renewable energy generator =====

The term "renewable" or "regenerative" energy is used to summarise those energy sources that are considered inexhaustible under human time horizons. However, "renewable energy" should not be understood in a strictly physical sense, as energy can neither be destroyed nor created according to the law of conservation of energy. Energy can only be converted into different forms or exchanged between different areas of a system.

Renewable forms of energy are categorised into three areas based on the original energy source. Geothermal energy via the heat in the earth's interior and planetary energy as a reciprocal force between the moon and the earth play a comparatively minor role. Solar energy is by far the largest source of renewable energy. On the one hand, this can be utilised directly or converted using technology (example: solar thermal systems and photovoltaics). Hydropower, wind energy and biomass are alternatively defined as indirect solar energy, as these energy sources make the natural conversions of solar radiation usable (e.g. evaporation, precipitation, biomass, wind systems).

Global renewable energy availability is virtually inexhaustible. Various studies differ in their quantitative estimates. What they do have in common however is that the amount of energy irradiated annually by the sun alone corresponds to a multiple of the world's primary energy demand. By comparison, the total amount of conventional energy sources available worldwide only amounts to a fraction of this potential.
===== Development of renewable energy in Germany =====

The contribution of renewable energy sources to total primary energy consumption in Germany has grown continuously since the 1990s and amounted to 14.8 % at the end of 2019 ([AGEE-Stat. 2020]). For one thing, this maximum value so far is due to the ongoing expansion of renewable technologies; for another, the annual availability of wind power and solar energy varies, which means that a consecutive peak value at the end of each production year is not guaranteed.

A separate consideration of the individual sectors of heating and cooling, electricity and transport reveals major differences. At the end of 2019, renewable energy accounted for 42.1% of the gross electricity supply, 14.5% of final energy consumption for heating and cooling and 5.6% in the transport sector ([UBA 2020]). The latter two figures have continued to stagnate since 2012, with the socalled German energy transition taking place almost exclusively in the electricity supply in recent years.

===== Renewable energy potentials in Germany =====

When determining the potential of renewable energy sources, a distinction is made between the following potentials: the starting point is the theoretical potential, which describes the maximum possible utilisation quantity of an energy source regardless of limiting practical boundary conditions. In contrast, the technical potential defines the utilisation of the theoretical potential, taking into account current technologies. This includes losses during energy conversion. Structural and ecological restrictions such as the conservation of natural resources and cycles and general nature conservation concerns also play a role. The economic potential, on the other hand, is determined by the economic framework conditions and generally reduces the existing technical potential under financial aspects. The potential that can ultimately be realised is the actual realistic contribution to the renewable energy supply subject to other limiting factors such as production capacities or other administrative or legal restrictions.

The potentials of key renewable energy sources in Germany are stated below under the technical assessments of potential of the available published studies. It should already be pointed out at this point that these figures are purely estimates that are subject to extreme uncertainties. The last three decades have shown that future scenarios for both conventional and renewable energy supply can differ drastically from subsequent developments due to changing political framework conditions or singular events (e.g. the nuclear disaster at Fukushima).

==== Hydroelectricity ====

Alongside wind energy, hydropower is the oldest source of energy utilised by humans. A distinction is made between run-of-the-river plants, hydroelectric power plants with reservoir storage and pumped storage power plants.

The latter consist of an upper reservoir and a lower reservoir and are used to store electrical energy. Energy is stored in the upper reservoir and can be recovered by discharging the water into the lower reservoir via a turbine system. Pumped storage power plants can be used flexibly and are utilised to cover peak loads and to maintain frequency. However, they are only considered a renewable energy source if renewable energy is actually used to pump the water from the lower to the upper basin.

Run-of-the-river power plants utilise the flow of a river or canal to generate electricity. On account of the continuous water supply, they are used for the base load, but are subject to seasonal fluctuations due to the varying water supply. Hydroelectric power plants with reservoir storage, on the other hand, have the advantage of additional water storage using a weir and can be used flexibly.

There are currently around 7,300 hydropower plants in Germany [BEE 2020]. The regional focus is on the Mittelgebirge (low mountain range) region in Bavaria and Baden-Württemberg. Over the past 10 years, the share of gross electricity consumption has remained almost constant at four per cent with a supply of approx. 20 TWh per year [AGEE Stat. 2020]. Possible expansion is considered limited in this country, which means that the current figures are assumed as a projection into the future. The main obstacles to expansion are comparatively high investment costs and, in some cases, huge interventions in ecosystems and the natural water balance. The construction of large hydropower plants is often associated with forced resettlement.

==== Bioenergy ====

Bioenergy sources are energy carriers produced from biomass which are differentiated according to their aggregate state and have universal application. This is because solid, liquid or gaseous biomass can be used to generate both electricity and heat, and can also be used to produce biofuels.

Solid bioenergy sources include wood and wood products (logs, wood chips, wood pellets), straw and energy crops. They are primarily used for heat supply and in large-scale plants for combined heat and power supply. The best-known liquid bioenergy sources are vegetable oils, biodiesel and bioethanol, which are used as a substitute for traditional fossil fuels (petrol, diesel, natural gas) in fuel and drive technology. Biogas, on the other hand, is produced when organic matter is decomposed by microorganisms in the absence of oxygen in a biogas plant. Liquid manure, silage or biogenic residues (e.g. organic waste) are used as starting materials for this biological fermentation. Biogas can be used for heat and energy, and is used often in combined heat and power plants for combined heat and power generation.

The utilisation of biomass for energy has always been a sociopolitical area of conflict. There is a debate about the competition between different forms of cultivation for agricultural land, with the cultivation of energy crops ("tank/fuel") competing with the cultivation of crops for food and feed production ("plate or trough/food or feed"). The social consensus in terms of resource-conserving provision of bioenergy is that the utilisation of natural waste that would otherwise not be used has priority over the targeted cultivation of biomass.

In 2019, biomass contributed to the energy supply in Germany more than all other renewable energies together (approximately 234 TWh) ([UBA, 2020]). This means that around 51.7 per cent of the renewable energy supply came from biomass. The largest share (around 90 %) of the newly added capacity related to the expansion of existing plants. Recent studies have assume a future technical biomass potential from residual and waste materials of approximately 275 TWh for the year 2030 [FNR 2016], which corresponds to a presently unutilised residual and waste material potential of approximately 125 TWh. If future biomass utilisation - as described above - is based exclusively on residual and waste materials, it becomes clear that the use of biomass can hardly be expanded.

In the past, bioenergy sources were primarily considered in terms of their base load capacity. The awareness that bioenergy is too valuable in material, energy and monetary terms to be used exclusively as a base load has now become reinforced. Its future use as a storage medium will therefore lie in demand-orientated, flexible peak load operation for balancing the fluctuation of other energy sources.

==== Near-surface heat sources and deep geothermal energy ====

Geothermal energy or ground heat is defined as the thermal energy stored below the solid surface of the earth. Geothermal energy can be used directly for heating and cooling, or indirectly to generate electricity or as a heat source/sink for a heat pump. The global average surface temperature is around 13 °C and the global geothermal gradient is three Kelvin per 100 metres. Geothermal energy sources are categorised into deep geothermal energy and near-surface/shallow heat sources.

Deep geothermal energy involves the extraction of energy from a depth of more than 400 metres. The reservoirs are tapped via deep boreholes and do not renew their energy content as quickly as the energy is extracted for economic reasons. An individual reservoir or the heat content of a rock layer is therefore not "renewable energy" in the strict sense.

In 2019, deep geothermal energy contributed 0.03 % to gross electricity generation with 196 GWh/a ([UBA 2020]). A significant increase is not foreseeable for a future assessment of potential, as very high investment costs and, in some cases, incalculable risks of seismic events represent huge obstacles.

Near-surface geothermal energy, on the other hand, uses the subsurface down to a depth of approximately 400 metres. The heat is extracted from the upper layers of earth and rock or from the groundwater and is available regardless of the weather and at any time of the year. With existing technologies, it is possible to utilise the geothermal energy potential practically everywhere by passing a heat transfer fluid through the ground in a closed circuit via geothermal probes or collectors and heating it up in the process. This natural geothermal heat can be raised to the required temperatures for heating purposes using heat pumps.

This heating technology is only considered a regenerative heat source if the heat pump is operated with renewable energy.

==== Wind power ====

As an indirect form of solar energy, air masses are constantly in motion on a global and local scale due to differences in temperature and pressure. Wind power plants can convert this movement of the wind into electrical energy. Due to its worldwide availability, comparatively favourable cost development and rapid technological progress in recent decades, wind power is already making a significant contribution to meeting the demand for electricity.

This rapid growth holds true both for the number of turbines installed and the output of the individual turbines. 29,456 onshore wind turbines with a capacity of 53.9 GW were in operation at the end of 2019 in Germany ([WindGuard 2020a]). The average capacity of newly installed turbines in 2019 was 3.3 MW, whereas in the early 1990s a capacity of 200 - 300 kW was still the standard. The average hub height also increased to 133 metres at present (average hub height of newly installed turbines in 2019).

The tall hub heights make it possible to utilise higher wind speeds at greater heights, as the wind speed profile increases with height depending on the ground conditions. Parallel to the height of wind turbines, the length of the rotor blades and thus the effective rotor area through which the wind flows has also been developed further technologically. The achievable output of a wind turbine increases with increasing rotor area, higher air density and the third power of the wind speed. Doubling the wind speed therefore means eight times the output.

As a parameter that varies with time, the wind speed already fluctuates at a resolution of one second, but also varies according to daily, monthly and annual cycles. Fig. 1 (top left) shows the year-round average of the diurnal cycle of a wind measurement at an inland location at measurement heights between 40 metres and 240 metres above ground. One can observe a wind field that is homogeneous at all heights during the night. With the onset of solar activity, there is mixing of the air layers: this energy input increases the wind speeds on the ground and rising air masses slow down the wind speeds at the top. This phenomenon depends on the intensity of solar radiation and can therefore primarily be observed in the months with high radiation levels. The same measurement provides an annual variation that is exemplary for wind availability in Germany, with high wind speeds in the winter half-year and sharply decreasing wind potential in the summer months (Fig. 1, top right). This effect intensifies with increasing measurement height.

In addition to seasonal fluctuations, wind speeds are also subject to interannual variation. Time histories of average year-round wind speeds for the period from 1990 to 2019 show a long-term trend of decreasing wind speeds for Germany ([UBA 2020, Appendix]). However, there are no reliable sources for assuming this trend for future years.

Offshore wind power offers great potential. Offshore wind farms become profitable only with larger turbine volumes so that the very high infrastructure costs can be compensated for. The offshore market became established in Germany around 2010. 1469 turbines with a capacity of 7.5 GW were in operation in Germany in the North Sea and Baltic Sea at the end of 2019 ([WindGuard 2020b]). The Baltic Sea is less important for the wind industry than the North Sea due to the lower wind speeds and the smaller size of the appropriate area. Offshore wind availability is significantly higher compared to locations on land and the wind also blows more consistently and evenly with increasing distance from the coast.

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Fig. 1: Above: diurnal (left) and annual (right) variation in average wind speeds on a one-year average at an inland location in Germany by way of example, measurement with a LiDAR device between 40 metres (light blue) and 240 metres (dark blue) above ground, source JUWI AG.

Bottom left: diurnal variation in global radiation in Karlsruhe ([Quaschning 2009]). Bottom right: annual variation in the daily totals for direct and diffuse radiation in Berlin ([Quaschning 2019]).

Generally, the wind potential over land may already vary significantly within a few metres, especially on complex terrain. High-resolution model simulations are therefore necessary for small-scale wind farm planning. For large-scale planning, there is a characteristic north-south gradient for the wind potential in Germany: wind maps of long-term average wind speeds show the described higher wind speeds in the coastal states down to comparatively low values in the Alpine foothills (see Fig. 2). According to calculations by the Fraunhofer Institute for Wind Energy Systems [BWE 2011, IWES 2013], the future potential of wind energy in Germany is estimated at around 200 GW on land and 54 GW offshore. This means that the gross electricity supply presently in 2019 could be almost met through wind energy alone in the future. However, current obstacles such as the increase in lawsuits against planned wind farm projects and discussions about regulations governing distancing already show the great uncertainties underlying such scenarios.

Even in 2019, so-called small wind turbines with a total height of less than 50 metres and an output of less than 100 kW still have the status of a niche sector or only being considered by enthusiasts. Their quality, market maturity and economic viability mean that this potential also seems to be manageable in the future.

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Fig. 2: Left: global radiation in Germany, average annual totals, period from 1981 to 2010 ([DWD 2004]). Right: average wind speed in Germany, 140 metres above ground, period from 1997 to 2016 ([anemos 2017]).

==== Solar energy ====

Solar energy is globally available in more than sufficient quantities and can be utilised directly in a variety of ways: solar cells in photovoltaic systems, solar thermal power plants and solar collectors convert the radiant energy into electricity or heat.

As large-scale plants for generating electricity, solar thermal power plants are mainly located in sunny regions of the world with high levels of direct sunlight. The USA and Spain have established themselves as the biggest markets. The direct rays of the sun are focussed onto a receiver through which a fluid flows via point focus or linear focus concentrating mirror systems. The heated fluid then drives conventional gas or steam turbines.

Non-concentrating solar thermal energy is used to convert solar radiation into heat using solar collectors. Examples of applications include hot water generation, provision of hot water for heating systems, heat recovery for industrial processes and the operation of air conditioning systems using absorption chillers. The central component of a solar thermal system is the collector. Here too, the solar radiation energy is converted into heat and transferred to a heat transfer fluid inside the absorber. This is located in the collector housing, which is usually covered by a glass plate and enclosed by thermal insulation. Depending on the insulation technology, a distinction is made between flat-plate collectors with conventional insulation material and vacuum tube collectors, which achieve insulation by means of a vacuum. Since there is only space for small amounts of heat transfer medium in the collectors, these are usually combined with heat accumulators, which have the task of providing the desired amount of heat when solar radiation fluctuates.

Photovoltaics can also be used to convert direct and diffuse solar radiation into electrical energy. Photovoltaic generators are characterised by their modular design, which allows them to be implemented across a wide power range. The spectrum ranges from a few mW for supplying the smallest electronics to systems in the kW range and large GW power plants. The key element of photovoltaic systems are solar cells, which are interconnected to form modules. The global market is dominated by solar cells made of silicon.

The solar cells on the market differ in terms of structure and efficiency. The highest efficiency is currently achieved with monocrystalline solar cells, although theoretical maximum efficiencies under laboratory conditions are sometimes significantly lower in practice due to the installation location and orientation of the modules, shading, overheating and soiling of the modules.

The main areas of application for photovoltaics are installation on roof surfaces and as so-called ground-mounted systems. According to [ISE 2020], photovoltaic modules with a nominal output of approx. 49 GW were installed in Germany at the end of 2019, of which rooftop photovoltaics accounted for approx. 75 %. In 2019, photovoltaics covered approximately 8.2 % of gross electricity consumption in Germany with a total electricity generation of approximately 46.5 TWh.

The PV potential achievable in practice is estimated at around 400 GW - roughly equally divided between 200 GW roof-mounted ([Quaschning 2011]) and 200 GW ground-mounted photovoltaics ([ISE 2020]). As with wind energy expansion, problems with acceptance by the public are also expected in the case of ground-mounted systems, whereby the criticism relates to land use conflicts and aesthetic aspects in general. Integrated photovoltaic technologies represent an innovative addition with which photovoltaic technologies can be integrated into the building shell, on and around driveways, in vehicles and on water surfaces (for example in flooded open-cast mines). In addition, special photovoltaic mounting systems can be installed on arable land as so-called agrophotovoltaics with simultaneous utilisation of plant and electricity production to increase land efficiency. According to a study by Fraunhofer ISE ([ISE 2019]), the technical potential of integrated photovoltaics amounts to a gigantic output of 3,400 GW.

The annual total of the average annual solar radiation for the period from 1981 to 2010 is shown in Fig. 2 for the surface area in Germany. The values from the German Meteorological Service are based on long-term measurements and additional satellite data. The general decrease in total irradiation from south to north is obvious. Low annual totals (around 950 kWh/(m²a)) in the north and west of Germany contrast with the highest values (around 1,220 kWh/(m²a)) in the Allgäu. Thus there is a difference of some 20 per cent in available solar radiation within Germany.

Long-term time series for averaged global radiation over Germany show a fluctuation range of individual annual totals around the long-term average. On average, these deviations are around 10 %, although individual extreme years can also cause significantly higher outliers. Statistical long-term analyses show a slight increase in global radiation for Germany and Europe in the order of 2 - 3 W/m² per decade since the 1980s ([Wild 2012]). This so-called "global brightening" was preceded by a "global dimming" of approx. 3 W/m² per decade between 1950 and 1980. Increases or decreases in air pollution and an associated change in the aerosol composition of the air are considered to be the primary causes of these long-term trends. Volcanic eruptions or changes in extraterrestrial radiation play a subordinate role.

Fig. 1 shows the average seasonal distribution of solar radiation in kWh per m² and day that is typical for our latitudes based on the example of the locations Berlin and Karlsruhe. It shows the proportion of direct radiation that reaches the earth's surface directly through the atmosphere and the diffuse radiation that results from the scattering of solar radiation due to aerosols and air molecules. Approximately 90 per cent of the annual radiation on a horizontal surface is measured in the 8 months from March to October. Fig. 1 also shows the typical daily course of a typical sunny day in July and in December (22 December) over Karlsruhe. In addition, a low-radiation daily cycle is shown for 28 December with minimal solar radiation.

==== Anticorrelation between wind and solar availability ====

From Fig. 1 and Fig. 2 it is evident that the energy availability from wind and solar energy in Germany complements each other in principle due to the given anti-correlation. Firstly, there are opposing north-south gradients of wind speeds and solar radiation across Germany. Secondly, Fig. 2 shows that on a rough average, the annual and diurnal variations behave in opposite ways. However, the highly volatile behaviour of both forms of energy and the actual energy demand, especially for coping with the "winter peak", require innovative, complex storage solutions in order to come close to the goal of a 100% energy supply based on renewable energies (see also the article by Wolfgang Feist in this Protocol Volume).




===== Summary =====

The massive expansion of wind and solar energy is seen as a key driver of the electricity-based energy transition. Assuming the stabilisation or only moderate expansion of power plant capacities for biomass (as a flexible energy source), geothermal energy and hydropower, the current estimates for the technical potential of offshore and onshore wind power and the photovoltaic potential offer realistic opportunities for almost doubling the gross electricity generation of approximately 600 TWh currently in 2019.

Even an expected increase in electricity consumption in Germany due to increasing electromobility, the increased use of heat pumps, hydrogen production and the additional demand in energy-intensive industries could be met by the given expansion potential of wind and solar energy. These scenarios are based on the latest technological developments. Foreseeable technological progress will significantly increase the renewable energy potential even more, with the applications in the field of integrated photovoltaics in particular holding enormous potential.

All of the scenarios and potentials presented here are based on technical estimates and are subject to a high degree of uncertainty. The past has shown that the history of renewable energy supply has been characterised by political framework conditions, economic crises, technological innovations and social change processes, so that reliable forecasts cannot be made for its further development even for 2020.

===== Literature & References =====

[AGEE-Stat. 2020] Development of renewable energy sources in Germany in 2019, charts and diagrams based on the latest findings of the working group on renewable energy statistics of February 2020.

[anemos 2017] anemos Gesellschaft für Umweltmeteorologie mbH (association for environmental meteorology): Windatlas project commissioned by the Federal Ministry of Economics and Technology BMWi, March 2017

[BEE 2020] Bundesverband Erneuerbare Energie e.V (German Renewable Energy Federation), [[https://www.bee-ev.de/unsere-technologien/wasserkraft|https://www.bee-ev.de/unsere-technologien/wasserkraft]], accessed on 29.07.2020

[BWE 2011] Bundesverband Windenergie (German Wind Energy Association): Potential of onshore wind energy utilisation, 2011 [DWD 2004] German Meteorological Service, climate and environmental consulting Hamburg, 2004, download from [[http://www.dwd.de|www.dwd.de]], accessed on 29.07.2020.

[FNR 2016] Fachagentur Nachwachsende Rohstoffe e.V., Biomass potentials of residul and waste materials, status quo in Germany, Series of publications on renewable raw materials, Volume 36, 2015

[ISE 2019] Fraunhofer ISE: Integrierte Photovoltaik - Flächen für die Energie¬wende, Positionspapier (Integrated photovoltaic systems – areas for the energy transition, position paper), October 2019.

[ISE 2020] Fraunhofer ISE: Aktuelle Fakten zur Photovoltaik in Deutschland (latest facts on photovoltaic systems in Germany), version on 26.03.2020.

[IWES 2013] Fraunhofer-Institut für Windenergie und Energiesystemtechnik: Energiewirtschaftliche Bedeutung der Offshore-Windenergie für die Energiewende (Fraunhofer Institute for wind energy and energy system techonology: Relevance of offshore wind energy for the energy transition from the energy industry perspective, 2013.

[Quaschning 2009] V. Quaschning: Regenerative Energiesysteme. Technologie - Berechnung – Simulation (regenerative energy systems: technology, calculation, simulation). 6th edition, 2009

[Quaschning 2011] V. Quaschning: Wie viel Solarstrom brauchen wir? Sonne, Wind und Wärme (How much solar power do we need? Sun, wind and heat) 03/2011.

[Quaschning 2019] V. Quaschning: Regenerative Energiesysteme. Technologie - Berechnung – Klimaschutz (regenerative energy systems: technology, calculation, climate protection), Hanser Verlag München, 10th edition, 2019 [UBA 2020] Renewable energy sources in Germany, data on the development in 2019, Federal Environmental Agency, March 2020.

[Wild 2012] M. Wild, 2012: Enlightening Global Dimming and Brightening, Bulletin of the American Meteorological Society 93(1):27-37,

[WindGuard 2020a] Deutsche WindGuard, 2020: Status des Windenergieausbaus an Land in Deutschland (status of onshore wind energy expansion in Germany), 2019.

[WindGuard 2020b] Deutsche WindGuard, 2020: Status des Offshore-Windenergieausbaus in Deutschland (status of offshore wind energy expansion in Germany), 2019.

===== See also =====

[[phi_publications:nr.56_energy_efficiency_and_renewable_energy]]

[[basics:energy_and_ecology]]