Overview of the results
In a nutshell
- There are alternatives to almost every technology, almost all can be replaced at reasonable additional costs – provided that the right steps are taken in time and that bad investments are avoided.
- In any case, gas power plants are the backbone of a stable energy system able to secure the energy supply even during several dark and windless weeks. In future, they will be operable with natural gas as well as biogas, hydrogen or synthetic methane.
- The most cost-efficient means of compensating for short-term fluctuations in the power generation is for households and the industry to adapt their power demand more effectively to the current supply (demand-side management). In future, batteries in electric vehicles and photovoltaic systems in households will be the standard and can serve as storage units that are charged in times of power surpluses. The hot water tanks of electric heating systems (power-to-heat) likewise offer buffering capacity, as well as refrigerators and cooling houses. By adjusting their energy consumption to current power prices, large companies avoid purchasing expensive electricity at peak load times.
- Economically, long-term storage is only viable if the power supply has so far been switched to renewable energy sources that carbon emissions are reduced by more than 80 percent. In the case of less stringent climate protection requirements, it is more cost-effective to bridge dark and windless periods of several weeks with natural gas power plants. Power surpluses from wind and photovoltaic plants are then either fed into the heating sector or curtailed.
More than 100 experts from academia and industry took part in the Working Group. Under the chair of Peter Elsner, Dirk Uwe Sauer and Manfred Fischedick, they evaluated the different flexibility technologies and assessed their development potentials by 2050 as well as the expected costs. From amongst a variety of existing scenarios, the group’s scenario experts selected eight exemplary cases with different power requirements and shares of wind and solar power. For these scenarios, the hourly residual load was calculated.
The results served as a basis for model calculations: Based on the calculated residual load, the portfolio of flexibility technologies was computed so as to keep the total annual electricity generation costs as low as possible.
In total, the Working Group computed about 130 possible constellations for the energy system. They are based on various different assumptions regarding the basic socio-political conditions, such as different carbon reduction targets, preferences for certain technologies and geopolitical risks.
In order to keep the voltage stable, it is imperative that electricity generation and consumption are kept in balance at all times. The central factor defining the flexibility requirement of a power system is the residual load. It is equivalent to the difference between the (fluctuating) amount of power generated from wind and solar energy and the power demand.
In the event of a positive residual load, wind and solar power are not sufficient to cover the demand. Either additional power is provided from dispatchable power plants or storage devices, or the demand is reduced by switching off flexible consumers (demand-side management). These include, for instance, batteries in electric vehicles and photovoltaic systems or electric heating systems with heat storage. Due to their respective storage capacity, these devices can shift their power demand to a later point in time.
A negative residual load occurs when the produced electricity exceeds the demand. The power surplus can either be used for storage, to operate flexible consumers, or to convert power into other forms of energy (e.g. heat or combustible gas). Alternatively, individual wind power or photovoltaic plants can be curtailed or completely switched off.
Gas-fired power plants
Flexibility technologies must compensate for the volatile feed-in from wind and photovoltaic power plants. Simple-cycle and combined-cycle gas turbine power plants are easily dispatchable and can therefore offset such fluctuations. In future, they will be used to stabilise the energy system and to ensure a reliable power supply, even if the feed-in from wind or photovoltaic plants should be scarce for several weeks.
Depending on the amount of carbon dioxide to be avoided and on the share of renewable energies, these power plants are operated with natural gas, biogas, hydrogen or synthetic methane. If engineered with a variable gas firing, these plants allow for an incremental transition to renewable fuels.
Share of renewable energies
With a high wind and photovoltaic share of 80 to 95 percent, the remaining power demand can be covered by bioenergy. However, this would imply almost doubling the current biogas consumption. The amount of biomass that is to be available for the power sector can only be determined in a national biomass strategy. Such a strategy must take possible competing uses of biomass as well as the ecological and social consequences of its cultivation into account. An ESYS Working Group examines possible strategies for a sustainable use of biomass in the energy system.
An alternative would be to install considerably more wind and photovoltaic plants than are necessary to cover the annual power demand (installed overcapacity). Together with electrolysis plants and large gas storage facilities, which can store sufficient electricity for dark and windless periods of several weeks, the current biogas share could be halved. During particularly windy and sunny periods, the additional wind and photovoltaic plants would be curtailed.
A low share of wind and photovoltaics could be complemented by solar thermal power plants with integrated heat storage. However, they are profitable only in very sunny regions such as southern Europe or North Africa. In order to transport the power to Germany, the trans-European grids would have to be significantly expanded. As a further prerequisite, the political and legal conditions in both the producer countries and the "transit countries" would have to be stable enough to ensure a reliable and secure power transport at all times.
Should a small amount of residual emissions be permitted, the additional power demand could be covered most cost-efficiently by means of natural gas power plants. Excluding natural gas and solar thermal energy, geothermal energy could close this gap. However, geothermal energy is comparatively expensive as a means of power generation and is therefore more suitable for heat generation.
Centralised vs. decentralised power supply
Owing to the use of renewable energies, the power system is increasingly evolving from a centralised system, in which electricity is produced in large power plants, to decentralised supply structures. The central technologies are complemented by many small, decentralised generation units – such as wind power and photovoltaic systems – feeding power into the grid.
As a rule, systems featuring a strong expansion of transmission grids as well as a mixture of centralised and decentralised technologies are more profitable than purely decentralised systems. The lower the wind and photovoltaic share, the higher the additional costs of decentralised supply structures. If the expansion of the transmission grid is to be kept at a low level, it is therefore advisable to build wind and photovoltaic plants all over Germany, especially near metropolitan areas and other main centres of consumption.
Surveys show that most citizens prefer small, decentralised power generation units to centralised supply structures. In addition, grid expansion frequently meets with resistance. The decision between a centralised supply structure and a decentralised organisation of the power system must not least take the preferences of the population into account.
The crucial difference between the numerous storage technologies consists in the length of time for which they can take in or release energy. If only a few hours need to be bridged and short-term energy storage is therefore sufficient, pump and compressed air storage systems as well as batteries are the choice devices. A much more cost-efficient means of meeting a short-term storage demand, however, would be demand-side management (DSM) measures, i.e. the specific control of power consumption in households or the industry. By the middle of the century, the number of photovoltaic and electric vehicle batteries, electric heating and hot water systems with thermal storage and controllable household appliances will probably suffice to cover all short-term storage requirements. The challenge lies in the fact that DSM requires the comprehensive equipment of household and industrial devices with intelligent control technologies allowing for their remote control and that the consumers need to accept these interventions.
Several weeks with little wind and solar radiation (dark and windless periods) can be technically bridged, either with long-term energy storage devices or with flexible producers such as gas-fired plants. For long-term storage, electricity must be converted into hydrogen or methane (power-to-gas), which are later used to fire according power plants.
Long-term storage is only profitable if carbon emissions are reduced by more than 80 percent. Otherwise, the economically better option is to use excess power for the thermal market while curtailing surplus generation by up to ten percent. However, with a sufficient number of wind and photovoltaic plants, long-term storage systems can likewise be used to mitigate the dependency of our power supply on natural gas imports.
By resorting to the power-to-heat technology, excess power from renewable sources can be a cheap energy source for heating. For this purpose, the hot water tanks of classic heating systems are equipped with immersion heaters. These heat the water when the sun is shining and the wind is blowing.
Using power to generate high-energy gases (power-to-gas) is a comparatively expensive option. It is only economically viable if the number of wind and photovoltaic plants is sufficient to likewise supply the heat and transport sector with renewable energy: Electrolysers and methanation plants require such large investments that they are profitable only at a high utilisation rate.