What happens in winter or on rainy days without wind? Where will the electricity come from then?

What happens in winter or on rainy days without wind? Where will the electricity come from then?

There are three options to ensure the continuous supply of electricity with 100% renewables, which should be combined with each other so that it does become not too expensive:

1. from the generation side: modification of the generation systems. If a solar power system is installed at a steeper angle, it produces less electricity during summer, but more in winter. Wind turbines can be designed in such a way that they still produce power when the wind is low, but less during a storm.

2. storage: use of electrical, mechanical, chemical, but also thermal storage, because heat and cold can be stored very well. There are also excellent locations for pumped hydro-storage close-by, such as in Austria, Switzerland, and Norway. Lithium batteries cost only a tenth of what they cost 10 years ago. A similar price reduction is expected for electrolysis for power-to-gas.

3. on the demand side: Demand-side management via dynamic electricity prices: Often the prosumer has the possibility to shift his consumption, for example, switching on his washing machine, charging his e-car or his e-bike. If the price is right, he will do so.

Thank you very much.

Further reading

To 1.)

Wind and solar plants have so far been designed in such a way that they produce the highest annual energy yield possible, but not in such a way that they would generate electricity (during the course of the day or over the year) timely to meet the course of consumption (mismatch of generation vs. load). 

Measures for solar power systems to achieve a better matching are presented here (e.g.: East-West orientation of modules, steeper installation, tracking, selective anti-reflection layers):

S. Krauter: Simple and effective methods to match photovoltaic power generation to the grid load profile for a PV based energy system. Solar Energy, Volume 159C (2018) pp. 768–776. https://doi.org/10.1016/j.solener.2017.11.039

Wind turbines can be modified in such a way so that they would start at lower wind speeds, but get saturated more often, leading to a lower overall variability and a higher capacity factor, which reduces the need for backup and storage requirements: https://news.stanford.edu/2019/07/01/steering-wind-power-new-direction/

“In the new study, the power improvement at low wind speeds was particularly high because turbines typically stop spinning below a minimum speed, cutting production entirely and forcing grid managers to rely on backup power. In slow winds, wake-steering reduced the amount of time that speeds dropped below this minimum, the researchers found. “Notably, the biggest gains were at night, when wind energy is typically most valuable as a complement to solar power.

To 2.)

2.1 In theory, gigantic wind, solar and biomass capacities could be installed, so that there is ALWAYS a power surplus, but a large proportion of the energy generated would be wasted.  It is much more economical to allow some overproduction, but to install different types of storage. 

A.A. Solomon, D. Bogdanov, C. Breyer: Curtailment storage penetration nexus in the energy transition. Applied Energy, Vol. 235 (2019), pp. 1351-1368. https://doi.org/10.1016/j.apenergy.2018.11.069

For the daily fluctuations these can be batteries (chemical storage), but also thermal storage (hot water boilers or storage of ice), if heat or cold is needed later. The prices for lithium batteries have fallen to about one tenth within the last decade: https://about.bnef.com/blog/behind-scenes-take-lithium-ion-battery-prices/


Note: Lithium batteries are already purchased by the car industry for 100 €/kWh only, considering 3000 full cycles lifetime, storing costs are at 3 cents/kWh which is similar to advanced P2G (Power-to-Gas). However, if the storage size is designed to overcome dark lull that occur only 2-3 times a year, the 3000 cycles take a VERY long time.

2.2 Pumped storage power plants (mechanical storage) are suitable for daily and for seasonal storage. This technology has existed for more than a century. In Germany, the possibilities are limited, but not all potentials have been exploited to date, as market prices for electricity are at dumping levels, this inhibiting profitable investments. In Austria, in Switzerland, and particularly in Norway excellent storage potentials are available (up to 35 TWh): https://www.statkraft.de/globalassets/old-contains-the-old-folder-structure/documents/hydropower-09-eng_tcm9-4572.pdf/

Lake Blåsjø in Norway would have a storage potential of 7.8 TWh, which is enough for a (hypothetical) absolute dark lull for almost a week in Germany.


In order to be able to import electricity from foreign pumped storage facilities, appropriate transmission line capacities are needed: Therefore, the Netherlands are operating a 700 MW power line to Norway since more than 10 years:


The German equivalent of 1.4 GW is ready, but will start operation at the end of 2020 / beginning of 2021:



However, to counter-measure all fluctuations at 100% Wind & Solar, that transmission line needs to be extended by a factor of 20-30, if no other measures (as described) are used.

2.3 For seasonal storage, electrolysis of water is a suitable method. This produces hydrogen, which can be stored for many months. Or the hydrogen can be converted to methane (natural gas) by adding carbon dioxide: This would have a lower efficiency, but the pipelines and storage facilities are much less costly, or even already exist in a sufficient scale. This process is called Power-to-Gas (P2G). In recent years, great progress has been made in conversion efficiencies – up to 76%: 

M.Gruber, P. Weinbrecht, L. Biffar, S. Harth, D.Trimis, J. Brabandt, O. Posdziech, R. Blumentritt: Power-to-Gas through thermal integration of high-temperature steam electrolysis and carbon dioxide methanation – Experimental results Fuel Processing Technology, Vol. 181 (2018), pp. 61-74. https://doi.org/10.1016/j.fuproc.2018.09.003.

O. Posdziech, K. Schwarze, J. Brabandt: Efficient hydrogen production for industry and electricity storage via high-temperature electrolysis. International Journal of Hydrogen Energy. International Journal of Hydrogen Energy, Vol. 44, Issue 35 (2019), pp. 19089-19101. https://doi.org/10.1016/j.ijhydene.2018.05.169

Hydrogen or methane could be used directly as a raw material (e.g. for steel production or other industrial processes), for heat generation and for vehicle propulsion, or converted back into electricity. Fuel cells or gas turbines are suitable for this purpose.  Gas turbines could also cover permanent needs, but often the number of “full-load-hours” are reduced, making operation unprofitable – at least for actual gas turbine designs. Even worse: The more renewables are installed, the shorter time periods the gas turbines will be used. However, there are efforts to modify gas turbines accordingly: 






To 3.)

Just as on the generation side, a better simultaneity of generation and consumption (temporal matching) can be achieved on the consumer side also, which would significantly reduce storage requirements (and its costs). This could be controlled via dynamic electricity tariffs: If there is an oversupply of wind & sun, the prices would be lowered; if there is a dark lull, the prices would rise. The consumer can decide for himself whether he needs electricity immediately or a little later. 

Shiftable consumption is often available via: Charging procedure of electric vehicles (Grid-to-Vehicle: G2V), dryers, ice machines, freezers, hot water boilers, ovens, dishwashers, washing machines. Shifting is also possible in industry for various processes – here dynamic electricity prices are actually already being used for load control.  These load shifting processes do not have to be carried out manually, but can be automated via least-cost routers – taking into account minimum requirements that can be individualised (e.g. minimum charge level of the electric car battery, minimum temperature of the refrigerator, etc.). One step further go applications in which the storage of an electrical vehicle storage is used to stabilize the grid (Vehicle-to-Grid: V2G):

M. Child, A. Nordling, C. Breyer: The Impacts of High V2G Participation in a 100% Renewable Åland Energy System Energies 2018, 11(9), 2206. https://doi.org/10.3390/en11092206

A. Ameli: Applying a smart management system for EVs in electrical power grids using smart grid capabilities. Dissertation University of Paderborn 2019. https://digital.ub.uni-paderborn.de/hs/content/titleinfo/3065817

S.Krauter, D. Prior: Minimizing storage costs by substituting centralized electrical storage by thermal storage at the end user, also suppling balancing power for grid operation In: Energy Procedia 135 (2017) pp. 210-226. https://DOI.org/10.1016/j.egypro.2017.09.505

M. Child, C. Kemfert, D. Bogdanova, C. Breyer: Flexible electricity generation, grid exchange and storage for the transition to a 100% renewable energy system in Europe Renewable Energy, Vol. 139, 2019, pp. 80-101. https://doi.org/10.1016/j.renene.2019.02.077

Load management could also take place on a seasonal basis: e.g. reducing consumption during winter by extending winter holidays at the expense of summer holidays. In summer, renewable energy is often available in abundance (e.g. PV), but industrial customers can’t use it because of general holiday periods.

An equalization of daily and of seasonal variation of electricity generation, as well as for load variation, could also be carried out via a global electricity grid. For solar power generation, an East-West extension of the grid can equalize generation during day- and nighttime partly or entirely (e.g. in Eurasia from Vladivostok (132° E) to Lisbon (9° W), solar time difference is 9.2 hours, so from the end of April until the end of August, a 24 h solar supply can be secured without storage necessities. State Grid Cooperation of China is planning such an Eurasian grid along the Silk Road, with a possible global extension until 2050.

Seasonal variation can be fully compensated via a North-South grid extension crossing the equator (e.g. from Berlin (52.5° N) to Capetown (34° S), thus offering reversed seasons). 


English lectures of course „Energy Transition“ (“Energiewende”)

1. Introduction to the Transition of the Energy System
2. Conventional power supply
3. Solar Irradiance:  Astronomical basics: Positioning of the sun during a year, Elevation and azimuth of the sun during a day and during the year from a terrestrial observer, Spectra of the sun for different “Air Masses” (AM)
4. Solar Thermal Energy Systems
5. PV – Basics: Principle of photovoltaic energy conversion. Characteristics of solar cells: I-V-curve, influence of irradiance level, spectrum of irradiance, spectral efficiency, MPP, Form Factor, influence of parasitic resistors, weak light effect, operation temperature, STC
6. PV Energy Systems: Installation, Mounting, Foundation, Substructures, Inverters, Conversion efficiency, Energy yield, Off-grid vs. On-grid systems, Configuration of PV systems, Wiring, Balance of System costs (BoS), Optimization of PV power plants
Solutions to Exercise 6 – PV systems –  plus homework
7. Wind Energy: Historical and actual development of wind power conversion. Extractable power from the wind. Parameters of wind energy: Height, pressure, temperature, density. Weibull- and Rayleigh-distribution of wind velocity. Theoretical conversion efficiency of wind power conversion, Betz efficiency. Types of wind power converters, tip speed ratio, cost, complementarity, software tool, literature.
7.1 Wind Energy
7.2 Correlation of wind and solar power
8. Hydro Power: Historical water wheels, type of turbines, run-of-river-plants, hydro storage power plants
9. Geothermal Energy: Potential, Geology, Conversion Processes, Exploration, Costs 

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