Electrical grids - How do they need to change for the energy transition?

Author: Prof. Dr.-Ing. Christian Rehtanz, ie³-Institute for Energy Systems, Energy Efficiency and Energy Economics, TU Dortmund University (as of February 2015)

The electrical grids are the marketplace and integration platform for renewable energies, conventional power plants, storage facilities and electricity customers. The grids must be used to spatially balance electricity generation and consumption. Care must be taken to ensure the stability of the system. This means that exactly as much power is fed in as is taken off. The individual fluctuations of feeders and loads balance each other out via the grid. Everything that is not balanced by this must be compensated by controllable feeders or storage and by load interventions.

From this it follows that large-scale network structures, tend to offer better compensation possibilities especially for the strongly fluctuating feeders from renewable energies and thus require less storage and balancing power plants. Ultimately, it is a political and economic decision how large-scale the electrical grid structures and thus the energy balancing are designed. It is already possible to build energy self-sufficient homes that supply themselves with electricity and heat throughout the year. However, the costs for this are exorbitantly high compared to a conventional supply and therefore not economical. For the power supply here is no reserve for technical failures built in and would cause further extreme costs, if not on the public supply of electricity would be resorted to. Precisely in order to be able to intercept failures of individual technical components, the electrical energy system has become networked throughout Europe. It is precisely this networking that today makes possible a European electricity market that promotes the most economical use of energy sources in each case and at the same time ensures a high level of supply reliability. For the further development of renewable energies in Germany and Europe, however, today's network structures are reaching their limits, as network capacities are not developed in such a way as to connect optimal locations for different renewable energies with the load centers.

If one considers such load centers, such as the Ruhr area or the greater Munich or Berlin areas, then these cannot be supplied with renewable energies on their own. Photovoltaics and waste power plants can contribute significantly to this, but must be supplemented by wind energy from the large-scale surrounding area, i.e. also from other German states, and hydropower. For dark slack periods, balancing power plants or storage facilities must also be available.

It can be stated that energy system structures and thus electrical grids are necessary for Germany as a business location in order to develop supra-regional optimal locations for renewable energies, to optimize the natural balance between producers and also to enable economic trading and balancing of energy provision with its European neighbors. This inevitably results in structures that, for example, combine wind energy from northern Germany with photovoltaics from southern Germany and hydropower from the Alps and also from Norway together with balancing power plants at favorable locations in an economically optimal way. If one deviates from this network structure in the sense of a stronger regionality, the network expansion requirement decreases and at the same time, however, the balancing requirement increases due to much more expensive balancing power plants and storage facilities, so that the overall system and thus the power supply would become more expensive.

Flexibility options to stabilize the system

The balancing via the networks is ultimately always spatial in nature. Only if there is generation capacity available somewhere at the right time can they compensate for a power shortage in other regions. If you look at the European large-scale weather phenomena, there can certainly be very large-scale lull situations with low temperatures in the dark winter months of up to two weeks. Renewable generation is very low over a wide area. Power plants in countries with electricity heating, such as France, are needed there themselves. To enable balancing via the grids, it would be necessary to leave the European borders and obtain electricity from North Africa, as has been proposed in the DESERTEC project. Politically, however, one certainly encounters comprehensible limits here.

Such scenarios therefore require flexibilities in the system to compensate for these lull periods. Storage capacities, including water reservoirs, are not sufficient for this purpose. Load shifts are not possible to that extent, but can contribute to peak smoothing to a certain extent. Today, peak load power plants would be the only viable economic option. The power plant park of nuclear and coal-fired large-scale power plants will develop more and more towards more flexible and faster controllable gas-fired power plants. Today's very low price on the power exchange does not yet reflect this need. However, necessary capacity mechanisms, such as an obligation for suppliers to also provide balancing when supplying renewable energy, would create suitable market incentives. Such concepts are currently being evaluated and need to be implemented urgently.

Such incentives would allow all flexibility options to compete economically with each other and contribute to the provision of the necessary balancing.

In addition to covering power gaps, the rates of change of renewable energy are also of particular importance for system operation. The fluctuation of power from renewables will continue to increase extremely in the coming years. This must also be taken into account in the overall system. It can be shown that relatively small storage units, such as those just entering the market for self-supply of photovoltaic power in households, help to significantly reduce the gradients due to solar feed-in in the overall system. However, these storage systems do not reduce the need for grid expansion, nor do they reduce the need for balancing power plants, but they do reduce the requirements for the control speed of balancing power plants.

Transport and distribution grid expansion needs

The strong growth of wind and photovoltaic systems is creating new challenges in the operation and planning of electrical grids. Depending on the structure and regional location of counties and municipalities, distribution grids face different requirements. While urban grids usually still have large absorption capacities for renewable energies, they are already at their load limits in extensive rural regions. Supra-regionally, the transmission grids must provide for balancing.

If the feed-in of renewable energies becomes significantly greater than the peak load for which the grids were planned in the past, grid expansion must take place. Photovoltaic systems in particular will require grid upgrades at the low-voltage level. In the medium-voltage level, larger PV systems as well as individual wind turbines are also connected, so the problem is exacerbated. If larger regions use the potential of renewable energies and these accumulate at the same time, the high-voltage networks must also be expanded. This can be seen today especially in rural states with high wind energy or photovoltaic potential, where in some cases the existing grid structures have to be doubled in terms of capacity. Still superimposed on these networks, the extra-high voltage networks provide the supra-regional balance.

In the publicly available dena distribution network study, the Technical University of Dortmund has estimated the investment requirements in the distribution networks on the basis of real networks from over 3,000 German municipalities and scenarios for the development of the addition of renewable energies. Depending on the scenario, this results in an additional 27 to 42 billion euros by 2030 for upgrading the distribution grids from low to high voltage. The order of magnitude is thus at a similar level to the costs for the electricity highways at extra-high voltage level in the German network development plan, which is estimated at around 22 billion euros by 2025.

The future challenge will be to reduce these investments through innovative network technologies and planning methods. Intelligent control of feeders and loads can mitigate extreme situations that rarely occur in distribution grids and thus reduce expansion there. New grid control resources can reduce or at least delay the construction of new lines in certain situations. Overall, however, the potential of renewable energies to supply Germany as an industrialized country can only be exploited if a correspondingly efficient grid infrastructure is further developed. Without regional and supra-regional grid expansion, the energy turnaround will not be able to take place as planned, or will at least be much more expensive.

Further reading:

dena distribution grid study "Expansion and innovation requirements of electricity distribution grids in Germany up to 2030"  (http://www.dena.de/projekte/energiesysteme/verteilnetzstudie.html)

dena study "System services 2030" (http://www.dena.de/projekte/energiesysteme/dena-studie-systemdienstleistungen-2030.html)