The base load is the minimum electricity demand for day-to-day operations. This base load is covered by power stations which generate electricity at low variable costs and which can (generally) only be regulated with difficulty. In Germany these are currently predominantly hydroelectric, nuclear and lignite-fired systems.

In this connection, however, consideration is also being given to the contribution which renewable energies (other than hydroelectric power) can make to securing the base load. A key term in the discussion is the »secured output«. This describes the contribution which a technology – conventional or renewable – can make towards covering the power demand with a defined level of certainty.

While wind power and, in particular, photovoltaics (PV) only have a small secured output in themselves, the figures for biomass, hydroelectric power and geothermal power are within the range of conventional systems. If the various renewable electricity generation technologies are viewed as a whole, the secured output is higher overall because, for example, PV and wind power are balanced out.

However, the secured output alone says but little about the question of how well electricity demand can be covered by renewable energies and the nature of the power station fleet which must be kept available to cover the demand. When examining the contribution of renewable energies to securing the base load, therefore, account must also be taken of the real load profile of the demand. The difference between renewable generation and demand represents the load that has to be covered by regulatable power plants.

By its very nature, the issue of »base load« does not lend itself to being addressed in isolation; rather, it has to be incorporated in an overall examination of the structure of electricity generation (power plant fleet, economic and environmental determinants of power plant operation, »virtual power stations«, investment decisions etc.) and the demand for electricity (e.g. load management, energy-saving measures and efficient energy use).

In order to address these questions, a methodology is required which covers both short-term power plant operation and long-term investment decisions in the energy market. For that reason, model-based studies on future electricity supply scenarios have been carried out as part of the project in addition to a review of the existing literature.

Results

A dynamically advancing expansion of electricity generation using fluctuating renewable energies means that it is absolutely essential for the electricity system to be capable of responding significantly more flexibly to different feed-in and demand situations to ensure that the security of supply is maintained at all times. The flexibility of the system must therefore be substantially increased at every level in terms of electricity generation, the grids and in the area of demand.

The various enhanced flexibility options can complement each other, but also to a certain degree replace each other. What is important overall is to identify from the available portfolio of enhanced flexibility options the combination of measures which guarantees the long-term security of supply at the lowest financial cost with the highest possible environmental and social compatibility. This means organising a social search process with scientific support.

Although the analysis shows that the existing and concretely planned options for enhancing flexibility will be more or less adequate up to 2030, government can nonetheless contribute towards further improving the system integration of renewable electricity generation, particularly in the long term, by providing additional flexibilisation options.

Electricity generation

As fluctuating feed-in from renewables plays an ever more important role in the system, the differentiation in load ranges (base, average and peak loads) is becoming increasingly obsolete. Similarly, increasing blurring will occur in future in the assignment of certain power plant types to individual load ranges. The options for operating power plants which are designed for very long periods of full-load operation are diminishing. In the scenario studied, the demand for base load power plants will fall from the current level of approx. 29 GW (installed power output of lignite and nuclear power plants) to just 6 GW by 2030.

On the other hand, flexible conventional power plants will be needed to maintain the security of supply. A debate is currently under way as to whether the construction of new (or even the retention of existing) such power plants also needs additional support – in the form of so-called »capacity mechanisms« – to ensure that the market players do so to a sufficient extent.

The greater emphasis on the demand side on electricity production from renewables also enables the flexibility of the generation process to be increased. Biomass power plants are well suited to this from a technical viewpoint, as are hydroelectric plants and geothermal systems. If at all possible, regulatable systems for renewable electricity generation should not be incentivised by a subsidy system to operate continuously, as is currently the case with the fixed feed-in payment system. The introduction of the optional market bonus is one way of making electricity production from renewables more flexible. However, it remains to be seen to what extent this new subsidy scheme will actually result in a change in the feed-in behaviour of renewables and thus to a greater degree of flexibility in the electricity system.

Electricity grids

The electricity grids play a key role in the integration of a dynamically rising proportion of renewable energies. Supply shortages in high-voltage and ultra-high-voltage grids already occur regularly in certain regions of Germany. This will further increase in future unless appropriate expansion measures are implemented. The performance capability of the transmission grids can be increased by optimising grid operations, enhancing grids and expanding grids, with the costs of the measures increasing in the order cited.

A number of analyses have already been carried out into the need to expand the transmission grid, including the two »Grid Studies« by DENA, the German Energy Agency. Robust estimates of the demand for expansion are not currently available for the distribution grids, though these are currently being drawn up. A shift of large parts of electricity production to the distribution grid level as a result of small decentralised systems (for example, photovoltaic systems) is a very recent trend which poses considerable challenges for grid operations. The appropriate expansion of the distribution grids is therefore a key area if the electricity system is to be successfully transformed.

Storage systems

Storage systems are just one of many options available for making the electricity system more flexible. The tasks which storage systems can handle can always also be achieved in other ways.

Pumped-storage systems are currently the most commonly used energy storage means at a system level. Until recently, pumped-storage systems represented a profitable business model in the liberalised electricity market. If, however, the recent trend continues, which has seen the price differential between off-peak and peak-load electricity shrink, this business model will find itself under ever greater pressure. If no other more efficient measures for enhancing flexibility are available, consideration may have to be given in this case to support measures which go beyond the current exemption from the network charges and from the apportionment scheme of Germany's Renewable Energy Sources Act (EEG).

Many of the other potential storage technologies (compressed-air or electrochemical storage systems, power-to-gas) are still in the development stage. Storage systems which link with other sectors – particularly the heating sector and also, in the longer term, the gas and fuel sectors – could open up attractive options and offer synergies. Particularly noteworthy in this regard are, for example, the incorporation of heat storage systems in combined-heat-and-power plants which enables their electricity production to be matched to electricity demand, or the possibility of producing methane (»wind gas«) or hydrogen by means of electricity. Storage systems should therefore remain a priority area for research funding.

At present it is not possible to reliably quantify the level of long-term storage which is economically and technically feasible. It is therefore recommended that the relevant knowledge base be expanded and that a detailed study be conducted into whether and, if so, what sort of, political support in addition to research grants is appropriate before taking any action – for instance, in the form of large-scale subsidy schemes for constructing storage systems.

Load management/flexibilisation of demand

By enhancing the flexibility of demand, it is possible to reduce the differential between electricity generation from renewables and electricity consumption. Industrial and large industrial consumers in particular (e.g. chlor-alkali electrolysis, aluminium production, large refrigerated warehouses) offer attractive potentials in overall financial terms where the cost of saving electricity at peak-load periods (or saving balancing energy) is lower than the cost of the additional electricity production. Relevant appropriate framework conditions need to be created. These include, for instance, further opening-up of the regulating power markets, but also a greater emphasis on the introduction of electricity tariffs in which the electricity price fluctuates with the power exchange price.

By contrast, the load management potentials in the domestic sector (and in large sections of the trade, industry and service sector), for example by means of household appliances which can be switched on or off intelligently or load management of electric vehicles, require closer study before coming to a definitive assessment. In particular, the extent to which the potential savings can justify investments in smart grid infrastructures needs to be determined.