F. Verheij & W. de Boer, KEMA, and A. Quist, Bureau Lievense, the Netherlands

Cost-effective energy storage has been hailed as the holy grail of the power industry. The Netherlands’ consultancy group KEMA and compatriot civil engineers Bureau Lievense have developed the concept of a sub-surface power station lake off the Dutch North Sea coast to achieve just that.

Electricity storage offers considerable added value for the energy sector. It increases the technical reliability of the power supply, stabilizes the cost of electricity and helps to reduce carbon dioxide emissions. Electricity storage and offshore wind energy is an excellent combination.

The temporary storage of electricity is a well established practice. Good results have been achieved with pumped storage facilities in several countries, e.g. Germany, Austria, Norway, the UK and the USA. In the Netherlands too, electricity storage is attracting increasing attention.

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The integration of electricity storage within the power supply system has numerous environmental benefits, particularly if storage is combined with wind-powered generating capacity on a large scale. For example, the storage system enables the capture of electricity produced by wind farms in periods of low loads (i.e. at night). This way, there is less need to shut down wind farms or conventional power plants during these periods. During the daytime, stored electricity can be utilized.

This reduces the overall amount of CO2 associated with electricity production and increases the energy efficiency of conventional power plants. Furthermore, the storage system can be employed for regulating power in order to increase the reliability of electricity production, and also in the case of strong variations due to wind energy. Last but not least, investment in large-scale storage can substitute for investment in the replacement or new development of peak production capacity.

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Energy consulting company KEMA and civil engineering firm Bureau Lievense, both of the Netherlands, have been working to take advantage of this situation. A preliminary design has been produced for an artificial ‘Energy Island’ – an innovative concept for large-scale energy storage – off the Dutch coast. The benefits of creating such an island would be long-lasting and not confined to the energy sector. It could provide coastal protection and harbour facilities, and opportunities for aquatic biomass and tourism.

On behalf of a number of Dutch (or Netherlands-based) utilities and TSO TenneT, KEMA and Bureau Lievense have investigated the technical feasibility and economic viability of large-scale offshore electricity storage.

The energy island concept

The ‘Energy Island’ concept is as follows. An artificial island would be created, incorporating a pumped energy storage facility that reverses the principle on which a conventional PAC facility works. When the supply of electricity exceeds demand, seawater is pumped out of the lake enclosed within the island’s perimeter dyke into the surrounding sea. When demand exceeds supply, seawater is allowed to flow back in, driving a generator.

The Energy Island would essentially consist of a ring dyke, sealed with bentonite and enclosing an area approximately 10 km long and 6 km wide. To prevent groundwater entering the enclosed lake by percolating through the substrata, the energy island would be sited at a location where there is a layer of clay tens of metres thick beneath the seabed.

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The water level in the ‘inner lake’ would be between 32 m and 40 m below that of the surrounding North Sea. It is envisaged that the lake would have a surface area of approximately 40 km2 and a storage capacity of more than 20 GWh, sufficient to supply an average of 1500 MW to the onshore power grid for at least twelve hours. Calculations have also been performed for a larger variant with 50 per cent greater capacity.

The first step of the feasibility study was to determine the technically optimal storage system size from a power supply viewpoint. Next, on the basis of a number of scenarios for the period 2015 to 2020, the storage capacity likely to generate the greatest cost savings and the associated reduction in CO2 emissions were calculated. The overall time required for the construction is estimated at six years.

The cost of large-scale electricity storage is to a significant extent determined by the preferred storage system’s power and capacity. KEMA has determined the optimal dimensions for a system of this kind by calculating the capacity requirements associated with several possible applications.

Looking at the Dutch energy supply system in 2020, the basic inner lake power plant design appears to provide adequate download capacity for conventional production units and certainly for wind energy imbalance compensation. It is anticipated that if a storage system were available, it would frequently be used for these two applications.

The storage of wind farms’ overnight output, the third possible application, is likely to be fairly common as well. Although the basic design is too small to cope with all the power potentially available from this source, economic analysis indicates that a larger storage facility would not be viable. The final two applications, provision of additional production capacity and the provision of primary action, require considerably less storage capacity and are not therefore decisive in relation to the size of the storage facility.

Economic considerations

The ideal size for a facility is dictated as much by economic considerations as technical considerations. The economic analysis actually involves a determination of the cost savings attainable relative to a zero-storage scenario. The calculations were performed using the ProSym simulation programme, which enables the most economic deployment of available production capacity to be determined on the basis of the marginal cost of fuel (gas, coal, wind etc), the start/stop costs of conventional power plant operation, the operating costs and the maintenance costs.

Impeccable green credentials

The model output indicates what the cost savings and CO2 emission reduction benefits of a particular storage system would be. Given a low gas price and 6000 MW of offshore and 3000 MW of onshore installed wind-powered capacity, the subsurface lake power plant would yield approximately €95 million ($128 million) per year. CO2 emissions from the Netherlands’ centralized power plants would also be significantly reduced, by roughly 2.5 per cent. Assuming a similar situation, but with gas prices at the high end of the anticipated range, the cost savings associated with a fall lake power plant would be much greater, at €190 million per year.

However, the reduction in CO2 emissions would not be as great as with a low gas price, since the storage facility would be used for overnight output from both wind-powered and coal fired production capacity. The storage of power from wind farms would nevertheless predominate, and CO2 emissions associated with centralized production in the Netherlands would therefore fall by 1.5 per cent.

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From the feasibility study, it is clear that a large-scale storage facility in the form of an energy island is technically feasible. Key factors in this regard are the presence of a layer of clay tens of metres thick beneath the bed of the North Sea and the fact that the technical feasibility of the engineering work involved has already been demonstrated in practice, e.g. suitable pump generators are already available.

From a power supply perspective, assuming the Dutch energy supply situation forecast for 2020, the technically optimal size for such a facility is roughly 2250 MW/30 GWh. From an economic viewpoint, a smaller power plant of 1500 MW/20 GWh is more attractive. The annual cost saving attainable by storing power produced overnight and returning electricity to the grid by day would be significant. Assuming a storage facility life expectancy of 40 years, the saving is likely to be €1.3-1.6 billion compared with an energy supply system without energy storage. The dredging costs are not included.

Additional multifunctional attractiveness

The Energy Island is a very attractive large-scale electricity storage option. One important difference between this innovative concept and other storage options is that the island could perform other functions besides energy storage. This might include coastal protection, the accommodation of industrial facilities, an LNG terminal and/or other port facilities such as emergency docks, beach/recreational facilities, hotels and housing, aquaculture, wind turbines on and within the island, the harvesting of energy from algae cultivated in the inner lake and the sale of sand dredged from the seabed.

These possibilities would all contribute to the value of the Energy Island. In practice, however, the inclusion of any activity would depend upon its economic ability to repay the capital cost of creating the ‘bare’ island. In a subsequent stage therefore, a detailed location study is planned and the technical capabilities and economic and ecological values of the other functions will be investigated.