Experts bank on energy storage
The quest for suitable storage systems for wind and solar power is running in top gear.pump and compressed air storage have a clear lead on hydrogen.modern battery technology could accelerate the development of electric cars.
Experts bank on energy storage
For hundreds of years rulers, merchants and travellers would make pilgrimage to the Apollo temple in Delphi to have the Pythia priestesses receive and pass on the prophecies of the Greek gods. Befuddled by vapours rising from a mysterious chasm at which they sat, they heard only incoherent bits of sentences spoken by the oracle. Most times what the Pythia would say would not be under-standable to those waiting for her answers, so the male priests of the temple would interpret the replies.
We know now that the priests were the true authors of the prognoses. They had built a sophisticated knowledge grid that gave them control over reading the future, without being responsible to anyone.
Almost two and a half thousand years on, anonymous surveys of experts, so-called Delphi studies, have become an esteemed method of scientific trend research. The idea is to have scientists make honest prognoses without pressure from colleagues, superiors or funders. The findings of such surveys quite often deliver big surprises.
So it was with the outcome of the first Delphi study on the future of the European energy supply, published recently by the Institute for Futures Studies and Technology Assessment (IZT) in Berlin. A large majority of the 670 experts polled anonymously assume that before 2030 renewable energies will produce a quarter of Europe’s primary energy supply. For comparison: The EU Commission projects in its study “European Energy and Transport – Trends to 2030” a renew-ables share of only 8.9% for the 15 EU states by the same year.
No less suspenseful is another outcome of the Delphi study: Even before the 25% target is reached, the experts see big growth in the importance of modern storage technologies for cleanly sourced power. The problem is well known. Because wind and solar converters produce power irregularly as they depend on the weather, backup power stations have to be ready to make up the slack when there is no wind or sunshine. On the other hand, when the wind is strong and demand low, there is surplus power that has to be stored, exported abroad or throttled at the source. As wind energy production expands on land and sea the problem is likely to increase. “Storage technologies will become more important,” predicts Johannes Lackmann, President of the German Renewable Energy Federation (Bundesverband Erneuerbare Energie, BEE).
The debate over energy storage concepts is now attracting public attention – not least due to the hydrogen fuel initiative by George W. Bush. Like the US President, many politicians and industrialists see the entry into a hydrogen economy as the golden path into the energy future. What is often ignored in the euphoria is that it takes a lot of energy and big losses to produce hydrogen. But who bothers about niggling details when the really big breakthrough beckons? Consequently, plenty of capital is flowing into large-scale research programmes and pilot projects such as the construction of a “hydrogen highway network” in California with which Governor Arnold Schwarzenegger wants to set himself a memorial. He's calling for the creation of a network of hydrogen fuel stations along major Californian highways.
But is hydrogen really the cure-all? Does the lightest element in the periodic table really have what it takes to be the energy currency of the future? The experts who took part in the Delphi survey are sceptical. They also see hydrogen becoming more important, but long before that other energy storers will be in front. Cheaper alternatives such as pump storage, compressed air storage or batteries could overtake the hydrogen economy early on.
The most efficient method of storing electricity has been in use inGermany for more than 70 years. The first pump storages were built to keep coal-fired power stations delivering steadily when their output fluctuated. The principle couldn’t be much simpler. Excess electricity is used to pump water into a reservoir located higher up. When power demand is big, this water runs down to drive turbines connected to generators. In Germany more than 30 such reservoirs with a joint capacity of some 6,000 MW are in use.
As recently as autumn 2003 Vattenfall Europe AG started operating Germany’s biggest pump storage power plant in Goldis-thal, Thuringia (290 km northeast of Frankfurt on Main); its capacity is 1,060 MW with an unloading period of eight hours. But the plant does not contribute to offsetting the fluctuating uptake of wind power. Vattenfall fills the storage reservoirs at night by using cheap brown coal power so as to produce expensive peak load electricity in daytime.
Pump storage power stations consist of upper and lower basins linked by pipes.In storage mode surplus power is used to pump water into the upper reservoir.In generating mode the water is released and drives water turbines.Pump storage power stations can be switched within minutes to full-load operation and achieve up to 85% efficiency.Because of the serious ecological strains in the building phase,new pump storage projects are practically ruled out in Germany.
The main advantages of pump storage systems are the high effi-ciency rate of up to 85% and the fast availability. Within minutes the plants can switch to full load operation. The price is right, too.
“Pump storage power stations are the cheapest technology for storing energy,” says Martin Hoppe-Kilpper, head of the power stations and grids desk at the German Energy Agency (Dena). The water reser-voirs are almost perfect for balancing the fluctuating wind power feed-in – if capacities weren’t so scarce in Germany.
Expansion is unthinkable. No-one nowadays wants to be responsible for blasting away whole mountain tops to build reser-voirs. Another drawback is that pump storage plants only make sense in mountainous regions and are located far from the main present and future wind power production sites at and off the German coast.
SALT DEPOSIT STORAGE:Air can be pressed into underground caverns with compression of as much as 100 bar.
There is, however, plenty of potential left for optimising existing stations, notes BEE president Lackmann. For example, the many snowmelt water power stations in the Alps are currently designed for seasonal operation, meaning that a complete charging and discharging cycle takes a whole year. Instead of waiting for the snow to melt, the plants could be refitted for pump storage operation and the reser-voirs could be filled more quickly by using wind-sourced power. “Using shorter cycles and bigger turbine capacity, the output of the existing reservoirs could be increased manifold,” Lackmann empha-sises. The drawback is that the lower reservoir facilities in place now are insufficient.
Potentials are greater in northern Europe. A study by the Kassel-based Institut für Solare Energietechnik (ISET), finds that Norway, Finland and Sweden have combined water storage capacities of 123 terrawatt-hours. The real wind power feed-in in Germany last year was close to 26 terrawatt-hours. That could give rise to the idea to build power lines to Scandinavia to fill the pump storages there with wind power at least in phases.
And such plans do exist. The second part of the German Energy Agency’s grid study is to examine whether the construction of high-voltage direct current transmission lines to Norway are technically doable and financially viable (see page 18). But German electricity utilities resist the idea because the price level on Scandinavian markets is substantially lower than across Germany.
Moreover, the construction of such new transmission lines would also be strongly resisted in the regions affected – even though they are capable of transmitting greater quantities of power over long dis-tances with less loss than conventional alternating current lines.
The alternative to such scenarios could lie in salt deposits thousands of metres underground in northern Germany. They offer incredible potential for building so-called compressed air energy storage (CAES) power plants. In principle these plants work similarly to pump storage. Off-peak energy is used to compress air to as much as 100 bar, i.e. 100 times atmospheric pressure, and pressed it into underground caverns. In power station operation the compressed air drives a conventional gas turbine and replaces its compressor, which causes as much as two thirds of the performance loss.
“With conventional CAES power stations we can cut natural gas consumption by 40% to 60%,” says the project developer, Fritz Crotogino of the Hanover-based KBB Underground Technologies GmbH. His company is specialised in the construction of sub-surface gas storages in salt deposits, depleted gas and oil deposits and aquifers. In the natural gas industry the use of sub-surface storage caverns has been standard practice for decades. Salt deposits are espe-cially suitable because the hollows can be created fairly simply with water injections which dissolve the salt in the depths.
Crotogino played a part in the planning of the world’s first CAES power station, built in 1978 by a predecessor enterprise of the Eon Kraftwerke AG in Huntorf – right in the neighbourhood of the Un-terweser nuclear power station. In turbine operation this plant can make available 290 MW within two hours. Its two caverns with a joint volume of 310,000 cubic metres can be recharged to maximum pressure within eight hours.
The Huntorf compressed air plant reaches a maximum efficiency of just on 42%. Modern plants recycle waste heat, thereby reaching 55% efficiency. The vast potential of the compressed air technology for storing electricity is demonstrated by a project in Norton in the US state Ohio. The CAES plant planned there is to be able to deliver 2,700 MW in a span of eight days.
Europeans are researching the development of compressed air storage that works without a gas turbine and hence without addi-tional fossil fuel. Such systems are not only to store the compressed air in underground salt caverns but also the heat generated by compression. In conventional CAES power stations the hot compressed air, which can reach 700 degrees, has to be expensively cooled before it is pressed into the cavern. In the new plants this energy is to be kept available in liquid or solid heat storage and to be admixed with the compressed air as it streams to the expansion turbine. The so-called adiabatic compressed air storage power stations are to reach as high as 70% efficiency, which would put them in an efficiency category alongside pump storage systems.
In addition to Alstom and MAN Turbo, the energy utilities RWE and Eon are among those supporting the research in this field. “There’s interest on the energy utilities side,” says Crotogino. The engineer sees great potential for the new technology. “In the immediate neighbourhood of the planned German offshore wind farms there are enough salt structures to build storages.” Even at sea the construction of compressed air storage systems is no problem, he says.
In contrast to pump storage power stations, geology sets no limits on storage capacity. In north Germany and many other parts of Europe salt deposits are available in almost limitless volume. The problem is, who is going to build the CAES systems? KBB engineer Crotogino suggests it would make sense for wind farms to form pools to jointly operate a storage system in order to regulate their power production. “Unfortunately there are hardly any incentives for such solutions in the present promotion scheme under the Renewable Energy Sources Act.”
Underground salt caverns are especially suitable for storing compressed air.Surplus energy is used to compress air to 100 bar.In conventional compressed air power stations the compressed air streaming out replaces the compression stage of a gas turbine.The technique cuts 40 to 60 per cent of fossil fuel consumption.In future adiabatic compressed air storage systems are to be driven only by cleanly-sourced power.The heat generated by the compression is also to be stored and be mixed with the compressed air streaming out to the expansion turbine.Adiabatic compressed air storage systems are to reach up to 70%efficiency.
BEE President Lackmann is ready for talks on this issue. “It would make sense if the wind power operators would contribute to the reg-ularisation of their power output.” It is quite conceivable that the EEG Act would be adapted accordingly, he adds, arguing that storage of wind energy close to where it is produced could one day be a real alternative to building transmission lines. It remains to be clarified, Lackmann adds, whether the operation of compressed air storage is economically viable. According to Crotogino’s estimates the invest-ment costs of conventional CAES power stations are on a par with comparable peak-load power stations. The same holds for operating costs.
The KBB expert is optimistic that the adiabatic compressed air storages will also function cost-efficiently. That is not least indicated by the great interest in the technology of the energy suppliers. Given the rising fuel prices it could soon pay off for the big power companies to convert cheap off-peak output into expensive balancing power by using compressed air storage technology.
Despite the euphoria about the potentials, Crotogino warns about too great expectations. “I assume that CAES stations will be used mainly to cover short-term reserve needs which come about through flawed prognoses of wind power feed-in.” He does not believe that pump or compressed air storage will ever meet the complete need for backup power stations.
That is not necessary, anyway, says Joachim Nitsch of the German Aerospace Center (DLR). “We need an overall strategy in which energy storages will play a big part.” In addition, the grids need to be optimised and the changeover to gas-burning power stations, which are quick to regulate, must begin, he adds. Thought also needs to be put into developing effective supply management by which load fluctuations can be regulated not just on the supply side, but also in terms of demand.
And what role will the much-touted hydrogen economy play in this overall concept? Ulf Bossel, one of the most stringent critics of hydrogen storage and initiator of the European Fuel Cell Forum (EFCF) in Switzerland, is pessimistic about it. In his view, converting electricity into hydrogen is pure waste of energy. “If wind power is to be stored as hydrogen, three out of four wind turbines would deliver only lost energy,” Bossel fumes. And the low efficiency of 25% in the complete conversion chain, including compression and liquefaction, is not optimisable because of the physical properties of the lightest gas, hydrogen, he says.
To Bossel the idea to turn electricity into hydrogen and possibly use it to create kinetic energy expresses outdated thinking. “It would be a lot better to use the electricity as electricity,” demands the me-chanical engineer. Two hundred years after James Watt converted coal into kinetic energy for the first time, it was time to say farewell to the concept of the steam engine and head in new directions, he says. Bossel sees the direction set. As the fossil sources peter out, the electricity age will begin – electricity generated mainly by wind, water and sun.
This change of paradigms would irrevocably revolutionise the structure of the energy supply. Not hydrogen, but electricity would be the energy currency of the future. Bossel believes that ultimately the mobile sector will also follow this course. Rather than building hydrogen fuel stations, the Swiss citizen calls for a changeover to electric cars that are connected to the mains supply in the garage and in parking lots and which with their battery storages can act as small, decentralised power stations.
The vision of a renaissance of the electric car is not off the mark. “Fifty per cent of all car runs are less than five kilometres and 80% of the petrol is used for drives of less than 100 km,” Bossel calculates. For a long time modern electric cars have had ranges of up to 200 kilometres and more optimisations are in the pipeline, Bossel says.
Hydrogen can be won either by electrolysis of water or by steam reforming of natural gas or other hydrocarbons. However,using steam reformation doesn ’t make much sense in energy storage.The primary energy sources like coal, petrol or natural gas are clearly more suitable.In electrolytic hydrogen production about half of the energy input is lost.Until the hydrogen is converted into electric power by the consumer,another 50%is lost,so that the overall efficiency is only 25%.
The message hasn’t clicked yet in Europe and America. There the motor industry pulled the plug on developing the electric car years ago. It’s different in Asia. In China alone, millions of electric vehicles are in use. Most of them as yet are light motor bikes and scooters, but the progression to the car is no longer far. No wonder that China produces the lion’s share of the global output of lithium-ion batteries.
Lithium-ion batteries that are familiar to us in mobile phones and laptop computers are a great deal superior to predecessors like lead batteries in capacity and recharging time. Other new battery concepts have similarly good qualities.
Batteries can store electric energy in the form of electrochemical energy.Accumulator batteries can be recharged with electricity after their chemical energy has been exhausted.Every cell consists of two electrodes connected with an electrolyte.A great number of battery systems has been developed.In addition to lead batteries,nickel-cadmium and nickel-metal-hydrid accumulators,more and more lithium ions are being used; they are characterised by high energy and performance densities.The general disadvantage of batteries is the limited number of loading cycles.
“Every year millions of electric vehicles and only a few dozen hydrogen vehicles are built,” Bossels states, clarifying the relationship of the two energy storage concepts. BEE President Lackmann also sees the changeover to electric cars as the way to go. “The perform-ance of the mobile sector is 15 times as great as the output of conventional power stations. That is potentially a gigantic resource for balancing power.” To avoid the long charging times – one of the major drawbacks of electric cars – Lackmann suggests developing exchangeable accumulators that can be swapped at service stations. Enercon CEO Aloy Wobben proposed a similar idea at the World Wind Energy Conference in Berlin in 2002.
But it will be a long while yet before electric cars with state-of-the art batteries go into series production in Germany. In the mid term companies like Daimler Chrysler and BMW will not be able to avoid at least a hybrid technology, that is a combination of batteries with a combustion engine. “The hybrid technology is the entrance to the age of the electric car,” Bossel believes. If the engineer’s prognosis turns out to be right, the hydrogen era would end without ever having started.
This article appeared in the Issue 3 / 2005 of new energy and is republished with permission of the authors and publisher.