Denmark juggles heat demand with wind output

A working group from the Danish energy ministry is charged with presenting a possible solution to balancing Denmark's electricity system with high levels of both wind power and decentralised combined heat and power. This article discusses progress so far. The country is aiming for 50% of its electricity to come from wind energy by 2030. This accompanies the main article in this issue, "Fading fears about fluctuations."

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Turning off one or more of Denmark's five planned offshore wind plant to regulate supply to demand will be a tactic of last resort in Danish power system planning for the future. Though it is technically possible to flip the switch on wind farms, it is an economically and environmentally expensive solution to balancing supply and demand. On the contrary, electricity overspill, a product of supply exceeding demand, will be put to good use through a combination of technical and regulative (political) solutions, expects Hans Christian Sørensen, the wind representative on an energy ministry working group charged with presenting a possible solution to balancing Denmark's electricity system with high levels of both wind power and decentralised combined heat and power (CHP). The country is aiming for 50% of its electricity to come from wind energy by 2030.

An excess of supply compared with demand -- three times in the space of 20 hours -- was first registered by regional utility Elsam in the west of Denmark in 1992. The Danish energy agency is now predicting that excess supply could reach up to 11% of demand because of the development of CHP (co-generation) and wind power, both seen as an essential ingredient of a sustainable and affordable energy system.

Central control the key

The widespread use of CHP to reap both heat and electricity out of the same fuel adds a particular dimension to supply in Denmark since demand for heat can often exceed that for electricity. At the same time, wind turbines produce power irrespective of the actual demand. The worst possible scenario in Denmark is a windy winter night when workers and households are in bed with lights and computers switched off, but in dire need of heat to keep warm.

The working group of which Sørensen is a member (he also heads an EU working group on offshore wind plant) is half way through its study of solutions and has begun to separate the possible from the unrealistic. The aim of the group's work on "more sensible use of electricity" is to present a series of recommendations for the short, medium, and long term, or for 2005, 2012 and 2020. It must report by October 1.

"The problem is that the risk of imbalance in the system will grow as more and more electricity production gets a priority tag, meaning that it can't be regulated or predicted and thus integrated into the system sufficiently in advance of when it is needed," says Sørensen. He adds that better wind speed prediction, which could help solve some of the problem, is not among the solutions being studied by the working group. The whole area of wind forecasting is being studied separately and this work is unlikely to be complete by October 1. Better predictions, however, would allow better scheduling of thermal plant and thus greater potential for power exports, although this will not always be possible, or desirable.

Even though the working group is just a few months old, Sørensen says there already appears to be broad agreement that increased use of electrical heating and electrical heat pumps in homes would not have a measurable effect on the electricity overspill problem. "The decisive factor is that they cannot be controlled centrally -- and planning and control are vital," he says. "At the same time there is also agreement that no single solution will do. The solution must be a combined one."

System bottlenecks

Eastern and western Denmark, with about 1.6 million households in the east and 1.4 million in the west, have never been connected with an underwater electricity cable. A cable connection is one of the solutions being discussed. "The problem is that a cable is a not an optimal economic solution, even though it could move electricity produced by the many wind turbines on Jutland to the more wind-poor eastern part of Denmark. That could be a political solution, nonetheless. But there is often little difference between how much it blows in the west and the east, and imbalance in the east would still exist," says Sørensen.

If the cable is built, however, it would open up the option of sending the power to Sweden via the existing link between east Denmark and southern Sweden, where power is needed. Sweden and western Denmark are already linked further north via a cable under the Kattegat, but power is not needed in that area of Sweden. But linking the east and west of Denmark to get power to southern Sweden is probably too expensive and too restrictive, explains Sørensen. Indeed, Denmark's limited cable connections appear to be the electricity system's future bottleneck, not least when the political decision to close Sweden and Germany's nuclear plants is realised, sparking demand for electricity from alternative resources. The existing cables are not of sufficient dimensions to keep the system stable with large amounts of wind penetration, says Sørensen.

Intelligent demand control

Another solution being seriously considered by the working group is the potential for installation of "intelligent" electrical devices in households as a means of controlling demand by moving it in time. Large scale use of intelligent control of refrigerators, freezers and district heating could do just that. "Here the practical solutions for reducing system imbalance are apparently quite good, but it is a solution which will only work in the long term when consumers and industry have bought control devices and new electrical equipment," says Sørensen.

A third solution has already been tested, in theory, with success. For a large provincial town in Denmark the working group has applied a computer model which uses an electrically driven 150 MJ/s water pump (50 MW electricity capacity) in the town's district heating system. "Here we had a result which showed that it is possible to reduce system imbalance considerably, but it demands precise planning and a capacity which can swing from zero to large scale depending on where in the system the pump replaces conventional capacity," he says.

"The impact would be much increased if it was combined with heat savings in district heating areas and supplemented with flexible heat storage. In my view this combination is one of the important solutions because it reduces imbalance in the system and contributes to energy saving."

Technical potential

In the long term, Sørensen predicts that electronics will be further developed so that both wind turbines and decentralised CHP units can contribute to frequency and voltage control on the grid, a task carried out by the big thermal plant on today's network. Nearly all of these have been converted to CHP operation over the past 20 years, raising the efficiency with which they burn fuel from 40% to 80%. The environmental benefits associated with greater efficiency mean the power can be exported as "priority electricity" and sold to consumers as such at a higher rate.

As to other solutions, Sørensen sees long term potential in today's experiments with using excess wind electricity to produce hydrogen, which can be stored for use as a source of energy for transport. Construction of further high voltage transmission lines in Denmark as a solution is one that Sørensen regards as sheer utopia. "Export of electricity, in my opinion, is only sensible if it is possible to decide when the export is to take place -- in other words, when the price is high enough," concludes Sørensen.

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