These are the two main conclusions of a study into how imports and exports of electricity can be used to optimise the use of renewable energy in Denmark. The study concentrates on trading wind through the Nordic power pool. "Based upon hourly simulation of the Scandinavian electricity system, with connections to the European continent, we track the fate of wind power in satisfying Danish demand, selling the surplus to and buying deficits from the pool," explain authors Peter Meibom, Torben Svendsen and Bent Sørensen of the mathematics and physics institute at Denmark's Roskilde University.
"What is significant is that wind forecasting is accurate enough to allow trade of excess power to the point where wind power generation and consumption closely follow one another," adds Meibom. "We do not need to rely very much on hydro at all."
The study uses two simulation models: one that meets the scenario laid down in Denmark's official Energy 21 policy for 2030, where 37% of production (and 54% of consumption) is met by wind, with the remainder mainly supplied by gas-fired combined heat and power plants; and a second scenario in which close to 100% of production is from wind energy. Stability is achieved in both simulation models by matching supply and demand through trade on the hydro-based Nordic Pool -- excess generation is sold and deficits are made up by electricity purchases. The key to making it work is the existence of hydro, where excess electricity is used to pump water into reservoirs for later use.
The two models use a series of hour-by-hour simulations where data on wind speeds, electricity consumption and combined heat and power production in Denmark are used in conjunction with data on electricity consumption and hydro production in Norway and Sweden. Simulations are made for good and bad wind years combined with good and bad years for rainfall.
In the first scenario, the study reveals that the extra cost of trading wind in the pool, once the envisaged power system for 2030 is in place, is no more than DKK 0.025/kWh. This is the cost of the margin of error in accurately forecasting wind production in a 24 hour period, which amounts to 12% of the average pool price obtained. In this scenario, "present transmission capacity turns out to be sufficient, provided that all remaining fossil plants in the system are regulated correspondingly, but still within the bounds set by the heat delivery requirements," states the report.
In the 100% wind scenario, the average extra cost of matching supply and demand rises to DKK 0.043/kWh, representing 21% of the average power pool sales price. This includes the cost of a new 1800 MW transmission cable between eastern Denmark and southern Sweden, though 72% of the DKK 0.043/kWh is made up of "incorrect bidding" in not being able to match wind forecasts perfectly with trade in the pool -- and thus not being able to sell all the wind. In effect, says the report, the cost constitutes that "of being able to use the pool as an effective energy store" and should be compared to that of "establishing other energy storage facilities of comparable capacity."
Trading the excess
In both simulation models, wind plant generate considerably more power than Denmark consumes. In the Energi 2001 model, the extra generation -- the difference between the 37% of production from wind and the 54% consumption that wind meets -- amounts to an excess of about 50%. "Since the main excess occurs from November to April, where water flow to reservoirs and turbines in hydro systems is small and consumption large," potential for selling it to the hydro systems of Sweden and Norway is good, says the report. The effect would be to reduce Sweden's thermal generation. In the all-wind scenario, the excess can amount to as much as 39 TWh in a year of good winds and plenty of rain. But even today that power can be absorbed by Denmark's power hungry neighbours, Germany, Sweden, Finland or the Netherlands, assures Meibom. In the liberalised market of the future, where renewables will most likely have "must run" priority, he does not envisage a problem.
In neither of the scenarios is supply stability hit. In a year of poor winds and little water, reserves of hydro can be brought into play and extra power bought from other European systems.
Meibom stresses that the study deals with whether it is possible to profitably trade wind in the Nordic power pool. It does not take account of all the technical ramifications of high wind power penetration -- or their costs. The extra cost of requiring Denmark's combined heat and power plants to frequently increase and decrease their output, for example, is not calculated. Among the report assumptions, however, is that development of wind turbines will move towards variable speed models of larger capacities than today, with frequency converters with potential for regulation of reactive power consumption
Contributions to the report have been made by Danish utilities Elsam and Elkraft as well as Stattkraft and Stattnet in Sweden, Nord Pool and the Norwegian oil and energy department.