14 May 2005-Germany would seem to be caught between conflicting environmental policies: the country plans to both phase out its nuclear reactors and reduce its greenhouse gas emissions, which cannot be done without also reducing coal emissions. The Social Democrat-Green Party governing coalition is pushing wind power as a way to keep the lights on. But can enough wind energy be tapped to make up the difference between a growing demand for electricity and decreased supply from big central generating plants? And even if there is enough wind power available, can Germany's power grid handle a much larger fraction of intermittent power-power that cannot be simply dispatched on demand?

The answer to both questions is a resounding yes, at least for the next decade, according to the Berlin-based German Energy Agency (Deutsche Energie Agentur, or DENA). Established to implement Germany's renewable energy policies, DENA is a consortium of German grid operators, transmission companies such as E.ON AG (in Dosseldorf), and wind power interests. In a 514-page study released this February, it concludes that by 2015-when Germany hopes to have shut down 7300 megawatts' worth of nuclear power-the country can more than double its wind capacity to 36 000 MW, from 16 600 MW in 2004.

Germany's total grid capacity is close to 120 000 MW, and wind currently supplies 6-7 percent of the country's electricity. (Though wind accounts for about 14 percent of capacity it supplies a smaller proportion of total electricity because of its intermittency.)

Half of the new wind power that DENA's study anticipates would be installed offshore in the North and Baltic Seas. DENA found that 850 kilometers of new high-voltage transmission lines-a 5 percent increase in Germany's grid-would be needed to accommodate the added wind turbines. Some lines would bring that offshore power to load centers farther south, while others would strengthen links between the German grid's eastern and western halves, which power-grid simulations predict would overload in certain high-wind conditions.

One surprise finding is that no additional thermal power plants would be required to ensure sufficient supply when winds are low. In fact, the plan predicts that doubling the capacity of Germany's wind sector actually reduces the overall need for reserve power on the grid by 2000 MW. The reason is that the wind turbines will be broadly distributed and therefore can be expected to continually generate at least 2000 MW of "statistically guaranteed power."

DENA predicts that total power-generation capacity will rise by more than 20 000 MW over the next decade, accommodating a 7.3-terawatthour increase in average yearly demand. In addition to the policy-mandated rise in wind capacity and drop in nuclear power, DENA predicts the addition of over 9000 MW from natural gas turbines and about 7000 MW from additional renewable sources such as geothermal and biomass installations.

The trajectory for coal and oil-fired power depend on what, if any, carbon penalties the German government imposes on new power plants. With no carbon penalty, coal power would drop by 500 MW, from about 48 500 MW at present; with penalties rising to euro 12.5 a ton by 2015, coal generation would plummet by 12 700 MW. With the tax, oil, because of its smaller carbon intensity per unit of energy produced, displaces more than 7000 MW from coal; but without the tax its role actually decreases. Depending on the scenario, annual carbon dioxide emissions from power production are predicted either to remain flat at the current level (279 million tons in 2003) or to drop, with the carbon tax, to as little as 228 million tons.

Grid upgrades, plus federal incentives to encourage the wind developments, would add no more than euro 15 (US $20) to the average household's annual electric bill. The government has declared victory, calling this a small price to pay for clean energy. German opposition parties point out that, according to the DENA study, cutting CO2 emissions by using wind power will cost more than euro 40 per ton of carbon, making it less cost-efficient than many energy conservation options. Germany's utility association agrees: "The DENA study shows that the programs for the promotion of renewable energies need to be revised," says Eberhard Meller, general executive manager of the German Electricity Association, in Berlin.

Martin Hoppe-Kilpper, DENA's head of power plants and grids, says Germany's Renewable Energy Act will be up for reconsideration in a follow-up study looking beyond 2015. He points in particular to the law's mandate requiring network operators to accept all power generated by renewable sources. Hoppe-Kilpper says that this mandate occasionally leads to excess power on Germany's grid when the winds are high and power demand is low, causing unscheduled power exports to Germany's neighbors: "It's like a bathtub with too much water in it." This minor headache for grid operators threatens to become a system-threatening illness, if wind capacity is built out beyond the 36 000 MW envisioned for 2015.