One of the smart things the Chinese have done—for decades now—is to take waste heat from coal power plants and use it to heat homes and businesses, something that’s done in the United States, but not widely. The system works by using some of the steam produced at a power plant to heat water, which is distributed in pipes through a city to radiators or floor heaters (pipes in the floor). Called cogeneration, it’s a relatively easy way to get far more use out of the energy in coal or other fossil fuels.

But as it turns out, cogeneration can make it more difficult to shift from fossil fuels to renewable energy. When cities depend on heat from power plants, there’s a limit to how much grid operators can lower power output from coal plants, while still providing enough heat. Especially in winter, then, power grid operators have to keep coal plants running, even when there might be sufficient wind power to shut them down, or at least turn them way down. As a result, some of the wind power can’t be used.

To use more wind power, the grid operators in northeast are building “pumped hydro” systems, which use excess wind power that’s generally produced at night, to pump water uphill into a reservoir. Later, when there’s less wind power, the water flows downhill and spins turbines to generate power again. This way, spikes in wind power that couldn’t be absorbed by the grid because of the minimum power requirements at coal plants can still be used by spreading the power generated throughout the day. But adding storage for wind power will drive up costs, a major challenge for technology that is already more expensive than coal-fired power plants.