This letter is in response to Maui Weekly's series of photovoltaic and electric utility articles.
When I was a student studying for my Master of Science in Renewable Energy Systems, I spent the summer at Riso, a research institute in Denmark, looking at the issues around keeping the power grid of Western Denmark stable.
At the time, this region of Denmark, which has its own power grid with limited interconnects to the surrounding nations and to the grid of Eastern Denmark, drew around 20 percent of its annual electricity supply from wind turbines.
As you can imagine, there were occasions when wind power only supplied a few percent of the load, and other times when wind met over half of total demand. This caused considerable difficulty for the grid operator, who needed to balance supply and demand on a moment-by-moment basis, and was threatening to put a tight technical limit on the future expansion of intermittent renewable energy sources.
My task was to review as many sources of information as possible and identify options for integrating more wind power without making the grid unstable.
I identified three strategies with potential to resolve the issues surrounding balancing supply and demand.
Store energy for later use as electricity--pumped hydro, batteries, compressed air energy storage, flywheels, ultra-capacitors, etc.
This strategy offers options for achieving grid stability across all time frames, and also for local grid stability where the load or power delivered to a particular circuit threatens to make the supply unstable.
Manage load--bring forward or defer electricity use when time of use is not critical; e.g., pump water out of boreholes and into water treatment reservoirs when electricity is in surplus, or turn off the pumps when supply is short (so long as the pumps work for the required number of hours each day, and the reservoirs remain at acceptable levels, everyone is happy).
Run heat pumps on district heating networks on a similar basis--store heat when it is windy, use stored heat when the wind dies down. (This was the strategy I recommended for Denmark, as there is huge capacity in the district heating networks, which supply around 70 percent of Danish homes and businesses.)
Gas separation--use cryogenic methods to separate oxygen, nitrogen, carbon dioxide and argon for various industrial and medical uses when power is in surplus.
Run the refrigeration equipment in frozen food warehouses harder when power is in surplus and less hard when power is not so available; if the temperature drops by a couple of degrees on a windy afternoon and rises by a couple of degrees (back to its normal operating temperature) when the wind dies down in the evening, this is fine so long as the temperature remains in the required range.
Make ice for the fishing industry when the wind blows, or in Maui's case, when the sun shines.
All of the above involves choosing the time at which electricity use takes place or is increased or decreased using loads that are not time-critical. For the most part, these loads are already being serviced by the grid, so little capital expenditure is needed--just smarter ways to decide when to use electricity for tasks where time is not critical.
Increase grid transmission capacity within and through the local grid to allow greater imports and exports over longer distances in order to level out some of the fluctuations, and find loads for any surplus electricity supply. Increasing transmission capacity on a circuit will result in a flatter voltage profile around the circuit. As for a given load, line drop is reduced.
Increasing the transmission capacity of a local circuit will make voltage more stable, while increased interconnection with other grids will allow for more power to be generated in a given location, and safely transmitted to customers across the whole interconnected area.
There is, of course, a forth strategy--to greatly increase the amount of fast-ramping fossil fuel generation to adjust to the variability of large amounts of intermittent renewable energy sources.