Renewable energy: The 99.9% solution

A combination of wind and solar power and sophisticated energy storage systems could keep a power grid fully supplied between 90 and 99.9% of the time, at costs comparable with today’s fossil fuel and nuclear mix, according to a new study from Delaware in the United States.

Computer simulation measured the performance of inland and offshore wind farms and photovoltaic cells, backed up by battery and fuel cell storage, under the lowest cost conditions, for a 72 gigawatt grid system (one gigawatt will typically provide power for about 750,000 to a million US households)..

Researchers from the University of Delaware and Delaware Technical Community College will report in the Journal of Power Sources for March 2013 that they tested 28 billion combinations of renewable energy systems and storage, under four years of real load and weather data from a working commercial system.

“These results break the conventional wisdom that renewable energy is too unreliable and expensive,” said Willett Kempton of the University of Delaware, one of the team. “The key is to get the right combination of electricity sources and storage – which we did by an exhaustive search – and to calculate costs correctly.”

Power demand and supply is an engineering headache: demand fluctuates according to hour, day of the week, the weather and the season, while wind power is vulnerable to calm weather and solar power is not supplied at night.

Storage costs are huge, and increase with the need to store for each extra hour. So right now, most electrical generators burn more fossil fuel to meet extra demand.

“The common view is that a high fraction of renewable power generation would be costly and would either often leave us in the dark or require massive electrical storage,” say the researchers.

But they found quite a different result. They tweaked their computer model and varied the conditions where they could: they found that consistent wind power could be obtained if the turbine fields were dispersed at distances greater than 1,000 kilometres.

They exploited not just hydrogen fuel cells and batteries for storage, but also grid-integrated vehicles: electric cars and trucks which when not being driven also served as sources for the grid.

They calculated the cost of renewable electricity generation without subsidies from either state or federal government, and when they made comparisons with fossil power, they factored in the external health and other costs of fossil fuel pollution.

The researchers worked with the prices for 2008, and with the projected costs of power for 2030, and they did not allow for any future advances in renewable technology.

They found that the cheapest solution was to generate far more power than consumers could demand. If they generated 180% of the necessary load, renewable sources could supply all that the grid needed for 90% of the time. If they generated 290%, then they could rely on renewable resources 99.9% of the time: that is, for all but nine hours a year.

And renewable energy, on this model, is the least-cost option, or close to it. “At expected 2030 technology costs, the cost-minimum is 90% of hours met entirely by renewable,” the team report. “And 99.9% of hours, while not the cost-minimum, is lower in cost than today’s total cost of electricity.”