You've probably heard the argument: wind and solar power are well and good, but what about when the wind doesn't blow and the sun doesn't shine? But it's always windy and sunny somewhere. Given a sufficient distribution of energy resources and a large enough network of electrically conducting tubes, plus a bit of storage, these problems can be overcome—technologically, at least.

But is it cost-effective to do so? A new study from the University of Delaware finds that renewable energy sources can, with the help of storage, power a large regional grid for up to 99.9 percent of the time using current technology. By 2030, the cost of doing so will hit parity with current methods. Further, if you can live with renewables meeting your energy needs for only 90 percent of the time, the economics become positively compelling.

"These results break the conventional wisdom that renewable energy is too unreliable and expensive," said study co-author Willett Kempton, a professor at the University of Delaware's School of Marine Science and Policy. "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."

By exhaustive, Kempton is referring to the 28 billion combinations of inland and offshore wind and photovoltaic solar sources combined with centralized hydrogen, centralized batteries, and grid-integrated vehicles analyzed in the study. The researchers deliberately overlooked constant renewable sources of energy such as geothermal and hydro power on the grounds that they are less widely available geographically.

These technologies were applied to a real-world test case: that of the PJM Interconnection regional grid, which covers parts of states from New Jersey to Indiana, and south to North Carolina. The model used hourly consumption data from the years 1999 to 2002; during that time, the grid had a generational capacity of 72GW catering to an average demand of 31.5GW. Taking in 13 states, either whole or in part, the PJM Interconnection constitutes one fifth of the USA's grid. "Large" is no overstatement, even before considering more recent expansions that don't apply to the dataset used.

The researchers constructed a computer model using standard solar and wind analysis tools. They then fed in hourly weather data from the region for the whole four-year period—35,040 hours worth. The goal was to find the minimum cost at which the energy demand could be met entirely by renewables for a given proportion of the time, based on the following game plan:

When there's enough renewable energy direct from source to meet demand, use it. Store any surplus. When there is not enough renewable energy direct from source, meet the shortfall with the stored energy. When there is not enough renewable energy direct from source, and the stored energy reserves are insufficient to bridge the shortfall, top up the remaining few percent of the demand with fossil fuels.

Perhaps unsurprisingly, the precise mix required depends upon exactly how much time you want renewables to meet the full load. Much more surprising is the amount of excess renewable infrastructure the model proposes as the most economic. To achieve a 90-percent target, the renewable infrastructure should be capable of generating 180 percent of the load. To meet demand 99.9 percent of the time, that rises to 290 percent.

"So much excess generation of renewables is a new idea, but it is not problematic or inefficient, any more than it is problematic to build a thermal power plant requiring fuel input at 250 percent of the electrical output, as we do today," the study argues.

Increasing diversity, reduced need for back-ups

The jump from 90 to 99.9 percent provision does require greater diversity in the renewable sources used, requiring "significant amounts" of inland wind, offshore wind, and photovoltaic solar power. However, that greater diversity actually reduces the need for both energy storage and fossil fuel back-ups compared with 90-percent provision because the chances of usable energy coming from somewhere are greater. To meet the demand only 30 percent of the time, inland wind alone is sufficient, the study finds.

Efficiencies improve further if, rather than being stored, surplus energy is used to offset gas to provide heating. This is viable, the argument goes, despite the inferior efficiency of electric heating because the costs of renewable energy is largely capital. Once built, the cost of "fuel" is effectively zero, unlike gas. It's cheaper to use it than store it, the researchers argue, even if you use it relatively inefficiently.

But this is all moot if renewables can't compete with fossil fuels. The key to finding out whether they can is the researcher's estimate that the total cost today of providing 1kWh of electricity via the PJM Interconnection is 17¢. The researchers used 2008 costs to calculate what it would take to supply all power from renewables for 30 percent of the day. By the researcher's calculations, the cost is already cheaper than the figure, coming in at 10 ¢/kWh.

However, for 90 percent, the cost jumps to to 19 ¢/kWh best case (which uses hydrogen storage), while the cost for 99.9 percent coverage rises to a best case (using vehicle storage) of 26 ¢/kWh. These rates include the cost of procuring and installing the energy infrastructure in the first place.

Wormhole your way to 2030, however, and it's a different story. A target of 90 percent coverage falls to to 9¢/kWh (vehicle storage) or 10 ¢/kWh (hydrogen storage), while a 99.9-percent target falls to 17 ¢/kWh for either vehicle or hydrogen storage. Central battery storage is more expensive, at 15 or 25 ¢/kWh for 90 or 99.9-percent coverage. And, according to the research, you can knock a few cents of all of these figures if you use energy surpluses to offset gas for heating. Then, the cost of meeting 90-percent coverage using vehicle storage comes in at 6 ¢/kWh at 2030 tech prices, for example. Costs are adjusted to 2010 dollar-value for ease.

"Aiming for 90 percent or more renewable energy in 2030, in order to achieve climate change targets of 80-90 percent reduction of CO 2 from the power sector, leads to economic savings, not costs," the researchers find. They suggest the sensible approach is to strive for a minimum target of 30 percent now, rising to 90 percent by 2030. Remember, that's not meeting 30 percent of your energy demand with renewables—it's meeting 100 percent of the demand with renewables for 30 percent of the time.

Because the renewables will inevitably contribute at other times, this amounts to about 60 percent of energy demand, the researchers claim. However, they argue that that subsidies for renewable energy, nuclear power and fossil fuels, ignored in the study's cost calculations, provide a barrier to the market finding the least costly technology mix.

It's worth remembering that the findings apply to a very specific case: the PJM Interconnection. How transferable and how scalable—down as well as up—the lessons are is open to discussion. And the research does weigh projected technology costs against current fossil fuel costs, so we are firmly inside the realm of the hypothetical. However, the researchers argue that by using energy surpluses to offset gas for heating rather than selling it to other markets, they are actually under-valuing it. It's also worth noting than one author declared an interest in a solar education startup, another in an grid-integrated vehicle startup.

Ultimately, the researchers are effectively proposing a brand new paradigm for energy provision. Today, fossil fuels are consumed at a rate conversant with hourly demand. More recent emerging wisdom, tied in with the "smarting" of the grid, holds that for renewable energy to be viable, non-essential loads must be shed (i.e. turned off) at times of peak demand. So, having provided enough renewable energy to meet minimum demand, additional needs are met with fossil fuels, while non-essential demands are delayed or denied. A modern washing machine might delay its spin cycle, a warehouse might turn off its AC for a while (and perhaps get energy discounts for doing so). But this approach is flatly dismissed in this research.

"If we applied the findings of this article, in the future we would build variable generation, designing for enough capacity to make electric load for the worst hours, and as a side effect we will have enough extra electricity to meet thermal loads," the study concludes.

Journal of Power Sources, 2012. DOI: 10.1016/j.jpowsour.2012.09.054 (About DOIs).