Two of the major renewable energy sources, wind and solar power, are intermittent in that they can't always be relied on for power. Although there are many strategies for mitigating the impact of the interruptions, the intermittency is thought to place a limit on the percentage of renewable power that can be easily integrated into the electric grid (although the precise point at which it becomes a problem is a bit of a moving target).

One approach to intermittency is to store electricity from these sources for use when they're not active. GE even offers an integrated wind energy/battery system that's meant to do precisely that. But a new analysis by Stanford researchers suggests that this approach might be misguided. Wind turbines are so cheap to build energy-wise, compared to batteries, that it's better just to discard the energy. By contrast, solar power pairs well with batteries in this analysis.

The problem with intermittency is matching supply with demand. The authors of the study cite data from Texas, which has seen a boom in wind generating capacity but hasn't built transmission lines fast enough to get the energy where it's needed. As a result, 13 TeraWatt hours of electrical energy have been discarded over the last five years. In some years, as much as 17 percent of the wind energy produced in Texas went unused.

There are a variety of means of storing electricity for future use, however. Some require large facilities like pumped hydro power or compressed air storage. Others, like batteries, can be distributed. But producing the storage capacity also takes money and energy, which cuts into the production from renewable power. The new study takes a look at this balance from the perspective of a value called energy return on investment (EROI).

The basics of EROI are pretty simple: it costs energy to produce energy, whether the cost is involved in mining coal or purifying silicon to produce a photovoltaic power. To be effective, the energy you get access to as a result will have to be higher than your initial investment. For renewable energy sources, this varies a great deal based on the precise technology used (thin film vs. silicon solar panels) and how that technology is put to use. For example, depending on the precise location of a wind turbine, the break even point for energy may be as little as months or as much as a few years, resulting in very different EROI values.

The same thing is also true for energy storage. Dams, generators, and pumps are all required for pumped hydro storage, all of which take energy to make. The same applies to batteries. The authors generated an equivalent of EROI for storage technologies, taking into account the amount of energy that can be stored, the efficiency with which it can be retrieved, and so on.

The good news is that both pumped hydro and compressed air storage provide excellent returns on the energy invested and can be successfully paired with both wind and solar power. The bad news is that even the best lithium batteries are almost two orders of magnitude worse, and things like lead-acid batteries come up short by yet another order of magnitude. This changes the results for the different sources of renewable energy significantly. Solar panels take a lot more energy to produce, so preventing the waste of that invested energy pays off, even if it's done with a low-energy payback using batteries.

By contrast, the authors conclude, "Attempting to salvage energetically cheap power (e.g., wind) using energetically expensive batteries is wasteful from a societal perspective." Elsewhere, they note, "if society aims to increase output of (say) wind energy with the least energetic investment, it is better in many cases to just build another wind turbine, or possibly transmission lines, than to build a battery to store power that arrives at off-peak times."

After looking over why their model gave this output, the authors conclude that the simplest way to change the situation is to give batteries a longer usable lifetime. Simply doubling the usable life of a lithium battery would be enough to start shifting battery storage of wind power into the break-even territory (increasing capacity and the ability to sustain larger drains would also help).

By the authors' own admission, their model doesn't account for everything. For example, they note that distributed storage can improve the stability of the electric grid and provide backup power to key facilities in emergencies, things that produce non-energetic value. The geological features that enable pumped hydro and compressed air aren't available in all locations either, which means that it may be a choice of battery storage or none at all. The authors' estimates can also be thrown off if the amount of storage capacity built is a poor match for the typical excess of electricity.

The last thing they suggest, however, is that having excess electricity may be a net societal benefit, allowing us to do things like desalinate water or produce materials that would otherwise be energetically prohibitive.

Energy & Environmental Science, 2013. DOI: 10.1039/c3ee41973h (About DOIs).