A number of renewable energy technologies are poised for significant growth. Wind turbine production is booked for several years, while several companies have reached the point where they're able to produce a Gigawatt of capacity annually. Although the US has started from a small base, these power sources have grown at an annual rate of about 20 percent for most of the past decade, a period in which demand only grew about one percent annually. The US National Academies of science has now examined the prospects for continued growth, and sees no limits within the next decade and beyond, but, should growth continue, there are going to have to be significant changes to our national grid.

The report was prepared as part of the America's Energy Future Project, which is supported by everyone from General Electric to the Kavli and Keck charitable foundations. It's the second of several planned reports; the next one will target prospects for energy-efficient technology.

The report excludes hydropower, which is renewable, but constrained by the availability of appropriate water resources. At the moment, these other sources—geothermal, solar, biomass, and wind—account for about 2.5 percent of US electricity generating capacity, and estimates are that, under a business-as-usual scenario, they would reach eight percent by 2030. The report addresses the question of whether they'd be capable of scaling, should the US determine it wanted to increase reliance on these technologies (the total available solar and wind energy within the US, at 13.9 million TWh, dwarfs any reasonable future projections of demand). The authors limited their consideration of biomass use because they felt it was likely that the government would promote its use as a transportation fuel.

The authors point out that it's a much harder question to ask than it might appear, because renewable energy sources have properties that are radically different from traditional, fossil fuel energy sources. So, while a traditional power plant can be built in reasonable proximity to the places where the electricity is needed, renewable plants generally have to be built where the energy supply is. That means they require a large investment in transmission infrastructure, some of which might wind up spending part of its time underutilized, given the intermittent nature of most renewable sources. Those interruptions in production, of course, add their own set of costs and challenges.

The good news is that the sorts of changes that will be needed are already areas of active R&D. These include things like demand-based pricing and response control through a smart grid and the development of on-grid storage through technologies like advanced batteries and compressed air.

Solar and wind energy are also relatively diffuse. Although they don't create significant pollution concerns, the production facilities can cause tensions in terms of land use, both for the plants themselves and the transmission lines. The technology itself is also a moving target, especially in the case of photovoltaics, which is affecting both the cost and the amount of land required for a facility of a given capacity.

Against all that complexity, the authors provide some reason for optimism in their analysis: past estimates of the cost of renewable power have proven to be reasonably accurate over time. Where things have gone wrong is in predicting the consequences of those price changes for our use of the technologies. In general, efficiency gains in fossil fuel production, transportation, and burning have helped them stay ahead of the price curve for longer than expected, although they exact their own costs in terms of price volatility.

As a result, the authors are less convinced that economics alone will necessarily drive renewable adoption unless the externalities of fossil fuels—pollution, environmental degradation, and importing from politically hostile nations—are factored in. This could either happen via some form of a carbon tax or cap, or through requirements for renewable power production. Both have been adopted by a number of states, and there is a chance the US government will join in in the near future. The extent of renewable uptake, ultimately, will probably depend on the extent to which various governments formulate consistent policies to promote it.

If the government goes all out for renewables, what are the primary limitations on renewable power production? Aside from sustained policies and their consequences on prices, the authors view the primary limitation as being transmission for at least the next decade. We simply don't have the infrastructure in place to get the power from sites of efficient production to where it's needed. Solar can scale more or less indefinitely, and wind could account for 10 to 20 percent of the total electric needs in most areas; other sources, like geothermal, may contribute in some specific areas. Overall, even with the limits imposed by the grid, the authors conclude that the current manufacturing and technology improvements can get us to 10 percent renewable by 2020, and double that in the decade afterwards.

There are a number of ways to limit the impact of the transmission problem. The authors suggest combining generating capacities through joint solar/wind or wind/natural gas projects, which would allow the transmission lines to be more fully utilized during times when the intermittent nature of one form of production would otherwise leave them with nothing to transmit. But somewhere between the 20 percent mark they think is possible and the point where half of our power comes from non-hydro renewables, we're going to need to change the electric grid.

The good news is that the sorts of changes that will be needed are already areas of active R&D. These include things like demand-based pricing and response control through a smart grid and the development of on-grid storage through technologies like advanced batteries and compressed air. Efficient long-distance power transmission will also be key.

That doesn't mean that the non-grid work will be easy. The authors note estimates that getting onshore wind up to a 20 percent share of the power supply will require 100,000 turbines and another $100 billion in infrastructure work. But, should the US decide to pursue the necessary policies, it appears to be in position to significantly change its energy mix.

The thing that concerns me about the report, however, is that all of this was obvious to someone without the resources available to the National Academies at least a year ago, and some of it was apparent decades ago. The fact that we still need these expert reports to state the obvious provides some indication of the degree of inertia at work.