Renewable energy is generally limited by the weather. Wind, solar, and hydroelectric are all sensitive to the local conditions. Given that the US doesn't have a national electric grid, that means they're very intermittent; if the sun's not shining in California, then the golden state doesn't get much photovoltaic power.

Expanding the source of power over much larger regions can overcome the weather dependence; it's essentially unheard of for the entire US to be experiencing low-wind conditions. But this runs up against the structural limits of the US grid, where shifting power over large distances is either impossible or highly inefficient.

A new study released Monday looks into what would happen if that limitation were eliminated. It envisions a massive web of high-voltage, direct-current transmission lines, hooked up to 32 nodes spread across the US. This allows a massive spread of renewable power that could be dispatched anywhere in the nation. The result is a grid with dramatically lower carbon emissions and the bonus of lower costs to consumers.

Benefits of DC

There are a couple of serious problems with transferring electricity across the US. Existing transmission lines are often alternating current, which doesn't cover long distances efficiently. In addition, while all the US grids operate on the same frequency, those frequencies aren't always aligned—the peak in the AC of one grid may line up with the trough in a neighboring grid. If California were to generate an excess of photovoltaic power, the only way to export it to other states is to convert it to DC and then back to AC. Expanding this capacity means building this capacity at the interface of each of the neighboring grids.

Direct current can transfer power over long distances efficiently. And, since it's specifically used for this transfer, all the hardware needed to convert AC to DC or back can be located where the DC power lines start.

In the new study, the authors envision a nation-wide DC transmission grid, with 32 nodes linked by a web of long-distance transmission lines. This would allow renewable power capacity to be distributed across the country. This makes generating sufficient power less dependent upon the local weather. And it makes renewable power more cost-efficient, as the hardware can be located at sites with the best resources, rather than the ones located closest to major population centers.

Optimizing the grid

To find out how well this would work, the authors examined the 2030 grid. In keeping with expectations, they assume a 14-percent increase in demand. And they assume existing nuclear and hydroelectric plants would still be in use.

From there, software kicked in. The authors have developed a model they call the National Electricity with Weather System (NEWS). It takes hourly weather data (it was fed the years 2006–2008 for this work) and can use it to estimate both electricity demand and renewable energy production for the entire United States. Then, it finds a way to create an electrical grid to feed the demand, minimizing the total system cost, including both generation and transmission.

The authors examined four different scenarios. One included coal having a substantial role in the future grid. This kept prices low ($.0854 per kiloWatt-hour) but boosted carbon emissions by 37 percent compared to 1990 US emissions, when grid prices were typically $0.1176/kWhr. The rest eliminated coal and varied renewable and natural gas prices. With high-cost renewable energy/low-cost gas, carbon emissions dropped by 33 percent, while the cost was similar to having coal present at $.0857/kWhr.

For cheap renewables/expensive gas, renewables were used more heavily, and carbon emissions dropped by 75 percent. Prices were higher, at $0.1004/kWhr, but that was still cheaper than 1990 and below the projected cost in 2030. Having moderate prices for both gas and renewables resulted in a similar price ($0.1021/kWhr), while emissions dropped by more than 60 percent.

All of those numbers assume that there will be no significant changes in the technology we use to obtain renewable energy, which could make the economics better. And all of them include the cost of creating the DC distribution network.

Since the high gas prices/low renewable prices scenario resulted in the largest deployment of renewables, the authors looked at this in more detail. The optimized grid built by the NEWS software included 371 GigaWatts of solar capacity, 461GW of natural gas, 100GW of nuclear, and 74GW of hydroelectric. Wind power would have 523GW of capacity and produce nearly 40 percent of the electricity used in the US. Almost all of that capacity would be onshore, as the DC network ensures that wind turbines can be installed where they're cheapest, rather than close to population centers.

The system would require about 0.08 percent of the US' land. But it would come with a significant bonus: the elimination of so many boiling water plants would cut water consumption for electricity by 65 percent. The authors also assume any excess electricity is discarded, rather than being used for things like hydrogen production.

Overall, the authors conclude that a single, national distribution network would go a long way toward meeting our climate goals with current technology, largely by allowing that technology to be deployed extremely efficiently. And, compared to most predictions of future electricity prices, it's cheap. By 2030, they estimate, US consumers would be saving $47.2 billion a year.

Nature Climate Change, 2015. DOI: 10.1038/NCLIMATE2921 (About DOIs).

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