Running the Numbers, and the Country, on Renewables

January 15, 2013

There is general agreement across the (sane, rational) political spectrum that renewable energy is the future. In 2009, Scientific American published a detailed vision of how a renewable planet could be achieved, and what such a world might look like.

Now, in a major effort at the University of Delaware, a much more detailed and specific look at how a shift to renewables could replace, watt for watt, the current system in a large regional grid called the PJM Interconnection, representing 13 states and one fifth of the US grid.

IEEE Spectrum:

Intermittency may be a problem for an individual wind farm or solar power plant, but a diverse array of renewable energy systems—coupled with storage in the form of batteries or hydrogen tanks—apparently wouldn’t suffer such issues.

study by researchers at the University of Delaware modeled how well renewables could sustain a big chunk of the U.S. grid—72 gigawatts worth, where the entire country has a capacity just north of 1000 GW—and found as high as 99.9 percent reliability at reasonable costs.

The Delaware researchers evaluated 28 billion combinations of renewable energy and storage, modeled out over a theoretical four-year period using historical weather and electricity load requirement data. “At 2030 technology costs and with excess electricity displacing natural gas, we find that the electric system can be powered 90 to 99.9 percent of hours entirely on renewable electricity, at costs comparable to today’s,” the authors wrote. Senior author Willett Kempton has long pushed forvehicle-to-grid (V2G) systems in which plugged in electric vehicles can provide power back to the grid.

The 99.9 percent figure can be achieved with, for example, 17 GW of solar power, 68 GW of offshore wind, and 115 GW of onshore wind. The most cost-effective solutions featured huge excesses of generation capacity—up to three times the load requirements at times—in order to minimize costly power storage additions. The authors wrote that “at 2030 technology costs, 90 percent of load hours are met at electric costs below today’s.”

University of Delaware:

One of several new findings is that a very large electric system can be run almost entirely on renewable energy.

“For example, using hydrogen for storage, we can run an electric system that today would meeting a need of 72 GW, 99.9 percent of the time, using 17 GW of solar, 68 GW of offshore wind, and 115 GW of inland wind,” said co-author Cory Budischak, instructor in the Energy Management Department at Delaware Technical Community College and former UD student.

A GW (“gigawatt”) is a measure of electricity generation capability. One GW is the capacity of 200 large wind turbines or of 250,000 rooftop solar systems. Renewable electricity generators must have higher GW capacity than traditional generators, since wind and solar do not generate at maximum all the time.

The study sheds light on what an electric system might look like with heavy reliance on renewable energy sources. Wind speeds and sun exposure vary with weather and seasons, requiring ways to improve reliability. In this study, reliability was achieved by: expanding the geographic area of renewable generation, using diverse sources, employing storage systems, and for the last few percent of the time, burning fossil fuels as a backup.

During the hours when there was not enough renewable electricity to meet power needs, the model drew from storage and, on the rare hours with neither renewable electricity or stored power, then fossil fuel. When there was more renewable energy generated than needed, the model would first fill storage, use the remaining to replace natural gas for heating homes and businesses and only after those, let the excess go to waste.

The study used estimates of technology costs in 2030 without government subsidies, comparing them to costs of fossil fuel generation in wide use today. The cost of fossil fuels includes both the fuel cost itself and the documented external costs such as human health effects caused by power plant air pollution. The projected capital costs for wind and solar in 2030 are about half of today’s wind and solar costs, whereas maintenance costs are projected to be approximately the same.

Interview with study author Willet Kempton below:

Midwestern Energy News:

Midwest Energy News: Earlier studies showed that in theory there’s far more than enough wind and solar power to meet the world’s electricity demands. But many believe that wind and solar are too intermittent to be reliable as a source of baseload power, and our limited ability to store that energy until it’s needed will keep us continually reliant on fossil fuels for baseload power. You found that wasn’t the case. If renewables and storage were adopted as you describe in this study, what would the electrical grid look like in 2030?

Kempton: You have a diversity of sources because you’re more likely to have power generated when you need it if you have onshore wind and offshore wind and some solar. A lot of the time you’re generating more power than you need. And when you are doing that you store it, but before long your storage fills up, so most of the time you’ve got excess power.

Sometimes, when you don’t have enough power being generated by renewables, you discharge your storage and run on that, plus whatever renewables you’ve got. And a few times per year, you actually have to look to some other source. In our analysis we used fossil, using legacy plants that are already in existence, and just running them much less frequently.

So, that’s what the system looks like: Lots of apparently excess renewables, a very small amount of storage, and some older fossil plants that are being kept around for these situations.

Earlier computer modeling efforts by renewable energy analysts had tried to match wind, solar and hydro generation to electricity use to see if renewables could provide reliable electricity. Your model instead tested 28 billion combinations of renewables and storage and sought out those that were least expensive. Why did you seek to minimize cost rather than maximize reliability?

We did set a reliability limit, so we said you have to enough power to run the system 30 percent of the time, enough for 90 percent of the time, enough for 99.9 percent of the time. For each of these, we ran for minimum cost. The reason we did that is that we really were trying to match two fluctuating things. People talk about renewable energy fluctuating, but load also fluctuates. So, unless you really understand whether the fluctuations are in sync or out of sync, it’s very hard to know how much renewable generation you need to make load.

[It’s also hard to know] which types, because wind on land tends to peak in production more in the evening, though that varies with location. Wind offshore tends to be more constant, but tends to peak when you have storm patterns moving through.  And solar, of course, peaks at noon. So, what’s the least-cost combination of those three and storage? We couldn’t know that in advance. We really had to try all combinations.

Your model found that the most affordable renewable-dominated grid was one with more than twice the generation capacity than would seem to be needed. Does that mean the excess energy would be wasted?

If we only had today’s uses of electricity, and didn’t change anything about how we use electricity, than yes it would be wasted. But what we saw when we did this model is that the excess primarily occurs in the cold months. That’s not necessarily something we expected. I mean, we knew there was more wind in the winter. We’re getting lots of excess electricity, especially September, October through May.

And lo and behold, that’s when we’re using a lot of fuels for heating. So . . . we asked the question, suppose we displaced natural gas for heating with this excess electricity? And when you calculate the energy value of that excess electricity, it’s pretty close to the same as the amount of energy burned for natural gas.

In this study, you sought to minimize all the costs of burning fossil fuels, and you included costs that ratepayers don’t pay for today, such as the damaging health and environmental effects of harvesting and burning coal and natural gas. Given the political power of utilities and fossil fuel companies, that seems like a big assumption. Why do you think it’s justified?

We’re not saying this is going to happen. We’re not saying this is a prediction of the future. We’re just saying let’s just look at what the costs are. Because people say, Renewable energy is expensive, or electric cars are expensive. Let’s figure out what the cost actually is.

Say I opened up a new business. I want to buy some things, manufacture a product and sell it, but I’m going to take some of the costs and I’m going to put it on somebody else’s ledger. So I’m not going to actually pay for the steel I’m using. I’m going to charge Dan for that. Well, I’m going to be able to offer my product at a lower price.

That’s what the fossil fuel industry is doing right now. Especially with health costs, which is an immediate, current cost that actually just goes right over on the ledger of health.

We’re not saying that’s going to change, just like I wouldn’t have said in 1960 people are going to stop smoking cigarettes. We’re just saying, what is the actual cost of this? So don’t tell me that cigarettes are cheap, or that electricity from coal is cheap–it is, by market price–but that’s not the total cost. We were trying to calculate total cost.

At the same time, we did not subsidize the renewables. We didn’t say, hey, there’s a production tax credit now and that’s a cool policy and you can get the taxpayers to pay for part of your wind turbine. We took away all the subsidies. We just put the actual costs of renewables and the actual cost of fossil, and put them together.

To achieve the sort of all-renewable grid that you write about, do we need new or improved generation or storage technologies?

We did not assume any technological changes. We took the numbers that were projected for 2030–what the same technologies would cost then, with the kind of minor refinements you get when you manufacture a product over 20 years. And, we took the cost projected for storage. I don’t think that’s realistic. I think that we will have step changes in both storage and in renewable generation, and they’ll probably occur before 2020, much less by 2030. But we didn’t assume that. We just assumed current technologies with refinements, but not new discoveries.

What policy changes would have to happen to make the grid you describe a reality?

My first answer would be let’s just charge what stuff costs. So, a new technology gets subsidized for 10 or 15 years, but if you’ve got external costs, they ought to be included in the price. So, we can estimate, when you build a coal plant, you don’t know if Jones is going to die of cancer vs. Smith, but you know from epidemiological studies that it will cause approximately this many deaths and lost work days and so forth. So that should be part of the cost of generation. If you do that, then the market will just do this stuff by itself.

In the policy area, the other way to do it is what we’re doing now, which is to subsidize renewables until they get to enough volume that they’re actually able to compete without subsidies. But that’s a policy answer.

The other way to answer the question is, What would we do to get there? I think I would say we would need some analysis by the energy planners to ask not just what’s cheaper today–onshore wind or offshore wind or solar–but to ask what kind of systems do we want when we build this out to 30 percent or 50 percent of our energy production from renewables. Our study shows you don’t want to keep just picking the cheapest source. You want to pick sources that go together so that one that might be a bit more expensive, but produces power when your cheaper ones are not producing much power–you want to have that as part of the mix.

That’s not the way we do planning now. You need another 100 MW? What’s the cheapest way to do it? That [describes] all state energy planning and all [utility] planning. Nobody’s doing this kind of analysis like we have done here.

30 Responses to “Running the Numbers, and the Country, on Renewables”


  1. […] are several scenarios out there for a renewable future in which no significant grid storage is needed. These are […]


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