Overcoming the Intermittency of Wind and Solar Power
The Drawback of Renewables
If we are going to discuss the possibility of making solar power and wind power a large part of the United States’ energy production mix then we need to be honest about some of the drawbacks. As we’ve already discussed, one of those drawbacks is the current cost of solar. Maybe more important however is the variable and unpredictable nature of wind and solar’s production capacity, also known as intermittency.
If we kick fossil fuels and go all-out for renewables, or all-out for nuclear, or a mixture of the two, we have a problem. Most of the big renewables are not turn-off-and-onable. When the wind blows and the sun comes out, power is there for the taking; but maybe two hours later, it’s not available any more. Nuclear power stations are not usually designed to be turn-off-and-onable either. They are usually on all the time, and their delivered power can be turned down and up only on a timescale of hours. To have an energy plan that adds up, we need something easily turn-off-and-onable.
This easily turn-off-and-onable something needs to be a big something because electricity demand varies a lot. The demand sometimes changes significantly on a timescale of a few minutes. However much we love renewables, we must not kid ourselves about the fact that wind does fluctuate. (MacKay)
And its not just the hourly fluctuations of wind that will cause issues. There will also be times of the year where lulls will occur that will last for days. Consider your own experience with the variability of the wind. Some days its really windy and some days it just isn’t. Intermittency is something that we will have to learn to deal with.
Fortunately, power engineers already cope with substantial everyday demand fluctuations on the national grid. The intermittency of wind and solar will cause similar issues on the supply side of electricity. It is not a problem that will be easily solved but it is one that is similar to the problems encountered every day by the engineers overseeing the energy grid.
In order to solve these issues, there are two approaches that will be used: (a) remote demand reduction and (b) storing excess energy produced during those periods when the installed capacity is working at peak efficiency.
Demand reduction is likely to come from technological advancements. Specifically, through the deployment of “Smart Grid” technology. From the Utility Solar Assessment (USA) Study:
The deployment of the smart grid—the integration of information technology into the electric grid in a way that allows more control by both utilities and customers over how and when energy is generated, stored, and used—is essential to the rapid scale-up of solar. But utilities need to understand how the smart grid and solar fit into their development plans and have the regulatory support and tools to deliver new products and services. The smart grid will also be essential to the mass deployment of other renewable sources, such as wind, and for road-based efficiency and conservation efforts.
In its most simplistic form, deployment of the Smart Grid means that the power company will be able to turn off your refrigerator for a few minutes or hours a day in order to reduce demand at specific times. It will mean washing machines and dishwashers that will run on timers at night. The Smart Grid will allow the power company to take some demand off of the grid as necessary.
But demand regulation will only go so far. In order to make renewables viable, steps will have to be implemented on the supply side as well. There are a number of ways that this will be handled.
Stabilize the Grid With More Renewables
Interestingly, the first solution is to put more renewables on the grid. One of the things that the engineers have found is that as more wind power is brought online the overall intermittency of wind power on the grid declines. Essentially, lulls in one area are made up for by strong winds in other areas. If all of these sources are connected and efficiently managed then they tend to cancel each other out. Not completely of course, but they do have an effect.
But even with demand regulation through Smart Grid technology and decreased intermittency through pervasive deployment, intermittency of solar and wind will still be an issue. How will this be dealt with? One option of course is to have enough easily turn-onable backup generating capacity. Initially this probably means retaining some of our fossil fuel plants and having them lie in idle waiting for when they are needed. But we’re trying to get rid of the fossil plants right?
Backup That Renewable
Another way of handling intermittency would be to install hydro-plants as a backup. The excess power generated at peak operation can be used to pump water up hills and into large storage basins. When the intermittent source is unavailable, the hydro capacity would be. By way of example, Denmark copes with the intermittency of its wind power by turning to its neighbors hydroelectric capabilities.
The Danes effectively use other countries’ hydroelectric as storage facilities (and pay for this service). Almost all of Denmark’s wind power is exported to its European neighbours, some of whom have hydroelectric power, which they can turn down to balance things out. The saved hydroelectric power is then sold back to the Danes (at a higher price) during the next period of low wind and high demand. Overall, Danish wind is contributing useful energy, and the system as a whole has considerable security thanks to the capacity of the hydro system. (MacKay)
The final and maybe most promising option involves the integration of the electric car and the Smart Grid. Known as vehicle-to-grid or V2G, cars and light trucks powered by batteries have the
potential for a synergistic connection between such vehicles and the electric power grid. The aggregate power rating of the US vehicle fleet is much larger than the total US electric generating capacity. If even a small fraction of vehicles are harnessed as generating assets, benefits would accrue both to the electric power grid, vehicle owners, and aggregator/service providers. The potential exists for the economic value generated to significantly offset the costs of electric, hybrid, and fuel cell vehicles and to bring a new source of economic returns to vehicle manufacturers.
US passenger vehicles are, on average, parked and idle for about 23 hours each day. During this time, they represent an idle asset and actually create negative value due to parking costs. The advent of electric, hybrid, and fuel cell vehicles, introduces the prospect that parked vehicles can become assets that create value. By connecting such vehicles to the electric power grid, a large scale, dispatchable, electric power generating resource is created.
By itself, each vehicle is small in its impact on the power system. But, in aggregate, a large number of vehicles will represent significant generating capacity. For example, five percent of California’s vehicle fleet could provide 10 percent of the state’s peak power requirement. This geographically-dispersed capacity could be controlled remotely in order to provide power when and where it was needed. Power or energy from an electric vehicle could be sold from a vehicle connection point located at the vehicle driver’s home or place of work. Such connected vehicles could provide a variety of services that have value to utilities and power grid operators.
Obviously V2G is a ways off. We don’t have a Smart Grid and we don’t electric cars. But then again, we don’t have much wind or solar capacity installed yet either. All of these things will take time, planning and technological innovation, but they are possible.