Hydro and Wind Capacity Factors

I enjoy looking at statistics on wind turbines. Typically, I hear that a turbine will produce enough energy for a certain number of homes… and then I hear the wind advocates telling others that their capacity factor is as good or better than hydro… and in some cases, it actually looks like they might be right. But there is a very large difference between the capacity factor of a hydro plant and the capacity factor of a wind turbine or solar collector.

I saw one statistic that suggested that a wind turbine might have an average capacity factor of 30%… A 3 MW wind turbine would generate about 7,900 MWh in a year. That SOUNDS like a lot of energy, but in fact, if you assume that the average home uses about 10 MWh per year, that is enough energy for about 790 average homes. That amount of energy provides a North American home with electricity for most needs, but generally does NOT include heating, which more often uses a fossil fuel such as natural gas.

But the number also ignores a few facts. The wind turbine does generate enough annual energy for 790 average homes, but is it there when needed?  As an extreme example, if the wind blew and the generator operated at full power continuously for 1/3 of the year, and did not operate at all, outside those times, there might be a serious problem. Those homes would have power to spare for a part of the year, and none for the other larger part.

But batteries are supposed to be the solution! If one assumed that the turbine ran 1 full day, and then not at all for the next 2, it would have a capacity factor of 33%, and that might need a very big battery… in fact, in one fill day, the turbine would generate 72 MWh, and the homes around it might use about 1/3 of that – leaving a surplus of 48 MWh for storage for the following 2 days. The cost of a battery, at current prices for that amount of storage would be (based on $200/kWh) almost $10,000,000, or a capital cost of about $12,000 per home.  That price is expected to drop, but it might go down to about $5,000/home by 2025.

But again, that misses a few factors…

First, the wind is not fully predictable in the longer term. Much of the time, it may be far better than a day on and two off; blowing for at least a few hours every day, meaning that the battery might be smaller, but on the other hand, we have all seen the hot summer days in the summer when there may not be a puff of wind for a week, meaning that it also might need to be a much bigger. The message  here is not intended to degrade wind, but to make it clear that there are challenges that can be overcome to store and use the energy more efficiently.

The real issue for emissions is the fact that this ONLY converts the existing household electrical use to renewable power. The two biggest sources of carbon emissions are electricity generation (largely coal) and transportation (60% personal vehicles), What about the heating, and fuel for the car, plus all the energy needed to supply goods and services for all that we all need to live. The bad news is the fact that the existing electric grid only supplies about 1/5 of the energy that we all use. The balance is almost all natural gas, gasoline or other fossil fuel. We use a lot of energy, and we are all accustomed to turning up the heat when it is cold…. driving to town for dinner or a night out, or a vacation to the beach a days drive away. A battery to smooth the needs for all use might cost much more. One problem that most of us face is a seasonal variation in fuel use. In my case, I use almost 75% of my annual energy between November and late March. Air conditioning, believe it or not, is actually relatively efficient. Solar energy is heavily biased to arrive in summer months when it may be needed least. Storage from summer to winter is a real challenge.

A hydro plant, on the other hand is a little different in its operation. It can be turned up quickly if needed, and shut down just as quickly if the power is not needed. In the years before about 1950, many hydro plants were built, based on “firm power capacity” and that meant that they could generate their maximum power virtually continuously for an entire year. There were times that water spilled over the spillway, and other times when it did not. But the plant could generate continuously and provided a reliable source of energy for the customers at the time.

In recent years, with all of the interconnections between utilities, and the wholesale trading that goes on, some smart companies have bought a bunch of old hydro plants and replaced the generators with much larger ones. These companies can now store water behind their dams for much of the day, simply by not using the water for generation, and not spilling it. This happens when market prices are low, but when the wholesale price spikes during peak periods, they can generate an entire days worth of energy in a few hours, and they make very good money in doing just that. This type of operation appears to have a low capacity factor, as the generators only run when the prices are high. The key is that an operator can choose when that is needed, and is not forced to wait for wind or sunny weather.

One example that comes to mind. As a student, I worked at a hydro plant on a river that is a tributary of the Columbia River. The dam had a height of over 200 ft, and a forebay behind the dam was only about 5 miles in length. It was a raging torrent of a river before the dam was built – dropping over 400 ft in the 14 miles of river that is in Canada.Capture

When first built, the plant was equipped with two generators – 90 MW each, but the powerhouse was equipped to accommodate 4 generators. In that plant, the 2 generators could run almost continuously at 180 MW with a few short periods, when flows were very low, that capacity might be reduced to 150-160 MW. But for almost half of each year, the spillway discharged much more water than was going through the generator turbines.

After a few years later, my employer signed an “equi-change” agreement with another utility that had opposite capability, or storage. Soon, two generators were added, and for half of the year, we ran 4 generators, and exported the equivalent of 1 generator, while for the other half of the year, we ran 2 generators, but imported the equivalent of 1 generator. We had a plant that provided about 270 MW of firm power.

You can imagine my surprise a few years ago, when I learned that they were building an extension to the plant, that would generate a further 335 MW of capacity. The total capacity, after upgrades to the existing powerhouse totalled more than 700 MW. I wondered how they would ever make that pay, as there were only a few weeks each year when there was enough water to generate that much power. Graph

It didn’t take long to find out what they were doing. When I worked there, I became an operator, and we were always careful to keep the forebay at a constant level – as high as possible, to get the most power from every bit of water than went through the turbines. We were rarely down more than an inch or two from the full pond level.

I can now see the forebay level on the internet, and what has happened is remarkable. The graph shows the forebay level over a 2 day period, and one can see that they are using the storage behind the dam, small as it is, to generate energy for peak periods and store during off peak periods. The old generators and turbines have been  upgraded and the plant now has a firm capacity of as much as 200 MW, but the capacity is over 700 MW. For those in the US, the readings are in M, (1 M is a little over 3 ft) so the forebay level was run up and down by more than 6 ft in 1-2 days. So much for our 2 inch limits. The average capacity factor is far less than 50%. The wind advocates may want to claim that their wind turbine can beat this… but there is one key difference. This capacity is fully dispatched. It can be called when needed and stopped – to store energy when not needed. They generate when it makes economic sense, and reduce capacity dramatically when prices are low. What is interesting is the fact that this plant has a small reservoir (5-6 miles in length) and the storage in that forebay has justified the construction of a second almost equal capacity powerhouse that cost almost 30 times the total cost of the initial dam and powerhouse.

Most hydro plants are now built with far more capacity than they can run continuously, but these plants can all be dispatched when and as needed. In this way, they are very different than wind or solar. The key difference is that they can be dispatched at any time to whatever capacity is needed, provided there is water available behind the dam.

What is really needed is a way to have the intermittent generators work in an optimized way with plants such as hydro that can be dispatched, to get all of the energy that we can get from the renewables, and store it efficiently. This form of storage is near to 100% efficient. I would suggest, that most of these plants were built, based on the value of the energy produced and not on the storage provided. The storage is almost free. The picture is now rapidly changing and storage may soon have much more value than the energy generated.

And that brings up the real definition of storage in the grid. Electricity used to be used in the very instant that it was generated. Storage allows the separation of the generation time and the consumption time.

There is a great future for batteries, but they remain a dedicated, and relatively costly device. There are many other forms of storage that can be leveraged; devices that were bought and paid for to provide another service, much like the old generating station, that can be used to do the same thing, but at the grid edge, where there is almost no energy loss between a storage device and the load the process may be even more efficient. We see the rapid advance of Virtual Power Plants (VPPs) that manage a wide variety of devices that can effectively deliver a large impact from many small storage devices, such as domestic water heaters, EV chargers, air conditioning units, water pumps etc.

We are entering a time when the entire grid operation will need to be optimized to gain maximum benefits from every source available, including the most intermittent sources, and the VPP will play a huge role in this. Some utilities are currently participating in imbalance markets that match shortages with surpluses. But this will need to grow to include behind the meter devices. The missing link may be the need to have all players around a single table, all agreeing to solve the problem in the best way, and a means to fairly share the benefits.

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