Tag Archives: Wind

Electricity Basics (and some advanced)

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I received my second request for a post! This time the submitter asks for information about electricity, transmission, and how intermittent renewables like wind and solar fit in.

So, the first question:

1. Electricity, for the most part needs needs to be consumed the instant it is produced?

Yes. Storage of electricity can be done in batteries, or with pumped-water energy storage, but these are all just ways of being able to make electricity at some moment later in time. In short, electricity, once produced, is either used immediately or stored. Massive storage is not practical at the moment, so it's used.

2. Wind or Solar electricity is essentially in addition or parallel to the base load, and do little to lessen the use of coal, NG, or nuclear derived electricity!

This bring up an interesting point about electricity production. In the US, we have 60hz electricity. It's made 60hz by the generator design (in the US, Europe and other places use 50Hz power). Thermal power plants, those that burn things to produce power, rely on spinning a turbine in a magnetic field to produce power. The magnetic field is part of the turbine design, and is too complicated for this post to discuss in further detail. The turbine is spun because water, turned into steam by the heat from burning things or other reactions (coal, natural gas, or even heat from fission), expands rapidly from water to steam. It creates pressure, and then pushes through the turbines to spin them. The turbines spin at the exact rate they need to in order to produce 60hz electricity.

If we produce slightly too much electricity, the turbines start spinning slightly faster. To keep the grid at the right speed, electricity production is reduced at plants. If there is too little electricity, the turbines will slow down, and we'll fall below 60hz. There is a constant dance of the power plants and the electricity users to make everything balance. It's mostly automated, and happens very quickly.

What does this have to do with solar and wind? A lot. Solar and wind power output can be predicted, but not perfectly. If we want to maintain a perfect 60hz grid, we need to be able to adjust for wind and solar output. Because, again, electricity is used when it is made, and not stored. Coal and nuclear power plants aren't great at changing how much electricity they produce in a short timescale, so if we are going to have power plants to make the balance necessary, we need hydro and natural gas to account for the variability of the solar and wind. There isn't enough hydro to do that all over the country.

In  other words, if we want to maintain a 60hz grid, we are always going to have some amount of natural gas power plants.

But beyond that little wrinkle, solar and wind power absolutely offset coal-fired power plants. The more solar and wind we have, the less nuclear and fossil fuel power we need, in general.

In practice, do renewables offset much? See the chart below.

US primary energy consumption. Source: eia.gov info

Short version: Wind was about 1.2% of primary energy (primary energy counts burning oil for cars as well), and solar is 0.16%. So wind and solar can replace coal and nuclear, but it barely does currently.

Longer version: We can let the 60hz grid go from exactly 60hz to let it slide between 58 and 62. And then we can fairly easily do away with a lot of other power plants, as long as we have enough wind and solar. Note, however, that there aren't enough good wind sites in the US for this, and solar is currently too expensive and resource-demanding to replace fossil fuels.

3. Electricity is bought and sold just like a commodity?

In some ways, yes, but not exactly! There is a complicated daily bidding process, and several factors are brought into play.

This one is a bit confusing. I'll do my best. Power plants bid on the day-ahead market. They submit their bids to what is typically called an ISO, for Independent System Operator (some places call it differently, like RTO for Regional Transmission Organization. The ISO/RTO looks at the bids, looks at their best guess for power the next day, and then figures out how many of the power plants they need to hire for the day. Those that don't get hired don't actually burn anything or produce power. Those that do get hired, get hired at the rate of the highest bidder. Let's do an example to explain better.

Note that a MWh is one hour of one MW production. So a 600MW plant produces 600MWh in one our, and 1800MWh in 3 hours.

A plant says, "I can produce this many megawatts at this many dollars per megawatt." Power Plant 1 might say, "I can produce 600MW of coal power at $80/MWh." Power Plant 2, "I can produce 1000MW of natural gas power at $100/MWh." Power plant 3, a nuclear power plant, doesn't shut down. They just keep running. They say, "I can produce 1200MW at $0/MWh." Why? Cause they have to run anyways. They are delivering that power at any price. Power plant 4 is an old coal-fired power plant that has already paid for itself, so it's really cheap, and says, "I can provide 300MW at $50/MWh"

Let's assume it is determined that all of the less expensive power plants, along with Power Plant 2, need to run in order to satisfy electricity demand. They want $100/MWh. Power plant 1, despite bidding in at $80 per MWh, gets $100/MWh, nuclear plant 3 also gets $100/MWh, and coal plant 4 also gets $100/MWh.

On another day, it is determined that only enough electricity is needed for power plant 4 (and all the ones who bid below it). So Power plants 1 and 2 do not produce electricity, power plants 3 and 4 each get $50/MWh.

Should inputs become more expensive, then the power plant has to raise its price. Natural gas, for example, became a lot less expensive in the past 5 years. So they now produce electricity for less than a new coal fired power plant would. So they bid in for less.

A bit confusing, right? It gets more complicated than that. This is a great example to show that electricity is not exactly treated like a commodity.

Now what about solar and wind? Pretty much, if solar and wind is produced in the US, it is purchased, pretty much outside the normal bidding system. What happens to the bidding system if all power becomes solar and wind? There probably will still be some version of it, changed to fit the new system!

That's all for now, thanks for reading!

- Jason Munster

Wind Power

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I haven't posted anything in a while because I am teaching this semester (Earth Resources and the Environment), which has made me incredibly busy, and also I was playing a computer game for the past month. Anyways, some friends told me they actually read this, so I am gonna start up again.

Today we discuss how wind power works, how a wind turbine works, and limitations on placement of wind turbines.

Block diagram of a wind turbine. Wind spins the blades, which in turn goes through a transmission to spin a turbine to produce electricity.

Block diagram of a wind turbine. Wind spins the blades, which in turn goes through a transmission to spin a turbine to produce electricity.

Wind power harnesses the power of wind to turn a turbine. Unlike every other power plant we have discussed, this is not a thermal plant. How does wind even happen? As we all know, the sun shines more directly near the equator than it does the poles. And so the equator is heated more than the poles. The Earth doesn't like having one part heated and another not, so the major prevalent winds are the way the Earth redistributes this uneven heating from the equator to the poles. Smaller winds are local manifestations of this phenomenon. In short, wind power is extracting the energy deposited by the sun.

Next: the design

The mechanical design of a wind turbine. Link

The rotors of a wind turbine catch the wind, and thanks to Bernoulli's principle, the wind forces the turbine to spin. Think of it as creating an area of low pressure behind the blade, so the blade is getting sucked, or pulled, rather than pushed in a circle. These blades are attached to a hub, which spins with it. The entire box behind the turbines is called the Nacelle, and contains all the parts that produce power. The hub itself spins somewhat slowly, but thanks to a gearbox, the shaft that goes to the generator spins much more rapidly. The windvane senses the wind direction, and a motor beneath the hub rotates the entire turbine to face directly into the wind.

Wind power plants face four primary limitations. First, they don't work when wind isn't blowing. So you aren't placing these things in windless or low-wind locations. Second, depending on the design of the turbine, each has a maximum wind speed where it most efficiently extract energy. In fact, during high winds, they have to shut down to prevent damage. Third, there is a factor called the Betz limit that indicates that the most energy you can extract from wind is about 60%. In reality, the best might be 45% efficient. A corollary fourth limit is that you cannot place wind farms too closely, because they become far less efficient if you place them nearby. They literally suck out the power from the wind. In the end, availability of location is the most important

This photo from NOAA uses LIDAR to track the turbulence produced in the wake of wind turbines. It visually depicts the limitations of putting turbines in the same place. The turbulence behind the turbines can damage the props on the next turbine, requiring further replacement. It also reduces the efficacy of the next turbine. link to NOAA.

Ultimately, the largest problem is where to site wind farms. You can't put them in places without wind, or you spend a ton of money on them and they don't return the payment.

This map indicates regions and their use for wind farms. It shows that many areas are not great for wind farms. Click the link for a more detailed image.

There are two closely linked issues associated with wind power. In most places, wind does not always blow. When the wind is not blowing, power cannot be extracted. This is called intermittency. It means that wind power cannot provide baseload power. In some places, like California, the intermittency is dealt with by power up peaking gas-powered power plants. In other places, the intermittency is seen as an insurmountable issue (California surmounted it. Those other places are foolish.) Other methods of dealing with it are compressed air storage (more on that later), batteries, and pumped hydro (more on that later).

Another important feature of wind turbines is size. To get more power from a single turbine and reap larger economies of scale, you build a taller turbine. Also, taller turbines reach farther up into the part of sky where wind it a bit more constant. But those huge turbines, that can produce up to 5MW each (recall a larger power plant is 1000MW), are relatively new. We are not sure how long they last in the wild. Maybe 50 years (like a normal power plant) or maybe 20. This is important, because per MW, wind power used more resources to build than almost anything else.

Now, offshore wind is a different beast entirely.

Offshore wind turbines become progressively more expensive as you move to deeper waters.

Offshore wind turbines become progressively more expensive as you move to deeper waters.

These things need to be moored to the ocean floor, or have very expensive floats. It can increase production cost by a factor of three. Ameliorating this fact is that wind is often more consistent offshore. But these things face waves, corrosive ocean water, severe storms, etc., and need to be built very strong, increasing costs. Moreover, they need a way to connect them together, and then very powerful regional lines to transfer the power to mainland. Expensive. If you remember my post comparing the cost of nuclear power to other types, offshore wind is mad expensive.

Another interesting point about wind power (and solar): they produce DC power. This is direct current, like a battery. The power we get from the wall is AC power. It alternates. Anything with a motor likes AC power a lot. Many electronics prefer DC power, hence needing AC adapters for all your electronics. Batteries use DC. Another fun fact about AC vs DC? Electricity make your muscles constrict. If you grab something with AC, since it alternates you let go. You grab something with DC, like a car battery or a taser while being arrested, that stuff causes constriction and you can't let go. Point is, stay away from DC electricity.

Back to the point: Somewhere in the process, whether at the turbine or at a collection station, this electricity needs to be converted to AC to use on the electrical grid. More expenses. In short, offshore wind is incredibly expensive, and only for countries that are afraid of nuclear power. In a later article, I hope to compare the resource costs per MW of constructing each of these types of power plants.

Thanks for reading again!

-jason munster