Wednesday, December 18, 2019

The Reality of Renewables





Generating power for the entire country comes with one fascinating quirk: the national grid produces the energy that is demanded at that particular moment. Phrased another way, power is used as it is created. In Xcel’s Pawnee coal plant in Brush, CO, a piece of coal is burned to drive a steam turbine, and some microseconds later, that energy is used to boil a kettle in nearby Wiggins.

From this reality of the grid, several consequences follow. The country has to generate more energy when the Superbowl goes to commercial and Americans turn on stoves and microwaves, so we have to have power plants that can quickly ramp up their production to meet the sudden demand. This need has been problematic in the UK when unexpected breaks in soccer games drive millions to energy-hungry tea kettles.

The power plants that have the capability to ramp up and down their production in response to demand are called load following power plants. These load followers carefully track the energy needs of the grid they are supplying and ramp their production up and down to meet the demand at that moment. When the Superbowl starts, these are the plants that ramp up or turn on to supply electricity to millions of TVs.

As well as the load followers, other forms of energy generation are the more laid-back base load power plants. A great example of a base load plant is nuclear energy. If you asked a nuclear power plant to quickly ramp up production, they would ask how you got in, then say they can’t easily change their output. These plants are kept on 24/7 to meet the base load: the lowest expected daily energy demand.

The extreme example of load following plants are peaking power plants, known as “peakers.” These plants are by default not producing power and only turn on when the base load cannot be met by the base load power plants. Their sole purpose is to smooth out intermittencies and ensure that just enough power is being produced. These plants operate inefficiently to prioritize a fast ramp up and ramp down time, and so the power they provide commands a premium price, as demand is necessarily high.

So what happens if this system goes wrong and there is more or less energy on the grid than is demanded? You might not have heard of the electricity wars between Edison and Tesla (the inventors, not the companies), but you have certainly heard of the band AC/DC. AC/DC got their name after someone saw the initials on a sewing machine. It stands for Alternating Current or Direct Current. Direct current is the kind you get from batteries, and alternating current comes from wall outlets. It is called alternating because it switches direction 60 times a second – 60Hz.

When more or less energy is produced than is needed, this frequency changes, which could begin to wreak havoc on things plugged into the national grid that are expecting a particular frequency. The grid is expected to deliver within 1% of 60Hz frequency to businesses and residences here in the US. One solution to this problem of shifting frequency is to have consumer devices like refrigerators monitor the incoming AC current and when it deviates too much from 60Hz, the device responds by powering down to reduce the demand. Enough consumer devices doing this will reduce demand back to the operating limits, and save consumers a few dimes a year to boot.

At this point it is helpful to look at some data. The California Independent System Operator (CAISO) is the main grid operator in California, responsible for about 80% of the energy usage of the state. Helpfully, they put all their data online, and update it every five minutes, and keep records going back months.

To start, here is the demand curve: how much electricity is needed throughout the day. Note that the y axis does not start at 0:


No surprises here. Overnight, less energy is needed, and there is a peak in the morning as people are getting up and in the evening after getting back from work. A lot of this is from energy-intensive appliances that heat up food and water, and relatively less from lights and TVs, as might be expected.

Next let’s take a look at the Net Demand, or what happens when you subtract out renewables (again, note the y axis):
The purple here represents the non-renewable resources. As expected, there is a dip starting around 7 am, lasting until about 5pm. This midday dip in non-renewable energy demand comes from the sun hitting solar panels.

One thing that can be seen by looking at the Net Demand graphs from many days is that California has been heavily investing in megawatts of solar energy, and less so in wind energy. The effect of heavy investment in solar over wind is that there is a narrow margin of renewables available when the sun isn’t out. December 16, 2019 (the day the data from these graphs was collected) wasn’t a particularly windy day, but the point still holds when looking at relatively overcast and windy days (look at Sep 9, 2019 for an example of a cloudy and windy day).

Next is the graph I want to dig into a little bit:

This graph shows how California met its demand on December 16th. I didn’t include coal, batteries, or “other” as they are insignificant to the overall supply, and I didn’t include Imports for clarity. The imports curve mirrors the natural gas curve, reduced by about 50%.

First to note is nuclear. The Diablo Canyon Nuclear Power Plant provides a constant 1,100 MW to California’s overall supply 24/7. On December 16, its output varied between 1,102 and 1,106 MW. This nuclear plant is a base load plant that cannot ramp. The Diablo Canyon Plant is due to shut down in 2024 or 2025.

Next is large hydro, or water behind dams. These can be ramped quickly, but not indefinitely. The water level cannot vary too much, so there is a natural limit to how long these plants can be ramped up - once the water level falls, it's time to shut down. On the other hand, if snowmelt from the neighboring Sierra Nevadas comes through the dam, operators have no choice but to let the water through. This leads to cheap energy prices in spring months. Large hydro is somewhere between a base load and a load following power plant.

Renewables including wind, geothermal, biomass, biogas, and small hydro account for a about 2,000 MW, and these can be considered base load plants. Solar is intermittent, and so can be load-following, but the manner in which they can be load following seems a shame. To make a solar farm load following, build many more solar panels than you need, and connect or disconnect them as the load changes. This seems wasteful, however solar panels do have an unmatched ramp up and ramp down speed; turning them on and off takes nearly no time. California routinely overproduces and curtails solar power at certain facilities during summer months.

Lastly is the hero of California’s load following energy production: natural gas. Natural gas plants can ramp up and down production to follow the load far quicker than coal or nuclear plants. They are comparatively nimble and reactive. Natural gas is at this point absolutely indispensable. Without it, California would be entirely reliant on energy imports from Nevada to match demand.

One more curiosity from the CAISO database. There are times when the cost of energy is negative. Before you get excited, you're not going to see a check from the energy company unless you are a wholesale buyer of energy, which you aren't. This negative cost arises due to excess energy being produced, and the fact that energy is expensive to transport. In some cases, it is cheaper for the grid operator to pay middlemen to offload excess energy than it is to transport or ramp down production. Living nearby a large hydro installation in springtime benefits an energy consumer. A typical large hydro plant might have fifteen room-sized turbines, 5 of which are usually on, but while snowmelt is coming through all fifteen might be producing power to get the water through the dam. This energy is cheaper to offload locally than transport elsewhere. Note to supervillains with energy-hungry doomsday weapons: find a valley in California and run your tests in the springtime to save money on a big electric bill.


The whole reason to have base load plants, load-following plants, and consumer appliances that turn of and off depending on incoming AC frequency is that we don’t have any way to store large amounts of energy. This is analogous to trying to run a restaurant with no refrigerators, or a bar with no kegs – a logistical nightmare. The challenge of large-scale energy storage is what stands between our current power grid, and one that runs off intermittent sources of energy like wind and solar.

There are some ideas floating around that may be able to scale to meet some of the eventual demand for energy grid storage:

1)    Batteries. Creating batteries that power cities rather than cars, watches, or laptops is difficult. Just wiring a lot of LiPo batteries together isn’t going to cut it. These city batteries will need to be charged in the summer, and hold that charge until winter with a near zero failure rate, and be made from earth abundant elements.

<sidenote> "Rare earth elements" are poorly named. Most aren’t rare at all. Every "rare earth element" is more common than gold, and there is about as much selenium as there is nickel. The thing that makes them hard to extract (rare) is that they don’t clump in exploitable deposits like gold or gemstones – they are more evenly distributed. To get an appreciable amount, you need to dig up an process a lot of earth. </sidenote>

2)    Pump Hydro Storage. This is essentially pumping water uphill and behind a dam during the day when you have energy, and letting it flow through a turbine at night when you need it. You can also pump up extra water in the summer and it stores perfectly well until it is needed in the winter. There is a related idea that replaces the dam in a valley with an underground reservoir, and trades out gravity for pressure: pump water underground at high pressure, and release the pressure to drive a turbine when you want the energy back.

3)    Molten Salt. Using power generated during the day to heat up salt until it is molten, as well as chilling antifreeze. This creates an extreme thermal gradient and you can use this to drive a heat exchanger to get energy out. This method doesn’t have a long shelf life – heat up salt during the summer and it will be cold by winter.

The three above are highlighted in this video.

4)    Flywheels. During the day, use solar power to spin up an enormous circular weight called a flywheel, storing energy in the form of angular momentum. At night, apply the brakes to spin up a turbine.

5)    Vehicle-to-Grid. We are starting to heft around pretty big batteries in cars and sometimes in wall mounted batteries in garages. The idea is to use these batteries for ‘peak shaving:’ sending energy to the grid when demand is high, and ‘valley filling:’ charging batteries at night when demand is low.

Without large scale and reliable energy storage solutions, the only way to move fully to renewable resources would be to fully supply our power needs even in the dreary January. Meeting the demand in January using California’s current ratio of renewable energy sources would result in overproduction in summer months several times over.

In the meantime, we need to meet the base load, follow the load as it changes, all while not releasing too much CO2. The path ahead seems clear:
·      Innovate and install energy storage solutions
·      Continue to invest in diverse renewable power sources
·      Build more nuclear plants to meet the base load
·      Keep our natural gas infrastructure in place until it’s no longer needed to follow the load

Cheers,

  - Scott

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