Electricity supply, part 1 - the need for storage

Keeping the lights on isn't easy

Electricity can't really be stored, as such.

It can also be quite dangerous, and so our electricity delivery systems have been designed with a lot of safety features to prevent electrocution, fires and equipment damage. These things do happen from time to time of course, but mostly, the safety features activate to shut down the affected circuits before any damage is done. This is critical to recovery, as preventing equipment damage means that faults can be corrected and service restored in  minutes or hours rather than the days, weeks or months it would take to replace damaged equipment.

To keep the system safe and the lights on, the delivery sytstem - "the grid" - needs to stay within quite tight tolerances for frequency and voltage (50Hz and 240V in Australia). If things get too far out of whack, or stay out of whack for too long then the safety systems start to activate and shut down parts of the grid to prevent damage. Managing the grid and keeping things from failing is actually a fantastically complex task. A great illustration of the sorts of things that can go wrong are the story of the 2003 blackout in the northeastern US, and the Texas power grid failure of 2021.

This means that supply and demand must be exactly balanced at all times. In the National Electricty Market (NEM) for Australia, which covers the eastern states and South Australia, the market operator (AEMO) operates a planning and bidding process every 5 minutes to ensure this balance is maintained on a minute by minute basis. 

This delicate balance between supply and demand has historically been achieved by letting people use electricity when they want it, and then the market operator AEMO organises the dispatch of supply to meet whatever that demand happens to be at that point in time. This means that total demand varies according to the time of day with our wake / sleep cycle, and also with the weather as we turn heaters and air conditioners on or off, use more or less lighting, watch TV in the evening or go for a walk in the park.

Figure 1, below, shows the total NSW demand for the first week of April 2022. All the data used in this series of posts are freely available to the public via the Australian Energy Market Operator (AEMO) although they are not particularly accessible or in a user friendly format. The NEMOSIS software is free and open source window onto that data that improves accessibility considerably, but I used the commercial package NEMsight, which makes things even easier. The OpenNEM project is the easiest of all, but unfortunately has only the most recent week of data available.

Figure 1 Electricity demand in NSW from 00:01 Friday 1st April to 23:59 Thursday 7th April. Total demand is highest during the day, and lowest in the wee small hours of the morning. Average for the state is about 7,000 MW (megawatts), which is about the same as having around 3 million electric kettles, or 3 thousand electric trains running simultaneously.

The demand shown here includes that which is met through rooftop solar panels "behind the meter" so is closer to the the electricity being used, than to the demand which is "seen" by the market operator (anything that happens behind the electricity meter is essentially invisible to AEMO).

Renewable energy sources don't make it any easier

We have some needs to move away from fossil fuel use for electricity production. Yes climate change, but also fossil fuel depletion. We aren't making any new coal, ever. The rate that we're making new oil and natural gas is so slow as to be zero for all intents and purposes. So for both of these reasons and more, we would really like to stop relying on fossil fuels for electricity generation in the grid.

This causes a real problem because as noted above, supply and demand is balanced by letting people use what they like, and then dispatching the supply to meet that. Fossil fuels are dispatchable, in the sense that you can  (more or less) turn them on or off, up or down, in order to meet the electricity demand and keep the whole grid stable.

Renewable energy generation isn't like that. It relies on energy flows in the environment, like the wind and sun. You can't dispatch those sources to the grid if the natural flow isn't available at that point in time, and so there is no guarantee that they will be available to help generate the electricity demanded. The only thing you can do with renewable sources is to "spill" them - turn them down or off when they are not needed, forgoing electricity generation in order to maintain the required balance.

Figure 2 shows how this renewable generation worked in the first week of April. The graph shows electricity supply from different sources. You can see that solar generators, both domestic rooftop units and utility scale commercial plants, only generate during the day and even then that depends on the season and the weather. Wind power can make electricity at night, but is not consistent over the week. Note that on the 5th April there was almost no wind power generated anywhere within the state of NSW.

Figure 2 - Electricity generation over the same period as Figure 1. "Other" generation is mostly fossil fuels, used to fill in the gaps between the total demand, and the generation from these renewable sources.

Let's go to 100% renewable generation

Figure 1 shows us the instantaneous power requirement for NSW in the period 1 - 7th April 2022. Power multiplied by time (the total area of blue shading in the graph) represents the energy requirement for that week - 1.23 million megawatt-hours, with a wholesale value of about $270 million. Of that energy, about 21% was generated from either solar or wind, the remaining 79% mostly from fossil fuels  (74%) and hydro (5%).

We can do some experiments with the data to test what might happen as we build more renewable generation. Perhaps we can even get to the goal of 100%. For example, suppose that we go on a massive program of wind farm building, so that we have 5 times as much wind farm capacity installed as we currently have in NSW. What would that first week of April have looked like? See Figure 3 below.

Figure 3 - Electricity generation over the same period with a five-fold increase in wind power capacity. The vertical red line is 5:30 AM on 5th April. The sun is still below the horizon and almost no wind power is being produced across the state. To meet demand at this time using wind, we would have to expand the NSW wind generation capacity by more than 120 times what's already installed!

Now we are up to 57% renewable generation but if you look closely, you can see a couple of occasions on the third day, and on the 6th and 7th evenings,  when we had to spill some renewable generation because the total exceeded the demand. This electricity has to be wasted because there is nowhere it can go. If it stays on the grid the voltage will start to rise and then the safety systems will start blacking out parts of the network. So we have to turn off some wind turbines before that happens.

In the scenario shown in figure 3, we have to spill about 3000 MWh of electricity, worth perhaps half a million dollars.

We get a similar situation with the solar - expanding commercial solar farms by a factor of 5 also gets us close to spilling electricity (shown in Figure 4).

Figure 4 - Electricity generation with a five-fold increase in commercial solar farms. No spill, but only 39% of energy generated renewably.

This shows why a renewable electricity grid needs to be able to store energy somehow. There are periods (such as 5:30 AM on the 5th April - see the vertical red line in Figure 3) when none of the renewable resources are available. We will need to be able to draw from hydro, from batteries, or from some other form of storage at those times. 

But exactly how much storage do we need? That's a question we'll explore next time.

P.S. I've made quite a few simplifcations here, in the interests of not obscuring the big picture. We'll address some of those in future posts.