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The Duck-shaped Barrier to Solar Energy

Posted by Zoe Bodinetz on 14 January 2020 at 11:29 am

The sun generates enough energy in one hour to satisfy global energy use for an entire year. With growing popularity and falling costs, it is difficult to understand why solar energy has not yet been fully incorporated into the electrical grid as a method of power generation. Yet, when we take a closer look at the way the grid operates, we begin to uncover what has become known as ‘The Duck Problem’, and what it means for renewable energy.

Grid Basics

The electrical grid transmits electrical power at a set frequency which varies from country to country. For example, in the UK, it is 50Hz, but in the US, it is 60Hz. The actual number doesn’t particularly matter, but the frequency must be kept as close to this value as possible. To keep the frequency of the transmitted power at this set value, the amount of power generated needs to be equal to the amount of power consumed. This is known as ‘load balancing’. If not enough electricity is produced, the frequency will fall, and customers won’t be able to use their appliances. If too much electricity is generated, then the frequency will increase, and there may be damage to the grid, or power outages. Power generation companies are able to accurately predict the amount of power consumed and generate the correct amount of power to keep this important balance.

Sources

Electricity is usually supplied to houses from a remote power plant. If they have solar panels, they can use the electricity supplied from the solar panels as long as it is sufficient, and the electricity from the power plant to back it up if needed. Traditional fossil fuel power plants take a long time to turn off and on and cannot rapidly increase the amount of energy generated. They are excellent in terms of stable energy supply and can adapt to smaller fluctuations in demand. Solar panels can be turned off and on incredibly quickly, but cannot supply consistent levels of energy, because of seasonal changes, cloud cover and sunrise/sunset.

Demand

When we look at how much energy a house is using over the course of a day, we can see predictable changes. For example, more energy tends to be used in the evening, as it typically gets colder and darker. An average household follows an energy demand curve like this:

Figure 1 source

Solar availability

If we look at the same sort of graph, but for solar energy available, it is again predictable. There will be more solar energy around the middle of the day, and none once the sun has set. Something like this:

Figure 2 source

What do ducks have to do with this?

The problem comes because these graphs do not match up very well. During the middle of the day, most of the energy needed by houses is able to be generated from the solar panels, and they use less power from the power plants. Later on in the day, the solar panels aren’t producing enough, and the house needs to get all its electricity from the power plant. All fine? Not so much. This change is incredibly hard for the power plants to deal with. It increases the risk that the supply and demand might not match up, changing the electricity frequency, and causing havoc for the grid. It becomes increasingly difficult for the energy generators to predict the demand, because they have to now also factor in solar energy use. Another problem comes with the huge ‘ramp’ of power generation needed in the evening, when electricity demand is at its highest, and solar power is beginning to drop off.

Figure 3 source

 
   

If the number of people using solar panels increases, this ‘dip’ in electricity use from the power plant will get bigger and bigger. If we plot increasing levels of electricity generated by solar panels, we end up with a graph that supposedly looks like a duck, and this is why it is referred to as ‘the duck problem’. The illustration by CAISO below illustrates the 'duck crurve':

Figure 4 source 

What can be done?

The simplest way to combat this, is to incorporate some sort of storage solution into the solar panels, so that any excess generation from the daytime can be stored and used at night. No power plant necessary. This sounds easy but relies on the idea that enough electricity can be generated from solar panels alone, with no input from the power plant. Until this is the case, the ‘Duck Problem’ remains a very real threat for the integration of solar energy, until the utilities managers find a solution.

 

If you have a question about anything in the above blog, please ask it in the comments section below.

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Comments

1 comments - read them below or add one

MuppetKeeper

MuppetKeeperComment left on: 21 January 2020 at 1:46 pm

Some of this is being addressed already, a solar farm opening near York in December 2019 with its own battery storage. They don't sell to the grid during the generation peak, they charge their batteries, and then they sell their electricity during the peak.  It makes them more money, and helps solve the issue.
The market price for electricity helps engineers address the problems, market prices seem to work, which genuinely surprises me!

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