As night turns to day and day changes back to night, our electricity demand goes up and down. At night, people turn off their lights, and some of the factories and stores close their doors, and as a result, about 2 GW of demand goes away in the Finnish power grid. That’s roughly two mid-sized nuclear reactors’ worth.
In the graph below, you can see the hourly electricity demand of 2018 in Finland. It looks a bit like someone took a marker and did some (not so careful) colouring. This effect is due to the electricity demand going up and down with the changes of day and night. Another change happens with the changing of the seasons. The graphs starts with the calendar year in January, so the demand is high because of winter when there’s more need for heating. From there it slides down for the summer because demand is smaller as the weather gets warmer and then goes up again with the arriving autumn.
Even though there is variation, there is always some electricity demand, whether it’s summer or winter, night or day. In Finland, a minimum demand on a summer night is roughly 7 gigawatts, and the maximum demand on a cold winter day can be double to that. For comparison, the Hanhikivi-1 nuclear reactor will have a capacity of 1.2 GW, so it will produce roughly one tenth of Finland’s electricity demand.
Even as the lights go out and stores close for the night, the demand stays a bit higher thanks to many Finnish houses having hot water boilers that are heated with night-time electricity. This is called demand flexibility, where demand is shifted from one point in time to another. In this case, daytime heating and hot water demand is partly done by storing heat in the boiler at night. Traditionally, this has been done to even out the night and day fluctuations in demand, as it is more cost-efficient to run power plants at stable, near-maximum capacity all the time, instead of ramping their power up and down.
This minimum level of electricity demand in society is called baseload. Usually the most cost-effective way to provide baseload power is with a reliable power plant that uses fuel, instead of energy sources that have variable production depending on weather or time of day. Nuclear power is an excellent way to produce baseload, as it is based on fuel, but since the fuel is not burned, it emits no greenhouse gases nor air pollution like combustion-based power plants do.
Wind and solar have variable output
Wind turbines and solar PV harvest energy from the energy flows around us. This means that they only produce energy when these energy flows – sunlight and blowing winds – are available. This causes their production profile to be variable.
Solar panels and wind turbines have a nameplate capacity, which is the amount of power that they can deliver in ideal conditions. When this is evened out throughout the year, solar panels usually produce between 10 and 20 % of their nameplate capacity on average, while wind turbined average 25 to 45 % - depending on the place and technology used.
This annual average production can give a misleading picture. In a country the size of Finland, solar production drops to practically zero every single night. Although in the midsummer Lapland, there might be some sun around the clock, as the sun won’t set for several weeks. With wind, the production rarely goes to zero, but as we can see on the graph below, it drops low quite often.
There are big plans to increase the amount of wind and solar production to help us with climate change mitigation, and their share and capacity has grown fast in many countries. When this is combined with our society’s extreme dependence on continuous and reliable supply of energy – especially electricity but also heat and liquid fuels for transportation – we can start to see some of the problems. As the share of wind and solar grows to several tens of percent of electricity (depending on place), it becomes increasingly hard and expensive for us to deal with their variability and maintain a stable, reliable energy service.
The demand for power tomorrow
There is one more graph below showing us what the situation might look like. In it, I have deduced wind power multiplied by 5 and solar power multiplied by 50 from the 2018 demand curve we saw in the beginning of the article (1). What we have left is the demand curve that the rest of the electricity system – other production capacity, energy storage and demand flexibility – would need to adapt to and provide for as we grow the share of wind and solar to more significant levels.
In this scenario, wind and solar produce a bit over one third of the electricity demand of Finland, or around 31 terawatt hours of the 85 TWh of total demand Finland had in 2018. Peak demand in winter is around 11 to 13 TWh, so we can’t close much of the other capacity. In windy days, the demand can drop by eight, even ten gigawatts, which means that most of the other capacity needs to be closed down.
In the summer, the residual demand after wind and solar are deducted can change rapidly between eight and minus two gigawatts. Even if most of the other power plants would shut down, we would need to export electricity to our neighbours with transmission cables running hot. And this would happen at a time when the neighbours might also enjoy sunshine and decent winds.
This scenario is a simplified one, for sure. But we will start to see significant issues even with relatively low penetrations of variable capacity. And electricity is just a part of our energy system – in Finland it accounts for around a fourth of our final energy demand, while the global share is around a fifth. Heating, industrial processes and transportation also needs to be cleaned up somehow. Hybridization of different energy systems, such as combined flexible production of heat and power, use of heat and electricity storages and production of hydrogen and synfuels with surplus electricity are expected to solve a big part of the problem. But even these investments need to make sense and be profitable. It is hard to make something profitable if one runs the expensive facility only during times of excess electricity production.
This load following of demand and production can be done much more easily if there is a baseload of firm and reliable production capacity available, such as nuclear. Hydro power can be used for load following within their capacity and depending on the water situation. A less known fact is that nuclear reactors can also ramp up and down quite nimbly, and they can also provide many of the other energy services we need.
1. It is a simplification to just multiply current production, as in reality, the new production would be sited somewhat differently from current capacity, and therefore the production profile would be somewhat different. Therefore, this graph should not be used as solid evidence for something, but more like a tool to help understand the situation and the general magnitude of the potential problems.