This is a summary of the article Meteorological conditions leading to extreme low variable renewable energy production and extreme high energy shortfall published in Renewable and Sustainable Energy Reviews in 2019 (https://doi.org/10.1016/j.rser.2019.04.065) as part of the Climate and Energy unit).

A note on the structure of academic journal articles

Academic journal articles are written according to a set format. Journal articles start with an abstract that gives a general overview of what the paper covers. An introduction covers the issue being investigated and can refer to previous research in this area. The next sections are about methodology. This allows readers to see how the team achieved their results. Following this, the researchers outline their findings and this can be broken into several sections. This paper then includes a section called sensitivity of results. The sensitivity refers to whether any other factors outside those being investigated could have influenced the findings. The paper ends with a discussion looking at how the findings fit with the larger issue under investigation, and then conclusions and outlook that summarises the findings and their implications.

Introduction

One of the ways that Europe will be able to tackle the climate crisis is by moving to zero or low-carbon energy production. Renewable energy is already part of the power mix in Europe and its use is set to grow.

In a future with a higher proportion of renewable energy, there are challenges and risks. The drawbacks of wind and solar energy are that they do not provide a reliable and consistent power supply, as the amount of production depends on the weather conditions.

If the amount of energy needed on a given day were to outstrip the amount of energy being produced by renewables, even if just for a number of days, this would result in an energy shortfall. This means that energy planners will need to consider what back-up plans to put in place.

While this is a known issue in theory, this research is the first time that accurate figures have been put to the nature of the risk and scale of the challenge. This research looks at two risks. First, it examines the natural variability of wind and solar energy production over the year and how this matches demand. Second, it predicts which types of weather event may increase the chance of an energy shortfall.

Data and methods

Predicting the weather and climate is not easy. There are many factors that influence weather and general trends are easier to describe rather than whether it will be a sunny holiday in a few months’ time. To work out the predicted wind and solar energy production, the team ran a series of computer models to calculate the wind speed, solar radiation and temperature over a grid of 100 kilometre by 100 kilometre squares across north western Europe. To ensure accuracy, the computer ran the climate simulation many times, resulting in 2,000 years’ worth of climate data.

To calculate predicted energy demand, the team considered that the change in energy demand over the year is influenced by temperature, i.e. we use more energy in the winter to stay warm, and potentially a little more in the summer with air-conditioning. This variability was then combined with data on historical energy demand and population data to predict how energy demand may fluctuate over a year in the future.

Weather and energy production

When the research team computed the mean average renewable production, they found that the average wind energy production for the year was 2.1TWh per day and 0.7TWh per day for solar in Western Europe in total^[1]. However, these figures varied over the year. With more wind during winter months (December, January and February) energy production was computed at 3.0 TWh per day, but this fell to just 1.2 TWh per day in the calmer summer months (June, July and August). This annual cycle is further complicated by the highly variable pattern of wind speed, with winter wind energy production ranging from 0.2 TWh per day to a high of 7.9 TWh per day.

Solar energy understandably saw peaks in the summer months (June, July and August) with longer daylight hours and less cloud cover, with a mean average for these months of 1.0TWh per day. During the winter (December, January and February) this fell to a mean average of 0.3TWh per day.

The overall picture saw combined mean total renewable energy production in winter exceeding that of the summer (3.3 versus 2.1 TWh per day), but energy production from these sources was also most variable during the winter months.

Energy production from wind and solar is not only affected by this seasonal variation between summer and winter, but also by weather events. The research identified instances of low energy production days. The research identified instances of low energy production days that had a 1-in-10 year chance of occurring. The team worked to see what weather conditions were causing these low energy days, which saw North Sea wind energy production fall by up to 63%.

The research showed that high pressure systems over continental Europe reduced wind speeds significantly and the team found that these events had a significant impact on energy production, with lower wind speeds reducing the amount of energy produced by wind turbines.

From the data they learned that longer periods of high pressure, up to 14 days long, also occurred, this would mean that energy production is low for 14 days in a row.

Potential energy shortfall

Any variation in energy production only matters for society if it means production cannot keep up with demand. The modelled European total energy demand peaks early in February due to cold temperatures (December, January and February mean of 8.5 TWh per day) and is lowest mid-summer (June, July and August mean of 7.2 TWh per day)[2].

Average renewable energy shortfall, i.e. the energy demand not met by renewables, is fairly constant throughout the year at 5.1TWh per day. There is, however, large variability over the year. The research focused on events where the energy shortfall exceeded 8TWh per day. All of these occurred during the winter months from November to March, with 81% in December and January. This is because of the effect of low temperature on demand, coupled with low energy production.

Similar to low renewable energy production events, high energy shortfall events happen during periods of high pressure. In this case it is a combination of low wind speeds limiting energy production with colder temperatures driving up demand. During these energy shortfall events, the entirety of the region was colder than normal, in some places 6°C below average.

Discussion and Conclusion

The main objective of this study was to investigate meteorological situations in relation to renewable energy generation.

It was found that there were particular meteorological conditions that cause energy shortfall. Findings showed that periods of high pressure cause atmospheric blocking which resulted in shortages of renewable energy production. It was found that events (7-14 days) tend to occur in the heart of winter.

The results were robust to changes in the spatial distribution of wind turbines and solar panels, the high-pressure weather events cover most of Europe, affecting renewable energy production of the whole region.

It was assumed that wind turbines over land were 80m and the offshore turbines were 120m. With increases in the height of the turbines more wind will be harvested, leading to higher absolute energy production.

Future studies should include interdisciplinary research teams as they provide a new way of analysing climate data. There should also be a focus on discovering the ‘true’ societal impact, with more studies identifying interactions between climatic drivers, hazards and societal risk. The research from such studies will help us become more resilient and implement more effective systems of energy production and storage.

The basic question for policy-makers and citizens is how to deal with the variability of the weather in a future with a high proportion of energy coming from renewables.

^[1] TWh stands for Terrawatt hour. A terawatt is 10¹² watts, i.e. 1,000,000,000,000 or 1 trillion watts. So, 2.8 TWh per day of energy production is equivalent to 1,022.7 TWh per year. In comparison, the UK currently generates 338.6 TWh of electricity (2017 UK Government figures), and The Netherlands generated 114.9 TWh in 2016 (US Government figures).

^[2] To put this into context, the average domestic electricity consumption in the UK in 2018 was 3.4 MWh per year (2018 UK government figures). So, the energy demand looked at in this paper is enough to power 923 million UK homes or 2.36 billion TVs left on for 24 hours a day (NB domestic consumption is only a fraction of overall electricity use).