Updated: Feb 24, 2020
Welcome to the first in a series of ROOTT Insights looking at this question “from the ground up”. The series has been designed for environmentally conscious users of power who want to know more about the issues involved and also those in the energy business who perhaps have no background in renewable power generation.
As renewable power gains popularity in the mix of power sources in many countries, it becomes more important to understand how these carbon-friendly supplies can be integrated into a typical power grid run on traditional fuel sources. These existing supplies might include some nuclear, fossil fuels (natural gas, oil and coal) and perhaps some pumped hydro power.
Over the series, we shall be concentrating on two renewable sources, wind and solar power; the two sources of renewable power which are currently available at any scale in addition to large pumped hydro power. “Run-of-the-river” hydro and wave power are still very small contributors to the mix.
We will look at the situation in both temperate climates and also in those countries nearer the tropics to show how the situation can vary in different environments.
Two markets Electricity grid companies have to perform a delicate balancing act to provide industries and residential consumers with the power they need. Supply has to match demand, both seasonally and also on a minute-by-minute basis. This not only means that the capacity available (expressed in Megawatts, MW) has to balance the demand, but also the quality of the power provided must be within certain tolerances, for example within a narrow frequency range (e.g. 50 or 60 Hertz (Hz) +/- 0.5 Hz) and similarly for grid voltage.
Consumer demand, however, is anything but constant over the year or over the day, as the examples below for the UK and in Abu Dhabi show. In the UK, at about 53 degrees north, the demand for electricity for space heating in the colder winter months is substantially higher than in the summer. Whilst offices and factories may have air conditioning for the summer months, few homes are similarly equipped, so currently demand for space heating/cooling is lower in the summer.
Abu Dhabi, at about 24 degrees north and near the Tropic of Cancer, has almost the reverse pattern, with high demand in the hottest months of the summer to fuel air conditioning systems and lower demand in the winter when the temperature is at a more pleasant level. This demand pattern is repeated in many parts of the USA and the Far East.
Abu Dhabi is fortunate to have plenty of land to build solar farms and plenty of sunshine, so not surprisingly, the country has turned to solar power as its preferred renewable source. Wind is a far less reliable source.
UK seasonal demand and supply
The UK total electricity demand (in TWh) in 2017 is shown in the graph below and shows about a 25% increase in the winter over the summer months.
Source: statista 2019
Here, by comparison, are the monthly outputs of a group of UK wind farms in the period 2016 – 2017, normalised over the year. There is some increase in output during the winter, but the seasonal fluctuation is also quite moderate.
The (normalised) annual shape of the wind farm output, when superimposed on the demand curve for the same period, is very similar, as the figure to the left shows.
Abu Dhabi seasonal demand and supply
Turning to look at solar power in Abu Dhabi. The annual demand curve for a number of recent years is shown in the Abu Dhabi Water and Electricity Company (ADWEC) summary below. As you would expect in a country where much of the power is used for air conditioning, the peak demand is in the summer months:
Source: ADWEC (now EWEC)
Measured amounts of solar radiation over the year in Abu Dhabi are shown in the figure below, drawn from a local academic paper, and show almost a doubling of energy per unit area between the winter and the summer months.
Source: Assi, A. et.al. (2012)
Again, normalising both the demand and the supply curves over the year, there is a good correspondence between the two, albeit with a two-month delay in the demand:
UK daily demand and supply
In both the UK (wind) case and the Abu Dhabi (solar) case, you might think that the reasonable seasonal correspondence between supply and demand could form the basis of a scaling up of these forms of renewable power to provide the majority of power for the whole of the grid during the course of the year. However, these figures are all monthly averages and electricity supply and demand must be matched on a minute-by-minute basis, not over the month.
The devil, as they say, is in the detail, so looking at the UK and Abu Dhabi again, but this time over the course of a typical day. This is the UK graph for a recent spring month, the curve being the average of 30 daily profiles for the month.
Comparing this with two months of daily wind data in February and March 2015 for one large wind farm in the UK, gives an indication of the variability of wind output from day to day.
Source: Brouillette 2018
There is clearly a significant variation in generation output between successive days over this typical period. Even allowing for some averaging of wind output across multiple wind farms, it is apparent that there is a major mismatch between daily supply and demand in UK wind, as given by these data, without having to superimpose the two curves.
Abu Dhabi daily demand and supply
Daily electricity demand in Abu Dhabi, for 2017, is shown in the graph below. In contrast to the almost doubling of demand over 24 hours seen in the UK profile above, the increase during the day in Abu Dhabi is only about 25% in the example shown. This is because air condition load is much more constant over a typical 24 hour period than heating load is in the UK.
Source: ADWEC (now EWEC)
Unfortunately, it is to early for any published results from Abu Dhabi's Al Noor 1.2GW solar array, which was commissioned in the middle of 2019, but the figure below shows typical diurnal output for a PV array based on the orientation of the array:
So the conclusion from this analysis, both in the UK and Abu Dhabi examples, is that the power grid company has a difficult balancing act to perform, both on a seasonal and more importantly daily and hourly basis, to accommodate renewable sources on the grid as part of a mixed source system.
The next part of this series will look at an important added complication for those wishing to introduce wind and solar power onto a national electricity grid; namely the difference between “variation” and “uncertainty” in these forms of power output and how operators seek to manage this complexity.
Subsequent parts of the series will address how wind and solar are integrated at low concentrations of renewable power, which has by and large been the case up to now and the challenges grids are facing as the proportion of renewable supply increases further.
Assi A., Jama, M. & Al Shamisi, M. (2012) “Prediction of global solar radiation in Abu Dhabi”, ISRN Renewable Energy, 2012