Electricity supply and demand for beginners

One of the ambitions I have for Energy Matters is to write some simple posts aimed at providing members of the public, media and politicians with the basic information required to make value judgements about energy supplies and climate change. Next week I’ll be taking a look at the history of UK electricity generation from 1920 to 2012 and as a prelude to that post some basic facts about electricity supply and demand are given below.

Figure 1 The pattern of electricity demand in the UK plotted on an hourly basis for the months of January and July 2009. Demand follows a very specific and predictable pattern and electricity supply must match that demand exactly. Data from National Grid.

Figure 1 shows the cyclical pattern of electricity demand in the UK for the months of January and July 2009. Three major cycles are present. The daily cycle shows how demand increases during the day time, peaking normally around 6 pm. The weekly cycle shows lower demand at the weekend when much of commerce is shut down. And the annual cycle shows higher demand in winter than in summer; shorter days and colder weather means we spend longer indoors with stuff switched on. The pattern for the UK will be similar in most N European countries, but in warm climates, such as the southern USA, electricity demand may be higher in summer as consumers turn up their air conditioning.

The key observation is that peak demand was 58.9 GW at 6 pm on a cold January day whilst trough demand was 22.3 GW on a warm Saturday night in July. Peak is almost 3 times the trough and the electricity supply system needs the flexibility to be ramped up and down, or switched on and off on a continual basis for supply to match demand exactly.

Note that the Y axis is given in Mega Watts (millions of Watts). Using 1 MW for 1 hour results in 1 MW hr of consumption. Other common units are Kilo Watts (thousands of Watts), Giga Watts (billions of Watts) and Terra Watts (trillions of Watts).

Figure 2, borrowed from Clive Best shows how different fuel sources were used to balance the grid over a two week period in December 2012. Nuclear provides stable base load supply. Wind is available when the wind blows and has no correlation with demand. Gas and coal are both cycled to provide the load balance and are reduced at times when wind is available. The dashed arrow shows the point of maximum demand when wind was zero.

Figure 2 UK electricity supplied from various sources for a two week period in December 2012. Chart from Clive Best. Data from BM reports at Elexon Ltd as reported by Gridwatch.

Flexibility, control and reliability are therefore key variables in electricity supply and on this basis electricity sources can be ranked.

Class 1 electricity, derived from energy stores, can be switched on and off and ramped up and down with little penalty to efficiency and this includes:

  • Combined cycle gas turbines (CCGT)
  • Coal fired power
  • Hydro electric power
  • Geothermal power

Class 2 electricity, also derived from energy stores, tends to be on all the time, providing stable base load, but not contributing much to load variance and this includes:

  • Nuclear power

Class 3 electricity is intermittent energy flows from renewable energy sources and these include:

  • Solar power
  • Tidal power
  • Wind power

Of these, Solar is by far superior since it is predictably intermittent and on during the day, every day, when demand is highest (Figure 3). Tidal power is also predictably intermittent but supply is not so well correlated with demand as solar. Wind is worst of all and is best described as randomly intermittent. Class 3 electricity can be converted to Class 1 if large capacity storage is available, but this remains an elusive goal.

The classification given above is based upon the engineering requirements of balancing the grid. If you were to perform the same exercise from the perspective of CO2 emissions then you would clearly derive a different hierarchy. Solar may actually deserve to be elevated to Class 1. On the daily cycle, solar performs very well (Figure 3) but on the annual cycle it does not at the high-mid-latitude of W Europe. Solar is on during the day when it is most needed, but largely off in winter, when it is also most needed.

Figure 3 From a large presentation by Prof. Bruno Burger. Germany has gone further than any other country installing solar and wind power. According to BP, it consumed 93 TWh of solar in 2012 and 46 TWh of wind power. The capital cost is high, but the fuel costs for 20 years or more are low. There is also a significant impact upon the profitability of traditional fossil fuel based generators.

The pdf from Prof Bruno Burger has 205 display slides and is one of the most comprehensive analyses of the integration of renewables into the electricity grid of a large economy that I have seen. I have not had time to digest all this data, but may do a post on this at a later date, time permitting. It should be quite clear that solar has the ability to provide peak day-time power, but at what cost? And we must remember that southern Germany is sunnier than Scotland!

According to BP, Germany’s gas consumption peaked at 87.2 billion cubic meters (bcm) per annum in 2006 and has since fallen to 75.2 BCM in 2012. That 14% drop in imported gas is worth a moment’s reflection.

Finally, for those who want to learn more about electricity supplies I can warmly recommend the online book Sustainable Energy Without the Hot Air. The author, Prof David MacKay is currently Chief Scientific Advisor to the UK Department of Energy and Climate Change.

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5 Responses to Electricity supply and demand for beginners

  1. clivebest says:

    Nice article Euan.

    Figure 3 is interesting. Firstly it leaves out the 9 Nuclear power plants that are still operating in Germany and which in 2013 met 18% of demand. If we add that into the mix then I get a total figure for energy production in July 2013 of 42 TWh.

    The first column in Figure 3 gives the maximum power achieved for both wind and solar which look impressive. However this is really the same as quoting the installed capacity, and we know that load capacity for wind is <30% and probably even lower for solar when averaged over a full year. Nevertheless taking the total generation figures given for the full month I get the following net contributions by fuel type:

    Solar – 12% Wind – 4% Coal+Gas – 68% Nuclear – 18%

    The solar figure for July is impressive but I wonder what is the figure for December ? Germany reacted to Fukushima by announcing the closure of their remaining nuclear plants by 2020. I cannot see how they can avoid not increasing gas and coal energy production to meet the shortfall. It is inconceivable IMHO that they could increase wind power by a factor of 5 without bankrupting the country.

    • Euan Mearns says:

      In January, solar is virtually off, but there is more wind. But in the middle week, there is neither wind nor solar and all the legacy generating stuff needs to be there to be brought on when it is needed. Greatly increasing generating capacity seems a strange reaction to me for an environmental problem.I hadn’t picked up that Prof Burger had excluded nuclear – I wonder why? Sweden took a decision to close down their nuclear plant years ago but then reneged on the decision.

  2. Hi Euan,

    Thank you for the interesting post. Another thing that strikes me, in addition to the problem of there being little sun in the winter when the demand is highest, is that the summer solar production does not appear to be a good match either. The German demand is actually pretty flat during the summer weekdays. There is little air conditioning peak that is so distinct in many hot US states. So often the shape of the electricity that gets exported has a similar shape to the solar production. This is obscured because Dr. Burger plots the data as if it were fossil-fuel production that is exported. It does mean that to calculate the solar share, one should consider a wider area than just Germany.

  3. Hi Euan,

    A question. In the British electricity data there is a tick up at the end of the day in the winter but not in the summer. Why? Is this differences in lighting, space heating? I assume people put on a pot of tea both in winter and summer.


  4. Euan Mearns says:

    Hi Dave, I’m not 100% sure what causes the very significant up tick in winter but would make a wild guess that it has to do with being dark and cold. Lights and electric heaters going on when folks get home from work ahead of commerce winding down into the evening / night time trough.

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