THERE ARE THREE commodities that symbolise human success and achievements in manipulating nature to meet our needs – food supply, water and electricity. No matter where we travel, apart from the most remote regions, we expect these commodities to be available. Not only are they expected to be available, but to be accessible any time of the day or night, inexpensive and of guaranteed quality. Unlike food and water, however, electricity has one particular quality that makes its supply more challenging than the others. It has proved impossible to store electricity in large quantities. The others, as well as items such as, petrol and medication can be stored in small packages in a variety of forms… in our houses; in containers in buildings or on a massive scale in freezer warehouses and storage facilities.
Electricity however is unique in being such a vital resource, yet having this drawback. It can be stored in the form of batteries but these are expensive for the quantity of energy they can store and often inconvenient – even when rechargeable. This storage aspect is easily overlooked by the public, as our supply is interrupted very infrequently. Yet the prospects for any advanced society are dire if electricity supplies are not available for long periods.
Large-scale electricity generation began in the nineteenth century and developed quickly for industrial and commercial applications as its advantages became clear. It has proved possible for control engineers to harness its power, with supply matching demand for over 100 years with a high degree of reliability. Why therefore should we have any concerns about its future reliability? To understand the risks that are developing, it is necessary to examine its use in more detail and to look at the developing trends in electricity generation.
Although an individual’s usage of electricity varies from minute to minute and day to day, the aggregated demand from a population of tens of millions is predictable with surprising accuracy and is the key to our reliable electricity supplies. There are regular diurnal and seasonal patterns of changing demand, that are anticipated by control centre engineers who have perfected systems over decades of experience to achieve three principal objectives.
· Providing continuous high quality power with at least 99.5 per cent security of supply
· Accommodating both predictable and unexpected increases in demand
· Accommodating sudden loss of station faults.
Until the 1950s, coal was the fuel employed exclusively in power stations, but from that time other fuels have been introduced. In the 21st century the engineers can call upon a variety of power sources… gas CCGT plants, nuclear power stations, coal-fired stations, wind turbines, hydro, pumped storage, imports and biofuels. National Grid engineers combine these to suit their particular advantages to provide our supplies. Consider a typical day.
Electricity demand is at its lowest level (but still around 25000 MW) in the early hours, then from about 6 am, more power is required as homes, businesses and industry begin the working day. Coal and gas-fired power stations can detect small changes in demand 10 - 100MW via the governing system and increase their output. Eventually however full power is reached and additional power plants are started. These are generally either CCGT or coal-fired. The CCGT plants frequently treble their output increasing from 5,000 to 15,000 MW in about 2 hours. Nuclear plants in the UK do not generally follow load changes but rather operate at full power 24 hrs per day throughout the year. The wood burning plant at Drax and the hydro plants are also used most days and there are also imports every morning from France and Holland.
Between 6 am and 9 am, the increase is typically about 10,000 MW… requiring about 10 power stations to be added at full power. On most days, demand is steady through the afternoon, then increases around 6 pm, especially in the winter. Power plants are brought in or closed down as necessary without most consumers being aware of events. If electricity is not generated to match demand, then the system frequency and voltage can fall outside the legal limits, causing equipment malfunctions. In the case of a serious mismatch, supplies are cut off in a sector to keep power flowing to other parts. For most consumers however the security of supply is about 99.5 per cent for the year.
Not all changes in electricity demand follow a clear pattern. There will be some much more substantial changes outside the diurnal and annual predictable variations. For example – sudden blizzards or heat waves, solar eclipses, special or highly popular television programmes; football finals or royal weddings pose challenges. These may involve power increases of several thousand MW in half an hour. To keep the grid stable during such large changes it may be necessary to use the stored power in hydroelectric plants such as Dinorwic and start up open circuit gas turbines. There are developments however making it ever more difficult to keep the grid stable.
Over the last 25 years, there have been novel changes in generating capacity. These have occurred, not in response to economic or engineering needs, or plants being retired, but driven by concerns about environmental damage. These are beginning to have an impact on our electricity system because their nature is very different from traditional power plants. Some 10,000 MW of wind turbines have been installed – about 7000 onshore and about 1700 offshore - plus large numbers of solar panels have been installed on houses or in open fields.
Unlike gas, wood or coal-fired power stations or hydro, these renewable generators are not controlled by engineers, but operate at the mercy of the weather. On windy and sunny days, their contribution may reach 30 per cent of electricity demand, but on still nights however it can decline to 1 per cent or less. On frosty mornings, when electricity is needed desperately, wind and solar contribute very little. This forces the coal and gas fired power stations, hydro and imports to boost their output to prevent power cuts. Wind power is sometimes in significant decline when demand is increasing rapidly, forcing the conventional plants to ramp up even more quickly, but so far they have coped. Accommodating the larger changes in power demand is becoming progressively more difficult as more renewables are connected whilst simultaneously coal plants are closed to meet European Union directives.
The UK has agreed to close the remainder of its coal-fired power stations over the next few years. Previous closure programmes have been balanced by the construction of new gas-fired power stations. This is unlikely to happen in coming years because the new gas plants cannot be guaranteed sufficient operating hours to repay the investment. Could another novel development come to our aid. Will batteries and electric vehicles help?
Electric vehicles have batteries within the range 30 to 100 kWh, with perhaps an average storage capacity of 50 kWh. One million EVs would therefore have an energy storage capacity of 50 GWh. With the present level of sales, it will take decades to have such a large fleet in the UK.
Electricity consumption in the UK is about 800 GWh /day. If owners of half of the electric vehicles would set aside half of their storage that would equate to 12.5 GWh. That storage would provide electricity for about 25 minutes, even if we could co-ordinate the actions of half a million drivers! Large-scale battery storage in static facilities also looks to be almost impossible, although it is offered as a glib solution by green pressure groups.
For battery storage to offer a feasible option, there must be regular episodes when renewables generation is well above daily demands. In the winter, when electricity supplies are most crucial, the margin between maximum generating capacity and maximum demand is now only a few thousand MW. At this time, renewables may be generating about 20 per cent of demand – 10,000 MW. To produce the excess for short term storage would necessitate current renewable plant capacity being increased considerably. As a minimum, it would have to be multiplied three or four fold. That would require the current fleet of almost 9000 wind turbines to be increased by about another 30,000. Since there is a moratorium for on-shore installations, the extra units would have to be built off-shore. That implies a twenty fold increase in the number of off-shore machines. Expansion on that scale is almost certainly beyond the capacity of coastal waters. In addition, the huge changes in their power output in changeable weather would overwhelm the balancing power available to grid controllers.
Such are the threats to one of our most vital commodities.
Paul Spare CEng, FEI FIMechE © 5 April 2018