The UK electricity system is facing a difficult, expensive, and possibly rather dirty summer as high output from uncontrollable generators such as wind and solar combines with very low peak and minimum demands, forcing National Grid to take remedial measures that are economically and environmentally undesirable.
The UK’s gas and electricity Transmission System Operator (TSO), National Grid, has this week published its Summer Outlook, a summary of demand and any system operating difficulties that it may face in the coming season, April to October.
This year’s publication is of particular interest, since it brings into sharp focus what power system engineers have always known, namely that a system with a large proportion of weather dependent, uncontrollable generation such as wind and solar will experience significant difficulties in periods of low demand, and may even experience these well before problems at the winter peak. This runs contrary to the general focus of public discussion of the renewables fleet, which has focused on whether windpower will be able to help meet the need for electricity on cold, dark winter afternoons.
The summer difficulties arise from the low load factors of wind and solar, which mean that in order to meet the annual targets set by policy for quantities of electrical energy (joules) it necessary to have renewable fleets with a very high installed capacity. A conventional generation fleet with high load factors could generate the same quantity of energy from a much smaller installed capacity. The necessarily large capacity of renewable generators means that the fleet has the potential to deliver a very high peak rate of instantaneous output (joules per second) in certain weather conditions. So large in fact, that the market pattern of instantaneous consumer demand (usage of electrical devices and lighting etc) may not always be sufficient to soak up all the available renewable generation at a particular moment. Essentially this is a stock and flow problem. In order to get the required stocks, the potential rates of flow have to be very high and cannot always find a correspondingly high rate of consumer demand. Of course, this failure to find a market also means that the stock targets may not be met.
The problem is complicated by the fact that wind and solar are not capable of providing the high quality electricity generation necessary to preserve stable voltage and frequency on the grid. (Technically, they are asynchronous and provide no inertia to stablise the system.) Much wind and solar is also relatively inflexible, and unable to increase or decrease output rapidly in response to unexpectedly sharp rises or falls in supply or demand.
Consequently, a minimum of conventional, so-called “synchronized” generation with these vital characteristics must be kept running, and that means there is a limit to the volume of wind and solar electricity that can be accommodated on the grid at any one time.
All these problems, well advanced in Germany, are now coming to head in the United Kingdom. The minimum expected demand is strikingly low, at 18.1 GW, partly because of the 12 GW of solar and 4 GW of wind generation that will be embedded on the distribution network. Moreover, National Grid is expecting a peak load of only 35.7 GW this summer, the lowest summer maximum on record. The market opportunity described by these minimum and maximum loads is the ever opening and closing window through which the generation fleet has to gain access to consumers, and at present there is simply too much generation trying to get through such a small aperture. National Grid reports that there is 67.43 GW of Transmission System connected generation, including 9 GW of wind, and obviously even at peak system demand of 35 GW, much of this potential output is surplus to requirements. The conventional, controllable generators will, except for those required to provide grid stability, be unable to sell their output. But the renewable generators will do their utmost to stay in the market, since they lose their subsidy if they do not generate. By contrast, renewables will without difficulty find purchasers since electricity suppliers must obtain, over the year, a high percentage of their energy from renewable sources or face penalties.
This puts National Grid in a difficult position. They need to find ways of maintaining electricity grid stability at reasonable cost. Unsurprisingly, they are struggling. For example, the Summer Outlook makes reference to the “opportunities for large energy users to voluntarily shift their electricity usage in exchange for payment”. That might resolve the problem for a while, but of course it is bad for other consumers who have to pay subsidies to encourage this demand shifting. And in the long term it must be very bad for the UK economy, since an environment in which available energy supply drives demand is unlikely to be an attractive place in which to invest or do business.
Elsewhere in the Summer Outlook National Grid refers to “significant capital investment programmes” in equipment to assist with stabilizing voltage. (They probably mean ‘synchronous compensators’, electric motors that operate without any load.) Again, this is perfectly sensible engineering, but it is very bad news for the consumer who must ultimately pay for these measures.
And of course National Grid can simply pay renewables to stop generating, but Grid is obviously worried that renewables will exploit their market power. Indeed, at one point they write “We encourage wind farms and generators to submit cost reflective prices to the Balancing Mechanism to manage low demand”. This will be immediately intelligible to anyone familiar with the record on wind farm constraint prices so far, where wind farms are still, in spite of negative publicity, asking for and receiving much more in compensation than they lose in subsidy when constrained off.
Perhaps the most remarkable observation of all appears in the last paragraph:
Conventional generation may be left running overnight at a time when wind farms may be curtailed to resolve energy imbalances. This is because the conventional generators are providing frequency response, reactive power support or an inertia contribution. Actions to operate the system are undertaken on an economic basis.
That is hardly surprising, but it does tend to confirm long-standing suspicions that the overall grid emissions per kWh in a renewables driven system are not nearly as low as might naively be expected.
And if one needed proof that the gas fired generation system is itself under strain due to the renewables fleet, the Summer Outlook section on the gas network contains the remarkably interesting admission that in part due to fluctuating demand from gas fired electricity generators there are emerging concerns regarding pressure in the gas pipelines. As is well known, gas generators are ramping up and down as rapidly as they do because they are working to make room for and to compensate for the absence of wind power.
None of these problems are insoluble, but all of the remedies are expensive, and some of them are not very green.