By Dave Elliott
If the use of renewables is to expand further, ways have to be found of compensating for their variability. Fortunately there are many, as I have outlined in a new book ‘Balancing green power’, produced for the Institute of Physics. It sets out to show how, taken together, they can help balance grid systems as increasing amounts of renewable capacity is added, helping to avoid wasteful curtailment of excess output and minimising the cost of grid balancing. The options include flexible generation plants, energy storage systems, smart grid demand management and supergrid imports and exports.
There have been a wide range of assessments of the viability of the various approaches, and I review the key ones, drawing on technical studies and strategic analysis from the UK, EU and US. What emerges is a clear and quite positive picture, which contradicts some of the views promoted by those concerned about the negative impacts of intermittency and the allegedly high costs of dealing with it. For example, in response to a criticism of his paper on wind energy costs for the Global Warming Policy Foundation, Prof. Gordon Hughes said that ‘to maintain secure reserve margins, each MW of wind generating capacity has to be backed by approximately 1 MW of generating plant which can be run on demand’: http://www.thegwpf.com/gordon-hughes-response-to-goodall-lynas/.
In similar vein, a report ‘Wind Power Reassessed: A review of the UK wind resource for electricity generation’ produced for the Adam Smith Institute and the Scientific Alliance, looked at a UK mix with 10 GW wind capacity and claimed, ‘the model wind fleet would require a conventional generation fleet of equal nameplate capacity to be built and operated alongside it to mitigate the wind fleet deficiencies’: http://www.adamsmith.org/wp-content/uploads/2014/10/Assessment7.pdf
In contrast, there are many studies suggesting that, if the appropriate balancing systems are available, it will be possible to operate a grid system reliably with large amounts of variable renewable capacity included, without a large backup from fossil fuel-fired plants. Certainly, at present levels of renewable input (up to around a 20-30% share), the existing system can cope quite happily. Gas plants are ramped up and down to balance the variations, much as they do anyway to meet the regular changes in supply and demand. No new ‘back-up’ plants are need – they already exist. They just have to ramp up and down more often, adding a small cost penalty. And, whereas critics have sometimes claimed that the extra emissions from using gas plants in this way will wipe out the carbon saved from using renewable sources like wind energy, the penalty is actually small. As a House of Lords Select committee put it, in a detailed review, ‘the need to part-load conventional plant to balance the fluctuations in wind output does not have a significant impact on the net carbon savings from wind generation’ : http://www.publications.parliament.uk/pa/ld200708/ldselect/ldeconaf/195/19502.htm Moreover, this small emission penalty can be avoided entirely if the gas plants use green gas (e.g. biogas produced from wastes) rather than fossil gas.
However, as the share of renewables rise, replacing conventional plants, unless we are prepared to keep, replace and expand these gas plants, other balancing systems will be needed. There are many possibilities. More use can be made of pumped hydro storage reservoirs and other storage options. One of the cleverest of these is Power-to-Gas conversion. Since renewable sources like wind vary, as more capacity is added to meet demand, there will be times when there is significant surplus production. Rather than dumping this excess energy very wastefully (by so called output curtailment), it can be used to produce a new fuel – hydrogen gas – by the electrolysis of water. That can be stored ready for use in a gas turbine or fuel cell to make electricity when there is a lull in renewable output and/or a demand peak. Or it can be fed direct into the gas main. Or used as vehicle fuel. So the problem of intermittency is turned into a solution – a valuable new fuel, possibly converted into methane, using carbon dioxide captured from the air or from any remaining fossil plant exhausts. The overall conversion process may not be very efficient, but the fuel input is free – it would have been wasted otherwise.
There are many other balancing options looked at in the book, including the development of long-distance supergrid links to allow local surpluses and shortfalls to be traded, for example across the EU, there being a degree of geographical smoothing across wide areas. Critics, looking just at wind across Northern Europe, have sometimes suggested that this smoothing effect is small, but with a wider footprint and the inclusion of other renewables, there can be significant benefits. Studies have suggested that the need for backup plants could be halved. Managing demand peaks can also reduce the need for backup and curtailment. Smart grid ‘demand response’ systems can shift peaks to times when supply is more available, by switching off some loads for a while. There are industrial, retail and domestic devices that can happily coast for an hour or so without power.
All of this is about electricity, storing it, swapping it, delaying its use, but one of the biggest end-uses of energy is for heat, and that too can play a role in balancing. Combined Heat and Power (CHP) plants, feeding district heating networks and heat stores, can vary their power to heat output ratio. So if there is a surplus of renewable electricity, they can produce less power but more heat, and store it for when it is needed later. If there is a shortfall in electricity, they can increase power output, and if there is still demand for heat, draw on the store. Storing heat is much easier than storing electricity and CHP plants can and increasingly do make use of biomass, reducing net emissions substantially. Solar heat can also be fed to the stress, as can heat produced using excess electricity from wind turbines.
As can be seen, there are many options for more flexible system operations to cope with variable energy inputs. Some may save money by avoiding wasteful curtailment, reducing peaks, creating new fuels and trading net excesses. Some critics have claimed that the cost of balancing will be high and rising, as renewables expand, adding 10-15% or more to energy costs. However, the International Energy Agency has concluded that, in time, the cost of balancing could fall as the technology improves and, if the cash, health and climate costs reductions from avoiding the use of fossil fuels is taken into account, then it can be negligible: http://www.iea.org/publications/freepublications/publication/the-power-of-transformation—wind-sun-and-the-economics-of-flexible-power-systems.html
The bottom line is – could fossil fuel use be totally avoided, even for balancing? The UK Energy Research Partnership has recently looked very critically at the various balancing options and was not that convinced by them, but even so it says that, in a hypothetical 100% renewable system, with wind and solar meeting most power needs most of the time, there would only be a requirement for a 12% fossil input: http://erpuk.org/wp-content/uploads/2015/08/ERP-Flex-Man-Full-Report.pdf That is more or less the same conclusion as was reached in a 2011 study of a 94% renewable scenario by Poyry for the UK Committee on Climate Change. By 2050 it only had 21GW of new gas ‘peaking’ plant plus a residual of 13GW of other gas plants – in all, just over 11% of the total capacity: https://www.theccc.org.uk/archive/aws/Renewables%20Review/232_Report_Analysing%20the%20technical%20constraints%20on%20renewable%20generation_v8_0.pdf
With the full development of the balancing systems described above, it seems possible that much of that residual could be eliminated, although, as I found in my work for the Pugwash High Renewables UK scenario, squeezing fossil fuel out of the transport sector might be harder, depending on how much biomass can be used: http://britishpugwash.org/pathways-to-2050-three-possible-uk-energy-strategies/
Note that nuclear power plays no role in these high renewables scenarios: it is too inflexible. Instead, as the new book shows, based on a range of national and global scenarios, given proper attention to balancing, flexible system development and energy saving and the use of multiple sources, renewables can supply the bulk of the energy needed in the years ahead on a reliable basis, and possibly all of it.
My new IoPP book, Balancing Green Power