Can grid-scale storage solve intermittency problem, asks Global Warming Policy Foundation – physicsworld.com
(Image courtesy: Shutterstock/hrui)
A briefing paper from the Global Warming Policy Foundation (GWPF) dismisses the idea that grid-scale electricity storage can help bring about a UK renewables revolution. According to the paper’s author, Jack Ponton, an emeritus professor of engineering from the University of Edinburgh, current approaches are either technically inadequate or commercially unviable.
Some have suggested that “intermittent” power from wind turbines could be balanced with batteries or pumped hydro storage but, according to the GWPF press release, Ponton says this approach is unlikely to be viable: “You need storage to deal with lulls in wind generation that can last for several days, so the amount required would be impracticably large. And because this would only be required intermittently, its capital cost could probably never be recovered”.
He also thinks that hydrogen storage has been unjustifiably hyped: “A major problem with hydrogen is its low volumetric energy density. The only practical way of storing the large volumes required would be in underground caverns or depleted gasfields. We are already short of this type of storage for winter supplies of natural gas.”
Views differ on renewable energy futures
The GWPF concludes that a lack of suitable storage technologies means that intermittent renewables cannot replace dispatchable coal, gas and nuclear power and so a sensible energy policy cannot be based on them. It quotes Ponton’s view that “wind and solar power are not available on demand and there are no technologies to make them so. Refusing to face these inconvenient facts poses a serious threat to our energy security”.
It can’t be done.
That is a fairly forthright conclusion, reinforced by the formal conclusions in the report itself:
(1) “There seems to be no possibility that any existing storage technology can handle the intermittency of wind generation and make it effectively dispatchable. There are not enough sites for pumped storage, batteries are likely to remain too expensive and both processing cost and availability of storage sites would rule out storage as hydrogen.
(2) Solar plus battery storage is probably already cost-competitive for locations in or near the tropics, where year-round load factors are acceptable and so only overnight storage is required. In the UK, low winter load factors mean that essentially no useful generation takes place in December and January. Only storage as hydrogen could provide summer-to-winter storage, but cost and lack of suitable storage sites would rule out this approach.
(3) The predictability and relatively short length of the tidal cycle make the combination of tidal stream generation and pumped storage worth consideration. However, the number of tidal sites with sufficient stream velocity to provide useful generation in the neap tide season may be limited. There are also questions about the reliability, maintainability and lifespan of turbines in a very hostile marine environment.”
Plenty of room
There are so many assertions here, and in the full text, that it’s hard to know where to start. But just taking the lack of cavern storage space issue, it’s interesting that a joint Edinburgh/Strathclyde University modelling study recently suggested that wind-derived power could be used to compress air for storage in porous sandstone strata offshore during the summer, ready for use to generate power again in the winter. It claimed that the potential storage capacity for the sites identified was equivalent to around 160% of the UK’s electricity consumption for January – February 2017 (77–96 TWh), with a round-trip energy efficiency of 54–59%. However, it would be expensive, at the very least doubling the cost of electricity. But no-one is seriously suggesting that we use that route for all UK spare variable power: it is just one of several options. Then again it might get cheaper. A 2014 study saw Compressed Air Energy Storage Systems (CAES) as offering “good performance, long lifetime, low net environmental impact and reasonable cost compared to rechargeable batteries”.
In addition, the Energy Technologies Institute has said that there are tens of GW-equivalent salt cavern sites in the UK, some of which could be used for hydrogen storage, and that could make the hydrogen production and storage route cheaper than pumped hydro storage. Ponton dismisses hydrogen electrolysis as too expensive, but as with many of his other claims, there are counterviews, with the “Power to Gas” (P2G)/hydrogen storage option now looking likely to be economic for widescale use. Similarly for P2G/electrolysis efficiency, which he says is too low. That’s about to be tested by ITM Power and others in a 100 MW power-to-gas project at Runcorn, including hydrogen storage in a salt cavern near Lostock. And moving away from direct power storage, what about solar heat storage and, indeed, heat storage generally? That, with combined heat and power (CHP), could provide useful flexibility. I could go on! For example, what about interconnectors, exporting surplus power and importing some back to meet lulls? But see the forthcoming new edition of my IOP book on Renewables for chapter and verse.
A net zero emissions plan for the UK
Ponton may be right about the limits of conventional batteries, at best offering short- term storage and frequency support: they are unlikely to ever be used for long-term bulk energy storage. However, as he admits, new ideas are emerging. For example, in terms of medium/large volume storage, what about the newly emerging large flow battery systems, like the 120 MW/700 MWh redox binary system using two salt caverns, each 100,000 cubic metres in volume, planned in Germany? Or the 200 MW/800MWh flow battery system planned in Dalian, China? Or for larger and longer-term storage, the 1 GW underground salt dome compressed air energy storage (CAES)/hydrogen/flow battery project in Utah, US.
Nuclear also needs back up
Ponton’s analysis, though offering surprisingly few references, is none the less worth reading to see how traditionalists think. Predictably, he seems to favour nuclear. That’s fair enough, views do differ, and it is not unusual from the GWPF stable. For example, the GWPF recently published a report by Capell Aris that claims that the newly emerging energy system, based increasingly on renewables, “will deliver significant carbon emissions cuts but will double electricity price”, whereas “a system based on gas and nuclear would deliver similar emissions cuts at around half the price”. The GWPF says the Aris report finds that “with a system based on gas and nuclear power, emissions reductions could continue out to 2030 while maintaining consumer power prices at their current level. This result holds even if the very high prices of the planned Hinkley C power station apply in practice.” That’s one assertion that takes a bit of swallowing.
So too does the claim by Aris that, in his proposed nuclear-dominated system, “all the ancillary service requirements could be served by the operation of the Dinorwig, Ffestiniog, Cruachan and Foyers pumped-storage stations at very low cost”. Nuclear plants can and do go offline unexpectedly, and sometimes for more than a few hours. We might need a lot more fast-start-up hydro pumped storage, or some other form of back-up, to cope with a large nuclear component.
That specific problem is not something that faces the very ambitious “100% by 2050” renewables scenarios produced by Jacobson et al. in the US, and by LUT in Finland and EWG in Berlin. See my last post. Although these scenarios have to provide flexible balancing systems for a range of renewables, to deal with their short- and long-term output variations, they do not need to have a large amount of instantly available back-up capacity to deal with sudden major nuclear plant shutdowns. Even so, although generation and storage/balancing costs are falling, there are still worries about the implementation costs of ambitious renewables programmes like this: see my next post.