In a white paper ‘Net-zero GB electricity: cost-optimal generation and storage mix’, published in June, a team from Imperial College London projects a need for 108 GW of offshore wind by 2035, to achieve net-zero carbon electricity. That’s two and a half times the UK government’s target for 2030.
The report says that this finding ‘is robust to changes in the Levelised Cost of Energy (LCOE) of offshore wind of ±£5/MWh on the Central case of £35/MWh’. As an alternative, the Imperial team looked at the option of building 10 GW more nuclear power and 18.5 GW more solar generation than is optimal in their Central case. That, it said, would ‘reduce the required build of offshore wind for 2035 by 24 GW, but increases the total cost of the system by about 3.3%.’
That’s a slightly odd way to look at it, but the Imperial College team seem convinced that offshore wind is the way ahead since everything else will be more expensive. Since it has a lower load factor than offshore wind, they put the LCOE for onshore wind at £50/MWh, and assume a build limit, due to local siting issues, of 30 GW. They say ‘nuclear energy would need to halve in cost to warrant major new builds within a cost-optimal system. At a cost of £45/MWh, approximately 50% of the strike price of the Contract for Difference (CfD) for Hinkley Point C, 13 GW of additional nuclear capacity would be included in the cost-optimal system. Finally, solar PV would need further and very significant decreases in cost to play a major role in the net-zero Great Britain system. Costs need to fall 60% from the central estimate, from £50/MWh down to £20/MWh, before a major expansion of its use is justified purely on cost grounds’.
So it’s offshore wind all the way, with just a small role for PV solar in the central case and offshore wind supplying over 75% of UK annual power needs. Indeed, it claims that the UK could go further: ‘In view of its large and very good potential sites for offshore wind, Great Britain could expand offshore wind capacity beyond that needed to supply its own demand and export zero-carbon electricity to neighbouring countries. This could involve an additional build of 18 GW of offshore wind farms with little increase needed beyond existing interconnections. The cost of this expansion would be compensated by the revenues earned by exporting energy’.
There are of course back-up issues ‘A substantial increase in the volume of energy storage is needed to support a system dominated by wind and solar energy. Not only do batteries help demand-supply balance when wind and solar output varies but they also assist with security of supply, obviating some expansion of electricity grids and meeting the needs of heat-pumps during spells of especially cold weather. Nevertheless, the volume of battery storage that would need to be built is over 140 GW, which would be more than a hundredfold increase on the 1.1 GW of grid-connected batteries of today’.
Well yes, it would certainly need a lot of backup and, arguably, not just short-term stuff for small perturbations or cyclic peaks in demand that batteries are good at. You would also need longer term storage, longer term than pumped hydro, to meet possible multiple day-long lulls in renewable availability. One idea would be to make storable hydrogen, using the large surplus power outputs there would at times be available from the huge wind capacity. That would arguably be better than having a lot of spare fossil generation capacity to meet shortfalls, presumably with CCS to cut its emissions when used. However, the Imperial report see batteries as best for grid balancing, although, a bit oddly, it also looks to Direct Air Capture (DAC) of CO2 which, it says, ‘could have a large impact on how electric heating is supported’. It wants to have most heating done by heat pumps, but it says ‘with DAC available as a negative-emissions technology, hybrid heat pumps could use natural gas during the coldest days of the year, which would reduce battery energy storage capacity by 45 GW’.
Well, as it admits, DAC is as yet untested at any scale (so too is secure carbon storage) and it is likely to be costly- and energy using. Surplus power-to-gas (P2G) green hydrogen conversion/use might be an easier balancing option- avoiding any need for curtailment of excesses! However, in Imperial’s central case, batteries soak up all this power and there is very little need for curtailment- just 3.6% of the annual total wind output.
So the Imperial team seem convinced that battery storage is best, least cost, option, and are not very impressed with the long duration energy storage options, including hydrogen. As with Demand Side Response (DSR), these options are not seen as reducing the need for batteries much. So, on that basis, for Imperial College, it’s wind, batteries and maybe DAC!
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It is good to see a study which avoids new nuclear and radically limits fossil fuel use, even if some CCS is then needed, but in overall terms we seem to have lost some ground in this analysis. There was a time when Imperial Colleges’ studies stressed diversity and flexibility. They still say flexibility is important, but now the optimal system mix is beginning to be defined, with some old flexible favourites, like DSR and P2G, evidently being seen of less importance. However, I’m not sure their replacement in the mix with energy using DAC is much of a step forwards! And on diversity, it is true that offshore wind has by far the best load factors, with some developers claiming 63%, compared to around 30% for on-shore wind and 11% for PV solar in the UK, but do we want, effectively, a monoculture of mainly just one type of technology- large scale offshore wind? In system terms, a wider range of sources/types, including those, like tidal energy and geothermal, that are not weather (solar) dependent, may offer operational advantages.
It is interesting to compare the Imperial College ‘central case’ scenario with that produced recently by Good Energy. That too has no new nuclear, but it has a lot more PV solar- 210 GW, as opposed to just 18 GW in Imperial’s scenario. Good Energy also only has wind set at 150 GW in total and it also has 40 GW of biomass and 34 GW of other renewables such as marine and geothermal. So it’s much more diverse and also more reliant on smaller-scale decentral power sources. It also has less battery storage- around 100 GW, mostly local.
As noted earlier, although PV is marginal in the the Imperial central case scenario, the team do include a scenario in which low cost PV expands dramatically- to over 150 GW. However, they say that this increases the need for curtailment, which rises to 14–18% of PV capacity annually. They note that ‘the opportunities to export excess PV generation are likely to be limited due to similar PV output profiles in continental Europe,’ but, interestingly, they add that a high level of curtailment ‘is acceptable from the economic point of view, given the very low nominal LCOE of PV generation’. You might say that this would also make P2G hydrogen production a winner, but that’s another story- left mostly unexplored by both Imperial and Good Energy.
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