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The green power switch – all to solar

By Dave Elliott 

A new book ‘The Switch’ (Profile Books) by Chris Goodall suggests that the combination of cheap solar photovoltaics and cheap batteries will be a global winner. It is certainly true that the cost of PV solar has been falling rapidly, outstripping predictions, and even confounding most of the PV optimists, as the technology has improved and markets for it have built. Goodall sees this as a continuing process, at maybe up to a 40% annual growth rate, with PV soon becoming the dominant energy source globally, a view that he notes even some conservative oil companies now share. Lithium ion battery costs have also fallen significantly. So, with wind also providing inputs when there is no sun, we are all set!  A similar line was taken in Tony Seba’s book ‘Clean Disruption of Energy and Transportation’. PV and batteries are going to boom worldwide, and electric vehicles too.

However, as ever, the devil is in the detail. PV cells have energy conversion efficiencies of 10-24% depending on the type and overall ‘load factors’ of at best 20%  – that’s a measure of how much of their theoretical output (if they could run all the time at full power) is achieved in practice. For wind turbines the load factors range up to 40% depending on location, with 50% expected soon offshore. Increasing PV cell efficiency won’t help much to change that big imbalance: it’s mainly due to the fact that it gets dark at night! Hence the importance of storage.

Goodall and Seba are enthused by batteries. Tesla’s much touted Powerwall domestic battery is being marketed in the US at $3,000 for a 7kWh rated unit. Installation and inverter costs would be extra. That would store enough electricity to run a 1kW heater overnight (7 hours), assuming the battery had been charged up from a suitably sized roof-top PV array during the daytime. If there was little sun the next day, or you need more energy, then what? You would still need a grid link – or more batteries and maybe a larger PV array to charge them. This doesn’t mean that battery storage is silly, but it will have to get a lot cheaper to make sense for significant domestic use.

Goodall and Seba think it will, and they may be right. They also see local storage as reducing the need for transmission and for extra generation to meet peaks – Seba says storage will herald an end to peaker plants. Indeed, he sees storage meeting 50% of demand.

However, although storage may expand dramatically, it’s not clear if domestic PV plus storage and offgrid prosumers will really have that big an impact on the overall power system or the grid.  Most people will have to stay grid-linked so as to cope with shortfalls between their demand and what they can generate and store themselves. Some of what they use then may come from large grid-linked utility storage options (fed by wind and large scale PV), which will be used to help balance local variations in demand and supply across large numbers of grid-linked users and a range of generators. These big stores could indeed replace some peaker plants, but not grids or grid flows. As Goodall notes, the big storage options include large-scale pumped hydro reservoir storage – the cheapest energy storage option at present. That can use surplus wind or solar-derived electricity to pump water uphill behind a hydro dam, ready to run out through hydro turbines when electric power is needed. There is also the compressed air option, stored in large underground caverns, and liquified air, cryogenically stored, the air being released/revapourised to drive turbines when electricity is needed.

Goodall also looks at the hydrogen option, which he sees as very significant. Surplus wind or solar electricity can be used to electrolyse water to generate hydrogen gas, which can be stored ready for use in a turbine or fuel cell when electricity is needed. In theory this can be done on a small scale, but there are economies of scale, making this ‘power to gas’ idea most suited to utility-level operation. That’s what is being done in Germany, with some hydrogen being converted, using captured carbon dioxide, to methane gas or other synthetic fuels. These can be used for heating or as a vehicle fuel. So can hydrogen. And they can also be added to the gas main. Clever stuff.

So there is an energy revolution underway, with new technology opening up many flexible options, at all scales, including for storage. Goodall says that means solar can play a major role even in countries with poor solar regimes like the UK. So too can long-distance electricity transmission from areas where solar is more available, traded with surplus power from wind and other renewables. That’s something he doesn’t look at. He does look at demand response (to time-shift demand peaks), but there are also some other options which Goodall doesn’t mention. For example, it’s easier to store heat than electricity, so you can use any spare PV power to heat water in your domestic boiler, with an immersion heater. Though it may be cheaper just to use good old-fashioned roof-top solar heat collectors to do this!  And sell any excess PV power back to the grid to defray the cost of installing PV. If you store it, as heat (or in a battery), you lose that option. Though the balance could tip in favour of domestic heat storage if PV generation spreads, so that there is too much available on the grid at peak times, reducing the opportunity for home generators to export any surplus power, at least then. So it may at some point be attractive to store it as heat. Even so, it’s more efficient to store heat at larger scale – with large hot water stores, the volume-to-surface-area ratio is higher, so the heat losses are lower. There are several large community-scale solar-fed heat stores in use in Denmark and elsewhere, some of them storing summer heat for winter use.

Gaps like that apart, Goodall’s book is generally very good – maybe a touch too solar obsessed, but though his mathematical projection that in theory it could head for 150TW globally by 2035-40 takes some getting used to (a factor of at least ten too high?), he may be right that PV will dominate. And Seba’s arguments on that and batteries are also very convincing – costs for both really are falling fast. See his interesting presentation, which also looks at the prospects for electric vehicles.

We could be in for some big changes. As the Carbon Tracker/Grantham Institute report    covered in my next post also argues.

  • 2016 PV solar contracts reported by BNEF include projects in Peru at 4.8 US cents/kWh, Mexico at 3.6 cents, Dubai 2.99 cents and Chile 2.91c/kWh.  And in much less sunny Germany, 5.38 euro cents/ kWh.




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