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The limits of PV solar

By Dave Elliott

Solar PV has been talked up a lot of late. Its costs have certainly fallen and it has expanded to reach around 300GW capacity globally so far. But is it really going to be the dominant renewable as some have suggested? For example, a recent report from the Grantham Institute/Carbon Tracker has PV supplying 29% of world power by 2050 (PDF), with a massive 10,000GW or so in place. Is that realistic?

PV solar’s load factor is low (10-15%), reflecting the simple fact that it gets dark at night, and in winter at times there may not be much available. Hence the need for so much capacity to get a reasonable average output – and for storage. By contrast, wind has a load factor of 30-40% or more, maybe 50% offshore soon, and it is often high in winter when demand is high. And it’s often available at night. You could say that wind and PV are complementary over the year, wind in the winter and solar in the summer – certainly PV can be well matched to day time air-conditioning loads in hot countries. But at some other times, PV arrays will not be producing anything.

Does that matter? Under the old fossil and nuclear power regime, while some plants (nuclear especially) were run continually to meet base-load, the majority of the rest were often not used for much of the time, some being kept on spinning standby, to meet peaks, the others just held in reserve. So why can’t we accept having a lot of PV not being used at times? There might be a need for quite a large surplus of PV capacity to ensure that it could meet peaks, but so what?  It’s now cheap and will soon be even cheaper, so we can afford to plaster it over every roof space available. Add some substantial energy storage, to deal with the fact that, given this large capacity, surpluses would sometimes be generated, and at times there would be no solar input available, then that might provide a balanced system. With battery storage getting cheaper, some see this as a viable option e.g. for domestic scale PV and battery systems – the excess power being used at night. Tesla’s rooftop PV tile system plus Powerwall batteries are certainly interesting for new build – although still expensive.

However, in most cases there are problems. Domestic-scale batteries can’t be used to store power for very long. Overnight is fine, using daytime PV, topped up again next day if it’s sunny enough, but you would need huge battery packs for longer term storage – and larger PV arrays to provide the input. It would be cheaper for longer lulls to import power from the grid, supplied by wind farms, or perhaps from large, more efficient and longer term energy stores like pumped hydro reservoirs. That’s also the case if you opted for storing heat rather than electricity. You could use excess output from domestic rooftop PV to heat water in your boiler for later use, but it would arguably be more sensible to export the excess power via the grid so that it could be bulk stored more efficiently for import back when you needed it.

Once you move away from total self-generation and self-use, power grids remain important. That worries some greens. There have been objections to new grids being installed to distribute wind and solar energy. Some would prefer a more decentralised system, possibly at the village scale, based mainly on PV. However, as we have seen, to balance that there would have to be a lot of PV over-capacity and also a lot of storage.  In some visions of the urban future, everything is plastered with PV, with huge PV canopies covering spaces in between buildings. We could end up with what looks almost like a space colony, but on Earth, with sunlight nearly all being intercepted. A life in the shadows!

That may be extreme. Although roof space in high-rise blocks is limited, it is possible to have some PV on vertical walls and some buildings can be designed with sloped surfaces to increase the solar take. And translucent PV materials can allow windows to be used.

Even so, given the high population density with multiple occupancy of high-rise buildings, there may still be a shortage of space in cities for effective solar energy capture on a very large scale. So even assuming that energy demand can be reduced, cities may need to get more energy. But what’s wrong with importing power from wind farms in good high wind speed locations? Or from wave or tidal farms? Or from big biogas-fired CHP plants?

These rotating generators also deal with another problem that faces PV – it offers no rotational inertia. If we want to continue with the AC grid system, so that we can balance local variations in supply and demand by linking up generators and demand centres over wide areas, then maintaining synchronous frequency stability is vital. If we want to use a lot of PV on the grid some “synthetic inertia” via smart inverters will have to be provided – at an extra cost, on top of the cost of the extra storage and capacity needed.

Synthetic inertia can nevertheless hopefully help, and we can also pursue clever new storage systems – for example, based on hydrogen production via electrolysis, although that would probably be best done on the larger scale, with hydrogen stored in bulk in safe locations and perhaps being distributed via the gas main. In such a system, solar may be one choice for cheap power in some locations (e.g. with huge solar PV farms or CSP plants in sunbelt/desert areas), but wind is arguably more credible in most others along, in some locations, with wave and tidal power. Geothermal too. And some types of biomass/biogas. We have plenty of choices for this and also for direct heat production – rooftop or community-scale solar thermal being the obvious example. There are also some hybrid options – e.g. PV combined with solar thermal: getting the best of both heat and power.

PV solar certainly has many attractions. Its cost is at an all-time low and still falling. It’s quick to install, silent, clean and has no moving parts and low maintenance requirements (cleaning apart). And it’s not too invasive, if designed to fit into rooftops, with interesting tile-like versions also now being available. Although it’s more cost-effective to do large projects in one go, in a community setting, PV can be deployed piecemeal as funding allows and can help support community development – it’s a good community-scale technology. But it’s not the magic solution to all our energy problems. It’s good for homes in rural locations, in villages and towns too, and it can help many cities to generate a significant proportion of their electricity internally, inside their boundaries but, as we have seen, not all of it. Cities will need to import power from some other green source. That could include solar power from solar farms in rural areas, though there is a limit to how much land can be used for that. There is also a limit to the land available for onshore wind farms. But offshore wind, wave and tidal power is less of a problem. There are some opportunities for floating solar arrays on lakes and reservoirs or even the sea, but it’s not until we move to desert areas that solar really takes off in resource-scale terms.

None of this means that solar is a bad idea. The rooftop resource is not small and there are other possible locations, some of them huge – if we are prepared to use grids to bring it to users. All in all, it’s one of the best and biggest options for many locations. However, it’s not the only one. We need an integrated approach, making use of whatever best suits the overall system and local needs. In some cases, that may well be mostly solar – but not in all.

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