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PV solar: big isn’t always better

By Dave Elliott

PV solar is now big – with 227GW installed around the world. But large solar farms apart,  much of it is in small roof-top units. Would bigger arrays be better? Certainly economies of scale suggest large-scale projects are generally more cost-effective than small ones. That holds up well for wind, but does it also hold for PV solar?

It is certainly true that large diameter wind turbines are much more efficient than smaller ones – the energy capture is proportional to the square of the blade length, so a single large 1MW turbine will produce much more output than 1000 small 1kW units. That’s the case even assuming the wind speed is the same, though in practice, whereas small units may often be located in low wind speed urban areas, large machines tend to be located in rural areas of higher wind speed – and the output is proportional to the cube of the wind speed.

However, PV solar does not have direct scale effects like this – the efficiency is the same at all scales, although the costs/kW may vary, since it may be cheaper to build, manage and maintain large arrays. And it may be easier for large companies to finance large arrays. It is often claimed therefore that large utility schemes are lower cost/kWh than smaller schemes. But recently, Lawrence Berkeley National Lab (LBNL) did an analysis of solar projects of various sizes that came on-line in 2014 and found that there wasn’t a simple scale-to-cost relationship. Their study collected information about ground-mounted, utility-scale projects and found that smaller utility-scale projects (5-10MW) had a lower cost per watt than the largest projects (100-1000MW). However, even the smaller projects are still more than 1,000 times larger than the average residential rooftop system (typically below 10 kW), which they didn’t look at, so we don’t know if that trend continues down to that scale –  although domestic schemes do have the advantage that they deliver power direct to users from their roof tops, avoiding grid costs and losses. But the report suggests that the very large projects face regulatory and interconnection complexities that drive up costs. Smaller projects (~25-50 acres) have an easier time clearing these hurdles. Some (~3MW scale) can be connected direct to the local distribution grid.

Medium scale projects like this are becoming popular in the US and elsewhere, some of them being commercial solar farms in rural areas. There are also some municipal projects:

Of course roof-top PV is much less invasive – ground mounted solar farms are contentious in some locations, with local planning battles sometimes resulting, for example in the UK:

In the UK, DECC is especially concerned if the projects are on agricultural land, although they can and do still allow for sheep grazing and create a wild flower/life haven: However, DECC sees marginal land/brown field/industrial sites as preferable for large scale projects. Several have been installed in the UK and elsewhere on factory/warehouse roof tops.

Whatever the scale, the overall trend around the world is dramatic expansion of total capacity as unit costs fall. A series of new reports issued as part of the US Department of Energy ‘On the Path to SunShot’ study looks at the DOE SunShot Initiative that was launched in 2011, with the goal of aggressively driving down the cost of solar energy. It seems to have worked, reaching around 70% of the 2020 price reduction target.

And it looks likely to continue. One of Berkeley Labs’ contributions finds that a future US electricity system in which solar plays a major role – 14% of demand in 2030, and 27% in 2050 – would result in enduring environmental and health benefits; another that the immediate elimination of Net Metering (NEM) in all states could reduce residential PV deployment by roughly 30% over the long-run, while universal availability of NEM would increase residential deployment by roughly 40%, relative to current state NEM policies.

However, there are still some issues. The load factors for PV are relatively low, 10-15% typically, depending on type, scale and location, although improving for advanced cells and systems, with cell energy conversion efficiencies rising. The best devices so far have reached 34.5% efficiency, and that’s without solar focusing/concentrator boosting: Even so, given also the relatively high energy input of cell fabrication, the Energy Return on Energy Invested ratio is relatively low for most current PV, whatever the scale, although improving – with EROEI’s of up to 20:1 or more. However a recent study claimed that the EROEI ratio for complete PV energy systems, including storage, can be below 1 in some locations, making them a net energy drain: See this quite enthusiastic relaying of it: But not everyone was convinced. See this comment:   A lively debate has ensued.

Clearly EROEI energy output/energy input ratios are hard to calculate (how widely do you draw the boundary?) and certainly some of the data, framing and assumption in this study can be challenged. Other studies have put the ‘cradle to grave’ EROEI ratio for good PV systems at up to 34:1 in some cases, with new cell materials and manufacturing technology likely to improve that:

Certainly several studies have put energy or carbon payback time at 10 years at the worst and for many PV systems nearer to 1 year or less: one anticipates that paybacks can be reduced to below 0.5 years by 2020. Also see

It is of course possible that in some locations overall system returns can be lower, depending on the type and scale of PV and if storage is assumed. However, large hydro had a very high EROEI ratio, typically 200: 1 or more, and although pumped hydro storage usage cycles may reduce that, the overall PV system EROEI surely won’t be reduced to below 1! That said, smaller-scale storage is usually less efficient and more expensive per kWh than large storage systems like this, and some domestic systems with batteries may have low overall EROEI’s at present, although that is changing fast as battery technology improves.

Whatever the scale, PV certainly at present has one of the lower EROEIs of any renewable, and although it’s improving, we do need to take note of EROEIs. But this study seems too extreme and pessimistic, with some arguably odd assumptions. Nevertheless, it’s led to lively debate, with some commentators still insisting that PV in the north may have problems due to low capacity factors: But, if so, is that really the case for Spain, as this earlier study argued? 

And looking to the future, what about solar breeders? PV manufacturing powered by PV could become self-sustaining, with no carbon debt:

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