The Sums on the Sun

I’m absolutely not anti-renewables. I love renewables. But I’m also pro-arithmetic.

Sir David J C MacKay, FRS, FInstP, FICE, Regius Professor of Engineering, Department of Engineering, University of Cambridge


There are two considerations which should be coming to the fore by now when promoting solar energy. And now is the time, because Australia finally has operational knowledge of performance and costs at an appreciable scale.

The first is what role it can realistically play in our energy supply mix (i.e. weather-dependent daytime power). The second is honest appraisal of the necessary enhancements involved in expanding that role – the effort and cost of overbuilding generators and storing energy for later use.

It especially behoves groups like the Australian Greens to shoulder such responsibility. In their recent energy proposal, they present photovoltaic solar as a third (approximately 130 million megawatt hours, MWh) of Australia’s supply in 2030. They also state

Energy storage, while not specifically separated out in this generation forecast will play a significant role in managing the network and matching dispatch times to consumption.


102 MW Nyngan solar farm, NSW

By applying the Rule of Thumb, this planned national solar capacity is the equivalent of roughly 540 Nyngan solar farms. With full operation in 2030, and a demonstrated build time of 1.5 years (leaving 12 years to begin commissioning the first farms if we start building NOW) it means the Greens have proposed the construction of an average of 45 solar farms per year.

For South Australia alone, the equivalent of 30 Nyngans by 2030 are demanded – two or three per year, starting today. This would be in addition to rooftop solar installation kept at last year’s rate.

These are the same campaigners who insist no one will want a nuclear storage facility in their backyard. But, apparently, the consen processt to site thirty 250 hectare solar farms is a non-issue?

Alkimos batteryThis is the Alkimos storage project in Perth.

  • 1.1 MWh capacity
  • Lithium ion
  • $6.7 million

Lithium ion storage has a lifespan dependant on depth and frequency of discharge, but suffice to say such batteries will not last as long as the rated 25 year lifespan of the Nyngan solar farm.

It is also being increasingly realised that the economics of storage at grid scale are predominantly determined by arbitrage opportunities and less so as dedicated support for intermittent generators.

So, if groups like the Greens want to replace the electricity supply with a third solar supported by storage, what would that look like?

aa10ab8c-6937-4961-b3e9-50019122c286On a good day the output profile of Nyngan is a symmetrical flattened peak and almost exactly a third of the 24 hour period by area. Intuitively, the battery capacity must be sufficient for the other two thirds – 16 hours. Since Nyngan is rated at 102 megawatts, this would call for 1,632 MWh.

1,632 ÷ 1.1 = 1,483.6
1,483.6 × $6,700,000 = $9.94 billion

It hardly matters that a further two Nyngan-size farms are needed to charge this battery (which would be a yard of nearly one and a half thousand shipping containers) since their extra cost is dwarfed.

As noted by ARENA, there’s no Moore’s Law for batteries but costs may well fall as production increased in large centralised overseas factories. A recent assessment by DNV GL in Europe included a specific estimate for lithium,

PwC calculations and recent market data indicate that battery costs will be reduced by at least around 55% by 2030.

potentially more than halving the price of this solar backup. However, the Greens expect to be finished building these solar farms by then, with scheduled gas capacity retirement clearly laid out in their document. Storage’s “significant role” isn’t really optional while they wait for the cost to drop from astronomical to merely unaffordable.

South Australian rooftop PV capacity is currently 659 MW, with roughly 60 MW added in 2015. Yesterday was overcast and peak generation reached 238 MW – 36% of of state capacity. Centralised solar farms operate more efficiently but are more prone to dropping out from intermittent cloudiness.

This model is clearly an idealised attempt to provide 24 hour scheduled supply from solar PV capacity (ignoring bad weather), what many would call baseload supply. This comparison is useful where proponents are simplistically proposing that renewable sources like solar should be replacing fossil fuels like coal, which are inherently dispatchable. However, other proponents categorically insist that baseload is a myth and unnecessary in a renewable energy dominated supply system. Much has been written on the subject over many years, but the willingness of such campaigners to sacrifice the baseload aspirations of alternative renewable energy technologies like geothermal and wave power is a grievous ideological lapse.

But there’s one other matter to examine here: the unspoken assumption that the above approach is sufficiently low emissions to be part of a decarbonised future. There is a terrible dearth of research on this. What was recently estimated by a group at Stanford is that the current combination of solar PV and lithium batteries may have a life cycle emissions intensity approaching 200 grams of CO₂ (or equivalent) per kilowatt hour. This would be well short of decarbonised.


This superficial analysis is just an illustration based on recent real world costs. Independent present value analysis would provide much better information. Solar has a large role to play in sunny Australia, which will only be optimised by getting the sums right. Can we love renewables enough to respect their limitations while making the most of their strengths?

Perhaps proponents of renewable energy mixes critically relying on scaled-up storage technology, like the leaders of the Greens, should consider commissioning such thorough and responsible economic studies instead of waving their hands.

We need a plan that adds up. We need to stop shouting and start talking, and if we can have a grown-up conversation, make a plan that adds up and get building, maybe this low-carbon revolution will actually be fun. Thank you very much for listening.


This blog was not originally intended as a dedication to Sir MacKay, who passed away during its writing. His book Sustainable Energy: Without the Hot Air is an essential reference for energy analysis. It is UK-centric but when I read it the potential for solar in Australia was left unbounded. This blog’s continuing analysis and comms concerning solar energy in Australia is just my amateur effort at supporting the commitment to arithmetic which Sir MacKay taught us is so crucial. As Mark Lynas noted beautifully,

David left us everything we need to figure out for ourselves how to proceed with solving climate change – and other problems, however huge and complex they may appear.



5 thoughts on “The Sums on the Sun

  1. I think PV farms can only work with cheap gas backup. The operators of Nyngan type plants get half their capital cost paid in grants and another $80 per Mwh LGC on top of that. As the sun fades and customers cook their evening meal with electricity the power comes from fossil generators. That’s regardless of whether there has been a week of rain or sunshine. Onsite batteries at the solar farm with storage capacity say 3X daily output won’t be enough. It needs at least a week. LCOE with batteries maybe $200 per Mwh or 6X uncarbontaxed brown coal.

    I suspect ARENA will fund a solar tower at SunDrop Farms near the soon-to-close Pt Augusta coal station. If so expect a debacle.

  2. PV solar and Li batteries are not the perfect utility scale solution. It is only good for residential use in remote places for lighting.

    Concentrated solar with molten-salt thermal storage is the way to go. But as Stewart Brand says, it should be done in mineral desserts like Sahara, Gobi not on green deserts.

    Even high power density concentrated solar uses lot of materials, but solar thermal will use commonly used material in large quantities not use exotic materials.

    Also, we need to include the cost of transmissions from solar-rich central and northern parts of Australia to the demand centers mostly located in southern parts.

    Reference, Sustainable Energy without the hot air: Chapter 25: A technology that adds up

  3. Solar Reserve’s Crescent Dunes solar thermal plant near Las Vegas with 10 hours of hot salt storage is the first of a kind and can generate after sunset for 10 hours at a cost of around 13 US cents/kWh. Copiapa, in Chile includes solar PV and the average price is below 10 US cents/kWh. A combination of colocated solar PV and solar thermal with storage ought to be ideal for Australia. There will be some gaps (single digit percenage maybe) which have to be covered by gas turbine generators.

    Another winning combination is wind, solar PV and batteries. A model for Texas shows wind, solar PV and 12 hours of battery storage can meet 95% of demand at less than 10 US cents/ average kWh, even pricing in gas generation plant capital cost and fuel to cover the other 5% (which consists of longer gaps measured in days). You can even use renewable gas as the fuel for a zero operational carbon system.

    Using solar PV on its own, then insisting on batteries to get it to cover the full 24 hours is about the most expensive way to design a renewable generation system. Yet that is what always seems to be suggested.

  4. “It would therefore be interesting to compare how much subsidy solar power would require to match the electricity output from Hinkley Point C (HPC) over the latter’s 35 year subsidy period. Recognising that solar is a variable but predictable energy source, we made allowance for significant additional investment to match solar’s output with electricity demand through the use of storage and other balancing mechanisms.”

    Generally filling the gaps with batteries to make it baseload seemed to be good enough methodology for the UK Solar Trade Association.

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