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Energy storage and management

Energy storage and management

In praise of (total) demand response

27 Jan 2018 Dave Elliott

Operating on the energy supply side has limits – trying to balance the variable inputs from renewables is hard. So why not operate on the demand side – tailoring demand to supply? Dave Elliott asks how far we are prepared to go

“If we could manage to adjust all energy demand to variable solar and wind resources, there would be no need for grid extensions, balancing capacity or overbuilding renewable power plants. Likewise, all the energy produced by solar panels and wind turbines would be utilised, with no transmission losses and no need for curtailment or energy storage.”

So says an interesting, wide ranging but well-referenced article in Low Tech Magazine. It goes on: “Of course, adjusting energy demand to energy supply at all times is impossible, because not all energy using activities can be postponed. However, the adjustment of energy demand to supply should take priority, while the other strategies should play a supportive role.”

The Low Tech Magazine article first looks at the problems with trying to balance the variable inputs from renewables by adjusting supply. On back-up supply it says: “For a power grid based on 100% solar and wind power, with no energy storage and assuming interconnection at the national European level only, the balancing capacity of fossil fuel power plants needs to be just as large as peak electricity demand. In other words, there would be just as many non-renewable power plants as there are today.” It concludes: “Such a hybrid infrastructure would lower the use of carbon fuels for the generation of electricity, because renewable energy can replace them if there is sufficient sun or wind available. However, lots of energy and materials need to be invested into what is essentially a double infrastructure. The energy that’s saved on fuel is spent on the manufacturing, installation and interconnection of millions of solar panels and wind turbines.”

Another way to avoid energy shortages is to have overcapacity: “If solar power capacity is tailored to match demand during even the shortest and darkest winter days, and wind power capacity is matched to the lowest wind speeds, the risk of electricity shortages could be reduced significantly. However, the obvious disadvantage of this approach is an oversupply of renewable energy for most of the year. During periods of oversupply, the energy produced by solar panels and wind turbines is curtailed in order to avoid grid overloading.”

That certainly has been a problem in China, with wind curtailment running at 15% or more, but also in the UK, with large constraint payments having been negotiated to compensate wind generators for lost income. Although provocative, this may be cheaper in the short term than building extra grid links, but Low Tech Magazine says, longer term, “Curtailment has a detrimental effect on the sustainability of a renewable power grid. It reduces the electricity that a solar panel or wind turbine produces over its lifetime, while the energy required to manufacture, install, connect and maintain it remains the same. Consequently, the capacity factor and the energy returned for the energy invested in wind turbines and solar panels decrease.”

Curtailment would become even more of an issue if renewables expand so as to be able to meet peak load, without any balancing (via back-up plants, storage, and/or imports) being available. That’s an unreal assumption: some would be. But if not: “In the case of a grid with 80% renewables, the generation capacity needs to be six times larger than the peak load, while the excess electricity would be equal to 60% of the EU’s current annual electricity consumption. Lastly, in a grid with 100% renewable power production, the generation capacity would need to be 10 times larger than the peak load, and excess electricity would surpass the EU annual electricity consumption.”

What about supergrids to trade this excess and balance shortfalls with imports? It says that for a European grid with a share of 60% renewable power, grid capacity would need to be increased at least sevenfold – and 12 times for a 100% share. And even so, that wouldn’t deliver full reliability: there would still be a need for some backup – supergrids would reduce this to at best 15%, at very high cost and with some transmission losses. So more renewable inputs, or supply/storage backup, would be required to compensate.

Finally, storage. Well, like supergrids, storage systems are expensive and lossy. And the scale needed to balance renewables fully would be gigantic: “It has been calculated that for a European power grid with 100% renewable power plants (670 GW wind power capacity and 810 GW solar power capacity) and no balancing capacity, the energy storage capacity needs to be 1.5 times the average monthly load and amounts to 400 TWh, not including charging and discharging losses. To give an idea of what this means: the most optimistic estimation of Europe’s total potential for pumped hydro-power energy storage is 80 TWh, while converting all 250 million passenger cars in Europe to electric drives with a 30 kWh battery would result in a total energy storage of 7.5 TWh. In other words, if we count on electric cars to store the surplus of renewable electricity, their batteries would need to be 60 times larger than they are today (and that’s without allowing for the fact that electric cars will substantially increase power consumption). Taking into account a charging/discharging efficiency of 85%, manufacturing 460 TWh of lithium-ion batteries would require 644 million terajoules of primary energy, which is equal to 15 times the annual primary energy use in Europe. This energy investment would be required at minimum every twenty years, which is the most optimistic life expectancy of lithium-ion batteries.”

So all these options require major efforts and just raise the cost of renewable supply. By contrast, not using power does not. And Low Tech Magazine then looks at the prospects for demand management. It says demand management is “usually limited to so-called ‘smart’ household devices, like washing machines or dish-washers that automatically turn on when renewable energy supply is plentiful”. Actually, there are many others, not just domestic time-of-use tariff systems, but also large company-based demand response interruptible supply schemes. However, it is true that, as the article says, “these ideas are only scratching the surface of what’s possible”. For example, it says “If the UK would accept electricity shortages for 65 days a year, it could be powered by a 100% renewable power grid (solar, wind, wave and tidal power) without the need for energy storage, a backup capacity of fossil fuel power plants, or a large over-capacity of power generators.”

Well maybe, but here’s where it gets a bit dubious – and extreme. Could we really adjust personal, domestic and industrial activities in response to natural energy flows? As a second article notes, we certainly did in the past. Corn grinding mills only ran when there was wind, ships were often becalmed. But an advanced global economy was nevertheless sustained e.g. using the trade winds and, increasingly, water power.

New energy storage technology might make parts of this less problematic now, and some aspects might actually be socially welcome – days off work when there were wind lulls! But modern economies are based on 24/7 production and consumption. Can we undo some of that? Make hay while the sun shines?

Overall, a bit OTT and pessimistic on supply-side balancing, knocking down each option separately. In reality, a synergistic mix of the options, including flexible smart grid demand-side response, backup supply and storage (including of Power to Gas-derived hydrogen), along with supergrid balancing, may be able to reduce system costs. Indeed, one study suggested that “flexibility can significantly reduce the integration cost of intermittent renewables, to the point where their whole-system cost makes them a more attractive expansion option than CCS and/or nuclear”. But it’s fun and paradigm challenging! And here’s a full-on off-grid decentralist view, living fully on the wild side, with a lot of daylight-only power use.

Or try this for a somewhat different, and arguably equally contentious, view. Surely energy waste is always bad: yes, the renewable energy resource is large and free, but the technology is not and using it has costs. We need to use renewable energy efficiently – to limit the use of materials and land.

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