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US green energy: store, curtail – or export?

by Dave Elliott

The US is pressing ahead with renewables, with around 60GW of wind and 10GW of PV solar already in place.  But that means some system operation issues are coming to the fore.  Since these sources vary, as does demand, when there is surplus output from wind of PV, should it be stored or just dumped?

A paper from Stanford University, ‘The energetic implications of curtailing versus storing solar- and wind-generated electricity’, by Brandt and Sally M. Benson (Energy Environ. Sci., 2013, 6, 2804), looks at how storage affects the energy return on energy investment  (EROI) ratios of wind and solar resources. It suggests that in some cases it is more energetically favorable to store energy than to simply curtail electricity production. But it depends on the type of storage. It says that electrochemically based storage technologies result in much lower EROI ratios than large-scale geologically based storage technologies like compressed air energy storage (CAES) and pumped hydroelectric storage (PHS). But it adds, all storage technologies paired with PV generation yielded EROI ratios that are greater than for curtailment.  However ‘due to their low energy stored on electrical energy invested (ESOIe) ratios, conventional battery technologies reduce the EROI ratios of wind generation below curtailment EROI ratios. To yield a greater net energy return than curtailment, battery storage technologies paired with wind generation need an ESOIe > 80’. They saw improvements in battery cycle life times as the most feasible way to increase battery ESOI, though ‘reducing embodied energy costs, increasing efficiency & increasing depth of discharge will also further improve the energetic performance of batteries’.!divAbstract

It does seem that current types of battery aren’t much help for wind, but can be with PV, as can other storage media, which are generally better, and better than curtailment, in both cases. But of course there are also cost issues to consider- storage is expensive. Mostly you would curtail if there is no demand for the power, but also sometimes because weak grid links can’t take it, in which case it might be cheaper to improve them than go for storage, especially if it can be transmitted to places where there is demand.

That is one interpretation of a parallel key new study by the National Renewable Energy Labs, which looks at the impact of wind and PV solar on the grid in the Western USA. It’s Phase 2 of the Western Wind and Solar Integration Study (WWSIS-2). The Part I study had concluded that it was ‘operationally possible to accommodate 30% wind and 5% solar energy if utilities substantially increase their coordination of operations over wider geographic areas and schedule their generation and interchanges on an intra-hour basis’. And that ‘the integration of 35% wind and solar energy into the electric power system will not require extensive infrastructure if changes are made to operational practices’. So grid upgrades may not cost too much.

Moreover, in terms of the need for back up plants to cycle regularly from full to low power, the new Part II study says that ‘the negative impact of cycling on overall plant emissions is relatively small. The increase in plant emissions from cycling to accommodate variable renewables are more than offset by the overall reduction in CO2, NOx, and SO2. In the high wind and solar scenario, net carbon emissions were reduced by one third.’  And crucially it adds ‘Operating costs increase by 2–5% on average for fossil fueled plants when high penetrations of variable renewables are added to the electric grid’. So if you can export to the grid, the  carbon and operational costs of  balancing variable supply  are low- and, by implication, that option is probably better than storage, or of course curtailment.

All of which should be good news for the USA. The case for expanding renewables further has been strengthened by a study by Laurie T. Johnson, Starla Yeh and Chris Hope, (J Environ ‘The social cost of carbon: implications for modernizing our electricity system’, Stud Sci DOI 10.1007/s13412-013-0149-5). This claims that wind, solar and natural gas produce more cost-effective low carbon electricity than coal, when factoring in estimates of the health and environmental costs. At one level that’s not too surprising -coal is dirty! But it is cheap and still very widely used in the USA, so the paper’s use of a US government ‘social cost of carbon’ (SCC) measure is helpful. So is its suggestion that new power generation from wind could be more economically efficient than natural gas with Carbon Capture, when the SCCs are included. That may even be true without the SCC added in; the paper quotes these official (DoE/EIA) figures for raw generation costs, excluding CO2 and SO2 damages, in cents/kWh (2007$):

Existing coal fleet (average)     3.0

New conventional natural gas   6.2

New onshore wind                    8.0

New natural gas with capture   8.6

New conventional coal             9.3

New coal with capture             12.6

New solar photovoltaic            13.3

With SCCs added, and depending on discount rates (1-5%) and carbon costs, the paper says that coal costs, with CCS, range from 12.7-15.1c/kWh, gas with CCS from 9.8-13.3c/kWh, while of course PV and wind (with no carbon emissions) stay at 13.3 and 8c/kWh. So wind beats both coal and gas, and under some conditions PV can also beat coal, or match gas! And without CCS, coal rises to 34.5, gas to16.3c/kWh, so then PV and wind both win out.

Penn Energy noted that ‘The study comes on the cusp of stricter emissions standards for power plants set to be released by the U.S. Environmental Protection Agency and following an aggressive new Climate Action Plan set forth by the Obama administration. These initiatives have met with strong opposition from much of the power industry, which is predicting a significant rise in costs and impacts on reliability under the new regulations’.

The paper can be accessed via:

Shale gas may be cheap, but the addition of social and environmental costs changes the picture, and even without that, US utilities are racing to buy into wind and even PV, which, as the raw figures above suggest, are looking increasingly competitive with coal, and unlike shale gas they wont run out!

Some say that, with wind and PV spreading,the traditional power utilities in the USA will be flattened by distributed power, eating into their markets and profits

Certainly, with shale gas further undermining it competiveness, nuclear seems to be taking a beating, with typical costs being put at $0.11/kWh in the USA.  Many existing plants are closing early and new projects are being abandoned. For example, EDF has pulled out of the US nuclear market , saying it saw ‘no room for nuclear to expand in the U.S. at this time’ and would instead focus there on renewables. Although some new nuclear projects are still continuing, it does look like the energy mix in the US is changing.

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