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Balancing variable renewables

By Dave Elliott

There is now a range of books looking at the technical and policy options available for managing the use of variable energy resources such as wind and solar energy. The pioneering text in this area was Earthscan’s “Renewable Electricity and the Grid” from 2007, edited by Godfrey Boyle , with contributions from many of the UK top experts. But the field has since expanded with, for example, a lot of new work being done in the US.

A recent book, “Variable Renewable Energy and the Electricity Grid” by Jay Apt and Paulina Jaramillo (Routledge) and some 20 contributors, focuses on the US situation. It is based on the technical knowledge gained during the four-year ‘RenewElec’ (renewable electricity) project at Carnegie Mellon University, the University of Vermont, Vermont Law School, and the Van Ness Feldman environmental law firm. It discusses the options for medium-scale (20-30%) integration of variable electric power generation, including issues of predictability, variability and efficiency.

Here are some of its key broad conclusions: electrically combining the output of several wind plants in a region can reduce variability in the aggregate power output, but by how much depends on the timescale involved. Impact from short-term variations (~1 hour) can be cut by 95%, 12 hour variations by only 50%. The benefits also fall off sharply as more plants are added in the same area. However, expanding the area and aggregating wind power generation over a large geographical region is beneficial to reducing variability – although making the links may cost more (in the US context) than building new CCGT gas plants. So major new grid links are not seen as the best option. Storage may be viable for short-term rapid ramp-up requirements (e.g. hydro), but compressed air energy storage is not seen as likely to be viable in the US. Nor is EV vehicle-to-grid (battery) storage.

It’s quite a conservative analysis, looking to just a 20-30% contribution from wind and solar. So, whereas they often now figure increasingly in European studies, no “wind-to-gas” or detailed smart grid and long-distance supergrid ideas are discussed. But the level of detail is high and there are some very useful comparison sections looking at the many other US integration studies, including NREL’s ambitious 2012 “80% renewables” study, which relies heavily on long-distance transmission and also significant curtailment of surplus green power. With bold scenarios like this, back up/reserve capacity and grid balancing become key issues, but by staying mostly in the easier to manage 20-30% range, this book mostly avoids them. However the basics are covered at length, so it’s a useful text for those wanting to get into the details from a US perspective.

By contrast, another new book, “Renewable Energy Integration” (Elsevier) edited by Lawrence Jones from Alstrom, with some 50 contributors, has much wider international coverage, with many good EU, US, African and Asian case studies and a wider, more pragmatic engineering approach. But while it pays more attention to the practical management issues, this book is also quite visionary in its approach. For example, a global super-grid is amongst the many topics covered. There are helpful scene-setting accounts of renewable energy policies past, present and planned around the world, and detailed analysis of the grid balancing, system flexibility and energy storage options, including coverage of wider technical issues and possibilities e.g. in relation to smart grids, demand response and HVDC supergrids. So, overall, it’s a quite comprehensive book – less of a research study and academic text, more of a practical solution-orientated guide to development, covering a staggering range of topics. And it’s not just wind: there is also coverage of solar PV, wave and tidal power integration.

This wide coverage does, of course, mean that it is a little hard to digest and navigate through – there is a lot of diverse material, although thankfully a good index, much more substantial than in the other book. But then if you are a devotee of the e-book format that doesn’t matter so much. About the only major surprise was the low level of coverage of China, surely the country where these issue will soon become central.

Neither of these two new books offers a “silver bullet” – a simple resolution of the variability issue. It’s clearly much more complex and multifaceted than that, as the continuing deluge of papers and studies indicates – see below for some examples. But both of these books are valuable in offering helpful explorations of a range of approaches and as guides to the literature and practices so far. Though, as with Boyle’s text, in both the focus is on electricity, while issues relating to gas and heat transmission and storage, and their roles in balancing variable renewables, are coming to the fore, as we begin to think about more holistic approaches to energy generation and use. Electricity grids may be rivaled by gas grids, and storing heat and gas is much easier than storing electricity. That widens the field a lot. We need a bigger book!

For the moment, however, we have quite enough to be going on with just trying to make sense of the balancing options for electricity, with, in the UK, a current focus being the new capacity market – how it will work and what sort of capacity will be included. Just gas-fired turbines or some additional options like storage and demand management systems? In Germany, the rapid expansion of renewables, PV especially, has led to some system problems, but the main focus seems to be on costs and whether or not the continued rapid expansion of PV is a problem: http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2477731.

In the US, the emphasis has been on wind, whereas PV has not expanded so much yet. But it looks likely to, and there are some interesting new studies of the implications, including this from Berkeley Lab: http://emp.lbl.gov/publications/integrating-solar-pv-utility-system-operations

One thing seems certain: the issues raised in these books will continue to present challenges as we try to limit the costs of using variable energy sources: http://emp.lbl.gov/publications/strategies-mitigating-reduction-economic-value-variable-generation-increasing-penetrati

 

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