Monday, May 21, 2007

Making the electric grid renewables-friendly

Back in the 1970's skeptics wondered if renewable energy technologies like wind and solar would ever be able to generate electricity reliably. When engineers and developers responded to that challenge, skeptics pondered if renewables could ever become economically viable. Now that wind energy, biomass, geothermal and other renewables compete favorably with fossil fuels in the marketplace, skeptics worry that the introduction of large quantities of renewable energy into the electric grid will destabilize the system.

A response to the question of just how much renewables can the grid handle follows. It appeared about a year ago in Refocus magazine.
I think that it's time for the skeptics to refocus their attention on something else -- or better yet, join the renewables revolution. (GW)

Wind and other renewables: How Much Can the Grid Accommodate?

By George Marsh
Jan/Feb 2006

Wind is a potent source of energy - in north-western Europe and the UK as much as anywhere. Debate rages on however, about its reliability and intermittency and regarding the nature and level of back-up required to support increasing amounts of wind and other renewables being added to the current grid infrastructure. George Marsh reviews the issues

Points of view range from 'wind is a waste of time because you need 100% back-up from conventional plant' to 'the need for back-up is minimal if you intelligently integrate power available from all existing resources over a wide enough area'. Judging precisely where, between these extremes, the truth lies is a difficult call.


Wind opponents argue that large investment is needed in conventional back-up, typically gas turbines because the rate at which this type of plant can react offers some hope of following the peaks and troughs of wind generation. But such plant is inefficient unless it can run continuously at near full load. Leading-edge combined cycle gas turbines (CCGTs), in particular, suffer thermal and mechanical fatigue if they are repeatedly cycled on and off line or have their output changed frequently. This increases support costs and, as a result, CCGTs are less useful for peak and trough following than their less efficient turbine predecessors.

Critics further argue that, in any case, carbon emissions from occasionally used back-up plant would cancel out emission reductions achieved by turning to wind power in the first place. Some highlight the dangers of 'ramping' and 'spiking' when substantial wind capacity is connected to electricity distribution grids. Asserting that, because the output of a wind turbine is generally proportional to the cube of wind speed, small changes in wind speed can result in large changes in power output. For a tightly clustered multi-megawatt wind farm these ramping and spiking variations can be of the order of megawatts per minute. This makes it difficult for supply authorities to maintain grid stability. These difficulties are exacerbated at the extremes when, for instance, a large area is left bereft of wind or when a storm with high winds leads to widespread automatic turbine shut-downs.

The Tyndall Centre for Climate Change states, in a new report, that 'in order to accommodate intermittent generation, it will be necessary to retain a significant proportion of conventional plant to ensure security of supply.' However, its researchers conclude that, at least up to a 20% wind power contribution level, the additional operating costs involved need not be prohibitive. Beyond that, they accept, the costs multiply and, in terms of whether they can be justified, much will depend on what happens to prices of conventional fuels.

This stance exemplifies a broad consensus among experts that having up to around 20% of total capacity accounted for by wind does not raise insoluble issues. But beyond this critical point, problems escalate for grid systems, at least as presently constituted. Grids are designed for high levels of constancy and predictability, chiefly because relying on fossil fuels as the major energy integrator has caused them to evolve that way. The costs of adapting grids to accept intermittent renewables on a large scale, say critics, will make generation of power from such resources uneconomic. There are other issues, like the disparity between times of peak demand and peak supply. In the UK, for instance, it would be necessary to close down turbines on warm summer nights, unless the energy could be stored or exported. This, again, would add to the costs of exploiting wind.

Some of the arguments outlined encapsulate the technical case against wind. Additionally, there are the familiar objections to the actual presence of wind turbines based on visual intrusion, turbine noise, hazards to birds, radar interference, loss of amenity, navigational hazard (offshore) and so on. But, in practice, the points presented are potentially the most damaging to wind prospects.

Countering doubt

Wind proponents are naturally keen to counter the above arguments point by point. Focus on limited back-up possibilities, primarily gas turbines, discounts potential contributions from biomass-fired combined heat and power (CHP) plant, differently phased renewables such as solar, wave and tidal; and renewable options regarded as 'firm' such as hydro-electric and geothermal. Denmark, for instance, which can already generate a fifth of its energy from wind, integrates its supply via a grid link across the border with Norway. By exporting its plentiful hydro-electric power when wind is scarce and, conversely, by offloading hydro-electric plant and absorbing excess wind-generated power from Denmark at other times, Norway can stabilise the supply in both countries. In response to ramping and spiking concerns, Tim Foster of Smartest Energy told a workshop on wind variability held at the Open University, Milton Keynes, UK in January that, while these concerns are valid, the almost instantaneous departure from the grid of a major conventional power station, which can happen today, is a far more momentous event.

The British Wind Energy Association and other authorities believe that the benefits of grid connection in balancing supply variations can be substantial. Grid balancing difficulties, it is claimed, can be ameliorated by feeding into the system outputs from turbines located over a large area. To cite again the UK example, Dr Bob Everett from the Open University argues in his 2005 report for NATTA (Network for Alternative Technologies and Technology Assessment) on 'The UK Electricity System - Transforming the Elephant', that such spatial diversity helps because if the wind is not blowing in one place, it is likely to be blowing in another. It is rare indeed for wind to be absent from both the south and north of the country since even large-scale summer anticyclones seldom provide this result.

Total calm rarely exists across the lesser east-west span either as wind tends to traverse the country in waves as depressions track across. A government commissioned study involving examination of 35 years' worth of meteorological records revealed no time during that period when the entire country had been becalmed. Furthermore, as Graham Sinden of the Environmental Change Institute at Oxford University points out, wind power available across Britain is not random, but is greatest at the times it is most needed, notably during daytime in winter.

Even spread

Achieving the necessary geographical spread of turbines would require effective national planning policies, along with strengthening of existing grid links between England, Scotland and Ireland. In this respect, the London Array in the south east would make good sense, helping to balance the growing preponderance of capacity in the windy north west, both on and off shore. A higher level of spatial diversity and grid integration can be envisaged for mainland Europe and the Nordic countries where land continuity pertains over a wider area. Because wind is no respecter of national boundaries, bringing this about requires trans-national planning policies but, thanks to the European Union and other regional groupings, the necessary political infrastructure exists. The grid in western continental Europe already operates as a unified synchronous entity with electricity being bought and sold across the EU. Similar extended grid solutions are conceivable for other parts of the world, particularly the Far East and the Americas.

Other renewables

Furthermore, it is not just wind that can be included in the energy mix. Other renewables can be integrated also. Thus Denmark is too small to rely on spatial diversity within its own borders but, as previously alluded to, exploits a strong grid link with Norway to utilise a rapid-response hydro-electric resource as the balancing medium. Similarly Germany could, given some grid strengthening, balance the substantial wind generating capacity it has in the windy north of the country with its own 5GW of flexible hydro-electric capacity which is predominantly in the south. German energy agency DENA has concluded that if some 400km of the existing 380kV grid were to be upgraded and about 850km of new line added, it would be possible to accommodate a 20% share of electricity generated from renewable energy by 2015-2020, so preventing annual emissions of 20 to 40 million tonnes of CO2.

Beyond this time period, further growth in renewable generation would require conventional back-up, at least pending other infrastructure such as the much debated hydrogen economy with fuel cells to deliver energy. During this interim period, fatigue problems associated with variable operation of CCGTs could be avoided by keeping their heat recovery steam generators at a high temperature when not in use. This, it is suggested, can be achieved using waste heat from fuel gasifiers, and therefore without great penalty in efficiency.

Hydro and geothermal are regarded as firm renewables whose reliable, predictable and rapid-response characteristics can be invaluable for grid stabilisation. However, smoothing can also be achieved by integrating other not-so-firm renewables which, while no less variable than the wind, are likely to vary out of phase with it. Wave power, for instance, lacks both constancy and predictability but, since it is a product of wind and tends to follow it after a time lapse, it can have a smoothing influence. Tidal power, though highly variable, is utterly predicatable and follows cycles unrelated to weather. Photovoltaic power is reliable and predictable in some areas, but less so in others. As the associated technologies are further developed, these sources may complement each other and exert a useful smoothing effect within a diverse energy mix. More smoothing is potentially available from pumped storage and from such facilities as that intended by the ill-fated Regenysis project, effectively a large-scale battery/fuel cell in which energy would have been stored chemically.

Meanwhile, another means of balancing wind on the grid is offered by CHP systems, fuelled ideally by carbon-neutral biomass. Biofuels and wastes currently account for the bulk of UK renewables and, though being overtaken there by wind, their expansion should continue. A report prepared by consultancy Ilex in 2002 concluded that a mix of windpower distributed throughout the UK and biomass base-load plant could meet that country's energy needs more cheaply than other solutions. Costs would be involved in providing some open-cycle gas turbine plant as back-up in the short term, plus other plant held as spinning reserve, but this need not be excessive as a proportion of total generating costs. Extra distribution costs could be limited by locating biomass plant close to points of demand. The Ilex report took no account of future smoothing contributions from other 'non firm' renewables.

Gas fired CHP, too, can be a potent balancing force because of its ability to tailor electrical output to demand, delivering the balance of its output energy as heat, which can be stored. Not only is gas CHP more flexible in helping the electricity grid cope with intermittent supplies than conventional power stations, it also has lower CO2 emissions. In Denmark, some 60% of electricity is generated by this type of plant, compared with wind's 20%. A current European Union backed research project, DESIRE, seeks to demonstrate more widely how CHP can solve intermittency problems. An international consortium of universities, research institutes and software companies is working in several countries to spread knowledge of the system. In the UK, the University of Birmingham is collaborating with engineering company PB Power to demonstrate the Danish-inspired technique at three co-production sites, and show how it could be integrated with the British electricity grid.

Another point made by wind adherents is that there is already enough conventional generating capacity 'out there' to back up renewables, in the form of standby generating plant possessed by businesses, utilities, public facilities, essential services and other bodies, in order to cover their needs during power outages. About 20GW of emergency diesel generation capacity is estimated to exist in the UK alone. Much of this is hardly ever used and, though it might be lacking in terms of peak efficiency, it is inherently flexible and in aggregate represents a powerful back-up resource. Banks of diesel generators could also help in moderating the ramping mentioned earlier. Marshalling this resource would require national consensus and policies, possibly involving edicts - though inducements in the form of payments for connecting to the grid and for power produced might solicit the necessary

Winner takes all

As wind power opponents are fond of pointing out, any large-scale introduction of intermittent renewables presents us with the 'second place' conundrum. As presently constituted, national electricity supply arrangements are predicated on a 'winner takes all' basis. Market forces and regulation ensure that plant/fuel combinations seen as being the most economically efficient capture the main base load, leaving less efficient but more flexible 'support players' to come in and out of the system as required to meet demand peaks.

In order to maximise operating efficiency, base load plant is designed to run continuously, almost flat out, typically at high operating temperatures - continually being made higher by use of advanced alloys and other new technologies. Running such plant in a variable way or at reduced load degrades its efficiency and adds to maintenance costs. The role of load following, cutting in and out to cater for demand peaks, is therefore allocated to second-place players such as open cycle gas turbines, whose lower operating efficiencies are accepted because they are used infrequently. Any move to shift base load responsibility to wind or other intermittent renewables would alter this model by relegating conventional plant to second place, giving it more of a load following role. This would reduce efficiency and, once again, appears to add to the overall cost of relying on wind generation.

No revolution required

Most experts accept that significant infrastructural changes are needed to accommodate contributions from intermittent renewables beyond the critical 20% level. But these need not result from a 'big bang' revolution with commensurate expense. Professor David Elliott of the Open University believes that they will evolve anyway and that, over the next two or three decades, the present fixed monolithic system will give way to a more adaptable and less centralised successor. Incorporating emerging technologies, the new system will accommodate diverse generators ranging from micro power systems to massive grid-connected renewable complexes. Fortified and extended grid systems will co-exist with semi-independent 'islanded' sub-grids and small-scale generators not grid connected at all. Fresh emphasis will be given to pumped and other storage. Load will be better managed to match supply, while load/demand forecasting and modeling will be improved.

Costs involved in achieving all these measures will clearly be substantial. But the global community has already demonstrated, through carbon trading and comparable measures, some willingness to adjust economic and accounting mechanisms in order to value environmental and social benefits. What price, for example, a clean atmosphere free of CO2 'fallout' from fossil fuels, of a slowing of climate change to natural proportions, and a freedom from destructive competition, to the point of war, for remaining fossil fuel reserves? Giving monetary value to such less tangible benefits may be key to unlocking the necessary investment. In the final analysis, the question of how much wind and other renewable energy we can accommodate is not merely a technical or short-term economic one, it is important for the future of human sustainability.

The author is indebted to the help of Professor David Elliott, co-director of the Energy and Environmental Research Unit at the Open University for his help and input in preparing this article.

DENA (2005) Planning of the Grid Integration of Wind Energy in Germany Deutsche Energie Agentur,

Everett R. Why Wind can Work, 'Renew' Issue 159 and various extracts, from 'Renew', the newsletter of the Open University led Network for Alternative Technologies and Technology Assessment, issues 157 and 159.ILEX (2002) Quantifying the System Costs of Additional Renewables in 2020

Sharman H. (2005) Smoothing and Extreme Ramping Offshore, Paper to UKERC Workshop on 'Intermittency', Imperial College, London, 5 July 2005.


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