Thursday, March 18, 2010

Grid connection and intermittency of renewable energy

It's still said that when the wind doesn't blow you need other sources of power, so wind farms are really a waste of money.

This topic - grid integration of renewables and variability - has been extensively investigated.

One conclusion - from the UK Government report quoted below - is: "For penetrations of intermittent renewables up to 20% of electricity supply, additional system balancing reserves due to short term (hourly) fluctuations in wind generation amount to about 5-10% of installed wind capacity."

Peter Freere, formerly an engineering professor at Monash University in Australia says:

"It is correct that in normal electric grids (without energy storage - eg pumped storage, charging electric cars, etc.), a single wind farm would not work well on its own and some conventional energy sources are also required.

"The same applies to the conventional energy sources, especially nuclear, whose response time is so slow that they must have a fast responding generation system in parallel.

"It is also true that due to the large sizes of modern generators in conventional systems (eg. 500 MW per generator), to allow for maintenance and breakdowns, it is necessary to have a complete spare generator ready to take over when another stops working (for whatever reason)."

"Hence the risk with wind farms is not so great - no more than many conventional systems."

The largest study on grid integration was done in Germany by DENA. The gist is that investment in the grid will be necessary to integrate the new renewable generation needed meet the German target of 20% of supply from renewable energy by 2020. The investment required is less expensive than additional grid expansion if new central-station plants are built, and this investment will result in making the grid more stable with or without the renewable generation.

A March 2006 UK Energy Research Centre report analysed the results of 200 studies on the grid integration of intermittent renewables. The following are excerpts from the summary of The Costs and Impacts of Intermittency: An assessment of the evidence on the costs and impacts of intermittent generation on the British electricity network. Below are its conclusions:

  • It is sometimes said that wind energy, for example, does not reduce carbon dioxide emissions because the intermittent nature of its output means it needs to be backed up by fossil fuel plant. Wind turbines do not displace fossil generating capacity on a one-for-one basis. But it is unambiguously the case that wind energy can displace fossil fuel-based generation, reducing both fuel use and carbon dioxide emissions.

  • Wind generation does mean that the output of fossil fuel-plant needs to be adjusted more frequently, to cope with fluctuations in output. Some power stations will be operated below their maximum output to facilitate this, and extra system balancing reserves will be needed. Efficiency may be reduced as a result. At high penetrations (above 20%) energy may need to be 'spilled' because the electricity system cannot always make use of it. (However it is hoped that in the future elecric cars will be charged sing this energy.) But overall these effects are much smaller than the savings in fuel and emissions that renewables can deliver at the levels of penetration examined in this report.

  • None of the 200+ studies reviewed suggest that introducing significant levels of intermittent renewable energy generation on to the British electricity system must lead to reduced reliability of electricity supply2. Many of the studies consider intermittent generation of up to 20% of electricity demand, some considerably more. It is clear that intermittent generation need not compromise electricity system reliability at any level of penetration foreseeable in Britain over the next 20 years, although it may increase costs. In the longer term much larger penetrations may also be feasible given appropriate changes to electricity networks.

  • The introduction of significant amounts of intermittent generation will affect the way the electricity system operates. There are two main categories of impact and associated cost. The first, so called system balancing impacts, relates to the relatively rapid short term adjustments needed to manage fluctuations over the time period from minutes to hours. The second, which is termed here 'reliability impacts', relates to the extent to which we can be confident that sufficient generation will be available to meet peak demands. No electricity system can be 100% reliable, since there will always be a small chance of major failures in power stations or transmission lines when demands are high. Intermittent generation introduces additional uncertainties, and the effect of these can be quantified.

  • System balancing entails costs which are passed on to electricity consumers. Intermittent generation adds to these costs. For penetrations of intermittent renewables up to 20% of electricity supply, additional system balancing reserves due to short term (hourly) fluctuations in wind generation amount to about
    5-10% of installed wind capacity. Globally, most studies estimate that the associated costs are less than £5/MWh ($0.0087/kWh) of intermittent output, in some cases substantially less. The range in UK relevant studies is £2 - £3/MWh ($0.0035-$0.0052/kWh).

  • Unless there is a large amount of responsive or controllable demand, a system margin is needed to cope with unavailability of installed generation and fluctuations in electricity requirements (e.g. due to the weather). Conventional plant - coal, gas, nuclear - cannot be completely relied upon to generate electricity at times of peak demand as there is, very approximately, a one-in-ten chance that unexpected failures (or "forced outages") in power plant or electricity transmission networks will cause any individualconventional generating unit not to be available to generate power. Even with a system margin, there is no absolute guarantee in any electricity system that all demands can be met at all times.

  • Intermittent generation increases the size of the system margin required to maintain a given level of reliability. This is because the variability in output of intermittent generators means they are less likely to be generating at full power at times of peak demand. The system margin needed to achieve a desired level of reliability depends on many complex factors but may be explored by statistical calculations or simplified models. Intermittent generation introduces new factors into the calculations and changes some of the numbers, but it does not change the fundamental principles on which such calculations are based.

  • Intermittent generators can make a contribution to system reliability, provided there is some probability of output during peak periods. They may be generating power when conventional stations experience forced outages and their output may be independent of fluctuations in energy demand. These factors can be taken into account when the relationship between system margin and reliability is calculated using statistical principles.

  • Capacity credit is a measure of the contribution that intermittent generation can make to reliability. It is usually expressed as a percentage of the installed capacity of the intermittent generators. There is a range of estimates for capacity credits in the literature and the reasons for there being a range are well understood. The range of findings relevant to British conditions is approximately 20 - 30% of installed capacity when up to 20% of electricity is sourced from intermittent supplies (usually assumed to be wind power). Capacity credit as a percentage of installed intermittent capacity declines as the share of electricity supplied by intermittent sources increases.

  • The capacity credit for intermittent generation, the additional conventional capacity required to maintain a given level of reliability and thus the overall system margin are all related to each other. The smaller the capacity credit, the more capacity needed to maintain reliability, hence the larger the system margin. The amount by which the system margin must rise in order to maintain reliability has been described in some studies as "standby capacity","back-up capacity" or the "system reserves". But there is no need to provide dedicated "back-up" capacity to support individual generators. [Emphasis added] These terms have meaning only at the system level.

  • This assumes around 20% of electricity is supplied by well dispersed wind power. Current costs are much lower; indeed there is little or no impact on reliability at existing levels of wind power penetration. The cost of maintaining reliability will increase as the market share of intermittent generation rises.

  • The aggregate 'costs of intermittency' are made up of additional short-run balancing costs and the additional longer term costs associated with maintaining reliability via an adequate system margin. Intermittency costs in Britain are of the order of £5 to £8/MWh ($0.0087-$0.0139/kWh), made up of £2 to £3/MWh from shortrun balancing costs and £3 to £5/MWh from the cost of maintaining a higher system margin. For comparison, the direct costs of wind generation would typically be approximately £30 to £55/MWh ($0.052-$0.0958). If shared between all consumers the impact of intermittency on electricity prices would be of the order 0.1to 0.15 p/kWh ($0.0017-$0.0026/kWh).


Below are a few links to other reports on grid integration of renewables.

4 comments:

Rick Row said...

Thanks for your posting. As it indicates there is a lot of information on the capacity credit of wind power, but I'm finding very little on the capacity credit of solar energy. Can you suggest any references on this?

Low Carbon Kid said...

Solar photovoltaics on the UK power system have no capacity credit. As this source suggests it's "because the triad peak periods are always on cold winter evenings after dark when there is no sunshine. Many other places (like New York, Paris and Vancouver) will be the same when their peak demand is in mid-winter but in countries with tropical climates, where the peak demand is in mid-summer, due to air-conditioning loads (like Florida, California and Greece), this will obviously not apply and photovoltaics may then have nearly 100% capacity factors."

More gnerally, in oral evidence to the Commons Environment Select Committee, Malcolm Wicks, then energy minister at DBERR, said they will expect to allow a capacity credit for renewables of around 10%-20%. To which Nigel Lawson bitingly replied: "That is an unusually optimistic estimate." Well, I dispute that as an average figure: wind is generally assumed to be around 40%. Solar will be therefore close to zero.

This does not mean it is worthless - clearly, when generating, it is displacing something. Any proper model of how we move to over 20% renewable energy penetration has to account for what credit will be available at all times of the year, and wind and solar are a good match in general - in a highly varied scenario.

The same article cited above confirms the viability of wind's contribution.

Low Carbon Kid said...

This debate has spilled onto Lightbucket's blog. There, Andrew Smith posted:

"As I wrote over here in the section “So what should Britain do?”, PV can make a positive contribution to UK energy security. Capacity Credit was once a useful back-of-envelope proxy for energy security 20 years ago, when there were few computers around, and almost all our electricity generation was by thermal plant. These days, it’s really past its sell-by date.

"It was a proxy for the Loss of Load Expectation – a probabilistic estimate of energy security. Clearly a system X + some PV is less likely to lose load than system X without PV, but you need a good energy model to tell you by how much.

"Looking at a putative 2030 grid, with say 60% mean energy coming from wind power, and 5% from PV, that PV gives you a lot of energy security, because insolation is negatively correlated with wind, over the year. And getting, on average, 5% of your electricity from PV, means your installed PV capacity is about 50% of mean demand. So if we had another long hot summer like 1976, we’d have plenty of PV to see us through the windless times."

Also Lightbucket underscored this point from the link i posted for you: "It argues that storage is pointless for grid-connected wind and PV, because they have zero marginal cost and would be the last to be stored. The energy that got stored would have come from the last generating units to be brought on line, i.e. those with highest marginal cost.

"It’s a subtle and non-obvious point."

Alexander said...

Good post very interesting