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.

Sunday, March 14, 2010

The cost-effectiveness of low or zero carbon energy generation

This update is following from my previous blog and George Monbiot's attack on feed-in tarriffs and responses to it.

Reliable independent figures on cost-effectiveness of low or zero carbon energy generation based on real monitored examples are yet few, and I'm trying to collate them, because this kind of evidence is what we need to help determine policy.

Crucially, page 37 of the 2009 impact assessment of the Community Energy Saving Programme (CESP) (which places an obligation on energy suppliers and electricity generators to meet a CO2 reduction target) ranks the effectiveness of non-large-scale generation measures in kgCO2 per pound sterling spent as follows:

1 Existing community heat to CHP 88 (kg CO2 score per £ spent)
2 Electric to community CHP 39
3 Wood pellet boilers (primary) 24
4 Micro Hydro (0.7kWp, 50% LF) 16
5 Ground source heat pumps 14
6 Air source heat pump 13
7 MiniCHP (revised) 9
8 Mini-wind 5 kW, 20% LF 4
9 Solar Water Heater (4m2) 4
10 Photovoltaic panels (2.5 kWp) 3
11 Micro Wind (1 kWp, 1% LF) 0

From this it is quite glaringly obvious that for both heat and power the community scale is by far the most efficient level for interventions. Right at the bottom are the single-dwelling only solutions (I dispute the figures for wood pellet boilers since data on their carbon content is disputed) except where hydro is available (not many places).

The Electricity and Gas (Carbon Emissions Reduction) Order 2008 (CERT) looked into the cost and carbon reduction effectiveness of various measures. The document Explanatory Memorandum To The Electricity And Gas (Carbon Emissions Reduction) Order 2008 contains a further Evidence Base.

In this community CHP with woodchips comes out at nine times more cost-effective in ££ per tonne of carbon saved than solar water heating and about the same as ground source heat pumps.

The figures are (- with suppliers’ cost to save one tonne C02 (£/tC02) for the Priority Group):
1 Community heating with wood chip 3
2 Ground source heat pumps 42
3 Wood chip CHP 49
4 Wood pellet boilers (primary) 58
5 Micro Hydro (0.7kWp, 50% LF) 60
6 Log burning stoves 110
7 Mini-wind 5 kW, 20% LF 125
8 Wood pellet stoves (secondary) 126
9 mCHP 176
10 Photovoltaic panels (2.5 kWp) 218
11 Solar Water Heater (4m2) 346
12 Micro Wind (1 kWp, 10% LF) 685
13 Community ground source heat pumps 697

The above underscores that renewable energies are frequently site-dependent and sensitive to economies of scale, because you have to cost the whole system.

Only 2% of UK homes can have a small wind turbine. This Energy Saving Trust report suggests the best sites and how to pick them.

Solar electricity

In my previous blog I link to actual surveys of real PV installations and the figures show that they do not generate sufficient power in the UK when we need it for the reason that there is not enough sunshine in the winter - unless you have a huge array, which is currently very expensive.

If PVs could become as cheap as a low-e coated window unit, with spray-on or printed nano-scale circuitry or similar it might be worthwhile. This is a technological advance not a deployment advance. They would also need to capture a greater range of frequencies of light.

I would suggest that the standard for measuring and marketing the rated output of a panel or a system is changed to make it more realistic and easier for buyers to understand. As discussed in the blog above, the test conditions are way different from European field conditions and led to unreal expectations, or to potential obfuscation by the industry/ unscrupulous installation companies.


All of this suggests that while renewable heat can work on a community level, only CHP can universally provide electricity, whatever the power source, and then not that much in relation to demand since there is a limit to the available waste, waste heat and biomass.

Therefore we have to conclude that for renewable electricity generation larger scale wind and marine power are what is required at a massive scale. Of these, only wind is currently cost-effective and that is why it is being aggressively pursued offshore and onshore.

Monday, March 01, 2010

Does PV - solar electricity - work in the UK?

The Government's Feed-in-Tariff (FIT) programme - energy cashback - will soon give subsidies for householders installing renewable energy in their homes.

George Monbiot told me last night he's going to publish tomorrow a critique of them (it's online now, here), arguing that when subsidising solar electricity (photovoltaic/PV) panels they are a waste of money and a rip-off to taxpayers.

As I have in the past campaigned for FITs, I was initially taken aback. But then I recalled I'd looked into the cost-effectiveness of PV systems before and agreed.

The truth is they do have very long cash payback times, even with a subsidy, which is an artificial way of reducing payback and doesn't reduce its actual real-world cost.

The question then arises - shouldn't we spend the money on other technologies which can have the same impact on reducing carbon emissions but more cost-effectively? George quotes the same McKinsey report comparison table Ive used before and which is in the Stern Report, to indicate which technologies do offer value for money.

What's the evidence for saying they are a waste of money?

Firstly: Here are the results of a 2006 study from Bartlett School of Graduate Studies, University College London:

This paper compared two solar systems, an actual building integrated, photovoltaic roof (BIPV) and a notional solar thermal system for a residential block in London, UK.

The carbon payback for the solar thermal system is 2 years, the BIPV system has a carbon payback of 6 years.

Simple economic payback times for both systems are more than 50 years. Calculations considering the current UK energy price increase (10%/yr), reduce the economic payback time for the PV roof to under 30 years.

The costs to reduce overall carbon dioxide emissions using a BIPV roof are £196/tonne CO2, solar thermal individual systems at £65/tonne CO2 and community solar thermal at £38/tonne CO2.

Secondly: A BRE / DTI 2006 UK Photovoltaic Domestic Field Trail (PV DFT) made recommendations on installing PV but did not even look at payback.

Thirdly: Here's something I wrote in my forthcoming book on environmental refurbishment that was taken out by an editor:

Photovoltaics would not need the high level of financial support that the technology clearly does if there wasn’t a problem with generating enough power in extreme latitudes to justify their installation.

Many people have had their enthusiasm for photovoltaics curbed when they find out exactly how much power they can expect to generate for their cash. Therefore it is important to be quite clear on this: you are unlikely to generate more than a fraction of what you need and if you’re looking for quick paybacks, forget it.

The best way to demonstrate this is through a worked example.

Lesson 1:
Solar panel manufacturers quote figures for the “peak power” and “installed capacity” of their products. According to industry standards these are the amounts of electrical output in watts that they would generate if one kilowatt per square metre of the sun’s energy were to fall on them. But how close is this to the amount of sunshine at your location? These figures can be found out from the same source on insolation given in the section on solar water heating. For most of the latitudes that cover England and Wales, the summer insolation is a fraction of that figure. Even Europe’s sunniest place, Limassol in Cyprus, only gets 325 W per square metre. London gets 198 and Edinburgh 172 in July. In December, the figures are 96, 22 and 13 respectively. So in the winter, it’s a lot less -- and that’s when you need more power because the lights will be on for longer.

On average, Edinburgh receives just 9% of the solar energy required by the panel to generate what it says it will on the box.

Lesson 2: Suppose you installed 30m2 of panels that were quoted by the manufacturer as having a peak power or installed capacity of 5.7kWp. Suppose they were installed in London, which has an average insolation figure over the year of 109W/m2. In that case you wouldn’t get 5.7kW averaged over the year, but 0.109 x 5.7kW = 621W. However that is the average figure.

In darkest December they were generating just 125W, or enough to power 10 low energy light bulbs. In fact it might be even less than this, because of shading, downtime and other system inefficiencies.

Lesson 3: How much would this cost and what would your payback be? Here are some figures from 2004 for an installation on the roof of The Insolvency Service, in Bloomsbury London. (There aren’t that many case studies where the figures are all available -- this one comes from a UK government report - Large-Scale Building Integrated Photovoltaic Field Trial: URN Number 07/1316, BERR, 2007:

Size  Annual Output Project Cost Cost per peak W  Cost per kWh Payback time

kWp MWh/year £ £/Wp p/kWh (years)

25.4 11.8 318,760 12.6 108.2 237

237 years payback? This is a good example of how not to do it - and perhaps ironic given the purpose of the institution (if we did this we would go bankrupt).

The best performance figures in the report come from a water park in the Cotswolds.

These are from 300 building-integrated monocrystalline modules rated at 85W on a 150m2 sloping roof, with a yield of 51kWp. The system cost €397,500 in 2003 and the following year generated 44.3MWh. The figures in the report are:

51.0 42.8 2 65,000 5.2 24 .8 54

54 years is still a long time to wait to get your money back, especially when the modules only came with a 20 year warranty from BP. And 24.8p per kWh is still twice the current average electricity price.

(These examples illustrate how to work out PV systems’ cost effectiveness. To work out how much carbon emissions they save, simply multiply the megawatt-hours by the carbon dioxide emissions figure given for fossil fuel electricity, which presumably the panels will displace, given on page x - 550 kg/MWh.)

Lesson 4: Although the figures from the field trials report a mean/peak power ratio of over 7%, equivalent to an annual yield of over 610kWh/kWp, if we accept a grid-purchased electricity price of £0.114/kWh (the 2008 UK average), and require a 5 year payback period, the break-even cost of a PV system - the total installation cost per peak watt - is £0.35/Wp.

But the UK Energy Saving Trust, in its brochure on solar electricity published in 2007, quotes installation prices for domestic rooftop PV of £5–£7.50/Wp. This seems a bit steep, but: it is 14–20 times higher than the break-even value.

And you’re only generating a fraction of the power you need.