Friday, 29 June 2012

How to balance electricity supply and demand using renewables (in the USA)

The National Renewable Energy Laboratory in the USA has published a study [1] showing several scenarios in which 80% of their electricity consumption comes from renewable sources by 2050. This is the most detailed plan I have yet seen: they modelled how demand could be met from a mix of mainly renewable sources from hour to hour using statistical models of regional weather patterns. It is interesting to look at the mix of renewable technologies they have selected, how they allowed for the intermittent availability of wind and solar power, and whether or not the same strategies could work here.

NREL looked mainly at technical and practical issues, including upgrades to the transmission grid as well as power generation but they also looked at costs: they estimate the retail price for electricity would rise by at most 5.3 cents/kWh due to the use of renewable electricity. Since their electricity prices are about half ours, this is more of a big deal for them that it would be for us. Continuing to rely on fossil fuels could also be a risky option because of the unpredictable cost of oil and gas on international markets.

The NREL scenarios involve no new nuclear power stations. Their target was 80% renewables, not just 80% less carbon emissions and in their plans nuclear power continues to be used for base load only.

Demand factors

In most of the NREL scenarios overall electricity demand increases only slightly by from 3700 TWh/year to 3920 TWh/year in 2050 (cf current UK demand about 340 TWh/year). This is achieved by energy efficiency improvements in new and existing buildings (30-40% improvements in existing buildings) and in industry, which don’t quite offset increases in demand from population and economic growth and electric vehicles. In the USA a great deal of building energy demand is from air conditioning (a/c) and efficiency improvements in this one area could contribute a great deal to the savings needed. In the UK electricity demand comes from a range of appliances and so a range of efficiency improvements or behaviour changes would be needed to achieve similar savings.

In the USA peak demand is in the summer, at up to 750 GW, compared to winter peaks around 550 GW. This is due to a/c loads which is convenient for using solar energy because solar power is available when it is sunny and hot. In the UK, the peak electricity requirement is in the winter: around 60 GW compared to 40 GW in the summer.

We could reduce our winter electricity demand by using low energy lighting and upgrading electric heating systems to use heat pumps. However, winter demand will increase if many people switch from gas heating to electric heat pumps.

Variable energy sources

Wind and solar energy capacity varies by the minute according to the weather. However, the USA is large enough that the weather varies greatly from one place to another and a shortage of power in one region can be offset by surplus in another. Also, when the weather is not so sunny it is likely to be windy and vice versa so solar and wind power can balance each other. The same is true in the UK though not to the same extent simply because the UK is smaller. Grid connections to the rest of Europe would help.

In the most likely NREL scenario, wind supplies more of the energy than any other source: 40% of electricity over the year. Solar panels contribute only 6% or so.

On the rare occasions when the sun doesn’t shine and the wind doesn’t blow then controllable renewables (pumped storage, geothermal and biomass) and fossil fuel power stations make up the difference.


Concentrating solar power

Concentrating solar power (CSP) is more controllable than solar panels. With CSP, solar energy is captured as heat which is used to raise steam and drive turbines; heat can be stored over a few hours, for example overnight or even a day or two. In the NREL scenarios, CSP contributes 10% or so of capacity. However, CSP requires direct sunlight that can be focussed onto a heat store – it does not work so well on cloudy days when the sunlight is diffuse. The USA has large desert areas with reliably clear skies but in the UK our weather is less reliable so CSP is less useful.

Controllable renewables: pumped storage, geothermal and biomass

Some renewable energy technologies are fully controllable in that you can turn them up or down according to need. This applies to geothermal power where heat from deep underground is used to drive turbines, and to biomass power stations which burn only when needed, or to pumped storage power stations where water is stored in a reservoir and allowed to flow when needed. In the NREL main scenario these together supply around 30% of the energy. Unfortunately, our pumped storage capacity is much less than in the USA and we don’t have any useful geothermal resource (the down side of having a stable geology with little risk of earthquakes). We do have the option of using biomass, using waste material from agriculture and wood products and energy crops. To generate 30% of our electricity we would have to dedicate a significant amount of land to energy crops: perhaps 10% of all our land area[2].

Storage technologies

Storage is important for load balancing over short time scales, because power stations take a while to get going even if they are already idling. The NREL study considers only three energy storage technologies. In order of increasing expense they are:
  • Compressed air storage in underground caverns which can drive gas turbine
  • Pumped storage in reservoirs
  • Batteries

Compressed air storage on a significant scale needs appropriate geology such as salt mines that have airtight caverns and pumped storage needs conveniently shaped mountains. Unfortunately, here in the UK we have currently only 3 GW worth of pumped storage (5% of peak load) and little potential for increasing it [2]. Batteries are the most expensive option (4 times the capital cost of compressed air) but they are practical and can be used anywhere.

There are a number of other storage technologies which NREL did not consider because they are not yet commercial such as superconducting and flow batteries, smaller compressed air systems, hot/cold gravel stores from Isentropic. See also my earlier post: Large scale energy storage options

Demand side management

When energy supply is short, rather than generating more the electric companies can ask their customers to use less: this is demand side management. It can operate over a range of time scales from a few minutes to hours. It is already happening: some homes in the USA have ‘smart a/c’ systems which automatically reduce power when requested by the electric company. Since the summer peak demand is mainly a/c this tactic has a lot of potential. In commercial buildings, larger a/c systems can store cold as ice or chilled water; when the electricity demand needs to be reduced the stored coolth can be used to keep people cool. Similarly hot water systems can store heat for later.

I can’t tell from the study report how much of the building energy is supposed to be load-shifted in this way but the UK could adopt similar measures using smart meters. We could allow our fridges and freezers to store coolth and run washing appliances off peak. Electric heating systems could adapt similarly to the a/c. I have touched on this before: Smart meters will save us money, if we take advantage.

Operating reserves

A well-managed electrical grid needs operating reserves to handle unexpected demand, unexpected failures and inaccuracy in predicting the weather. The NREL plan allows for up to 100 GW of operating reserve which comes from:
  • 40 GW spinning reserve (i.e. power stations on standby)
  • 20 GW storage
  • 40 GW interruptible load

I have already discussed the storage technologies; Interruptible load is a severe sort of demand side management. It is not a new concept: commercial and industrial users can take a contract that allows them to be cut off at short notice, though this should be rarely required – typically only a few hours in a year. NREL reports that there was nearly 16 GW of such interruptible load contracted in 2009 so 40 GW is not impractical. 40 GW would be 6% of peak load. In the UK we currently only have 0.5 GW of interruptible load – less than 1% of peak load but this could be increased [3].

Higher demand scenario.

NREL also modelled scenarios where demand increased 30% and found the 80% renewables was still achievable, though this required more gas power stations backup, more interruptible load and more renewables capacity which would be a challenge for the construction industry to deliver.

How would this work for the UK?

The NREL tactics do not work so well for us because
  • Our peak demand is in the winter rather than the summer
  • We have less reliable clear skies for CSP
  • We are smaller, so variation in weather across the country is less
  • We are short on pumped storage potential and geothermal energy

However DECC have an analysis, to be published soon [3], which shows how we could achieve 80% reduction in carbon emissions in various ways (based on the 2050 Pathways scenarios published last year - 2050 Pathways looked at all energy usage, not just electricity and was not so detailed.) The new analysis for UK electricity supply is similar to the NREL report in that it relies on  4 key mechanisms to balance load and demand:
  • Grid connections to Europe - to make use of the range of weather across the continent, although this will reduce our energy security
  • Storage
  • Demand side management – which depends largely on how we use our new smart meters
  • Thermal peaking plant - biomass as well as fossil fuel or nuclear power stations.

We can ramp up our capacity with nuclear power stations, if we accept the safety risks. This misses the NREL 80% renewables target but not the 80% reduction in carbon emissions. Financially, nuclear may or may not be cheaper: the 2050 Pathways report found that using more nuclear power did not reduce costs much. Nuclear power becomes a lot more expensive when it is used for variable loads (see Nuclear power is not just for base loads). It remains to be seen what a more detailed analysis for the UK will find.

[1] Renewable Elecrtricity Futures Study NREL

[2] 2050 pathways analysis assumes 300 TWh energy could be generated from wastes plus 2.4 million ha of land (10% of the UK). This could generate around 100-150 TWh electricity.

[3] Balancing electricity supply and demand, Smart   
  Grids And Clean Power conference, Cambridge, 2012 Rachel Crisp DECC

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