Colin Axon, Gary Taylor, and Malcolm Irving forecast sparks ahead

An important factor has been overlooked in the current energy debate – it is not a straightforward matter to ‘bring on-line’ large quantities of renewable generation.

The commonest barriers to the deployment of renewable energy sources, whether large- or small-scale, include the aesthetic impact on the landscape or skyline (as David Cameron found with his ultimately successful planning application to mount a small wind turbine on his house in a conservation area), and doubts that they will supply enough energy.

A more significant impediment is the configuration and performance of the present UK electricity grid infrastructure. The UK network consists of two separate (but linked) systems – the high voltage transmission grid, characterised by the pylon network, and the lower voltage distribution grid for the UK’s 22+ million domestic and commercial consumers.

This network was designed for a relatively small number of large power sources, not a large number of small distributed ones. The difficulties of integrating large-scale sources (wind farms, wave or tidal generators and so on) into the transmission system are related, but different, to those of adding domestic scale ‘embedded generation’ into the distribution system.

Transmission issues

The transmission grid consists of approximately 180 large power stations and 10,000 sub-stations, connections and so on. As there is no cost-effective way of storing electrical energy, the demands made by consumers must be generated and managed minute by minute; this is one of the most complicated control problems imaginable.

By using statistical analysis of usage patterns, it is possible to predict and cope with short-term peaks and troughs from daily patterns, in addition to the slowly altering seasonal demand. What is much more difficult to deal with is inherently unpredictable variability.

The intermittent nature of renewables, particularly wind turbines, will make control of the transmission grid especially difficult because all generators in the system have to be synchronised. Furthermore, the system must be actively controlled and balanced to exacting standards for frequency and voltage, set and regulated by Ofgem, the body which regulates the electricity and gas markets in Great Britain.

It is widely accepted that the transmission grid will remain stable and reliable with a rise in renewable generation from the present three per cent to the Government’s desired ten per cent.  However, it is not known what percentage of renewables might adversely affect the grid’s behaviour. This figure is likely to be above 50 per cent of operating capacity, though if the strategies to decrease power demand and to grow the number of large-scale renewable sources are successful, we might get to that figure faster than anticipated. Grid engineers have been very resourceful in meeting demands, but we are at only the start of the grid-connected renewables learning curve.

Down your street

The greatest concern is how the distribution grid will behave once embedded with many district- and domestic-scale generators.

We do not know whether the behaviour of such a complex network could exhibit unpredictable characteristics and even unexpected emergent behaviour. The proportion or number of distributed generators that the system can comfortably tolerate is simply not known. The interaction between the transmission and distribution grids is currently straightforward, but once the distribution grid also becomes a ‘generating’ grid, this interaction will cease to be simple.

Complex networks can be very sensitive to small changes. In 2003, north-eastern USA and Canada experienced a widespread blackout as the result of a single cable failure. This uncontrollable cascading outage affected around 50 million people. The UK system, presently able to meet expected demands, will need to be flexible in future to cope with increased levels of intermittent generation and more variable loads.

One possible solution is to reconfigure the distribution grid into so-called micro-grids based on localised areas such as housing estates or neighbourhoods. Any micro-grid will need to be finely balanced between generators and users, meaning that there must be a mix of domestic, public (for example, schools), and commercial users.  There is much current research and development to work out how these schemes would function, but practical experience to date is very limited.

What now?

The transmission and distribution network operators need clear support and firm policy implementation from Government. Given the current situation the renewables targets are at best over optimistic, and at worst arbitrary. The Government’s recent energy review acknowledges many of the transmissions issues, but seems to assume that the distribution grid will cope with all eventualities.

The Government can set whatever targets it chooses, but unless there is a very large investment in infrastructure and research, there is little possibility of properly harnessing the obvious advantages of renewables.

Further reading
Tom Markvart and Ray Arnold, ‘Micro-grids: Power Systems for the 21st Century’, Ingenia, September 2005, Royal Academy of Engineering.

‘The Energy Challenge’, The Department of Trade and Industry Energy Review, July 2006, London, www.dti.gov.uk

Colin Axon is in the Energy and Environment research group of the School of Engineering and Design at Brunel University

Dr Gary Taylor and Professor Malcolm Irving are with the Brunel Institute of Power Systems, also at Brunel University