This is an edited extract from Mark Diesendorf’s new book, Sustainable Energy Solutions for Climate Change, published in Australia and New Zealand by UNSW Press in December 2013; to be published overseas by Earthscan in April 2014.
Since renewable energy is intermittent, it is too unreliable to be the major source of electricity supply. Large penetrations of renewable energy into the grid must await either the development of baseload renewable power stations or a vast amount of storage.
Computer simulation modeling of electricity systems with 80-100 per cent renewable electricity (RE) refutes this traditional assumption.
The simple conception of balancing baseload demand with baseload supply over a 24-hour period is outdated in the context of a RE future. It was appropriate when inflexible baseload power stations, coal or nuclear, were the only choice for 24-hour power. However, when large penetrations of RE are integrated into the grid, baseload power stations are not needed and their inflexibility becomes a liability. The computer simulations show that baseload demand can be supplied reliably by a mix of RE sources, none of which is, strictly speaking, a baseload power station, as shown in the figure.
As originally pointed out by David Mills, it’s more relevant to speak of a mix of ‘flexible’ and ‘variable’ RE power stations. The output of a ‘flexible’ station can be varied rapidly as required. Furthermore a ‘flexible’ technology is dispatchable, meaning it can be switched on rapidly, unlike a coal-fired or nuclear power station, and its electricity can be fed into the grid upon demand with a high degree of certainty.
The following renewable electricity systems are flexible to varying degrees:
– a gas turbine with an assured source of fuel derived from biomass (organic materials);
– a hydro-electric station with a large dam;
– a geothermal power station; and
– a concentrated solar thermal power station with thermal storage.
As with any definition, there are grey areas. The flexibility of an RE source with storage increases with the amount of storage.
On the other hand, wind, solar PV without storage and run-of-river hydro are much less flexible and are classified as ‘variable’ sources. Although the electrical outputs of variable power stations can be decreased when required, they cannot be increased beyond the level offered by weather or flow conditions at the time. In other words, they cannot be dispatched reliably, but that does not matter if they are part of a mix with flexible renewable power stations.
Some people label variable RE technologies as ‘intermittent’. I avoid this term, because it can be misleading, implying incorrectly that the large-scale renewable electricity production generally cuts off abruptly when the wind drops or a cloud blocks the sun. While the output of a single wind turbine or solar PV module may indeed be intermittent in this sense, a wind farm spanning tens of kilometres and a solar power station spanning a kilometre or so will take much longer to cut-off. A system of geographically dispersed wind farms and solar power stations may take many hours. Furthermore, short-term changes in wind and sunshine can be predicted quite well from weather observations.
The challenge for a predominantly RE system is to supply the peaks in demand, not baseload, and this can be achieved with both geographic dispersion and a mix of flexible and variable power stations.
The concept of flexibility can be extended to the transmission network and to demand management as well, as shown by the results of detailed hour-by-hour computer simulations of supply and demand in the US by the National Renewable Energy Laboratory, which finds that:
In the 80 per cent renewable electricity scenario by NREL, about half the annual electricity generation comes from fluctuating wind and solar PV. The other renewable sources are concentrated solar thermal, hydro, biomass and geothermal. Coal, gas and nuclear comprise the remaining 20 per cent, reduced to 10 per cent in NREL’s 90 per cent renewable electricity scenario. Vast amounts of storage are found to be unnecessary for 80-90 per cent renewable electricity in the US.
The results of scenario modelling are supported by observation. Two large-scale electricity systems already have more than one-quarter of their annual electricity generation coming from variable RE sources, predominantly wind. In 2012 Denmark generated 30 per cent of its annual electricity supply from the wind, while South Australia generated 27 per cent. Neither jurisdiction has significant amounts of storage on the grid and each has a relatively small power capacity (compared with its wind capacity) of transmission lines joining it to neighbouring countries or states. As a result of the high penetration of wind energy into its grid, South Australia’s two coal-fired power stations are operating for only half the year and are likely to be shut down permanently.
The myth is busted!
Mark Diesendorf is associate professor and deputy director of the Institute of Environmental Studies at UNSW