Mark Diesendorf

Opponents of renewable energy, from the coal and nuclear industries and their political supporters, are disseminating the fallacy that renewable energy cannot provide base-load power to substitute for coal-fired electricity.

If this fallacy comes to be widely believed, renewable energy would always remain a niche market rather than achieving its true potential of becoming a set of mainstream energy supply technologies.

Electricity grids are already designed to handle variability in both demand and supply. To do this, they have different types of power station – base-load, intermediate-load and peak-load – and reserve power stations.

A base-load power station is in theory available 24 hours a day, seven days a week, and operates most of the time at full power. In mainland Australia, base-load power stations are mostly coal-fired while a few are gas-fired. Coal-fired power stations are by far the most polluting of all power stations, both in terms of greenhouse gas emissions and local air pollution. Overseas, some base-load power stations are nuclear powered.

An electricity supply system cannot be built out of base-load power stations alone. These stations take all day to start up from cold and in general their output cannot be changed up or down quickly enough to handle the peaks and other variations in demand. They also break down from time to time.

A faster, cheaper, more flexible type of power station is used to complement base-load, handle the peaks and handle unpredictable fluctuations in supply and demand on timescales ranging from a few minutes to an hour or so. These peak-load stations are designed to be run for short periods of time each day. They can be started rapidly from cold and their output can be changed rapidly. Some peak-load stations are gas turbines (like jet engines) fuelled by natural gas. Hydro-electricity with dams is also used to provide peak-load power.

Some renewable electricity sources have identical variability to coal-fired power stations and so they are base-load. They can be integrated into the electricity supply system without any additional back-up. Examples include the following:
* bioenergy, based on the combustion of crops and crop residues, or their gasification followed by combustion of the gas;
* hot rock geothermal power, which is being developed in South Australia and Queensland;
* solar thermal electricity, with overnight heat storage in water or rocks or a thermochemical store; and
* large-scale, distributed wind power, with a small amount of occasional back-up from peak-load plant.

Moreover, energy efficiency and conservation measures can reliably reduce demand for both base-load and peak-load electricity.

The inclusion of large-scale wind power in the above list may be a surprise to some people, because wind power is often described as an ‘intermittent’ source, that is, one that switches on and off frequently. While a single wind turbine is certainly intermittent, a system of several geographically separated wind farms is not. Total wind power output of the system generally varies smoothly and rarely falls to zero. Nevertheless, it may require some additional back-up, for example, from gas turbines.

When wind power supplies up to 20% of electricity generation, the additional costs of reserve plant are relatively small. For widely dispersed wind farms, the back-up capacity only has to be one-fifth to one-third of the wind capacity. Since it has low capital cost and is operated infrequently, it plays the role of reliability insurance with a low premium.

Of course, if a national electricity grid is connected by transmission line to another country (for example, as Western Denmark is connected to Norway), it does not need to install any back-up for wind, because it purchases supplementary power from its neighbours when required.

By 2040, renewable energy could supply over half of Australia’s electricity, reducing greenhouse emissions from electricity generation by nearly 80 per cent. In the longer term, when solar electricity is less expensive, there is no technical reason to stop renewable energy from supplying 100 per cent of grid electricity. The system could be just as reliable as the dirty, fossil-fuelled system that it replaces.

The barriers to a sustainable energy future are neither technological nor economic, but rather are the immense political power of the big greenhouse gas polluting industries – coal, aluminium, iron and steel, cement, motor vehicles and part of the oil industry.

A longer, referenced version of this paper is posted at <www.energyscience.org.au>. Dr. Mark Diesendorf is a Senior Lecturer in Environmental Studies at University of New South Wales. His latest book, 'Greenhouse Solutions with Sustainable Energy', is published by UNSW Press.