Renewables: can they fit in a modern power grid?

25 September 2008

Technology to utilise the forces of nature for doing work to supply human needs is as old as the first sailing ship. But attention swung away from renewable sources as the industrial revolution progressed, on the basis of the concentrated energy locked up in fossil fuels. This was compounded by the increasing use of electricity based on fossil fuels and the importance of portable high-density energy sources for transport – hence the era of oil. Yet attention has now returned to the huge sources of energy surging around us in nature – sun, wind, and seas in particular. There was never any doubt about their magnitude; the challenge was always in harnessing them.

There is today an unprecedented interest in renewable energy, particularly solar and wind power, which provide electricity without giving rise to any carbon dioxide emissions. Harnessing these for electricity depends on the cost and efficiency of the technology, which is constantly improving, thus reducing costs per peak kilowatt.

Renewable energy is sometimes misleadingly regarded as a competitor to nuclear, but in reality the two are complements, both contributing to security of supply and to abating carbon emissions. Yet utilising electricity from solar and wind in a grid requires some backup generating capacity due to their intermittent natures. If utilised in a stand-alone system they require corresponding battery or other storage capacity. If we move to a distributed electricity system with localised supply, this may eventually happen, but for now we are still left with grid systems based on centralised generation. How can the key renewable sources fit into this?

Most electricity demand is for continuous, reliable supply that has traditionally been provided by baseload electricity generation. Some is for shorter-term (eg peak load) requirements on a broadly predictable basis. Hence if renewable sources are linked to a grid, the question of backup capacity arises – for a stand-alone system, energy storage is the main issue. Apart from pumped storage hydro systems, no such means exist at present, at least on any large scale. Pumped storage is used to even out the daily generating load by pumping water to a high storage dam during off peak hours and weekends, using the excess baseload capacity from low cost coal or nuclear sources. During peak hours this water can be used for hydroelectric generation. But relatively few places have scope for pumped storage dams close to where the power is needed, and overall efficiency is low. Alternative means of storing large amounts of electricity as such in giant batteries or by other means have not yet been developed.

A distinct advantage of solar and to some extent other renewable systems is that they are distributed and may be near the points of demand, thereby reducing power transmission losses if traditional generating plants are distant. But this same feature sometimes counts against wind, as the best sites for harnessing it are sometimes very remote from population centres.

Hydro, wind and solar energy are the three most prominent renewable sources today and have different features. Geothermal, wave and tidal systems are also important, but either limited in scope for electricity (geothermal) or still under active development (wave and tidal).

Hydroelectric power, using the potential energy of rivers, is by far the best-established means of electricity generation from renewable sources. It now supplies 16% of world electricity (99% in Norway, 58% in Canada, 55% in Switzerland, 45% in Sweden, 7% in the USA, 6% in Australia). Apart from those four countries with an abundance of it, hydro capacity is normally applied to peak load demand, because it is so readily stopped and started. This also means that it is an ideal complement to wind power in a grid system, and is used thus most effectively by Denmark. The chief advantage of hydro systems is their capacity to handle seasonal (as well as daily) high peak loads. In practice the utilisation of stored water is sometimes complicated by demands for irrigation, which may occur out of phase with peak electrical demands.

Utilisation of wind energy has increased spectacularly in recent years, with a 27% increase in installed capacity during 2007, capping similar rises in previous years. This brought total world wind capacity to 94GWe, with tens of thousands of turbines now operating. Germany leads the field with over 22GWe installed, Spain has over 15GWe and the USA has over 16GWe. The average size of new turbines in the USA in 2007 was 1.65MWe. Where there is an economic backup which can be called upon at very short notice (eg hydro), a significant proportion of electricity can be provided from wind. Depending on site, most turbines operate at about 25% load factor over the course of a year (European average), but some reach 33%. With increased scale and numbers of units, generation costs have been diminishing. They are still greater than those for coal or nuclear, and allowing for backup capacity and grid connection complexities adds to them. However, government policies in many countries ensure that power from them is able to be sold.

Solar energy is readily harnessed for low temperature heat, and in many places domestic hot water units (with storage) routinely utilise it. Several methods of converting the sun’s radiant energy to electricity are the focus of attention, the best known method utilising sunlight acting on photovoltaic (PV) cells. For a stand alone system, some means must be employed to store the collected energy during hours of darkness or cloud – either as electricity in batteries or in some other form. An extra stage of energy conversion is therefore involved with consequent energy losses. In some systems there is provision for feeding surplus PV power from domestic systems into the grid as contra to normal supply from it, which enhances the economics. With solar input being both diffuse and interrupted by night and by cloud cover, solar electric generation has a low capacity factor, typically less than 15%. Power costs are two to three times that of conventional sources, which puts it within reach of being economically viable where carbon emissions from fossil fuels are priced.

“Introducing renewable energy unavoidably leads to higher electricity prices”

Having solved many problems of harnessing renewable sources for power generation, there is a significant challenge of integrating them into the supply system. All but hydro cannot be controlled to provide directly either continuous baseload power, or peak load power when it is needed, so how can other, controllable sources be operated so as to complement them?

There is some scope for reversing the whole way we look at power supply, in its 24-hour, 7-day cycle, using peak load equipment simply to meet the daily peaks. Today’s peak load equipment could be used to some extent to provide infill capacity in a system relying heavily on renewables. The peak capacity would complement large-scale solar thermal and wind generation, providing power at short notice when they were unable to. This is essentially what happens with Denmark, whose wind capacity is complemented by a major link to Norwegian hydro (as well as Sweden and the north German grid). The major interconnections are of some 1000MWe, 600MWe and 1300MWe respectively. The Norwegian hydro resources can normally be called upon, which are ideal for meeting demand at short notice (though not in 2002 after several dry years). Denmark is a good example of how a renewable resource can be well utilised, but the circumstances are far from typical.

If hydro is the backup and is not abundant, then it will be less available for peaking loads. If gas is the backup it may be the best compromise between cost and availability. If a brown coal generating plant is the backup and thus has to be run at lower output to accommodate the wind-generated input, then the CO2 emissions per kWh increase, eroding or even eliminating any emission advantage from wind. In Germany brown coal plants need to be run inefficiently to back up wind capacity and this has both cost and CO2 implications.

Grid management authorities, faced with the need to be able to dispatch power at short notice, treat wind-generated power not as an available source of supply which can be called upon when needed, but as an unpredictable drop in demand. Improved ability to predict the intermittent availability of wind enables better use of this resource. In Germany it is claimed that wind generation output can be predicted with 90% certainty 24 hours ahead. This means that it is possible to deploy other plants more effectively so that the economic value of that wind contribution is greatly increased.

Ensuring both secure continuity of supply (reliably meeting peak power demands) and its quality (for example, no voltage drops) means that the actual potential for wind and solar input to a system is severely limited. Doing so economically requires a low-cost backup such as hydro, or gas turbines with cheap fuel. For the UK, with little interconnection beyond its shores, a 20% renewables target is clearly difficult to achieve. Meeting up to 20% of electricity demand need not compromise reliability, but is likely to have a significant cost.

So introducing renewable energy unavoidably leads to higher electricity prices. Not only are production costs substantially higher than for conventional energy, but in the case of intermittent energy sources like wind energy, grid extensions and additional balancing and backup capacity to ensure security of supply imply costs which add considerably to the end price for the final consumer. However, the economic disadvantage referred to will be reduced as carbon emissions costs become factored into fossil fuel generation.

One other important development will be the development of advanced metering systems. Current systems, based on no more than different tariffs for peak and off peak power, are relatively primitive and more sophisticated charging mechanisms can be developed to manage demand and integrate renewables into a grid system.

In practical terms, non-hydro renewables are able to supply up to some 15-20% of the capacity of an electricity grid, though they cannot directly be applied as economic substitutes for most coal or nuclear power, no matter how significant they become in particular areas with favourable conditions. Nevertheless, they will make an important contribution to the world’s energy future, even if they cannot carry the main burden of supply.

Author Info:

Steve Kidd is Director of Strategy & Research at the World Nuclear Association, where he has worked since 1995 (when it was the Uranium Institute). Any views expressed are not necessarily those of the World Nuclear Association and/or its members

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