Hourtohour variation in output
A key aspect of variable-output renewable resources is the rate of change in power output over time. The rate of change in output of a wave, wind or tidal stream electricity generator depends upon two factors - the sensitivity of the device to changes in environmental conditions and the degree to which environmental conditions change. Nordex N80 wind turbine power function Nordex N80 wind turbine power function Figure 3.4 Power transform curve for a typical large wind turbine Consider the power...
Integrated gasification combined cycles in a renewable scenario
This analysis suggests that, because of the incorporation of CO2 capture processes, the need to run at higher temperatures, and the temperature changes resulting from intermittency, current designs of plant may not be ideal. The need to find a more promising alternative is more pressing for coal. One such option is a modification of the Integrated Gasification Combined Cycle IGCC , a gasification-based process that can easily be adapted to capture carbon dioxide, while at the same time...
Effects on plant components and reliability
Plants that have to 'two-shift' i.e. be operated in intermittent rather than continuous mode have to be kept in very good condition. Maintenance that is neglected risks unscheduled shutdowns. Even if this does not cause serious problems with the grid, it is likely to result in substantial financial penalties. Unfortunately, two-shifting is extremely damaging rule-of-thumb estimates suggest that each start-up and shutdown is equivalent to about 20 hours of normal operation. The basic cause of...
Implications for conventional plant capacity needed
All study results indicate that for low levels of penetration, the firm power capacity displaced equals the mean power delivered by wind generation i.e. measured by the total wind generation average load factor , but decreases with increasing penetration of wind Rockingham, 1980 Halliday et al, 1983 Grubb, 1986, 1988 Swift-Hook, 1987 . This diminishing return in the value of wind capacity reflects the increasing importance of the possibility of little output from all sources of wind generation....
Conclusions Vbj
The author supports the opinion that the effect of intermittency on the present set of UK generating plants will not cause operating problems and is not substantially different from what happens when steam or CCGT plants are designated as two-shift units. The main implications are that irregular operation will lead to increased maintenance costs and unreliability. These factors are well recognized within the power generation sector, and their impact can be minimized by improved detail design...
Reserve and standby generating capacity on the UK National Grid
On the UK National Grid system there is approximately 1.5GW of 'spinning reserve' - typically, this takes the form of a large power station that is paid to produce at less than its full output. Such a station might have four generating sets each of 660MW, giving a total output of 2.64GW, but might only be operating at 2GW with the steam boiler full but the steam valve not fully open. On request from the National Grid control centre, this valve can open and deliver an extra 640MW in 20 to 30...
Overall flattening of daynight demand
There is considerable variability in electricity demand throughout the day, even without the introduction of variable renewable electricity supplies. It would thus pay to 'flatten' the day-night electricity demand curve by encouraging the use of off-peak electricity. This would probably require an increase in the use of electric heating. Given the availability of cheap North Sea gas, this has not been economically viable in the UK since the 1970s, but could become so again in the future. It...
Wind energy penetration levels
Although the British and Danish system operators have both stated that the limits to wind energy penetration are economic rather than technical, this has gone largely unnoticed. There is still a feeling that high wind energy penetrations will cause severe technical problems but this is simply not the case. Extra costs are incurred but these can be quantified. A recent Danish study Pedersen et al, 2006 has suggested that these extra costs reach a maximum value of around 15 MWh of wind. If...
Wind predictability
In general, wind is perceived as unpredictable but this is not strictly true. Weather forecasting techniques enable average wind speeds to be predicted with reasonable accuracy, but not necessarily the hour-by-hour variations. In some parts of the world, however, such as the Californian passes, the winds do have an established diurnal cycle, which benefits system operators. Although there are significant differences in the way in which electricity jurisdictions operate, there is a reasonable...
Triads A revenueearning opportunity
So-called 'triad' periods provide a further revenue-earning opportunity, separate from the Reserve Service. Triad periods are the three half-hour periods of maximum electricity demand during winter. Diesel generators can earn substantial sums by reducing a site's peak demand during these peak periods. National Grid Transco is funded principally by means of a capacity charge levied on the energy suppliers, who then pass it on to their customers in a more or less transparent way. The charge is...
Security of power supply and planning margins
Historically, in the development of national electricity supply industries, the planning process has sought to ensure that sufficient spare generation capacity is available, over and above that needed to meet the maximum peak load demand, in order to account for contingencies. To do so requires having sufficient spare capacity in order to meet not only expected generating plant outages for repair and maintenance, but also unexpected events causing breakdown of plant and, thus, non-availability...
Smoothing effect
The power output of wind farms fluctuates. These fluctuations are very difficult to forecast, and even if the power output on a particular day is predicted well, the fluctuations will cause a forecast error. The larger the wind farm, the smaller will be the fluctuations and the corresponding forecast error. If many wind farms are forecast together, the forecast error decreases further. In addition, the aggregation of large regions with several gigawatts of installed capacity will lead to a...
References
Bell, K., Ault, G. and McDonald, J. 2006 'All eyes on wind', IET Power Engineer, June July, pp30-33 Boyle, G. ed 2004 Renewable Energy, 2nd Edition, OUP and the Open University, Oxford E.ON Netz 2004 E.ON Netz Wind Report Wind Year 2003 - An Overview, E.ON Netz Gmbh, Bayreuth, info eon-netz.com Grubb, M. J. 1986 The Integration and Analysis of Intermittent Sources on Electricity Supply Systems, PhD thesis, Cambridge University, Cambridge Grubb, M. J. 1987 'Capital effects at intermediate and...
Combinedcycle gas turbine generating plants
CCGTs consist of a gas turbine s that produces about two-thirds of the power from the plant. A single steam turbine set utilizes the steam generated in a Heat Recovery Steam Generator HRSG to produce the remaining third. The gas turbine operates by compressing air to a pressure of about 20 to 25 bar, burning natural gas in the compressed air and then expanding this through a combustion turbine, which then drives a compressor and an alternator. Some forms of CCGT have the set of steam turbines...
Power demand and supply Daily load curves
In the British system covering England, Wales and Scotland, the total daily power demand varies between a minimum summer load of about 22.4 giga-watts GW and a winter peak above 59.4GW National Grid, 2006 . Figure 1.8 illustrates the demands during the two days, with minimum and maximum demands, respectively, over the 12-month period of 1 July 2005 to 30 June 2006, the areas under the curves representing daily energy demands supplies and the heights of the curves representing the average...