http://hdl.handle.net/10468/280 2013-05-04T06:35:47Z 2013-05-04T06:35:47Z Leahy, Paul G. Kiely, Gerard http://hdl.handle.net/10468/707 2012-09-19T02:00:11Z 2012-08-01T00:00:00Z

The effect of introducing a winter forage rotation on CO2 fluxes at a temperate grassland Leahy, Paul G.; Kiely, Gerard Temperate grasslands have the potential to sequester carbon, helping to mitigate rising atmospheric CO2 concentrations. The ability of grasslands to absorb CO2 is influenced by site elevation, soil type, management practices, climate and climatic variability. There is a need for long-term observations and field experiments to quantify the effects of the key drivers of management and climate variability. This paper presents over 4 years of eddy covariance measurements of CO2 flux over a managed temperate grassland site in south-east Ireland. For the first 2 years the entire study area was under grass. During the second 2 years a winter forage crop was grown over part of the site. The site was found to have a net uptake of CO2 during all years. However, the magnitude of the CO2 uptake varied considerably from year to year, with a maximum net uptake of 1.32 kg CO2 m−2 in 2004, a year with no winter forage crop. Net uptakes were much lower in the 2 years of mixed grass and kale cultivation, but detailed analysis of the measurement footprint and statistical comparisons showed that this was not due to the introduction of the forage rotation. For a short period following sowing of the forage crop, daytime CO2 uptake was less than that of the area under grass, but over subsequent months daytime CO2 uptake of the kale areas recovered strongly and exceeded that of the grass areas. The net effect over the year following kale planting is close to CO2-neutral.

2012-08-01T00:00:00Z Leahy, Paul G. Foley, Aoife M. http://hdl.handle.net/10468/718 2012-10-11T02:00:10Z 2012-03-01T00:00:00Z

Wind generation output during cold weather-driven electricity demand peaks in Ireland Leahy, Paul G.; Foley, Aoife M. Recent cold winters and prolonged periods of low wind speeds have prompted concerns about the increasing penetration of wind generation in the Irish and other northern European power systems. On the combined Republic of Ireland and Northern Ireland system there was in excess of 1.5GW of installed wind power in January 2010. As the penetration of these variable, non-dispatchable generators increases, power systems are becoming more sensitive to weather events on the supply side as well as on the demand side. In the temperate climate of Ireland, sensitivity of supply to weather is mainly due to wind variability while demand sensitivity is driven by space heating or cooling loads. The interplay of these two weather-driven effects is of particular concern if demand spikes driven by low temperatures coincide with periods of low winds. In December 2009 and January 2010 Ireland experienced a prolonged spell of unusually cold conditions. During much of this time, wind generation output was low due to low wind speeds. The impacts of this event are presented as a case study of the effects of weather extremes on power systems with high penetrations of variable renewable generation.

2012-03-01T00:00:00Z Leahy, Paul G. McKeogh, Eamon J. http://hdl.handle.net/10468/709 2012-09-19T02:00:09Z 2012-01-01T00:00:00Z

Persistence of low wind speed conditions and implications for wind power variability Leahy, Paul G.; McKeogh, Eamon J. As the penetration of wind generation increases on power systems throughout the world, the effects of wind variability on power systems are of increasing concern. This study focuses on sustained occurrences of low wind speeds over durations ranging from 1 h to 20 days. Such events have major implications for the variability of energy yields from wind farms. This, in turn, influences the accuracy of wind resource assessment. The frequency analysis techniques commonly used to study wind variability cannot represent the autocorrelation properties of wind speeds and thus provide no information on the probabilities of occurrence of such sustained, low wind events. We present two complementary methods for assessing wind variability, runs analysis and intensity–duration–frequency analysis, both with emphasis on characterising the occurrence of continuous, extended periods (up to several days) of low wind speeds. Multi-annual time series of hourly wind speeds from meteorological stations in Ireland are analysed with both techniques. Sustained 20-day periods corresponding to extremely low levels of wind generation are found to have return periods of around 10 years in coastal areas. Persistent, widespread low wind speed conditions across the entire country are found to occur only rarely.

2012-01-01T00:00:00Z Arrigan, John Pakrashi, Vikram Basu, Biswajit Nagarajaiyah, Satish http://hdl.handle.net/10468/279 2011-04-15T02:00:04Z 2011-04-01T00:00:00Z

Control of flapwise vibrations in wind turbine blades using semi-active tuned mass dampers Arrigan, John; Pakrashi, Vikram; Basu, Biswajit; Nagarajaiyah, Satish The increased size and flexibility of modern multi-Megawatt wind turbines has resulted in the dynamic behaviour of these structures becoming an important design consideration. The aim of this paper is to study the variation in natural frequency of wind turbine blades due to centrifugal stiffening and the potential use of semi-active tuned mass dampers (STMDs) in reducing vibrations in the flapwise direction with changing parameters in the turbine. The parameters considered were the rotational speed of the blades and the stiffness of the blades and nacelle. Two techniques have been employed to determine the natural frequency of a rotating blade. The first employs the Frobenius method to a rotating Bernoulli-Euler beam. These results are compared with the natural frequencies determined from an eigenvalue analysis of the dynamic model of the turbine including nacelle motion, which is developed in this paper. The model derived considers the structural dynamics of the turbine and includes the dynamic coupling between the blades and tower. The semi-active control system developed employs a frequency-tracking algorithm based on the short-time Fourier transform technique. This is used to continually tune the dampers to the dominant frequencies of the system. Numerical simulations have been carried out to study the effectiveness of the STMDs in  reducing flapwise vibrations in the system when variations occur in certain parameters ofthe turbine. Steady and turbulent wind loading has been considered.

2011-04-01T00:00:00Z