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UNIVERSITY
OF MALTA INSTITUTE FOR ENERGY TECHNOLOGY Triq il-Port Ruman, Marsaxlokk, ZTN 09, Malta TEL: (+356) 21650675/21652249 FAX: (+356) 21650615 |
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UNIVERSITY
OF MALTA INSTITUTE FOR ENERGY TECHNOLOGY Triq il-Port Ruman, Marsaxlokk, ZTN 09, Malta TEL: (+356) 21650675/21652249 FAX: (+356) 21650615 |
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Wind Power and the Local
Wind Potential
Ing. Robert N. Farrugia
BACKGROUND Mankind’s quest for ways and means to harness nature’s forces has been going on since time immemorial. The first documented reference to windmills date back to at least 644 AD where wind-driven sails mounted on a vertical main shaft were employed to grind grain in ancient Persia. In principle, these primitive wind machines remained unaltered until the twelfth century when the horizontal axis Dutch-type windmill made its appearance in both France and England. These machines were to undergo only superficial changes for quite some time. The next milestone for wind driven devices was brought about by the western type wind pumps associated with the new continent’s pioneering days. Locally, windmills were used for grinding corn till early last century whilst water pumps are still much in evidence across our rural landscapes. The first ‘modern’ windmills capable of producing electricity were designed and built in Denmark at the end of the 19th century. By 1908, several hundred wind turbines rated at anything between 5 and 25 kW were operational in that country. The technology also bloomed in rural North America prior to 1930’s, when the Rural Electrification Act allowed farmers to access cheap and convenient fossil-fueled electricity. More than six million small-scale wind machines were installed producing a cumulative 1 million MWh of energy as early as 1860. One of the first true forerunners of modern Wind Energy Conversion Systems (WECS) was built and operated on the Black Sea Coast back in 1931. The 30-metre diameter rotor was fixed on top of a 30 metre lattice tower. At a rated wind speed of 11 m/s, this particular machine was capable of producing 100 kW [1]. What probably gave wind energy its greatest boost was the energy crisis of 1973. The oil embargo and the quadrupling of prices on oil imported to the United States imposed a quest for new electricity generation methods. Wind along with other renewables found increasing acclaim and support. Myriad machines in all shapes and sizes were designed and erected. Amongst the more familiar are the vertical axis Savonius and Darrieous types. With horizontal-axis machines, the array of models was equally mind-boggling with multi- or single-bladed machines, upwind and down-wind designs. Turbines had generators rated from a few Watts up to 3 MW. The smaller machines found favour in battery charging stand-alone mode at locations detached from the utility grid. The larger variants were connected directly to the utility grid in what is termed grid-connected mode. The stand-alone scenario remains practically unchanged although the larger prototype behemoths were to suffer greater setbacks within the first few years of operation, eventually ending up in the inevitable scrap heap. These tottering and painful first steps induced wind designers to go for smaller and better-understood designs. At present, the typical medium-sized wind turbine consists of a tubular steel (preferable in Europe) or lattice tower (more favoured in U.S.A.), two or three bladed rotors, and generators rated between 500 and 750 kW. Also worth mention is the fact that the last few years have seen a resurgence of the MW-class machine. Expertise gained on R & D of medium size turbines has now pushed a number of wind turbine manufacturers to produce models rated at up to 2.5 MW. These veritable power plants are targeted for offshore wind generation projects. FIELD STUDIES There is the popular conception that the Maltese Islands enjoy more than their fair share of wind. Unfortunately, it is not realistic to assess the quantity and quality of wind solely on personal experiences and observations. Historical evidence should however augur well if only due to the number of wind mills and water pumps which still dot our villages and countryside. Concrete and definitive results indicating the feasibility of wind energy systems in the local contest could only be attained through a scientific study to assess the islands’ wind resource. Wind parameters are highly site specific and locations within a few hundred meters of each other may enjoy a different characteristic wind finger-print. It is this long-term wind climate, which effectively dictates whether a site is attractive for wind energy conversion applications or not. The Institute for Energy technology of the University of Malta has, as one of its key objectives, the prerequisite of evaluating renewable energy sources in the local context. To further this aim, a wind monitoring programme was initiated in mid-1995 whereby a number of masts around the islands were rigged with wind monitoring equipment [2]. Dedicated anemometer and wind vane clusters were mounted at different heights above ground level to measure wind speed and direction with specific sampling periods. Key parameters recorded include 10-minute mean wind speeds, standard deviation, average deviation, maximum gust and direction of maximum gust amongst others. The data gathering process was made all the much easier and cost effective by the use of data loggers in the field. At all locations wind parameters were monitored at 10 metres above ground level, which is the World Meteorological Organisation’s (WMO’s) standard. Other heights were also monitored, these being chosen so as to gain a better insight to conditions within the lower boundary layer. Wind data was taken at 25 metres, which is a typical height for small wind turbines and 45 metres above ground level. The latter height is more representative of the hub height of typical medium-sized wind machines. MATHEMATICAL MODELLING In order to cover as many locations as possible, the study also employed established computer software. The WasP programme is a modelling package with great and proven capabilities in this respect. The variation of the wind climate may be attributed to a combination of factors such as the position of the reference site in relation to the prevailing wind direction, obstacles, terrain roughness, as well as the monitoring height. Working with long-term records from a meteorological station as a reference, the WasP methodology allows for data cleansing from these localised effects, to generate a ‘virgin’ or geostrophic wind climate. This data is especially important as it can subsequently be transposed to the candidate sites under scrutiny. This capability allows the wind engineer to generate the predicted wind climate for candidate sites within a reasonable radius around the reference meteorological station. The results emanating from the WasP analyses were further consolidated and correlated by actual on-site measurements at selected candidate sites. This methodology was established so as to determine the operational characteristics of WasP in the local context [3]. WIND TURBINE PERFORMANCE ESTIMATES Based upon the results of this research project, it is evident that there are quite a number of locations on the Maltese Islands enjoying appreciable wind characteristics. A single typical 600 kW turbine erected at one of the best locations would produce an estimated 1.7 GWh per year with a capacity factor of 30%. These estimates were derived from measured local wind data in combination with a typical turbine power curve. Turbine availability (due to maintenance), transformer/line losses, wake effects, turbulence and blade contamination were all considered in this analysis. From this data it is also possible to calculate the total annual down time, which cumulatively adds up to 13%. One of the most attractive results, which favour wind energy applications in Malta is that the cost of energy generated is comparable to that of conventional power stations. OFFSHORE WIND GENERATION Another growing offspring of the technology is offshore wind generation. Offshore wind farms are gaining ground, and although the costs related to such an installation are still relatively higher than that of the land based counterpart, land availability and other socio-environmental factors may make this option a very viable alternative. Offshore wind turbines may be larger in size with production models rated between 1 to 2.5 MW now coming into the market. Larger machines can be used at sea as the impact of higher towers and larger rotors are reduced. Wear and tear on off-shore machines is also less due to lower turbulence levels and steadier wind speeds off the coast. CONCLUSIONS Electrical power is essential for a country like Malta, which is constantly upgrading its infrastructure and with ever improving standards of living. The need for more and more installed generating capacity will continue if the trends over the past few years persist. Unfortunately conventional electricity generation causes environmental pollution and subsequently has a detrimental impact on the ecosystem. There will be a time when every country must conform to conventions and directives on pollution levels, to the benefit of all mankind. Whilst none of the renewables can fully replace conventional generation methods, it is equally evident that technologies such as wind power will have an important and significant role to play in our country’s future generation mix. REFERENCES: [1] Windpower (1981), Daniel Hunt V., Van Nostrand Reinhold, New York, N.Y., U.S.A. 1981 [2] A Wind Resource Assessment for Malta (1998), Farrugia R.N., University of Malta, Malta, 1998 [3] A Wind Resource Assessment for the Maltese Islands (1999), European Wind Energy Conference and Exhibition, Nice, France, 1999. |
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