A.J.M. van Wijk

The attainment of target position during step-tracking movements despite a shift of initial position.

SummarySubjects performed a step-tracking forearm movement at maximum velocity without visual guidance. A considerable shift of initial forearm position, which remained unnoticed by the subject, was induced by vibration of the biceps tendon. Notwithstanding the shift of initial position subjects attained the correct final position, irrespective of whether the vibration was switched off before the movement or continued during the movement. Recordings of biceps and triceps electromyograms show that muscle activities were programmed to produce a movement from the actual initial position to the target position. The findings indicate that correct information on forearm position is available to the central nervous system at a subconscious level even if position perception is disturbed through vibration.

Analysis of wind speed observations over the North Sea

Abstract Wind speed observations of three offshore platforms over the period 1985–1992 have been analysed. These platforms located in the southern North Sea off the Dutch coast, are: K13 Platform (K13), Euro Platform (EPF) and Measuring Post Noordwijk (MPN). First, a statistical analysis of the data was performed, including calculation of mean wind speeds, availability of data, annual and diurnal variations, the Weibull parameters and the distribution by wind direction. Then, in order to compare the wind speeds of the platforms measured at different heights, Monin-Obukhov similarity theory was applied. Therefore, air and seawater temperature data were used from observations on the platforms (if available) or from Voluntary Observing Ships. The seawater temperatures were fitted to a sine function. Although, on a yearly average, (very) unstable conditions prevail, the calculated wind profiles showed hardly any difference between the logarithmic and the diabatic wind profile. On a seasonal basis, however, the logarithmic and diabatic wind profile differ significantly, for the spring season in particular. Wind speeds increased with increasing distance from the coast.

Evaluation of stability corrections in wind speed profiles over the North Sea

An improved meteorological method for estimating the wind speed profile over sea is developed. In the proposed “diabatic” method, the Monin-Obukhov similarity theory, which takes into account the stability of the planetary boundary layer, is applied. By choosing parameterization of several components, the over-sea profile parameters (u∗, θ∗, q∗, z0and L) can be expressed in terms of routine weather data only (wind speed, sea water temperature and air temperature). With these parameters the wind speed as a function of height can be calculated. The diabatic method has been tested using a 4-month data-set consisting of wind speeds measured at heights of 18 and 70 m and temperatures of the air and the sea water. The mean wind speed measured at a height of 70 m over this period was 9.6 m s−1. Using the diabatic method the estimated mean wind speed at the same height was 9.3 m s−1, with an r.m.s. difference of 1.1 m s−1. The estimated mean wind speed using the logarithmic profile (only neutral stratification) was 8.7 m s−1 with an r.m.s. difference of 1.4 m s−1. This shows that the proposed diabatic method gives a better estimate of the mean wind speed than does the logarithmic method. The diabatic method also yields a more accurate estimate of the frequency distribution of the wind speed as characterized by the Weibull shape and scale parameters and of the average diurnal variation in the wind speed. If the two methods are applied to a 4-year period, it is found that the mean wind speed estimated by each method is almost the same. However, the diabatic method gives a much better simulation of the mean seasonal wind speed.

Capacity credit of wind power in the Netherlands

Abstract The Dutch Government has stated that by the year 2000 a total amount of 1000 MW wind power should be installed in the Netherlands. The penetration of wind power into the electricity supply system poses questions about the costs and benefits of wind power. One of the parameters affecting the benefits is the amount of conventional capacity that can be saved by wind power, the so-called ‘capacity credit’. In this study the capacity credit of wind power in the Netherlands is analysed. The capacity credit is calculated using a probabilistic method which evaluates the loss of load expectation (LOLE) of the total electricity generating system. In these evaluations the available wind power is treated as ‘negative load’. The capacity credit is evaluated with respect to the Dutch electricity generating system and the electricity demand that is projected for the year 2000 by the Dutch utilities. Special attention is given to modelling the hourly wind power production. The model incorporates detailed siting information, wind speed data for several meteorological stations and the power curves of five different types of wind turbines. The average amount of electricity produced by wind power can be expressed by the capacity factor. For the set of assumptions and for the meteorological conditions for the years investigated the capacity factor has a value of 22%. The mean value of the capacity credit of 1000 MW of wind power is calculated to be 184 MW or 18.4% of the installed wind turbine capacity. This value depends on several parameters, the influence of which is investigated. It is shown that there is a strong correlation between the capacity factor of wind power and the related capacity credit. The capacity credit is also influenced by the penetration level of wind power. The capacity credit, expressed as a percentage of installed wind turbine capacity, decreases with increasing wind turbine capacity. When 100 MW of wind turbine capacity has been installed the capacity credit is 28%. For 2000 MW of wind power this value is 13.6%. Furthermore, it is shown that geographical dispersal of the wind turbines over the Netherlands has a positive effect on the capacity credit. The capacity credit of 1000 MW dispersed over the Netherlands is 34 MW higher than the capacity credit of 1000 MW imaginarily installed at one location.

Optimization of the final waste treatment system in the Netherlands

Abstract The potential for optimizing, in both economic and energetic terms, the final waste treatment system in the Netherlands is evaluated in the light of the performance of new technologies. Projections of the final waste supply and waste treatment technologies are combined to construct several scenarios for waste treatment in the year 2010. Technologies include processes currently in the demonstration or pilot phase. It is concluded that final waste treatment could be performed at lower cost and with substantially greater energy recovery than at present. In a minimum cost scenario, the final waste treatment might cost 300–600 MECU/year, compared to 1000–1600 MECU/year in a reference scenario, on the assumption that conventional, but improved waste treatment technologies are used. A maximum energy recovery scenario might save 80–90 PJ primary energy per year compared to 39–47 PJ/year for the reference case. Two major competing technologies are gasification, both for biomass waste and integral waste, and fluidized bed incineration. Further development of these technologies integrated with electricity production is recommended.

Reliability analysis of generating systems including intermittent sources

Abstract A new method for the calculation of expected unserved energy of large-scale power systems including intermittent energy sources like hydro power units, wind turbines and photovaltaic units is presented. The correlation between hourly load and intermittent energy supply is accurately accounted for by the hourly application of a mean capacity outage table. The propose method is efficient, because it eliminates the need for hourly computation of a system negative margin table. In addition, expressions are given for computing the loss of load expectation and loss of load frequency. The method is applied to the IEEE reliability test system which is extended with intermittent sources in the form of wind turbine units.

Potentials for electricity production from wood in Ireland

Greenhouse gas emissions in Ireland are expected to be 25% higher in 2010 as compared to 1990, whereas Irelands Kyoto commitment is a maximum rise of 13%. One of the measure taken to reduce this gap (about 7Mtonne CO2-eq.) could be the development of biomass based electricity production. This study focusses on various biomass resources and a range of conversion technologies, in order to select promising routes for biomass based electricity in the short term. This article extends previous analyses by: (i) including the use of forest residues and sawmill residues as potential biofuels; (ii) considering both co-firing in retrofitted existing peat stations and co-firing in the new to be built Europeat power plant; and (iii) including other promising conversion technologies to generate electricity from biomass. Although the potential for energy crops in Ireland could be large, their cost is relatively high, 4.7–8.2 GJ−1, dependent on the type of land used. Lower cost biomass can be obtained from currently unutilised forest residues. It was estimated that about 3.4PJ could be extracted from the forests at a cost of 2–3 GJ−1. The present market price of sawmill residues (in total about 7PJ) is 1.4–3.5 GJ−1. Three promising routes for electricity generation were selected: co-firing with peat in the new to be built Europeat power plant, small scale CHP generation with fixed bed gasifiers, and co-firing in the Moneypoint coal fired plant. Co-firing in the Europeat plant is the most attractive. The electricity production costs are 0.033–0.053 kW−1h−1 as compared to 0.041 kW−1h−1 for peat based electricity. Costs per avoided tonne of CO2 are between −7 and 12 . Electricity generation from wood could fill about 14% (about 1.0Mtonne CO2-eq.) of the Irish ‘Kyoto gap’.

Analysis of wind speed observations on the North Sea coast

Abstract As part of the objective to describe the wind climate of the North Sea and the coastal zone, specifically with regard to wind energy applications, we study the observations of two meteorological stations on the North Sea coast: Hoek van Holland (HVH) and IJmuiden (IJM). Statistical analysis of the 8-year datasets shows that the mean wind speeds are 7.8 m/s (at 18.5 m height) for IJM, and 7.6 m/s (at 15.0 m height) for HVH, respectively. The coastal observations are compared to the observations of three offshore platforms in the North Sea analysed in earlier work. It is found that diurnal variations are very similar in autumn and winter, but differ in spring and summer. Furthermore, wind speeds with fetch over sea do behave very similar at the coastal stations and offshore platforms. It is shown that the effects of thermal circulation leading to the sea breeze are distinctly present, at least up to 30 km offshore. These effects can be attributed to relatively low wind speeds (less than 7 m/s at 10 m height), and manifest themselves as a shift, on average, of wind directions from easterly to westerly directions during the day. Higher wind speeds (more than 7 m/s), most relevant for wind energy applications, compare well at coastal stations and offshore platforms.

The energy crop growth model SILVA: Description and application to Eucalyptus plantations in Nicaragua.

Abstract Energy crop yield is an important parameter for the performance of a biomass energy system, both from a financial and an environmental point of view. A crop growth model is a tool that can be used to estimate yields of energy crops in regions without present experience with these types of crops. This study describes the crop growth model SILVicultural Actual yield model (SILVA), which estimates potential and water-limited yield for a certain region on the basis of solar radiation and daily data for precipitation and pan-evaporation. With the help of already available commercial yields of the same energy crop at a reference location, it makes an estimate for the actual yield that can be expected. The model was applied on plantations of Eucalyptus camaldulensis established for electricity generation at the San Antonio sugar mill in Nicaragua. On the basis of the harvest results of about 10tonne dm ha −1 yr−1 at the reference location, the Nicaraguan sugar mill Victoria de Julio, it was estimated that the E. camaldulensis plantation at San Antonio could produce about 13tonne dm ha −1 yr−1, mainly because precipitation was higher. The data requirement of the model matched well with data availability in Nicaragua.

Description of the North Sea wind climate for wind energy applications

In The Netherlands it is foreseen that wind turbines will be installed on offshore locations in the North Sea before the year 2010. Therefore adequate knowledge of the offshore wind climate should be obtained, both for the estimation of energy yields and for the determination of wind load parameters. For this purpose databases of two offshore locations have been analyzed. One is the K13 platform, near the Dutch coast, with a database of 8 years with wind speed measurements at one height, the other is the West Sole platform near the English coast, with a database of only 1 year, but with both wind speed and turbulence intensity measurements at seven heights. It is concluded that at K13 the mean wind speed at 74.9 m height during the period 1982-1989 is 9.0 m/s, and that the Weibull form factor k is 2.04.