An Investigation into Power from Pitch-Surge Point-Absorber Wave Energy Converters.

There is a worldwide opportunity for clean renewable power. The results from the UK Government's "Marine Energy Challenge" showed that marine energy has the potential to become competitive with other forms of energy. The key to success in this lies in a low lifetime-cost of power as delivered to the user. Pitch-surge point-absorber WECs have the potential to do this with average annual powers of around 2 MW in North Atlantic conditions from relatively small devices that would be economically competitive with other technologies and would be relatively easy to install and maintain. The paper examines the factors governing the performance of such devices and outlines their underlying theory Preliminary laboratory test results from a 1/100 scale pilot design are presented. It is hoped that more extensive development work will follow these promising early results. Engineering designs for devices based on these findings are outlined

Solar cycle variations of large frequency separations of acoustic modes: Implications for asteroseismology

We have studied solar cycle changes in the large frequency separations that can be observed in Birmingham Solar Oscillations Network (BiSON) data. The large frequency separation is often one of the first outputs from asteroseismic studies because it can help constrain stellar properties like mass and radius. We have used three methods for estimating the large separations: use of individual p-mode frequencies, computation of the autocorrelation of frequency-power spectra, and computation of the power spectrum of the power spectrum. The values of the large separations obtained by the different methods are offset from each other and have differing sensitivities to the realization noise. A simple model was used to predict solar cycle variations in the large separations, indicating that the variations are due to the well-known solar cycle changes to mode frequency. However, this model is only valid over a restricted frequency range. We discuss the implications of these results for asteroseismology.Comment: 9 pages, 11 figures, accepted for publication in MNRAS, references updated, corrections following proof

Reliability of P mode event classification using contemporaneous BiSON and GOLF observations

We carried out a comparison of the signals seen in contemporaneous BiSON and GOLF data sets. Both instruments perform Doppler shift velocity measurements in integrated sunlight, although BiSON perform measurements from the two wings of potassium absorption line and GOLF from one wing of the NaD1 line. Discrepancies between the two datasets have been observed. We show,in fact, that the relative power depends on the wing in which GOLF data observes. During the blue wing period, the relative power is much higher than in BiSON datasets, while a good agreement has been observed during the red period.Comment: 7 pages, HELAS II: Helioseismology, Asteroseismology, and MHD Connections, conference proceedin

Misleading variations in estimated rotational frequency splittings of solar p modes: Consequences for helio- and asteroseismology

The aim of this paper is to investigate whether there are any 11-yr or quasi-biennial solar cycle-related variations in solar rotational splitting frequencies of low-degree solar p modes. Although no 11-yr signals were observed, variations on a shorter timescale (~2yrs) were apparent. We show that the variations arose from complications/artifacts associated with the realization noise in the data and the process by which the data were analyzed. More specifically, the realization noise was observed to have a larger effect on the rotational splittings than accounted for by the formal uncertainties. When used to infer the rotation profile of the Sun these variations are not important. The outer regions of the solar interior can be constrained using higher-degree modes. While the variations in the low-l splittings do make large differences to the inferred rotation rate of the core, the core rotation rate is so poorly constrained, even by low-l modes, that the different inferred rotation profiles still agree within their respective 1sigma uncertainties. By contrast, in asteroseismology, only low-l modes are visible and so higher-l modes cannot be used to constrain the rotation profile of stars. Furthermore, we usually only have one data set from which to measure the observed low-l splitting. In such circumstances the inferred internal rotation rate of a main sequence star could differ significantly from estimates of the surface rotation rate, hence leading to spurious conclusions. Therefore, extreme care must be taken when using only the splittings of low-l modes to draw conclusions about the average internal rotation rate of a star.Comment: 10 pages, 7 figures, accepted for publication in MNRA

Control systems for WRASPA.

The paper discusses the need for a wave energy converter (WEC) to sense and respond to its environment in order to survive and to produce its maximum useful output. Such systems are described for Wraspa, a WEC being developed at Lancaster University and first reported at ICCEP in 2007. The main control system that continually monitors and optimises the power-take-off is termed ldquoStepwise Controlrdquo and seeks to continually adjust the damping force applied to the collector to suit the wave force that drives it. The complete instrumentation and control system that will be needed is considered briefly, including the above PTO control system; direction sensing and heading control; tide level compensation; condition monitoring and provisions for access and maintenance

The Octave (Birmingham - Sheffield Hallam) automated pipeline for extracting oscillation parameters of solar-like main-sequence stars

The number of main-sequence stars for which we can observe solar-like oscillations is expected to increase considerably with the short-cadence high-precision photometric observations from the NASA Kepler satellite. Because of this increase in number of stars, automated tools are needed to analyse these data in a reasonable amount of time. In the framework of the asteroFLAG consortium, we present an automated pipeline which extracts frequencies and other parameters of solar-like oscillations in main-sequence and subgiant stars. The pipeline uses only the timeseries data as input and does not require any other input information. Tests on 353 artificial stars reveal that we can obtain accurate frequencies and oscillation parameters for about three quarters of the stars. We conclude that our methods are well suited for the analysis of main-sequence stars, which show mainly p-mode oscillations.Comment: accepted by MNRA

A new efficient method for determining weighted power spectra: detection of low-frequency solar p-modes by analysis of BiSON data

We present a new and highly efficient algorithm for computing a power spectrum made from evenly spaced data which combines the noise-reducing advantages of the weighted fit with the computational advantages of the Fast Fourier Transform (FFT). We apply this method to a 10-year data set of the solar p-mode oscillations obtained by the Birmingham Solar Oscillations Network (BiSON) and thereby uncover three new low-frequency modes. These are the l=2, n=5 and n=7 modes and the l=3, n=7 mode. In the case of the l=2, n=5 modes, this is believed to be the first such identification of this mode in the literature. The statistical weights needed for the method are derived from a combination of the real data and a sophisticated simulation of the instrument performance. Variations in the weights are due mainly to the differences in the noise characteristics of the various BiSON instruments, the change in those characteristics over time and the changing line-of-sight velocity between the stations and the Sun. It should be noted that a weighted data set will have a more time-dependent signal than an unweighted set and that, consequently, its frequency spectrum will be more susceptible to aliasing.Comment: 11 pages, 7 Figures, accepted for publication in MNRAS, Figure 6 had to be reduced in size to upload and so may be difficult to view on screen in .ps versio