This paper presents the geometry optimization of a floating oscillating water column (OWC). The device consists of a floater pierced by a small thickness tube open at the bottom to the sea water and ...at the top to the OWC chamber. The dimensions of the floater and tube are optimized in order to maximize the wave energy extraction under certain geometric constraints. The formulation considers linear water wave theory. A boundary element method code is used to calculate the hydrodynamic coefficients. The floater and the water column are assumed to oscillate only in heave. The top of the water column is modeled as a piston. The compressibility effect of the air inside the chamber is accounted for. A Wells turbine with a linear characteristic curve is considered as a power take-off system. The system is modeled in the frequency domain, assuming two degrees of freedom. The power extraction from real sea waves is simulated through a stochastic model, using an energy spectrum and the wave climate conditions off the western coast of Portugal. The dimensions of the floating OWC are optimized using two distinct optimization algorithms. Results have shown that the diameter of the floater, the submerged length and the air chamber height have a large impact on the annual average power extraction.
► Axisymmetric floating oscillating water column with variable tail tube cross section. ► Optimization of geometry and turbine damping for a given wave climate. ► Use of a frequency domain and stochastic model of wave energy absorption. ► Optimal geometries were found applying gradient-free methods. ► Floater diameter and submerged length have a large impact on the power extraction.
The objective of this paper is to present the wave energy resource assessment having in view the construction of an OWC (oscillating water column) to be integrated into a new breakwater at the mouth ...of the Douro River in Porto (northern Portugal). The paper starts with the evaluation of the offshore wave energy resource from measured data. This is followed by the wave transformation from offshore conditions to the plant location in about 11 m water-depth. Such transformation was carried out using an inverse-ray refraction model that is described in detail. As expected, the wave power level is lower, whereas the wave energy period is higher, at the plant coastal site as compared with deep water conditions. The average wave direction rotates from approximately NW offshore to nearly West at the plant location, and the directional spread becomes smaller, which are effects of refraction as the waves propagate in waters of decreasing depth.
•Evaluation of the offshore wave energy resource from measured data.•Inverse-ray refraction model to compute the wave energy resource at the shoreline.•Wave energy resource for a breakwater-integrated oscillating-water-column.
The Bacillus subtilis endospore coat protein CotA shows laccase activity. By using comparative modeling techniques, we were able to derive a model for CotA based on the known x-ray structures of ...zucchini ascorbate oxidase and Cuprinus cereneus laccase. This model of CotA contains all the structural features of a laccase, including the reactive surface-exposed copper center (T1) and two buried copper centers (T2 and T3). Single amino acid substitutions in the CotA T1 copper center (H497A, or M502L) did not prevent assembly of the mutant proteins into the coat and did not alter the pattern of extractable coat polypeptides. However, in contrast to a wild type strain, both mutants produced unpigmented colonies and spores unable to oxidize syringaldazine (SGZ) and 2′2-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS). The CotA protein was purified to homogeneity from an overproducing Escherichia coli strain. The purified CotA shows an absorbance and a EPR spectra typical of blue multicopper oxidases. Optimal enzymatic activity was found at ≤pH 3.0 and at pH 7.0 for ABTS or SGZ oxidation, respectively. The apparent Km values for ABTS and SGZ at 37 °C were of 106 ± 11 and 26 ± 2 μm, respectively, with corresponding kcat values of 16.8 ± 0.8 and 3.7 ± 0.1 s−1. Maximal enzyme activity was observed at 75 °C with ABTS as substrate. Remarkably, the coat-associated or the purified enzyme showed a half-life of inactivation at 80 °C of about 4 and 2 h, respectively, indicating that CotA is intrinsically highly thermostable.
A
bstract
The NEXT experiment aims at searching for the hypothetical neutrinoless double-beta decay from the
136
Xe isotope using a high-purity xenon TPC. Efficient discrimination of the events ...through pattern recognition of the topology of primary ionisation tracks is a major requirement for the experiment. However, it is limited by the diffusion of electrons. It is known that the addition of a small fraction of a molecular gas to xenon reduces electron diffusion. On the other hand, the electroluminescence (EL) yield drops and the achievable energy resolution may be compromised. We have studied the effect of adding several molecular gases to xenon (CO
2
, CH
4
and CF
4
) on the EL yield and energy resolution obtained in a small prototype of driftless gas proportional scintillation counter. We have compared our results on the scintillation characteristics (EL yield and energy resolution) with a microscopic simulation, obtaining the diffusion coefficients in those conditions as well. Accordingly, electron diffusion may be reduced from about 10 mm/
m
for pure xenon down to 2.5 mm/
m
using additive concentrations of about 0.05%, 0.2% and 0.02% for CO
2
, CH
4
and CF
4
, respectively. Our results show that CF
4
admixtures present the highest EL yield in those conditions, but very poor energy resolution as a result of huge fluctuations observed in the EL formation. CH
4
presents the best energy resolution despite the EL yield being the lowest. The results obtained with xenon admixtures are extrapolated to the operational conditions of the NEXT-100 TPC. CO
2
and CH
4
show potential as molecular additives in a large xenon TPC. While CO
2
has some operational constraints, making it difficult to be used in a large TPC, CH
4
shows the best performance and stability as molecular additive to be used in the NEXT-100 TPC, with an extrapolated energy resolution of 0.4% at 2.45 MeV for concentrations below 0.4%, which is only slightly worse than the one obtained for pure xenon. We demonstrate the possibility to have an electroluminescence TPC operating very close to the thermal diffusion limit without jeopardizing the TPC performance, if CO
2
or CH
4
are chosen as additives.
A
bstract
The measurement of the internal
222
Rn activity in the NEXT-White detector during the so-called Run-II period with
136
Xe-depleted xenon is discussed in detail, together with its ...implications for double beta decay searches in NEXT. The activity is measured through the alpha production rate induced in the fiducial volume by
222
Rn and its alpha-emitting progeny. The specific activity is measured to be (38.1 ± 2.2 (stat.) ± 5.9 (syst.)) mBq/m
3
. Radon-induced electrons have also been characterized from the decay of the
214
Bi daughter ions plating out on the cathode of the time projection chamber. From our studies, we conclude that radon-induced backgrounds are sufficiently low to enable a successful NEXT-100 physics program, as the projected rate contribution should not exceed 0.1 counts/yr in the neutrinoless double beta decay sample.
A
bstract
Natural radioactivity represents one of the main backgrounds in the search for neutrinoless double beta decay. Within the NEXT physics program, the radioactivity- induced backgrounds are ...measured with the NEXT-White detector. Data from 37.9 days of low-background operations at the Laboratorio Subterráneo de Canfranc with xenon depleted in
136
Xe are analyzed to derive a total background rate of (0.84
±
0.02) mHz above 1000 keV. The comparison of data samples with and without the use of the radon abatement system demonstrates that the contribution of airborne-Rn is negligible. A radiogenic background model is built upon the extensive radiopurity screening campaign conducted by the NEXT collaboration. A spectral fit to this model yields the specific contributions of
60
Co,
40
K,
214
Bi and
208
Tl to the total background rate, as well as their location in the detector volumes. The results are used to evaluate the impact of the radiogenic backgrounds in the double beta decay analyses, after the application of topological cuts that reduce the total rate to (0.25
±
0.01) mHz. Based on the best-fit background model, the NEXT-White median sensitivity to the two-neutrino double beta decay is found to be 3.5
σ
after 1 year of data taking. The background measurement in a Q
ββ
±
100 keV energy window validates the best-fit background model also for the neutrinoless double beta decay search with NEXT-100. Only one event is found, while the model expectation is (0.75
±
0.12) events.
A
bstract
In experiments searching for neutrinoless double-beta decay, the possibility of identifying the two emitted electrons is a powerful tool in rejecting background events and therefore ...improving the overall sensitivity of the experiment. In this paper we present the first measurement of the efficiency of a cut based on the different event signatures of double and single electron tracks, using the data of the NEXT-White detector, the first detector of the NEXT experiment operating underground. Using a
228
Th calibration source to produce signal-like and background-like events with energies near 1.6 MeV, a signal efficiency of 71
.
6
±
1
.
5
stat
±
0
.
3
sys
% for a background acceptance of 20
.
6
±
0
.
4
stat
±
0
.
3
sys
% is found, in good agreement with Monte Carlo simulations. An extrapolation to the energy region of the neutrinoless double beta decay by means of Monte Carlo simulations is also carried out, and the results obtained show an improvement in background rejection over those obtained at lower energies.
Although the latching control strategy has been recognized as an important mean of increasing the efficiency of one-body point-absorbing wave energy converters (WECs), its effectiveness in two-body ...floating point-absorbers has been questioned in some studies. The current work investigates the increase in annual absorbed energy achieved with a simple threshold unlatching control strategy when applied to a generic two-body heaving WEC. The WEC performance is evaluated for a set of sea-states characteristic of the wave climate off the Portuguese west coast.
To achieve this computationally intensive task, a new high-order numerical method for the solution of the Cummins equations is presented and used. This approach is based on a polynomial representation of the solution, whose coefficients are computed using a continuous least-squares approximation. The code has been parallelized and computations were performed at the IST cluster.
► Time-domain modelling of a two-body wave energy converter with latching control. ► Annual energy absorption maximization regarding a wave climate. ► Optimum mass ratio between the floater and the submerged body. ► New high-order numerical method for the solution of the Cummins equations. ► Continuous polynomial approximation of the solution using the least-squares method.
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