• Energy Research
• 14. Life underwater close
• Applied Energy close

The climatic trends of Wind Speed (WS) and wave height play a key role in wind and wave energy assessments, climate change analyses, and air-sea interactions, among many others. Using ERA5 reanalysis, this study reveals the climatic trends of global oceanic WS and wave height for 1979–2018, including the overall trends, regional and seasonal differences of the trends, with a special focus on the differences and similarities between the trends in wind-sea wave height (Hwind) and swell wave height (Hswell), as well as the contributions of climate indices to WS, Hwind, Hswell and Significant Wave Height (Hs) respectively, by employing linear regression and correlation analysis. The results show an overall global oceanic increase for 1979–2018 in WS (+0.47 cm/s/yr), with increases of +0.13, +0.28 and +0.32 cm/yr in Hwind, Hswell and Hs respectively, and a stronger increasing trend in the Southern Hemisphere than in the Northern Hemisphere. There is good agreement between the spatial distribution of annual and seasonal trends in WS and those of Hwind, as well as between Hswell and Hs. Areas with strong increasing trends of WS and Hwind are mainly located in the tropical South Indian Ocean and tropical Pacific Ocean. Hswell and Hs exhibit significant increases in most global oceans. The months with the broadest and strongest increase in Hswell and Hs are June-July-August (JJA). There is a close relationship among the WS, wave height climatology and the modes of climate variability. The wind has the strongest response to climate indices, followed by the wind-sea, with swell having a minimal response. This work was supported by the developing projection of Dalian Naval Academy, the open fund project of Shandong Provincial Key Laboratory of Ocean Engineering, Ocean University of China (No. kloe201901), the National Natural Science Foundation of China (No. 41490642). Cesar Azorin-Molina was supported by Ramon y Cajal fellowship (RYC-2017-22830) and the grants no. VR-2017-03780 and RTI2018-095749-A-I00 (MCIU/AEI/FEDER, UE).

Abstract This study examines the use of water-use fees in California’s bidding-based power markets to balance freshwater conservation and reduction of the marine ecosystem impact of coastal once-through-cooled power plants. An hourly power dispatch is simulated using the state’s 2014 demand and generation capacity data. Fees on ocean water withdrawals of $5–120/acre-ft are simulated in three scenarios that test the grid’s ability to simultaneously mitigate its impact on marine ecosystems, conserve freshwater, and incentivize recycled water use. Although fees modeled represent a small share of generator fuel costs, results show that they trigger declines in ocean water withdrawals of up to 11% that are almost always cost-effective if accounting for effects on system-wide fuel costs and CO2 emissions. An appropriately designed fee-structure reduces ocean water withdrawals by 9% without increasing freshwater consumption elsewhere. Wholesale electricity price increases of 5–10% are concentrated in Northern California, and marine ecosystem benefits are partly offset by increases in NOx and SO2 emissions inland. Overall, this study finds that water-use fees could be an effective strategy for reducing the marine ecosystem impacts of California’s power sector, particularly because they can also address short term fluctuations in freshwater scarcity. Keywords: Energy-water nexus, once-through cooling, scarce water, environmental pricing, energy policy, electricity dispatch, power systems.

Abstract EU directives RESD (2001/77/EC) and WFD (2000/60/EC) can be considered as partially conflicting. Achieving a good qualitative and quantitative status of waters, what presumes “non-deterioration principle” of the existing ecological status in line with WFD, is conflicting with the construction of new hydropower plants that are promoting renewable energies, what is in line with RESD. Several projects have been developed in order to minimize conflicts between the two Directives, often providing a list of key criteria to be taken into consideration when deciding on the impact minimization of new ones or certification to existing plants. One example is CH2OICE, aiming at developing a technically and economically feasible certification procedure for hydropower generation facilities of high environmental standard. This paper aims to evaluate applicability of multicriteria decision aid to decision makers during the design process, decisions on site selection and plant technical and operational parameters, based on both economic and environmental criteria selected.

Abstract An innovative system for recovering energy from tidal currents is proposed. The system is composed of a blade immersed in sea water and connected to a vertical bar which, moving up and down under the action of the sea current, transfers energy to a double effect pump. The latter feeds a pressurized reservoir providing a water flow rate, at suitable pressure, to a hydraulic turbine. The basic configuration involves a four-bar linkage connecting the vertical bar and the piston pump. The system can be employed in all the sites whose seabed quickly deepens and whose tidal currents are parallel to the coast. In this paper, the authors provide a methodology for designing the system, once found the bathymetry of the chosen site and the field of variation of the tidal current velocities, taking into account the morphology of the costal boundary layer. A case study, developed for the Calabrian site of Punta Pezzo (Strait of Messina – Italy), demonstrated the technical feasibility.

Abstract Better understanding of the global wave climate is required to inform wave energy device design and large-scale deployment. Spatial variability in the global wave climate is analysed here to provide a range of characteristic design wave climates. K-means clustering was used to split the global wave resource into 6 classes in a device agnostic, data-driven method using data from the ECMWF ERA5 reanalysis product. Classification using two sets of input data were considered: a simple set (based on significant wave height and peak wave period) and a comprehensive set including a wide range of relevant wave climate parameters. Both classifications gave resource classes with similar characteristics; 55% of tested locations were assigned to the same class. Two classes were low energy, found in enclosed seas and sheltered regions. Two classes were moderate wave energy classes; one swell dominated and the other in areas with wave action often generated by more local storms. Of the two higher energy classes; one was more often found in the northern hemisphere and the other, most energetic, predominantly on the tips of continents in the southern hemisphere. These classes match existing regional understanding of resource. Consideration of publicly available device power matrices showed good performance was primarily realised for the two highest energy resource classes (25–30% of potential deployment locations); it is suggested that effort should focus on optimising devices for additional resource classes. The authors hypothesise that the low-risk, low variability, swell dominated moderate wave energy class would be most suitable for future exploitation.

Abstract Natural gas hydrates in the South China Sea generally exists in clayey sediments, whose permeability is significantly affected by the swelling-shrinkage characteristics of clay that are caused by hydrate dissociation and reformation, and these characteristics influence the efficiency of gas production. In this study, montmorillonite with the most significant swelling characteristics and kaolinite with weak swelling characteristics in South China Sea sediments were investigated experimentally, respectively. The gas permeability was experimentally investigated for varying hydrate saturations during hydrate dissociation and reformation. The results showed the following: The clayey samples after hydrate dissociation had lower permeability than those without dissociation due to the clay particle swelling and the existence of bound water, but the swelling and hydrophilicity of kaolin are lower than for montmorillonite, which is confirmed by observation of SEM photography. Furtherly, the hydrate reformation improved the gas permeability of clayey samples to a certain extent except for those kaolin samples with initial ice saturations of 5% and 10% because of the increased pore space due to dehydration shrinkage of clay particle and the formation of typical aggregate structures, which is in contrast to the results for sand. Furthermore, the damage value of permeability after hydrate dissociation and the ratio of the permeability after reformation of hydrate to the initial permeability in both the montmorillonite and kaolin were obtained in this study.

Abstract As a point absorber, the solo Duck wave energy converter (WEC) shows high power capture efficiency within a narrow bandwidth around the natural period. In this paper, real-time latching control is applied to the solo Duck WEC in irregular waves to improve its performance in sea states away from the natural period. Two predictive latching control strategies, in which one is close-to-optimal and the other is sub-optimal, and one non-predictive strategy are considered. The improvement of the WEC performance due to latching control is studied. Compared to the performance under simple resistive control, the three latching control strategies show almost equivalent control effect, leading to an average increase of the maximum relative capture width by around 70% and an average decrease of the optimal power take-off (PTO) damping coefficient by around 60%. Since the non-predictive strategy requires no prediction of future excitation force and WEC motion, it can be identified as the best choice for the solo Duck WEC under latching control. Although latching control leads to significant increase of fatigue load on the WEC hull like other advanced controls, it does not cause additional fatigue damage to the PTO.

Abstract This study describes the modelling of the thermal behaviour of aquacultural ponds through nodal discretisation and the use of correlation formulae for exchanges at the water-air interface. The direct resolution of the resulting system was effected by an interconnected numerical method. The experiments carried out on a test pond provided the database necessary for refining the model. The satisfying comparison of experimental and simulated responses for the pond water justify the choice of theoretical approach. A data-processing tool was developed on a PC computer to determine the water temperature of a parallelepiped pond of variable geometry and composition, subjected to different demands and with or without thermal protection covering.