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Online ISSN 1752-1424 Print ISSN 1752-1416

IET Renewable Power Generation

Volume 6, Issue 3, May 2012

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Volume 6, Issue 3

May 2012

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    • Experimental study on a heat-pipe photovoltaic/thermal system
      • Author(s): H.D. Fu ; G. Pei ; T. Zhang ; H.J. Zhu ; J. Ji
      • Source: IET Renewable Power Generation, Volume 6, Issue 3, p. 129 –136
      • DOI:  10.1049/iet-rpg.2011.0142
      • Type: Article
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      • p. 129 –136 (8)
        In this study, a novel heat-pipe photovoltaic/thermal (HP-PV/T) collector is proposed. Compared with the traditional water-type PV/T collector, a heat-pipe photovoltaic/thermal collector does not freeze when used in cold regions by careful selection of a heat-pipe working fluid. Two types of heat-pipe photovoltaic/thermal collectors, with the tube spaces of heat pipes at 80 and 140 mm, were constructed. Two individual heat-pipe photovoltaic/thermal systems using each of the two types of collectors were also assembled. Outdoor tests were performed to study the performance of the two systems with solar collectors installed at tilt angles of 32 and 45°. A semi-empirical system efficiency model with variable (Ti–T̄a)/Ht was used to correlate the results of the daily thermal efficiency, total PV/T efficiency and primary-energy-saving efficiency. Comparison analyses between the two systems and between the two solar collectors tilt angles were also carried out.
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      Integrated structural and electromagnetic design of direct-drive linear machines for wave energy
      • Author(s): R. Crozier  and  M. Mueller
      • Source: IET Renewable Power Generation, Volume 6, Issue 3, p. 137 –148
      • DOI:  10.1049/iet-rpg.2011.0002
      • Type: Article
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      • p. 137 –148 (12)
        Combined electrical and structural models of four types of permanent magnet linear electrical machines suitable for direct-drive power take-off in wave energy applications are presented. Electromagnetic models were developed using polynomial approximation to finite-element analysis results. The structural models are based on simple beam theory and other classical techniques. Unoptimised example designs are provided for each machine using the integrated tools, demonstrating their potential usefulness in an integrated optimisation process.
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      Experimental results from the operation of aggregated wave energy converters
      • Author(s): M. Rahm ; O. Svensson ; C. Boström ; R. Waters ; M. Leijon
      • Source: IET Renewable Power Generation, Volume 6, Issue 3, p. 149 –160
      • DOI:  10.1049/iet-rpg.2010.0234
      • Type: Article
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      • p. 149 –160 (12)
        Wave energy comes in pulses and is unsuitable for direct conversion and transmission to the grid. One method to smooth the power is to deploy arrays of wave energy converters (WECs), the geometrical layout and damping optimisation of which many have studied analytically and numerically, but very few by experiments at sea. In this study, the standard deviation of electrical power as function of various parameters is investigated. Two offshore experiments have been conducted. During the longer run, three WECs were operated in linear damping during 19.7 days. It is shown that the standard deviation reduces with the number of WECs in the array up to three WECs. The reduction compared to single WEC operation was found here to be 30 and 80% with two and three WECs, respectively, as a mean for an arbitrary array member. It is found that in sea states above ∼2 kW/m, the standard deviation is independent of sea state parameters. This is contradictory to a previous study on the same device. The results are, however, in accordance with numerical results of the SEAREV device but show larger reduction in standard deviation with number of WECs. This could be because of suboptimal damping conditions.
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      Designing the C-GEN lightweight direct drive generator for wave and tidal energy
      • Author(s): O. Keysan ; M. Mueller ; A. McDonald ; N. Hodgins ; J. Shek
      • Source: IET Renewable Power Generation, Volume 6, Issue 3, p. 161 –170
      • DOI:  10.1049/iet-rpg.2009.0213
      • Type: Article
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      • p. 161 –170 (10)
        The C-GEN is a novel permanent magnet generator aimed at reducing overall system mass in direct drive power takeoff applications. The design of a C-GEN generator requires the combination of electromagnetic, structural and thermal models. Models used in the development of design tools applicable to both rotary and linear C-GEN generators are described in this study. The design tool is verified with the experiment results obtained from a 15 kW prototype. A genetic optimisation algorithm is developed combining the analytical model with economical issues to search for most suitable designs for specific applications. Designs are presented using the optimisation design tool for two marine renewable applications: a wave device called Oyster developed by Aquamarine Power and a tidal current device developed by Scotrenewables.
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      Multi-phase air-cored tubular permanent magnet linear generator for wave energy converters
      • Author(s): N.P. Gargov  and  A.F. Zobaa
      • Source: IET Renewable Power Generation, Volume 6, Issue 3, p. 171 –176
      • DOI:  10.1049/iet-rpg.2011.0190
      • Type: Article
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      • p. 171 –176 (6)
        Direct driven permanent magnet linear generators (PMLGs) are an alternative solution for wave energy converters (WECs). Generally, problems such as high magnetic attraction forces between the permanent magnets and the magnetic core are associated with direct driven PMLG. To eliminate the attraction, air-cored generators can be used. They do not contain any stainless steel in either the stator or the rotor and therefore there is no magnetic attraction between the moving and the stationary parts. In this study, a novel design of multi-phase air-cored PMLG is proposed. The main advantage of the generator is the reduction in the Lorentz forces acting on the bearings by addressing the force in the direction parallel to the motion axis and elimination of cogging forces. Additionally, in the study a new system bypassing inactive coils is proposed and simulated as part of the grid integration system. The system achieves implementation of a small number of elements connected in series with the coils and hence the thermal losses in the grid integration system are reduced. All simulations are made by means of finite-element (FE) software working simultaneously with Matlab/Simulink.
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      Thermo-mechanical stress in tubular solid oxide fuel cells: Part I – transient operating behaviour and the relevance of material creep
      • Author(s): K. Fischer  and  J.R. Seume
      • Source: IET Renewable Power Generation, Volume 6, Issue 3, p. 177 –193
      • DOI:  10.1049/iet-rpg.2011.0095
      • Type: Article
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      • p. 177 –193 (17)
        A spatially discretised thermo-electrochemical model is developed to calculate the temperature distribution in a tubular solid oxide fuel cell (SOFC). Model validation is accomplished based on the operating data from a demonstration plant. Using a mechanical model of the ceramic membrane-electrode assembly, the distribution of thermo-mechanical stress is calculated from the temperature profile. The resulting risk of fracture failure, being one of the crucial life-limiting factors of SOFC, is determined by means of Weibull analysis. The methodology and results are presented in two parts: Part I covers the dynamic operating properties of the SOFC and the time scale of material creep in its ceramic components. Part II deals with the risk of fracture failure related to transient operating scenarios, discusses its dependency on the operating conditions and derives a low-risk operating strategy. The dynamic operating behaviour is found to be dominated by the large thermal inertia of the solid cell components. An analysis of the creep relaxation indicates a significant relief of mechanical stress in the electrodes within a few hours of operation. This justifies a novel assumption regarding the stress-free state in the mechanical analysis of the fuel cell, which significantly increases the plausibility of the resulting risk of fracture failure.
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      Thermo-mechanical stress in tubular solid oxide fuel cells: Part II – Operating strategy for reduced probability of fracture failure
      • Author(s): K. Fischer  and  J.R. Seume
      • Source: IET Renewable Power Generation, Volume 6, Issue 3, p. 194 –205
      • DOI:  10.1049/iet-rpg.2011.0109
      • Type: Article
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      • p. 194 –205 (12)
        A spatially discretised thermo-electrochemical model is developed to calculate the temperature distribution in a tubular solid oxide fuel cell (SOFC). This is used in a mechanical model to compute the distribution of thermo-mechanical stress in the ceramic membrane-electrode assembly of the cell. The resulting risk of fracture failure is determined by means of Weibull analysis. Part I of this work covers the dynamic operating properties of the SOFC and the time scale of material creep in its ceramic components. This work, Part II, deals with the risk of fracture failure related to transient operating scenarios, discusses its dependency on the operating conditions and derives a low-risk operating strategy. Contrary to the common perception, thermal gradients are found to have little impact on thermo-mechanical stress in the studied SOFC. Failure-relevant stress levels arise merely due to thermal mismatch of the ceramic layers. Regarding the operating strategy, the dynamics of changes in operating conditions are of minor importance for the resulting risk of failure, while operating strategies aiming at a constant mean cell temperature prove to be advantageous. The consideration of material creep is shown to be essential for a sound analysis of thermo-mechanical stress and risk of fracture in the investigated SOFC.
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