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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Innovative concepts for drive train components' taking place on Thursday, 13 March 2014 at 11:15-12:45. The meet-the-authors will take place in the poster area.

Daniel Buhagiar University of Malta, Malta
Co-authors:
Daniel Buhagiar (1) F P Tonio Sant (1)
(1) University of Malta, Msida, Malta

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Abstract

Combining offshore wind and thermocline energy production: going beyond electrical generation

Introduction

The viability of offshore wind energy is presently hindered by various issues associated with existing marinised wind turbine technology. These include gearbox failures [1], high dependency on copper [2] and expensive nacelle-based electronics [3]. There is today a consensus to implement radical changes in offshore wind turbine technology to reduce the costs of offshore wind power [4].

This paper deals with a novel concept whereby offshore wind turbines are utilised to concurrently exploit the thermal energy available in deep seas below the thermocline together with offshore wind, thus enabling the offshore turbines to enhance their energy yield.


Approach

The developed conceptual system, referred to as the Offshore Wind and Thermocline Energy Production (OWTEP) system, consists of a hydraulic-based offshore wind turbine that uses an open-loop pipeline to draw in cold seawater from below the thermocline layer and transmit it to shore for electrical generation and thermal energy extraction [5] [6]. A basic schematic of the system is shown in Figure 1.



This approach builds on work undertaken at the Delft University of Technology (TUDelft) [7] [8] on the use of hydraulic-based wind turbines that pump pressurised seawater to a centralised hydro-electric power station in lieu of directly generating electricity within the nacelle. There exist vast offshore areas, such as in the Central Mediterranean, where the viability of present offshore wind technology producing solely electricity is prohibitive due to relatively low to medium wind speeds and the availability of deep waters. In the subject work, it is shown that the OWTEP concept increases the total energy yield significantly under such conditions. A case study is taken in the Central Mediterranean Island of Malta where a significant proportion of the energy demand in buildings is required for cooling in the hot summer months [11] [12].

A new simulation tool developed to model the steady-state performance of a single turbine OWTEP system is presented. The mathematical models of the various sub-systems are illustrated and brought together using the physical network approach [13]. Of particular note is the modelling of thermal stratification in coastal seawater columns from meteorological data for prediction of the thermal boundary conditions. These mathematical models are integrated into a MATLAB®-based computational tool and an in-depth analysis is undertaken. The NREL 5 MW reference wind turbine rotor [14] is utilised as the rotor for the OWTEP wind turbine.

Main body of abstract

Operating characteristics are analysed for a variety of parametric variations of the system. This parametric analysis is carried out by simulating the performance characteristics of the single turbine OWTEP system using the aforementioned computational tool. Wind speeds from 0 to 25 ms-1 are fed into the model so as to observe the electrical power generated and the rate of thermal energy extraction, along other performance attributes such as: line pressure, system flow rate, temperatures and pipeline stresses. This process results in establishing ideal values of the fundamental system parameters pertaining to the wind turbine positioning, pipeline, hydroelectric turbine and heat exchanger.

An annual energy yield analysis of the system is also undertaken. Meteorological data for Malta is used to simulate boundary conditions for the model, which in conjunction with hourly-readings of wind speed are fed into the model. Results indicate that the proposed hybrid system can produce a combined 22.35 GWhr/annum, consisting of electrical generation (14.32 GWhr/annum) and thermal energy extraction (8.03 GWhr/annum). The combined energy production is 54% higher than the equivalent standard wind turbine, which uses an identical rotor to only generate electrical energy.

The concept of using hydroelectric turbine angular velocity regulation to control the system pressure is proposed. Throughout the parametric analysis, it is observed that higher line pressures favour electrical generation by reducing the flow rate required to transmit a fixed amount of hydraulic power. Flow rate reduction results in lower frictional losses. Conversely, lower pressures and hence higher flow rates favour higher rates of thermal energy extraction. It was also observed that the by regulating the angular velocity of the hydroelectric turbine, one can directly control the line pressure. Figure 2 shows the effect of hydroelectric turbine angular velocity on the monthly electricity and thermal energy yields.



Different seasonal pressure regulation schemes therefore result a means for tailoring the energy mix (electric/thermal) to meet a particular demand, as well as marginally increasing the overall annual yield. It is proposed that the angular velocity of the hydroelectric turbine undergoes seasonal variations that reflect the meteorological conditions and energy demands of that period. Based on the typical demands of Central Mediterranean dwellings [12], it is evident that the angular velocity should increase in the winter months, where electrical energy is in demand. It should then be reduced in the summer months, to favour thermal extraction, which is in demand due to high cooling requirements. A number of control schemes are considered, results for the most favourable scheme are shown in Figure 3. This scheme successfully shifts the energy mix based on seasonal demand without resulting in precariously high line pressures. Moreover, it results in 2.4% more energy (electrical and thermal) being generated/extracted per year.



The energy yield analysis also serves to compare the use of a standard wind turbine control scheme and a novel “high-speed” control scheme developed at TUDelft [9] for hydraulic-based wind turbines. In this scheme, the wind turbine rotor angular velocity is allowed to increase beyond that corresponding to the rated wind speed. By having a rotor operate at a higher angular velocity, while limiting torque, the power harvesting at higher wind speeds can be substantially increased (around 43% for wind speeds beyond 17 ms-1). However, results from the present study indicate that the latter scheme would not be particularly beneficial in a central Mediterranean context. This is due to relatively lower wind speeds that rarely exceed the rated speeds of typical large-scale rotors.

Conclusion

With the shift to a hydraulic system at the core of the OWTEP system design, the mechanical aspect of the model provides a means for assessing hydraulic transmission and energy conversion with an adequate level of accuracy. The objective being to answer a relatively simple question: “In terms of the output power, is the shift to hydraulic transmission a viable alternative to the traditional concept of electrical generation and transmission?”
A particular challenge is to model the boundary conditions of the pipe as it travels from the deep sea to the centralised generator platform. Here the modelling takes on a more rigorous approach, the aim being to develop a better understanding of the physics behind thermocline formation and what meteorological factors affect it. The objective is however to answer another simple question: “Is the favourable temperature of deep seawater maintained as it travels through the pipeline?”

The implementation of mathematics into a rigorously validated system model leads to a preliminary answer to these two basic research questions and others that developed along with the OWTEP concept. The simple answer is that the simulations indicate that yes, a seawater open-loop hydraulic system can be a viable alternative to traditional electrical transmission. Moreover, despite thermal losses along the pipeline, the seawater still has a favourable temperature difference of up to 13˚C with respect to the ambient conditions, and can therefore provide a means for cooling or heating. In the Central Mediterranean context, the addition of cooling energy extraction to the system implies that an appreciable amount of energy is now directly obtained in the form in which it is required.

Such a radical hybrid system, in conjunction with a method for energy mix regulation implies a substantial contribution to grid-integration and the overall viability of wind energy: one can now extract substantially more energy for the same rotor dimensions and footprint, with a means for regulating the combination of thermal and electrical energy to be extracted based on the demand.



Learning objectives
- Describe the novel concept that is the OWTEP system,

- Develop an adequately verified and validated computational tool that uses mathematical models to simulate its performance,

- Identify key aspects of system behaviour and ideal parameters,

- Compute annual energy yield for a single turbine OWTEP system in the Central Mediterranean,

- Assess the use of a unique approach to regulate energy mix (thermal/electrical) using line pressure regulation through hydroelectric turbine angular velocity control.



References
[1] Adam M. Ragheb and Magdi Ragheb, "Wind Turbine Gearbox Technologies," Fundamentals and Advanced Topics in Wind Power, pp. 189-206, July 2011.

[2] R Ackerman, "A Bottom In Sight For Copper," April 2009.

[3] American Wind Energy Association, "Wind Energy Fact Sheets: Economics and Cost of Wind Energy," AWEA, 2005.

[4] M.M. Hand et al., "Renewable Electricity Futures Study," National Renewable Energy Laboratory, USA, NREL Report 2012.

[5] Tonio Sant and Robert Farrugia, "Performance Modelling Of An Offshore Floating Wind Turbine- Driven Deep Sea Water Extraction System For Combined Power And Thermal Energy Production: A Case Study In A Central Mediterranean Context," in ASME 32nd International Conference on Ocean, Offshore and Arctic Engineering, Nantes, 2013.

[6] Daniel Buhagiar, "Performance Modelling and Analysis of an Offshore Wind Powered Hydro-Geothermal Energy Plant," Department of Mechanical Engineering, University of Malta, Msida, MSc Thesis 2013.

[7] N Diepeveen and J Van der Tempel, "Delft Offshore Turbines, the future of wind energy," Delft University of Technology , 2008.

[8] N. Diepeveen, "On the Application of Fluid Power Transmission in Offshore Wind Turbines," Delft University of Technology, Delft, PhD Thesis 2013.

[9] Antonio Jarquin Laguna, "Steady-State Performance of the Delft Offshore Turbine," Faculty of Aerospace Engineering, Delft University of Technology, Delft, M.Sc. Thesis 2010.

[10] Daniel Buhagiar, "Analysis of a Wind Turbine Driven Hydraulic Pump," Department of Mechanical Engineering, University of Malta, Malta, B.Eng. Thesis 2012.

[11] Enemalta, "Annual Report and Financial Statements," Enemalta Corporation , 2009, 2010, 2011.

[12] Vincent Buhagiar, "Sustainable Development and Building Design in Malta," in Commonwealth Peoples Forum, Valletta, 2005.

[13] Steve Miller, "Modeling Physical Systems as Physical Networks with the Simscape Language ," Mathworks, 2008.

[14] J Jonkman, S Butterfield, W Musial, and G Scott, "Definition of a 5-MW Reference Wind Turbine for Offshore System Development," U.S. Department of Energy, National Renewable Energy Laboratory, Golden, CO, Technical Report 2009.