Research Papers

Modeling the Energy Yield Enhancement From a Wind Turbine at a Deep Offshore Low Wind Site Through Combined Power and Thermocline Energy Production

[+] Author and Article Information
Tonio Sant

Associate Professor
Department of Mechanical Engineering,
University of Malta,
Msida MSD 2080, Malta
e-mail: tonio.sant@um.edu.mt

Robert N. Farrugia

Assistant Lecturer
Institute for Sustainable Energy,
University of Malta,
Marsaxlokk MXK 1531, Malta
e-mail: robert.n.farrugia@um.edu.mt

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: Including Wind Energy and Building Energy Conservation. Manuscript received July 23, 2013; final manuscript received June 30, 2014; published online July 29, 2014. Assoc. Editor: Yves Gagnon.

J. Sol. Energy Eng 137(1), 011002 (Jul 29, 2014) (8 pages) Paper No: SOL-13-1211; doi: 10.1115/1.4027963 History: Received July 23, 2013; Revised June 30, 2014

This paper presents steady-state performance modeling and analysis of a novel wind powered system that concurrently exploits thermocline thermal energy through deep sea water extraction in conjunction with offshore wind energy for combined power and thermal energy production. A single offshore wind turbine rotor directly coupled to a large positive displacement pump is modeled to supply deep sea water at high pressure to a land-based plant, the latter consisting of a hydro-electric generator coupled to a heat exchanger. The steady-state power-wind speed characteristics for the system are derived from a numerical thermofluid model. The latter integrates the hydraulic characteristics of the wind turbine-pump combination and a numerical code to simulate the heat gained/lost by deep sea water as it flows through a pipeline to shore. The model was applied to a hypothetical megawatt-scale wind turbine installed at a deep offshore low wind site in the vicinity of the Central Mediterranean island of Malta. One year of wind speed and ambient measurements were used in conjunction with marine thermocline data to estimate the time series electricity and thermal energy yields. The total energy yield from the system was found to be significantly higher than that from a conventional offshore wind turbine generator (OWTG) that only produces electricity. It could be shown that at sites having less energetic wind behavior and high ambient temperatures as a result of a hotter climate, the cooling energy component that can be delivered from such a system is relatively high even at periods of low wind speeds.

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Fig. 1

Thermoclines in Central Mediterranean basin [1]

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Fig. 2

Schematic diagram for the single turbine OWTEP system. Main parameters for the performance model are indicated.

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Fig. 3

Pressure rise of deep sea water across wind turbine pump at different wind speeds

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Fig. 4

Volume flowrate of deep sea water across wind turbine pump at different wind speeds

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Fig. 5

Numerical model for the temperature gain/loss across a pipeline element

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Fig. 6

Simplified model for the thermocline, Tsea(h)

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Fig. 7

Predicted performance characteristics of the single turbine OWTEP system. The performance curve for the NREL 5 MW OWTG [6] is also shown.

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Fig. 8

Variation of the pipeline exit sea water temperature with pipeline thermal conductivity. The units for the values k are W/m K.

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Fig. 9

Variation of the monthly average values of the 10 min wind speed at ambient temperature data using in the single turbine OWTEP model. Month 1 denotes Feb. 2010. The error bars denote the one plus/minus values in the monthly standard deviation.

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Fig. 10

Histogram depicting the distribution of the hours per annum at different values of (Tamb − T1)

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Fig. 11

Effect of temperature difference cut-off, ΔTc, on the energy yield from the OWTEP system. The energy yield from a conventional 5 MW OWTG that produces only electricity is also included.

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Fig. 12

Variation of the monthly average energy yields as predicted by the single turbine OWTEP model. Month 1 denotes Feb. 2010. ΔTc = 7 °C.

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Fig. 13

Dependence of the annual average thermal energy yields for cooling and heating with pipeline thermal conductivity. ΔTc = 0 °C.

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Fig. 14

Dependence of the annual average thermal energy yields for cooling and heating with the water flow rate from the district heating/cooling system entering the heat exchanger. ΔTc = 0 °C.

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Fig. 15

Dependence of the annual average thermal energy yields for cooling and heating with the overall heat transfer coefficient for the heat exchanger. ΔTc = 0 °C.

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Fig. 16

Dependence of the annual average thermal energy yields for cooling and heating with the heat exchanger area. ΔTc = 0 °C.




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