An innovative approach to heat extraction from a salinity gradient solar pond to enhance overall efficiency

Yaakob, Y 2013, An innovative approach to heat extraction from a salinity gradient solar pond to enhance overall efficiency, Doctor of Philosophy (PhD), Aerospace, Mechanical and Manufacturing Engineering, RMIT University.

Document type: Thesis
Collection: Theses

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Title An innovative approach to heat extraction from a salinity gradient solar pond to enhance overall efficiency
Author(s) Yaakob, Y
Year 2013
Abstract A solar pond is a simple and low-cost solar collector with long-term thermal storage. It utilizes a large body of salinity gradient water to absorb radiation from the sun and stores it in the form of heat at the bottom of the pond with temperature between 70-90oC. The pond consists of three different layers. The cold thin upper layer is known as the upper convective zone (UCZ) and consists of low salinity water and has temperature close to the local average ambient temperature. The second layer is the gradient layer known as the non-convective zone (NCZ), where salinity increases from the top of the NCZ to the bottom of the NCZ. The bottom layer or lower convective zone (LCZ) has homogenous high salinity water which absorbs and stores solar thermal energy which reaches the bottom of the pond.

Conventionally, heat has been successfully extracted from the LCZ. This study concerns an attempt to extract the absorbed heat from the NCZ using external heat exchangers with the aim of enhancing the overall efficiency of the solar pond. External heat exchangers were fabricated and installed to extract heat from different levels in the NCZ. The process used a 12V pump to withdraw hot brine from the NCZ and re-inject it to the same level. Cold brine from the UCZ was used as a cold heat transfer fluid to extract heat from each of the heat exchangers. The results show that by extracting heat from the NCZ, the thermal efficiency of a solar pond could potentially be improved to 50% as compared with the conventional heat extraction method from the LCZ only.

An adverse effect associated with active heat extraction from the NCZ was the development of instability in the salinity gradient layer. Optimisation of the technique for withdrawal and re-injection of the cooled brine into the NCZ is essential to overcome this issue. It needs to be addressed as a future study.

The other part of the present study was the investigation of a semi-passive thermosiphon heat extraction system for heat removal from the NCZ or LCZ of a solar pond. The thermosiphon heat extraction system relies on buoyancy effects to remove heat by the effect of temperature difference. Theoretical governing equations have been developed based on principles of conservation of energy and mass. Theoretical analysis revealed great potential for this system to be implemented in solar ponds. Experimental results from a single thermosiphon heat extraction system have been presented for various cooling water mass flow rates. The experimental and theoretical performance estimates were compared and the results showed a good agreement. The experimental findings showed that a thermosiphon heat exchanger has the potential to minimize the use of pumps in heat extraction from solar ponds.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Aerospace, Mechanical and Manufacturing Engineering
Keyword(s) Solar Pond
Non Convective Zone
Heat Extraction
Heat Exchanger
Overall Efficiency
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Created: Thu, 13 Feb 2014, 11:20:51 EST by Brett Fenton
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