Solar thermal desalination - a modelling and experimental study

Development of additional clean water supply options, such as desalination, will increasingly be required to meet the growing demands for fresh water. If desalination is to be conducted more widely, the challenge is to focus on truly sustainable desalination technologies that rely on renewable energy sources with low or zero greenhouse gas emissions. The aim of this research project has thus been to investigate both theoretically and experimentally the technical and economic feasibility of producing potable water from seawater or brackish water using desalination systems powered by renewable energy in the form of low-temperature (less than 90 o C) solar thermal sources. The focus has been on salinity-gradient solar ponds and evacuated tubular solar collectors. In this thesis, a number of new Visual Basic - Excel computer simulation models of the different solar multi-stage flash (MSF) and solar multi-effect evaporation (MEE) configurations are presented. In addition, transient modelling of the salinity-gradient solar pond (SGSP) and evacuated tube solar collector (ETSC) is conducted to determine the required collection area and the overall performance of the solar-thermal desalination systems. A comparative theoretical analysis of the specific thermal energy consumption of solar-MSF and solar-MEE systems showed that the solar-MEE system uses up to 55% less energy than the solar-MSF for the same number of stages/effects. The experimental studies have involved the design and construction of three small-scale experimental desalination units – a solar single-stage flash system, a solar multi-effect evaporation system with three effects, and a novel integrated solar collector and evaporation system – for performance measurement and evaluation. The key design and operating parameters controlling the cost of fresh water are distillate production, recovery ratio, specific thermal energy and solar collection area. These parameters were determined from the computer simulation and compared with experimental results. Overall, the comparative analysis showed that the simulated results were within the error ranges of the empirical data. The models developed can therefore be used with confidence for further analysis and performance evaluation of solar-thermal desalination systems. The solar desalination systems studied have been compared with conventional desalination systems powered by energy from fossil fuels, and other renewable-energy based systems, using a technical and triple bottom line evaluation. Four zero-emission desalination technologies have been investigated: SGSP coupled with an MEE system, with PV panels and battery storage providing the electrical energy by the pumps; ETSC coupled with an MEE system and thermal energy storage (TES) tank, with PV panels and battery storage; and PV-RO and Wind-RO systems. On the basis of the assumptions made in the financial analysis, the cheapest zero-emission technology for Melbourne location was found to be Wind-RO with a water cost of AUD $1.69/m 3 , followed by the solar-MEE ($3.31/m 3 ) and PV-RO ($5.15/m 3 ). The solar-thermal MEE option has an economic advantage over PV-RO. Further analysis showed that the cost range for solar-thermal MEE desalination systems is competitive with the other main zero-emission desalination options in areas of high insolation