Desalination with a solar-assisted heat pump: an experimental and analytical study
M N A Hawlader, Tobias Bestari Tjandra and Zakaria Mohd. Amin
Dept. of Mechanical Engineering,
National University of Singapore
9 Engineering Drive 1
Singapore 117576
Abstract
The Solar Assisted Heat Pump (SAHP) desalination, based on the Rankin cycle operates in low temperature and utilizes both solar and ambient energy. An experimental SAHP desalination system has been constructed at the National University of Singapore (NUS). The system consisted of two main sections: a solar assisted heat pump and a water distillation section. Experiments were carried out under the different metrological condition of Singapore and results showed that the system had a performance ratio close to 1.3. The heat pump has a Coefficient of Performance of about 10, with solar collector efficiencies of 80 and 60% for evaporator and liquid collectors, respectively. Economic analysis shows that to achieve a high production rate while maintaining a low investment cost, a system, without using liquid solar collector, is preferred. This system, at a production rate of 900 liter/day with an evaporator collector area of around 70 m[1] [2], will have a payback period of about 3.5 years.
Keywords: Desalination, heat pump, solar collector, evaporator collector, economic analyses, payback period.
applications at temperatures less than 100oC but the most promising source is the solar energy [2]. Experimental work on heat pump assisted water purification has been carried out in Mexico since 1981 [3], where electrically driven mechanical vapour compression pumps were first used. Absorption heat pumps were then tested in large-scale purposes and Siqueiros and Holland [3] found that the cost for desalination to produce potable water for cities was competitive to that of RO and ED.
Ozgener and Hepbasli [4] has performed energy and exergy analysis on solar assisted heat pump (SAHP) systems. Torres-Reyes and Cervantes [5] studied both theoretically and experimentally on a SAHP with direct expansion of the refrigerant within the solar collector and performed a thermodynamic optimization. The maximum exergy efficiency was determined by taking into account the typical parameters and performance coefficients.
The feasibility of a solar energy system is determined not only from its performance but also from an economic analysis, which must be carried out to evaluate its performance. Usually, solar energy systems require an initial high investment followed by a low maintenance and operation costs [6]. The economic Figure of merit used in the economic optimization is the payback period, as it shows how soon the initial investment can be returned by accumulated fuel savings [7].
At National University of Singapore (NUS), a direct expansion solar assisted heat pump (SAHP) system was designed and built,[8]. Studies performed on the system indicated the effectiveness of small-scale application. Modifications were made to incorporate the SAHP into a single effect MED desalination system and a series of experiments were performed. In this paper, experiments and economic analyses performed on a novel solar assisted heat pump desalination system is presented and discussed. [3] water tank. A thermostatic expansion valve regulates the refrigerant’s mass flow rate. After passing through the expansion valve, the refrigerant is divided into two branches, one through the evaporator-collector, and the other to a cooling coil located at the top of the desalination chamber to condense water vapors. These two streams are then mixed before entering the compressor.
In the desalination section, a commercial solar collector is used to preheat incoming feed water. An electrical heater is positioned at the outlet of this solar collector to provide auxiliary heating to ensure the feed water to maintain the desired temperature, when solar radiation is inadequate. The electrical heater will maintain the water temperature to be not less than 70°C. After passing through the electrical heater, feed water enters the desalination chamber. The chamber is evacuated to a pressure of 0.14 bar and at this pressure the corresponding saturation temperature for water is 52.6°C. Thus, feed water entering the chamber will undergo thermodynamic flashing. The remaining part of water that does not evaporate will flow down to the bottom of the chamber, where it will be heated further by the heat pump’s condenser coil, thus evaporating the water. Vapors generated from flashing and evaporation will be condensed at the top section of the chamber by a cooling coil of the heat pump. Distillate water produced will flow down to a collection tray.
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