2.6
Summary
There have been several pilot trials of solar powered MD systems which have aimed to establish the performance of such systems in various configurations. These systems have typically included two separate solar collectors, one thermal array to heat the saline feed solution and one PV array to power the pump and meet any other electrical
requirements of the system. However, to date, there has not been a study of an
MD system powered by a combined Photovoltaic/Thermal collector with the use of a parabolic concentrator. A single CPV/T collector would be capable of heating the feed and producing the electrical power needed, reducing the cost of materials and improving efficiency.
However a CPV/T system would present challenges of its own, as the output from the system is intermittent. In all cases, a solar powered MD system would not be capable of running continually; the modules are shut down over night allowing them to cool. During the day the output from a CPV/T system fluctuates, resulting in short periods of heating and cooling. Several researchers have noted fluctuating distillate conductivity during intermittent use of an MD module. However this effect has not been fully characterised experimentally. This research presents experimental data from an MD module while subject to fluctuating temperature, ranging from periods of 5 minutes to 12 hours.
Chapter 3
Experimental set up and
measurement techniques
This chapter gives details of the experimental rig and measurement techniques used throughout this research. The configuration of the membrane module is described and the properties of the membrane material are discussed. Specifications of all equipment used in the rig are given, including the pump, sensors and data acquisition systems. The rig was designed to enable testing of an MD module’s performance during intermittent use, such as applications with an overnight shut down period. It was also tested with short term fluctuations, such as those from a solar energy source. The experimental procedures used to test the performance of the module in these conditions are detailed
3.1. Membrane System overview
here. The procedure for assessing the effects of temperature on the structure of
membranes used in the module is also given.
3.1
Membrane System overview
The experimental rig is shown in figure 3.1. Saline feed solution is stored in a 40 litre tank. The feed solution is pumped from the tank into the cold channel of the membrane module by a 505 Watson Marlow peristaltic pump. The feed is preheated as it flows through the cold channel of the module. After exiting the cold channel, the feed solution passes through a heater coil placed inside a water bath, where is it heated to the desired temperature. The feed then enters the hot channel of the module, where it flows along the membrane. A percentage of the feed evaporates at the surface of the membrane and the vapour formed defuses through the membrane pores, where it condenses against the plate that forms one wall of the cold channel. After condensing, the distillate solution flows out of the module where it is collected. The feed solution that remains liquid, and therefore can not pass through the hydrophobic membrane, flows out of the hot channel where it can be collected and recirculated.
Four T-type thermocouples were used to measure temperature at the entrance and exit of the hot and cold channels. A digital manometer manufactured by Omega was used to measure the pressure drop across the module. A conductivity probe measuring from 0-2000 S/cm, was placed in the distillate outlet pipe. The distillate output flowed into a beaker on a weighing scale; the scale logged the weight to a computer via
Windows‘T M HyperTerminal program. The rate of increase in weight was then used
to determine the distillate flow rate. Further details of the sensors used in the rig, including their positioning, precision and accuracy, are given in section 3.3.
In this setup, a water bath is used to heat the feed solution and simulate the energy input from a solar source. A GD120 stirred water bath from Grant Instruments was used for this purpose. The GD120 immersion thermostat has a heating range from
3.1. Membrane System overview Storage tank Water bath Rotameter MD module Peristaltic pump Weight scale
Figure 3.1: Image of the MD system
feed solution was heated as it flowed through the coil. This method is preferred as the feed solution used in the experiments has a high salt concentration. Pumping the feed solution directly into the bath would cause corrosion of the heating elements.
The Watson Marlow peristaltic pump was also chosen to avoid corrosion. It is a positive displacement pump with a rotor and 3 rollers. A platinum silicon tube with a bore size of 8mm and a wall thickness of 1.6mm was placed through the rotor. The tube is compressed by the rollers as the motor turns and fluid is pushed through the tube. The feed solution does not come into contact with the metal components in the pump, preventing corrosion. The RPM of the motor determined the flow rate and this value can be set, however positive displacement pump does not always maintain a constant set flow rate. Over time the tube can become compressed and the flow rate can decrease. To ensure that the flow rate was maintained throughout the course of all experiments a rotor-meter was placed after pump and the flow rate could be checked and maintained.