Saltwater Purification

Background:

Water purification is a field that will only develop in the coming decades. Already, mechanisms like reverse osmosis promise to obtain potable water from less-than-ideal sources. However, all solutions are incredibly energy/resource intensive, requiring high energy input in the case of simple distillation, and complex components and pressurization to achieve results. A combination electrolysis/fuel cell set up may circumvent some of these issues. 

The motivation behind this experiment comes from the mechanisms of heat pumps. Heat pumps function using the ambient air temperature as a heat source/sink. Rather than heating or cooling through direct supply of energy, exchange with the atmosphere allows for ambient heat to be used, taking advantage of the natural occurrence of thermal gradients. Rather than ‘paying’ directly for heat transfer, we instead use thermodynamics for actual heat exchange, and only have to pay for the pressure change of the processing fluid. This core principal, paying the energy for transport rather than the direct effect, is what this electrolytic set up aims to replicate. 

Construction and Operation

The set up has 3 main components: The electrolytic cell, the fuel cell, and the transport mechanism. Electrolysis of water containing some dissolved salt is performed, and gasses are collected from each electrode. The salt, concentration, and applied voltage are responsible for selectivity of ions at each electrode. Ideally, the output streams should be pure hydrogen (cathode) and oxygen (anode). The gasses are collected through a makeshift flashback arrestor. The output streams of the electrolytic cell are placed such that they must bubble through water before being collected in reservoirs. Its unlikely that the arrestors will serve their ‘intended’ process, as no flames are being used in this experiment. However, the ability to collect gas without any chance of flowing back/escaping allows for a more accurate experiment. Both gas streams are collected this way, but the hydrogen stream contains an additional component: a peristaltic pump. This is to deliver the hydrogen at higher pressure. The fuel cell has two input streams, separated by proton exchange membrane (PEM). As hydrogen moves from one side to the other, electricity and water are generated. The membranes function when there is a driving force pushing the hydrogen through, caused by a pressure difference. The voltage is governed by thermodynamic properties of the gases and the fuel cell, and the amperage is a function of how much hydrogen actually moves through the membrane. The output stream from the fuel cell is water vapor, which is condensed and collected through a heat exchanger.

At first, it may be hard to see the validity of comparison with heat pumps. However, upon closer examination, one can see the fundamental similarities in each setup:

• Two opposite processes tied to each other. In heat pumps, this is the evaporator and condenser, here it is electrolytic and fuel cells. (EC and FC)
• Operating fluid: In the case of heat pumps there is a refrigerant that is used for heat transfer. In this case, the working fluid is effectively a stream of ions. I don’t think characterizing it as just ‘gas’ or ’electricity’ would be accurate; the output of EC’s/input of FC’s is gas, and the input of the EC’s/output of FC is electricity, but in each half, the reaction is that of ion exchange.
• Using natural gradients to extract from the ambient processes: This is the similarity that spawned the idea for this project. Heat pumps pay to drive a temperature gradient, and use the natural tendency of heat to flow to its surroundings to extract utility. In this case, it is a function of concentration. The electrolytic side loses water, increasing in salt concentration, and the fuel cell side reduces in concentration as water in generated. Using a ambient ion solution source on the electrolytic side (such as saltwater) allows the increased concentration input stream to be continually replenished, and extracts useful output in the form of water. 

Operating specifications:

Specific pumps may introduce variations on this mechanism, such as heat exchangers between the heat pumps and their output locations. This may be used as the working fluids in the heat exchanger may be too volatile/hazardous, or if the range of temperatures between hot and cold sides makes a multi-stage process necessary. I