Light Bulbs from Mixing

Assistant professor Ngai Yin Yip (EEE department) delivered an excellent talk for the Chemical engineering department on his PhD thesis and current research, which delves in the intersectionality between renewable energy and climate change. I was quite amazed because his research harnessed energy from the most unlikely source—mixing. Who would ever think that you can get energy from mixing salt in water? What I also found cool was that the energy from salt gradients does not come mostly from enthalpy of mixing, but the entropy of mixing. Salt prefers to dissolve because it can be more places when dissolved as opposed to in a crystal. I find that a really fundamental idea that makes so much sense. In a way, this research is the opposite of desalination—rather than putting in energy and separating salt from water, it is using the natural release of energy of mixing. I have since told multiple people (friends and family) about the idea that mixing is a renewable energy source and they were all immediately confused. It seems that it is a very non-intuitive idea for most of those who do not have a science background.

But what is the mechanism for gathering this energy? It’s none other than pressure driven flow! Basically, there is less salty water and very salty water separated by a semi-permeable membrane that only allows water to traverse. The water from the less salty water then crosses to the other side and the energy of this pressure driven flow is harnessed through a piston. The talk primarily focused on details about how to improve this process, and included some interesting techniques including running the process upside down in order to clean the membrane of gunk (called fouling). But what I found interesting, and a little suspicious about the whole thing, was that Professor Yip only spoke about the mechanical energy harnessed. Yet he compared the mechanical energy harnessed from this source to electrical energy from solar power and other renewable energy sources. Albeit, doing my own research, the efficiency in converting water turbine energy into electrical energy is as high as 90%, this still marks an important further loss that he didn’t take into account.

Lastly, how can this source of energy be used practically? Professor Yip gave one way, which is when fresh water and sea water meet, and there is a water potential between them. However, I found that scientists have proposed another use—which is how this research applies to climate change as well. Desalination results in salty water, and chemically treated wastewater has very little salt content. So scientists have proposed combining the two, and harnessing energy from that! Thus this process has real applications and can be a great energy source for the future.