CHON: REN Direct air capture of water

2021.05.24 – Ian Page

I was musing on the fact that many regions of the world have continuous sun in huge quantities (Australia) but no access to water.

Given that one of the things you want to do with the very cheap electricity from solar PV in such places is to generate hydrogen and perhaps then make ammonia for fertilizer or fuel, where do you get the water from?

Being far from the sea, unable to use rivers or clean water aquifers as they have better uses and using brackish water aquifers involves energy intensive clean up before use and is not a sustainable source, it seemed obvious to me that the eventual route might be via direct air capture.

There is of course  a literature on this ( I discovered it’s called AWS  https://pubs.acs.org/doi/pdf/10.1021/acsmaterialslett.0c00130 for a review), and membrane separation (https://bmcchemeng.biomedcentral.com/articles/10.1186/s42480-019-0020-x)

Even desert air contains significant humidity, but the issue with AWS is that it is designed to produce liquid water and thus uses more energy than necessary to provide the necessary cooling to get rid of the large heat of water condensation.

Despite this a MOF based system is mentioned that can deliver liquid water on a kilogram scale using only sunlight as the energy source.

It’s not clear that there are any existing large scalable systems, but there is lots of potential particularly in arid regions and perhaps it’s just waiting for a company to pick a technology intercept, a market and productize it in sufficient volume to get the costs down. It is more likely to be of value as an industrial input and for drinking, than for agriculture, although it’s possible that very dry regions with small-scale appropriate low water demand crops and expensive import alternatives might make sense. Where there are AWS issues with potential leaching of materials not suitable for human drinking, these might not be a problem for other industrial uses. 

Costs of water need to be considered vs other local sources, not obviously the sort of costs in the UK (where the main problem recently has been keeping dry!) Thus, the holistic cost of AWS + solar + electrolyser is the important metric to be compared with the local imported cost of hydrogen or fertilizer.

In a visionary sense I note that if you have lots of very cheap energy, CO2 DAC+ AWS has the potential to create useful industrial chemicals via various electrolytic routes, anywhere. The thing that struck me is that the feed to some gas permeable catalytic systems is wet CO2, not liquid water and CO2, which raise the question as to whether suffering the energy loss of disposing of the heat of condensation in AWS to make liquid water and then again to make it a gas, could be designed out leading to overall lower costs