Jun 01 2023

Harvesting Energy from Water Vapor

I did not plan to write yet another post about energy, but this popped up and I had to write about it. UMASS researchers have produced a device that generates electricity by harvesting charge from water vapor. They write:

The common feature of these materials is that they are engineered with appropriate nanopores to allow air water to pass through and undergo dynamic adsorption–desorption exchange at the porous interface, resulting in surface charging. The top exposed interface experiences this dynamic interaction more than the bottom sealed interface in a thin-film device structure, yielding a spontaneous and sustained charging gradient for continuous electric output.

Mainstream reporting, which made national news, has been mediocre, although some better than others. Author Jun Yao states that the device is like an artificial cloud, but instead of building up a charge that gets released as lightening, it harvests a tiny amount of current. The key is not in the material itself, but its structure – which requires nanopores less than 100 nm wide. Obviously this is a proof-of-concept laboratory phenomenon. Yao claims this can be a source of large amounts of continuous clean energy.

I have lots of questions, and reporting so far has only touched on them. My first question is – how will this scale? Yao reports you could stack a billion such devices into an appliance the size of a refrigerator which could generate 1 kw of power. The average American home requires an average of 1.2 kw of power, but can draw 3-5 at peak demand. So we are in the ballpark with 1 kw. Assuming this all works and scales (a big assumption I am not willing to make) I can envision appliances perhaps the size of a mini-fridge, producing 200 watts power, which can be chained together to produce whatever power a particular building needs.

This still runs into a problem of meeting demand, similar to the problem of intermittent energy. You can solve this problem in a number of ways. You can use the grid as backup, sending excess energy to the grid and drawing energy from the grid when needed. This works at low penetration, but will quickly break down if such devices are cost effective (another big assumption I am not willing to make), You could also have overcapacity, having enough devices to meet peak demand then shutting them off when not needed. Or you can have battery storage, with each device having a modest battery that can store energy when not needed and release it during high demand. Likely a combination of overcapacity and battery storage would work.

But let’s get to those assumptions – will such devices scale? There are technical hurdles to scaling, and who knows if they can be worked out, but even assuming they can be, I have some theoretical concerns. First, how much water vapor is available in any one location at a time? Obvious that depends on the humidity. I would like to see a calculation of how much energy such devices could theoretically create in an average-sized home based on different humidity levels. In addition to the question of how much water vapor can move through the device at any one moment, there is the question of how much total water vapor there is available in the building. I assume such a device would also functionally be a dehumidifier. Will the water collect in a basin, perhaps to be drained outside? What happens as the humidity level predictably drops? There is therefore the question of how much power such devices could generate at any time, and the total energy they can produce over time.

I strongly suspect that herein lies the deal-killer, and none of the reporting I saw even raised the issue. Perhaps such devices will find a niche as dehumidifiers that power themselves, but not much else. Or perhaps these might work well for campers, or other off-grid applications. But I seriously doubt we will be powering a significant amount of our civilization with them. And that’s OK – often new technology is initially hyped for a purpose that it is ultimately ill-suited for, but comes to be used for other applications. This might turn into a nifty technology that finds lots of creative uses, even if it never runs our homes. It may find a more limited use of providing a small amount of power, mainly used for backup.

Any application will largely depend on the other question – cost effectiveness. As a source of electricity, this is somewhat easy to determine, because we can just make a straight comparison of cost per kw hour and compare that to other sources of electricity. We can also calculate upfront costs, lifetime costs, carbon and energy efficiency, and pay-back time. Cost-efficiency is often hard to know until we get large-scale manufacture and efficiencies of scale kick in. If the stacking of these units works, then I suspect some company will try to make it work and will either succeed or fail. If it works we may see devices popping up in 5-10 years, aimed at early adopters and priced out of range of most consumers.

Overall, while this is a tantalizing idea, just playing the odds I would say it’s unlikely this technology results in any consumer products. If it does, such products are likely to be limited in application, but could have some interesting uses. What I really need to see is some assessment of how much energy is potentially available in any space over time given the level of humidity. If someone know where to find this, let me know in the comments. Again, this is likely the deal-killer of this technology, which can doom it to be nothing more than a laboratory curiosity.

No responses yet