Exploitative evaporation technologies such as solar water vaporization have been touted as a pragmatic means of alleviating the water shortage crisis. Nevertheless, emerging investigations are bringing some basic assumptions with regards to these systems into question.
A group of scientists operating under the Northeastern University affiliation has focused its study on the efficiency with which solar evaporators work and how this has been measured in a recent publication in the journal Science Advances. In particular, they focused on the absolute or relative refrigeration retention vaporization enthalpy which is associated with the energy taken up to change water, certain liquids, into vapor that is being made.
Dark Evaporation Test: An Outdated Practice
Multiple reports have come up with the production of evaporators that appear to exceed the limits of water evaporation under sunlight conditions. The explanation that is usually available is that the evaporative materials such as hydrogels weaken the hydrogen bonds of water and thus it is easy to vaporize. Researchers usually carry out this through two means.
- Using a dark room to estimate the evaporation rate
- Quantitative vaporization enthalpy determination using differential scanning calorimetry
However, the new paper on the other hand makes a case that the former dark evaporation tests is where the problem lies.
As lead author Andrew Caratenuto notes, “It is not the case, as it is often assumed, that either the water system, or the evaporator system equally receives energy input from the environment.” “On the contrary when the advanced evaporation occurs, more cooling occurs resulting in greater temperature differences within which more environmental energy can be harvested by the evaporator.”
Subtly altering a few experimental conditions and performing appropriate modeling, the authors were able to show that this temperature difference explains the enhancement of the evaporation rate quite satisfactorily without the assumption of having a lower heat of vaporization.
Cultural Context: Contradictions and Implications for the Field
More curious, however, was the fact that their results also showed a contradiction between the two measurement methods. Some materials that did not exhibit any trace of modified water bonding or decreased enthalpy in DSC tests still presented with improved evaporation even at dark conditions.
“Therefore, these studies imply that, either DSC or dark evaporation tests are measuring discrepant parameters in compassion to vaporization enthalpy,” explains Caratenuto. “Such a proposition may be silly since we are, in essence, trying to get across a novel point in the field.”
The implications could even be dire as far as the studies dealing with the solar evaporation device are concerned. Importantly, it has been highlighted it is so that there is much literature available to attest to its use in solar evaporation systems. This work indicates those results may need to be re-evaluated.
Nonetheless, the authors of the study note that it is not the case that such solar evaporators should have limited advantages because they cannot go beyond the theoretical limits through such means. For example, there are some concepts that could be capable of utilizing some excess environmental power while running.
“We are not saying that the vaporization energy on water is not reducible,” says senior author Yi Zheng. “But the facts available to us point that maybe we should be more careful and consider other possibilities of the phenomena we are observing in these systems.”
Given the threats of increasing water scarcity all over the world, there is still a need to enhance current solar desalination and purification methods. This research is an important progression in making sure that explorations in this area are done in a logical manner.
The full study, entitled ‘Critical assessment of water enthalpy characterization through dark environment evaporation’ has been published in the Science Advances.
