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Solar Cooling
November 22, 2021
After introducing the
chemical elements and the
periodic table,
chemistry textbooks proceed to combinations of the elements to form
chemical compounds. Such
chemical reactions are classified as
exothermic or
endothermic, either releasing
heat to, or absorbing heat from, the
surroundings. By releasing heat, the
products of the exothermic reaction decrease their
enthalpy (ΔH is negative), and by absorbing heat, they increase their enthalpy (ΔH is positive). The terms, exothermic and endothermic, derive from the obvious
root for
temperature and the
Greek prefixes,
exo, (ἔξω, "external), and
endo (from ἔνδον, "internal").
How can we
estimate whether a reaction will be exothermic or endothermic? An easy way is to compare the
sums of the
atomic bond energies of the reactants and products. As a simple example, consider the reaction of
hydrogen and
oxygen to form
water.
2H2 + O2 -> 2H2O
These combinations of elements have the following
bonds and bond energies.[1]
When we sum the two hydrogen bonds and the oxygen
double bond of the reactants, we get 1359
kJ/
mol. The four oxygen-hydrogen bonds of the product give 1868 kJ/mol, with sum of reactants minus products of negative 509 kJ/mol. The water formation reaction is exothermic, since the energy needed to break the bonds of the reactant molecules is less than the energy of the bonds formed in the product molecules.
Our
households are frequent hosts for major chemical reactions, such as
combustion in
furnaces and
automobiles. There are also some minor reactions, such as those that occur in
electric batteries and those involved with
leavening agents such as
baking soda (
sodium bicarbonate, NaHCO3 and
baking powder. Each of these release
carbon dioxide gas by an
acid-base reaction of sodium bicarbonate with the
hydrogen ion (H+) of an acid; viz.,
NaHCO3 + H+ -> Na+ + CO2 + H2O
Our instant world no longer tolerates the use of a water bottle as a heating or cooling device. Instead, we have disposable chemical heating pads based on an exothermic chemical reaction, such as dissolving calcium chloride in water.
Ammonium nitrate, NH4NO3, is used in instant cold packs, since its ionic dissolution in water is an endothermic reaction that consumes 26.2 kJ/mol of heat.
I received this ammonium nitrate cold pack from a dentist after some minor oral surgery. The pack contains a pouch of water that wets the ammonium nitrate when it's broken.
(Photograph by the author.)
The endothermic reaction of
ammonium nitrate with water has been proposed as a method for
air cooling.[2-5]
Global warming has increased the need for cooling, and much of this cooling is needed in areas with lack of access to
electricity. The proposed cooling method does not require electricity, and the ammonium nitrate
salt is subsequently
crystallized using
solar energy.[2-5] This
research was
published in an advanced issue of the
journal,
Energy & Environmental Science, by
scientists from the
King Abdullah University of Science and Technology (KAUST, Thuwal, Saudi Arabia) and the
Hong Kong Polytechnic University (Hong Kong, China).[2] They call their
process NESCOD from "no electricity and sustainable cooling on-demand.[2]
One other approach to passive cooling whose
principle I wrote about in an
earlier article (Solar Reflecting Paint, November 19, 2018) is to
radiate heat to the cold of
outer space through a narrow
optical passband in
Earth's atmosphere at
mid-infrared wavelengths (8-13
μm). However, this approach has a low
intrinsic thermodynamic cooling power limit of about 160
W/
m2, and it can reduce the temperature of the
area beneath the cooling material by a mere 10°C.[2]
The endothermic nature of ammonium nitrate mixing with water arises from the disassociation of ammonium nitrate into an
ammonium cation and a
nitrate anion.
NH4NO3 -> NH4+ + NO3-
Ammonium nitrate has a cooling power that's greater than four times that of
ammonium chloride (NH4Cl), its closest
competitor.[4] The exceptional cooling power of ammonium nitrate is a consequence of its high
solubility in water, which is about 2
kilograms per kilogram of water.[4-5] Another advantage of ammonium nitrate is that it's
inexpensive, and it's widely used as a
fertilizer.[4-5]
The continuous cooling device was constructed as a a tall container made of a sunlight absorbing material wrapped with a water-wicking fabric that's floated on top of the salt solution.[5] The top portion is coated with polytetrafluoroethylene (a.k.a., Teflon®) to prevent the salt crystals from climbing too high on the container.[5] (Created using Inkscape. Click for larger image.)
The continuous cooling device was constructed as a a tall
container made of a
sunlight absorbing material wrapped with a water-
wicking fabric that's
floated on top of the salt solution.[5] The salt solution wicks up the fabric, is warmed, and the water
evaporates.[5] This produces ammonium nitrate salt crystals that grow on the container's outer wall.[5] It's then possible to automatically collect the crystallized salt using
gravity.[4] The system's cooling power could reach up to 191 W/m
2 under normal solar conditions.[2]
It was found that The temperature of the cooling solution can reach about 2.4 °C in just 20
minutes.[2] This cooling system can also serve in
food storage applications. When the ammonium nitrate was gradually dissolved in water in a
metal cup placed inside a
polystyrene foam box, the temperature of the cup dropped to around 3.6°C, and it remained below 10°C for 8 hours, and below 15°C for more than 15 hours.[4-5]
References:
- Bond Energies, Chemistry Library of the LibreTexts project.
- Wenbin Wang, Yusuf Shi, Chenlin Zhang, Renyuan Li, Mengchun Wu, Sifei Zhuo, Sara Aleida and Peng Wang, "Conversion and storage of solar energy for cooling," Energy and Environmental Science, (Advanced Online Publication, September 1, 2021. https://doi.org/10.1039/D1EE01688A. This is an open access publication with a PDF file here.
- Supplementary information for ref. 2 (PDF file).
- Strong sunlight powers passive cooling device, KAUST Press Release, September 19, 2021.
- Prachi Patel, "Salt + sunlight powers an innovative electricity-free cooling system," Anthropocene Magazine, September 23, 2021.
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