So I guess most of us up here in the northern
hemisphere all had bouts of pretty sweltering weather this summer didn't we? We even
broke some all-time records over here in Europe. And I imagine you folks in southern
hemisphere climbs like Australia have the word 'sweltering' pretty much seared
into every one of your summer seasons. Now, I'm not going to use this video to get into
the whys and wherefores of whether those high temperatures are being affected by climate
change...(they are!). No, far be it from me to inculcate such a contentious notion! But I am
going to use this video to consider what could be regarded as a somewhat counter-intuitive solution
to the problem of increased warming from our sun, and that's the concept of using the sun's heat to
cool us down. Pretty weird eh? It's a technology called Solar Cooling, and I'm not talking about
simply using solar panels to provide electricity to run an air conditioner, although using
renewables for that purpose is a pretty good idea in itself. No, I'm talking about a system
that can collect heat directly from sunlight outside and apply some clever physics to it to
produce cool air inside. There are some commercial operators already offering various versions
of Solar Cooling in hotter parts of the world, but now a team at the Massachusetts Institute
of Technology in the States has published a research paper demonstrating a system that can
provide passive cooling to preserve food crops and improve the efficiency of conventional air
conditioners using the heat of the sun and a very small amount of water and with no need at all
for any additional electrical power. So as usual, the question has to be - is this yet another piece
of academic genius from smart MIT post doctorates and professors that'll never actually work in the
real world, or is it a piece of radical insight that could dramatically reduce energy requirements
for cooling in a future more sustainable world? Hello and welcome to Just Have a Think. Here's a couple of stats for you courtesy of
the International Energy Agency. Half of all the air conditioners in use in the world
today are in China and the United States. More than half a billion households in
those two countries own an AC system. But of the nearly 3 billion people living in
the hottest, mid-latitude parts of the world only eight percent have access to their own
air conditioning. That's changing fast though, and by 2050 the IEA projects that around
two-thirds of all homes in the world will have an air conditioning system installed, half
of which will be in China, India and Indonesia. Current air conditioning technology is incredibly
energy hungry, and if it's powered by electricity generated by coal or gas power plants then
the rapid increase in the number of units will inevitably result in a similar increase in
greenhouse gas emissions. Plus those systems only transfer heat from one place to another. They
don't actually cause heat to leave the overall planetary system, which means local external
temperatures, especially in tightly packed urban environments, continue to increase and more and
more cooling power is required. So anything that can be done to mitigate that effect must surely
be welcomed. Obviously moving rapidly away from fossil fuels and towards low carbon alternatives
like wind and solar is arguably the most important step, but if we can make the cooling process
itself more efficient than all the better, eh? Nature's way of keeping us cool is the process of
evaporation, which is what happens when we sweat. Water droplets on the surface of our skin get
heated by the ambient air temperature until they change phase from a liquid into a gas or vapour.
That phase change involves something called the latent heat of evaporation which requires about
600 extra calories of energy to liberate the molecules in each gram of water and allow them
to escape as vapour. And that loss of energy is what cools our skin down. You can achieve a
similar effect by filling a container with water and letting it evaporate as it removes energy from
the surrounding warm air. It's a way of passively cooling your immediate surroundings and if you
place a circulating fan next to the Bowl then you can direct the cooler air to where you want
it, but you need an awful lot of water to make an appreciable difference, so you'll be constantly
refilling your container, and all that water vapor will increase the humidity of the air which in
many parts of the world is the last thing you want to do. Nature also uses radiative cooling to
keep the entire planet at a liveable temperature. Some of the energy reaching our planet's surface
is radiated back out as infrared light. That energy then escapes back up into the cold empty
vastness of outer space. If the planet couldn't lose that unwanted energy then temperatures
would quickly reach uninhabitable levels, and by the way it's the light in the infrared
range that's being captured in increasing quantity by the greenhouse gases than our modern lifestyles
are spewing out into the atmosphere, which is why our scientists are so worried about global
warming. But I promised I wouldn't go down that road today so let's park that and get back to the
main point, which in this case is that infrared heat radiation represents another potentially
powerful way to achieve passive cooling. It's a technique that's been used for centuries in
tropical and subtropical regions for cooling and water harvesting during the night time when
the temperature differential between the Earth's surface and the skies above is at its greatest
especially on a clear night with no cloud cover. The effect is much less pronounced during the
daytime though, so it's not so easy to reap the benefits of infrared radiation for daytime
cooling, which is often when most people need it. The folks at MIT say they've taken both
these somewhat limited thermodynamic principles and combined them with a thermal insulation
layer to produce a much more efficient cooling system about the size of a standard solar PV
panel. Essentially you've got three layers of material. The top layer is an aero gel which is an
extremely porous sponge like polyethylene material containing a proportionately large volume of
air in its cavities. It's an inherently very good insulator but crucially it also allows both
water vapour and infrared light to pass through. Below that is a second layer containing hydrogel.
It's another sponge-like material but as the name suggests the pores of this one are filled with
water. It's apparently very similar to materials used in cooling pads and wound dressings. The
hydrogel acts as the source of water for the evaporative part of the cooling process. The
bottom layer is a mirror-like material that reflects any remaining sunlight that managed
to make its way through the top two layers. That heat reflection prevents the entire device
from warming up too much which would otherwise negatively affect its overall performance.
The top layer of air gel is also a very good solar reflector so it helps to keep the system
cool as well even under strong direct sunlight. The effect of this triple layer sandwich is both
evaporative cooling from the hydrogel and infrared radiative cooling from the reflective base
layer. And unlike traditional air conditioners, which simply spew unwanted hot air out into
the surrounding external environment, the infrared radiation from this cooling system goes
straight up through the atmosphere and out into space which according to the MIT team means it
really is removing heat from the Earth's system. One of the paper's authors, MIT postdoc Zhang
Mao Lu explained... "the challenge previously was that evaporative materials often do not deal
with solar absorption well when they're under the sun. They get heated so they're unable to get to
high cooling power at ambient temperature." The novel Insight that the team has shown here is
simply to bring together the three principles of evaporation radiation and insulation into
a single design architecture to overcome these previous deficiencies. The system was tested on
a rooftop at MIT using a small version about four inches across and it demonstrated that even in
sub-optimal weather conditions it could achieve just over 9 degrees Celsius or about 19 degrees
Fahrenheit of cooling. The design is so slimline that it could theoretically be incorporated into
the lid of a food container, keeping food cold and fresh for much longer periods of time without
the need for electrical power. And that really could be absolutely transformational in remote
off-grid areas or in parts of the developing world where many folks simply can't afford
electrically powered cooling technologies. But larger roof mounted panels could also be
used to send chilled water through pipes to the condenser of an air conditioning system.
Condensers in those systems remove heat from highly compressed refrigerant gas allowing it to
convert back into a liquid and carry on through the AC pipe work, so by Design condensers get
very hot. If chilled water could be channelled around the condenser then heat could be dissipated
much more quickly and the overall efficiency of the AC system would be greatly improved, which
in turn would mean a significant reduction in energy requirements to run the equipment. The
only maintenance required by the MIT design is the addition of water from time to time to ensure
that evaporation is happening. But the MIT team reckoned this system's water consumption is
so low that this would need only to happen about once every four days in the hottest driest
areas and only about once a month in wetter areas. Now there is an inevitable caveat of course,
because life is never quite as simple as we'd like it to be is it? While the majority of the
materials in the system are readily available and relatively inexpensive, producing the air gel is
currently not a cheap process. It turns out that the size of the pores in the air gel is a very
specific and absolutely critical parameter in the overall efficiency of the system. The pores are
produced by mixing the polyethylene with solvents and allowing it to set like a block of jelly,
or Jello if you're of an American persuasion, until it reaches something called a critical
drying point, or CPD, where the solvent can be removed from the polyethylene without
damaging its very delicate structure. That requires specialist equipment which of course
costs money. The research team is looking at other less expensive techniques like freeze drying and
they're experimenting with alternative materials that might provide the same insulating function
at lower costs such as membranes separated by an air gap, but right now it sounds like the
air gel is a limiting factor that's going to increase the timeline for transforming
this technology into a commercial reality. But if they can get that wrinkle ironed out
then this one does look like a very promising development especially for food storage and
safety in those parts of the world with limited access to the resources we all enjoy in Western
societies. And if it can improve the efficiency of the billions of additional air conditioners
I mentioned right at the start of the video then it could make a significant contribution
to climate change mitigation. And while we're on the subject of climate mitigation strategies I
just wanted to mention this brand new publication called the Carbon Almanac. It's not something I've
been involved with personally, and I'm not getting any money to plug the book, but I felt it was
worth bringing it to your attention if you haven't already seen it because it's absolutely jam-packed
full of just about every conceivable option for reducing the effects of climate change in the
coming decades. Every technology and data point in the book is supported by comprehensive references
that can be looked up on the accompanying website, so you can check out the scientific robustness of
each technology idea for yourself. It's well worth a read. That's it for this week though. Thanks to
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