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 all our Patreon supporters out there who allow me
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very much for watching, have a great week and remember to just have a think. See you next week.