One of the greatest gifts our modern human technologies have brought us
is the ability to maintain safe comfortable temperatures in our living and working spaces
without the need to find blocks of ice to cool down or firewood to keep warm.
But of course we all now know the environmental and climate costs of those technologies,
most of which use fossil fuels in one form or another, either directly for heating or indirectly
via national electricity grids. According to the International Energy Agency the use of air
conditioners and electric cooling fans accounts for nearly 20% of the total electricity used in
buildings around the world, and that number is much higher if you include mobile cooling
in private vehicles and on public transport plus industrial and commercial refrigeration
for keeping food fresh and for chilling critical products like pharmaceuticals. There's something
like 3.6 billion cooling appliances in use today all over the world and that number is going up
by about 10 devices every second. And as we move rapidly away from gas and oil
and towards electric heat pumps for space heating even greater demand will be placed on our grids.
These things draw a lot of energy and many of them use hydrofluorocarbons or HFCs as a refrigerant
gas. HFCs aren't quite as disastrous as the CFCs that were banned by the Montreal Protocol
30 years ago. They don't wreck the ozone layer. But they're still extremely potent greenhouse
gases - often hundreds of times more powerful than carbon dioxide. So the race is on to find better
solutions to the rapidly growing challenge of heating and cooling with better energy efficiency
and without potentially harmful emissions. Now a British company looks like they've achieved
precisely that with an elegantly simple system inspired by this thing - the sterling engine
Hello and welcome to Just Have a Think. Stirling engines are an example of an external combustion
engine. The energy that drives them comes from outside the closed system, as opposed
to an internal combustion engine where the fuel is burnt inside the combustion chamber. So how does
it work? Well in this little model I've got here we've got two metal plates at the top and bottom
and then we've got this little block of foam sitting in an enclosed chamber in the middle.
If I put the model onto something hot like a cup of coffee the heat energy from the
coffee warms up the bottom plate and that makes the air inside the chamber expand.
And as it expands it pushes this little piston upwards. The piston is attached to the wheel
via this rod, so as the piston goes up, the wheel starts to turn. But the wheel is also
attached to the foam block, so as it turns it pushes the block downwards. That forces
the air to flow past the block and come into contact with the top plate, which is cooler.
The cooling air contracts, pulling the piston back down, which turns the wheel a little bit more.
And as the wheel continues to turn it pulls the foam block back up, forcing the air back
down to the hot side, and the whole cycle starts again. As long as there's a temperature difference
between the top plate and the bottom plate then the engine will keep running. In fact you could
put this thing on top of a block of ice and it'd still work, albeit with the wheel turning in the
opposite direction, because the top plate would then be warmer than the bottom plate. The greater
the temperature difference between the two plates the more efficiently the system will run, and
conversely if there's no temperature difference at all the whole thing grinds to a halt.
But Stirling's invention is one of the few cycles that can be run in reverse to effectively make it
a heat pump instead of a heat engine. If I use the energy in my arm to manually turn the wheel,
or in a more sophisticated version if I attached a drive shaft to it powered by a little electric
motor, then the expansion and compression of the air would make one plate hot and the other plate
cold. And if that system was scaled up then you could theoretically draw the heat or the cold off
from one plate or the other depending on whether you wanted to heat a space or cooler space.
Sounds simple right? So why don't we run all our heaters and coolers that way instead of using air
conditioning units that contain all that horrible hydrofluorocarbon refrigerant fluid stuff?
Well it all has to do with something called the Carnot cycle and whether a gas is expanding
isothermally or adiabatically. I know - science jargon! I know. The full explanation of the
Carnot cycle is beyond the scope of this video, but suffice to say it represents
the theoretical maximum efficiency of a heat pump or a heat engine. Isothermal just means constant
temperature, so if a gas is expanded or compressed isothermally it stays at the same temperature all
the time. It's much more normal though for a gas to be compressed or expanded adiabatically,
which means its temperature increases or decreases as it's compressed or expanded. To achieve the
ideal efficiency of the Carnot cycle a Stirling heat pump would have to run 100% isothermally so
that the gas was able to lose all its heat energy as it was being compressed and draw in enough heat
energy to stay at a constant temperature while it was being expanded. That kind of energy exchange
might be possible for the gas molecules right next to the outer wall of the chamber,
but the molecules in the middle of the chamber are a very long way away, so to achieve a fully
isothermal cycle you'd have to run it extremely slowly to allow time for all those molecules to
either give up or take in energy. That's just not practical for any real world application.
You'd be waiting hours or days for your room to cool down or heat up. So instead air conditioners
and refrigeration units around the world today use vapor compressor heat pump technology that's been
around for more than 160 years and which relies on those nasty refrigerants with very high global
warming potential or GWP. And they still only achieve about 40% of the Carnot cycle efficiency.
But now this new system has been created that takes the elegant simplicity of the Stirling cycle
and applies some good old engineering lateral thinking to the challenge of heat energy transfer
to produce a working heat pump that can reach 60% of Carnot without using any gases with high GWP
values. The design is the brainchild of hydraulic engineer Michael Crowley and is being developed
by his company Fluid Mechanics who are specialists in the design, analysis and modelling of hydraulic
systems. Several prototypes have been built and tested since the beginning of the project back
in 2015. All of them are based on the principle of pistons and cylinders to achieve a similar effect
to this version of a Stirling engine known as an Alpha Type. Essentially the gas in the expansion
cylinder is heated externally and the gas in the compression cylinder is cooled externally. That
sets up the temperature differential needed to make the pistons move to rotate the drive shaft,
just like in my little model version. The gas is able to flow between the two cylinders, which
it does via this channel known as a regenerative heat exchanger. The heat pump developed by Fluid
Mechanics also has two cylinders, just like the Alpha Type Stirling engine. In fact the working
model will actually have another pair of cylinders at the back to optimize the mechanical balance of
the system. Each set of two pistons are attached to each other via something called a Ross Yoke,
which is a clever piece of existing technology designed to keep them out of phase with each other
by about 120 degrees. Again, very similar to the Alpha Type Stirling engine. The gas
is expanding and compressing, and we've also got a regenerative heat exchanger moving the gas between
the cylinders. So far so samey! But remember the Fluid Mechanics system is set up to be a heat pump
not a heat engine. That means the drive shaft is providing the input power to move the pistons up
and down rather than vice versa. So now we've got to find a way of removing the heat from one
cylinder and the cold from the other. The Fluid Mechanics system achieves that in two ways.
Firstly by using helium as the working fluid rather than ambient air,
and secondly by adding a series of thin metal fins to the bottom of each piston.
Helium molecules are much smaller than air molecules so they can move much faster and that
means they can transport energy more quickly too. In fact the thermal conductivity of helium is 10
times greater than air, and according to Michael you could just as effectively use hydrogen gas
for exactly the same reasons. The helium gas is held between the metal fins at the base of each
piston. The distance between the fins is just two millimetres, so the average distance that
the helium molecule has to travel to get to a fin is just 0.5 millimetres. And that means heat
transfer out of or into the gas is extremely rapid indeed. As each piston completes a cycle
the fins are plunged into a sump of silicon oil where they're quenched. Depending on which side
of the system the quenching takes place, the oil is either heated up a bit or cooled down a bit.
A circulating pump moves the oil constantly around an external circuit via a heat exchanger
where it heats up water on the hot side and cools it down on the cold side. To measure the basic
system efficiency, Fluid Dynamics created this test rig which looks a bit like something out
of Back to the Future, but is in fact a very carefully calibrated scientific instrument.
Based on measurements of the pressures inside the chambers, the rig demonstrated that it was
achieving 95% isothermal efficiency. Very close to that theoretical maximum of the Carnot cycle, with
an overall real world working efficiency of about 60% once all the system losses are accounted for.
The performance of all heat pumps improves as the temperature difference between what
you've got outside and what you want inside gets smaller. It's a function known as the
Coefficient of Performance or COP. So if the ambient temperature is 19 degrees C and you
want say your room or your car to be at 21C, then your system will have no trouble at all. but if
it's 45 degrees Celsius outside and you're trying to get to 18 degrees inside then the COP number
will drop significantly. That's the same for all heat pump systems, and Fluid Mechanics' design
is no different. Nevertheless, early test results suggest that the Fluid Mechanics system
will achieve a higher COP than a typical air source heat pump available on the market today.
And their current 2.7 kilowatt prototype is capable of generating a temperature difference of
60 degrees Celsius between hot and cold. The next development model will be a 10 kilowatt unit with
improved technology that Fluid Mechanics expect to reach temperature differences of about 80 degrees
Celsius. That could happily run an industrial freezer in a building where the external
temperature was over 30 degrees Celsius, or if it was configured as a heating device it could
provide hot water at about 60 degrees Celsius when the external temperature was less than minus 10!
In principle the design can be scaled all the way up to one megawatt of cooling capacity.
Something that's attracted interest and funding from the navy, who currently cool their ships with
air conditioning units that leak refrigerant gases all the time, making it extremely difficult for
them to achieve their emissions reductions targets as part of the Paris Agreement. So there it is.
A system that achieves a 30% energy saving versus the current technology with absolutely no
high GWP refrigerant gases and which, by the way, also operates much more quietly than a
traditional refrigeration unit. And the potential applications are everywhere, from air conditioners
in buildings to refrigeration units on lorries and even cabin heaters in electric vehicles.
Fluid Mechanics reckon they're about three years from full production capacity
but if they can successfully bring this innovation to market then it could potentially
have a hugely beneficial impact on the energy requirements of our rapidly warming world.
If you've got feedback and views on this one, or if you work in the HVAC or refrigeration industry
and you've got insight that you can share, then jump down to the comments section below
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