Every now and then a technology comes along that at first sight looks completely crazy
but which on closer inspection starts to make quite an awful lot of sense. And that's kind of
how it is with thermophotovoltaic or TPV technology. Now to avoid any confusion,
I'm not talking about the photovoltaic thermal or PVT panels we looked at a couple of weeks ago. No,
the idea with TPV is to plunge photovoltaic cells into a super hot material at about 2400 degrees
Celsius and instead of simply being instantly vaporized as you might logically think, the cells
would actually start producing electricity with an efficiency of at least 40 percent.
Completely bonkers right? Well maybe not. In fact, once the details of the overall system
are revealed it really does begin to look like something that could provide very long duration
utility scale energy storage at a fraction of the cost of current lithium-ion battery technology.
So let's just dive straight in and take a look.
Hello and welcome to Just Have a Think. Thermophotovoltaics or TPVs are not a new concept.
The basic idea is that they convert predominantly infrared wavelengths of light coming from a heat
source into electricity using essentially the same process found in normal solar PV cells that you
put on your roof. The challenge, until recently, has been getting them to a level of efficiency
that starts to compete with existing combined cycle gas turbines, which are generally thought
to be the most efficient heat engines currently available. The first part of the problem
is finding a way to efficiently heat a material to a high enough temperature
to produce light at just the right intensity and wavelength. The second hurdle is how
to avoid the whole TPV cell vaporization thing I mentioned a moment ago. Ironically,
that means somehow keeping the cells cool while they're inserted into a white hot environment.
Sounds a bit like a contradiction in terms doesn't it, but that's exactly what a team from MIT has
been working on in recent years. And they've just published details of their breakthrough technology
in this paper. To get the inside track on how the whole system works in practice I got in touch with
one of the paper's main authors, Asegun Henry, who is a mechanical engineer and Associate Professor
at MIT. Professor Henry explained that the big advantage of TPV cells is that they can facilitate
very large utility scale energy storage at very low cost compared to existing technologies.
Here's how it works. The system uses excess electricity generation from renewable energy
sources to power up a very large heating element, which in turn heats tin into a molten state that
can be pumped around a closed-loop system. Now here's where the MIT folks were able to make
their first major breakthrough before we even get to the TPV technology. You see if you asked most
engineers to pump molten metal around a system at very high temperatures they'd probably tell you
it was impossible, mainly because there'd be no way of effectively sealing the various components
in the loop at those sorts of temperatures. So the MIT team developed a carbon-based sealing
solution that overcomes this problem. This video capture shows the laboratory setup pumping molten
tin at 1400 degrees Celsius. The team has now demonstrated the ability to pump at temperatures
in excess of 2000 degrees Celsius. The methodology was so ground-breaking that it merited an entire
scientific paper in its own right, which I'll link down in the description section below.
The super hot liquid metal travels through a network of carbon composite
pipes into a sealed and insulated warehouse size building containing massive blocks of graphite.
The super hot pipes radiate heat into the graphite blocks and the whole building is
filled with argon gas which keeps the environment as inert as possible and prevents the carbon and
the tin from oxidizing. Once that heat is transferred into the graphite blocks
they stay hot for a very long time indeed, not just as a result of the good quality insulation,
although that obviously does play a role, but mainly as a result of the laws of nature which,
as Professor Henry explained, dictate that the bigger the blocks are the smaller the losses will
be. That's because the rate at which heat is able to leak out of the blocks is proportional to their
surface area, whereas the amount of energy stored in the blocks is proportional to their volume. A
small object has a large ratio of surface area to volume. That's why the cup of coffee on your
desk only takes about an hour to cool down to room temperature. But very large objects
have a much smaller surface area to volume ratio. The graphite blocks in this system cover an area
the size of half a football field, which means they will literally take months to cool down.
So now we've got a very large scale thermal energy storage facility that can be located,
for example, on the site of a decommissioned gas peaker plant. Decommissioning expensive,
highly inefficient and massively polluting gas peaker plants is all the rage these days,
so there'll be no shortage of locations where this technology can be utilized. Anyway, I digress.
So, here's where the TPV technology comes in. Whenever there's a power demand from
the grid the heat from the graphite blocks is transferred to a second warehouse type building,
also filled with argon, containing an array of large hollow rectangular carbon based chambers
lined with a tungsten foil. The tungsten foil glows white hot as it heats up, exactly like
the filament in an incandescent light bulb. The TPV cells sit inside the chambers, which the MIT
refer to as a Power Block, and depending on how much electricity they're required to produce at
any given time the cells can be raised up and down inside the block, exposing more or less of
their surface to the light. We'll come back to how that works in a second, but first of all,
why don't the cells just vaporize on contact with 2000 degrees heat like we said earlier?
Well, if we take a closer look we can see that the cells are mounted onto four sides of a heat sink,
which is essentially a block of metal with channels of water flowing through it.
The water is continually drawing heat away from the cells and as long as it flows quickly enough
it'll always stay liquid and not change phase into steam. That fast flowing water then circulates
through what is effectively a massive version of a car radiator, where the heat energy is dissipated,
allowing the cooled down water to flow back around the closed loop and back into the
heat sink underneath the TPV cells. So what about those TPV cells then? Well,
if we delve inside them we find that there are two junctions with different band gaps. We've looked
at the concept of multiple junctions and band gaps in a previous video, which you can jump back to by
clicking up there somewhere. So I won't go into the minute detail of that here, suffice to say
that the band gap of a material determines the wavelength of light that can interact with it to
knock its electrons out of their position and send them into an electrical circuit.
Rather than using silicon as the light capturing material, which is what you find in most solar
PV cells, the MIT team have opted for gallium arsenide, which is the same stuff that NASA
and satellite operators use for solar panels in space. It's a much better quality material
with far fewer defects, which is capable of coping with the high intensity of light produced
by the tungsten power blocks. In this case the top junction in the cell has a band gap that captures
light in the visible part of the spectrum with wavelengths around 0.8 microns or 800 nanometers.
That's represented by the dark blue section over here on the left hand side of this graph.
The bottom junction has a slightly modified molecular structure which enables it to
capture light in the infrared part of the spectrum, represented by the narrower lighter
blue section of the graph. Right at the base of the cell is a mirror that bounces any remaining
light in the infrared part of the spectrum back through the cell and into the tungsten material
where it's reabsorbed to help keep the tungsten as hot as possible.
The higher the peak on the graph, the greater the power output it represents. So you can see that
the power produced by the cell is derived from the visible and near infrared light, while the
majority of the light simply goes through the cell and is reflected back to the tungsten emitter.
That means the quality of the mirrored surface plays a massive part in the efficiency of the
cell. Previous research had already achieved mirror reflectivity of 98.5 percent.
Using that methodology, plus their own technological breakthroughs,
the researchers in the MIT team plan to demonstrate an overall cell efficiency close to
50 percent. They've already achieved 40% using a mirror that was only 93% reflective, and Professor
Henry explained they have a well-defined pathway for reaching the 98% reflectivity target to eke
out that extra 10 percent of overall efficiency in the cell, and that'll be getting close to the
efficiency achieved by combined cycle turbines. This animation gives you a rough idea of the size
of a typical configuration compared to the pickup truck in the bottom right hand corner. Each TPV
cell module will have about one square meter of area and will have a generating capacity of around
100 kilowatts. So an array of 30 by 33 modules will give an overall capacity of 100 megawatts.
There's enough heat in the graphite to keep the cells generating electricity for 10 hours which
means a site like this would provide a thousand megawatt hours of energy, or one gigawatt hour if
you prefer. That's enough for tens of thousands of homes. One very significant additional advantage
of setting the system up with this kind of modular configuration is that everything is decoupled.
In other words each component can be modified in isolation from the other parts of the system.
So if a grid operator needed very fast charge times they could simply increase the size of the
electrical heater. If they required more storage capacity they could just add more graphite blocks,
and of course the same principle applies to the number of TPV cells, allowing discharge
capacity and rates to be fully adjustable too. That's an extremely attractive feature for grid
operators who are always looking for maximum flexibility on their systems, especially as more
and more intermittent renewable power sources come online over the next couple of decades.
And because the electricity is being generated by a PV cell rather than a steam turbine,
you get instant response times just like a lithium-ion battery, which means this system is
much better than a gas peaker plant at following the peaks and troughs of the daily demand cycle.
Professor Henry and his fellow mechanical engineers on the project
have crunched the cost numbers and unlike some other research projects they've really considered
all of the variables that would come into play to get to real-world commercial production.
Once all those elements are factored in they reached an overall cost of less than ten
dollars per kilowatt hour. Compare that to the fully installed cost of a typical utility scale
lithium ion battery configuration, which can be more than $300 per kilowatt hour,
and you start to see the very compelling economic advantage of the TPV system. Plus
you've got all that much greater flexibility that I just mentioned. Unsurprisingly then,
Professor Henry and his colleagues are quite keen to get cracking on ramping this thing up.
A start-up business called Thermal Battery Corporation has already been created to get
the concept from laboratory prototype to full commercial scale. They're already working with
an existing producer of gallium arsenide based PV cells with a view to manufacturing a one megawatt
hour pilot system during 2022. Following that a 50 megawatt hour commercial scale demonstrator will
come online in 2026, with the first full-scale one gigawatt-hour system planned for 2028. That should
be pretty good timing to slot nicely into place as all those gas peaker plants I mentioned earlier
get decommissioned. Now of course this technology will be entering a very competitive marketplace
with all sorts of energy storage solutions coming online all the time,
many of which we've featured on this channel. So Professor Henry and his colleagues have
certainly got their work cut out! But commercial partners are already being secured and discussions
are already taking place with electricity grid operators, so there's a decent chance we could
see this innovative energy storage system cropping up all over the world in the next decade or so.
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