After decades of improvement, solar and wind are now some of the cleanest and cheapest
ways to produce elecricity. But there’s a major problem.
For renewables to really make an impact, we’ll need a reliable and cost effective way to
store the energy they produce so we can access it any time, anywhere.
We’ve covered tons of energy storage technology on this channel, from traditional lithium-ion
batteries to mechanical energy storage — even a sand battery.
But one recent breakthrough by researchers at MIT may finally present a real solution.
The technology is called Thermophotovoltaics, and at its core it uses alot of familiar technology
but in some truly innovative ways.
So how does this technology work? How does it stack up to other energy storage technology?
And can it really provide the energy storage solution we’ve been waiting for?
Let’s dive in.
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Right now, more than 90% of the world’s electricity comes from heat sources — like
coal, natural gas, nuclear energy, and concentrated solar energy. For centuries, these technologies
have relied on steam turbines. But the truth is, this method isn’t particularly effective.
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For one thing, these systems rely on complex machinery that can be difficult to operate
and maintain. But the biggest drawback is efficiency.
While the highest efficiency turbines operate around 60% efficiency, on average, steam turbines
only operate at around 35% efficiency. One main reason is that most industry standard
turbines can only handle so mch heat. Anything at or above 1500 degrees C and the machinery
starts to break down. This means that lots of heat energy ends up simply going to waste.
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This is why so many researchers have been looking into solid-state alternatives. Heat
engines with no moving parts that could work efficiently at significantly higher temperatures.
Enter thermophotovoltaics.
… Thermo…photo…what now?
I know, its a mouthfull. Thankfully the shorthand is TPVs. What are TPVs?
TPV cells have alot in common with traditional solar cells. But, instead of converting solar
radiation into electricity, these cells absorb radiation from a heat source in the form of
thermal radiation. See thermo — photo — voltaics. We’ll dive into the specifics in a bit,
so stick with us! LINK
This particular breakthrough comes from MIT researchers led by Asegun Henry. Associate
Professor of Mechanical Engineering in partnership with the National Renewable Energy Lab.
Really, it’s two breakthroughs. The first is a revolutionary way of storing renewable
energy.
It starts by feeding surplus wind or solar energy into a semiconducting material, like
tin, heating it to incredibly high temperatures — we’re talking well above 2,000 degrees
C (over 3600 F). At that temperature, the tin melts. LINK LINK
The molten tin then gets pumped through a closed-loop system of carbon composite pipes.
Those pipes lead into a sealed, insulated building filled with massive blocks of graphite.
The whole room is filled with argon gas which helps keep the room inert — or chemically
inactive — which helps prevent the carbon and tin from oxidizing.
Once the heat transfers into the graphite, it can stay there for a long time.LINK
Why graphite? Well, if you remember from out graphene videos we did a while back, Graphite
is an excellent heat conductor due to its layers of carbon atoms and free electrons.
But, there’s more.
According to Professor Henry, the rate at which heat leaks out of the blocks is proportional
to their surface area, where as the amount of energy they can store relates to their
volume. A small object has large surface area to volume ratio.
That’s why a hot cup of tea cools down to room temperature in a relatively short amount
of time. But large objects have a much smaller surface area to volume ratio. The graphite
blocks cover an area about half the size of an American football field. They will literally
take months to cool down, making them the perfect medium for storing thermal energy.
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But wait, Ricky, you ask — haven’t you covered concentrated solar batteries before?
Didn’t one utterly fail, costing billions of dollars? What makes this different?
The researchers at MIT have found some innovative ways to fix the shortcomings of other similar
storage technology.
If you remember from our video about the Crescent Dunes plant in Nevada, one reason that system
failed was because the high temperature working fluid kept springing leaks causing massive
stalls. And stalls mean lost time and money, which is why it ultimately shut down.
One way the MIT researchers resolve this issue was bydeveloping a carbon-based sealing solution
which can effectively trap the hot liquid in the closed loop system. In laboratory experiments,
the team was able to pump molten tin at temperatures well above 2000 degrees celsius with no leaks
or other issues. The technology was so groundbreaking that it warranted an entire scientific paper
in its own right — and that’s not even the main breakthrough!
Still… we’ve seen “breakthroughs” before that use high temperatures and moment
materials. While the team has so far shown the resilience of the materials in the lab,
its still worth noting how many systems have literally crashed and burned. Even the tiles
on space fairing craft, are often replaced after a few trips. Because this technology
is still int he experimental phase, we can’t know for sure how they’d hold up in real
world situations.
So that’s how this systems sotres energy. But… what about when we need to extract
power?
This is where the TPV cells come in.
Whenever the grid demands power, heat from the graphite blocks transfers into a second
building which houses an array of large, hollow, rectangular carbon-based chambers lined with
tungsten foil. LINK
Why tungsten? One reason is that tungsten tends to radiate photons across a broad spectrum,
from high-energy ultraviolet to low-energy far-infrared. Basically, it emits more useable
light that the TPV cells can absorb and turn into electricity.
Once the heat gets transferred, The tungsten glows white hot, like the filament in an incandescent
light bulb.
The cells sit inside a chamber, called the Power Block. Depending on how much electricity
is needed, the cells can be raised up and down inside the Power Block increasing or
decreasing exposure to the light emitted from the glowing hot tungsten.
Now, as we mentioned up top, these cells have a lot in common with the solar cells you have
on your roof, but they also have some key differences.
One major difference is the materials they’re made out of. The solar panels on your roof
are made out of silicon and are tuned to have a specific band gap that determines the wavelength
of light that can interact with it to knock its electrons out of position and send them
to an electrical circuit. All photovoltaic cells, including TPV, are tuned to absorb
photons in a narrow range, which often means light with higer and lower frequencies get
wasted.
Instead of of silicon, the MIT team opted for a material called gallium arsenide, the
same stuff NASA uses for their solar panels in space! GaN ​​can absorb relatively
more energy from the incident solar radiations because of the relatively higher absorption
coefficient. This also means, in this context, in can absorb energy from light emitted from
the glowing tungsten instead of the sun.
But the TPV cells also have one other key difference. The cells used on your roof are
called “single junction” cells, meaning ​​they use only two different semiconductor
materials for its p-n junction. While single junction cells are generally considered low-cost,
they also suffer from lower efficiency due to what’s known as the Shockley–Queisser
limit, essentially the maximum efficiency of solar cells based on the principle of detailed
balance. It places the maximum solar conversion efficiency at 33.7% for a single-junction
solar cell with a band gap of 1.4 eV and AM1. 5 spectrum. LINK
The TPV cells, however, utilize multiple gap junctions in order to jump over this limitation.
Henry’s team laid down more than two dozen thin layers of different semiconductors to
create two separate cells stacked one on top of another.
We actually covered multi-gap junction solar cells in our “Solar Panel World Record”
video which we’ll link below if you want to check that out.
Each layer has a slightly different bandgap to absorb a wider range of the light spectrum.
The higher bandgap captures the highest energy photons from the heat source. In this case,
the top junction can capture light in the visible part of the spectrum with wavelengths
around 0.8 microns or 800 nanometers.
The bottom junction is modified to capture infrared light. Together, they can capture
a much wider spectrum of light than typical solar cells. LINK
How do the cells endure such intense heat? The solution is brilliant. The cells are mounted
onto four sides of a heat sink, a block of metal with water channels flowing through
it. The water draws heat away from the cells, but the key is that it moves quickly enough
that it stays as a liquid instead of turning to steam. LINK
But it doesn’t stop there. While these calles can absorb a wider array of light compared
to their silicon siblings, there’s still a large amount of light that escapes.
But the team doesn’t want that precious energy to go to waste. So, right at the base
of the cell the team placed a highly reflective mirror . This mirror bounces any remaining
light that managed to escape the multi-junction gauntlet back through the cell and into the
tungsten material where it's reabsorbed to help keep the tungsten as hot as possible.
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This not only helps avoid wasting energy, but maximizes the extraction of luminescence,
which in turn boosts the voltage. This has enabled the creation of record-breaking solar
cells.
That means the quality of the mirrored surface plays a massive part in the efficiency of
the cell. The mirrors currently used by the team
reached a maximum reflectivity of about 93%. With these numbers, the group says that their
TPV cells converted about 41.1% of the energy from the 2400 degree C tungsten filament into
electricity, a sizeable lead over even today’s highest rated silicon, single junction cells.
But it gets even better! Professor Henry explained that his team already have a well-defined
pathway for reaching 98% reflectivity squeeze out an extra 10 percent of overall efficiency
in the cell — reaching an over-all efficiency of 50%!
How much power would that make?
Well, if these cells could reach that 50% efficiency mark — a 1 square meter cell
could generate around 100 kilowatts. If you’re thinking 100 kW that can’t be right, after
a similarly sized solar panel only makes about 300-400 watts, or 1/25th as much, that just
goes to show you how much energy and subsequently light tungsten puts out when heated to 2400C!
So, an array of 30 by 33 modules — about 990 square feet — will have a total capacity
of around 99,000 kilowatts, or close to 100 megawatts. The graphite blocks can hold enough
heat to keep the cells generating power for roughly 10 hours. 100 Megawatts per hour for
10 hours means a site could provide a thousand megawatt hours, or one gigawatt — enough
to power tens of thousands of homes! [LINK]
And, compared to traditional steam powered turbines, which again suffer from having complex
moving parts, TPV cells can deliver energy instantly, the same way traditional solar
cells would. This makes them more ideal for grid scale applications.
Now, it’s worth pointing out that while 50% efficiency is nearly double the efficiency
of the average silicon panel today, which tops out around 22%, AND more than the 35%
achieved by the average turbine, it still falls quite short of some other storage systems
— specifically lithium ion batteries. The reason Lithium Ion has been the front runner
in the energy storage race for so long is because it is incredibly efficient — converting
about 90% of energy back into electricity.
BUT some of the key reasosn we have yet to see Lithium Ion batteries really find their
way into grid storage in large volume, is expensive raw materials like lithium and nickel,
complex expensive supply chains, and ultimately cost.
According to the EIA, average battery energy storage capital costs in 2019 were $589 per
kilowatthour (kWh), with Lithium Ion hovering around $300 per kilowatt hour. At the time
of this video, Professor Henry recently launched a venture — Thermal Battery Corp. — to
bring this technology to the commercial market. After considerable research, Henry and his
team estimate this system could store energy at a fraction of the cost — around $10 per
kWh of capacity! LINK LINK
Remember that gallium arsenide solar panels are way more expensive than silicon panels
we use for our homes, but with such an intense light source, these cells are producing so
much electricity and the economics totally shift and they become quite cost effective.
Remember that our sun is 93M miles (150M km) away from us and the strength of the light
falls of by the square of the distance. So if the sun were ½ the distance from us, the
intensity of the sun’s light, would be 4x higher. This is the key to the superheated
tungsten that is placed very close to the TPV panels.
And the best part is, installation could be incredibly simple. The team imagines rolling
out their TPV storage systems by retrofitting decommissioned gas peaker plants.
One final benefit of the system is that each component operates independently of the others.
Meaning the system is modular and easily adaptable to the needs of a given grid system. Need
more storage? Simply add more graphite blocks. Need more energy delivered? Add more solar
cells.
As of right now, Thermal Battery Corp. plans to roll out a 1 mWh pilot system by the end
of 2022, with plans to eventually implement a 50 mWh commercial scale system by 2026.
And all these numbers are based on current technology. We’ve talked about the potential
applications of graphene instead of graphite — perhaps this could be an excellent application.
And, as we mentioned, we did another video not long ago about record-breaking multi-junction
solar cells. Who knows, maybe a storage system like this could be th perfect home!
Now all of this sounds super promising. But there are still lots of challenges that Thermal
Battery Corp need to solve, and prove they can manage. Moving heat around from graphite
blocks, through model tin to separate chambers to heat tungsten is all complex, requires
very advanced complex pumps and systems to cope with all that heat. How reliable would
such a system prove to be? How much down to for maintenance will be required? Plus blasting
gallium arsenide pv cells with such intense heat and light is another new challenge engineers
haven’t faced before. Even with sufficient cooling to keep the panels from overheating,
how long would such a cell last? These are all incredibly challenging questions that
are yet to be determined.
I think the potential for such a system is pretty incredible, especially because the
thermo-photo-voltaic energy generation concept, can be coupled with all sorts of heat storage
sources. Like the Finnish Sand Battery we’ve talked about in this video. It’s almost
better to think of these systems are blocks that can be combined in different ways. Based
on region, and natural resources, heat can be stored in graphite or sand, or some other
interesting medium, and the thermal photo voltaic system could be the missing ingredient
to convert this stored energy back into electricity for the grid. Remember also, that there are
tons of industrial applications that produce excess heat that is currently just wasted.
Could we start to see TPV systems co-located at power plants, or smelters, to improve efficiency
and absorb some of that lost heat? The opportunities abound, but before we get ahead of ourselves,
we need to see how this pilot plant performs.
But let us know what you think. Could this breakthrough finally provide the energy storage
system we’ve been waiting for? Does the cheaper price make up for the lower efficiency?
What other Grid Energy Storage Systems should we look into?