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. 1000000 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. LINK 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. LINK 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. (INSERT AD) 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. LINK 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?
