Molten Salt Heat Energy Storage and Efficiency of Conversion of Heat to Electricity

MIT researchers have a new energy storage design that will store heat generated by excess electricity from solar or wind power in large tanks of white-hot molten silicon. It converts the light from the glowing metal back into electricity to get energy back out. It should be vastly more affordable than lithium-ion batteries. It should cost half as much as pumped hydroelectric storage which is the lowest cost form of grid-scale energy storage to date.

Goatguy Provided the Link to the Article and Analysis

Goatguy notes, that however interesting this idea seems, it is vexed by the low-conversion of heat (light in this case) back to electricity.

Use 1 kWh of electricity to heat already white-hot molten silicon a bit more.
Use the light produced … to power PV. At maybe 25% (dual-layer conversion).
Get 1/4 kWh back out.

Sure, with parasitic (good way) co-generation, the imposed heat could also be tapped to vaporize a closed loop of Freon, pushing turbines or cylinder heads, creating rotational energy to drive a generator too … “for free”.
Not really free, since the amortized cost of all that stuff needs be taken into account.

But still, the 25% could be pushed to perhaps 40%.

Is it “good” now to lose 60% of one’s municipal power to storage loss?
I think not.
Even if it is cheap. And easy.
Which it is not, especially with cogeneration.

Multijunction thermophotovoltaic Cells Offer the Possibility of Super-Efficient Heat to Electricity Conversion

Nextbigfuture recalls and has followed up something that we wrote about in 2009 which might be relevant. Perhaps the efficiency of the conversion can be increased much more if there is a way to tune the wavelengths of the thermal radiation. I have not been able to find the current efficiency of heat to electricity micron gap technology. They are working on a quantum version that is more efficient. Their existing product is attacking waste heat. An ultra-high temperature system would have the potential for very high efficiency.

* tune the heat frequency of the heated material and then make your thermal PV as efficient as possible for the conversion of that particular heat source.
* Controlling the variables and materials could allow for vastly higher efficiency.
* MTPV is still around getting to market for waste heat recapture. Working on a gen 2 quantum leveraging version.
* Claims from 2009. 85% theoretical max, 50+% practical max and probably 15% in prototypes in 2009.

In 2009, Nextbigfuture covered micron gap thermal photovoltaics. Linked here. Thermal photovoltaics use solar cells to convert the light that radiates from a hot surface into electricity. In a thermal photovoltaic system, light is concentrated onto a material to heat it up. The material is selected so that when it gets hot, it emits light at wavelengths that a solar cell can convert efficiently. As a result, the theoretical maximum efficiency of a thermal photovoltaic system is 85 percent.

In practice, engineering challenges will make this hard to attain, but DiMatteo says that the company’s computer models suggest that efficiencies over 50 percent should be possible. The prototypes aren’t this efficient: they convert about 10 to 15 percent of the heat that they absorb from the glass-factory exhaust into electricity, which DiMatteo says is enough to make the devices economical.

MTPV corp had gotten $10 million in funding back in 2009. They are still around.

MTPV has developed an advanced technology solution to directly convert heat to electricity. The EBLADE™ Power Platform is our solution to meet the global demand for electricity, using a 100% clean energy source, while also reducing industrial thermal emissions.

Current approaches to energy recapture are large, costly and complex, so more than 95% of manufacturing waste heat has no energy recovery solution deployed to capture it. The result – most industrial waste heat is allowed to escape into the atmosphere.

MTPV’s EBLADE™ solutions use semiconductor technology to directly convert waste heat into electricity. Our unique approach deploys photovoltaic chips tightly coupled with emitters to create a highly efficient energy conversion engine that excels in environments where temperatures exceed 600°C – over 1100°F. Future product generations will be effective in temperatures as low as 100°C.

MTPV’s system for thermal energy conversion is based on its proprietary micron-gap thermophotovoltaic technology. At its core, an MTPV system maintains a gap of less than one micron between a hot emitter chip and a cooler photovoltaic (PV) chip. The emitter chip is directly heated by the waste heat environment and radiates infrared light to a PV chip. The PV chip converts this radiated infrared light into electric power, similar to how a solar cell converts sunlight into electricity.

MTPV is working on a second-generation energy engine that exploits a fundamentally different phenomenon – Quantum Coupled Energy (QCE). The advances they have already made in Microelectromechanical systems, Energy Physics, and Thermal Technology to deliver the EBLADE™ Power Platform are all harnessed in the QCE energy engine design.

MTPV received $16 million in C round funding in 2018.

MTPV, who recently received Frost & Sullivan’s Global Technology Leadership Award for waste heat recovery solutions, creates semiconductor chips that convert heat directly into electricity.

MTPV is deploying commercial pilots today and anticipates full commercialization within the next 12 to 18 months and expects to be highly competitive in low-cost power markets within the first four to five years of commercial operation.

There was an MTPV presentation at a 2018 conference. The talk was called “Multijunction thermophotovoltaic cell for efficient power generation out of the waste heat in solid oxide fuel cell”.

Multijunction thermophotovoltaic (MTPV) cells, with integrated back surface reflector, are combined in solid oxide fuel cells (SOFCs) as a highly efficient novel hybrid system to exploit waste heat. The intellectual merit of using MTPV cell is verified analytically in terms of power output density and efficiency of the TPV cell, SOFC and designed hybrid system. Optimum performance analysis of the hybrid system is obtained based on the ratio of the emitter area to the surface area of the contact electrode, back surface reflectivity, the band gaps, as well as the number of junctions. Optimal key parameters of the systems were suggested at the maximum power density. Use of MTPV eliminated the historical tradeoff between efficiency and power density of TPV converters to provide practically-viable high-performance TPV systems. The proposed model significantly increases the employment of the waste heat produced in SOFCs for conversion to electricity. Results showed that, the innovative use of MTPV in the hybrid system, leads to a more efficient hybrid system than other SOFC-based ones because of obtaining higher efficiency and power density.

More Details on the MIT Proposed Energy Storage System

Concentrated solar plants store solar heat in large tanks filled with molten salt, which is heated to high temperatures of about 1,000 degrees Fahrenheit. When electricity is needed, the hot salt is pumped through a heat exchanger, which transfers the salt’s heat into steam. A turbine then turns that steam into electricity.

Molten silicon can withstand incredibly high temperatures of over 4,000 degrees Fahrenheit.

In 2017, the team developed a pump that could withstand such blistering heat, and could conceivably pump liquid silicon through a renewable storage system. The pump has the highest heat tolerance on record.

22 thoughts on “Molten Salt Heat Energy Storage and Efficiency of Conversion of Heat to Electricity”

  1. If this system can be produced and installed very cheaply it could be the “load of last resort” to be powered up after every electric water heater, and every hvac system within 100 miles was heated, or cooled to maximum allowable temperature, and all grid linked batteries were charged.

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  2. You can argue with the Goat, and some have proven him wrong at times. But you better have all your ducks in a row, your facts backed up and your math sound.

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  3. What you’ve cited above is one person’s blog post, and it’s deeply flawed. Perhaps unfair of me to say that, since I don’t intend to explain the various ways in which it’s flawed. Would take too long. So just take it as an unsupported assertion by someone (me) who may or may not know what he’s talking about.

    I will, however, comment on that “80% efficiency” figure.

    The 80% figure, as used in the cited post, is not for energy storage round trip efficiency. It’s only for the electrolysis step. There are electrolysis equipment manufacturers who will spec their equipment at that, but it’s misleading. For a figure that high, they have to compare the DC electrical energy consumed to the LHV (lower heating value) of the hydrogen produced. If they used the HHV (higher heating value), I believe the efficiency would be something closer to 65 or 70%.

    For energy storage, however, you have to figure in the efficiency of converting stored hydrogen back to electricity. That’s again at best only 65 to 70% efficient. Multiplying the (real) efficiency of electrolysis with the efficiency of the best fuel cells gives the 40% round trip efficiency that’s commonly taken as about the best that can be expected for electrolytic hydrogen as energy storage.

    It’s theoretically possible to do better, but there are pretty fundamental reasons why it’s unlikely, any time soon. They have to do with the overpotential required to drive the oxygen evolution reaction at a practical rate.

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  4. But gravitational potential energy isn’t subject to Carnot efficiency. In principle it can approach 100% efficiency.

    I personally like the idea of ultra large scale superconducting storage. But maybe that’s because I moved south, and miss seeing the aurora. 😉

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  5. Your proposal fundamentally violates the laws of thermodynamics, as it gets energy out of a hot body without having to have a heat sink.

    Let’s see, what “gotcha” did thermodynamics chose to use this time? Oh, yeah: A PV cell can’t work if the charge carriers have as much thermal energy as the incoming EM radiation; The leakage current consumes all the power.

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  6. Spinning has mostly been ruled out for more than a few minutes of energy stored, but people are still looking into the rocks idea. Issue is cost must be very low, because electricity is already very cheap. Gravitational potential energy is small (except if height is very large, like with some pumped hydro) compared to thermal energy over large temperature.

    https://cleantechnica.com/2018/08/21/energy-vault-proposes-an-energy-storage-system-using-concrete-blocks/

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  7. This system is expected to be 50% efficient, which turns out to be ok becsuse the cost is so low. The paper is free, feel free to take a look.

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  8. I would think that the real trick would be to put the PV inside the insulation. Any light not converted then doesn’t leave the system. So what they need is a PV that can survive the heat that it is exposed to. If the only thing energy leaving the system is the heat that leaks through the insulation and electricity, then the efficiency of the system is determined solely by the insulation.

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  9. I think the conversion efficiency could be better than 40%, or even better than 60%. The trick would be to store the molten silicon in a super-insulated tank, with a narrow passband optical window for escaping radiation. Radiation in a narrow band of near-IR optimal for PV conversion would pass nearly unimpeded, everything else would be reflected back into the tank.

    I’ve heard that band-matched PV cells illuminated by monochromatic light can achieve 80% conversion efficiency. Can’t vouch for that, however.

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  10. There’s been talk about making a photonic crystal, that would not pass wavelengths over a set length. I think you’re supposed to heat the inside of the crystal to the required emission of light, and frequency waves are blocked.
    Your PV module is of of course tuned to the critical wavelength, and a bit higher frequencies.

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  11. What happened to pushing rail road carts of rocks up a hill and sucking energy out when they come down?

    Or carbon nanotube fiber spinning wheels?

    concrete rings up a tower? Water up a tower? Compressed air under water, in a hole in the ground?

    Superconductor storage ring?

    There are a lot of ways to store energy. The problem is at what cost.

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  12. Conversion efficiency back to electricity not withstanding, white hot molten Silicon implies it is hot enough to use as thermal power in industrial applications.

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  13. Well the educational system says you can only get a PhD on ‘new’ science. So schools gin-up newish twists to existing plateaued technology. Then we shoot holes in it, but it did its job and got the guy or gal graduated.

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