[HN Gopher] A new heat engine with no moving parts is as efficie...
___________________________________________________________________
A new heat engine with no moving parts is as efficient as a steam
turbine
Author : WithinReason
Score : 394 points
Date : 2022-04-13 15:48 UTC (7 hours ago)
(HTM) web link (news.mit.edu)
(TXT) w3m dump (news.mit.edu)
| danans wrote:
| > The researchers plan to incorporate the TPV cell into a grid-
| scale thermal battery. The system would absorb excess energy from
| renewable sources such as the sun and store that energy in
| heavily insulated banks of hot graphite.
|
| The article doesn't provide the efficiency of this insulated hot
| graphite battery. Given that this device is meant to be paired
| pretty exclusively with such a battery, it seems like a narrowly
| applicable solution. Or are there other renewable sources of heat
| between 1900C and 2400C that could be used for this purpose?
| kragen wrote:
| The efficiency of heating things up is pretty much 100%. The
| efficiency of insulation can be arbitrarily high (up to 100%)
| or arbitrarily low; heat loss is proportional to surface area
| and storage time and inversely proportional to insulation
| thickness. If your heat engine is only 40% efficient you might
| as well design your insulation to be 80% efficient or so over
| the expected time span, which would be a few hours for grid
| storage systems.
| danans wrote:
| I was thinking more about the efficiency of the mechanism
| that transfers the heat from battery to the TPV engine. The
| article said that the device has to be exposed to photons
| coming off a white-hot source.
|
| I suppose that if the TPV devices are closely coupled with
| the white-hot source this should be minimal but it's not
| clear from the article whether there is an intermediary step,
| or how they plan to control the TPV's exposure to the heat
| source to discharge the battery.
|
| Given that this works via radiation (vs convection like a
| traditional heat engine), the path between the heat source
| and the TPV would have to be fairly direct, and not lose much
| heat to any adjacent non-TPV material.
|
| Maybe they have some kind of massively insulated
| opening/closing heat shield that they can use in a manner
| similar to the gates used to control water in hydroelectric
| plants.
| inetknght wrote:
| So does this mean I can attach one of these things to my CPU and
| get back some of the power used?
| lazyier wrote:
| Only if your CPU is producing around 1,900 to 2,400 degrees
| Celsius
| ziffusion wrote:
| My soup then.
| disqard wrote:
| It requires "a heat source of between 1,900 to 2,400 degrees
| Celsius, or up to about 4,300 degrees Fahrenheit" -- so your
| Nvidia gpu should work great for this purpose :)
| sfink wrote:
| Sure, if you're running your CPU at 1900degC.
| zionic wrote:
| Sure, you get ~40% back once you hit 1,900 to 2,400C
| jotm wrote:
| No, but you could realistically make a small Stirling engine
| that would act as a heatsink for your processor while
| generating electricity.
|
| Would be noisy as hell, but quite a cool project haha
| HPsquared wrote:
| It's not really fair to compare this, which takes heat at
| 2000degC, to a steam turbine that takes heat at say 550degC.
|
| A turbine system would have much better efficiency than 40% if
| its heat was available at that temperature. For example a closed
| Brayton cycle gas turbine + steam turbine system. Certainly
| complex and expensive, but could get much better than 40%
| efficiency.
| lovemenot wrote:
| If a steam turbine _could_ operate at those higher temperatures
| it would be more efficient. But it cannot do so under any
| reasonable condition. If you want the higher efficiency from
| storing high grade heat energy, it 's not feasible to use a
| steam turbine.
|
| Think of the steam turbine as a baseline. Like rating a vehicle
| in horse power.
| cormacrelf wrote:
| Also I wonder if, using similar principles to a heat pump's
| operation, you could still get juice out of stored heat at
| lower temperatures. Surely you can have this hot graphite
| sitting at under 2000degC, heat some fluid/gas to say
| 1000degC, and then compress the gas to increase its
| temperature? Surely that would be the ideal solution anyway,
| since you don't want your hot graphite to become a chunk of
| useless heat simply for dropping below temp briefly.
|
| On the topic of heat pumps, you could also use a TPV for
| geothermal power. Since there are no moving parts and
| presumably no huge steam engine installation, it would be
| more feasible to have one of these in your back yard. The
| grid powers a heat pump, you compress the fluid till it hits
| 2000degC, and your TPV extracts power. The heat pump itself
| is >100% efficient, so overall you can steal a fair bit of
| electricity from the ground. Right?
| bigcat123 wrote:
| omgJustTest wrote:
| The Carnot efficiency of a heat engine favors large delta T! This
| is the theoretical limit of efficiency. For steam turbines this
| efficiency should be around 60-70%, theoretically.
|
| While cool (1,900 to 2,400 degrees C cool) the Carnot effiencies
| should be closer to 86%.
|
| The idea that heat engines get more efficient as you increase
| delta T has been around for a while. The problem is constructing
| a delta work extraction loop that doesn't have more losses as a
| result of the delta T increase, ie the practicalities of
| extracting work energy.
|
| Warning: I am assuming they are working with an approximately
| room temperature cold side, as article doesn't say. The
| practicalities of allowing for the delta-t is usually where the
| efficiency losses are made.
|
| source: I am an electrical / chemical engineer.
| driverdan wrote:
| While this is far away from being a production device it's pretty
| exciting to see 40% efficiency.
|
| How would you go about converting the energy stored in a thermal
| battery to a high enough temp for this to work? Some kind of heat
| pump? It has never been clear to me how to concentrate heat like
| that.
| jandrese wrote:
| I think they're planning to heat up big chunks of graphite to
| thousands of degrees C with electric heaters powered by the
| grid and then attach these thermocouples when the renewable
| sources start to flag (windless nights) to cover the gap.
| diarrhea wrote:
| Heat radiation losses (scaling with the fourth power of the
| temperatures) would obliterate such a storage solution. I
| could only imagine it as very short-term, at which point
| heating using electricity and converting back shortly after
| quickly becomes pointless.
| danans wrote:
| I don't know how they mitigate the radiation losses (huge
| amounts of insulation?), but it appears that graphite has
| some interesting properties when heated, like it gets
| stronger 1000-2500C and doesn't expand much when heated
| [1]. Perhaps those properties help it to store heat
| effectively at those extreme temps.
|
| 1. https://nucleus.iaea.org/sites/graphiteknowledgebase/wik
| i/Gu....
| lazyier wrote:
| > I don't know how they mitigate the radiation losses
| (huge amounts of insulation?)
|
| My guess is a vacuum flask made of tungsten. Tungsten
| sheet metal is a thing and it has the second highest
| melting point of currently known elements, which is
| 3,422C.
|
| Then for the 'battery' you will want to find a material
| that has a phase change around the temperature required
| for this heat engine to operate. The energy required for
| water to go from 100c water to 100c steam is considerably
| more significant than the energy required to go from 0c
| water to 100c water.
|
| Graphite will likely come into play because you need to
| have electrodes to heat the material up to storage
| temperature. Graphite is a good for this sort of thing.
| Although gradual erosion will likely limit the life of
| the battery.
| danans wrote:
| If that's the case, would the tungsten flask heat up to
| 2000C and give off the photons that are absorbed by the
| TPV? I still don't understand how they will control
| transmission of the heat at those temperatures.
| jandrese wrote:
| Maybe insulate with reinforced carbon-carbon? I'm not an
| expert on this, but it doesn't seem outright impossible
| on the surface, but the details are always what get you.
| namibj wrote:
| You need vacuum super insulation. Basically take a
| thermos, and fill the vacuum with (mostly) non-touching
| reflective foil. It's the golden stuff satellites are
| wrapped in.
| danans wrote:
| The insulation I sort of understand. It's how they
| temporarily breach the insulation to let out the photons
| to strike the TPV that I don't understand. With normal
| heat->electricity conversion, you transfer heat to a
| turbine via a fluid (i.e. water) that is allowed to
| contact the exterior of a heated vessel, but in this case
| you need to somehow open a slot to let the photons
| radiate out.
| z3c0 wrote:
| I'm curious what the implications would be for solar panels, or
| for any device that outputs excess thermal energy (like our
| computers). Would a solid device like this allow recapturing
| some of the energy that would otherwise be lost?
| visarga wrote:
| I don't think it's hot enough.
| diarrhea wrote:
| Only at tremendously low efficiencies it seems.
| waynecochran wrote:
| How long/well can "insulated banks of hot graphite" hold heat?
| lacrosse_tannin wrote:
| What is it made of?
| arc-in-space wrote:
| 0 comments after 20 minutes on an energy post? How am I supposed
| to know why this won't work, won't be useful, won't be cost
| effective, won't scale, and that it's just a fad?
|
| Jokes aside, this seems impressive, I have no idea what the best
| applications would be but wikipedia claims that current similar
| devices have fairly bad efficiency(https://en.wikipedia.org/wiki/
| Thermoelectric_generator).
| willhinsa wrote:
| Thanks for the laugh! So true.
| javajosh wrote:
| _> why this won't work, won't be useful, won't be cost
| effective, won't scale_
|
| Not an expert, but reading this a few negatives popped out.
| Basically they are heating a black body to 2400C and then
| making electricity from gathering the emitted light in a cell.
| They get to pick a temperature to match the bandgap of the
| cell.
|
| The key problem is getting something that hot without using
| another (lossy) form of power. The Sun's surface is ~5600C so
| that's enough headroom to get there from solar. That's cool.
| But are there any fission reactors that get (or could get) that
| ridiculously hot?
| Animats wrote:
| _" The team's design can generate electricity from a heat
| source of between 1,900 to 2,400 degrees Celsius"_
|
| That's way up there. That's well above the melting point of
| steel. That's above the highest temperature jet engines made
| for experimental aircraft.[1] Most jet engines try for
| exhaust gas temperatures around 600C or so, for a long useful
| life. Typical nuclear reactors, around 300C.
|
| It's not impossible to operate up at those temperatures.
| Every steel plant does it. There are ceramic and brick
| materials that can deal with such temperatures.[2] The
| storage medium would probably be some molten metal.
|
| This seems way too much trouble just to store energy.
|
| Now if this thing worked at 600C or so, there would be more
| uses.
|
| [1]
| https://www.nasa.gov/centers/dryden/pdf/88068main_H-1375.pdf
|
| [2] https://www.ceramicsrefractories.saint-
| gobain.com/refractory...
| thehappypm wrote:
| I think they're envisioning a no-moving-parts kind of
| system, which makes dealing with difficult environments
| easier.
| walnutclosefarm wrote:
| No, they claim no moving parts for the generator, but
| refer to engineering designs that use pumped liquid tin
| to move the heat within the system. Clearly the challenge
| there is building a pump that can handle liquid tin at
| 2400C.
| Animats wrote:
| Liquid metals can be pumped with a linear motor type
| magnetic field, with the liquid metal being the moving
| part.[1]
|
| But all this just to replace a battery?
|
| [1] https://www.comsol.com/model/inductive-liquid-metal-
| pump-590...
| hetspookjee wrote:
| I think the sheer size would make it interesting. The
| heat energy potential of acres full of graphite is
| enormous and presumably much cheaper to construct than an
| energy equivalent battery. Now I wonder how it holds up
| to other methods of storing energy.
| [deleted]
| robotresearcher wrote:
| The article mentions the planned storage medium is
| insulated banks of graphite.
| giantg2 wrote:
| Chernobyl did.
|
| I think most try to keep temperatures under 1000C. I think
| many FAST reactor designs are looking at 600C operating temps
| with peak temp reaching maybe 1200C during emergency testing.
| But my memory might be wrong.
| sandworm101 wrote:
| Biggest key problem imho is how they expect to store this
| heat energy. It looks like this cell will, like a PV cell,
| constantly be absorbing photons. If those photons aren't
| creating electricity/voltage across a gap then they are being
| converted into heat. So to keep the medium at temperature you
| would need to insulate it, to wrap it in a mirror, only
| exposing the flux to the energy-collecting cell as needed.
| That means moving parts.
|
| As for available temperatures from fission reactors:
| https://en.wikipedia.org/wiki/NERVA
|
| "When the reactor was operating at full power, about 1,140
| MW, the chamber temperature was 2,272 K (2,000 degC)"
| thehappypm wrote:
| This is a bummer, but probably a reality. Perhaps some kind
| of LCD type tech could make for it to be more digital/less
| big metal moving around.
| DontGiveTwoFlux wrote:
| Not mentioned in the article is power density. How quickly can
| the energy be released? Consider solar panels, you need a table
| sized cell to get 100W. That can make for a big battery to get
| grid scale power output if these cells are only as power dense
| as solar panels. The energy density of a heat based solution
| can be very high- metals can get very hot and they are dense
| enough to store a lot of energy. But if you can't get the
| energy out of the battery fast enough that limits the
| applications. By comparison lithium ion batteries can dump
| power out extremely quickly, which is what makes them great for
| cars. Hydro is even better.
| walnutclosefarm wrote:
| The article in Nature quotes an energy density of 2.38
| w/cm^2. Which means a Gw battery would require 10e5 m^2 of
| absorber surface, exposed directly and at close range to the
| radiation from molten metal (which is the heat transfer fluid
| they propose). It has to be direct, and at close range,
| because the efficiency they quote relies on the absorber
| reflecting non-absorbed photons directly back into the
| emitter, where they are re-absorbed as heat and potentially
| re-emitted.
|
| That's about 25 acres of absorber, and an implied 25 acre
| surface area of the liquid metal emitter pool.
|
| There is a basic challenge here to the design - the energy
| storage density for the thermal battery they envision scales
| as the cube of the characteristic dimension of the plant, but
| the power density that can be delivered scales only as the
| square of dimension. Not saying that can't be dealt with in
| engineering, but it ain't going to make this easier or
| cheaper.
| matthewfcarlson wrote:
| I think their application is grid scale and you can scale
| across hundreds of batteries to provide the throughput you
| need. I don't know how I feel about having a small molten
| ball of metal inside the hood of my car. Turns my car into
| the most dangerous gusher in the case of an accident (for
| those who aren't familiar, gushers are a gummy like candy
| with juice inside).
| Retric wrote:
| Surface area is relevant for solar because the sun is so far
| away. A local heat source allows you to surround it with 3D
| shapes not just a flat plain.
|
| As to temperature this thing is for very high temperatures:
| _can generate electricity from a heat source of between 1,900
| to 2,400 degrees Celsius_. At 40% efficient you need a wide
| temperature difference which would suggest a high energy
| density.
| walnutclosefarm wrote:
| This design is photovoltaics, just like solar, but
| optimized for infrared photons. There is no avoiding the
| reality that energy storage density will scale as the cube
| of the facility size, but power density only as the square.
| And at 2.38 w/cm^2, the scale coefficient is not all that
| great.
| ethbr0 wrote:
| For long term energy banking and if we can get them
| working, flow batteries seem vastly superior to all
| alternatives, by scaling storage with regards to tank
| volume. Instead of some difficult-to-manufacture structure.
| smrtinsert wrote:
| Exactly why I clicked in, but man this technology sounds like a
| serious gamechanger.
| GoodJokes wrote:
| Manuel_D wrote:
| This device doesn't really change the energy landscape. Let's
| rephrase the title: "A new heat engine is as efficient as a
| steam engine but needs a thermal source 1,800 degrees celsius
| hotter to work". The device described in the article is
| interesting in that it has no moving parts and might have an
| application one something like a nuclear powered spacecraft.
| Actually trying to harvest energy from TEGs is exceptionally
| difficult, since renewable energy sources aren't nearly as
| energy dense as thermal fuels like hydrocarbons or fission. The
| thermal gradients produced through renewable sources like solar
| are tiny [1]. It could be used for something like geothermal
| power, but again it needs temperatures way hotter than
| conventional steam engines which already work fine for
| geothermal energy production.
|
| 1. https://www.youtube.com/watch?v=Tdge8vEODeY
| samatman wrote:
| > _" A new heat engine is as efficient as a steam engine but
| needs a thermal source 1,800 degrees celsius hotter to work"_
|
| But this title would not be correct.
|
| Methane combusts at 1,957 degC, right in the band for this
| thermal cell, and is a normal heat source for steam power.
|
| The steam doesn't reach this temperature, of course, but nor
| does it need to.
| zdkl wrote:
| Re. thermal gradients, have you considered plain old mirrors?
| https://en.wikipedia.org/wiki/Odeillo_solar_furnace
|
| > 54 metres (177 ft) high and 48 metres (157 ft) wide > more
| than 2,500 h/year [sunlight] > peak power of 3200 kW >
| Temperatures above 2,500 degC (4,530 degF)
|
| Sounds like it could be useful as a "default load" inside an
| otherwise inactive solar furnace at least.
| Manuel_D wrote:
| You're describing solar thermal energy [1]. Use solar
| collectors to turn light into heat, then use a heat engine
| to turn that heat into electricity. This TEG could be used
| as a heat engine for this task. But again, our heat engines
| are already capable of this task and don't need such high
| temperatures. A solar collector array even getting to this
| TEG's operating temperature might not be feasible.
|
| Photovoltaics just turn solar energy into electricity, and
| don't need the heat engine. This has made them way cheaper
| to deploy than solar thermal energy. So unless there's
| something very important about this new TEG, the solar
| thermal vs photovoltaic calculus doesn't really change.
|
| 1. https://en.wikipedia.org/wiki/Solar_thermal_energy
| guelo wrote:
| The point is to store the thermal energy for later use to
| smoothen out photovoltaic's intermittency issues.
| Manuel_D wrote:
| Right. But we already have that technology with
| conventional heat engines which have the advantage of
| much, much, lower operating temperature requirements. If
| you have a 3,000 degree vat of thermal storage material
| this new engine stops working after draining 1000
| degrees. Existing heat engines can usually work down to
| several hundred Celsius - though superheated steam
| engines need around 700 Celsius. But that's still an
| extra 1000 degrees you can bring it down, even in the
| conservative case.
| gibolt wrote:
| This is the first of its kind to reach this efficiency,
| correct?
|
| I assume that means there could be room to significantly
| improve its efficiency or operating requirements with
| more investment and research.
|
| Having one example, even if 'useless' or elementary, is
| key to developing new technology.
| Manuel_D wrote:
| Compared to other TEGs. Not compared to steam turbines.
| The article is actually being very generous in saying
| it's "as efficient" as steam turbines. Steam turbines are
| more efficient with scale, and industrial ones for power
| generation are over 90% efficient [1]. This new TEG's
| efficiency is "around 40 percent". Higher than the
| previous TEGs in the 25-35% range. But not compared to
| steam engines, that also benefit from much lower
| operating temperatures.
|
| 1. _Multistage (moderate to high pressure ratio) steam
| turbines have thermodynamic efficiencies that vary from
| 65 percent for very small (under 1,000 kW) units to over
| 90 percent for large industrial and utility sized units._
|
| https://www.epa.gov/sites/default/files/2015-07/documents
| /ca...
| jvanderbot wrote:
| This is an absolute game changer for planetary exploration,
| where RTGs are common.
| xyzzyz wrote:
| RTGs do not get anywhere close to 1800 degrees Celsius.
| Even if they did, it wouldn't be a game changer, because
| you can make up for loss of efficiency with a bigger RTG.
| jvanderbot wrote:
| OK, perhaps I stand corrected (or just tempered).
| pvarangot wrote:
| No news is good news, this one probably will work I guess.
| yetihehe wrote:
| > won't be useful, won't be cost effective
|
| > The team's design can generate electricity from a heat source
| of between 1,900 to 2,400 degrees Celsius
|
| Pretty high temperature for me, copper melts at 1800C.
| giantg2 wrote:
| That might be when copper _liquifies_. It will lose its
| structural integrity and deform easily at much lower
| temperatures.
| wishawa wrote:
| High temperature is the point. The efficiency of heat engines
| depend on the temperature difference (relative to ambient).
| The hotter you can go, the better. (granted this thing isn't
| really an "engine", but the trend still applies)
| dv_dt wrote:
| But for practical applications, the availability and cost
| of materials to survive the operating temperature is also
| relevant.
| diarrhea wrote:
| A Carnot heat engine operating between, say, 2600K and 400K
| can reach almost 85% efficiency.
|
| The higher the temperature, the higher the share of exergy in
| the heat flux. At high enough temperatures, it's no longer a
| feat to convert to electricity at high efficiencies.
| rowanG077 wrote:
| It would be pretty hard to create a carnot heat engine that
| can withstand 2600k. I'm not even sure if for example
| Tungsten has structurally integrity at that point.
| marcosdumay wrote:
| This is not similar to anything on that page, as it operates on
| temperatures of thousands of degrees. The comparison with steam
| engines is also quite bad, as the Carnot efficiency on that
| kind of temperature difference is way above 90%.
|
| So, it's just an overrating research PR piece, like the ones
| people like to complain. This thing probably scales just fine,
| and may be quite useful. The entire problem is that science
| gets divulged on those insane PR pieces where it's compared to
| completely different things, or promise completely impossible
| results.
| diarrhea wrote:
| At those temperatures, Carnot efficiency is between 80 and 90
| percent.
|
| The comparison to steam engines is misleading, but there's an
| important distinction. Steam or gas turbines _would_ reach
| very high efficiencies at these temperatures too, but won't
| because of material properties and limitations thereof.
|
| These limitations don't seem to exist for this new
| technology. Hence, reaching very high efficiencies becomes
| possible. In theory... In practice, I don't see how heat
| sources with temperatures that high are feasible or could
| stem from renewable sources. (something with the thermal
| battery? Wasn't explained much in the article)
|
| In any case, _in comparison to steam turbines_ , the
| technology presented here does absolutely nothing in terms of
| decarbonising the grid, as claimed. It's just potentially
| more efficient. But what's the source for the primary energy?
| sroussey wrote:
| Heliogen for solar, and Quaise for geothermal. Both have
| high temperatures to deal with.
| whimsicalism wrote:
| Not to beat a dead horse, but nuclear?
| fsloth wrote:
| "in comparison to steam turbines, the technology presented
| here does absolutely nothing in terms of decarbonising the
| grid, as claimed"
|
| I understood the claim to be that this would have close to
| no moving parts. That sounds it's cheaper to mass
| manufacure and operate?
| walnutclosefarm wrote:
| The actual generator has no moving parts. The "tanks" for
| storing the heat can be made from graphite, but the
| thermal battery made by combining the storage tank with
| the generator that they propose has to pump heat around
| using liquid tin (or perhaps liquid silicon) as the
| working fluid, at temperatures up to 2400C. That requires
| not just moving parts, but some pretty far out
| engineering. All of the metals we commonly build things
| like pumps out of are liquid at those temperatures, after
| all. And of course, you want pumps that run reliably for
| years in that hostile environment.
| diarrhea wrote:
| Of all things keeping the energy transition back, steam
| turbine manufacturing is probably very low on the list.
| I'm not aware it's an issue. It's an old and proven
| technology.
| intrepidhero wrote:
| I think the (unstated) idea is to use an arc furnace,
| during peak solar/wind output to heat graphite and recover
| the energy later using their fancy new TPV cell. That's
| going to require some really good insulation, since your
| heat source is intermittent and your temperature difference
| is huge.
|
| My first thought was let's use it in fission (and later
| fusion) reactors.
| marcosdumay wrote:
| It's too hot for a fission reactor. Probably for fusion
| too, but that's not clear.
|
| I got the impression they would heat the graphite with
| concentrated solar power.
| MisterTea wrote:
| The problem is it only generates electricity when the
| input temperature is 1900-2400 C whereas uranium melts at
| 1132.2 C.
| the8472 wrote:
| Most civilian reactors use uranium dioxide, which has a
| melting point of 2865degC. Or uranium carbide, which
| melts at 2350degC
| klyrs wrote:
| Passively slurping the heat off a critical puddle of
| molten uranium is the disruptive startup pitch I'd make
| to secretly record audience reactions.
| marcosdumay wrote:
| With unmoderated fast neutrons, and critical geometry
| your startup will be always exceptionally close to
| booming and taking over a huge flank of the market.
| MisterTea wrote:
| You can use this as an inspirational soundtrack for the
| presentation:
| https://sentientruin.bandcamp.com/album/wormboiler
| 13of40 wrote:
| I think they're talking about something like this:
|
| https://www.theverge.com/2022/2/22/22945975/rondo-energy-
| dec...
|
| Basically, in their words, "a large insulated shoebox
| full of brick". And I could be wrong, but I think you
| should be able to scale the amount of "brick" up to
| whatever size and keep the insulation the same thickness,
| so the storage capacity would increase by the cube of the
| scale and the amount of insulation would only increase by
| the square of the scale.
|
| That would allow you to minimize the fluctuation in
| temperature - i.e. if it takes 10 days to get up to
| temperature, because it's big, you don't have to cool it
| all the way back to room temperature when you take an
| afternoon worth of energy back out.
| biomcgary wrote:
| For non-mobile storage, it seem that the waste heat (from
| cooling the TPV) would still be at such a high temperature
| that it can be used for co-generation to improve total
| system efficiency. Do existing technologies exists to make
| optimal use of this "temperature bandgap"? Would direct to
| steam work efficiently?
| abeppu wrote:
| > the Carnot efficiency on that kind of temperature
| difference is way above 90%.
|
| This is definitely not my area, but is Carnot efficiency
| directly comparable to the efficiency numbers cited in the
| article? Or is the "work" in Carnot efficiency the mechanical
| work, prior to being converted to electricity?
| kragen wrote:
| Yes, it's directly comparable. You can interconvert
| mechanical work and electricity freely; electricity isn't
| like heat. Everyday machinery does it with 95% efficiency,
| but there's no fundamental limit.
| frankus wrote:
| The university where I did a post-bacc had a research project
| back in the late 1990s to build a TPV-powered hybrid electric car
| that used compressed natural gas to heat an emitter surrounded by
| water-cooled PV cells.
|
| With the technology at the time they weren't able to get the
| efficiency to be competitive with an internal combustion engine,
| but something like this probably could've made it competitive.
| I'm not sure if there's any need for it with today's battery
| performance/price, but maybe as a range extender or something.
|
| https://vri.wwu.edu/viking-series-cars-history/ (scroll down to
| Viking 29)
|
| https://www.sae.org/publications/technical-papers/content/97...
| (technical paper)
| KennyBlanken wrote:
| Toyota has had a 41% efficient engine in production for five
| years. If it were set up as a range extender it would likely be
| even more efficient (fixed RPM range and load can be optimized
| for.)
|
| https://en.wikipedia.org/wiki/Toyota_Dynamic_Force_engine
| jacquesm wrote:
| Those are all amazing vehicles. I played a bit with water
| cooled solar cells, I found that the hard part wasn't the
| cooling but to be able to draw that much current from a single
| cell, the attachment points for the wiring typically really
| weren't up to the job and having multiple attachment points
| became a necessity. You also need a pretty beefy pump and
| radiator to get rid of the excess heat, though I guess in a
| vehicle you would use an active system with a fan.
|
| What I don't understand about the Viking 29 article is how an 8
| KW generator is going to power a 75 KW motor, is there
| something I'm missing?
| opwieurposiu wrote:
| There was a battery used for peak acceleration power. 8KW is
| enough to cruise at highway speeds if you have an aerodynamic
| enough body.
|
| More details: http://fennetic.net/pub/viking_29_thermophotovo
| ltaic_electri...
| jacquesm wrote:
| Thank you! Very interesting design this.
| tinco wrote:
| If you point concentrated solar at a vantablack object,
| heatsinked to this 40% efficient TPV, do you get an easy 39.94%
| efficiency, easily outpacing mass produced photovoltaic or am I
| missing a loss?
| wishawa wrote:
| What you suggested is more similar to what Solar Thermal
| systems do
| (https://en.wikipedia.org/wiki/Solar_thermal_energy). They are
| known to be more efficient than solar photovoltaics. They have
| their own downsides of course.
| diarrhea wrote:
| They are more efficient because converting anything to heat
| is trivial. Heat is just losses basically. A process
| consisting of 100% losses is great for heat generation.
|
| However, extracting exergy (electricity is pure exergy) from
| a flow of energy is the tricky part and will always be
| associated with efficiencies way below unity, based on
| fundamental principles.
| kragen wrote:
| Yes, but labs have also produced 40% efficient multijunction
| solar cells that work directly from sunlight without the
| intermediate heat absorber. Off-the-shelf multijunction PV for
| space applications is I think 36% efficient.
|
| Also, you don't need Vantablack, a regular cavity absorber
| would be fine.)
| jayd16 wrote:
| Does vantablack have some kind of clear coat that makes it
| suitable for industrial use and cleaning?
| JaimeThompson wrote:
| I don't think such a coating has been developed, but I could
| be wrong but I do know that by default just touching it can
| really damage vantablack.
| tinco wrote:
| You could encase it in glass under a vacuum or in some inert
| gas.
| prewett wrote:
| It sounded like it wasn't the heat that got converted to
| electricity, but rather the photons emitted by the hot object
| glowing, and since your vantablack object is not glowing you
| would expect to get nothing.
|
| But maybe not, the glowing comes from black body radiation, so
| the vantablack material would presumably glow as well
| (ironically). As long as the heatsink coupling did not block
| the visible "white" light produced, or glowed itself, then at
| least the photons from the back would get used. I expect that
| getting a heatsink paste rated for 2200 C is ... challenging,
| but, conveniently, you'd do better if you just skipped the
| paste.
| wongarsu wrote:
| > I expect that getting a heatsink paste rated for 2200 C is
| ... challenging, but, conveniently, you'd do better if you
| just skipped the paste
|
| Liquid metal is some of the best performing thermal paste
| around. In computer applications that's normally an alloy
| made from Gallium, Indium and Tin, but at 2200C the majority
| of metals should work. Maybe Gold to reduce oxidation.
| multiplegeorges wrote:
| The vantablack material would indeed "glow", as everything in
| the universe glows. It just does so outside the visible
| spectrum. The difference between this thermo-voltaic cell and
| a photo-voltaic cell is that photo is visible spectrum and
| thermo is IR. It's all just photons!
| rosetremiere wrote:
| The article says, if I'm not mistaken, that the heat source
| must be between 1900 and 2400 degrees (Celcius), and I would
| bet that Vantablack loses its blackness at such temperatures?
| chucksta wrote:
| Wikipedia indicates the melting point of vantablack being
| 3000C. I would think a black coloring is pretty heat
| resistant. It looks like it needs to be 500-750C to create it
| as well
| sbierwagen wrote:
| In vacuum, maybe. Vantablack is made of carbon nanotubes,
| and those oxidize away in atmosphere above 750C.
| VBprogrammer wrote:
| One minor issue is diffusion due to clouds, similar to all
| concentrated solar power systems it needs direct sunlight.
| Normal solar panels can produce some power even under a thin
| cloud layer.
| voakbasda wrote:
| I'm guessing the vantablack would be destroyed by the
| concentrated heat or UV. Or is there an industrial formulation
| that could withstand the high temperatures?
| dioxide wrote:
| You'd need the solar to heat something white hot.
| adrian_b wrote:
| This is just a multi-junction photovoltaic cell optimized for
| temperatures of the radiant body between 1900 and 2400 Celsius
| degreees.
|
| Such multi-junction photovoltaic cells, but optimized for the
| higher temperature of the Sun, have existed for many years and
| efficiencies over 45% are well known.
|
| So there is no point in heating anything, the concentrated
| solar light must be directed to an appropriate multi-junction
| photovoltaic cell, for the best efficiency.
|
| Despite their very high efficiency, the multi-junction
| photovoltaic cells are seldom used for solar energy, because
| they are expensive, so they can only be used together with
| light-concentrating mirrors, to achieve a reasonable cost.
|
| Even with mirrors, the price is still much higher than for
| normal solar panels, so they might be chosen only when space
| constraints would prohibit the use of a larger area with solar
| panels.
| moffkalast wrote:
| Might be useful for electric planes or cars where you have
| very limited area? How high of a price multiplier are we
| talking about? 2x, 10x, 100x?
|
| I can't seem to find any place that sells these with a brief
| search, so I'm thinking 1000x.
| tinco wrote:
| Electric cars don't have enough area for solar power, and
| planes have even less.
| djrogers wrote:
| No - you'd get nothing - concentrated solar gets you in the
| 800-1000* range, and per TFA the TV cells work at 1900-2400*.
|
| [0]
| https://www.sciencedirect.com/topics/engineering/concentrate...
| jandrese wrote:
| Isn't that because the current designs use liquid sodium as
| the working fluid at the boiling point is near 900C?
| WJW wrote:
| Apart from the vantablack heating issues sibling comments have
| already mentioned, you'd also need to take into account the
| energy used by the cooling system for the cold end of the TPV.
| In practice you'd need to pump either water or air past some
| form of heatsink and the energy consumption of the pumps would
| reduce the efficiency below that of the 40% of just the TPV.
| jandrese wrote:
| This would be a heck of a lot of waste heat to deal with. You
| could probably boil water with the leftover energy to turn a
| steam turbine to power the cooling apparatus. A dual stage
| solar plant.
| rowanG077 wrote:
| If water is viable this would indeed seem almost too good
| to be true.
| tshaddox wrote:
| It would be less waste heat than a system that captured the
| same amount of sunlight but converted it to electricity
| less efficiently, right?
| jandrese wrote:
| The reason for the high level of waste heat is that the
| system has to operate at thousands of degrees C. There is
| still a huge potential above room temperature. Most
| systems work closer to room temperature so there is space
| to squeeze them in after this system has extracted all of
| the energy it can.
| kragen wrote:
| Yes, but the pumping power is about 0.2% of the total power,
| so this is not a significant consideration in practice. If it
| was, people would use solar chimneys instead of mechanical
| fans and pumps.
| nicoburns wrote:
| > The researchers plan to incorporate the TPV cell into a grid-
| scale thermal battery. The system would absorb excess energy from
| renewable sources such as the sun and store that energy in
| heavily insulated banks of hot graphite.
|
| It's clearly not ready for production yet, but storing energy as
| concentrated heat seems like one of the plausible proposed grid-
| scale storage solutions to me. I'm interested to see where this
| goes.
| diarrhea wrote:
| Concentrated energy is not a technical term. But high-
| temperature internal energy storage (aka heat storage) is
| terrible because of the losses.
|
| For example, low temperature floor heating is very efficient.
|
| You don't want high temperature deltas because of the
| associated exergy losses.
| CSSer wrote:
| Does anyone know if we already use things like this to increase
| the efficiency of existing energy usage in applications which
| require high degrees of heat output by recycling energy that is
| otherwise lost as excess thermal output?
|
| The example that springs to mind for me is a steel mill. The
| temperatures required there easily meet and exceed what is
| required to generate and store energy with this device.
| 323 wrote:
| Probably not in a steel mill, because otherwise you could have
| just heat water to steam and use it to run a generator. Since
| this is not done, there is a catch.
|
| An example where this idea works is a condensing boiler where
| the burned gases heat is used to increase efficiency by 10-30%.
| tomxor wrote:
| > can generate electricity from a heat source of between 1,900 to
| 2,400 degrees Celsius [...] plan to incorporate the TPV cell into
| a grid-scale thermal battery. The system would absorb excess
| energy from renewable sources such as the sun and store that
| energy in heavily insulated banks of hot graphite. When the
| energy is needed, such as on overcast days, TPV cells would
| convert the heat into electricity, and dispatch the energy to a
| power grid.
|
| Heating graphite based thermal batteries to >1900C using the Sun
| for long term storage? I'm not sure why the article is refraining
| from being explicit, but i'm guessing the intended application
| here is to replace the steam turbine usually found at the centre
| of high temperature solar thermal collectors.
|
| I wonder how feasible and cost effective it is to insulate a
| battery well enough to maintain over 2000C for multi day periods
| without substantial loss? The heat storage strategies used for
| steam turbines doesn't require such high temperatures.
| lapinot wrote:
| relevant lowtechmag article:
| https://solar.lowtechmagazine.com/2020/05/thermoelectric-sto...
|
| tldr: thermoelectric generators don't have great efficiencies,
| but by cogenerating heat and electricity they can get viable (if
| you need heat and electricity, make a bit more heat than you need
| and put a thermoelectric generator, the non-converted heat will
| just end up as useful heat). This would be adapted to households,
| which typically need a lot of heating.
| Simon_O_Rourke wrote:
| This is great, but all of this feels like it's coming 15 to 20
| years too late. I worked with a guy in Berlin who had previously
| worked in PV energy. He predicted that unless the costs were
| reduced by a factor of half the price point of fossil fuels that
| it would go nowhere. We're addicted to cheap, no hassle sources
| of energy right now, and it's depressing.
| GoodJokes wrote:
| andrepd wrote:
| Millions of people already die yearly from pollution. It's
| definitely not hassle free.
| Maursault wrote:
| You have to wonder what PV would cost if even a tenth of the
| resources invested in developing nuclear energy were instead
| invested in solar energy development. I expect by 1980, solar
| energy would have been the cheapest way to generate
| electricity, President Carter would have been reelected, no
| Iran-Contra affair, and no deregulation of banks leading to the
| 2008 recession, and likely would have avoided 911, both Iraq
| wars and the one in Afghanistan. We could have saved all kinds
| of money, had cheap energy, still had plenty of nuclear power,
| and it wouldn't be so damn hot all the time.
| TSiege wrote:
| This is a great timeline to imagine :)
| outworlder wrote:
| > You have to wonder what PV would cost if even a tenth of
| the resources invested in developing nuclear energy were
| instead invested in solar energy development.
|
| Not enough was invested in nuclear. Fossil fuels received
| massive funding (and subsidies). Had we deployed more nuclear
| power for baseline and industry, we would have been in a far
| better place. The Cold War also messed up things, and caused
| the reactors that were deployed to be the ones better suited
| for weapons first, energy second.
|
| Note that our solar panels are similar to computer chips.
| Investing more money would have sped up the development but
| not in time for it to be viable in the 80s.
| Maursault wrote:
| > Not enough was invested in nuclear.
|
| The Manhattan Project cost, adjusted for inflation, $22B.
| That was just to blow something up. Had the United States
| and Great Britain not subsequently developed nuclear
| energy, it is likely it never would have been developed
| because it would have been impossible due to cost. Only a
| nation can develop nuclear energy, it is not something that
| could have been developed privately, again, due to cost.
|
| When factoring the cost of the energy produced by nuclear
| fission, the cost of that electricity, the cost of the
| development of nuclear energy is never factored in. If it
| were, it would be clear there has never been a more
| expensive way to generate electricity than nuclear fission.
| Nuclear energy development was a freebee, and the biggest
| freebee in the history of civilization: nuclear energy
| development, paid for by tax payers, was _given away_ and
| the tax payers ' investment _never had one penny of
| return_. "Electricity too cheap to meter," never
| materialized, and not even close. The tax payers were
| bamboozled.
|
| When all is said and done, the treehuggers have the weaker
| argument. Sure, nuclear energy has some environmental
| concerns, but these kinds of arguments pale in comparison
| to the economic argument: nuclear energy has never been and
| will never be economically viable. There are reasons, but
| they can be ignored, because we can see the result, no
| investor will touch nuclear. And complaining about the type
| of nuclear plants serves no purpose because the fission
| plants we built are the cheapest designs there are. Seeder
| reactors would be cool, but, you see, they're even less
| economically viable than the fission steam turbine plants.
|
| We could invest everything, every dollar earned, every
| possible value society could produce, into nuclear energy
| development, but even if we did, nuclear energy would
| remain economically unviable.
|
| Again, if we could just invest a small portion of what we
| wasted on nuclear energy development into solar energy
| development... well jusT look at how cheap solar has gotten
| in 20 years with _private_ development. Imagine if it was
| 80 years instead of 20 and included massive, mind-boggling,
| government subsidies. Forget any government investment in
| solar, if solar subsidies could merely match nuclear
| subsidies, dollar for dollar, no one would be talking about
| nuclear power anymore.
| soperj wrote:
| >The Manhattan Project cost, adjusted for inflation,
| $22B.
|
| China spends 4 times more than that yearly on solar.
| Maursault wrote:
| And well they should, as they can expect a 20% ROI,
| meanwhile, nuclear fission, since inception, has yet to
| be profitable. Not even once.
| 8bitsrule wrote:
| Agreed ... but I'll posit the existence of technical
| 'treehuggers' who knew it early on (and kept quiet after
| what happened to Oppenheimer.)
|
| Further posit: they were aware that nukes were more about
| making Pu than heat. No insurance company back in the day
| would accept the risk either (thus the 1957 'Price-
| Anderson Nuclear Industries Indemnity Act').
|
| Anyway, I'll quote a 1951 expert: "t is safe to say ...
| that atomic power is not the means by which man will for
| the first time emancipate himself economically.... At
| present, atomic power presents an exceptionally costly
| and inconvenient means of obtaining energy which can be
| extracted much more economically from conventional
| fuels.... This is expensive power, not cheap power as the
| public has been led to believe." -- C. G. Suits, Director
| of Research, General Electric, who who was operating the
| Hanford reactors.
| [https://www.ieer.org/pubs/atomicmyths.html]
| nynx wrote:
| Thankfully, the price of PV has dropped by far more than a half
| over the past ten years.
| k__ wrote:
| Can it also absorb excess heat from me?
| outworlder wrote:
| Unless you are a member of the Fantastic Four, no.
| mfrieswyk wrote:
| Could something like this directly harvest the heat from a
| traditional nuclear plant? Or would it degrade from the
| radiation?
| philipkglass wrote:
| This thermophotovoltaic cell is tuned to operate with radiant
| energy from a body heated to between 1900 and 2400 degrees
| Celsius. That is much hotter than any current reactor core. It
| wouldn't work with a traditional nuclear plant; it would
| require (at a minimum) developing reactors that operate at a
| much higher temperature. The most common power reactor design,
| the pressurized light water reactor, heats water to around 315
| degrees Celsius:
|
| https://en.wikipedia.org/wiki/Pressurized_water_reactor#Cool...
| worldsayshi wrote:
| Could this maybe be used as part of a fusion reactor?
| short_sells_poo wrote:
| I think the problem isn't that we can't achieve 2000
| degrees with fission, but the entire design would need to
| be re-thought to handle such temperatures.
|
| You need a completely different sort of materials and need
| to consider new types of risks for a reactor that's
| supposed to operate at such temperatures.
| aidenn0 wrote:
| Most semiconductors don't like ionizing radiation. In addition,
| the bottom-end of the operating range for this (1900C) is well
| above what google suggests is the typical coolant temperature
| (300C).
| aidenn0 wrote:
| TFA says turbines can't operate for heat sources around 2000C.
| I'd believe they can't operate with _steam_ at 2000C, but can 't
| you decrease the temperature of the steam relative to the heat
| source by increasing the flow rate?
| wayfarer1291 wrote:
| There's a silicon-valley based company that's building a combined
| thermal storage and TPV system: https://www.antoraenergy.com So
| these ideas may be get to market sooner than a typical university
| press release.
|
| https://www.nrel.gov/news/features/2021/new-projects-move-th...
|
| (some of the same people involved in this paper at NREL seem to
| also be collaborating with Antra, which is great to see)
| pooloo wrote:
| Dyson sphere, maybe?
| mwattsun wrote:
| Sometimes I like to take a step back and imagine a world with
| nearly free energy. Converting sea water to fresh water to
| irrigate the deserts is always the first thing that comes to
| mind, but that's mundane compared to the changes nearly free
| energy will bring, which is the direction we seem to be heading.
| civilized wrote:
| It will be unimaginable. I truly hope it is closer than I think
| it is.
| kibwen wrote:
| As long as proof-of-work cryptocurrencies exist, free energy is
| impossible. Any time the price of energy falls below a certain
| point, the energy consumption of the network will increase to
| compensate. PoW is a floor on the price of energy.
| synalx wrote:
| I love that the article talks about realizing a "decarbonized
| grid" by storing energy in literal chunks of carbon.
| after_care wrote:
| That's funny wordplay, but atmospheric carbo is clearly the
| intended implication.
| falcolas wrote:
| I'm not well enough versed in these kinds of devices - how does
| this differ from a traditional thermocouple device?
| djrogers wrote:
| Thermocouples are far less efficient - 5% or less, vs 40% for
| these.
| jandrese wrote:
| Traditional thermocouples have abysmal efficiency. 5% is about
| the best you can hope for. This device claims to be 8x as
| efficient.
| lisper wrote:
| More efficient by an order of magnitude or so.
| XorNot wrote:
| I'm struggling to figure out what this is. If you can capture
| heat from a "white hot" object, why is it not just a good solar
| cell? The sun is easily in this range.
| icodestuff wrote:
| If I'm reading this right, this actually has a larger usable band
| gap range than traditional photovoltaics -- the article talks
| about capturing the high-energy photons -- in the first pass.
| This means that the cell can actually capture the electrons
| knocked out by those high-energy photons, which is something we
| haven't been able to do.
|
| Silicon's band gap (1.11 eV) corresponds to 1110 nm (NIR), and
| any photons with more than 3 eV energy (413 nm) are lost (and all
| the excess energy in photons in between is lost as heat). Newer
| cells are around 0.6-0.7 eV, but I don't know their maximum
| capture energy. That's all the violet light and UV. There's a
| startup that makes a polymer film that can create two lower
| energy photons in the band gap from a high-energy photon to
| capture some of that wasted light. This would seem to be a cell
| that could capture it directly. Very very cool; that's a lot more
| energy captured per photon.
|
| What I'm unclear on is why you need to heat it up so much to get
| to those efficiency levels, and it wouldn't just work as an
| ordinary solar cell.
| MatteoFrigo wrote:
| The article is confusing, but I read it the same way as you:
| these guys have figured out a way to capture a larger part of
| the blackbody spectrum than a normal photovoltaic cell. I don't
| think they are saying that the cell needs to be heated up. I
| think they are saying that they capture 40% of the power of a
| blackbody at 2000K. I am not sure why they don't mention what
| happens for a blackbody at 5600K such as the Sun.
| SnowHill9902 wrote:
| To everyone in this thread commenting about the Carnot
| efficiency: yes, the Carnot efficiency grows with T, but you must
| use a Carnot engine for that! The Carnot theorem is an upper
| bound not a property of a given engine. It's trivial to build a
| heat engine whose real efficiency (measured efficiency / Carnot
| efficiency) approaches 0.
| gene-h wrote:
| This would be interesting for powering spacecraft because it
| doesn't have any moving parts. For the most part, spacecraft are
| not serviced ever, so it's best to minimize things that can
| break.
|
| Nuclear powered spacecraft have been hard to develop because of
| the need for moving parts. NASA's cancelled Advanced Stirling
| Radioisotope Generator, which was supposed to be a more efficient
| radioisotope powered generator than the thermoelectric generators
| previously used, had trouble because the moving part based
| generator wasn't very reliable.[0]
|
| In addition to eliminating moving parts this is also interesting
| for nuclear powered spacecraft because it may be possible to pass
| light through a radiation shield to prevent damage to the
| converter. The problem is that the reactor would need operate at
| extremely high temperatures. While this probably isn't high
| enough to melt the fuel, the fuel might not be structurally
| stable. Although liquid uranium nuclear rockets are being
| considered[1].
|
| [0]https://en.wikipedia.org/wiki/Advanced_Stirling_radioisotope..
| .
|
| [1]https://www.uah.edu/news/items/bubble-through-nuclear-
| engine...
| beambot wrote:
| I was under the impression that NASA RTGs relied on solid-state
| Peltier / Seebeck thermocouples that had no moving parts:
|
| https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...
| blamazon wrote:
| Those thermocouples are less than 10% efficient, usually less
| than 5%.
|
| This means you have to dissipate more than 10 times more
| energy as heat than the energy you actually want to use as
| electricity.
|
| Dissipating heat in space is not easy because unlike earth
| there is no surrounding fluid to dissipate heat into through
| convection. This means you have to spend precious mass budget
| on huge (compared to your energy budget) direct heat
| radiation systems that cannot leverage convection
| efficiencies.
|
| Thus, a significant energy efficiency increase would be a big
| deal for RTG powered spacecraft design. It is curious that
| the article above does not consider this.
|
| Bonus thought: consider what this lack of convection problem
| means for "Hyperloop" type vehicle systems that operate in a
| vacuum tunnel, as most conventional trains dissipate excess
| braking energy through convection from resistive heating
| elements atop the roof.
| caf wrote:
| Conventional trains that run under wires can also dump
| excess braking energy into the overhead. That seems
| feasible even if you had to have your resistive heating
| elements somewhere outside the tunnel system.
| foxyv wrote:
| That is correct, I think the RPG design was to improve the
| efficiency of the RTG.
| tomxor wrote:
| Yup, Voyager 1 and 2 are still running off RTGs right now
| since 1977.
|
| However the thermoelectric converter used in those RTGs
| exploit a different effect, and appear to have quite
| different properties as a result... What I can tell from a
| quick dig:
|
| - Thermocouples ("Seebeck effect") operate on a temperature
| difference, and are more commonly used as temperature
| sensors. They have the advantage of working at a larger range
| of temperatures, but the disadvantage of needing to maintain
| a temperature gradient for power production... Any inherent
| efficiency is likely negated by the requirement for constant
| cooling. [0]
|
| - Themal Diodes ("Thermophotovoltaic") is more like a solar
| panel for a different wavelength (infrared). The principle
| they operate on suggests no temperature gradient is required,
| not sure about cooling for other reasons though, but the
| clear disadvantage is the requirement for a high operating
| temperature for effective use in power production. [1]
|
| Historically thermal diodes don't appear to have been
| particularly efficient compared to thermocouples either,
| obviously this particular one changes that.
|
| [0]
| https://en.wikipedia.org/wiki/Thermocouple#Power_production
|
| [1] https://en.wikipedia.org/wiki/Thermophotovoltaic
| the8472 wrote:
| > The principle they operate on suggests no temperature
| gradient is required
|
| A temperature gradient is absolutely required. Ignoring
| materials problems (such as melting) a photovoltaic cell
| ultimately does work off a temperature differential because
| it has to absorb photons. It cannot do that if its
| temperature is the same as the black body temperature of
| the lightsource illuminating it, otherwise they would be at
| an emission/absorption equilibrium.
|
| This is also why PV cells could theoretically work in
| reverse mode at night, emitting IR into space. They'd just
| do that paltry power ratings because _DT(earth, cosmic
| background)_ is much smaller than _DT(photosphere, earth)_.
| And they 'd have to be made of a material with a much
| smaller bandgap.
| tomxor wrote:
| You're right, I kinda knew it needed cooling, but I guess
| your point is even though the effect being exploited
| doesn't require a temperature gradient, in practice the
| heat dissipation requirements are the same if the TVP is
| to survive - or are they different... surely for the
| purposes of satisfying the first law the temperature
| dissipation requirements are dependent on the efficiency
| of conversion?
|
| Which would make the cooling requirements of this TVP
| lower (relative to input) due to it's higher efficiency?
| but still substantial.
| [deleted]
| jvanderbot wrote:
| OK, so perhaps it's not the de-carbonization future we all
| wanted, but this could be an absolute game changer for planetary
| exploration, where RTG (nuclear-decay-driven thermal engines) are
| common. The existing efficiency for those is abysmal, which is
| actually OK since heat is such a useful product in itself.
|
| I'm looking at Dragonfly, specifically, where an RTG provides the
| electricity and heat to keep everything alive. Imagine what 10x
| longer flights would do for that mission.
| tomxor wrote:
| The advantage does appear to be higher efficiency compared to
| themocouples, and the TVP should in theory require less cooling
| _relative_ to the input due to the higher efficiency - However
| the disadvantage is that TVPs generally require a far higher
| operating temperature for effective power production, which may
| actually require more sophisticated cooling.
|
| This particular one requires around 2000C, which appears to be
| above the critical temperature of most RTGs (though not all!):
|
| https://www.researchgate.net/figure/Critical-temperatures-to...
|
| I wonder if those RTGs also have any disadvantages or are
| simply more substantial.
|
| [edit]
|
| Corrected cooling requirements as pointed out by the8472 in
| another comment.
| nautilius wrote:
| Do RTG reach the >2170K necessary for this?
| jvanderbot wrote:
| Apparently not, as someone below pointed out.
|
| Oh well, I was briefly excited but there's a big enough gap
| that it probably won't work as is
| perihelions wrote:
| Apparently not. This 1991 document [0] says the GPHS heat
| source [1] has (?) an upper operating limit of 1,300 degC
| (1,573 K), at the iridium cladding that contains the Pu-238
| fuel. There's a suggestion this could be raised to 1,500
| degC.
|
| [0, pdf] https://ntrs.nasa.gov/api/citations/19910015359/down
| loads/19... (2.2 "Temperature Constraints")
|
| [1] https://en.wikipedia.org/wiki/GPHS-RTG
| WheelsAtLarge wrote:
| "OK, so perhaps it's not the de-carbonization future we all
| wanted, but this could be an absolute game changer"
|
| I hate to throw water on the situation but we constantly read
| articles about tech that will be a game changer only to never
| be seen again mainly because it can't be scaled to the size
| needed and provide the advantages we need.
|
| Yes, it sounds good. But what we need now is a proof of concept
| rather than theories on how much of a miracle the tech is. My
| question is, "How can we help to move it forward to a point
| where we can see actual advantages?"
| petermcneeley wrote:
| Keep in mind that Carnot efficiency at this temperature is nearly
| 90% https://en.wikipedia.org/wiki/Carnot_cycle
|
| 1 - Tc/Th = 1 - 295k/2000k
| SnowHill9902 wrote:
| Yeah but heat losses grow with the 4th power (!).
| mrfusion wrote:
| We could drastically shrink coal, and nuclear power plants if we
| can do away with the steam turbine portion.
| tenthirtyam wrote:
| Well now. We can have a heat pump with a COP of 3.5 or so. Let's
| say we put in 1kW of electricity, and take out 3.5kW of heat
| energy. Now let's take 2.5kW of that heat energy, push it through
| this gadget at 40% efficiency to get 1kW electricity out. Push
| that back into the heat pump and, viola, 1kW of free heat left
| over!
|
| What am I missing?
| danans wrote:
| > We can have a heat pump with a COP of 3.5 or so. Let's say we
| put in 1kW of electricity, and take out 3.5kW of heat energy.
|
| A heat pump COP of 3.5 or higher or happens only at relatively
| low delta-T between the source and destination temps of the
| heat pump [1] - like the delta-T typical for space or water
| heating. The COP degrades exponentially with increasing delta T
| as it has to work ever harder to pump heat against an ever
| higher temperature/pressure (assuming thermal storage with a
| fixed volume with few losses).
|
| The refrigeration cycle can't raise temperatures even hundreds
| (much less thousands) of degrees C - otherwise we'd already
| have heat pump stoves and ovens.
|
| This is coincidentally also one reason why all else equal, heat
| pump clothes dryers (which are great) take a bit longer than
| conventional technologies to dry clothes: they only reach about
| 50C (vs 70C-75C for standard gas or electric resistance
| dryers).
|
| > Push that back into the heat pump and, viola, 1kW of free
| heat left over!
|
| Because the COP degrades with the delta-T, it bottoms out at 1
| (an electrical resistance heater), and in your scenario, you
| end up with 1kWH in, and 400Wh out, so a theoretically 40%
| efficient battery.
|
| Minus the electrical generation, and at much lower
| temperatures, your scenario with a heat pump + thermal storage
| does however describe how the new domestic thermal heat
| batteries can work with heat pumps [2].
|
| 1. https://www.engineeringtoolbox.com/heat-pump-efficiency-
| rati...
|
| 2. https://sunamp.com/
| syntaxing wrote:
| Entropy. You cannot look at a system purely from an energy
| conservation stand point. Take Carnot efficiency for instance.
| That's the extreme case where it teeters on the fully
| recoverable entropy. For your example, the entropy price has to
| be paid somewhere in the system (usually in the heat to work
| ratio).
| audunw wrote:
| > What am I missing?
|
| Just guessing, but a heat pump that can actually output at
| least 1900degC of heat?
|
| If we had those kinds of heat pumps I guess we'd use them in
| all kinds of industrial processes
| usrusr wrote:
| If the stored heat is on the level of getting the emitter into
| the white-hot, how is the "battery" turned off, to hold the
| "charge"?
|
| The article mentions a mirror layer as part of the cell, for
| retaining the energy of out of band photons. Would that be the
| "off" solution, just bounce them back when they are not needed?
| Cell moved out of the path? Somehow that triggers "too simple to
| be true" heuristics in me, but on the other hand... yeah, mirrors
| (or just very white surfaces, precise direction is not needed)
| can be quite capable of not getting heated by incoming photons,
| and that must mean bouncing them back.
| karamazov wrote:
| You have a heat reservoir, i.e. a well-insulated and very hot
| object, that stores energy as heat. If you insulate it with
| mirrors, that can look like bouncing photons back into the
| reservoir.
|
| When you want to generate energy, you open the insulation and
| let heat out to hit this chip.
|
| It's like opening an oven door to let some hot air out.
| megaman821 wrote:
| Is there some sort of physical property when you are that hot
| (2400C) that you begin glowing? Then just open a slot in the
| insulation so the photons cell whenever you need electricity.
| robotresearcher wrote:
| That's what the article says.
|
| "The heat engine is a thermophotovoltaic (TPV) cell, similar to
| a solar panel's photovoltaic cells, that passively captures
| high-energy photons from a white-hot heat source and converts
| them into electricity."
|
| One detail is that objects radiate at all temperatures. The
| trick is to choose the temperature so that you get a lot of
| emission in the band that matches the best performance of the
| PV component.
| scythe wrote:
| https://en.wikipedia.org/wiki/Black-body_radiation
| tialaramex wrote:
| If your question is "wait, do all hot things glow?" then the
| answer is Yes.
| jacquesm wrote:
| Everything above absolute zero emits infrared photons, as
| something gets hotter it will also start to emit in the visible
| range because some of the photons are more energetic resulting
| in a shorter wavelength. That's why you see the progression
| from barely visible red to red to orange, yellow and eventually
| white when you make something hot enough. But it will always
| continue to output photons at lower energy levels as well,
| though not in equal proportion (and that's why a hot _object_
| is white and not blue, and why a hot _gas_ flame can be blue).
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