9 ms·

Solar plant has generated “supercritical” steam

188 points by roberthahn 12y ago

dang 12y ago
Url changed from http://sciencealert.com.au/news/20140506-25618.html http://sciencealert.com.au/news/20140506-25618.html, which points to this.
- roberthahn 12y ago
  Thank you, appreciate the consideration.
Sniffnoy 12y ago
Can anyone explain just what it means for the steam to be "supercritical"? The article doesn't seem to have explained it at all.
- austinz 12y ago
  http://en.wikipedia.org/wiki/Supercritical_steam_generator http://en.wikipedia.org/wiki/Supercritical_steam_generator has a helpful explanation and some links about supercriticality.
- deleted 12y ago
  [deleted]
- jjoonathan 12y ago
  Temperature-pressure phase diagrams (example linked below) http://images.flatworldknowledge.com/averillfwk/averillfwk-fig11_022.jpg http://images.flatworldknowledge.com/averillfwk/averillfwk-f... have a line that separates gas from liquid, but this line has a finite length. One endpoint is on the triple point (solid, liquid, gas coexist at one P,T) and the other is on the critical point. That's where the distinction between gas and liquid vanishes -- the viscosity, index of refraction, etc of one phase approach those of the other until differences vanish altogether at the critical point. It's a little arbitrary to say that steam at a given temperature or pressure is supercritical (there IS a rigorous definition, T>T_criticalpoint&&P>P_criticalpoint, I'm just saying that it's a bit arbitrary), but the gist of it is that you're in the part of the phase diagram where movement in the phase plane is going to avoid the gas/liquid transition. Nothing physical happens in a liquid->supercritical or supercritical->gas transition and there are no phase transitions in the supercritical region. This is exploited for the production of aerogel. Normally you can't dry out a gel and have it retain its shape because the liquid/gas interface during evaporation/boiling has enough surface tension to tear apart the microstructure of the gel. But if you scoot around the liquid/gas transition in phase space (e.g. by heating past T_criticalpoint, lowering pressure below T_criticalpoint, cooling below T_criticalpoint, and finally releasing any lingering pressure, or in other words liquid->supercritical->gas) then you can get rid of the liquid without ever boiling/evaporating it -> no nasty surface tension to tear apart the microstructure! Here is a video of CO2 being heated past the critical point. Since there is a gas-liquid equilibrium, the system will move more or less exactly along the curve separating gas from liquid until it "slips off the end" into the supercritical region: https://www.youtube.com/watch?list=FLAv56CgHL4P8vk6FDm9s1Zw&feature=player_detailpage&v=GEr3NxsPTOA#t=150 https://www.youtube.com/watch?list=FLAv56CgHL4P8vk6FDm9s1Zw&... EDIT: || -> && in the formal definition EDIT2: linked a phase diagram for those who don't stare at them all day :)
  - jwheeler79 12y ago
    in layman's terms anyone?
    - femto 12y ago
      If you heat water under a high enough pressure, when you release the pressure it instantly becomes stream. Normally, when you boil water the vapourisation happens piecemeal. That's why you see bubbles rising to the surface. For supercritically heated water, the vapourisation is a runaway chain reaction, triggered by a reduction in pressure, so the whole body of water flashes into stream. --- Edit: added note about reduction in pressure being the trigger.
      - jjoonathan 12y ago
        If you release pressure slowly you never see boiling/evaporation either, but at the end of the process you have steam.
      - neltnerb 12y ago
        Apologies, I don't want to be pedantic, but this has nothing to do with being supercritical. https://en.wikipedia.org/wiki/Supercritical_fluid#mediaviewer/File:Carbon_dioxide_pressure-temperature_phase_diagram.svg https://en.wikipedia.org/wiki/Supercritical_fluid#mediaviewe... You can note in this diagram that what you are describing is true whether or not the system is supercritical, and can be seen in how water will boil into steam when the pressure is decreased from atmospheric as well. What you are describing is how higher pressures allow you to add energy to the water while it remains liquid, and how if you add enough energy it will overcome the enthalpy of vaporization and cause it to convert to steam as the pressure is reduced.
        femto 12y ago
        Point taken! I'm not an expert. As I understand it, a container of boiling water will have liquid in the bottom half and steam in the top half. As the pressure and temperature rise, the steam/water goes supercritial, meaning the water/steam boundary disappears and the whole container becomes a homogenous mush of supercritical fluid. Am I right in thinking that this supercritical fluid can flash into steam faster than a combination of water and steam? My thinking is that for a water/steam combination to convert into steam, the water molecules have to take the time to break their bonds and separate into a gas. For a supercritical fluid it's faster because there are no bonds to be broken? I'd be grateful if you can correct the above, as I can learn something here.
- wycx 12y ago
  I think the most straightforward way is via a phase diagram that is contoured for density (lines of constant density are called isochores). For example: http://eurjmin.geoscienceworld.org/content/15/5/773/F3.large.jpg http://eurjmin.geoscienceworld.org/content/15/5/773/F3.large... A straightforward way to distinguish between liquids and gasses is their density. Liquid water is a high density fluid and steam is low density fluid. There is line on the phase diagram that separates the liquid field from the vapour field. On that line is the only place on the diagram where liquid and vapour can coexist, i.e. where boiling can occur. It happens to be the case that the surface of the earth is in the liquid field, but within vicinity of that line, so if you heat some water up, you can watch it boil. However, at ~250°C, you need to be at a pressure of ~4 MPa to observe boiling. At those conditions the density of the liquid will be ~0.8 g/cc and the vapour ~0.2 g/cc. The liquid-vapour phase boundary (the boiling curve) terminates at the critical point (647 K and 22.064 MPa). Above the critical point H2O is supercritical. On inspection of the phase diagram we see isochores radiating out from the critical point. Above the critical point the density of water can vary smoothly as a function of P and T, and there is no boiling, condensation, etc. I don't know anything about power generation, but presumably when you can maintain temperatures higher than the T of the critical point, you don't have to worry about losing energy to phase changes.
- angusb 12y ago
  Replying because I don't think the explanations you've got so far are easy enough to read || accurate. Here's my understanding: Supercritical steam is a special form of steam that can not be described as a gas or a liquid. It's somewhere between the two: molecules aren't bunched together in dense clusters that settle at the bottom of a container (as they are in a liquid), but they also aren't flying all over the place individually in a low density vapour (as they are in a gas). How's that possible? Water molecules have relatively strong intermolecular attractive forces between neighbouring molecules. They like to stick together, even though there's no permanent connection between them. They are like mini-magnetised marbles. This explains why water has a much higher boiling point than most tri-atomic molecules. When you increase the temperature of liquid water, the molecules in the liquid vibrate and move around within the liquid, and as you cross the boiling point, the vibration and movement of the molecules is so great that they are able to escape the pull of their attractive interactions with their neighbours en masse. When this happens, the molecules shoot off into the vapour, where there is an (almost) unlimited amount of space for them to shoot around in. Now consider what happens when you do this at high pressure. High pressure essentially means that there are lots of molecules in the gas phase moving around really quickly. Now, when the temperature gets high enough that molecules have enough energy to overcome their attractive interactions with neighbouring molecules, they leave the pack: but this time with nowhere to go to. The pressure is so high in the 'gas' phase (i.e. there are so many other molecules up there) that they are forced to just bump around where the liquid was but at extremely high speeds. This type of behaviour is pretty difficult to distinguish from the behaviour in the high pressure 'gas' -- in fact, after the system has time to equilibriate, they are exactly the same. Clearly then, the transition from 'liquid' to 'gas' at this point is pretty much indistinguishable. The liquid may begin to display the molecular kinetic behaviour of a gas, but the density stays the same. The end result is: When the pressure and temperature is high enough, to onlookers it appears as if the entirety of the fluid is half way between a liquid and a gas, and is stable in that state. That's called a supercritical fluid.
- walshemj 12y ago
  It is steam at a higher pressure than atmospheric would allow. http://en.wikipedia.org/wiki/Superheated_steam http://en.wikipedia.org/wiki/Superheated_steam. Its more efficient in driving steam engines and turbines
  - VLM 12y ago
    Well... yes technically anytime you have hotter steam its more efficient. A big gain is your stereotypical turbine blade doesn't like condensing conditions. Supercritical can't condense by definition, so its inherently good. Water droplets literally wear away the blade. Kinda sucks. They're expensive. So those expensive little things last longer if superheated steam is used. Its not so much that you can't make a condensing turbine, its that a condensing turbine will be less financially / economically efficient, its going to have to be much bigger and stronger for a given power output. Also the flow of steam is very predictable and constant, but once you start condensing no one really knows how it'll put vibration loads on, which can break the blades and wear out the bearings. Its false economy to use saturated or "wet" steam in a turbine to save money, usually. You can add a reheat stage to the middle of a turbine to prevent condensation. Of course that costs money and maint labor and energy. You can see the appeal of just using higher quality steam and avoiding all that. Sometimes you just have to eat the losses. Especially with nukes, they have relatively wet steam, well compared to coal plants anyway. Note that what some people call a condensing turbine doesn't involve condensation in the blades, at least not intentionally LOL. Its just a turbine with a huge condenser on the output instead of using a small condenser with an intermediate stage of process heat. Process heat is like, here's cruddy wet steam, but its hot, so how about using it in the office radiators, or to help heat preheat cooking ovens or something. Its hot by human standards but by power generation standards its only lukewarm and no longer economically useful to generate electricity. Its useless on the turbine floor, but perhaps a neighboring bread bakery would pay real money for it.
pinkskip 12y ago
On the other hand Abbott's government in their recent budget announced cuts to CSIRO and Clean Technology Innovation Program. Go Figure!
- infectoid 12y ago
  I know it's done the rounds but I want to make sure that everyone has seen it... John Oliver on Tony Abbott https://www.youtube.com/watch?v=c3IaKVmkXuk https://www.youtube.com/watch?v=c3IaKVmkXuk This guy man. I don't even know what to do about it.
  - kibibu 12y ago
    Encourage everybody you know to vote for an alternative. In particular, though, don't tell Abbott voters how terrible they are for voting for him. People will get defensive and actually strengthen their views under criticism. (http://youarenotsosmart.com/2011/06/10/the-backfire-effect/ http://youarenotsosmart.com/2011/06/10/the-backfire-effect/)
knowtheory 12y ago
I'm unclear on why supercritical steam is important for electrical generation. Anybody happen to know? Reading through http://en.wikipedia.org/wiki/Supercritical_steam_generator http://en.wikipedia.org/wiki/Supercritical_steam_generator indicates that Benson boilers are more fuel efficient (and perhaps less prone to explosion), but reduction in amount of fossil fuels used to turn a turbine seems to be sort of a moot point here. Would super critical steam generation mean a solar plant can produce more electricity? Or that a supercritical steam generating solar plant is cheaper to operate?
- cossatot 12y ago
  I'm not an expert here, but I believe that the maximum possible efficiency of all heat engines is governed by the temperature difference between the heat source and the heat sink [1,2]. Increasing the water T/P to supercritical both increases the T at which it can enter the system and, from what I gather by the link you posted and from [3], also results in some simplification of the plant mechanics, which likely increases efficiency as well. This should indicate that for a given amount of sunlight (or fossil fuels in a conventional power plant), a greater amount of electricity can be created. [1]: http://en.wikipedia.org/wiki/Carnot_cycle http://en.wikipedia.org/wiki/Carnot_cycle [2]: http://en.wikipedia.org/wiki/Rankine_cycle http://en.wikipedia.org/wiki/Rankine_cycle [3]: http://www.brighthubengineering.com/power-plants/32896-how-are-supercritical-boilers-different-from-subcritical-boilers/ http://www.brighthubengineering.com/power-plants/32896-how-a...
  - neltnerb 12y ago
    This is the correct answer. Higher temperature steam allows a higher maximum power extraction potential.
- jaggederest 12y ago
  For a given megawatt of thermal energy input, you get more electrical output by working in the supercritical steam domain than you do in subcritical systems. So not 'more electricity' since the sun is functionally unlimited, but 'more electricity per square foot' or 'more electricity per heliostat' or what have you.
quink 12y ago
> The $5.68 million research program is supported by the Australian Renewable Energy Agency and is part of a broader collaboration with Abengoa Solar, the largest supplier of solar thermal electricity in the world. Australian Renewable Energy Agency (ARENA). Let's see what the first Abbott budget had to offer up on this matter: > Finance Minister Mathias Cormann confirmed yesterday that the Australian Renewable Energy Agency (ARENA) will be discontinued in the 2014-15 Budget. Fun times, Australia. What fun!
- NamTaf 12y ago
  The CSIRO really is a gem in this country's crown. The amount they return on investment given is really quite impressive and their focus is directed towards our partiuclar problems (e.g.: agricultural research, ocean-based research, etc.) which may not necessarily be considered as equally by overseas research centres. To gut their funding is really very depressing.
  - Gustomaximus 12y ago
    Not trying to turn this thread into a political discussion but this announcement couldn't come at a better time for the Greens (for non-Aussies they are an environmentally focussed political party with influence via their holding balance of power to the major 2 parties). The Greens are looking to block cuts to the Clean Energy Finance Corporation. It's not the CSIRO but this showcases what some scientific investment can achieve and hopefully stops the planned cuts across our research facilities being too deep. http://www.sbs.com.au/news/article/2014/06/15/greens-move-block-clean-energy-cuts-sets-double-dissolution-trigger http://www.sbs.com.au/news/article/2014/06/15/greens-move-bl...
    - quink 12y ago
      Ahhh, the CEFC. http://www.afr.com/p/national/clean_energy_profits_up_in_smoke_qQFE6H3pjYYoCcKnOim9cL http://www.afr.com/p/national/clean_energy_profits_up_in_smo... $200 million annually in profit by 2017 for the government through investment in renewables. But making money from renewable energy is, one should remember, 'utterly offensive': http://www.abc.net.au/news/2014-05-02/joe-hockey-wind-turbines-utterly-offensive/5425804 http://www.abc.net.au/news/2014-05-02/joe-hockey-wind-turbin... The CEFC is a commercial investment in actual rollouts of renewable energy with a 7% return on investment. The Abbott government, however, says 'bugger that, we hate evil renewable-tainted money' (or something like that) and is currently looking forward to passing legislation to abolish the CEFC as one of the first things the new Senate will do once the new Senate meets in about two weeks' time. Note also that, if it weren't for that the $200 million might well grow to $520 million pretty handily - they've got $10 billion in applications: > The Portfolio Budget Statement does not include the significantly higher positive contribution to the Budget the CEFC would make if it was able to continue to carry out its investment function over the forward estimates period. Were it to do so, the CEFC would deliver a net cash surplus (profit) for the Government of more than $520 million, net of operating costs. This government is committed to throwing away $500 million and more for taxpayers in money companies are willing to borrow and pay back - with interest - to roll out renewable energy schemes. It's like "Dear government, if you give us x amount of money, this lender over there will give us 2x of money, and I'll pay you back both, with interest. This is for us to change all the lighting across this huge area to use a third of the energy.", and the government saying, instead of "let's have a look at your business case and assure everyone this is a sound investment for us and your claim actually checks out and we'll all make money" a "lol, no".
abdullahkhalids 12y ago
Please keep in mind that efficiency in itself does not mean much. The variable you have to optimize is cost per unit of energy. Supercritical steam is useful for fossil fuel generation because it is relatively easy to make closed off systems which can withstand high temperatures/pressures. The optimum for solar power might lie at the much lower temperature. Why? Because the cost of solar array as a function of desired temperature rises much faster than that of a fossil fuel generator.
- pcl 12y ago
  the cost of solar array as a function of desired temperature rises In a heliostat-based configuration, I'd assume that it's just the cost of the target of the array of mirrors that increases, right?
  - abdullahkhalids 12y ago
    To increase temperature you need to increase the power being transferred to the water. There are several ways you can do it. (a) increase size of mirror array (b) increase reflectivity of mirrors (c) improve sun tracking so that mirrors point towards the sun. (d) improve radiation absorption of receiver. Except the first, each of these is a very non linear. eg. taking reflectivity from 90 to 95 percent might double the cost. But then taking it to 97 might double the cost again.
    - ndonnellan 12y ago
      In addition, as your design steam temperature rises, your receiver's costs start to go way up. You need to use more expensive, less conductive metals - some (all?) of which are patented. Your radiative heat losses go up exponentially (T^4), so you need to hide as much surface area from the environment as possible (which means you need a cavity receiver). Now that you're a cavity, it becomes harder to concentrate light (smaller target) so your heliostat requirements go up. If I were a betting man, I'd say this goes nowhere. I think PV has already won the "war" versus thermal unless someone can come up with a great solar -> syngas receiver.
mmaunder 12y ago
This is important because it increases generator efficiency. http://en.wikipedia.org/wiki/Supercritical_fluid#Supercritical_fluid_in_power_generation http://en.wikipedia.org/wiki/Supercritical_fluid#Supercritic... Supercritical fluid in power generation The efficiency of a heat engine is ultimately dependent on the temperature difference between heat source and sink (Carnot cycle). To improve efficiency of power stations the operating temperature must be raised. Using water as the working fluid, this takes it into supercritical conditions.[20] Efficiencies can be raised from about 39% for subcritical operation to about 45% using current technology.[21] Supercritical water reactors (SCWRs) are promising advanced nuclear systems that offer similar thermal efficiency gains. Carbon dioxide can also be used in supercritical cycle nuclear power plants, with similar efficiency gains.[22] Many coal-fired supercritical steam generators are operational all over the world, and have enhanced the efficiency of traditional steam-power plants.
keypusher 12y ago
That's really great but the unsolved hurdle in solar power is energy storage. You can generate a large amount of energy on bright, sunny days but what happens at night when people go home and turn on their lights, television, and computers? Without a way to offer on-demand energy, solar will never be able to replace traditional coal and gas-fired plants. Wind power does fare better throughout the day, but is still subject to the whims of the weather. The big piece of the puzzle still to be solved is large scale battery/storage technology.
- cma 12y ago
  Air conditioning is a big power draw and peaks along with the sun.
  - hueving 12y ago
    That doesn't really help unless the panels are in the same location as the solar field. Anecdotally this isn't really true either. The peak for air conditioning is usually later in the afternoon since it takes a while for the sun to heat up the area from the cool of the night.
- TheSpiceIsLife 12y ago
  Why are people down-voting this comment? Is being correct not allowed? In Australia it is often not sunny for days on end, also it is often not windy for days on end, over huge areas of land - you know, like, bigger than a lot of European countries. For wind and solar to be economically competitive with coal / gas / nuclear they have to be at price parity including 24+hrs storage. This isn't going to happen any time soon without massive government incentive. Professor Barry Brook of Adelaide University has been blogging about this for a good few years now. We need to think critically about 'sustainable energy'. I encourage you all to read his blog [1], particularly the TCASE [2] (Thinking Critically About Sustainable Energy) series. Let's level the playing field: ultimately there are only two good metrics worth considering a) life-time cost per kWh and b) life-time CO2 emissions per kWh --- but to level the playing field you need to consider each generating technology on a base-load comparison. It's no good comparing a 1GW gas plant and a 1GW nameplate solar installation because the sun only shines about 6.5hrs per day averaged throughout the year in, say, Adelaide for example, so you typically need to over-build solar by a factor of 4 and then add storage. When you do that the life-time cost and life-time CO2 emissions aren't so crash got because of the massive amounts of energy intensive stainless steel, steel, and concrete required, plus new transmission lines. If you want to see a real-world example of how wind does work, check the UK National Grid Status site [3] - the wind hasn't been blowing in the UK for weeks, presently their 8GW of installed wind is generating 0.82GW electricity. The data speaks for itself, look at the graphs. A vote for wind / solar is a vote for new gas _because_ gas is easily load following. Of course, the gas plant owners don't like that because when the wind blows their plants sit idle. The whole renewable energy push is an expensive mess. I've commented elsewhere on HN about this, so I'll stop repeating myself now. 1. http://bravenewclimate.com/ http://bravenewclimate.com/ 2. http://bravenewclimate.com/?s=TCASE http://bravenewclimate.com/?s=TCASE 3. http://www.gridwatch.templar.co.uk/ http://www.gridwatch.templar.co.uk/ Edit: formatting
jeffdavis 12y ago
"The $5.68 million research program..." It struck me how tiny the amount of money involved here is. Let's say this engineering effort was a one one-thousandth step along the way to developing the technology into a major energy alternative. Then that would be, what, $5B? Perhaps I'm overestimating the significance of this discovery.
Gustomaximus 12y ago
Does 'supercritical' steam have any storage benefit? Or is it more efficient use of the solar energy? I ask as I read some articles about using a similar focused mirrors to create molten salt which then could keep solar energy running after the sun went down, one of the major drawbacks of solar. http://en.wikipedia.org/wiki/Solar_thermal_energy#Molten_salt_storage http://en.wikipedia.org/wiki/Solar_thermal_energy#Molten_sal...
achivetta 12y ago
Thinking aloud: could one build a combination fossil fuel and solar steam-powered turbine? When the sun is out, you use (supercritical) steam as they describe being able to generate. If the sun goes away, you feed the same turbine with steam generated with fossil fuels. You could use same turbine, condenser, etc but just switch the heat source (or use both) as the environmental conditions change.
rail2rail 12y ago
Amazing that even our most cutting edge power generation schemes are still just steam engines under the hood.

188 points by roberthahn 12y ago

dang 12y ago

Url changed from http://sciencealert.com.au/news/20140506-25618.html http://sciencealert.com.au/news/20140506-25618.html, which points to this.

roberthahn 12y ago
Thank you, appreciate the consideration.

Sniffnoy 12y ago

Can anyone explain just what it means for the steam to be "supercritical"? The article doesn't seem to have explained it at all.

austinz 12y ago
http://en.wikipedia.org/wiki/Supercritical_steam_generator http://en.wikipedia.org/wiki/Supercritical_steam_generator has a helpful explanation and some links about supercriticality.
deleted 12y ago
[deleted]
jjoonathan 12y ago
Temperature-pressure phase diagrams (example linked below) http://images.flatworldknowledge.com/averillfwk/averillfwk-fig11_022.jpg http://images.flatworldknowledge.com/averillfwk/averillfwk-f... have a line that separates gas from liquid, but this line has a finite length. One endpoint is on the triple point (solid, liquid, gas coexist at one P,T) and the other is on the critical point. That's where the distinction between gas and liquid vanishes -- the viscosity, index of refraction, etc of one phase approach those of the other until differences vanish altogether at the critical point. It's a little arbitrary to say that steam at a given temperature or pressure is supercritical (there IS a rigorous definition, T>T_criticalpoint&&P>P_criticalpoint, I'm just saying that it's a bit arbitrary), but the gist of it is that you're in the part of the phase diagram where movement in the phase plane is going to avoid the gas/liquid transition. Nothing physical happens in a liquid->supercritical or supercritical->gas transition and there are no phase transitions in the supercritical region. This is exploited for the production of aerogel. Normally you can't dry out a gel and have it retain its shape because the liquid/gas interface during evaporation/boiling has enough surface tension to tear apart the microstructure of the gel. But if you scoot around the liquid/gas transition in phase space (e.g. by heating past T_criticalpoint, lowering pressure below T_criticalpoint, cooling below T_criticalpoint, and finally releasing any lingering pressure, or in other words liquid->supercritical->gas) then you can get rid of the liquid without ever boiling/evaporating it -> no nasty surface tension to tear apart the microstructure! Here is a video of CO2 being heated past the critical point. Since there is a gas-liquid equilibrium, the system will move more or less exactly along the curve separating gas from liquid until it "slips off the end" into the supercritical region: https://www.youtube.com/watch?list=FLAv56CgHL4P8vk6FDm9s1Zw&feature=player_detailpage&v=GEr3NxsPTOA#t=150 https://www.youtube.com/watch?list=FLAv56CgHL4P8vk6FDm9s1Zw&... EDIT: || -> && in the formal definition EDIT2: linked a phase diagram for those who don't stare at them all day :)
- jwheeler79 12y ago
  in layman's terms anyone?
  - femto 12y ago
    If you heat water under a high enough pressure, when you release the pressure it instantly becomes stream. Normally, when you boil water the vapourisation happens piecemeal. That's why you see bubbles rising to the surface. For supercritically heated water, the vapourisation is a runaway chain reaction, triggered by a reduction in pressure, so the whole body of water flashes into stream. --- Edit: added note about reduction in pressure being the trigger.
    - jjoonathan 12y ago
      If you release pressure slowly you never see boiling/evaporation either, but at the end of the process you have steam.
    - neltnerb 12y ago
      Apologies, I don't want to be pedantic, but this has nothing to do with being supercritical. https://en.wikipedia.org/wiki/Supercritical_fluid#mediaviewer/File:Carbon_dioxide_pressure-temperature_phase_diagram.svg https://en.wikipedia.org/wiki/Supercritical_fluid#mediaviewe... You can note in this diagram that what you are describing is true whether or not the system is supercritical, and can be seen in how water will boil into steam when the pressure is decreased from atmospheric as well. What you are describing is how higher pressures allow you to add energy to the water while it remains liquid, and how if you add enough energy it will overcome the enthalpy of vaporization and cause it to convert to steam as the pressure is reduced.
      - femto 12y ago
        Point taken! I'm not an expert. As I understand it, a container of boiling water will have liquid in the bottom half and steam in the top half. As the pressure and temperature rise, the steam/water goes supercritial, meaning the water/steam boundary disappears and the whole container becomes a homogenous mush of supercritical fluid. Am I right in thinking that this supercritical fluid can flash into steam faster than a combination of water and steam? My thinking is that for a water/steam combination to convert into steam, the water molecules have to take the time to break their bonds and separate into a gas. For a supercritical fluid it's faster because there are no bonds to be broken? I'd be grateful if you can correct the above, as I can learn something here.
wycx 12y ago
I think the most straightforward way is via a phase diagram that is contoured for density (lines of constant density are called isochores). For example: http://eurjmin.geoscienceworld.org/content/15/5/773/F3.large.jpg http://eurjmin.geoscienceworld.org/content/15/5/773/F3.large... A straightforward way to distinguish between liquids and gasses is their density. Liquid water is a high density fluid and steam is low density fluid. There is line on the phase diagram that separates the liquid field from the vapour field. On that line is the only place on the diagram where liquid and vapour can coexist, i.e. where boiling can occur. It happens to be the case that the surface of the earth is in the liquid field, but within vicinity of that line, so if you heat some water up, you can watch it boil. However, at ~250°C, you need to be at a pressure of ~4 MPa to observe boiling. At those conditions the density of the liquid will be ~0.8 g/cc and the vapour ~0.2 g/cc. The liquid-vapour phase boundary (the boiling curve) terminates at the critical point (647 K and 22.064 MPa). Above the critical point H2O is supercritical. On inspection of the phase diagram we see isochores radiating out from the critical point. Above the critical point the density of water can vary smoothly as a function of P and T, and there is no boiling, condensation, etc. I don't know anything about power generation, but presumably when you can maintain temperatures higher than the T of the critical point, you don't have to worry about losing energy to phase changes.
angusb 12y ago
Replying because I don't think the explanations you've got so far are easy enough to read || accurate. Here's my understanding: Supercritical steam is a special form of steam that can not be described as a gas or a liquid. It's somewhere between the two: molecules aren't bunched together in dense clusters that settle at the bottom of a container (as they are in a liquid), but they also aren't flying all over the place individually in a low density vapour (as they are in a gas). How's that possible? Water molecules have relatively strong intermolecular attractive forces between neighbouring molecules. They like to stick together, even though there's no permanent connection between them. They are like mini-magnetised marbles. This explains why water has a much higher boiling point than most tri-atomic molecules. When you increase the temperature of liquid water, the molecules in the liquid vibrate and move around within the liquid, and as you cross the boiling point, the vibration and movement of the molecules is so great that they are able to escape the pull of their attractive interactions with their neighbours en masse. When this happens, the molecules shoot off into the vapour, where there is an (almost) unlimited amount of space for them to shoot around in. Now consider what happens when you do this at high pressure. High pressure essentially means that there are lots of molecules in the gas phase moving around really quickly. Now, when the temperature gets high enough that molecules have enough energy to overcome their attractive interactions with neighbouring molecules, they leave the pack: but this time with nowhere to go to. The pressure is so high in the 'gas' phase (i.e. there are so many other molecules up there) that they are forced to just bump around where the liquid was but at extremely high speeds. This type of behaviour is pretty difficult to distinguish from the behaviour in the high pressure 'gas' -- in fact, after the system has time to equilibriate, they are exactly the same. Clearly then, the transition from 'liquid' to 'gas' at this point is pretty much indistinguishable. The liquid may begin to display the molecular kinetic behaviour of a gas, but the density stays the same. The end result is: When the pressure and temperature is high enough, to onlookers it appears as if the entirety of the fluid is half way between a liquid and a gas, and is stable in that state. That's called a supercritical fluid.
walshemj 12y ago
It is steam at a higher pressure than atmospheric would allow. http://en.wikipedia.org/wiki/Superheated_steam http://en.wikipedia.org/wiki/Superheated_steam. Its more efficient in driving steam engines and turbines
- VLM 12y ago
  Well... yes technically anytime you have hotter steam its more efficient. A big gain is your stereotypical turbine blade doesn't like condensing conditions. Supercritical can't condense by definition, so its inherently good. Water droplets literally wear away the blade. Kinda sucks. They're expensive. So those expensive little things last longer if superheated steam is used. Its not so much that you can't make a condensing turbine, its that a condensing turbine will be less financially / economically efficient, its going to have to be much bigger and stronger for a given power output. Also the flow of steam is very predictable and constant, but once you start condensing no one really knows how it'll put vibration loads on, which can break the blades and wear out the bearings. Its false economy to use saturated or "wet" steam in a turbine to save money, usually. You can add a reheat stage to the middle of a turbine to prevent condensation. Of course that costs money and maint labor and energy. You can see the appeal of just using higher quality steam and avoiding all that. Sometimes you just have to eat the losses. Especially with nukes, they have relatively wet steam, well compared to coal plants anyway. Note that what some people call a condensing turbine doesn't involve condensation in the blades, at least not intentionally LOL. Its just a turbine with a huge condenser on the output instead of using a small condenser with an intermediate stage of process heat. Process heat is like, here's cruddy wet steam, but its hot, so how about using it in the office radiators, or to help heat preheat cooking ovens or something. Its hot by human standards but by power generation standards its only lukewarm and no longer economically useful to generate electricity. Its useless on the turbine floor, but perhaps a neighboring bread bakery would pay real money for it.

pinkskip 12y ago

On the other hand Abbott's government in their recent budget announced cuts to CSIRO and Clean Technology Innovation Program. Go Figure!

infectoid 12y ago
I know it's done the rounds but I want to make sure that everyone has seen it... John Oliver on Tony Abbott https://www.youtube.com/watch?v=c3IaKVmkXuk https://www.youtube.com/watch?v=c3IaKVmkXuk This guy man. I don't even know what to do about it.
- kibibu 12y ago
  Encourage everybody you know to vote for an alternative. In particular, though, don't tell Abbott voters how terrible they are for voting for him. People will get defensive and actually strengthen their views under criticism. (http://youarenotsosmart.com/2011/06/10/the-backfire-effect/ http://youarenotsosmart.com/2011/06/10/the-backfire-effect/)

knowtheory 12y ago

I'm unclear on why supercritical steam is important for electrical generation. Anybody happen to know? Reading through http://en.wikipedia.org/wiki/Supercritical_steam_generator http://en.wikipedia.org/wiki/Supercritical_steam_generator indicates that Benson boilers are more fuel efficient (and perhaps less prone to explosion), but reduction in amount of fossil fuels used to turn a turbine seems to be sort of a moot point here. Would super critical steam generation mean a solar plant can produce more electricity? Or that a supercritical steam generating solar plant is cheaper to operate?

cossatot 12y ago
I'm not an expert here, but I believe that the maximum possible efficiency of all heat engines is governed by the temperature difference between the heat source and the heat sink [1,2]. Increasing the water T/P to supercritical both increases the T at which it can enter the system and, from what I gather by the link you posted and from [3], also results in some simplification of the plant mechanics, which likely increases efficiency as well. This should indicate that for a given amount of sunlight (or fossil fuels in a conventional power plant), a greater amount of electricity can be created. [1]: http://en.wikipedia.org/wiki/Carnot_cycle http://en.wikipedia.org/wiki/Carnot_cycle [2]: http://en.wikipedia.org/wiki/Rankine_cycle http://en.wikipedia.org/wiki/Rankine_cycle [3]: http://www.brighthubengineering.com/power-plants/32896-how-are-supercritical-boilers-different-from-subcritical-boilers/ http://www.brighthubengineering.com/power-plants/32896-how-a...
- neltnerb 12y ago
  This is the correct answer. Higher temperature steam allows a higher maximum power extraction potential.
jaggederest 12y ago
For a given megawatt of thermal energy input, you get more electrical output by working in the supercritical steam domain than you do in subcritical systems. So not 'more electricity' since the sun is functionally unlimited, but 'more electricity per square foot' or 'more electricity per heliostat' or what have you.

quink 12y ago

> The $5.68 million research program is supported by the Australian Renewable Energy Agency and is part of a broader collaboration with Abengoa Solar, the largest supplier of solar thermal electricity in the world. Australian Renewable Energy Agency (ARENA). Let's see what the first Abbott budget had to offer up on this matter: > Finance Minister Mathias Cormann confirmed yesterday that the Australian Renewable Energy Agency (ARENA) will be discontinued in the 2014-15 Budget. Fun times, Australia. What fun!

NamTaf 12y ago
The CSIRO really is a gem in this country's crown. The amount they return on investment given is really quite impressive and their focus is directed towards our partiuclar problems (e.g.: agricultural research, ocean-based research, etc.) which may not necessarily be considered as equally by overseas research centres. To gut their funding is really very depressing.
- Gustomaximus 12y ago
  Not trying to turn this thread into a political discussion but this announcement couldn't come at a better time for the Greens (for non-Aussies they are an environmentally focussed political party with influence via their holding balance of power to the major 2 parties). The Greens are looking to block cuts to the Clean Energy Finance Corporation. It's not the CSIRO but this showcases what some scientific investment can achieve and hopefully stops the planned cuts across our research facilities being too deep. http://www.sbs.com.au/news/article/2014/06/15/greens-move-block-clean-energy-cuts-sets-double-dissolution-trigger http://www.sbs.com.au/news/article/2014/06/15/greens-move-bl...
  - quink 12y ago
    Ahhh, the CEFC. http://www.afr.com/p/national/clean_energy_profits_up_in_smoke_qQFE6H3pjYYoCcKnOim9cL http://www.afr.com/p/national/clean_energy_profits_up_in_smo... $200 million annually in profit by 2017 for the government through investment in renewables. But making money from renewable energy is, one should remember, 'utterly offensive': http://www.abc.net.au/news/2014-05-02/joe-hockey-wind-turbines-utterly-offensive/5425804 http://www.abc.net.au/news/2014-05-02/joe-hockey-wind-turbin... The CEFC is a commercial investment in actual rollouts of renewable energy with a 7% return on investment. The Abbott government, however, says 'bugger that, we hate evil renewable-tainted money' (or something like that) and is currently looking forward to passing legislation to abolish the CEFC as one of the first things the new Senate will do once the new Senate meets in about two weeks' time. Note also that, if it weren't for that the $200 million might well grow to $520 million pretty handily - they've got $10 billion in applications: > The Portfolio Budget Statement does not include the significantly higher positive contribution to the Budget the CEFC would make if it was able to continue to carry out its investment function over the forward estimates period. Were it to do so, the CEFC would deliver a net cash surplus (profit) for the Government of more than $520 million, net of operating costs. This government is committed to throwing away $500 million and more for taxpayers in money companies are willing to borrow and pay back - with interest - to roll out renewable energy schemes. It's like "Dear government, if you give us x amount of money, this lender over there will give us 2x of money, and I'll pay you back both, with interest. This is for us to change all the lighting across this huge area to use a third of the energy.", and the government saying, instead of "let's have a look at your business case and assure everyone this is a sound investment for us and your claim actually checks out and we'll all make money" a "lol, no".

abdullahkhalids 12y ago

Please keep in mind that efficiency in itself does not mean much. The variable you have to optimize is cost per unit of energy. Supercritical steam is useful for fossil fuel generation because it is relatively easy to make closed off systems which can withstand high temperatures/pressures. The optimum for solar power might lie at the much lower temperature. Why? Because the cost of solar array as a function of desired temperature rises much faster than that of a fossil fuel generator.

pcl 12y ago
the cost of solar array as a function of desired temperature rises In a heliostat-based configuration, I'd assume that it's just the cost of the target of the array of mirrors that increases, right?
- abdullahkhalids 12y ago
  To increase temperature you need to increase the power being transferred to the water. There are several ways you can do it. (a) increase size of mirror array (b) increase reflectivity of mirrors (c) improve sun tracking so that mirrors point towards the sun. (d) improve radiation absorption of receiver. Except the first, each of these is a very non linear. eg. taking reflectivity from 90 to 95 percent might double the cost. But then taking it to 97 might double the cost again.
  - ndonnellan 12y ago
    In addition, as your design steam temperature rises, your receiver's costs start to go way up. You need to use more expensive, less conductive metals - some (all?) of which are patented. Your radiative heat losses go up exponentially (T^4), so you need to hide as much surface area from the environment as possible (which means you need a cavity receiver). Now that you're a cavity, it becomes harder to concentrate light (smaller target) so your heliostat requirements go up. If I were a betting man, I'd say this goes nowhere. I think PV has already won the "war" versus thermal unless someone can come up with a great solar -> syngas receiver.

mmaunder 12y ago

This is important because it increases generator efficiency. http://en.wikipedia.org/wiki/Supercritical_fluid#Supercritical_fluid_in_power_generation http://en.wikipedia.org/wiki/Supercritical_fluid#Supercritic... Supercritical fluid in power generation The efficiency of a heat engine is ultimately dependent on the temperature difference between heat source and sink (Carnot cycle). To improve efficiency of power stations the operating temperature must be raised. Using water as the working fluid, this takes it into supercritical conditions.[20] Efficiencies can be raised from about 39% for subcritical operation to about 45% using current technology.[21] Supercritical water reactors (SCWRs) are promising advanced nuclear systems that offer similar thermal efficiency gains. Carbon dioxide can also be used in supercritical cycle nuclear power plants, with similar efficiency gains.[22] Many coal-fired supercritical steam generators are operational all over the world, and have enhanced the efficiency of traditional steam-power plants.

keypusher 12y ago

That's really great but the unsolved hurdle in solar power is energy storage. You can generate a large amount of energy on bright, sunny days but what happens at night when people go home and turn on their lights, television, and computers? Without a way to offer on-demand energy, solar will never be able to replace traditional coal and gas-fired plants. Wind power does fare better throughout the day, but is still subject to the whims of the weather. The big piece of the puzzle still to be solved is large scale battery/storage technology.

cma 12y ago
Air conditioning is a big power draw and peaks along with the sun.
- hueving 12y ago
  That doesn't really help unless the panels are in the same location as the solar field. Anecdotally this isn't really true either. The peak for air conditioning is usually later in the afternoon since it takes a while for the sun to heat up the area from the cool of the night.
TheSpiceIsLife 12y ago
Why are people down-voting this comment? Is being correct not allowed? In Australia it is often not sunny for days on end, also it is often not windy for days on end, over huge areas of land - you know, like, bigger than a lot of European countries. For wind and solar to be economically competitive with coal / gas / nuclear they have to be at price parity including 24+hrs storage. This isn't going to happen any time soon without massive government incentive. Professor Barry Brook of Adelaide University has been blogging about this for a good few years now. We need to think critically about 'sustainable energy'. I encourage you all to read his blog [1], particularly the TCASE [2] (Thinking Critically About Sustainable Energy) series. Let's level the playing field: ultimately there are only two good metrics worth considering a) life-time cost per kWh and b) life-time CO2 emissions per kWh --- but to level the playing field you need to consider each generating technology on a base-load comparison. It's no good comparing a 1GW gas plant and a 1GW nameplate solar installation because the sun only shines about 6.5hrs per day averaged throughout the year in, say, Adelaide for example, so you typically need to over-build solar by a factor of 4 and then add storage. When you do that the life-time cost and life-time CO2 emissions aren't so crash got because of the massive amounts of energy intensive stainless steel, steel, and concrete required, plus new transmission lines. If you want to see a real-world example of how wind does work, check the UK National Grid Status site [3] - the wind hasn't been blowing in the UK for weeks, presently their 8GW of installed wind is generating 0.82GW electricity. The data speaks for itself, look at the graphs. A vote for wind / solar is a vote for new gas _because_ gas is easily load following. Of course, the gas plant owners don't like that because when the wind blows their plants sit idle. The whole renewable energy push is an expensive mess. I've commented elsewhere on HN about this, so I'll stop repeating myself now. 1. http://bravenewclimate.com/ http://bravenewclimate.com/ 2. http://bravenewclimate.com/?s=TCASE http://bravenewclimate.com/?s=TCASE 3. http://www.gridwatch.templar.co.uk/ http://www.gridwatch.templar.co.uk/ Edit: formatting

jeffdavis 12y ago

"The $5.68 million research program..." It struck me how tiny the amount of money involved here is. Let's say this engineering effort was a one one-thousandth step along the way to developing the technology into a major energy alternative. Then that would be, what, $5B? Perhaps I'm overestimating the significance of this discovery.

Gustomaximus 12y ago

Does 'supercritical' steam have any storage benefit? Or is it more efficient use of the solar energy? I ask as I read some articles about using a similar focused mirrors to create molten salt which then could keep solar energy running after the sun went down, one of the major drawbacks of solar. http://en.wikipedia.org/wiki/Solar_thermal_energy#Molten_salt_storage http://en.wikipedia.org/wiki/Solar_thermal_energy#Molten_sal...

achivetta 12y ago

Thinking aloud: could one build a combination fossil fuel and solar steam-powered turbine? When the sun is out, you use (supercritical) steam as they describe being able to generate. If the sun goes away, you feed the same turbine with steam generated with fossil fuels. You could use same turbine, condenser, etc but just switch the heat source (or use both) as the environmental conditions change.

rail2rail 12y ago

Amazing that even our most cutting edge power generation schemes are still just steam engines under the hood.