|
the posted:One thing I've never really seen a clear answer to is how much it takes to power the average "nuclear" family to live in America. And how much it would cost for that family to produce their own power via solar panels, wind turbines, etc. Average electrical energy use for an American household is about 11.5 MWh annually.
|
#
¿
Sep 6, 2012 05:48
|
|
|
|
| # ¿ May 12, 2024 20:55 |
|
|
Hobo Erotica posted:Electric cars get way too much hype. They sound good at first glance, but when you think about it, they don’t actually accomplish very much by way of solving problems at all. Imagine if we somehow instantly changed every car in the world to an electric motor. You’ve still got traffic, you’ve still got parking shortages, you've still got people not getting fit, and unless you’re getting your electricity renewably, (which in most Australian cities isn’t the case), you’re still burning fossil fuels, with an emissions intensity which as bad or worse than petrol or diesel, let alone LPG. Power generation efficiency at a power plant is much higher than in an automotive engine: combined natural gas/steam cycles easily reach ~60% thermal efficiency (up to ~80% or more if they're cogeneration plants), and steam cycles alone reach ~40% efficiency, compared to ~25% efficiency for your average car engine. Even accounting for distribution and storage losses, centralized electrical generation with distribution to electric vehicles is going to be substantially more efficient than individual combustion engines for every vehicle.
|
|
#
¿
Nov 21, 2012 05:43
|
|
|
Quantum Mechanic posted:I think you're underestimating what there is to be gained from increasing the efficiency of solar plants - if we can crank the heat of the tower up another 100 degrees or so we could reasonably fit gas turbines to them instead of steam turbines for something like a 150% efficiency gain (40% steam turbine efficiency to 60% gas). Gaining what is effectively half a new plant from an increase in tower efficiency is nothing to sneeze at. Are externally-fired gas turbines in use anywhere? As far as I know, the only "gas turbine" in widespread use is the conventional internal-combustion type. In fact, when you said "gas turbine", an externally-fired version didn't even come to mind, and I don't see why low temperature would ever prevent you from using an externally fired closed-cycle gas turbine. John McCain fucked around with this message at 07:43 on Apr 2, 2013 |
|
#
¿
Apr 2, 2013 07:37
|
|
|
Quantum Mechanic posted:As far as I'm aware ~600C is where gas turbines in a combined cycle (i.e. connected to a traditional steam turbine running off waste heat) become markedly more efficient than a single steam turbine. Oh. You may want to do more research on generation technology, because conventional gas turbines can't be externally fired (they are internal combustion engines, whereas steam turbines are external combustion engines), and therefore are irrelevant in any discussion about running power generation from stored solar thermal energy. Basically, a conventional gas generator is an open cycle, inherently fossil-fuel based technology, while a steam turbine is a closed cycle that just requires some external heat source (and sink) to operate. The efficiency of either generator (indeed, of any generator) is always improved by pushing the temperature of the "hot side" of the heat engine higher. The reason gas turbines in a combined cycle are more efficient than steam turbines is because gas turbines get so much hotter than steam turbines that you can run a gas turbine and use its exhaust (so you've already extracted a significant amount of work from the gas) to run a steam turbine. John McCain fucked around with this message at 13:31 on Apr 2, 2013 |
|
#
¿
Apr 2, 2013 13:26
|
|
|
Flaky posted:Yeah it's so hilariously overstated. Don't they know benzene is colourless? That and the fact that there's absolutely no reason why benzene would be anywhere near a pure nuclear power plant (it will show up in real life since if you're dealing in hydrocarbons you can't escape it, and any power plant will have hydrocarbon-fueled backup generators), while it's a universal gasoline additive (and occurs naturally in most fossil fuels)!
|
|
#
¿
Apr 2, 2013 13:46
|
|
|
Quantum Mechanic posted:As far as I'm aware a closed-cycle gas turbine can be run off a heat exchanger? http://en.wikipedia.org/wiki/Closed-cycle_gas_turbine This is exactly why I asked you about externally-fired gas turbines, because they're never what people mean when they just say "gas turbine" and are a noncommercial technology at this point. But like I said, there's no obvious reason why there would be a temperature limitation with a closed-cycle gas turbine because there's typically no phase change or anything. You could run one from a 900K heat source or a 500K heat source or a 2000K heat source.
|
|
#
¿
Apr 2, 2013 13:57
|
|
|
Quantum Mechanic posted:As near as I'm aware the compression pressure of the turbine is related to the maximum temperature you can achieve, right? Quantum Mechanic posted:If so, because the thermal efficiency of a closed-cycle turbine vs. compression pressure isn't linear, you can get a marked difference in efficiency with a relatively small jump in temperature. You certainly CAN run one from a 500K heat source, but I don't think in that case it would be as efficient as the steam turbine. This is kind of true but it has very little to do with the working fluid. You can think of it this way: the choice of working fluid is essentially a compromise that reduces various engineering challenges (size of turbine, corrosion, knowledge of properties, desirable/undesirable reactions). But the limiting factor on the efficiency of an engine will be, for the foreseeable future, the temperature at the high-pressure turbine inlet, which is limited by the material you make the turbine from. We've picked water as a working fluid for much of our power generation because it has a number of convenient properties (notably, a relatively high boiling point, which allows us to take advantage of condensation/evaporation heat exchange, as well as allowing us to pump water (which is essentially "free" energy-wise) rather than compressing a gas, among many other properties). But that limits our turbine input temperature because supercritical water does all sorts of nasty things to turbine blades, so we tend to avoid it (although supercritical water has come into use relatively recently as metallurgy has advanced: from a GE publication, "GE designed the world’s most powerful USC steam turbine rated 1050 MW operating at 250 bar / 600 C / 610 C (3626 psi / 1112 F / 1130 F)", which is well above the critical point). Yes, it would be silly to use a non-water working fluid for most applications at low temperatures, but the reason why isn't really efficiency, or not directly (though in a fully-gas cycle, efficiency will suffer considerably from having to compress a gas rather than a liquid).
|
|
#
¿
Apr 2, 2013 16:14
|
|
|
Quantum Mechanic posted:See everything I can find is that the efficiency isn't directly proportional, it's logarithmic. That's my mistake, it's proportional to the temperature ratio (kind of: η = 1 - T1/T2), so it's log in the pressure ratio because of the power relationship between T and P. Quantum Mechanic posted:But the fundamental difference between steam and gas turbines isn't just the working fluid, it's that one's pressure-based and the other is flow-based? I have literally no idea what you're trying to say here. What exactly do you mean by "pressure-based" and "flow-based"? The turbine stage of gas and steam turbines is exactly the same, except conventional gas turbines run at much higher temperatures, and the expansion limit of steam turbines is limited by the fact that you don't want to expand too far into the saturation curve (need χ > ~0.9 because wet steam will tear the hell out of your turbine). In fact, the entire cycle is pretty drat similar: isentropic compression ---> constant pressure heat addition (for a conventional cycle, by internal combustion; for a CCGT, by a heat exchanger) ---> isentropic expansion ---> constant pressure heat rejection (via exhaust in a conventional turbine, in a heat exchanger for a CCGT) for the Brayton cycle and isentropic compression ---> constant pressure (and temperature!) heat addition in the boiler ---> isentropic expansion ---> constant pressure (and temperature!) heat rejection in the condenser for the Rankine cycle. Quantum Mechanic posted:Again, every resource I can find indicates that the efficiency v. temperature of a gas turbine requires higher temperature operation to be more efficient. The CSIRO CST research generator uses an air-fed gas turbine and runs at ~1100K, where more traditional solar plants run at about 850. If I get a chance tonight I'll ask one of the BZE engineers about it. This might be true because of the dramatic increase in compression work required in the compressor stage if you're dealing with a gas rather than with a liquid (which can be pressurized for very little energy), but the point I've been trying to make is that higher average heat supply temperatures will, as long as you're not breaking material limits, inherently lead to higher efficiency. It's not the fact that you're "able" to switch from a steam cycle to a closed cycle gas turbine, because (as far as I can tell), you can operate a CCGT at any arbitrary temperature range, it's just that because you've been able to boost the turbine inlet temperature by switching working fluids, you've been able to boost the efficiency. But the jump isn't going to be as dramatic as you've claimed because if CCGTs were economical everyone would be replacing their steam turbines with CCGTs.
|
|
#
¿
Apr 2, 2013 23:46
|
|
|
Quantum Mechanic posted:What I mean is the mechanism by which work is actually extracted from the heat cycles. I get that they're similar but that doesn't mean the same. It seemed that you were arguing before that using a gas turbine is irrespective of the input temperature, where you've just said there that gas turbines run at higher temperatures. Conventional gas turbines (i.e. fossil-fuelled turbines that make up probably 99.9999% of anything you'd ever call a "gas turbine") run at much higher temperatures than steam turbines simply because they are burning fossil fuels, which means they're inherently going to have a very hot combustion chamber. They are impossible (!!) to run from concentrated solar power because they are internal combustion engines. The type of "gas turbine" you'd have to run from a concentrated solar plant would be a closed-cycle gas turbine, an external combustion engine. "Pressure-based" vs "flow-based" is not a distinction that makes any sense. I can't come up with any possible reasonable definition of those terms that makes any sense. It's like if you said to me "Isn't it true that gas turbines are purple-based, while steam turbines are orange-based?". Quantum Mechanic posted:Just to check, by CCGT do you mean closed-cycle or combined-cycle? Because closed-cycle might merely be more efficient at 1100K but as near as I can tell combined-cycle effectively can't run at less than that. Combined-cycle power generation (it doesn't really make any sense to say "combined-cycle gas turbine", even though the term is in wide use) could conceivably run at almost any reasonable output temperature from the gas turbine. You could boil water at atmospheric pressure or near-atmospheric pressure and condense it through a steam turbine to extract some power. The question is not "Will sticking a steam turbine on the output of my gas turbine increase my efficiency?", because the answer is almost always "yes". The question is "Does sticking a steam turbine on the output of my gas turbine make economic sense?". Quantum Mechanic posted:To be fair I think I've been mixing up terminology as well, so I mostly think we've been talking past each other. Long story short though is as near as I can determine current market tech for concentrating solar wouldn't be able to run combined-cycle generation where with an extra 100-200K it could. Like I said, the question isn't really whether you will gain a positive Δη from sticking a steam turbine on the outlet of your gas generator, or whether you can even run one (for really low outlet temperatures you could even switch from steam to e.g. a hydrocarbon-based Rankine cycle if you were really obsessed with efficiency at any cost), the question is whether it makes economic sense to do so. Theoretical efficiency is going to be a smooth function of combustion temperature. And practical efficiency (which is a function of capital outlay vs marginal return) for large generators isn't going to ever see a dramatic jump from 45% --> 60% because it would become economical to add a steam turbine well before that. John McCain fucked around with this message at 00:47 on Apr 3, 2013 |
|
#
¿
Apr 3, 2013 00:45
|
|
|
Quantum Mechanic posted:
This is probably true at some point (although I'm becoming more and more skeptical about it for achievable concentrated solar temperatures as I think about it, since the work penalty associated with gas compression is very substantial, and a well-designed steam turbine can get into the low 40s for efficiency). Basically, though, the point I've been trying to make all along is that I strongly doubt that there is a magic number as far as solar collector temperature goes that suddenly produces an enormous jump in efficiency. Actually, I think the more likely option as we go forward will not be pure solar thermal plants, combined cycle or otherwise, but using solar thermal to substitute for some (but not all) fossil fuel use in more-or-less conventional power plants.
|
|
#
¿
Apr 3, 2013 04:05
|
|
|
Mined fossil fuels being cheaper than carbon-neutral synthesized fuels is a problem that can be addressed through the tax system (preferably "sin taxes" on fossil fuels rather than subsidies on carbon-neutral fuels, but either could accomplish the end of making the use of neutral fuels the cheaper option).
|
|
#
¿
Apr 15, 2013 08:49
|
|
|
For the low low price of probably trillions of dollars in capital investment, much of which would not see enough use to ever be profitable.
|
|
#
¿
Apr 17, 2013 02:41
|
|
|
Mostly what's stopping it from happening is that it's not profitable and the political will doesn't exist to maintain the level of taxation and/or debt that would be necessary to finance the plants publicly. Make no mistake, a program to replace 99.9% of the generative capacity of the US with renewable sources would be a capital investment on a scale the US hasn't seen since the Great Depression (and probably higher than even most of the New Deal). For example, Hoover Dam cost ca. $700m in 2008 dollars and has a maximum capacity of 2 GW. The existing summer generative capacity of the US is approx. 1 TW. The article suggests overbuilding generation to 300% required, so the needed capacity by 2030 would be at least 3 TW assuming no electrical energy growth needed over the next 20 years. That means you need to build 1500 Hoover Dams (!!!!) for a cost of ca. $1 trillion. Of course, you can't possibly build 1500 Hoover Dams (which, incidentally, is currently producing power for a cost of ca. 1.6c/KWh, which is dirt cheap compared to most renewables), so the real price tag is going to be significantly higher than $1 trillion. John McCain fucked around with this message at 03:32 on Apr 17, 2013 |
|
#
¿
Apr 17, 2013 03:21
|
|
|
It's not an issue of profitability for its own sake, it's simply the fact that you're never going to attract private investment for something that's unprofitable. And in order to adjust the market to properly account for the significant negative externalities of non-renewable generation would require a significant political sea change just to admit climate change as a problem, much less to agree to government subsidies (!) or, God forbid, new taxes (!!). And profitability issues aside, we're talking about huge expenditures of capital to build the drat plants. How much of the US GDP should be devoted solely to construction of new power plants? One percent? Five percent? Ten percent? That represents a titanic investment of concrete, steel, and labor. The mining for the steel and concrete alone would itself cause significant environmental damage.
|
|
#
¿
Apr 17, 2013 03:49
|
|
|
If you want people to invest in projects that they know very well aren't profitable and never will be profitable you're going to have to do something about that pesky "human nature" thing. And while switching government subsidies from fossil fuels to renewable power will provide a start, it'll be a drop in the bucket compared to the amount of investment required to reach 99.9% renewable by 2030. We're talking New Deal/WWII Reconstruction investment levels required.
|
|
#
¿
Apr 17, 2013 04:23
|
|
|
The problem is not convincing you that it's a good idea, the problem is convincing the people who have control of the trillions of dollars needed to get the project done. I don't think a gradual transition to renewables is impossible, but I do think that getting America on 99.9% renewables by 2030 is impossible. I think we're pretty much hosed at this point in terms of avoiding significant climate change, but that doesn't mean we should give up, because at this rate we're going to boil the oceans. Also, this is a thread about energy generation, not exclusively alternative energy generation. John McCain fucked around with this message at 05:00 on Apr 17, 2013 |
|
#
¿
Apr 17, 2013 04:47
|
|
|
One of my classmates' senior design projects was designing a rapid-charge system for cars based off of supercapacitors. Spoiler alert: Turns out a charging system for electric cars is probably a problem mostly for electrical engineers, not mechanical engineers.
|
|
#
¿
May 16, 2013 12:30
|
|
|
I can tell you straight off that the method they used to calculate h for the convective heat transfer off the cylinder is dumb as hell; they should have calculated it experimentally for a dummy cylinder. Also, an IR camera is a stupid way to determine surface temperature for something like this. They should have used actual temperature probes. e: basically, trying to perform an energy analysis on a cylinder just set up out in the open in a random apparently uncontrolled room is pretty loving stupid if you want to be at all rigorous John McCain fucked around with this message at 06:46 on May 25, 2013 |
|
#
¿
May 25, 2013 06:32
|
|
|
So did Lord Kelvin, which is why he thought the Earth was at most 400 million years old (that is, he assumed that the Earth initially formed as a giant ball of molten rock and has been cooling since then). http://physicsworld.com/cws/article/news/2011/jul/19/radioactive-decay-accounts-for-half-of-earths-heat
|
|
#
¿
Jun 9, 2013 01:31
|
|
|
All power plants that convert thermal energy to electricity have to dump a lot of heat to the environment (typically anywhere from about 80% to 150% of the electrical power generated), so, for example, a 500 MW solar thermal plant with 50% efficiency would also have to dump 500 MW of heat to the environment. This is normally handled by heating a huge volume of water a few degrees C (because the cooler the working fluid is when rejecting heat to the environment, the more efficient the power plant), but doesn't necessarily entail any actual consumption of water. If, for example, your water supply is 20 degC base and you heat it to 35 degC with your condenser, and your power plant is 500 MW electrical with 50% thermal efficiency, you need a flow of (500 MW / (4.18 kJ/kg/degK * 15 degK)) = ~8000 kg/second of water (which is about 8 m^3 of water per second). This sort of basic analysis neglects many factors, of course, not least of which is evaporation, but it should make it obvious that a nontrivial amount of water is required for thermal power plants of any appreciable size. According to the National Renewable Energy Laboratory, about 2.5% of the water used by thermoelectric power plants is actually evaporated so the above example would evaporate about 200 kg/sec of water, or about 53 gallons/sec (or about 4.6 million gallons per day). For perspective, from the above link, NREL posted:According to the USGS the total amount of fresh water used at U.S. thermoelectric power plants in 1995 was 132,000 MGD (500 x 10^9 L/d), of which 2.5%, or 3,310 MGD (12.5 x 10^9L/d), was evaporated The paper also notes that about half a gallon of water is evaporated per kWh produced by thermoelectric power plants. I have no idea what the water consumption for PV would be.
|
|
#
¿
Aug 6, 2013 05:15
|
|
|
Hobo Erotica posted:Can anyone tell me how much gas a 400 MW plant would use in a day though? And the price? That passage is super hosed. 1 GW = 1e9 W. not 1e3 W. The numbers are all correct but you need to fix those units.
|
|
#
¿
Sep 27, 2013 16:43
|
|
|
Pumped water storage is the worst form of energy storage except for all the others that have been tried. Pretty much the only thing it has going for it is that the energy is stored as mechanical energy.
|
|
#
¿
Jan 14, 2014 04:58
|
|
|
|
| # ¿ May 12, 2024 20:55 |
|
|
blowfish posted:Currently, hydro is being expanded even into biodiversity hotspots like the Balkan alluvial forests where it'll gently caress up the entire ecosystem. At least pumped storage has the advantage of just needing a body of water next to any hill (but we will find a way to put the largest pumped storage facility in the country on top of some exceedingly valuable habitats, I'm sure). Generally speaking, if you want to construct any significant amount of pumped storage capacity, you're going to gently caress up the surrounding terrain just like with dam construction. The size of the reservoir you would need for even a relatively small amount of storage given a reasonable height difference is just ridiculous. Gravity is so loving weak that it's difficult to store a lot of potential energy just by moving stuff around in a gravity well.
|
|
#
¿
Jan 14, 2014 20:09
|
|