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# Suggestions to reach high thermodynamic efficiency?3

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## Suggestions to reach high thermodynamic efficiency?

(OP)
I know that combined cycle power plants can go just a bit over 60%.

I would like to go that high for space based solar power plants. 60% thermal efficiency with a non-steam topping cycle reduces the size of the radiators. Potassium Rankine may be a good choice. One document makes a case for 54.6% and notes that a better vacuum on the steam condenser would add a percentage point or two. Other candidates for topping cycles include helium Brayton cycle, MHD and thermo-ionic (proposed in the original Boeing studies). There is also the possibility of using supercritical CO2 instead of water/steam. The reason to consider CO2 is the much smaller machine size and good efficiency of supercritical CO2 turbines.

Have I missed something?

Suggestions?

The most obvious thing you have missed is the economic viability of such a system. The cost of launching this device into the required orbit is currently around $10K/lb. And given its likely reliability and life span, even if it was 100% thermally efficient you'd still never make any money from it. ### RE: Suggestions to reach high thermodynamic efficiency? (OP) Tbuelna, your objection is economic rather than physics, but I am happy to discuss it. First, you are right, the current cost to GEO is close to$20,000/kg. Boeing used something close to your number in this study, http://www.sspi.gatech.edu/aiaa-2009-0462_ssp_alte... They got $145,000/kW. The levelized cost of electricity from no-fuel, low maintenance sources is the capital cost divided by 80,000. That's capital cost in dollars to kWh in cents. So the power would be about$1.80/kWh, about 100 times to expensive to find a market.

SpaceX will eventually bring the cost to GEO down to $1000/kg. That's still too high. The cost per kW capacity is the cost of the rectenna ($200/kW) the parts that go into space ($900) and the lift cost in$/kg x kg/kW. The original work came in at 10kg/kW, I have been using 5 kg/kW and the Japanese researchers think 7 kg/kW is more like it.

For 2 cent per kWh power, the power sat plus rectenna can't cost more than $1600/kW. For the cost estimates given, that would take lifting 5 kg to GEO at$500 or $100/kg, a 200 to one reduction over the current cost to get tons to GEO. Even at 10,000 flights a year, the lowest cost number I can get is$120/kg and that's only to LEO. (Reaction Engines, numbers for the Skylon.)

If you go up to 3 cents per kWh, then the max cost is $2400/kW. Taking out$1100 for the rectenna and parts leaves $1300 for getting it to GEO. At 5 kg/kW that's$260/kg or $140/kg over the cost to get a kg to LEO. Chemical rockets won't do that because of the reaction mass fraction. Ion engines will, but you have to power them and sunlight isn't concentrated enough to do that. The current thought is to use a reversed power satellite microwave link to send power to a transfer vehicle that goes from LEO to GEO and returns for another load. The loads would be ~1000 15 ton Skylon containers. Artwork here: Ground transmitter size https://drive.google.com/file/d/0B5iotdmmTJQsZlhne... Microwave rockets 3 pixs https://drive.google.com/file/d/0B5iotdmmTJQsNVZxW... https://drive.google.com/file/d/0B5iotdmmTJQsUkNUa... https://drive.google.com/file/d/0B5iotdmmTJQsYnVwa... This may not answer your objection entirely, but we have been thinking about the need for lower cost transport to GEO for a long time. ### RE: Suggestions to reach high thermodynamic efficiency? Peuter effect chips work, but how much might be the question. ### RE: Suggestions to reach high thermodynamic efficiency? hkhenson, Sorry, didn't mean to offend. While it would definitely be possible to build and deploy such a system using existing technologies, it would not be economically viable. Your proposal is interesting as an academic exercise, but it will never become a reality until you can prove the economics work. While I appreciate what SpaceX has accomplished, there is no way in heck they (or anyone else) will ever get (unsubsidized) GEO launch costs anywhere close to$1000/kg.

As for 10,000 launches per year, that's more than 27 per day. Mankind has been launching payloads into space for over half a century, and currently the total global number of commercial launches each year is less than two dozen.

### RE: Suggestions to reach high thermodynamic efficiency?

(OP)
The operative word in your post is "currently." If Skylon flies in 2021 (and they have money to develop the engines) it is expected to fly every other day. It takes off from a runway and you could probably fly them off at 6-8 per hour. But ignoring the transport cost, you do mention that it is interesting as an academic exercise.

If you had a limited transport budget where you needed to get a kW on the ground out of 5-7 kg of material in GEO, how would you do it?

### RE: Suggestions to reach high thermodynamic efficiency?

I don't have any idea how to bring launch costs down to the level your concept requires. But I'd suggest doing research on some of the incredibly creative (but totally impractical) ideas that have been proposed over the past 50 years for launching payloads into orbit. One was a concept proposed by some physicists to launch massive payloads using the reaction force of continuous nuclear explosions against a shield on the base of the vehicle. There is also the concept of the space elevator to hoist payloads into space using a cable connected from the earth's surface to an orbiting station 22K miles up.

### RE: Suggestions to reach high thermodynamic efficiency?

(OP)
I guess this site isn't as useful as I hoped it might be.

I am intimately familiar with space elevators, having presented a paper at the Microsoft sponsored conference some years ago, and with the Orion nuclear propulsion concept by Ted Taylor and Freeman Dyson. In fact, I know Freeman Dyson from O'Neill's space colonization conferences.

By the way, an earth elevator runs up against material limits, but a lunar elevator out through L1 can be built with current commercial grade Spectra fiber. A moving cable version would lift its own mass in 100 days. See the work by Jerome Pearson.

Using Spectra, it looks to be possible to build low temperature radiators for around a kg/kW. "The mass of the 22 deg C radiator, including a safety factor of 6 for the walls, is almost exactly 1 kg/kW. That makes the entire radiator ~6670 metric tons. The breakout is 1360 tons for wall material and 5300 tons for steam/water. That sounds like a lot, but it is 27% in the context of a total target mass of 25,000 tons for 5 kg/kW."

Steam, of course, will not get you 60% thermodynamic efficiency. You have to use a topping cycle. What I was after was suggestions and perhaps pointers to analysis of topping cycles.

### RE: Suggestions to reach high thermodynamic efficiency?

For a H2O steam plant it was proposed (for a ground based toping cycle), using an ammonia based steam toping cycle. I don't know of anyone who has done this though. The same ammonia based steam cycle was also proposed by EPRI as a method of extracting energy from a salt water solar pond. I also believe that never happened.

Not sure I can help with light weight systems other than the thremal chips.

### RE: Suggestions to reach high thermodynamic efficiency?

I'm sorry that you are disappointed in answers that do not support your idealism.

But, you see, we are engineers. Not politicians nor "scientists".

We HAVE TO come up with ideas that actually do work, that actually are safe, that actually can be maintained - do you seriously believe you can run a thermal plant in space, then add a ammonia topping plant to the thermal plant "in a vacuum" ???? - and that are cheap enough to build, operate and repair so the owner actually makes a profit over time.

I would support an ammonia topping plant operating in space. See, that way the repair mechanics and riggers and welders are already in full vacuum suits and respirators all the time! I don't have to worry about ammonia leaks or simple gasket failures killing the plant and everybody around it.

By the way, what's going rate for union pipefitters working in spacesuits?

### RE: Suggestions to reach high thermodynamic efficiency?

(OP)
If trying to figure out a technical way to get the human race out of the energy jam is "idealism" so be it.

Ammonia, however, is a candidate for a bottoming cycle, not a topping cycle. http://arpa-e.energy.gov/sites/default/files/docum...

Mercury was used to top steam cycles in a few places. Potassium has been proposed. http://web.ornl.gov/info/reports/1964/344560548342... Focused solar energy can get really not. The object is to get as much energy out of it as you can.

Re pipefitters, we need about 500 of them for a modest production rate of 100 GW of new power plants per year. Pay at a million dollars per year would hardly show up in a $200 B per year sales. ### RE: Suggestions to reach high thermodynamic efficiency? hkhenson: perhaps you should be working on the solar roadways project. Their source fuel seems to be idealism unfettered by commonsense, and they've gotten enormous press coverage and apparently quite a bit of crowdsourcing money out of it. You're asking a bunch of work-a-day engineers who spend their lives solving real-world problems, to spend their time puzzling over a scheme which presupposes the existence of a bunch of as-yet uninvented and in in some cases, fundamentally infeasible, technologies as a starting point, and you are surprised that they don't want to play along? Imagine for just a moment what the embodied energy of a space-based power system of any kind would be. Where would THAT energy come from, and how many years of perfect, zero maintenance operation would it take to pay back that deficit? What we need to get humankind out of its "energy jam" isn't powerplants in space, nuclear fusion or any other kind of deus ex machina big ticket megaproject technical solution from the sky: it's much, much simpler than that. What we need first is whole cost energy pricing including a fossil carbon tax. Once there's a clear market signal that the days of merely digging into the geological stores of past solar energy and spewing the effluent back into the atmosphere free of charge are coming to an end, the market will then fund energy generation/storage/distribution and energy use efficiency schemes using existing technology, as well as the development of new technology for such schemes, because there will be money in it for the developers. Not unreliable and economically unsustainable government subsidy, but real money- money given in trade by the users of energy, who derive benefit from its use, for its true and full value. Right now, the smart money steers clear of this whole market space. They've watched numerous biodiesel and corn ethanol and solar and wind producers get their collective @sses handed to them by changing government policy, BANANAism much less NIMBYism, and the absence of the required underlying political will to drive taxation or regulatory changes to make anything other than the status quo economically viable, and they're smart- they learn from other people's mistakes. Right now we don't even scratch the surface of the earth's renewable energy generation potential, and we are profligate wasters of energy. We're weak in both columns of David Mackay's "Renewable Energy- Without the Hot Air" energy balance. We can make great strides in both columns with minimal effort and without the invention of any new technology. Right now we use electricity derived from coal to make low-grade comfort heat with a resistance heater because we can "afford" it. We drag hundreds of thousands of people in the same direction at the same time two times a day in hundreds of cities in the world, each encased in their own two tons of steel, because each of those people feel that they can "afford" the benefit they derive from that expenditure. Building space-based powerplants to permit that sort of technological stupidity to continue is like putting a bigger engine in a car so it is less hindered by its square wheels. ### RE: Suggestions to reach high thermodynamic efficiency? molten, I sort of agree with your rant. The solar power in my calculator seems to last longer than the battery in my cell phone. Much of the energy waste is because of the trendy push by the salesmenship of the money people. No I don't really need side pipes on my lawn tractor, but they look cool. That said dosen't mean we should not look at new technology to accomplish the same things we are doing today. On the other hand, I could run my PU on wood (a little conversion), but I'm not going to. Coal may not be the fuel of the future, but don't phase it out before you have an economic replacment. That's just dumb. Turn the light off in your house and leave the rest of us alone. We do need new ideas, and new methods and I am happy that someone is asking questions. ### RE: Suggestions to reach high thermodynamic efficiency? (OP) moltonmetal, I am really disappointed in getting what amounts to political dogma responses from a group that's supposed to be technical heavies. Nothing new either, read the comments here http://www.theoildrum.com/node/5485 "Perhaps it is incorrect of me to assume they are in favor of a die off when they reject that there even could be a solution to the carbon/energy problems. Operationally though it's the same thing." or here http://www.theoildrum.com/node/7898 I am willing to consider other solutions. I worked almost two years on this http://www.theoildrum.com/node/8323 before we found it was going to be more expensive than power satellites. But solar roadways are just silly. What's fundamentally infeasible about power satellites? Re energy payback, if space elevators turn out to be feasible (I kind of doubt it, but who knows?) the payback is in single digit days. 15 kWh to haul a kg to GEO, so 5 kg/kW would take 75 hours or about 3 days to repay the lift energy. Triple that for the energy content of the parts and you are still under ten days. I worked out the energy payback time for a Skylon variation laser boosted ground to LEO and a second stage laser powered LEO to GEO and it came in at 53 days. If you can't get numbers in this range then the project *is not worth doing.* Have you actually *read* David MacKay's book? Or his subsequent analysis on roads? The point of the book is that the current ideas on renewable are not going to do it for the UK. Sorry. I have corresponded with Dave MacKay on these topics since 2008. ### RE: Suggestions to reach high thermodynamic efficiency? #### Quote (hkhenson) If trying to figure out a technical way to get the human race out of the energy jam is "idealism" so be it. There's nothing wrong with thinking about better ways of doing things or asking thoughtful questions. In fact the purpose of this website is to get engineers to provide answers to carefully considered technical questions. In fact, the quality of free technical advice you often get on these forums can be quite good. All it usually requires is for you to carefully think through the issue before you post your question. And then not act offended when someone posts a response you may not agree with. Remember that you are dealing with engineers here, and most professional engineers are by nature the polar opposite of an "idealist". Lastly, I'd just like to point out one other thing for you to consider about your proposal to construct a massive space-based electrical power generating system. Your basic concept requires developing a space launch system capable of putting payloads into GEO for less than 1% of what it currently costs. It seems to me that you are focusing on the wrong problem. Think about it this way, if you were able to prove you have a launch system capable of putting payloads into GEO for just 50% of current costs, you would become rich beyond your wildest dreams. Once you earned$billions solving that far simpler problem, you could use that massive wealth to build your space based power system.

Good luck to you, and I truly hope you succeed in your effort.

### RE: Suggestions to reach high thermodynamic efficiency?

I think that one of the things that make it unfeasible, aside from launch considerations is how the power is going to make it back to Earth in addition to the amount of material needed to be transported into orbit.

Assuming some sort of orbit that results in constant availability of sunlight, that gets you ~41 MBTU/m^2 annually. Worldwide energy consumption is > 500*10^15 BTU annually. Assuming you want to supply 1% of the world's energy, and you have your 60% generation efficiency, your transport efficiency back to the ground is going to be not much better than the same. Assuming 1364 W/m^2 solar constant, that results in 340 million m^2 of collection area required.

TTFN
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### RE: Suggestions to reach high thermodynamic efficiency?

If you consider my response to be "politically dogmatic", I doubt you'll bother to read what I have to say, but I'll say it anyway. I've read Mackay's book, thanks- and I'm pretty sure I understood it. The point of the book that I took away is simply this: we need to use energy GREATLY more efficiently BECAUSE the amount of energy we can generate both economically AND renewably is completely insufficient to replace the enormous amount we're getting out of the ground and spewing back into the atmosphere at present. In fact, in the UK, the amount that they could generate renewably is insufficient to match the amount they're consuming right now, even if price and capital expenditure were no object.

Mackay's book sets economics aside entirely to study the limits of renewable generation capacity- to put a plug in the ridiculous claims being made by people about the potential of (terrestrial) renewables to become simple swap-in replacements for fossil fuels. That's a very valuable exercise- especially if a few politicians listen. But we do live in the real world, and have to realize that we're not going to get anywhere with energy policy until we address the underlying economics that make energy so cheap that we can AFFORD to waste it in the profligate ways we do now.

I get it: there isn't enough terrestrial renewables capacity to match our growing energy "needs", so you're reaching for out-of-this-world ways to give us access to enormous amounts of renewable energy- you're trying to remove the limit, doubling the supply column in Mackay's ledger. My question is: even if we could do so both technically and economically -which I doubt sincerely- then why? So we can keep driving the car with square wheels? I just think that's utterly wrong-headed, especially if the solution you're working on is a mega-project to be operated in a location which pretty much defines the word "inaccessible", and which presupposes technologies that don't yet exist and may never exist.

My focus is on the valuation of the commodity itself, on both sides of the ledger, which will affect the consumption side AND the supply side. I'd argue that we actually don't NEED to consume nearly as much energy to live the lifestyles we enjoy now. We just need to waste vastly less of it by being smarter about how we consume it. And we actually don't need to invent anything new in order to make that happen, though we surely will do so. But we won't do that in earnest, nor will we make sincere and sustained investments in the renewable capacity which does exist here on earth, until we fix the valuation of energy. We need to stop dumping fossil carbon to the atmosphere free of charge if we actually want to meaningfully wean ourselves from fossil fuels. Right now, people whinge about the cost of electricity but in fact, it's too cheap to bother storing it. People whinge about gasoline prices but in fact this rarely determines what the average person drives much less where they choose to live. Until we fix that, indeed this entire discussion and your project both are more "hot air".

### RE: Suggestions to reach high thermodynamic efficiency?

Why do we need to only use renewable energy? Nucular Fission is not renewable, however it is going to happen even if we don't capture it. So I ask again, why do we need to only use renewable energy?

A good portion of natural gas will escape in some way if we don't use it, so why not capture it and use it?

The bet here is that technology will improve energy efficency, before we get to a point of having none.

If efficency and renewable were the only concerns, then where would Disney Land be? Why would we have sports, or even TV?

A little energy waste sure makes life easer to deal with.

### RE: Suggestions to reach high thermodynamic efficiency?

First and foremost ... Insulate

you must get smarter than the software you're using.

### RE: Suggestions to reach high thermodynamic efficiency?

Separating wants from needs is a value judgment and hence very personal. I may want TV, but the world would be better off in my opinion without Disneyland- my wife worked for Disney for a time so she knows from the inside what sort of business that company really is- you couldn't drag me there. But yes cranky you've got a very good point, even if you picked an example that's a sore spot with me.

Why do we need to use renewable energy? Because we're only here a while, and while we have every right to use the earth's resources to meet our needs, we don't have the right to squander them to meet extravagant wants. We have a responsibility to future generations. Again, that's all highly personal value judgment and your values are free to differ. As to fission, the issue I have with fission isn't the finite amount of uranium or thorium, or safety, or waste, or anything else- those are all manageable problems to me. It's the cost to build and operate the plants safely. They need to be so huge that waste and inefficiency, graft and corruption etc. are unavoidable. The only way it's possible for those plants to be built at all is if we mutualize the liability for an accident onto all of us as taxpayers, whether we're the ones using the energy wisely or squandering it. Call it what you will, but I call that a subsidy, and if we use energy more efficiently it's an utterly unnecessary subsidy. But I'd keep operating the ones we've already built, until the end of their realistic design life. Regrettably we're nearing that for our CANDU units here in Ontario and the combined decommissioning and replacement costs are coming due and whew, they're going to be enormous!

I don't want to care about your energy consumption choices and you shouldn't need to care about mine. The way to at least partially make that happen is to make sure that whatever energy you use, you pay the full and fair cost for it- including all its impacts on everybody else. Until you do that, everybody's consumption choices are my business because I'm paying the cost of adapting to the impact of your consumption- and vice versa with my choices. And the market for energy is distorted in favour of sources which put the cost of the impacts on others.

hkhenson: I recommend you read an AIChE paper titled, "Chemical Engineers Must Focus on Practical Solutions", by William Banholzer and Mark Jones (both from Dow). July 15, 2013, published in wileyonlinelibrary.com. A couple relevant quotes:

"Chemical engineers must do a better job explaining the difference between the subset of discoveries that offer practical solutions from the set that are simply possible."

"The world has a finite GDP and we must be exceptionally efficient so we do not waste it on ideas that require simultaneous miracles or violate thermodynamic principles."

I believe both of those points wholeheartedly.

### RE: Suggestions to reach high thermodynamic efficiency?

One current issue that seems to be often forgotten is that energy production plants can be classified as to the types of fuel they can accomidate. The steam plants seem to be the widest possible fule deversity, in that from the fuel nozel and fuel treetment is the only parts that need to be changed to adapt to a new fuel (the exception maybe the nucular steam plant).
Gas turbans on the other hand have a much narrower types of fuel they can accomidate, in that the fuels must be a gas.
The internal combustion engine may need a little more modification to change fuels, but again the fuels need to be a liquid or gas (although the orignal engines used wax).

### RE: Suggestions to reach high thermodynamic efficiency?

(OP)
tbuela, you are looking at a long conceptual development process. The low cost delivery of cargo to GEO depends on a UK project, Skylon, recently funded by $350 million, a combination of a grant by the UK and private investment. If you look at the price/volume market curve, there is a huge desert between communication satellites and power satellites. Comm sats can stand$10-20,000/kg, power sats need to get down into the range of $100/kg for the economics to work out. Cutting the cost by a factor of two doesn't help enough. That's what SpaceX is doing. molten, if we fail to replace fossil fuels with something of about the same capacity, a huge fraction of the human race will just starve. Consider if you will, the energy cost of making nitrogen fertilizer. Re "finite GDP for the world", this is going out of the world. ### RE: Suggestions to reach high thermodynamic efficiency? What's the heaviest comm sat ever launched? 3400kg. Assuming that the collection area calculated above requires 1 pound of material per m^2, that would require lifting 154 million kg into orbit, which makes that 45,000 equivalent launches. The entire world has launched a total of about 7,000 satellites, ever. And that gets us 1% of the world's current energy demand. TTFN FAQ731-376: Eng-Tips.com Forum Policies Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers ### RE: Suggestions to reach high thermodynamic efficiency? Actually to make nitrogen fertilizer you typically use natural gas, in the case of ammonia, and currently we are flaring great amounts of natural gas in the middle east because we don't have a use for it. If fertilizer is your concern, then I would be looking at our wastful flaring of natural gas. On the other hand, natural fertilizer is better for plants as it contains many trace minerals that plants consume. Where man made fertlizers are used it is common to have problmes with trace material depleation. Why not look more at technology for composting. ### RE: Suggestions to reach high thermodynamic efficiency? hkhenson, I direct you to the tables in Mackay's book. http://www.withouthotair.com/c13/page_79.shtml While fossil energy is definitely used, and VERY difficult to substitute, to both produce fertilizer and to replace human and animal labour in agriculture, in developed societies the amount of energy used in food production is significant at 15 kWh/d/person, but small relative to other items- heating and cooling (37 kWh/d/person), jet flights (30 kWh/d/person), and cars (40 kWh/d/person). And people in the developing world eat lower down the foodchain, which is more efficient in energy terms, so their consumption of food energy per capita is lower still. Furthermore, nitrogen fertilizers are made from the best (in fossil CO2 emission terms) fossil fuel we have- natural gas. We can focus on removing the waste in the higher energy consumption categories and by so doing, make plenty of room to continue making fertilizer. No space elevators or other magic required- just energy costing that makes this make sense, which will stimulate investments and provide a return on them that at present doesn't exist without unsustainable subsidy. Done right, a carbon tax will also provide a revenue stream to help people make those investments to get the fossil monkey off their backs. It was to counter statements like the one you just made- which have the ring of truth but which are not actually accurate, that Mackay did his work in the first place. ### RE: Suggestions to reach high thermodynamic efficiency? Still it makes me ill that we are flaring so much natural gas, and we turn around and spend so much on wind farms. Why not just use the natural gas for electricty production and avoid the wind farm problems. ### RE: Suggestions to reach high thermodynamic efficiency? #### Quote (bkhenson) Comm sats can stand$10-20,000/kg, power sats need to get down into the range of $100/kg for the economics to work out. Cutting the cost by a factor of two doesn't help enough. That's what SpaceX is doing. You completely missed the point I was making. To generate the tens-of-$billions you need to make your space power concept a reality, you don't need to develop a technology capable of launching payloads to GEO for 1% of current costs. You only need to develop a system capable of reliably launching payloads for just 50% of current costs, which is a far easier problem to solve. And if you do you will become extremely wealthy.

As for SpaceX, they have not proven they can launch commercial payloads for anywhere close to 50% of existing systems. And while the Skylon propulsion concept is creative, they have not shown viable solutions to other more difficult problems like airframe thermal protection systems. Unfortunately, $350M in funding won't get Skylon very far. Developing these very technically complex launch vehicles is incredibly expensive. Consider that NASA spent almost$400M on just a single Space Shuttle flight.

### RE: Suggestions to reach high thermodynamic efficiency?

(OP)
tbuelna, you completely missed the goal. Even if it is an easier problem to solve, reducing the cost of launching payload to GEO by 50% will not contribute to the goal of economical solar power from space. Also, I am not much interested in becoming extremely wealthy.

ESA's review of Skylon didn't indicate any thermal protection problems. Its much lower sectional density reduces the thermal protection difficulty considerably. For example, it goes through peak deceleration some 10 km higher than the Shuttle did.

Your comment on NASA is correct. It's a good reason for NASA not to be involved.

### RE: Suggestions to reach high thermodynamic efficiency?

hkhenson- I do understand what your ultimate goal is, but in order to achieve that goal you must formulate a plan that addresses both the technical and economic hurdles it faces.

Consider the example you brought up of how an efficient and nimble private company like SpaceX is able to reduce launch costs below what the big existing OEMs can offer. SpaceX would not exist without Elon Musk committing huge amounts of his own money to get it started. But the huge amounts of money Mr. Musk used to get SpaceX started came from another business venture he developed and sold off.

### RE: Suggestions to reach high thermodynamic efficiency?

(OP)
tbuelna, I have worked on this off and on since 1975. I have an *intimate* appreciation of the economics of the problem. I came here not to discuss economics or business financing problems, but to see if this engineering forum had any thoughts on thermodynamic cycles to skim off more of the energy from solar energy in GEO (which can get to really high temperature). Total power, efficiency and temperature determines the radiator area. The radiator mass is proportional to area. It's a little disappointing that you don't have suggestions. Most of the design work since the original studies in the late 70s have used PV.

This is part of design study document for a thermal power satellite.

General considerations

There are only two frequencies being considered for power satellites, 2.45 GHz and 5.8 GHz

For microwave optics reasons and the minimum forward voltage of the receiver diodes, power satellites have to be large. The higher the frequency, the smaller the optics, but atmospheric loses and rectification loses go up as well. This analysis uses 2.45 GHz, the same as the original designs investigated in the 1970s, and a power level of 5 GWe, that is power to the grid on the ground.

The electricity to electricity microwave path loss is 50% (3 db) which makes the power fed to the microwave transmitter at GEO 10 GW.

Turbines

Ten GW is a _lot_ of power. The largest generators constructed to date are 1.5 GW, and far too heavy to consider moving into space in one piece.

A GE90 engine on a Boeing 777 aircraft puts out 75,000 kW with a mass of ~7500 kg or 0.1 tons per MW. Given a 20 ton shipping limit, a turbine could put out 200 MW at this specific power. It would take 50 turbines of this scale to generate 10 GW. 10,000 MW of turbines at 0.1 tons/MW would mass 1000 tons.

Generators

The generators may mass more than the turbines. One example is an aircraft 400 Hz, generator, 40-50 KVA that massed 15 kg, or .33 kg/kW, or 330 tons per GW. http://books.google.com/books?id=QVlKAQAAIAAJ&...

Superconducting generators may be a lot lighter, and given that we may have to use super conductors anyway, may be acceptable. Tentatively we will assume the generator and power transmission mass to be 3300 tons with the understanding that this may be off either way by a substantial amount.

Thermal cycles

Thermal power satellite design is concerned with radiator area. For 50% efficient thermal engines (difficult but possible with two stages) the amount of heat radiated would be 10 GW and the sunlight input 20 GW. (This assumes no re radiation loss at the boiler.)

50% efficient thermal is beyond what is practical with steam (Rankine) cycles. It is possible for combined cycle plants (on earth) to exceed 60%. Efficiency isn't a direct economic concern (sunlight is free), but a substantial fraction of the mass is in the waste heat radiators. High efficiency reduces the size of the radiators. This analysis will assume 60% thermal efficiency with a non-steam topping cycle. Potassium may be a good choice. This document makes a case for 54.6% and notes that a better vacuum on the steam condenser (which we have) would add a percentage point or two. http://web.ornl.gov/info/reports/1964/344560548342... Other candidates for topping cycles include helium Brayton cycle, MHD and thermo-ionic (proposed in the original Boeing studies). There is also the possibility of using supercritical CO2 instead of water. The reason to consider this is the much smaller machine size and good efficiency of supercritical CO2 turbines. The cold end of the cycle (32 deg C) would be a good fit to a 10 deg C delta heat exchanger to a circulation of water/steam through the radiator tubes at 22 deg C (10 deg C delta T).

Efficiency and power set the collector area and radiator area.

Collectors and Radiators

For 60% efficient and 10 GW out, the input thermal energy will be 16.67 GW, and the radiator will need to dispose of 6.67 GW of low grade heat. The solar collecting area will be 16.67 GW/1.365 GW/km2 or 12.21 km2. The mass of the structure holding the reflectors and the reflectors will be taken as 0.5 kg/m2 or 500 tons per km2. The reflector surface will most likely be stretched aluminized plastic at no more than 0.1 kg/m2

Collectors would be ~6100 tons.

The ratio of collector to radiator optimizes with a radiator area of about twice the collector. (A. Bejan, Advanced Engineering Thermodynamics, 2nd ed., Wiley, New York, 1997, pg 495) Counting both sides of the radiator, the projected area is about the same.

For 6.76 GW / 12.21*2 km2 the heat level is ~273 W/m2. This number, substituted back into the Stefan–Boltzmann law is cooler than is actually useful for steam cycles (below freezing).

From previous work, (Drexler/Henson 1979) radiators tubes must be in a loop heated on both ends to eliminate massive return headers. Also, a minimum mass radiator has a square shape. A recent conceptual advance made while working on space based laser propulsion is to condense the low pressure working fluid (steam) only partway to prevent "water logging" in the radiator tubes and high fluid mass. There may be a better fraction, but the assumption in this analysis is that 80% of the steam condenses, an increment in density by a factor of 5. This increases the steam/water mass over the length of tube by an average of 3, (1+5)/2.

[And so on.]

I doubt you care, but the artwork on Google drive shows a Skylon cargo container being added to a LEO to GEO stage. The second slide shows the cargo under way using VASIMR engines making the purple glow. The engines are powered by microwave from the ground.

https://drive.google.com/file/d/0B5iotdmmTJQsamt1T...

https://drive.google.com/file/d/0B5iotdmmTJQsNVZxW...

### RE: Suggestions to reach high thermodynamic efficiency?

bkhenson-

I do appreciate the amount of effort you put into your posts. I also have some experience with the more practical issues of what it requires to put payloads into orbit, what is required to make a system reliable enough for extended use in a space environment, and how incredibly difficult it is to design a reusable space vehicle thermal protection system that is reliable enough for the duty cycle of a vehicle like Skylon.

I spent about 5 years working on the US Space Shuttle program, and I saw first-hand just how difficult the problems of thermal protection systems, efficient space radiators, and reliable space power generating systems can be to solve. The TPS used on the Shuttle was always a huge maintenance problem, and the radiator system used to reject waste heat in orbit was often a concern.

These are not easy problems to solve.

### RE: Suggestions to reach high thermodynamic efficiency?

(OP)
tbuelna, I agree with you re designing vehicles like Skylon. However, I am predicating my limited focus on an assumption that Reaction Engines knows what they are doing with (among other things) reentry thermal protection. I am trying to figure out how Skylon can be used to solve the energy, carbon, climate and economic problems.

I have been concerned with radiators to get rid of waste heat in space for a long time. (And on the ground too, waste heat is the bane of EEs.) Eric Drexler (of nanotechnology fame) and I wrote a paper on space radiators for the Space Manufacturing conference in 1979.

http://www.nss.org/settlement/L5news/L5news/L5news...
http://www.nss.org/settlement/L5news/L5news/L5news...

Recently, in the context of dumping a few GW of waste heat at a low temperature to cool propulsion lasers, I came up with an idea for low pressure, low temperature steam partially condensing radiators. If you worked on the Shuttle radiators, you night find the attached spread sheet interesting. Nobody else has checked it, so there is a decent chance you can find errors. The math gives the non intuitive result that hotter radiators mass more. The ISS radiators are rather heavy. Do you remember the kg/kW and the radiator temperature they used in the Shuttle doors?

### RE: Suggestions to reach high thermodynamic efficiency?

hkhenson- I don't know a whole lot about the radiators used on the Shuttle. I do know they were quite large and they covered almost the entire inside surface of the payload bay doors. I believe they used some type of freon as the working fluid. I recall one or two occasions where the system sprang a leak in orbit, but it was not too serious a problem since there were two independent systems used and the leak could be isolated.

### RE: Suggestions to reach high thermodynamic efficiency?

hkhenson- I imagine the reason for using different types of fluids had to do with which part of the vehicle the circuit was located in. If the circuit passed thru the crew space you'd use a fluid like water that would not pose a hazard to the crew if there was a leak. If the circuit was located in the external spaces of the vehicle, where temperatures can drop well below -100degF, water would not be suitable. Freezing water could easily damage the tubes/manifolds of the circuit.

### RE: Suggestions to reach high thermodynamic efficiency?

(OP)
tbuelna, the fluids used had more to do with the external regime the Shuttle was in. On the way up, before reaching orbit and being able to open the doors, they used water evaporating into vacuum or near vacuum. On the way back down, they used evaporating ammonia to get rid of waste heat when the pressure was high enough that water would not evaporate fast enough. Freezing didn't seem to have been a problem.

IRstuff, if you download that file, it should open in Excel.

### RE: Suggestions to reach high thermodynamic efficiency?

(OP)
Design of a thermal type power satellite proceeds. The big tubes are the radiator, the red hot object is the boiler.
https://drive.google.com/file/d/0B5iotdmmTJQsSWpzT...
Larger view showing the concentrating mirrors on tracks that follow the sun while the transmitting antenna faces earth.
https://drive.google.com/file/d/0B5iotdmmTJQsWVoyc...
Animation, not to scale
https://drive.google.com/file/d/0B5iotdmmTJQsNmVTa...
Another animation here. The camera is in an orbit just outside of GEO.
https://drive.google.com/file/d/0B5iotdmmTJQsSjRzN...

### RE: Suggestions to reach high thermodynamic efficiency?

The original posted question deals with space based solar device. If the location of the assembly is near the earth's orbit, then a Stirling engine could be used. The concentrating reflector/collector would face the sun ( duhh)and the exhaust would radiate to cold space. See the earth-based device at Infinia's website.

I would be surprised if NASA doesn't use this already.

"Whom the gods would destroy, they first make mad "

### RE: Suggestions to reach high thermodynamic efficiency?

(OP)
davefitz, as far as I know, PV and nuclear are the only power sources ever used in space.

Thermal cycles are possible, but it isn't easy to get rid of the waste heat. If you can start at a really high temperature, and take energy out in stages, then the size of the radiators is reduced. Sterling engines work over a restricted temperature range and for that reason are not very efficient. You can't get around thermodynamics (Carnot).

### RE: Suggestions to reach high thermodynamic efficiency?

hkhenson,

The stirling cycle as previously used by Infinia ( now called Qenergy) was a closed cycle , in that the working fluid was re-used . Its main profit center was emergency backup-power for defence-related consumers.

In 2005 its power conversion efficiency was over 20% when using solar concentrators and "exhausting" to ambient at 100 F ( 560 R). This had compared well to the 10% PV efficiency at that time. As you indicated the Carnot cycle efficiency limits can only improve when "exhausting" to space at 3 R. One earth-based issue may be that when a public observer looks at the glowing , focused acceptor there may be caused eye damage- TBD. But since PV may be over 20% efficient soon, the earth-based advantage is lost.

"Whom the gods would destroy, they first make mad "

### RE: Suggestions to reach high thermodynamic efficiency?

(OP)
davefitz,

Efficiency is only one consideration. The point of the entire design to cost effort is to get the cost of electric power delivered to the power grid on earth well below the cost of coal.

For microwave optics reasons and the minimum forward voltage of the receiver diodes, power satellites have to be large. The higher the frequency, the smaller the optics, but atmospheric losses and rectification losses go up as well. This analysis uses 2.45 GHz, the same as the original designs investigated in the 1970s, and a power level of 5 GWe, that is power to the grid on the ground.

The electricity-to-electricity microwave path loss is 50% (3 db) which makes the power fed to the microwave transmitter at GEO 10 GW.

Ten GW is a lot of power. The largest turbine/generator sets constructed to date are 1.5 GW, and far too heavy to consider moving into space in one piece.

A GE90 engine on a Boeing 777 aircraft puts out 75,000 kW with a mass of ~7500 kg or 0.1 tons per MW. Given a 20-ton shipping limit, a turbine could put out 200 MW at this specific power. It would take 50 turbines of this scale to generate 10 GW. 10,000 MW of turbines at 0.1 tons/MW would mass 1000 tons.

There is much more analysis if anyone wants to see it.

### RE: Suggestions to reach high thermodynamic efficiency?

hkhenson, I'm interested. This has been a great thread.

Best to you,

Goober Dave

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### RE: Suggestions to reach high thermodynamic efficiency?

This is a fun thread. I agree with moltenmetal on where the real problems lie, but still: The original question of the op, that I understand as "what would be the best medium and cycle for a steam engine in space?" is is interesting because it forces one to look at the usual design constraints of steam engine in a new light.
I can't add anything to this discussion. But in keeping with the blue-sky disussion happening here, some consideration:
Is high efficiency of the steam engine actually the biggest problem? I assume the collector to be relativly light weight, compared to turbine, generator and radiator. You can't escape Carnot, high efficiency will rewquire low temperatures, thus large radiators.
Turbine + Generator are heavy. Can we build a piston engine, wrap a coil around it and safe weight? I think linear generators are used on some stirling engines.

### RE: Suggestions to reach high thermodynamic efficiency?

Steam and CO2 are both problematic working fluids in space because they have the bad habit of freezing. Helium may be a better choice in a stirling cycle. Its a long wait until the service guy shows up to free up the frozen parts.

"Whom the gods would destroy, they first make mad "

### RE: Suggestions to reach high thermodynamic efficiency?

(OP)
Dave, freezing is a problem, particularly for water in the radiator. I don't know how to get away from using water in the radiators. It's also tricky because you can't let more than perhaps 80% condense before the radiators get waterlogged. CO2 as a working fluid can probably be kept in a small enough volume to keep it warm.

Martin, sterling engines are too heavy and too low in efficiency. I don't know enough about free piston engines to evaluate them. Any idea of how many kg/kW?

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