Suggestions to reach high thermodynamic efficiency? 3

[hkhenson]

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?

 

[tbuelna]

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.

 

[hkhenson]

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_alternatives_potter.pdf

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/0B5iotdmmTJQsZlhneC1BLTNicU42OWI5MHhpSEJXZWFOQjE0/edit?usp=sharing

Microwave rockets 3 pixs

https://drive.google.com/file/d/0B5iotdmmTJQsNVZxWWxRN0oyWXMzRlB0d0F4aFJOazRiZG9n/edit?usp=sharing

https://drive.google.com/file/d/0B5iotdmmTJQsUkNUak9hYmFoU3B5UFpmUVVYSEJ1VGF1cHVB/edit?usp=sharing

https://drive.google.com/file/d/0B5iotdmmTJQsYnVway1JRGRJVzVEUXloTXBwdDVLRllUNUZJ/edit?usp=sharing

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.

 

[cranky108]

Peuter effect chips work, but how much might be the question.

 

[tbuelna]

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.

 

[hkhenson]

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?

 

[tbuelna]

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.

 

[hkhenson]

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.

 

[cranky108]

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.

 

[racookpe1978]

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?

 

[hkhenson]

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/defa...or More Efficient Power Generation Agenda.pdf

Mercury was used to top steam cycles in a few places. Potassium has been proposed.

http://web.ornl.gov/info/reports/1964/3445605483429.pdf

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.

 

[moltenmetal]

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.

 

[cranky108]

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.

 

[hkhenson]

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.

 

[tbuelna]

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.

 

[IRstuff]

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
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[moltenmetal]

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".

 

[cranky108]

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.

 

[BigInch]

First and foremost ... Insulate

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

 

[moltenmetal]

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.