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?

 

[cranky108]

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).

 

[hkhenson]

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.

 

[IRstuff]

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
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[cranky108]

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.

 

[moltenmetal]

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.

 

[cranky108]

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.

 

[tbuelna]

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.

 

[hkhenson]

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.

 

[tbuelna]

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.

 

[hkhenson]

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=QV...a=X&ei=02rFU5ShOIfwoASF7oKYCw&ved=0CCYQ6AEwAg

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/1964/3445605483429.pdf

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/0B5iotdmmTJQsamt1Tk1BaXlwaXBqLUhubS10VTJ4MnItSFNr/edit?usp=sharing

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

 

[tbuelna]

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.

 

[hkhenson]

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/L5news7907.pdf

http://www.nss.org/settlement/L5news/L5news/L5news7908.pdf

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?

 

[tbuelna]

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.

 

[IRstuff]

An actual spreadsheet would be more useful than a picture of one.

TTFN
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[hkhenson]

tbuelna, this document

http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/eclss/atcs.html

could sure use diagrams. Freon, water *and* ammonia.

 

[tbuelna]

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.

 

[hkhenson]

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.

 

[hkhenson]

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/0B5iotdmmTJQsSWpzTHZUS2diQWdvY055WmdPVS1sOWt2MGJj/edit?usp=sharing

Larger view showing the concentrating mirrors on tracks that follow the sun while the transmitting antenna faces earth.

https://drive.google.com/file/d/0B5iotdmmTJQsWVoyclVNTFh4cHpDQjNvRm0wSnMtYzE1dFpN/edit?usp=sharing

Animation, not to scale

https://drive.google.com/file/d/0B5iotdmmTJQsNmVTaUR1T2h1NlNFMFJiS3lHd2ptR013dnhR/edit?usp=sharing

Another animation here. The camera is in an orbit just outside of GEO.

https://drive.google.com/file/d/0B5iotdmmTJQsSjRzNUpkVFAwYUpUVWJxNEtRUVBoczlJM0tV/edit?usp=sharing

 

[davefitz]

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 "

 

[hkhenson]

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).