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?

 

[davefitz]

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 "

 

[hkhenson]

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.

 

[DRWeig]

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

Best to you,

Goober Dave

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[hkhenson]

The article on how to get the transport cost down was posted today:

http://theenergycollective.com/keith-henson/485571/power-satellite-progress

 

[hkhenson]

moltenmetal re "Without hot air,"

http://withouthotair.blogspot.co.uk/2014/09/solar-power-from-space.html

 

[MartinLe]

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.

 

[davefitz]

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 "

 

[hkhenson]

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?