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March 21, 2012

Ways to make Space Solar Power Work

Tom Murphy a UCSD professor has put out an article that argues against space based solar power.

This is similar to an argument made in 2004 by Steve Fetter

Tom makes the arguments that launch costs are too high and it will be too expensive to make the space based solar power to justify about 3 times gain from being in orbit.

Tom uses the weight of ground based solar panels as a starting point of a standard rooftop panel delivers about 10 W per kilogram of mass (slightly better than this, but I will stick to round numbers). Let’s say a light-weighted version for space achieves an impressive factor-of-100 improvement: same power for 1% the mass. This gives 1 kW/kg.

Space has no gravity so you do not need the supporting structure of a ground based system. Al Globus provides the case for space based solar power in "Towards an Early Pro fitable PowerSat"

The Japanese Ikaros satellite, a solar sail, is producing power at about 0.8 kg/kw for the power production system. This extremely low value is accomplished by using a heliogyro thin film solar sail design, which has no structure, and thin film solar cells on part of the surface. Given reasonable improvements in thin film solar cells this figure could be 0.16 kg/kw for power production, not including other parts of the satellite.
A JPL Heliogyro design

This paper examines three technologies:
1. Power transmission in one of the atmospheric windows near 1 to 2 microns, reducing the minimum product of the power transmission emitter and receiver radius by a factor of up to 120,000 over traditional designs

2. Thin-film solar-to-electric conversion systems that can be made in large sheets, rolled up, launched, unrolled, and function in the space environment.

3. Heliogyro design for PowerSats, eliminating most of the structural mass associated with energy collection.

While there are substantial uncertainties and many unknowns, reasonable assumptions regarding improvements in these areas suggest that it may be possible to deploy a 5MW operational SSP system with a single launch of an existing vehicle. Furthermore, it may be possible to pay for this launch within one or a few years by selling power in high-priced niche markets.




Consider a PowerSat launched by a Falcon 9 assuming a mass of 100g/m2, which at 45g/m2 for the collection area leaves 2:6tons for all other systems. This leads to a square PowerSat 210 meters on a side. Assuming 8% sunlight-to-grid-power efficiency (20% solar cell and 40% transmission efficiency) this system would deliver roughly 5:28MW to the grid. A recent DOD report suggests that the U.S. military is willing to pay $1/kwh for power beamed to forward bases in Asia. Trucks transporting diesel can be ambushed, IR power beams cannot, and football-field sized receivers could fit on the larger bases. A 5MW system at this price would provide up to $46 million per year revenue, enough to pay for the launch in a little over a year. For commercial customers, the highest price this author could find world wide was $0.29 per kwh for industrial users in Italy in 2008. This could deliver up to $13.4 million per year { requiring a little over three years to pay for the launch.

The other big markets for space based solar power is for space applications. They can provide power for other space satellites and they can provide power for industrializing space. Having a lot of power to process lunar regolith enables the utilization of space based resources.

Al Globus had more in his article In Defense of Space Solar Power

Although purchasing one 12,500 kg per launch Falcon 9 costs $36.75 million, SpaceX representative Lauren Dreyer reports that packages of 1,000 launches can be purchased for $10 million apiece, which works out to $800/kg. At 5 kg/kw, 1,000 launches could provide roughly 2.5 gw -- a tiny fraction of current global energy demand.

A Spacex Heavy will launch for about $1000/kg but volume purchases of 1000 launches could bring that down to $500/kg. Also, Elon Musk is working on reusability which could bring costs down to $10-100/kg.

There could be cases based on the size of the space based power system where it makes more sense to use 60% efficient thermal heat engines instead of solar cells.

Existing turbines are 10 kW/kg but can be far cheaper than solar cells.

Beaming the Power Back

Al Globus provides the following information :

The product of the sending and receiving system minimum diameters is linearly dependent on wavelength. There are narrow transmission windows in the atmosphere at around 1 to 2 micron which, if exploited, could reduce the minimum antenna diameter product by a factor of around 60,000-120,000 at the cost of some efficiency. At 1 micron this would permit a fi ve meter on-orbit beam transmitting to a minimum 32m receiver on the ground. This enables small PowerSats, at the cost of much higher beam density and associated problems. Assuming the best conversion efficiencies demonstrated in the lab, for a 5MW-to-the-grid facility with a beam at 10x the power of sunlight the receiver must be about 45m in diameter. Also, as the near 1 micron windows do not survive cloudy conditions, such PowerSats may be most suitable for desert-like locations, where, fortunately, there are substantial electrical power markets such as Los Angeles, San Diego, Las Vegas, parts of Australia and North Africa.

Infra-red power beaming has been demonstrated by, among others, LaserMotive, which won first prize in a NASA sponsored power beaming competition. This involved a vehicle climbing a one km tether using power beamed from the ground. LaserMotive delivered 500w at 0.808 micron with around 10% efficiency over one km using o -the-shelf lasers, custom optics and custom solar cells. High efficiency was not essential to the project. The LaserMotive chief scientist suggests that 25% could be achieved today and perhaps 40% with near-term technology

Alfalight, Inc., a diode laser manufacturer, recently demonstrated 65% power
conversion efficiency as part of the DARPA Super High Efficiency Diode Sources program. This program has a goal of 80% efficiency. As lasers produce energy at essentially one wavelength, it should be possible to develop high efficiency 'solar' cells to convert this laser light back to electricity. The low efficiency of conventional solar cells is in part due to the difficulty of capturing energy at many wavelengths. Spire Semiconductor LLC produced a concentrator photovoltaic solar cell measured by the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) at 42% peak efficiency. These results also suggest that something near 40% end-to-end efficiency may be achievable. While this is less than the 63% estimated for microwaves, massive reduction in system size provides ample compensation.


Keith Henson has various approaches to make space solar power work

Tom Murphy has a history of projecting current technology without making proper adjustments to get to stupid conclusions. He wrote an article about galactic scale energy but does not have the civilization adapting to deal with waste heat. Tom starts from a conclusion that he wants and works backward to justify it as best he can

Keith Henson Current Plan

The solution to the economic problem as the NASA study pointed out is to reduce the cost to GEO to $100-200 per kg. At $100/kg, an installed watt cost about $1.60 based on five kg/kW, $900/kW for the parts and $200/kW for the rectenna. That’s low enough to make synthetic fuels for a dollar a gallon on off peak power.

The problem, as you point out, is the low payload fraction to GEO using conventional chemical rockets. You have to give up on chemical rockets and go to heated hydrogen to get the performance you need. (See http://en.wikipedia.org/wiki/Nuclear_thermal_rocket for performance data on heated hydrogen.)

But something that only recently occurred to me is that if you are building hundreds of GW of power satellites, then it’s easy to use the power from the first one for big propulsion lasers. I have run the calculation. Start with a Skylon (Reaction Engines’ design) derived vehicle that burns air and hydrogen until it runs out of air at 26 km and Mach 5.5. Then it switches to 2 GW of laser to heat hydrogen from Mach 5.5 to orbital speed. It will put 20 tons in GEO three times an hour or half a million tons per year. That’s enough to build 100 GW of new power sats per year.

The energy payback time (a more useful measurement than EROEI for renewables) is about two months. A previous version of this scheme is on The Oil Drum. http://www.theoildrum.com/node/7898

The problem is to get a 500 MW seed propulsion laser to GEO. That’s expensive, the laser at $10/watt is $5 Billion and the transport to GEO using Falcon Heavy is perhaps $20 Billion. The rest of the 2 GW ($15 Billion more for lasers, $1.5 Billion for transport) is brought up in 1/4-scale vehicles. The income from the completed transport system is about $50 Billion a year and the gross margin is at least $25 Billion, making the payback time less than three years.

If one part in ten of the new power sats was fed back into more laser propulsion capacity for a couple of years, the transport rate to GEO would go up to 5 million tons a year, enough for a TW of new power per year. With slightly more growth, the entire world could be off fossil fuels in less than a decade.

Combining Al Globus ideas with Keiths

Make profitable 5 megawatt Heliogyro systems and sell the for high value peak power applications like remote military bases. Scale it up to 500 megawatts and use that seed to provide the laser boosting in Keith's plan.

Solar Sails: Towards an Early Profitable PowerSat from Space Communication Journal on Vimeo.


Space Journal article on further work on the early space power satellite

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