Rocketdoc Notes – Week of November 22, 2020
Can Space Solar Power solve Global Warming?
Once again, I’m going to revisit the Global Warming problem. If you look at issues currently troubling our population, Global Warming is number two after the Corona 19 pandemic and is definitely worth exploring. One of my readers last week reminded me about Space- Solar Power Satellites (SPS) as a possible answer to the fossil-free electric power problem so I thought I would bring you up to speed on SPS derivatives and why I think they are a not the best solution for our current problems. I do have friends who will argue this point.
SPS goes back to the early 1970s when Peter Glaser was granted U.S. patent number 3,781,647 for his method of transmitting power over long distances (e.g., from an SPS to Earth's surface) using microwaves from a very large antenna (up to one square kilometer) on the satellite to a much larger one, now known as a rectenna, on the ground. Peter was a VP at Arthur D. Little, Inc., and based on that patent A.D. Little and four other companies got NASA contracts to explore the SPS concept in 1974 and 1975. One of those companies was Boeing Aerospace and that is how I became involved.
The Boeing SPS work was summarized in a 1976 Final Report Study titled “Systems Definition Space-Based Power Conversion Systems”, Boeing Aerospace Company Publication D180-20309-2. I’ve enclosed a number of figures from this report to show where State-of-the-Art with respect to SPS Technologies was in 1976 including advancements expected by 1985. All costs are in 1976$ (Note: a 1976$ is worth $4.576 2020$). Figure 1 below shows the overall schematic for SPS. There would be a chain of SPS in Geosynchronous orbit beaming down power to receiving stations (i.e., rectennas) all over the world (only US stations are shown in this figure).
Figure 1 – Solar Power Satellite Operating Characteristics
Sunlight in Geosynchronous orbit is almost constant (they are in darkness for 72 minutes at midnight at the changing of the equinoxes twice a year). Therefore SPS, in theory, can replace almost all of the stationary electric powerplants at latitudes below 50 degrees. The problem isn’t the physics, it is the costs involved.
The Boeing study looked at various ways to generate the electric power to be beamed down trying to find the lowest cost variant. They looked at two types of photovoltaic arrays (Silicon and Gallium Arsenide), two types of solar thermal power conversion (Thermionic and Brayton Cycle) and two advanced nuclear reactor power sources (Rotating Particle Bed Reactor and a Molten-Salt Breeder Reactor both driving a Brayton Cycle Powerplant). The relative sizes of the on-orbit infrastructure for 10 GW busbar power out of the rectennas is show in Figure 2 below.
Figure 2 Relative SPS sizes for 10 GW busbar power out of the Rectenna
As you can see these structures are gigantic, 20 to 30 kilometers long, with the one exception, the fission powerplant which doesn’t have to collect solar power. This answer was not popular with NASA because why launch a nuclear powerplant all the way to geosynchronous orbit when the same plant works just as well on the ground a few miles from its prospective customers.
The masses of each of the candidate power systems is shown in Figure 3 below. There are a few surprises in the weights because radiators are much heavier than photovoltaic panels, so some candidates have mass advantages even though they were much larger in size.
Figure 3 – Relative Mass Comparison for10 GW Busbar power on the ground
The three winners from a mass standpoint were the advanced Gallium Arsenide Solar panels, the Brayton Cycle Solar Thermal Cycle, and the Rotating Particle Bed Fission Reactor Powerplant (it operates at a much higher temperature than the Molten-Salt reactor). Remember mass is important because every pound much be launched into low earth orbit (LEO) and then transferred to geosynchronous earth orbit (GEO). After the initial design exercises the nuclear options were dropped for obvious reasons. Why would anyone pay to launch nuclear reactors into orbit when they could operate more efficiently on the ground?
From then on, the competition was between solar photovoltaic and solar thermal systems. The Life Cycle Costs for the remaining competitors are shown in figure 4 below.
Figure 4 – SPS Life Cycle Cost Comparison
Life Cycle Costs (LCC) here are the development, transportation, and maintenance costs for an average SPS unit (10 GW ground power). Note that the units are in Billions of 1976 dollars, so the cheapest system (Brayton Cycle Solar Thermal @ $33B) has a LCC unit cost of $151 B in todays dollars. This is despite the assumption that much of the ETO traffic was assumed to be carried by the Space Shuttle, which in 1976 was still five years from first flight and was bookkept at $4.8M per launch ($75/pound in 1976$).
This was essentially the showstopper for the SPS. When the development and operating costs of a new large reusable launch system were included the SPS option costs were about $200 B before the first watt could be delivered. This was about two orders of magnitude more than competitive ground-based systems and that is totally unaffordable to any utility companies.
Another issue is the beaming microwaves down to Earth. The rectenna design in 1976 is show in Figure 5 below. The rectenna is the silver-colored ellipsoid near the right bottom. The landmarks are visible through the rectenna because it a metallic matrix structure elevated off the ground so sunlight and rain pass through it, and plants can grow beneath it.
Figure 5 – Picture of Rectenna at Mid-USA Latitudes
The biggest problem with the rectenna approach is that the microwave frequencies originally planned for have been appropriated for various currently operating systems and are no longer available. Still higher microwave frequencies are still available, but they greatly complicate the orbital transmitter and rectenna construction. There has been some of talk of switching to laser beam transmission to reduce the size of rectenna, but this greatly reduces the transmitter and receiver efficiencies, and the loss of power during cloudy and rainy weather approaches 100%.
The bottom line is that Solar Power Satellites seemed like a good idea after the 1975 oil crisis when we had a serious energy shortfall, but today we have a surplus of oil and natural gas and relatively cheap renewable ground-based energy sources in the form of windmills and solar-voltaic farms. Two of my friends, John Mankins and Joe Howell, started the Space Solar Power Exploratory Research and Technology program (SERT) program in March 1999 to find a “magic bullet” technology breakthrough to make a SPS concept competitive with ground-based power generation. They found two critical technologies that were key, the improvements in the efficiency of solar cells (especially thin film cells) and reductions in the cost of ETO launches. Improvements in the efficiencies of solar cells over time is shown in Figure 6 below. Thin cell efficiencies have more than doubled since 1985 but the cost of ground-based power has dropped by over an order of magnitude during that time frame (in constant dollars) so SPS is still losing the race.
Figure 6 – Best Solar Cell Efficiencies over Time
The second magic bullet, reductions in ETO launch costs to about $100/kg, has continued to be a showstopper until recently. NASA dropped low-cost launch aspirations in the later 1990s and switched to heavy lift launch vehicles for Mars. But private industry recognized the need, and the new Space-X Starship can probably meet the $100/kg goal. Once Starship becomes operational, we may have to revisit updated SPS concepts. I still have friends waiting for that opportunity, but for myself, I believe geosynchronous orbit is already crowded enough and I’m focusing on small ground-based fusion powerplants to complement windmills and photo-voltaic farms.
Thanks for reading.