The topic of space solar power is very important this year. It is important to understand how space solar power works and its advantages compared to ground solar power. Mr. Elon Musk spoke about this comparison in a video interview available on YouTube. The above is a segment of a longer 2012 interview of Elon Musk with PopularMechanics.com. A number of topics were discussed in the nearly hour-long interview. Here is my transcript of his remarks on space solar power with extraneous comments removed:
Mr. Musk: “Let me tell you of one of my pet pieces: space solar power. OK, the stupidest thing ever. And if anybody should think, should like space solar power, it should be me. I’ve got a rocket company and a solar company. I should be like – I should really be on it, you know. But it’s like super obviously not going to work because if you have solar panels … First of all it has to be better than having solar panels on Earth, right? Solar panel is in orbit you get twice the solar energy, assuming that it’s out of Earth’s shadow, but you have got to do a double conversion. So you are going to covert it from photon to electron to photon back to electron. So you got to make this double conversion. What’s your conversion efficiency? All in you are going to have a real hard time actually even getting to sort of 50 percent. (Side comment on conversion efficiency responding to remark from the audience.) Take any given solar cell, is it better to have it on Earth or better to have it in orbit? What do you get for being in Orbit? You get twice as much sun, best case, but you have got to do a conversion – you’ve got to convert it to photons, the energy to photons or you have incoming photons that go to electrons but you’ve got to do two conversions that you don’t have to do on Earth which is you got to turn it into photons and turn the photons back into electrons – and that double conversion is going to get you back to where you started, basically. So why are you bothering sending them to bloody space? And, by the way, electron to photon converters are not free and nor is sending stuff to space. Obviously, super (likely meaning space solar power) doesn’t work. Case closed.”
To summarize, Mr. Musk’s view, as of 2012, is that space solar power is a bad idea. I have not found a more recent set of public remarks by Mr. Musk on this topic. With space solar power gaining increased public exposure, a critique of Mr. Musk’s remarks is appropriate.
The illustration above provides a top-level explanation of the space solar power concept. Located in geostationary Earth orbit (GEO) – the orbit used by communication satellites – the large space solar power platform converts the intercepted solar isolation (sunshine) into electrical power and feeds this electrical power to a transmitter array that sends the power to a ground receiving station. These are the conversions Mr. Musk spoke about. As this process is continuous for almost the entire year, this provides baseload electrical power much as do coal-fired and nuclear power plants.
Sunlight in space has more power (watts) per sq. mi. than on the ground because the atmosphere absorbs some of the isolation. Each sq. mi. of solar receivers in space “sees” about 3.6 GW of solar radiation continuously. Current estimates are that the conversion of the intercepted solar isolation into ground electrical power will be about 16 percent efficient overall. Thus, after going through the conversion processes, the 3.6 GW of captured sunlight yields about 0.6 GW of baseload electrical power provided to the ground utility grid.
A typical coal or nuclear baseload power plant generates 1 GW of baseload electrical power. To match this, the space solar power platform needs to intercept about 1.7 sq. mi. of solar isolation using solar arrays or mirrors. As noted earlier, because the space solar power platform’s mirrors or solar arrays are in continuous sunshine for almost the entire year, this sustainable power supply is provided as baseload electrical power. The exception is during brief periods at local midnight during the twice-annual spring and fall equinoxes as the GEO platform passes into the Earth’s showdown. Reserve power is used during this time. Also, not all GEO platforms experience this at the same time.
Mr. Musk’s explanation fails to distinguish the primary difference between terrestrial solar power and GEO space solar power. (The photograph above is of a typical US terrestrial solar farm.) The terrestrial approach is subject to the limited availability of sunlight due to the day-night cycle, obscuring cloud cover, and seasonal changes in the elevation of the sun above the horizon at solar noon. This means that terrestrial solar-electricity is variable, not baseload. It has a design “capacity factor” – the percentage of the total time that useful electrical power can be produced – of about 20 percent of the year on average. A year has 365 x 24 = 8,760 hours. Thus, terrestrial solar would only be expected to deliver about 1,752 hours of electricity directly while the sun is shining. The remainder of the time, another source of energy (gas-fired turbine generators) or stored energy (batteries or hydrogen fuel cells) would be needed to provide electricity to the customer. To be completely solar powered, the low capacity factor means that some means of storing the solar-electricity – batteries, thermal storage, hydrogen produced via electrolysis – is needed to enable dispatchable electrical power to be provided to the power grid when needed any day or any hour of the year. To collect sufficient solar-electricity when the sunshine is available to meet the electrical power needs the other 80 percent of the time means that the terrestrial solar farms must be oversized to produce the excess electrical power that must be placed in storage or used to produce hydrogen. Whereas it would require about 1.7 sq. mi. of solar arrays to yield 1 GW of baseload electrical power, ground solar farms would need about 78 sq. mi. of land to provide the same. Therefore, each sq. mi. of space solar array is roughly equal to 46 sq. mi. of ground solar farm land area in terms of total energy supplied per year. (As seen in the above photograph, only about 50-60 percent of the solar farm’s land area is covered with solar arrays.)
I have calculated the land area required for both terrestrial solar power and GEO solar power to meet the energy needs of the United States in 2100. (See this paper.) In 2100, the expected population of the United States will about double from that of 2010 to about 618 million. To meet the entire energy needs of the United States in 2100, about 473,000 sq. mi. of terrestrial solar farms would be needed to provide the annual needs for both dispatched electricity and hydrogen fuel. To build the required terrestrial solar farms, most of the flat land in the Southwestern United States would need to be converted into solar farms. (This is about 16 percent – about one out of every six sq. mi. – of the total US continental land area.) This is unlikely to be acceptable for both environmental and political reasons. For comparison, GEO solar platforms would require about 10,500 sq. mi. of solar arrays in geostationary orbit and about 60,000 sq. mi. of ground receiving stations. (This is about 2 percent of the total US continental land area.)
In conclusion, Mr. Musk’s 2012 remarks do not address the space solar power advantages in terms of reduced land area and ability to provide baseload electrical power when compared to the variable solar-electricity from ground solar farms. The world is moving from fossil fuels to sustainable electrical power as the primary power source and space-based power will be a major part of this transition. For a general understanding of this topic, please read these papers. The 2008 paper looks at the world situation. The other two focus on the United States national energy security needs and how space-based power is the only practicable solution.