Space-based solar power is not a thing

This post is part of a series on common misconceptions in space journalism. It’s also part of the sub-series on space resources, and why commercial exploitation of space resources is less inevitable than you might think. It is an expansion and update of a previous post on some of my unconventional space opinions, all of which will eventually be revisited in this series.

In this blog we’re going to avoid reasoning by analogy which, in space, will lead us astray. Space is so different from the familiar here on Earth that the only way to be sure we’re on the right track is to think like physicists and use first-principles reasoning.

My view is that space-based solar power is impossibly expensive and will never be used on Earth. There are no shortage of prominent and qualified people on both sides of this issue – my purpose here is to show why it’s hard and attempt to illustrate some limits on the concept.

To get the obvious out of the way, solar power is an obvious and vital source of electrical power for space-based applications, such as powering satellites and rovers, probes and space stations. It is, and will remain, the de facto source of energy for applications in space well into the foreseeable future.

What of Earth? Space-based solar power is not a new idea. Indeed, it was popularized by Gerard O’Neill in his visionary book “The High Frontier”, which I have reviewed. O’Neill advocated the construction of gigantic space stations in Earth orbit, and saw solar power as the killer product to pay for construction.

In another blog I will explore the detailed reasons why mining the moon or asteroids is a commercially tricky proposition, but fundamentally it’s due to the enormous expense of flying rockets to and from space, combined with their desperately limited cargo capacity.

Solar power, on the other hand, could be beamed to Earth using microwaves, which have no intrinsic mass and so only require rockets to install the systems in the first place. In addition, a gigantic solar array built in space could receive solar power 24 hours a day, unobstructed by clouds, night time, or the atmosphere. In California we are spoiled by solar availability, but much of northern Europe receives pitifully small quantities of solar power, particularly in winter.

The principle of wireless power transfer, first practically demonstrated by Nikola Tesla, is well understood physics. The underlying technology for space-based solar power has existed since the 1970s. Oil has been considered scarce since the same period, so why haven’t gigantic solar space stations been built to provide us with power?

As Elon Musk has concisely pointed out, the fundamental problem with space-based solar power is that it’s obtaining a commodity, power, somewhere where it’s expensive and selling it somewhere where it’s cheap. This is not a good business. Indeed, it might make more sense to beam power from Earth to space stations, if they needed it.

But why is power in space so much more expensive than Earth? Remember, there is 3-50x more solar power in space than Earth, depending on the location. If my opinion is valid, then we should expect ancillary system costs to outweigh the improved solar resource in space.

What are the extra costs? Broadly, they fall into the following categories: Transmission losses, thermal losses, logistics costs, and space technology penalty. Individually, any one of these issues cancels out the benefits, and combined they leave space-based solar power at least three orders of magnitude more expensive than the terrestrial equivalents. Because it’s not even close, I don’t have to be persnickety about decimal places – instead I can rely on generously drawn bounds.

For a baseline comparison, consider a GW-scale power station. For terrestrial solar, this consists of standard panels on single axis mounts, covering about 10 square miles. For the space-based solar case, an identical area of land is covered instead with an antenna, a mesh of conductive wire held above the ground, to absorb the transmitted microwaves and convert them to electricity. An identical area implies similar overall energy fluxes, which is correct. Even if it were physically possible (it is not) to transmit microwaves in some focused narrow beam with power densities of MW/m^2, it would be unacceptably dangerous to do so. In some orbit far above, a space station covering, say, 2 square miles, receives the sun’s light and converts the power to microwaves, transmitting to the ground through an antenna of similar size, necessary to keep the beam focused.

Transmission losses: The process of converting sunlight to electricity is about 20% efficient, depending on the type of panel – and this is a loss common to both systems. In addition, the space-based system has to convert the electrical power back into EM radiation, which is converted back into power on Earth. Proponents think that it should be possible to perform each conversion with 90% efficiency, but even beam-forming that well is not possible without a much larger antenna. My personal opinion is that the end-to-end microwave link efficiency would be lucky to exceed 40% efficiency, which erodes the competitive advantage substantially.

Thermal losses: The conversion efficiency of the high-power microwave transmitter has a nasty side-effect, namely that what isn’t transmitted is wasted as heat, and that heat has to go somewhere. If the transmitter is 80% efficient (which is being very generous), then it will have to radiate 200MW of thermal power. This is a different problem to the thermal losses in the solar panels, which are more like 4GW but spread over a huge area that is in radiative thermal equilibrium with its environment. Instead, the microwave power electronics will need a huge cooling system. If the electronics can operate at 350K, then the radiator power will be 850W/m^2, so the radiator will need a total area of 23ha, comparable to the total size of the solar array and the microwave transmission antenna. In contrast to the usual claims of perfect scaling efficiency with solar arrays in microgravity, a large space-based solar power system will also need a huge antenna and cooling system, which don’t scale quite as nicely.

Logistics costs: Consider transportation cost. Today, SpaceX has crushed the orbital transport market with a price of around $2000/kg. Compare this to the worldwide network of intermodal containers, which can transport anything in 20T units almost anywhere on Earth for about $0.05/kg. Even if all of Elon Musk’s wildest Starship dreams come true, transport costs will dominate the total capex of any space-based solar system, by many orders of magnitude. A factor of 10x improvement in resource does not make up for transport costs which are more than 10,000x higher. If logistics costs are more than 0.1% of current solar farm costs (they’re more like 20%), then increased transport costs completely negate the improved solar resource. It’s not even close.

One further aspect of logistics bears closer examination. In our baseline case, we considered an array of panels strung up on posts, compared to a mesh of wire strung up on posts. It turns out that (as of 2019) a substantial fraction of the overall cost of a solar PV station is the mounting hardware, which is also required by the microwave receiver. So if the mounting hardware costs 20% of the overall deployment cost for terrestrial solar, that places a strong upper bound on total system cost allowable for space-based. In other words, does anyone seriously believe that the microwave receiving antenna could cost 20% of the overall system capex, the other 80% to be used to launch thousands of tonnes of high performance gear into space? Put another way, the most cost-effective way to get a GW of power out of a microwave receiving antenna is obviously to tear down the wire mesh and sling up a bunch of solar panels, which can be ordered with a lead time of weeks from any of dozens of suppliers worldwide with widely available financing.

Finally, the space technology penalty. On Earth, we are living in an extremely exciting time for energy. Hundreds of major companies are competing on development cycles measuring only months to provide solar panels in an industry that’s growing at 20% a year. As a result, costs have fallen by 10% a year, and in the last few years, solar and batteries have neared, equaled, then utterly crushed all other forms of electricity generation. Initially, this process occurred on remote islands with high fuel import costs. Then the sunnier parts of the US. The rampage continues northwards at about 200 miles a year. The industry can sustain 30% deployment growth rate worldwide for another decade at least, before saturation occurs.

Today, I can pick up the phone and any of dozens of contractors in the LA market can fill hundreds of acres with panels, each built to survive 30 years under the harsh sun and sized perfectly for deployment using the latest tech, which is men in orange vests with forklifts.

In contrast, space technology has not benefited from such breakneck levels of growth, demand, and investment. Prohibitive maintenance costs demand perfect performance, and low rates of deployment ensure a slow innovation feedback loop. The result is that none of the current incredibly cheap solar panels could work in space, where thermal and vacuum, not to mention stresses of launch, would destroy their operation in days.

Instead, space operators rely on more traditional supply chains, with the result that building anything for space takes years and costs billions. Right now, a billion dollars invested will buy about 100MW of solar panels on the Earth, or 100kW of solar panels in space. This is a factor of 1000, and it also erases the advantages of more sunlight in space.

These four elements, transmission, thermal, logistics, and space technology, inflate the relative cost of space-based solar power to the point where it simply cannot compete with terrestrial solar. It’s not a matter of 5% here or there. It’s literally thousands of times more expensive. It’s not a thing.

I can relax assumptions all day.  I can grant 100% transmission efficiency, $10/kg orbital launch costs, complete development and procurement cost parity, and a crippling land shortage on Earth. Even then, space-based solar power still won’t be able to compete, because the antenna receiver alone is basically a solar plant in disguise.

I can grant a post-scarcity fully automated luxury communist space economy with self-replicating robots processing asteroids into solar panels, and even then people will still prefer to have solar panels on their roof, to avoid supply interruptions and utility bills. Or maybe they’ll all be post-humans living in some data center orbiting Jupiter. Let’s reel it back in a bit.

There is one additional reason why space-based solar power won’t be built, and that is investment risk. It is the same reason why nuclear power won’t solve climate change. Power plants traditionally front load much of their costs, and space-based solar is no exception. So what, advocates say, all infrastructure costs a huge amount up front, and that investment is paid off gradually over decades of use by everyone. Let’s say that despite all the above issues, a business plan emerges which can justify borrowing the necessary capital to build and deploy a bunch of space-based power stations, backed by a “purchase power agreement”, where a utility agrees to buy all the power at a currently acceptable price for some number of decades. Like nuclear power, which requires 50 or more years of operations to pay off construction and decommissioning costs, signing such an agreement in 2019 is an enormously risky thing for a utility to do, because of future price uncertainty.

Indeed, just last month a major solar farm was announced in Nevada with a power price of $35/MWh, including storage. This price would have seemed impossibly low even last year, and yet I am certain that we will not have to wait long to see solar projects built for less than $10/MWh. For the first time in nearly 50 years, energy is rapidly getting cheaper and there’s no limit in sight. Against a backdrop of supply costs dropping by 10%/year, it will not be possible to find financial backers for projects that have a ROI time measured in decades. It is simply not possible to predict whether they will be able to make any money.

This is not a bad thing! Sure, I would love to see a vibrant cis-Lunar economy, enormous space stations, and thousands of people living in space. It’s a beautiful vision. But if it occurs, it will not be funded by electricity, because climate-friendly energy has gotten very cheap here on Earth. If I had to choose between terrestrial solar power crushing coal and gas, or giant space cities, I would choose the former. So has the market.

As a sort of postscript, I will point out that it is possible to make money beaming microwaves at Earth from space, and nearly the entire civilian space industry is based on it. The problem with beaming power using microwaves is that the monetizable value per Watt is incredibly low, because essentially unmetered electricity comes out of the walls of every building. The trick is to increase the value per Watt, by increasing the value and decreasing the power. The value is increased by modulating the microwaves with high speed data, and the power can be reduced by a factor of a million or so without hurting this method. Indeed, customers pay only for the data, and not for the transmitted electrical power, which is pathetically low at the receiver. Communications satellites remain the killer app for the commercial space industry.