The Costs of Solar Power from Space

Delivering power from space using photovoltaic panels is incredibly costly and not a reasonable option for the collection of energy today, nor likely in future.  On earth, the cost of installing solar modules (BoS) nearly equals the cost of the module itself, and will become a bigger share of the total costs in the future. Presumingly, the costs of installing modules on the ground or on rooftops will always be significantly lower than in space.  An estimate for the price of sending cost-effective modules to space is at least $285/W. The deployment costs for weight-optimized modules can be much smaller, $17/W, but these modules are much more expensive. In either case the deployment costs alone are a magnitude higher than the current total cost of solar on earth (~$3/W). 

Sunlight is a very diffuse source of energy. To capture a useful amount of it, a large area is needed. To collect sunlight in a cost effective manner, it must be deployed incredibly cheaply, no matter the technology.
The cost of solar modules have been decreasing at a faster pace than the labor and other equipment and administrative costs of installing solar (called the balance of system costs, BoS). The BoS of rooftop PV installations already exceed the cost of the modules. Even for large utility-scale projects where economies of scale can be employed and workers don't have to trek up and down a roof, the BoS costs are nearly half the total cost and will quickly exceed the module costs.¹ 

Putting solar modules in space and then beaming the energy to earth would thus seem like a costly option, yet it's an idea that has existed for a while, most recently brought to the attention of the readers of the Economist in a recent article.

There is some rational behind the idea. The sun is brighter outside our atmosphere (see Figure 1 and the top of the post) because gases in our atmosphere adsorb some of the spectrum, especially in the higher energy part.² The brighter the sun, the more electricity or energy you can produce per unit area of the module.

Fig 1. The solar radiation spectrum for sun light outside the atmosphere (yellow) and at sea level (red)

Over the entire spectrum, the energy from the sun outside the atmosphere (called AM0 for 'air mass zero') is about 35% more compared to a sunny place near sea level (1,000 W/m² versus 1,353 W/m²). This doesn't tell the entire story, since today's modules cannot capture the entire solar spectrum, only those photons that have an energy above the bandgap of the material.  Two common materials for photovoltaics are Si and CdTe which have a band gap of 1.44 and 1.12 eV, respectively. This means that the solar energy (photon) must have a wavelength shorter than 862 nm or 1108 nm to be converted into electricity. If you look to the left of those two wavelenghts in Figure 1, the atmosphere is indeed blocking a significant portion of that light.  Photovoltaics in space could also lead to longer collection periods (aka, a capacity factor closer to 100%) since the satellite can be pointed towards the sun nearly 100% of the time. Putting modules in space also solves the "not in my backyard" problem, although the amount of room to stick more satellites in orbit around the earth is certainly not infinite.³ Another often quoted advantage of space is that it is much cooler (the efficiency of a module improves with lower temperatures). However, in a vacuum, the primary heat loss mechanism of convention doesn't exist and thus only conduction (to the rest of the satellite) and radiative heat losses remain, possibly wiping out this advantage depending on the design of the system.

What are the costs of solar power from space? For a simple approximation, two important inputs are needed, 1. the performance of the PV modules in space and 2. the costs of putting them in a rocket and shipping them up there. In terms of the performance, with today's modules we have nice spec sheets which give exactly the performance under standard test conditions (STC) on earth. As we talked about above, over the entire spectrum you gain about 35% more by being outside earth's atmosphere, but the actual improvement will depend on the module, so for simplicity, we'll use what's likely the upper bound, a 50% factor of improvement. Ignoring the cost of the modules themselves, a major cost will be sending the modules into orbit. An exact cost depends on a variety of factors including the type of rocket used. The price for sending payloads into space has been nearly constant the last couple decades, and several reports (although slightly outdated), put the lowest end of costs at $8000/kg, which we will use in this estimate.⁴

In table 1 we list some important parameters and the estimated cost to send a variety of modules into space (ignoring the cost of the module itself or any other costs associated with building the satellite and the infrastructure to receive the energy on earth). There are a variety of different modules we could have included in our comparison. Given the high costs of sending payloads into space, the best choice will be a module that gives the largest power with the smallest weight (the last module in the table).  This module, however, is many times more expensive than the cheapest modules (the first four listed in the table) that are currently used in a typical solar application. I include both types of modules for comparison, but if the goal is to approach a total system cost that comes even remotely close to the costs on earth, a relatively cheap solar module should be used.

Table 1. The power per weight (under STC conditions, except for the last module
labeled with a asterisk which was tested under AM0) and module area per weight for
a variety of modules. The last column is the cost to transport the module in space,
including a 50% improvement in performance from STC conditions and assuming
a cost of $8000/kg.

The first module listed from First Solar is made of thin film CdTe and is the industry leader in cost. It's lower efficiency and higher weight (CdTe is deposited on a glass substrate), however, make it a poor choice for space. The next two modules (SunPower and Suntech) are made of crystalline-Si and are slightly better choices because of their higher efficiency. Uni-Solar's module made of amorphous-Si has the lowest efficiency on the list, but it's thin-film is deposited on a flexible substrate, significantly reducing the weight. The last module from Emcore (made of InGaAs), is optimized for weight and is in fact currently used to power satellites.

What if we lived in a perfect world (that also broke the laws of thermodynamics) and we could get a module that had a 100% efficiency for free.⁵ In such a case we know the energy production would be about 1,353 W/m². To calculate the cost of sending this module to space, let's assume an area to weight ratio of about 1 m²/kg, similar to the Emcore module that is currently used in space. The cost for such a module would be $6/W, still over twice the cost of a utility scale project on earth today, without even including the cost of the equipment needed to send the energy back to earth or the loss of energy that would invariably occur during the process.  

The Economist article mentioned that while the costs are high (as we've seen, they're incredibly high), there are potential useful applications where the energy collected could be beamed down to anywhere on earth: "Military expeditions, rescuers in disaster zones, remote desalination plants and scientific-research bases". But I just don't see any real application.  First, they aren't getting the energy for free at the source, they still must bring some complicated collection equipment. Even if the equipment is light, the absolute costs are high. For the cost of solar from space, you could easily truck or fly in a much higher energy density fuel like oil.  Even better, you can deploy solar panels on the ground, a much more cost effective option and has already done in some of the above mentioned applications.^6,7

I'll be the first to admit that there are likely many vaults with the estimates in this post, but the solar intensity above the atmosphere is only about 35% greater than on earth. It is hard to believe that this will ever balance the large cost of putting the modules in space and the energy losses of transmitting the energy back to earth.  It's much smarter to stick to earth, preferably solid ground and rooftops. I'll have to reserve another post to describe the problems with this:

taken from NYTimes "Solar on the Water"

Sources and Notes:
1. http://www.greentechmedia.com/articles/read/solar-pv-balance-of-system-costs-to-surpass-modules-by-2012-according-to-gt/
2. http://en.wikipedia.org/wiki/File:Solar_Spectrum.png. Also, if you wanted a reason to keep ozone around (O₃), look at the right side of Figure 1. This is most energetic portion of the solar spectrum (UV) and as a result the most harmful. You can see how ozone is responsible for adsorbing much of the light.
3. http://www.economist.com/node/16843825
4. http://www.futron.com/upload/wysiwyg/Resources/Whitepapers/Space_Transportation_Costs_Trends_0902.pdf
5. The thermodynamic limit, assuming you can use the entire solar spectrum, is around 80%. Today's best laboratory multi-junction and concentrating cells can get around 44%.
6. http://www.army.mil/article/58781/Experimental_solar_shade_in_Djibouti_provides_constant_power/
7. http://www.army.mil/article/25391/