Distributed Satellite Network for Space-Based Solar Power (Research Notes)

This MIT paper "Space-based Solar Power Generation Using a Distributed Network of Satellites and Methods for Efficient Space Power Transmission" proposes a constellation of smaller solar power satellites rather than a single monolithic satellite. The following is my notes based on this research and is not my own work.

The authors describe their solution as "plug and play" due to each smallsat's ability to reach orbit via rideshare, redundant architecture allowing for uninterrupted performance, and of course size advantages in lighter, smaller array and electronics requirements. Mitigation for these problems are described later.

Furthermore, distribution takes advantage of economies of scale. The authors identify high launch costs + high PV cell costs as barriers to entry for space-based solar spacecraft, which they believe can be alleviated by multiple solar-collecting and -transmitting smallsats providing economies of scale.

The distributed approach has clear advantages, but naturally introduces new problems for space-based solar as well:

• precise coordination of the constellation, attitude control for multiple spacecraft

• reduced transmission efficiency from multiple beaming stages (required for full coverage of all smallsats)

• does not get rid of in-orbit deployment problems for individual smallsats

Spoke Architecture and Modular Design

The satellite constellation consists of 120 solar-collecting smallsats which transmit their power to 10 sub-beamer satellites that then transmit to 1 central beamer responsible for transmitting the collected power to receiving stations on Earth. Between satellites, power is beamed at a high frequency of 5.8 GHz, and the central beamer transmits power down to Earth at a lower frequency of 2.45 GHz.



12 collectors are arranged in spokes at a radius of 105 meters around their respective sub-beamer, and 10 sub-beamers are arranged in spokes at a radius of 350 meters around the central beamer. The authors propose 2 of these "clusters" of satellites.

The use of high and low frequencies for inter-satellite beaming and space-Earth beaming, respectively, is intentional. This, coupled with the spoke geometry of the collector satellites around beamers, prevent interference between the two stages of power transmission.

Collector satellites are made of Copper Indium Gallium Selenide (CIGS), a type of thin-film PV cell providing a 19.5% efficiency. Thin-film PV is preferable to more efficient triple junction cells like those manufactured by Emcore or Spectrolab due to being cheaper and lighter weight.

In order to deal with the excess heat created during solar power collection, the authors propose coating the backside of the solar arrays with a reflective Z93 paint to radiate excess heat to deep space.

Mass and cost breakdowns for individual satellites show a collector's power accounts for most of its mass and almost all of its cost, supporting the idea that leveraging a distributed network of satellites does not significantly add to the total mass and cost of the system.

Beamer satellites serve a dual purpose of power transmission and communications. The downlink rectenna (receiving antenna) aboard beamer satellites are smaller than ground-based rectennas due to transmitting higher-frequency power, but still larger in size than the collector satellites. The satellite constellation will occupy a geostationary orbit above a ground station on Earth that consists of a rectenna array estimated to cover 2 square kilometers.

It is noted that communications equipment on beamer satellites must be more reliable than on collector satellites due to their crucial role in the entire constellation's performance.

Rideshare Scheme

ESPA (Evolved Secondary Payload Adapter) is a standard set by the Department of Defense that allows the extra space not being used by payloads in rocket launch vehicles to be utilized by other projects, i.e. secondary/auxiliary payloads.



An ESPA ring. It can house up to 6 ESPA-class payloads in this cylindrical space (source)

These ESPA rings fit onto Atlas or Delta rockets. The smallsat collectors are envisioned to be ESPA-class, effectively "ridesharing" with other payloads.

The beaming satellites are expected to be larger and bulkier due to electronics, so Falcon 9 is the suggested launch vehicle.

ESPA dimensions are 35.5" x 28" x 24". Thin-film solar arrays are flexible, and can be rolled to fit payload constraints and unrolled during deployment in orbit. These dimensions call for small arrays on individual collectors, justifying the need for a network of these satellites.

Projected Costs

Such a project would cost $900M and is expected to be paid off in 80 years, a period which the authors acknowledge as unacceptable and expect to be reduced with new technologies and economies of scale.