From COSMOS (AU) : “Satellite solar-an explainer” 

Cosmos Magazine bloc


26 October 2021
Deborah Devis

In space, no one can complain that the sun is an unreliable source of renewable energy.

Credit: Mmdi / Getty Images.

As the world prioritises the transition towards clean, renewable energy to (hopefully) avert a climate catastrophe, solar is proving itself the prime candidate to replace our reliance on fossil fuels.

But defenders of fossil fuel energy sources often point to solar’s limitations, and the fact that cloud cover, let alone nightfall, reduces its availability.

“On Earth, solar power generation is erratic during the day,” says Stephen Way, an engineer and senior consultant at Frazer-Nash Consultancy Ltd. “Solar panels are affected by light and dark, and clouds can block the amount of solar power that reaches them.”

Which raises the question: where might solar panels be best positioned to completely avert these pitfalls? Scientists are now looking beyond the clouds to an uninterrupted source of solar power.

“In space, satellites are not affected by day/night cycles, atmosphere or weather in the same way, so they are able to collect solar power constantly,” says Way.

Solar collection

The theory is relatively straightforward.

Satellites powered by solar already routinely move around in their orbits of Earth. Plans are being devised to expand this harvesting potential, then direct the energy back to Earth as a constant, on-tap power source.

“Photovoltaic panels are obviously the most important part of a satellite,” says Way. “The solar panels capture the photons and convert them into electrons. This is the form that can be beamed back to Earth.”

This energy would be wirelessly dispatched via a large antenna down to a receiver – called a rectenna – on Earth, where the electromagnetic energy is converted into current and distributed.

“These beams can be microwave beams,” say Way. “People can get concerned about having a big beam like that, but they won’t hurt you. There are safety limits that control the beam’s maximum intensity.”

Of the models so far proposed, each satellite design aims to generate around 3.4GW of electricity, transmit the microwave power at 2.45GHz with a maximum beam intensity of around 230W/m2 (one quarter of the intensity of midday sunlight) to produce around 2GW of electrical power to the grid.

The antenna needs to be directed towards Earth all times, while the rectenna will need to be kilometres wide to capture the microwave beam.


There are several serious conceptual designs in circulation that have been proposed for solar satellites. They have a number of features in common:

Highly concentrated solar photovoltaics;
Minimal weight;
Wireless transmission back to Earth;
Robot assembly;
A plan to dispense with them when they die.

“These satellite solar stations would be massive – they would each weigh several thousand tonnes – so it would take a huge amount of resources to launch them into space,” says Way.

But their potential promises an abundance of clean energy for an entire planet.

Three concepts stand out.

SPS-ALPHA concept. Credit: The National Aeronautics and Space Agency (US).

1. Solar Power Satellite Via Arbitrarily Large Phases Array (SPS-ALPHA)

This satellite concept involves large multiple solar panels in a familiar shape.

“The mirrors are like a big umbrella that opens out towards the sun, and the photovoltaic cells are in the flat handle,” says Way.

Each mirror is a heliostat that is motorised to independently adjust position to best catch the Sun. All this highly concentrated light is reflected to the cells on a round disk positioned between the mirrors and Earth.

This hyper concentration acts like a magnifying glass, creating super intense heat. To mitigate that heat, a sandwich panel – made of an insulating material between two thin metal sheets – offsets the heat so the satellite doesn’t incinerate. If too much light and heat are accumulated, the mirrors can also be adjusted to reduce the load.

The entire satellite is estimated to weigh 8,000 tonnes and has a huge, 1.7km diameter antenna that beams energy back to Earth. It has an estimated life span of 100 years.

TSPS-ALPHA is geocentric – meaning it always stays above the same position on Earth so that the energy is delivered to the same location. The rectenna has a 6km diameter – clearly it would need to be positioned in an area with plenty of room.

“The good thing about this satellite is that it is modular and can be easily maintained,” says Way. “Parts can be robotically assembled and swapped out.”

CASSIOPeiA concept. Credit: I. Cash, “CASSIOPeiA solar power satellite,” 2017 IEEE International Conference on Wireless for Space and Extreme Environments (WiSEE), 2017.

2. Constant Aperture Solid-State Integrated Oribital Phased Array (CASSIOPeiA)

The CASSIOPeiA looks completely different to SPS-ALPHA. High concentration solar photovoltaic (HCPV) panels make up the bulk of its helices, which are topped by mirrors that reflect light back towards the panels.

“It is almost like a baked-bean tin, but both ends are open, and the lids are two huge mirrors reflecting light into the middle,” says Way.

Instead of one big antenna, several microwave antennae sit at right angles to the HCPV panels that send energy back as an array instead of a single beam. This means that it can transmit at 360°. Its mirrors remain facing the sun as the satellite moves around the Earth, but the intricate angles of the helix means there are sufficient antennae to constantly deliver power no matter its position.

Unlike SPS-ALPHA, CASSIOPeiA has no moving parts, but it is also modular in design and single units can be removed as they degrade.

The estimated mass is 2,000 tonnes, with a 1.6km diameter antenna beaming to a 5km wide rectenna.

3. Multi-Rotary Solar Power Satellite (MR-SPS)

Multi-Rotary Joints SPS

This design looks like most rooftop solar panels. It is comprised of two rectangular wings with a flat antenna in the middle. Each wing has sections with solar panels attached to rotating joints that can move independently to best catch the sun.

The energy collected passes through the rotating joints to a flat, 1km diameter antenna that beams the energy down to a 5km wide rectenna back on Earth.

Multi-Rotary Joints SPS – 2015 SunSat Design Competition.
Credit: China Academy of Space Technology[中国空间技术研究院](CN)

The satellite can send 1GW and is 11.8 kilometres wide, 10,000 tonnes, and geostationary, with an estimated life span of 30 years.

The benefits of this satellite are that it is relatively simple engineering compared to the other two. It also doesn’t heat up with concentrated light and doesn’t require the same thermal regulation. However, the rotary system requires a lot of energy to function, so it is not as productive in what it dispatches back to Earth.

What will be see in the future?

These designs are still concepts under review, and there is unlikely to be just one winner.

“We will probably have a whole constellation of satellites beaming energy back to Earth,” says Way.

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