As the world prioritises the transition towards net zero carbon emission 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 power output.
“On Earth, solar power generation is variable during the day,” says Stephen Way, an engineer and senior consultant at Frazer-Nash Consultancy Ltd. “Solar panel output is affected by the rotation of the Earth and weather conditions, such as clouds which 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 looking beyond the clouds to an uninterrupted source of solar power. In space.
Although this is a concept that has been around for decades, the recent concerted gaze towards the stars reignites this excitement of space as a place to gather energy.
“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 almost constantly,” says Way.
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 near-constant, on-tap power source.
“Photovoltaic panels are an important part of a satellite,” says Way. “The solar panels capture the photons and convert them into electrical power. This can be turned into electromagnetic energy 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 there are safety limits that control the beam’s maximum intensity.”
Of the models so far proposed, many satellite designs aim 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 more developed conceptual designs in circulation that have been proposed for solar satellites. They have a number of features in common:
- A way to resolve the difference between the direction of the Sun and Earth;
- Minimal weight;
- Wireless transmission back to Earth;
- Robot assembly;
“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 across the planet.
Three concepts were highlighted in the report Way provided to the UK Government last year.
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 arranged like a big umbrella that opens out towards the Sun, and the photovoltaic cells are in the flat disc at the other end,” 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.
The round disk is referred to as a sandwich panel, because it has a layer of PV cells on one side and of microwave beaming electronics on the other side. This leads to an efficient use of mass and structure, minimising weight. 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.
SPS-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 more easily maintained,” says Way. “Parts can be more easily assembled and swapped out by robots.”
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 helix, 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, thousands of 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.
Multi-Rotary Solar Power Satellite (MR-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.
Credit: China Academy of Space Technology
The satellite can send 1GW and is 11.8 kilometres wide, 10,000 tonnes, and geostationary, with an estimated life span of 30 years.
While this concept looks simple, the massive rotating joint in the middle would have to handle gigawatts of power – something which is very technically challenging. Additionally, while this uses conventional solar PV without concentration, the fact that the power is sent along cables to a central point means that there are resistance losses along the cables leading to their own thermal challenges from heat generation.
What will be seen in the future?
These designs are still concepts under review, and there is significant engineering work to be done.
“Should this technology continue to prove that it is feasible to address net zero, we will probably have a whole constellation of satellites beaming energy back to Earth,” says Way.
Deborah Devis is a science journalist at Cosmos. She has a Bachelor of Liberal Arts and Science (Honours) in biology and philosophy from the University of Sydney, and a PhD in plant molecular genetics from the University of Adelaide.
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