Why bother with space solar energy, when terrestrial solar farms and other sources of clean energy are well developed and growing fast in the energy transition?

‘Ultimately it comes down to reliability and uninterrupted power,’ says Dr Nicol Caplin, Exploration Scientist, ESTEC at the European Space Agency (ESA). ‘Though you can have massive solar arrays on Earth, you can never get around power disruptions half the day (at night-time) and weather. Whereas space-based solar power offers limitless, reliable and continuous power.’

Indeed, there is promise of 99% availability using solar panels in space to catch the sun. ‘You get 13 times more incident energy and that drives very compelling unit economics through the system,’ explains Sam Adlen, Co-CEO of Harwell-based Space Solar. According to their calculations the levelised cost of electricity will be $34/MWh (£27/MWh) ‘which means that the cost is similar to intermittent renewables, and a third of the cost of large-scale nuclear’, he says.

The other benefit is baseload, because the baseload power is continuous and complementary to terrestrial solar and wind power. ‘This allows grid balancing in interesting ways and is dispatchable and tuneable because the satellite arrays will be in higher geosynchronous orbit, where you can see a third of the Earth’s surface. The amount of power that is transmitted can be switched between different points on the Earth’s surface. So, you can completely reimagine the way the grid works on an international scale, which in terms of cost democratises energy access,’ remarks Adlen.

The economic case for SBSP and optimisation of the grid is also emphasised by Professor Goran Strbac, Professor of Power System Economics at Imperial College, London. His study is one of eight projects at leading UK universities which were awarded funding under the £4.3mn UK government’s Space-based Solar Power Innovation competition. Cambridge University is developing ultra-lightweight solar panels, and Queen Mary University, London, is working on a wireless system to enable the solar power collected in space to be transferred to Earth.

An independent study by Frazer-Nash Consultancy in 2021 suggested that SBSP could generate up to 10 GW of electricity a year by 2050, and potentially create a multi-billion pound industry.

The key driver  
The prime driver for SBSP is the energy transition and the need to move from dependency on fossil fuels. ESA’s Solaris project has the ambition to provide Earth with an almost limitless renewable energy source. A co-funded project between ESA and Columbiad Launch Services (Canada), with the University of Strathclyde and University of Glasgow, is looking at the green credentials of the Solaris programme. Research is underway to determine the potential impact of SBSP on the environment, human health, ionosphere and atmosphere, launch deployment, infrastructure and technology issues, prior to a Council Ministerial meeting in 2025 which will give the project thumbs up or not.

In early programme notes, ESA admits that Solaris is ‘a preparatory R&D initiative, not a full development programme’. No decision has been made yet about going ahead with a full development programme for SBSP. Current funding is a few tens of millions of Euros, aimed at ground demonstrations rather than in space. ESA estimates that a full SBSP development programme would require investment of €10–20bn spread over 10–15 years.

The notional roadmap for a European SBSP at GW-scale has a timeline for the first in-orbit demonstrator by 2030, a pilot plant scaled up by the mid-2030s for commercial-scale operation by 2040 or so, with 25–50 solar power plants providing 10% of Europe’s electricity needs by 2050.

According to Paul Febvre, Chief Technology Officer at the Satellite Applications Catapult: ‘SBSP makes it possible to deliver power to any city or urban conurbation within view of the space-based solar power station in response to a local surge in energy requirements at any time of day or night, irrespective of weather conditions – potentially circumventing the requirement for a resilient national grid and supplementing terrestrial solar power infrastructure with an additional source of energy.’

Established technology   
Although there are still technical, environmental risk, health and regulatory issues to address, Caplin maintains that an SBSP system will use well-established space technologies such as solar panels and devices which convert electricity into radio waves. ‘Space-based solar power is fundamentally an engineering challenge. Unlike other future energy solutions on the table [like nuclear fusion], the physics are well understood,’ Adlen adds.

For GW-scale power generation Solaris will require massive scale-up of these technologies, to create structures of kilometre-scale and weighing thousands of tonnes in geostationary Earth orbit. To receive and convert the solar energy, a massive rectifying antenna, also known as a rectenna, will be required on the ground for transfer of power to the grid. Febvre reckons some urban locations will be comfortable having a net-like rectifying array overhead if this means constant renewable energy availability.

Basically, two very large mirrors in space will point at the sun and focus the sun’s light on to high concentration photovoltaic cells which convert the solar power into high frequency microwaves that are beamed down to a rectenna on Earth.

The frequency of the microwave beam is chosen to minimise attenuation from the atmosphere and keep the maximum beam intensity to safe limits. A pilot beam is transmitted from the ground to the satellite to lock the microwave beam on to the correct target. The ground-based rectenna converts the electromagnetic energy into DC electricity which can be converted and provide power to the terrestrial electricity grid.

‘Space-based solar power is fundamentally an engineering challenge. Unlike other future energy solutions on the table [like nuclear fusion], the physics are well understood.’ – Sam Adlen, Co-CEO Space Solar, Harwell

According to one design, the CASSIOPeIA solar power satellite concept (see headline image and Fig 1), the solar power satellite will be assembled in space as a solid-state structure weighing about 2,000 tonnes, measuring 1,700 metres in diameter (almost six times the height of the Eiffel Tower), for geosynchronous orbit at 35,786 km, with disc-shaped solar reflectors, whose orientation can be controlled with respect to the sun to constantly reflect sunlight on to the solar panel array below.

Hundreds of thousands of lightweight ‘dinner-plate sized’ solar panels will be anchored into a helical structure, incorporating high-concentration photovoltaics to capture the sunlight from the reflectors and convert the solar radiation to electricity. Transmitting high frequency radio waves as a beam from the satellite to a receiver (rectenna) on the ground at a frequency of 2.45 GHz, locked on to a pilot beam from the ground station. The ground-based rectenna will be at least 5 km diameter, receiving 245 W/m² high frequency radio wave power, generating 2 GW into the grid at commercial scale.

Space Solar hopes to launch the first solar power station in six years to produce just 6 MW of intermittent power from a low Earth orbit, followed several years later by a 180 MW solar plant in higher Earth orbit, and subsequently a 2 GW solar plant in geostationary orbit, as baseload. Adlen estimates it will cost about $800mn to get the pilot station into space and is aiming to raise $70mn this year towards the project.

Fig 1: Space-based solar power satellite overview, being considered for development by the European Space Agency for Council Ministerial meeting in 2025
Source: Earth image: iStock.com; Frazer Nash consultancy 

Big lift required  
Key to success will be heavy reusable satellite launch vehicles to radically reduce launch costs.

Adlen estimates that 68 SpaceX Starship launches will be required for a 2 GW SBSP system. ‘The space launches have been a limiting factor until recently, but four recent studies have concluded that space solar plants are both technically and economically viable. All of the component technologies have been proven. Wireless power transmission is well-developed with the sort of efficiencies needed, but there is a need to scale both in space and on the ground, with high-concentration photovoltaics.’

The other big challenge is for regulations to evolve around this spectrum of wireless power transmission beaming down to Earth from large space infrastructure. ‘However, there’s quite a long lead time and key regulators from the UK government with international partners from ESA, the US and Saudi Arabia are now starting to make progress,’ he says.

Health and safety  
Obviously, safety is key. Adlen and Febvre both insist that the system is designed to be safe by design. ‘Anybody getting in the way of the beam will simply feel warmer!’ remarks Febvre. Adlen maintains that the maximum power intensity of the beam is a quarter of the intensity of the midday sun. ‘But clearly there’s still work to make sure that the benefits this will deliver are communicated to the general public,’ he says, aware of previous outcries during the deployment of 5G mobile phone networks.

Which brings us to the ground-based rectenna. This structure will likely encompass a 6 km diameter circle at the equator or a 7 km x 13 km ellipse offshore the UK, Saudi Arabia or elsewhere. ‘It will be a very sparse, net-like structure of rectifiers – covering about 40% of the land area for equivalent solar energy output and 10% of offshore wind. As it would be transparent, it could also be operated with dual-use for growing arable crops beneath the rectenna to help reach net zero,’ explains Febvre.

Global initiatives  
Other solar space energy initiatives under way across the globe include Caltech’s Solar Power Demonstrator (SSPD-1) launched into space last June (2023) to demonstrate wirelessly transmitting power in space and beaming detectable power to Earth for the first time, in the MAPLE (Microwave Array for Power-transfer Low Orbit) experiment. China is also developing a butterfly-inspired SBSP with a bowl-shaped body about 1 km wide, as a GW-scale solar power plant.

Collaboration will be key, and in the UK community about 90 organisations have teamed up in the Space Energy Initiative bringing together scientists, researchers, industry and government in a move towards space solar power.

Space-based solar energy no longer looks such a distant dream.