The Earth, a spaceship like any other - PART 1

From the Millennium Falcon and the Death Star in Star Wars to Endurance in Interstellar, Nostromo in Alien and the USS Enterprise in Star Trek, navigating the vast emptiness of space has been a theme of science-fiction for decades. The technical complexities inherent in space travel - i.e., those caused by life in a closed and controlled environment - are rarely addressed, but we can get a pretty good idea of what’s what simply by taking a closer look at good old planet Earth!

Indeed, both are closed systems populated by humans who survive thanks to an external source of energy (in our case solar radiation). What can we learn from this parallel?

In 1966 in his premonitory essay “The Economics of the Coming Spaceship Earth”, Kenneth Boulding wrote: “The economy of the future might similarly be called the ‘spaceman’ economy, in which the earth has become a single spaceship, without unlimited reservoirs of anything, either for extraction or for pollution, and in which, therefore, man must find his place in a cyclical ecological system”.

The "earth system"

Although this message was largely ignored at the time, when not simply disregarded as doom-mongering, today the resemblance between the system on Earth and a spacecraft is undeniable. This similarity explains a common drive for technologies able to create a holistic system that - based on circularity and efficiency – is capable of sustaining human life.

Of course, in space, strict technical constraints apply as there is very little room for manoeuvre and the safety margin is small. As a result, R&D focuses on technical rather than economic aspects. Back on Earth, the technology designed to be used in space can be adapted and further optimised to the benefit of both sectors. As we will see later on, the reasoning behind the adoption of these technologies may sometimes be completely different, but in any case, circularity and efficiency are always the priority. To set off on a voyage and discover the bridges that exist between the Earth and the stars, let’s put on our spacesuits and embark for the International Space Station (ISS). However, there are some things we will need onboard.

First of all, we are going to need energy! Spacecraft systems all need electrical power which must be generated onboard. Different primary energy sources can be used and the ones that are selected will depend on the type of mission. For example, Earth-orbiting spacecraft typically use solar photovoltaic systems, however such systems are inadequate for missions to the outer planets such as Mars, because solar irradiance decreases in intensity with the square of the distance from the Sun. In this case, spacecraft powered by radioisotope power systems (RPS) are preferable; they do not depend on the Sun and are highly reliable, which is important for autonomous operations such as the rovers exploring the surface of Mars.

To colonise the Moon, nuclear fission is considered the best option. On site, the various installations and equipment (from “accommodation” to the larger rovers) could be equipped with fuel cells, which convert the chemical energy contained in hydrogen (H2) and oxygen (O2) into electricity. Fuel cells have already proven their worth on the Apollo missions and the Space Shuttle programme and moreover they produce (drinkable) water as a by-product of their operation, which is obviously vital to keeping the astronauts alive. The similarities between energy systems on Earth and in space lie in some of the main challenges they are facing, notably sustainability, energy efficiency and intermittent resources.

Back on the earth 

Let’s take the example of the satellites, spacecraft and space stations orbiting the Earth. Their energy is supplied by solar arrays (backed up by lithiumion batteries to cover the periods when they enter the Earth’s shadow). This can be compared to the current situation of power generation on Earth, where the increasing amount of intermittent renewable energy sources requires a back-up solution for the periods during which the wind does not blow or the sun does not shine. This flexible on-demand power generation can be delivered by thermal power plants, but batteries are also rapidly gaining importance.

Storage solutions also have a role to play. Energy is stored in periods of excess renewable electricity generation and is subsequently used to make up for periods during which renewable electricity generation does not cover the demand. Smart energy management systems can predict demand, manage production and optimise the grid.

Today, most Earth-orbiting spacecraft rely on advanced solar cells with an efficiency around 30 %, which is higher than the efficiency of those used in the best performing commercially available solar panels on Earth (22-24 %). Could we imagine a transfer of technology between space and our planet? It’s difficult to say and only the future will tell as space-grade cells are currently too expensive for deployment on Earth. As we have already mentioned, in space applications cost is traded-off against efficiency, mass (which affects the cost of launching) and resistance to the higher levels of radiation present in the emptiness of space.

Li-ion batteries are widely used in the energy sector for applications such as back-up power, time-shifting and grid services, however space-grade batteries face some additional challenges compared to those used on Earth. They are generally designed to meet specific requirements and must be capable of operating in harsh conditions and in a vacuum. They have to be able to resist vibrations, shocks and acceleration during launching, extreme temperature variations and radiation. As the cost of launch is one of the most important contributing factors to the overall cost of an Earth orbiting spacecraft, it is important that batteries can provide maximum electrical energy for minimum weight and volume – in other words be exceptionally efficient. Another performance related aspect is that batteries in low Earth orbiting spacecraft are exposed to a high number of cycles (more than 30,000) compa- red, for example, to the li-ion batteries used in electrical vehicles whose lifespan is just 1,500 cycles. Could cars one day benefit from the technology used in space batteries?

At the end of the day, the only input into our closed systems - whether on the vessels we send into space or on Earth itself - is the energy that is continuously supplied by the Sun. On our planet, this energy travels along chains and loops, for example in food chains. Could Nature’s way of working be applied onboard a spacecraft? Could electricity be a part of a self- sufficient system that would sustain the astronauts? Maintaining air quality and producing food are two telling examples that would seem to confirm this hypothesis.

To be continued... read the second part of the article

This article was written by Jim Gripekoven - ENGIE Laborelec, Jan Mertens - ENGIE Research, Rob Verstreit - ENGIE Research and Michael E. Webber