Sustainable energy without borders?

Solar and wind farms, tidal and geothermal power plants. All of these derive energy from sources that had been untouched since the dawn of time. This raises an interesting question. How much of these energy streams can people use for their own purposes without upsetting the Earth’s energy system? Is there endless sustainable energy? A team from Empa – headed by Harald Desing from the Technology and Society Department – is aiming to answer this question.

Sustainable energy sources are often referred to as being “endless”. But is that really true? Is there really enough renewable energy available worldwide for a sustainable arrangement of material flows in our society without pushing the planet’s limits?

Fuel for the Earth

Essentially, planet Earth is a system that exchanges exclusively energy with its surroundings. The vast majority of the energy brought into the system is solar radiation. Accompanied by negligible amounts of energy from planetary movement and geothermal heat. In the past, these energy flows were always completely used up by the Earth itself. Its many subsystems such as the oceans, atmosphere, forests and reflective frozen surfaces, have effectively been “kept running” by them. Most of these subsystems convert the occurring energy into other renewable energy streams. Such as wind flows and water currents, or into the production of biomass. Exergy, the free energy available from the prevailing energy streams, is derived in the process. Irrespective of the way energy is used, whether in the Earth’s natural system or the man-made technosphere, all energy is ultimately radiated back into space.

Solar farms also have a climate impact

If humans increasingly siphon off renewable energy from these streams for their activities, the amount of energy available to the Earth’s own system will reduce. The planet can offset such imbalances up to a certain point. However, if they are too large, the risk of passing tipping points will increase. The result would be fast and irreversible changes to the Earth’s system: these include the polar ice caps melting, which would in turn accelerate climate change. To avoid going beyond these tipping points, the size of the land area used cannot be more than what the planet can handle. In addition, how this area is used also makes a difference. For instance, solar farms in place of forests disrupt bio-diversity, the evaporation of water and – in the process – the hydrological cycle, the reflection of heat into space, and much more.

The same upper limits apply for harvesting chemical energy – such as for agriculture and forestry, food and fodder, heating material, fuel, and construction materials – as for solar energy. In many areas, generating technical energy is in conflict with food production.

In order to compare and add up the various potentials in renewable energy, the Empa researchers converted them into electrical energy equivalents, expressed in terms of the efficiency levels of currently available power plant technology. Here, it makes a difference if electricity is generated from solar energy, from wood, or from hydropower. These conversion losses considerably diminish the potential yield of certain potentials once more.

The earth needs most energy for itself

The study’s conclusion is surprising: 99.96 per cent of the energy arriving on Earth from space is used to power the planet’s energy system or required for food production, leaving just 0.04 per cent available for technical use. That does not sound like a lot. However, in actual fact, this potential is still around ten times the world’s total energy requirements today.

The result of looking at conversion losses is less surprising. Available energy should preferably be harvested and used with solar cells. After all, almost all renewable energy resources – including wind power, hydropower, and biomass production – are ultimately driven by the sun. Using solar energy directly means fewer conversion steps and, therefore, fewer losses overall.

Even less available sustainable energy

So, will the problem be solved if we simply build countless solar farms? Naturally, it is not that simple. In their study, the team from Empa only looked at the first step – calculating the available energy potential. The amount of energy that is actually available will be smaller: limiting factors include the availability of raw materials, financial capital and manpower, the environmental impact of sourcing raw materials or production, managing waste from and operating the facilities concerned, and the need for additional energy distribution and storage infrastructure.


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