17/05/2011

Solar, wind, and wave power – natural limits of renewable energy

Their research suggests the availability of energy from wind and waves is renewable but limited and its extensive extraction might impede its own regeneration by the Earth system. However, the conversion of solar energy seems capable of fulfilling the growing demand for sustainable energy on Earth for quite some time.

Producing enough sustainable energy is a major challenge for the future of our civilization. While climate-friendly nuclear energy bears major safety risks, the burning of fossil fuels ultimately contributes to climate change. Alternative natural energy sources like sun, wind, and waves are considered risk-free, renewable, and without a climate impact. The latter two aspects have now been analyzed by scientists of the Max-Planck-Institute for Biogeochemistry in Jena, Germany, using a holistic approach. They used the well-established physical theory of thermodynamics to differentiate between energy and so-called free energy, the part of energy that is relevant for the availability of renewable energy.

“Free energy is a well-defined concept in physics”, says Dr. Axel Kleidon, head of the independent Max-Planck Research group ‘Biospheric Theory and Modelling’. It refers to the fraction of energy that is useful for performing physical work, such as lifting or accelerating mass, but also for energy conversion for human use. The natural limits on the rate of free energy generation are not often considered and discussion of them is practically absent in the fields of climatic change and renewable energy potentials. In two new publications (see references below), Kleidon and colleagues show that the natural ability of the Earth system to generate different forms of this useful energy is limited to roughly 1,000 terawatts (1 terawatt equals 1012 watt, the unit of energy use through time), compared to the massive solar influx of more than 160,000 terawatts. The physical theory of thermodynamics can explain this very low efficiency of conversion of solar radiation into free energy. “Indeed, only about 0.6 % of the huge energy from solar radiation can be transformed into usable free energy”, confirms Kleidon. Human energy consumption currently accounts for about 5% of the free energy available on Earth.

The implication of this new, thermodynamic look at renewable energy is that its different forms have different types of impacts. Incoming solar energy heats the Earth surface differently, generating wind, which then provides an energy input to waves. The efficiency for conversion of free energy into the physical work done by wind and waves is limited, with thermodynamics describing the mechanisms. Hence a renewable source of energy based on wind and waves is continually replenished by energy input from the sun, but ultimately finite. “We found that wind or wave power deplete the natural stock of free energy”, explains Kleidon. Thus, the extraction of wind and wave energy has an impact on the ability of the climate system to (re)generate these forms of energy.

The use of solar energy as a renewable energy form is different. When sunlight results in heating or is reflected back to space, its free energy is largely wasted. Photosynthesis and solar power technologies exploit sunlight directly and produce free energy before it is converted into heat, and hence allow the Earth system to generate additional free energy. Even solar energy has free energy extraction limits, but its placement at the top of the free energy hierarchy makes the human extraction limits proportionally much higher.

Source: Max-Planck-Institute for Biogeochemistry, Jena, Germany