Fuels, energy carriers and the thermodynamics of sustainable development

With the local pump prices of diesel and gasoline having nearly doubled in the last three years, it is not surprising that the large-scale use of alternative fuels in the Philippines is now being seriously considered by industry, the government and the general public. Interest has also been stimulated by the popular, award-winning documentary An Inconvenient Truth, which has managed to awaken public attention to the gravity of global climate change. In response to these trends, we now have legislation that requires blending of biofuels into conventional petroleum products — specifically, ethanol with gasoline, and biodiesel with diesel. Numerous LPG-fuelled taxis (and some private cars) now ply the streets of Metro Manila, doing their small share in cutting down emissions of greenhouse gases and other pollutants. Still, the transport sector remains heavily dependent on imported petroleum and continues to generate roughly a third of the Philippines’ carbon emissions. And it remains uncertain how far alternative fuels can displace petroleum with currently available vehicle technology.

Various technologies have been touted as the successor of the internal combustion engine now found in practically all commercial motor vehicles. Foremost among these are fuel cell vehicles which run on hydrogen. The hydrogen can be in free or elemental form or bound in the molecules of organic compounds such as methanol. It seems that the large car manufacturers of the US, the EU and Japan are placing their bets on this technology. Then there are electric vehicles, running on batteries that are meant to be charged in the same way as cellphones and laptops. Other, more exotic technologies have also been proposed — for instance, cars that run on compressed air or liquid nitrogen. Just how viable are these alternative technologies, exactly? Are any of them on the verge of imminent commercial breakthrough?

My main intention here is to dispel some common misconceptions in the popular media about these supposed alternative fuels. Let me begin with some fundamental physical principles, ones that are found in textbooks all over the world, and which no science-literate person hold in doubt; these are the laws of thermodynamics, and I describe them here in rough layman’s terms. Firstly, energy cannot be created out of nothing; it has to come from somewhere. What people deal with in everyday life is usually the conversion of energy from one form to another — for instance, conversion of chemical energy in fuels into heat, noise and motion when they are used in motor vehicles (Einstein’s famous equation from relativity theory, of course, allows for conversion of matter into energy, and vice-versa, which is the basis for the production of nuclear energy). Secondly, any real-life conversion of energy from one form to another inevitably leads to some degradation of its capability to do useful work.

Having reviewed these basic laws of nature, we now consider the question: where does the energy on Earth come from?  The answer is that the bulk of it comes from solar radiation. Other sources include tidal energy, geothermal heat and nuclear energy; however, their combined contribution is smaller than the total sunlight captured by the Earth. This sunlight, in turn, drives atmospheric convection, the hydrological cycle, and biomass growth through photosynthesis. These natural processes then provide the basis for wind energy, hydropower, and biofuels. Dead organic matter is also converted by extremely slow geological processes into fossil fuels — natural gas, petroleum, and coal. The now obvious conclusion is that, with a few exceptions, much of the energy we see and use is nothing but bottled sunlight. It also follows then, that the apparent wealth of energy available in fossil fuel reserves is the result of millions of years of sunlight being accumulated in underground deposits. Drawing on these non-renewable (or to be more precise, slowly renewable) resources is like trying to live off the contents of a piggy bank — eventually, the supplies will dry up.

The trouble with popular depictions of hydrogen, electricity or compressed air as alternative fuels is that they do not properly emphasize the underlying system energy balances. Note that these fuels do not exist in readily usable form in nature. I am not sure if this omission is intentional or not, but it does seem quite prevalent.  Consider the case of hydrogen.  It is very abundant in nature, as each kilogram of water will give you more than a hundred grams of hydrogen. I have heard people make the statement such as, “all you have to do is break water into its constituents, and you have all the hydrogen you need.” This surprisingly common argument misses the point completely. Breaking water down into oxygen and hydrogen requires energy. Burning hydrogen fuel merely recovers the energy originally invested to make the fuel in the first place. In fact, the laws of physics tell us that we cannot even recover all the energy invested in the process; some of it is inevitably lost. Similar arguments can be made for electricity (which needs to be generated in a power plant) or compressed air (which will probably have to be produced using electricity).

Hydrogen, electricity and compressed air are more properly viewed as energy carriers rather than energy sources. They allow energy to be transmitted and distributed in convenient, readily usable form that is compatible with future motor vehicle technology; however, the environmental benefits of these energy carriers depend on the ultimate energy source used for their production. For example, electric cars running on energy drawn from fossil fuel-fired power plants at best offer only marginal gains in total system efficiency compared to current technologies, and would not be truly sustainable. For such energy carriers to develop into really sustainable technologies, they must be derived from renewable energy sources such as solar energy (in its various guises), tidal power or geothermal heat. That is because the thermodynamics of sustainability make it very clear to us: there is no such thing as a free lunch.

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Dr. Raymond R. Tan works at De La Salle University-Manila’s Center for Engineering and Sustainable Development Research (CESDR) on the analysis of energy life cycle systems. He is an associate professor of the Chemical Engineering Department and a recipient of multiple awards from the National Academy of Science and Technology. For more information, visit his website http://www.geocities.com/natdnomyar/web.