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The Exergy Crisis

I do believe that most everyone is familiar with the term "Energy Crisis", or has at least heard about it. To the casual observer, the terms "energy crisis", "peak oil", etc. would seem to insinuate that we are "running out of energy". It is somewhat bothersome when I am asked "Are we running out of energy?" or someone tries to argue "We have enough oil sands/oil shale/coal/etc. to last centuries". Even more irritating are the proposals to build new condensing (meaning no recovery of the waste heat) power generating plants, or the belief that we could continue on business as usual simply by seamlessly switching everything over to run on renewable energy.

My views on these perspectives came to fruition upon realization of the importance of the Second Law of Thermodynamics. Most of the general public is familiar only with aspects of the First Law of Thermodynamics, which states that energy is neither created nor destroyed. The first law fails to identify the quality of energy, only the quantity of energy; a joule of electricity is treated the same as a joule of heat, the heat of combustion of municipal solid waste is treated the same as a the heat of combustion of gasoline, despite the fact that we cannot easily run our cars on municipal solid waste. Even more important than quantity of energy is its quality.

The sun produces about 365 yottawatts (365 x 10²⁴ joules/second) of power through thermonuclear fusion. The fraction of this power reaching Earth is around 174 petawatts (174 x 10¹⁵ joules/second). In terms of only the raw number of joules per day, this is about equivalent to all of the conventional oil reserves, past and present, known to mankind, every thirty hours. So with all of these barrels of oil constantly raining down on us, how in the world could there EVER be an energy crisis?

The answer is that there is no "energy" crisis. There is an exergy crisis. Exergy is a measure of the quality of energy. Think of it like being stranded on a boat in the middle of the ocean. Dehydration generally sets in fairly quickly, despite the fact that the passengers of the boat are surrounded by water. What good is water when you can not drink it? What good is energy when you can not really use it?

The average human body at rest produces about 100 watts of power in the form of body heat. Why, then, can we not use our body heat to power up a 100-watt incandescent light bulb? 100 watts of dilute HEAT is completely different from 100 watts of electricity, just as 100 kilos of lead is completely different from 100 kilos of gold.

EXERGY CONTENT OF VARIOUS FUELS BASED ON TYPICAL ELECTRICITY CONVERSION EFFICIENCIES.

As previously stated Exergy refers to the quality of energy. Modern human society requires an extremely high level of exergy, a level of exergy so high that the only form of energy capable of supplying it is electricity. Electricity, when controlled as is done by humans, has the highest level of exergy of any form of energy. It can even be argued that the "advancement" of a society can be measured by its exergy needs. Agrarian societies rely on the sun, direct burning of firewood and other biomass, and motive power from animals. These energy sources are of a much lower level of exergy than say the electricity guzzled by an urban industrial culture. I am expanding the official definition of exergy in thermodynamics by including such factors as "convenience of use". An automatic coffee machine powered by electricity is more convenient that building a wood fire in a cast iron cook stove just to heat up a kettle to make coffee.

Peak Exergy

Throughout human history, we have continuously moved towards forms of energy with higher and higher levels of exergy. Before the days of controlled fire, the only energy available to us was the energy in the food we consumed. Even in these days, however, it was surely known that meat yielded a higher level of energy per unit consumed than plant-based foods, hence providing an incentive for hunting. Meat is more exergetic than vegetable matter.

We then figured out how to burn biomass (wood, etc.). Fire gave us the ability to preserve food, provide light during night, provide heat in cold weather, and eventually make ceramics, bread, beer, and reduce metals from their ores. Combustion provided the exergy necessary for gathering into societies. The development of the steam engine and utilization of coal (which contains more energy per unit of mass and volume than wood, and in many cases is easier to use) provided huge amounts of motive power. Motive power has an extremely high level of exergy, and allowed for industrialization free from the bounds of nature.

No more was a factory dependent upon the flows of water over a waterfall, or the whims of the wind or schedule of the tides. The motive power of fire (as it was named by Sadi Carnot, the pioneer in thermodynamics) could be harnessed anywhere, anytime. The steam engine broke the bondage that man had to the natural world, and paved the way for the unprecedented population growth and massive industrial production levels seen today. The steam engine allowed us to siphon off the exergy from fuels and apply to whatever we desired. Later on, the internal combustion Diesel and Otto engines would once again revolutionize transportation utilizing high-exergy petroleum-derived fuel. Use of oil and natural gas to produce nitrogen fertilizer boosted agricultural productivity to levels high enough to support mass movement from rural to urban life, as well as extremely high population growth. The harnessing of electricity then allowed the mechanical output of the steam engine to be transmitted over long distance with little loss. Thin metal wires strung overhead invisibly carried the equivalent of thousands of coal carts and dozens of gas pipelines. The cleanliness, convenience, and versatility of electricity defines the modern world, and electricity is indisputably the single most influential accomplishment of human beings.

THE OLDUVAI ENERGY CURVE. INDUSTRIAL SOCIETY IS DEPENDENT UPON ENERGY SOURCES WITH A HIGH EXERGY LEVEL TO MAKE BREAKNECK GROWTH AND CONSUMPTION POSSIBLE. MANY OF THE "POST 2012" BLACKOUTS ARE ALREADY OCURRING IN THE DEVELOPING WORLD.

Today, however, we have reached the point where the exergy of our available fuels has hit a maximum. The oil left in the ground, while it may be substantial, requires far more processing to have it meet the exergy needs of modern society. Renewable resources, such as the wind, sun, and geothermal energy, also require much more effort and new ways of thinking to convert them into the high-exergy electrical energy upon which our society is based. Even nuclear fuel is limited; high-grade uranium ores are not plentiful and the low-grade ones will require much more effort to get into a form usable in nuclear reactors. For the first time in human history, we are forced to utilize energy sources which have less exergy available than the ones we had used previously. This forms the entire basis of the "energy crisis". Energy is not becoming scarce, but energy with enough quality to operate our gadget-centric lifestyles will get harder to come by. Creating electricity using renewables takes more effort and creativity, and it is likely to be a bit more expensive than the dirt-cheap fossil fuels we have always been used to. The fossil fuels will not even really be an option anymore, as they are finite resources which are disappearing into the air as their combustive energy is extracted and bypassed through iPods, Ford Explorers, and flat-screen TV sets.

"Upgrading" Energy

One of the aspects of modern society is that most of the energy that we use has been "upgraded" in some way. More exactly, "upgraded" energy is like meat with the fat trimmed away. Only the good stuff (exergy) is utilized, the rest is thrown away. Controlled electricity and gasoline don't exist in nature; we have built power plants and refineries which are able to upgrade the energy found in natural resources into a more convenient form with a much higher level of exergy. Few city dwellers in the western world today could ever imagine heating their home with a hand-stoked coal or wood stove, or having to light and tend a fire every time they wanted to cook a meal. We could never imagine having to get horses ready for the commute to work and feed them hay and clean up their excrement.

We are used to the "set it and forget it" forced-air heat powered by natural gas and electricity, or the coffee pot that kicks on and brews a cup of joe before we ever roll out of bed, or the car that is ready to go with the turn of a key. This is all made possible via the extremely high level of exergy available in modern fuels such as electricity and gasoline. When you have a lot of exergy available, things are much less of a hassle.

Energy, believe it nor not, can have "order"; exergy is a measure of this order. For example, the electrons flowing in a wire, water flowing in a pipe, or a rotating shaft, all represent a flow of energy. The energy is all flowing in a well-defined direction under some level of control. Heat energy, on the other hand, is represented by the vibration of molecules and atoms. These vibrations are random, and this randomness cannot be used to produce work unless some type of heat engine is used. Heat engines take advantage of the expansion and contraction of fluids due to their varying contents of heat energy. SOME of the heat energy is then converted into mechanical work. The process does not come without a price, however. Only a portion of the original heat energy is able to be converted to mechanical work (and possibly then electricity). The rest must be dissipated to a colder environment. Creating peace and order is always harder than creating chaos - ever so true.

Even with this waste, fossil fuels still have a much higher level of exergy than most of the renewable energy sources, with the exception of hydropower. Because industrial society requires such high levels of exergy, energy sources with a high level of exergy are desired. Fossil fuels simply give a whole lot of exergy without a lot of immediate hassle. Hence the rationale for their use.

A Decline in Exergy

Inevitably, we can expect the amount of exergy readily available to humans to decrease as fossil fuels become depleted. We will have to develop lifestyles which take more advantage of the dilute light and heat from the sun rather than using exergetic electricity and liquid fuels. The clothesline for clothes drying, open windows for fresh air, gardens to help grow food closer to home, passively heated houses and solar hot water systems will become mainstays of residential life. No more will it be sensible to use electricity and natural gas to create dilute space and water heating; the high levels of exergy in these fuels must be reserved for tasks which require high levels of exergy. Cogeneration of usable heat along with electricity at power stations will be a mandatory practice, as throwing away all of this energy rather than using it for space heating will be an incredibly wasteful proposition. The large amounts of upgrading and equipment necessary to convert non-conventional hydrocarbon resources into gasoline will ensure that it will be far to costly and inefficient to keep the "happy motoring" car-centric suburban lifestyle running. We can make cars that are powered by electricity, but realize that they are not going to be the heavy 8-passenger 4-wheel drive SUVs that American society has come to love, simply because basic thermodynamic constraints don't allow it. Oil simply provides us with such large amounts of exergy packed into such a small and easy to use package, and we have been spoiled by this. Once it is gone, it is gone.

People will need to become more educated on energy and how they interact with the natural environment which supports them. Consumerism will be impossible without sufficiently cheap exergy to drive the factories, container ships, trucks, and retail outlets, though I am not going to speak as if a reduction in consumerism is a BAD thing; it most certainly is not. A commentator on an article written about compact fluorescent lamps spoke of how it is "stupid" for people to have to become educated about which energy-efficient light bulbs are best, that they should not have to worry about such "petty" things. Well, as much as it may hurt, The Second Law of Thermodynamics is actively kicking stupidity, ignorance, and our futuristic consumer-capitalist aspirations in the butt!

Additional Reading

• Wikipedia - Exergy (Thermodynamic Definition)
• Encyclopedia of Earth - Exergy
• A Sunshine Limit to Growth
• Losing the Battle with Entropy

Last Modified: 01/27/2009
Created: 1/27/2009