Embodied Energy, Efficiency and Emergy

1. Embodied Energy H.T. Odum’s insightful concept, along with the diagram in Fig.1 was introduced in the earlier chapter Economics as If Energy Matters. Just as an orange has been referred to as “concentrated sunlight”, electricity can be thought of as concentrated solar energy. Whether generated by dams, solar collectors or coal power plants, the upstream source is still the sun.

            As you move from left to the right in the processes of Figure 1 , the embodied energy increases. The energy also becomes environmentally and (usually) economically more expensive. Therefore, it is reasonable to associate quality with energy. The larger the embodied energy, the higher the quality.

                                               Fig. 1 Transformation of Energy

            Fig. 1 indicates that it takes 160,000 Solar units of energy to generate one electrical unit of energy! It doesn’t matter what these units are -- Cal, BTU, or KW-Hr -- the ratio stays the same. The same 160K solar units generate four coal units, so it only takes 40K solar units to generate one coal unit. The diagram also indicates that eight wood units make four coal units, so it takes two wood units to generate one coal unit. As you move to the right, the embodied energy and quality increase.

            The ratio of input energy to useful output energy is the transformation ratio for the process. The transformation ratio for Sun-to-Wood is 160K/2 = 80K , while for Wood-to Elec it is 8/1 = 8. Altogether there are six transformation ratios. There are three for the one-stage processes, two for the two-stage processes, and one for the overall three-stage process.

Exercise 9.1   Calculate the other four transformation ratios.

Exercise 9.2  Calculate the efficiency for each of the six energy transformations and compare with the corresponding transformation ratio. What do you notice . . . ? Is it a coincidence?

            What are the units for transformation ratios -- are they dimensionless? Interestingly, the answer is Yes and No. If we take Calories as the energy unit, the Wood-to-Coal ratio becomes. 8 Coal Cal / 4 Wood Cal. Since for thermodynamics “A BTU is a BTU is a BTU”, this ratio has the units Cal/Cal, which is dimensionless.

            On the other hand, the embodied energy perspective requires us to distinguish energies, and the ratio Coal Cal / Wood Cal does have dimensions. There is a much deeper difference between conventional energy and embodied energy than dimension.

            Take a look at the three egg-shapes in Fig. 2:

A laboratory measure of energy with a bomb calorimeter would not distinguish between the barnyard and factory egg. Both will be reduced to inert atoms and molecules, and each will yield an thermal energy content of about 150 Cal. The two eggs are thermodynamically equivalent. Yet those eggs are enormously different from the biological, economic and environmental point of view.

            A few dozen factory eggs, like the barnyard eggs, could be used as food by animals, but they could not be used to raise a flock of chickens. The barnyard egg is the result of an endless line of roosters and hens who passed their genetic information on to their eggs. This chain is broken by factory eggs -- they are sterile. As you move from left to right, there is an enormous jump in embodied energy. There is more structure, more information, more life.

Exercise 9.3  Garfield the cat and Bugs the bunny weigh about the same. Who has more embodied energy. Why?.

Exercise 9.4  What has more embodied energy, a pound of chicken or a pound of shrimp?

Exercise 9.5  It costs about $1.25 per gallon for 87 Octane gasoline in the U.S. and about the same amount per liter for the this Octane gas in Europe. Comment on the embodied energy of equal quantities of fuel from the U.S. and from Europe.

2. Entropy In contrast to many researchers, Odum has chosen to downplay the role of entropy. However, entropy and the quality of energy are intimately, though inversely related. This is demonstrated by the miniature orange tree. It begins as an open system with sun and rain as external energy sources. It is then reduced to a closed system. The orange tree goes from fruit- bearing health to “heat death”, from an organized bio- logical system to a random chemical-physical system.

Exercise 9.6

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3. Efficiency Revisited It takes about ten pounds of vegetable food to produce one pound of cow. Producing beef via grains and legumes is one of the most inefficient ways to grow food

               Figure 4

The (American) sacred cow

for human consumption. Similarly, it takes about ten pounds of rodents to increase the weight of a fox by one pound. If we assume that the inputs and outputs have approximately the same energy per pound, the efficiency of these two-step processes is about 10%.

            The transformation of solar energy into vegetable matter has a an even lower effici-ency, only a small fraction of 1%. These biochemical processes have evolved over many years, so their efficiencies are probably at, or near the maximum. The food chain in Fig. 5 shows that it takes one million solar units to produce one unit of Tertiary Consumer (cats, hawks, snakes)! The five-step process has an efficiency of .000001 = 1/10,000 of 1%!

                    Figure 5 Energy and the Food Chain (After H.T. Odum)

Some examples of Primary Consumers (vegetarians) are rabbits, quail and mullet. Some examples of Secondary Consumers (mixed eaters) are chickens, foxes and grizzlies.

Exercise 9.7  Please refer to Figure 5. Verify for the following processes that efficiency and transformation ratios are reciprocals:

a) The two-stage process from Plant to Primary

b) The three-stage process from Plant to Secondary

            As suggested by Exercises 2 and 7, the efficiency (Eff) and transformation ratio (TR) are reciprocals for all processes. This follows right from their definitions:

                                    Eff = Output/Input                 TR = Input/Output .

Since Embodied Energy is proportional to TR, this means that it is proportional to the reciprocal of Eff. Another way to say this is:

Embodied Energy is inversely proportional to Efficiency.

4. Embodied Energy and Uniqueness According to Figure 1, an electrical energy unit embodies 160K solar energy units. This must be the same today as tomorrow, the same for you as for me. In other words, the concept of Embodied energy should lead to a unique result for a given process. Let’s check Natural and a Design (human) systems.

Natural Systems

            Natural processes have been going on for eons. In addition, when man interferes with the operations of natural systems, output drops, although it might take a while for this to register:

∙          For years, monstrous multiple-purpose dams were the US government vogue from the 1930's to the1960's. Now that the damage to upstream lands, erratic flooding, and damage to fisheries have become obvious, they are no longer being built in the US, and are even being decommissioned.

∙          For years, clear-cutting was all the rage with the US Forest Service; now that the damage by floods, mudslides, and the decline of species due to lack old wood have become obvious, the practice is being curtailed.

∙          For years, war has been declared on large predators; now that it has become obvious that top predators keep browsers and grazers from exceeding the carrying capacity of their environment, predators are being allowed to return to their ancestral ranges.

            Even though many of nature’s efficiencies are very low by engineering standards, the conclusion seems inescapable that natural systems are operating in an optimal, or near-optimal state. So embodied energy calculations will be remain the same, or almost the same for natural systems. In other words, the embodied energy is unique for a given natural process or system.                                                               Design Systems.

            For human-designed systems, the situation is very different. Very few processes or heat engines are optimized. Let us return to the simple example of a power plant. The power plant shown on the left side of Figure 6 has a thermodynamic efficiency of 25%. The more efficient power plant on the right has an efficiency of 33.3%. Notice that an increase in the

Figure 6 Efficiency of coal power plants

 efficiency of the coal-fired power plant will decrease the number of solar units required to generate a unit of electrical energy. The “upstream” requirement for the left plant is 160K solar units, while for the right plant it is only 120K solar units.

            Another way to say this is that one electrical unit from the left plant embodies 160K solar units, while one electrical unit from the right plant only embodies 120K solar units. In other words, embodied energy is not unique for business, commercial or industrial processes unless they are operating at top thermodynamic efficiency. Therefore,

An embodied energy calculation for a design process might not be unique.

There is another disturbing aspect of this. We have come to associate Embodied Energy with quality. But the above arguments show that embodied energy increases as the efficiency of a design process goes down! So it is important to capture uniqueness.

            Can the definition of Embodied Energy be modified so as to yield unique answers for design processes ...? The answer is yes.

5. Emergy*  Define emergy as the value of embodied energy when all processes are operating at maximum efficiency. We need to deal only with design systems -- business, commercial or industrial processes. Maximum efficiency can either be calculated or estimated for a design process. For example, the all-important heat engines have the upper bound

Eff_Max = (T_Hi - T_Lo)/T_Hi ,

where T_Hi and T_Lo are the source and sink temperatures, respectively. The temperatures are measured in Kelvin degrees, which are 270 degrees higher than Centigrade. For example, if the combustion in a gasoline engine is taking place at T_Hi = 550 C^o (= 1022 F^o) and the exhaust is at temperature T_Lo = 30 C^o (= 86 F^o) then    

                                             T_Hi = 550+270 = 820 K^o and T_Lo = 30+270 = 300 K^o.

Therefore, the maximum thermodynamic efficiency is

                                             Eff_Max = (820 - 300) / 620 = 520/820 = .634 = 63.4%.

(Notice that the 270's in the numerator canceled each other.)  

            As the efficiency of an engine goes up, the embodied energy gets smaller, but the emergy remains the same. The smallest possible value of embodied energy is the emergy.

The above discussion can be summarized as:

     Natural system -- Emergy = Embodied Energy

     Design system -- Emergy = (Eff/Eff_Max) x (Embodied Energy) ≤ Embodied Energy.

___________                                      .

* Embodied Energy was being used in some “creative” ways by researchers, so Odum chose

 this new term to replace it. An opportunity was lost to deal with uniqueness.

The above discussion suggests that Emergy is more reliable than Embodied Energy for measuring quality.

Exercise 9.8  The theoretically maximum efficiency for any “heat engine”operating under certain conditions can be calculated. Suppose that a coal-fired power plant has a maximum efficiency of 60%. If the actual operating efficiency of the power plant is 25%, what is the (solar) emergy of an electrical unit of energy produced by one this plant?

Exercise 9.9  If the actual operating efficiency of a power plant is 30%, what is the (solar) emergy of an electrical unit of energy produced by this plant? (See Ex. 8)

6. Quality vs Efficiency Efficiency calculations are independent of the quality of the energy flows. Suppose the two systems in Figure 7 perform the same services and have the same corresponding numerical output and inputs. Their corresponding energy qualities will also be the same, except that the top input is medium quality energy for the left system and is high quality energy for the right system.

Fig. 7 Two processes

Let us be specific and assign the following numerical values to the energy flows for the two systems --

Inputs: a = 5 BTU/sec, b = 55 BTU/sec

                                                Output: p = 12 BTU/sec .

Conservation of Energy requires that the sum of the output and the sink must equal the sum of the inputs:

                                    p + c = a + b or 12 + c = 5 + 55.

Therefore, the flow through the sink is c = 48 BTU/sec.

The thermodynamic efficiency for each system is –

                                           Eff = p/(a+b) = 12/60 = .20 = 20% .

            The medium quality energy flowing through the top input of the left system might be provided by coal or untrained personnel, while the high quality energy flowing through the top input of the right system might be provided by electricity or trained personnel. Although these are very different systems from the environmental (or economic) point of view, conventional thermodynamics can not discriminate between these very different systems. These are simple systems, but the principle can be seen at work in more complex systems. Traditional farming controlled pests by mechanical picking, in contrast to modern agribusiness which attempts to eradicate pests by airborne air-bourne spraying of petrochemical insecticides.

Exercise 9.10  Businesses need to keep expenses down in order to compete. Can you give an explanation why environmentally and (supposedly) economically inefficient practices such as massive applications of insecticides have driven out traditional methods?

Exercise 9.11  Environmentalists often suggest “Eating lower on the food chain”. Are they appealing to the same principle as discussed above? Be specific.