The Odum Brothers
Permanent Agriculture gets Technical
To understand the development of the post-World War permanent agriculture movement and the movements that followed, we need to follow the trajectory of the movement of the field of ecology, and we cannot trace this evolution without talking about the Odum brothers. Eugene and Howard T. Odum were the sons of sociologist Howard Washington Odum & Anna Louise Kranz and would go on to change the trajectory of agroecology, for better or worse.
Howard Washington, much like his sons, played a major role in understanding sociology, especially the development of regionalism in the American South. Much like the permanent agriculture movement’s leaders and much as his sons would do later on, Howard W. ambitiously attempted to bridge the gaps between various fields of study to create a comprehensive understanding of Southern identity. His goal had been unity through diversity— even if his understanding of race and class was poorly understood. He was obsessed with comparing “folkways” with “technicways” & “stateways”, and he described how traditional community was destroyed through technology and the resulting intellectualism, specialization, and centralization of the modern state.1 His concerns were the ‘achievement lag’, meaning specifically that ‘man has too often failed to apply his technical skills to prevent the social problems that have been created by the rapid expansion in technology’.2 A hobby jersey cow breeder, Howard W would spend his life trying to find a way in which humans reigned in their massive technological capacity to address social problems. As we will see, his overlaps with his sons are impressive, and the developments of his sons appear to continue his thinking into ecology.
Howard’s first son, Eugene Odum, was born on September 17, 1913, eleven years before Howard T. Odum would be born on September 1st, 1924. Eugene would head to school to study ornithology, graduating with his doctorate from the University of Illinois in 1939, and would take a faculty position at the University of Georgia in 1940 where he would spend the rest of his career. Howard T. would take a similarly mundane path, gaining a doctorate in biogeochemistry from Yale in 1951 and taking a position at the University of Florida at Gainesville.
In 1954, however, both were hired by the Atomic Energy Commission to study a coral reef at the Eniwetok Atoll atomic test bomb site.3 Just the year before, Eugene had published the first edition of Fundamentals of Ecology, the first textbook focused on the concept of the ‘ecosystem’. As they had refined their beliefs on ecology and systems thinking (while Eugene had been the primary author in the book, Howard T had contributed chapters to it), their time working at this test bomb site provided the foundation for both brothers and their belief around ecosystem energy. The coral reefs were described by the brothers as a steady-state system; it was their assessment that the coral reef system used most of the energy it consumed through photosynthesis to regulate the system. It would be the example that the brothers would point to of what a mature ecosystem looked like— self-regulating, self-maintaining, and a steady-state system. Both brothers would go on to study different ecosystems and each provided new data that the condition of stability was characteristic of all mature ecosystems.
Of course, this concept already existed in the idea of a climax community— the end of a forest succession. However, the Odums believed that the concept existed far outside of the visible plant communities— from puddles to humans themselves fell into the same systems thinking.
Fundamentally, energy was the common denominator for analyzing any system, a tool to study ecoenergetics— the flow of energy through a system. The significance of this, of course, was that energy could be understood and translated into human systems in a way that no longer separated the ‘natural’ world systems from the ‘human’, industrialized systems.
Based on their understanding of systems within an energy framework that could be translated into mathematics, the conventional methods applied by organizations like the Soil Conservation Service, which had sprung up during the Roosevelt administration, seemed woefully inadequate and lacked scientific rigor.
It was this lack of energy analysis that drove the Odums, specifically Eugene, to cite and critique the tools the SCS offered farmers. Further, much like many of the proposals of the post-WW 2 boom in agriculture and the systematic decommissioning of many of the New Deal’s progressive reforms in agriculture, the SCS had transitioned to an extension of the push to increase productivity in agriculture as the United States began its weaponization of food aid in its fight against communism.4 Instead of defending the environment, the SCS was overseeing engineering projects that were what drove its creation in the first place— making wetlands agricultural lands and straightening rivers.
This was all par for the course in the post-World War 2 boom; as we discussed here. Farms were producing record crops and exporting surpluses overseas as Europe recovered from the war. Extension programs and state agricultural programs were focused on increasing yields and showcasing those yields overseas as multiple green revolutions were proposed by the FAO across the global south (for more info, read about Erna Bennett and Efraím Hernández) which were moving in the opposite direction of the permanent agriculture movement that had formed during the Roosevelt administration.
As the movement withered, a hodgepodge mix of folks from the permanent agriculture continued to advocate for more ethical systems from the Back to the Land Movement to the Friends of the Land journal started by Russell Lord. Despite fundamental differences between these groups about the role of homesteading and the concept of scale, they all agreed fundamentally that there needed to be some sort of balance between humans and nature, that humans were part of nature, and that soil conservation and restoration were the basis of this philosophy. While seemingly similar, the Odums took a different position, and that instead, systems did not need to be ‘balanced’, but rather that human ecosystems simply needed to more closely ‘resemble’ natural systems.5
The other movement that sprung away from the collapse of permanent agriculture was the organic movement. This movement often overlapped with the first, as it was, in its nascent form, focused on aligning agriculture with natural processes— specifically around compost as a method for recreating the natural process of humus building on the forest floor. As we’ll discuss in another piece, the biggest barrier to its development as a serious alternative to conventional agriculture was due to its close proximity to pseudo-medical beliefs around the quality of both organic vegetables and the universal benefits of uncooked vegetables. Despite all of this, these three particular movements highlighted how the permanent agriculture movement fractured in the face of the petrochemical fertilizer industry which took off after World War 2.
The Odums believed that all three were incapable of addressing the issues facing ecosystems in their time— that alternative agriculture was naive and the continued growth advocated by the SCS & extension schools was not sustainable. Their beliefs were so strong that in their writing, they explicitly ignored contributions to American ecology by non-ecologists and simply committed credit for early pioneering work in any state-sponsored programs or alternative agriculture communities (for example, many of the names highlighted in this series).6 Further, they fully ignored any indigenous (in the United States or abroad) land management techniques and attempted to define agroecology within narrow parameters that centered capital-S Science and considered all other inputs invalid. In Eugene’s words:
“If biologists do not rise to the challenge, who will advise on the management of man’s environment— the technicians who have great skill, but no understanding, or the politicians who have neither?"7
Their perspective and alienation from a large swath of alternative agriculture movements, as they sought to define agroecology in their terms, had long-term ramifications. As the dust bowls of the 1950s rolled through the nation and the increasingly impossible task of farming for profit once again reared its head during years of astronomical growth due to fertilizers, there was a clear need for an alternative. Yet, their lack of willingness to find common ground with ‘failed’ agriculturalists kept agroecology separate as organics, biodynamics, and the Back to the Land movements grew.
Fundamentally, they believed that their job as ecologists was to “manage with actions”— create human systems based on ecological laws. Fundamentally, farmers were incapable of understanding ecology and their role was simply to be the labor of the ecologists. This was predicated on developing simple mathematical computations that could unify and compare the energy used— whether it be a wheat field, a forest, or a prairie.
To this point, we have spent more time talking about Eugene’s prescriptive understanding of agroecology, but when it came to modeling, Howard was the more skillful one.
These energy flow diagrams showcase how much energy in systems can be lost— in the above example, through heat— and are useful tools to understand how energy moves through ecological systems, and many of his early graphics are still used today in textbooks. As time continued and Howard worked to refine his method, he moved on, developing energy circuit diagrams for ecosystems.
While traditional energy circuit diagrams are used to show electricity, Howard felt they would be better at highlighting positive and negative feedback loops. While it took repeated attempts to refine the model, it eventually took off and the careers of both brothers exploded as this model was applied in much of the literature on ecosystems through the 1950s and 1960s. During this same period, both brothers worked with the Woods Hole Marine Biological Laboratory, where they won two prizes in ecology from two countries in a few short years.
Despite their rise to fame, both brothers still focused on showcasing how these diagrams would help develop modeling for their vision of agroecological systems. The graphic above, for example, was presented to the Panel on the World Food Supply Problem in 1967. For context, this was the same year that Erna Bennett had coined the term ‘genetic erosion’ addressing the shortfalls of conventional agriculture from an entirely different perspective.
What underscored the Odums’ analysis was how the laws of energy defined how effective different agricultural methods were. The first two laws we’ve discussed around complex systems science here; these laws define that all energy is accounted for and that energy degradation creates entropy, usually through heat sinks. The final law, which is still up for debate as an official ‘law’ in our understanding of energy flow, is the Darwin-Lotka law, or ‘the maximum power principle’. The basic principle of this law is that natural selection selects for maximum effectiveness in the use of available resources— for example, the plants that can survive with the least amount of rainfall take over dry regions.
Beyond this, they framed their work around what they had learned about mature ecosystems— that they tended to have production equal to respiration. This meant that the maximum power system eventually created stability within an ecosystem due to the increased efficiencies. In many ways, the maximum power principle can be understood as the mathematical framework behind natural selection, and as natural selection refines species diversity and conversely efficiency within an ecosystem, fewer species risk extinction.
With time, the Odums attempted to expand how all inputs could be converted into objective energy— even things such as money. All of these facets needed— in their perspective— to be converted into kilocalories to examine and compare systems to objectively identify the correct forms of agriculture for specific landscapes. This was further cemented as the Green Revolution was proposed as the solution to the population explosion taking place globally in the mid-twentieth century, which continued to push the food system that the Odums believed was the worst— the American agricultural system— which they felt had been misunderstood. Fundamentally, the successes of the Green Revolution were based on “energetic fallacies”, and that there would be a long-term cost for the short-term increases in productivity due to petrochemical fertilizers and pesticides.8
Fortunately for the Odums, developments in technologies such as computers and calculators had made it increasingly possible to fully account for the fossil fuels used in producing the massive crop yields the United States experienced. They believed this was the fundamental flaw in conventional agriculture, that there was little understanding of the actual energy cost of crop production. It didn’t matter, according to the Odums, what the gross energy produced by farms was (their yield), but rather what was the net energy of the farming system. While solar was largely ignored for determining net agricultural energy, everything else must be added up and compared to the amount of kilocalories produced in food. Their studies found that hunting and gathering and subsistence farmers showed small net yields, while highly industrialized farming tended to have a net energy loss and that there was a strong correlation between additional energy inputs outside of solar and decreasing net yields.9
As the global push for Green Revolutions continued, both Odums found themselves criticizing the American standard of agriculture and the idea of pushing it across the globe. Eugene worked to paint the future he felt would best meet our ecological limitations while attempting to maintain a similar food system for consumers, describing a world that focused more heavily on foods with fewer inputs (Mussels being one he advocated) and communities filled with more homesteads.10 Howard, in comparison, believed we would be returning to a point at which the intensive agricultural systems described by F.H. King would invariably take over our food systems.11
As the OPEC oil crisis of 1973 hit, the fears of peak oil became mainstream. The Odum’s believed it would drive acceptance of their ecoenergetic analysis of society and agroecology. And it did. However, it wasn’t who they had hoped. Instead, agroecology and their vision were found by folks like organic farmers and environmentalists. The Rodale Institute, for example, would build on the concepts of local food by developing what they’d called “foodsheds” based on ecologically important agricultural watersheds. The organic movement looked to agroecology for scientific support for their homestead-agrarian nostalgia and health concerns. The Odums also were a major influence on Bill Mollison & David Holmgren’s development of permaculture, which, as we will see in a future piece, takes their vision and runs with it— for better or worse.
From the environmental front, Howard’s energy-based perspective also backed the ‘deep ecology’ movement of the 1980s. For folks unfamiliar with the Deep Ecology movement, its framework is simple— that all life on earth, human and nonhuman, is of intrinsic value, and diversity itself is of particular value. Further, humans have no right to reduce this diversity except in order to survive, and humans have interfered with these levels too far already, and that a substantial decrease in human population would benefit both humans and nonhumans’ ability to thrive.12 There’s plenty to criticize about Deep Ecology, and if you’re familiar with our work you’ll notice that we’ve made criticisms about it without directly calling it out in regards to human-managed ecosystems. Bookchin in particular makes the point that Deep Ecology is misanthropic, and this is exacerbated by ignoring the roles of authoritarianism and hierarchy, which reinforces the most problematic issues of Deep Ecology.13
From a purely ecological perspective, their vision of bringing human ecosystems in alignment with ecological laws was backed by their contemporaries for a number of years. In fact, a 1974 Congressional act mandated the use of ‘net energy analysis’ in the assessment of energy resources.14 However, ecologists tended to stay away from the terminology proposed by the Odums as they were primarily anthropocentric, and many found these to be human-centered and ignored the actual needs of ecologies. Agroecology in the English-speaking world was less about living within ecological conditions and more about measuring the impacts of systems as they impacted ecologies instead of being tools to better develop these systems.
Part of the failure of the Odums’ vision was that their predictions of returning to the manual labor of the 19th century failed to come to fruition, despite the fact their claims that the sky was falling well into the 1990s. This was reinforced by the agricultural economists who claimed that the Odums poorly understood how farms actually worked at scale and that their analysis was based on sweeping generalizations from data.15
Another factor that influenced their inability to influence policy was that the Odums were more interested in understanding agroecology than understanding how and why American agriculture existed as it did. This poor understanding and lack of context alienated many potential supporters who otherwise sympathized with the Odums and their concerns, keeping their vision from ever engaging with actual policy the way Rexford Tugwell had successfully done only 50 years prior.
The Odum Legacy and the Maximum Empower Principle
With this in mind, we can explore in more depth one of the most important contributions that the Odums made to agriculture and science as a whole, and that’s the Maximum Power Principle. Howard had begun exploring this concept starting with his dissertation, which was based upon Alfred J. Lotka & Ludwig Boltzmann’s prior work who had separately worked to develop a framework for what they believed to be the fourth law of thermodynamics— the concept that helped ‘guide’ self-organization in open systems.16
H.T. Odum explained this idea by stating that:
Over time, through the process of trial and error, complex patterns of structure and processes have evolved… the successful ones surviving because they use materials and energies well in their own maintenance, and compete well with other patterns that chance imposes.17
This same concept guides much of how we understand concepts such as steps in scalability in economics, for example, where building a factory that will stay busy, but not too busy to the point that it is incapable of running effectively, guides our decisions. What should be clear here, however, is when we talk about maximum power, we are not talking about maximum efficiency. Instead, we are often talking about the idea of maximum potential across a multitude of expected positions. Another example would be driving a car— if you are in the middle of a power band in 3rd gear, for example— it is easy and efficient to decelerate or accelerate. While staying at the lower end of the power band might be more efficient, it is often much more difficult to accelerate and that is why you often drop into a lower gear to accelerate. Efficiency is not the same as maximum power; efficiency often operates in a static role while maximum power operates within a dynamic one.
In the 1950s, Howard spent his early days at the University of Florida attempting to further develop this concept, working with Dr. Richard Pinkerton. It was here that they began to take this idea and apply it to a number of unconventional systems— specifically around ecology. While many of the original studies done in the 60s and 70s offer a basic starting point for more complete and complex research to follow, we can see how this played into the energy circuit diagrams that came later.
By the 1970s, Howard narrowed his interests more closely to the quality of energy, which he later termed “Emergy”. He defined Emergy as “the available energy of one kind (usually solar) that has to be used up directly and indirectly to make a product or service”. Functionally, how does the primary source of all energy on Earth— sunlight— translate into stored energy that can be used more effectively? Maximum power and Emergy helped to explain why any system would dissipate energy already captured to create another form of energy— most frequently what is lost as heat. He argues that energy ‘hierarchies’ are created as resources that require more energy take a longer time to develop; that items that require more energy need to have a positive effect on the larger system to justify the emergy required for their development. Because of this, ecosystems reinforce the idea of maximizing the power output possible from the resources that are available.
I don’t want to get too tied up in framing up the science of this position, because this ultimately became a philosophic point for Howard, what he called ‘emergy morality’, which came with 15 guiding points:
1. Seek satisfaction in useful contribution
2. Help maximize real wealth (empower).
3. Reinforce environmental sources.
4. Treasure genetic and cultural diversity.
5. Adapt to natural hierarchy.
6. Minimize luxury.
7. Minimize waste.
8. Adapt to system rhythm.
9. Share information.
10. Optimize efficiency
11. Circulate materials.
12. Circulate money.
13. Fit the earth.
14. Reproduce only as needed.
15. Have faith in self-organization.
In reality, this list is merely a collection of personal opinions, which stands far aside from the mathematic construction of the world as energy in— energy out. But this list which includes some questionable positions isn’t surprising; his former students have written about the need to ‘out-populate’ the Soviet block, that the United States had an imperative to use all energy available first before the rest of the world would have access to it.18 In 2006, Howard and his wife Elisabeth wrote what could be considered his hopes for the future, outlining their positions on what needed to be done for society to survive the impending energy descent that he had warned about for decades.
In “The Prosperous Way Down”, we see a lot of recurring arguments made by Malthusians; that distribution of birth control is “necessary” in poorer countries, but “cannot be the only means for stabilizing or decreasing population”. Needless to say, you should be hesitant at the choice to include ‘only’. Cities must depopulate to locally-supported levels (without the aid of fossil fuels). The Odums argue this can only be done “when the governments of cities and counties be combined so they are managed as a single system”. Further, automobiles and fossil fuel reliance must decline and a focus should be placed on increasing efficiency of already existing infrastructure instead of attempting to scrap and rebuild or rely on future renewable energy policies.
The vision by the Odums is paradoxical; global information sharing can save us from territoriality but that information is energy-dense through the internet and storage, and universities should carry the weight of deciding which information is worth saving and which isn’t. He makes these types of arguments based on ‘objective’ analysis, for instance, arguing that there is more power from hydroelectricity— which they claim is the most reasonable electricity source in the future— than the value of salmon runs that would otherwise exist in those spaces.
The Odums wrap up the short essay with a critique of capitalism that doesn’t outright toss aside capitalism, but posits capitalism with a decrease in unearned income and more accountability, what they call “private enterprise with public control” and suggest that we find new ways to organize families in a world with fewer children. My point here is to highlight the complexities in these arguments and that often times objectivity without historical context can often open up a can of worms that are dangerously close to really terrible things. Without historical context, how do we best address who has more right to child-bearing? How do decide which communities allocate resources? As artificial intelligence has shown us thus far, the idea of objectivity cannot yet be coded even into a neutral object such as technology due to our own internalized biases.
Survival of the Fittest?
Today, studies are still being done to understand the concept of maximum power, and it’s worth understanding Maximum Power in relation to how we otherwise understand ecology and how ecological systems develop. The basis of much of our understanding of ecology is due to the work of the naturalist Charles Darwin’s Theory of Evolution, specifically how the work of Peter Kropotkin refined it in his quest to understand the cooperation and mutual aid he became exposed to when studying the landscape of Siberia. Now, the reason this comparison is so important to unpack is multifaceted— H.T. Odum was said to not believe in Darwin’s position on evolution, despite the idea of Maximum power seemingly offering survival of the fittest through energy efficiency.19 I’d be surprised if this wasn’t tongue-in-cheek, given that he does talk about evolution in his own writing, but it does give us a chance to understand maximum power in relation to pre-existing ecology. And, to me, most importantly, this comparison helps us better understand its utility and identify its lack of context, as we pointed out above when discussing the Odums’ vision for the future.
While the Odums looked to bigger governments with less financial influence from capital and top-down decision-making around birth rights and other quality-of-life issues, Kropotkin was inspired by the way birds, fish, and other animals cooperated in the absence of any formal organizational structure. While Kropotkin is an easy figure to point to because of the weight of his name as an anarchist, his prominence as a scientist is often understated. Kropotkin the scientist was one of the world’s most influential researchers in evolution for nearly 80 years, until the 1960s. He argued that evolution was not strictly about the survival of the strongest, and his work is the basis of the entire subdiscipline in biology devoted to the study of cooperation and altruism in animals. Kropotkin argued that British scientists misrepresented Darwin’s work on evolution and that Darwin’s position supported Kropotkin’s argument about cooperation and altruism.
Conversely, the question of how the work of the Odum brothers has been translated into ecological analysis has been much further on the periphery, with the exception of the permaculture movement. In the beginning of Retrosuburbia, David Holmgren’s most recent book, he discusses how the Maximum Power Principle is the basis for all of permaculture systems design. But as we’ve seen, there’s quite a bit of ambiguity around the ethics that underpin the ‘guiding points’ behind the Odum brothers and their translation of the concepts they developed (and the overlaps, as we’ll see, between the Odums and Retrosuburbia are concerning). The fact that the Odums didn’t believe we needed to support native ecosystems but rather ‘similarly designed systems’ further explains the use and defenses of invasive species that are so common in permaculture. All of this is worth examining further and is something we’ll be diving in detail in the future.
Despite the fact that the Maximum Power Principle is the keystone of permaculture systems design, its own name ‘permaculture’ dismisses the continuation of the Maximum Power Principle’s model for the most efficient systems, and that is the pulsing paradigm. The Odum brothers, in attempting to better articulate feedback systems and their role in how Maximum Power self-organizes, developed the concept of the pulsing paradigm. The origins of the pulsing paradigm were not dissimilar from their early work around coral reefs; they looked to the ocean and the ways that marshes and wetlands were influenced by consistent, medium frequency, and amplitude systems. Their conclusion was that ecosystem performance and species survival are enhanced when external and internal pulses (both biotic and abiotic components) of an ecosystem are coupled. In their words, “if pulsing is general, then what is sustainable in ecosystems, is a repeating oscillation that is often poised on the edge of chaos”.20 Years later, they would argue that “an alternation of production and consumption provide a better long-run coupling of energy intake for maximum power than a steady state can provide".21 In other words, systems maximize not by being the most efficient but by maintaining a higher consistent, long-term efficiency adapted for a multitude of consistent conditions.
Unlike permanent agriculture or permaculture, the need for pulsing is a necessary component for long-term sustainability. In much of North America, this ‘pulsing’ was overseen by the prescribed fires that reset ecological systems. These burns guided the landscapes and allowed for specific management practices to continue for thousands of years. Swidden agriculture, common across the globe, also follows this same ‘pulsing pattern’.
In short, the Odums offered a new way to think about ecology and agriculture. However, their consistent shortcomings around context and building bridges continuously hampered their efforts at making a difference outside of academic settings. Despite this, there is much to be learned from their work, as it reminds us not to think so much of going back to the past but rather finding ways to integrate ecological ethics into our agriculture of the future.
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Bannister, Robert C. (1977) “Dictionary of American Biography, Supplement 5”, American Council of Learned Societies. https://www.swarthmore.edu/SocSci/rbannis1/pubs/Odum.htm
As quoted in Eugene Odum, ‘The Attitude Revolution’, 1970: 14
Madison, M. G. (1997). “potatoes made of oil”: Eugene and Howard Odum and the origins and limits of American agroecology. Environment and History, 3(2), 209–238. https://doi.org/10.3197/096734097779555881
https://truthout.org/articles/the-us-has-a-long-history-of-weaponizing-aid-to-other-countries/
Odum, Eugene P. 1969. ‘The Strategy of Ecosystem Development’. Science 164: 262- 270.
Odum, Eugene P. 1968. ‘Energy Flow in Ecosystems: A Historical Review’. American Zoologist 8: 11-18.
Odum, Eugene P. 1964. ‘The New Ecology’. BioScience 14, No. 7: 14-16.
Odum, Howard T. 1971. Environment, Power, and Society. New York: Wiley-Interscience.
Odum, Howard T. 1983. Systems Ecology: An Introduction. New York: Wiley.
Odum, Eugene P. 1969. ‘The Strategy of Ecosystem Development’. Science 164: 262- 270.
Odum, Howard T. and Elisabeth Odum 1976. Energy Basis for Man and Nature. New York: McGraw-Hill.
Devall, Bill; Sessions, George (1985). Deep Ecology. Gibbs M. Smith. p. 70. ISBN 978-0-87905-247-8.
Bookchin, Murray (1987). "Social Ecology versus Deep Ecology: A Challenge for the Ecology Movement". Green Perspectives/Anarchy Archives
https://www.princeton.edu/~ota/disk3/1978/7801/780129.PDF
Connor, Larry 1977. ‘Agricultural Policy Implications of Changing Energy Prices and Supplies’. In William Lockeretz (ed.) Agriculture and Energy, pp. 669-681. New York: Academic Press.
Hall, C. (Ed.), 1995. Maximum Power: The ideas and Applications of H.T. Odum. University Press of Colorado, Niwot.
Odum, Howard T. 1983. Systems Ecology: An Introduction. New York: Wiley.
Brinson, M. M. (1996). Maximum power: The ideas and applications of H.T. Odum. Ecological Engineering, 7(2), 157–158. https://doi.org/10.1016/0925-8574(96)00009-2
https://www.emergysociety.com/through-the-macroscope/
Odum, W. E., Odum, E. P., & Odum, H. T. (1995). Nature’s pulsing paradigm. Estuaries, 18(4), 547. https://doi.org/10.2307/1352375
Odum, H. T., & Odum, E. C. (2006). The prosperous way down. Energy, 31(1), 21–32. https://doi.org/10.1016/j.energy.2004.05.012