Thermoeconomics: The Future of Life
by Troubadour
May 20, 2009
There is a far deeper and more profound transition occurring in the "green revolution" than is commonly appreciated. While the immediate benefit of humanity's shift to a sustainable energy economy is that existential crisis is averted and energy supplies stabilized, the far more staggering implications become clear when one looks further into the future. Our present challenge is to achieve a fully sustainable system, but if we ask ourselves the question, "What happens after we achieve it?" the answer is truly breathtaking.
I. Tracing Energy Pathways
A. From the Sun to the atmosphere
All life as we know it, and thus all economics, begins with the Sun*. Stellar fusion is a process driven by the continuous gravitational collapse of a star's matter, which produces temperatures in its core sufficient to drive atomic nuclei together at speeds high enough to fuse them. This event transforms a fraction of the original matter involved into electromagnetic energy (a photon), which ever-so-slowly wends its way through the opaque soup of the star, bouncing erratically for eons before reaching the photosphere and escaping.
photonimage
Only a small fraction of the EM energy released from the Sun (about 50 billionths of a percent) reaches the vicinity of Earth. The highest-energy photons of that small proportion, namely Gamma rays and X-rays, are absorbed directly by the terrestrial atmosphere, breaking down some of the gases in the higher regions into simpler molecules and contributing partly to atmospheric temperature. This energy is then re-radiated at longer wavelengths - particularly, infrared (IR) - both back into space and toward the ground, which in turn absorbs it and re-radiates upward at still longer wavelengths.
Top_of_Atmosphere
Energy directly from the Sun in the form of ultraviolet (UV) light is mostly not absorbed by the upper atmosphere, but is absorbed by water - hence the lack of need for sunscreen on a rainy day. However, UV from direct sunlight mostly reaches the terrestrial surface, and is absorbed and re-radiated by the ground at its characteristic IR wavelength. With few exceptions related to chemistry and electrical properties, the atmosphere is transparent to wavelengths longer than UV, and the same absorption and radiation occurs at the surface.
earth_sun_plank
For the atmosphere, we thus have a three-pronged energy process at work: (1)Direct absorption of sunlight by atmospheric molecules; (2)energy that bypasses the atmosphere, is then absorbed by the surface, and finally re-radiated to the atmosphere at IR wavelengths; and (3)the downward re-radiation of energy from process (1) feeding into process (2). Energy reflected directly back into space either by the atmosphere or the surface can be ignored, since it is of no immediate consequence - although in the far-future, capturing more of it will be of interest.
greenhouse_effect_comet_sm
The Greenhouse Effect that currently endangers civilization is a consequence of some pollutant gases creating feedback loops between processes (2) and (3), but all three of them are critical for maintaining livable temperatures at the Earth's surface. Without an atmosphere, there would be no unified thermal environment but two sharply-defined environments (shadow and direct sunlight) hundreds of degrees apart in temperature, with only the ground as a stabilizer - life, if there were any, would survive only in caves, as some have speculated it may exist on Mars. If too much sunlight were reflected back into space by the atmosphere, not enough would reach the ground to keep water liquid, and Arctic conditions would ensue at the surface - life would retreat to sea floor vents, hot springs, and active volcanoes*.
If, however, the atmosphere were too transparent, water could not be kept in a vapor state by absorbing solar energy, causing the world to become an arid, desiccated wasteland with no clouds, rain, or snow. This would turn the equatorial regions into Saharan furnaces, and the ice caps from both North and South would expand right up to the edge of them - life would subsist in the tiny strips along the borders of these opposite hells. Parallel nightmares would occur if the surface was too absorptive or too reflective. Such are the balances.
B. Direct Power Sources
This brings us to Energy Pathway #1a: Photovoltaic (PV) solar power. This is the most direct tap of human civilization into our fundamental power source, the Sun. Current technology depends on electrons for our power needs (i.e., electricity) because we have not yet developed practical, efficient ways to directly harness the photonic energy responsible for their usefulness - so we must move the whole cow (the electron) rather than just taking its milk (the photon) and moving that, at least for power purposes. PV directly uses the energy of sunlight to free electrons, so it is the most parsimonious energy pathway under current technology. Some day, I suspect, we will use sunlight itself for power without even having to generate electricity from it, but photovoltaics are a step along that path.
solarcell
Next is Energy Pathway #1b: Solar thermal (ST) power. I make the distinction with PV because solar thermal does not directly use sunlight to generate electrons, but rather to heat a medium (such as water) that drives a steam engine which in turn generates electricity. Now, do not mistake my intention in applying this numbering scheme: I make no claims about the current relative efficiency of ST vs. PV, but I merely state that PV involves fewer physical transitions of the energy before becoming usable electrical power, and indeed the fewest of all solar-based power sources*.
solarthermaldiagram
solarthermalfield
solar-thermal2
Call it a desire for elegance; call it intuition; call it philosophy; but I consider it almost common sense that a process with fewer energy transformations will ultimately become more efficient. Entropy requires that a certain amount of energy be lost with each transformation, so at least on a theoretical basis fewer such would appear to be more desirable. Also, it is notable that PV is a fundamentally new technology in the world, having been invented mere decades ago while the steam engine and the mirrors it uses to focus sunlight have been perfected over centuries: PV thus has a large potential for massive improvement over time, but ST is unlikely to make any great leaps. Still, ST is direct energy pathway, and I believe (by the same intuitive criteria) that it has more potential than those that follow.
C. Fluid Temperature <--> Motion
As not every part of the Earth can be equally exposed to sunlight simultaneously, and indeed half of it is in the shadow of the other half at any given time, this produces what is known as a temperature gradient - or what normal people would call "different places being different temperatures." The laws of thermodynamics tell us that energy naturally flows along this gradient from higher temperatures toward lower temperatures, and fortunately for life this process causes fluids (such as atmospheric gases) to convect and move around.
Seasonal differences, latitude, and altitude all factor into how much solar energy reaches a given part of the atmosphere. Moisture content and pollutant concentrations affect how much of that energy is absorbed by the air molecules, as well as how that energy is expressed (re-emitted, or turned into air movement). Meanwhile, surface characteristics - e.g., the presence of a large body of water, or a highly reflective or absorptive type of rock - can greatly affect how the lower parts of the atmosphere behave relative to the upper parts. All of these differences result in the characteristic motions we call wind.**
temperature
The same is true of wave motion, although only the first few hundred meters of the ocean surface are directly affected by sunlight, and waves have far more to do with wind than with the direct heating of water molecules - surface tension makes water strongly resistant to exhibiting such motions beneath the boiling point. The death and fall of photosynthetic algae and dependent organisms to the ocean bottom ties the Sun to the ecosystem below, but that is a Biomatter energy pathway, which I deal with later.
oceantempdepth
D. Fluid-deferred Power Sources
We now arrive at Energy Pathway #2: Wind. Whereas solar technologies use energy largely as it arrives, wind power is a more circuitous pathway, as described by the three-part atmospheric process described above. A wind turbine functions by the spinning of rotor blades shaped such that air molecules colliding with them will impart a torque along its rotational axis - thus transmitting, after a long journey, energy absorbed by the air from sunlight or from re-radiation by the ground. The rotor blades can be a variety of shapes, such as two or three common propeller blades, vertical helices, or closely-spaced sheets like a pinwheel, although two- or three-bladed propeller-type windmills are the most common design.
Again, I make no claim about the relative efficiency of current wind power technology with respect to direct solar, but I reiterate that it involves many more energy transformations and inherent losses. One could, I suppose, analogize wind power as being a subset of solar thermal, with the atmosphere being the heated medium used to power the generator - but in this case, one is only capturing a very small part of the involved solar energy, as it has been transferred between molecules countless times and gone through innumerable cycles of absorption/emission before reaching the wind turbine.Moreover, it has most of the same technological limitations as ST, with an even bigger problem in that its moving parts become massively large in order to achieve greater efficiencies. There is room for incremental improvement with wind power, but virtually no room for quantum leaps. I would say that wind is an important transitional step while we escape from unsustainability, but it is an ultimately sterile technology that merely defers energy that could be captured directly.
Then there is Energy Pathway #3: Wave power. First, let me distinguish wave power from tidal power - one is a consequence mainly of wind acting on the ocean surface, and the other results from lunar and solar gravitational attraction warping the shape of the world's oceans. Neither are especially prevalent today, nor do I expect them to ever become so in the future, but they will have some limited applications and I might as well mention them. A graphic explanation of tides:
tides
Wave power is #3 because it's caused by #2: After all of the energy that's been lost by the wind in purely atmospheric motion, some of it is transmitted to the surface of the ocean, and that movement can then be harnessed by human technology using principles similar to those applied in wind turbines, along with other, more specialized means. There are technical differences, since water is heavier, "sticky" (i.e., viscous) relative to air, and machinery operating in it tends to deteriorate faster than in air due to its stronger oxidizing and other chemical properties. Nevertheless, the basic operating mechanism for wave turbines is the same - molecules in a fluid imparting momentum by impacting rotor blades that spin a turbine. There also elongated buoys and other mechanisms:
wavepowerbuoys
There are theoretically more elaborate ways of doing this involving piezoelectrics (chemicals that store and release electricity when moved), but in principle we are talking about an even more deferred process than with wind, and the same underlying issues arise. On top of those issues, of course, is the fact that the ocean is a hostile environment for technology with moving parts compared to the atmosphere. Wave power will play only a very limited role in the mid-range future, and none over the distant horizon.
E. All Roads Lead to Rome - Biomatter
Life, in various forms, uses all available sources of energy. Ultraviolet sunlight powers photosynthesis in plants, which are then consumed by non-plant microorganisms and herbivorous animals, and those in turn are consumed by scavengers and predators (including humans). While this is occurring, each organism also maintains homeostatic (life-supporting) temperatures by consuming water and breathing gases - that is, life uses both direct and fluid-deferred energy to survive. The Sun both powers the food web with plant matter, and allows organic processes to occur by keeping water liquid and air sufficiently warm not to shut down the cellular processes that use it.
Photosynthesizing plants and microbes use the process to break down CO2 into carbon and oxygen, which can then be recombined with other elements to form sugars and other organic substances (the primary application of the carbon) or released as molecular oxygen. This is an enormously energy-intensive process due to the strength of the chemical bond between carbon and oxygen, and is only considered efficient in comparison (thus far) to human attempts to mimic the process technologically. As with all energy transformations, some is lost to heat, but a significant amount is "stored" in the sugars and oxygen that are thus formed.
As an aside, I would note that significant efforts are underway to achieve efficient artificial photosynthesis, and that it would be a revolutionary breakthrough for the current state of energy technology were it to be successful. Nevertheless, I would make a point that this process, while a remarkable method of energy storage, and perhaps a relatively efficient method of power generation, would merely be a deferred form of power applied through a chemical medium, and require larger minimum energy losses than a more direct method. It does, however, have one rather stunning implication that the next few paragraphs hint at.
From this point, there are two energy-relevant media released into the system: The organics and the oxygen. The oxygen will either be directly respirated by aerobic organisms, including humans, or be consumed by aerobic chemical processes used in artificial activities such as fire. Thus we see that a chemical process crucial to our second-by-second existence, oxygen respiration, is in fact fueled by an energy process deferred through the chemical medium of plants - separate organisms that invariably lose some of that energy maintaining themselves and propagating. If mankind were able to efficiently generate its own oxygen directly from solar power, we may eventually outdo plant life the efficiency of oxygen production.
But before anyone labels this hubris, allow me to make this qualification: Evolution has created organisms superb at serving themselves, but that does not mean they are superb at serving us - our technology can indeed do better for us than nature incidentally does by allowing us to exploit other species. Moreover, there are profound moral arguments for striving to extricate mankind's ever-growing desires from dependence on an ecosystem we increasingly endanger, and if we can find a way to create everything we need directly from sunlight without needing to use other living organisms, that would, I think, be a great and moral gift to the world.
Which brings us to the second plant-derived medium, organics. These form the "food" leg of the energy pathway, and are another chemical fuel critical to the immediate functioning of living organisms. We obtain these compounds in one of two ways: By directly eating plants, or by eating animals that eat the plants. The former (i.e., agriculture) is an energy-intensive, time-consuming, and sometimes catastrophically unreliable process, and the latter is even more so, in addition to being morally problematic.
If the weather doesn't cooperate; if a disease should happen to strike the crops; if a pest should happen to swarm over the fields and eat the produce, the farmer may find himself anywhere from financially ruined to condemned to starvation, as well as all who depend on him. Similar vicissitudes apply to animal farming, with the giant addition of the fact that more or less sentient creatures are living in anguish and terror before a grisly death just to transmit those organic compounds to your table. It is also pertinent to note that the plant matter used to feed meat animals would nourish more people than the meat animals do, illustrating the amount of energy wasted by the time it reaches the consumer through the medium of other animals.
Now imagine if the potential of technology to efficiently generate oxygen directly from solar power was extended to more complex compounds, such as sugars, carbohydrates, and proteins. Imagine that we bypass plants and other animal species entirely in creating these substances, meaning that (a)we no longer have to depend on / parasite off of other organisms, and (b)we would no longer have to pay the compounded energy price of cultivating them and every organism they consume.
Without all of those energy transformations standing between the source (sunlight) and the objective (food), the process would likely be much more efficient, and vastly more reliable. Yes, getting the taste right would take a while longer, but ultimately it would be achievable to completely end mankind's predatory behavior on other animal species, and allow the flora and fauna of the world outside our developed areas to return to an undirected natural state. We could study it from a detached distance, and watch evolution occur before our eyes. Wildness would be reintroduced to the world, yet we would be there to enjoy it.
F. Biomatter Power Sources
Presently we come to Energy Pathway #4: Biofuel. Food and biofuel are roughly the same concept - defer solar energy through other organisms and then release it, either by consuming them ourselves or processing them in a "biofuel reactor" of some kind that also involves organic chemical processes. This is, in fact, a deferred form of eating: Rather than consuming the compounds ourselves (since some of them can't be digested by humans) and doing physical labor, we feed them into a "reactor" and the machinery does the work. Examples include corn ethanol, cellulosic (wood) ethanol, algae, and various composting processes.
Without going into the current efficiencies of these technologies, they would naturally share the same issues and energy losses outlined above with relying on plant farming for food. Quite ironically, it also appears that using agriculture to generate electricity may actually reduce the food supply by either consuming edible produce or retasking arable land toward production of energy crops. This is not merely robbing Peter to pay Paul, but robbing Peter to pay a debt to Paul when Paul owes Peter - food is a primary energy source because it directly powers our very existence, while electricity is a secondary energy source that powers our lifestyle. This means that biofuel may not only be wasteful in the same sense as relying on other organisms for food, but basically unsustainable as energy technologies. I feel strongly that biofuel is just a stopgap, only somewhat less destructive than the worst approaches.
Did someone say "worst approaches"? Energy Pathway #5: Fossil Fuels (FF). A fossil fuel is a biofuel that takes millions of years of slow chemical change to create, followed by costly and environmentally destructive extraction and refining processes. It is also worth noting the political consequences of the fact that FF is heavily concentrated in regions plagued by militant religious fundamentalism (Saudi Arabia...and Texas), empowering cultures mankind would do well to avoid allowing into positions of influence.
From a buyer's perspective, FF is pure gold: It is energy-dense, which means it's easy to transport and child's play to use. The kick of energy one gets is practically free compared to the immediate price - much like crack cocaine. Of course, that is only because the time required to make it is not reflected in the price - human beings arrived on the planet with ample supplies of petroleum just sitting there waiting for us to discover them. If the time-value of money were reflected in the price at the pump, I would call it likely that only governments could afford gasoline. It, and indeed all fossil fuel products, are perhaps the most inefficient energy technologies in existence when time is accounted for, and that does not even begin to address the costs of climate change.
Fossil fuels have the exact same energy pathway as biofuels, but with the addition of eons of pressure and organic percolation - time whose economic value simply cannot be estimated since mankind has not even existed that long. Forgetting efficiency, geopolitics, sustainability, scalability, climate change, and every other consideration, fossil fuels are embarrassing to mankind - like realizing that your father had bought your college degree with a generous endowment. Humanity has been living off the energy endowment of the dinosaurs beneath whose feet our rodent ancestors scurried in terror, and virtually all of our progress to date has come directly from that generous, unintentional gift. We are just now beginning to imagine standing on our own two feet.
G. Categorizing Applications
At this point we have articulated two categories of entry point for energy into the human system: Our own bodies, and power plants. Arguably the latter is a deferral of the former, but alteration of human physiology is one inevitability I am not interested in exploring in this diary, so instead we can simply ask "What do we actually do with the energy we produce?" We shall now trace the broad strokes of where the energy goes once it reaches the human sector of the ecosystem.
First and most immediately, the energy we directly consume goes into doing whatever we do - that much is tautological and obvious, so we can (thankfully) ignore the complexities of human metabolism in this discussion. That which results from power, however, has more specific and reducible uses that can be categorized thus: (1)Production, (2)Transportation, and (3)Communications.
Production is any activity utilizing power that results in a qualitative material change in something: This could include putting pieces together to form a larger product, injecting plastic into a mold, or running an automatic car wash, among a virtually infinite variety of activities. However the electricity is generated, it is then transmitted through conducting wires to electrically-powered machines, which use the energy to perform productive work.
The generation process has an efficiency, x; the transmission of the electrons over power lines carries another efficiency, y; in transforming the electricity into work, the local machinery has its own efficiency, z. Over the entire process, xyz (x times y times z) is how efficiently the productive work was accomplished (all three are less than 1, so their product is smaller than any one of them). If the work in question is just a sub-process required to do yet further productive work, z will include multiple compounded efficiencies and successive energy losses. Maximizing whole-process efficiency and minimizing energy loss is a major concern of green technology, and the defining principle of Thermoeconomics.
Transportation is, quite intuitively, any process whose main intended consequence is a change in the relative location of some material object or person. Since physical work is involved, the same formula can be applied here as to Production; indeed, since moving things is often literally more straightforward than producing them, Transportation will tend to have much better efficiencies, and will comprise most of the sub-processes involved in Production. For instance, a conveyor belt, a high-pressure ink jet, or a spinning drum would be Transporation, although together they might be producing something. Also included would be automobiles, aircraft, boats, space rockets, etc.
Communications is the purest and most efficient application of energy in human civilization, since the physical work required is minimal. Due to the vast and increasingly incomprehensible layers of abstraction created by mathematicians, computer engineers, and other experts in the field, trivial physical changes in electronic components can transmit awesome quantities of information, allowing human minds to connect with minimal recourse to Production or Transportation. This category's efficiency is also the easiest to measure, since information (and even its own uncertainty) is perfectly mathematically quantifiable. Moore's Law is a consequence of this purity, and is a glimpse into the future of the other categories once they too become strongly coupled to Thermoeconomics.
The transition to sustainable (and particularly solar) energy has major consequences for all three of these categories. In particular, the capacity for dirt-cheap, decentralized power generation makes it feasible to imagine power generation largely occurring on-site or near on-site for most production, transportation, and communications activities.
For Production and Communications, this would take the form of factories and server farms having on-site power plants, thus minimizing the distance the electricity must travel, the reliability issues with an extended power grid, and the costs of both building and maintaining that system, and thereby maximizing efficiency. For Transportation, this could mean either direct and immediate power - e.g., a maglev train with generators feeding into the system every few kilometers - or otherwise closely-spaced access, such as an electric car with ubiquitous battery-swap stations. Over time, these power applications may become massively networked, perhaps wirelessly, and a Total System Efficiency could be realistically calculated and improved.
H. A Matter of Special Urgency: Water
Even with all the other promises of sustainable energy, a major immediate obstacle to resource-security for mankind is our reliance on fresh water aquifers and runoff for our drinking supply. This, like Fossil Fuels, is a matter of too much past convenience endangering our future: We are accustomed to rain feeding our crops, filling our lakes and reservoirs, and swelling our rivers, and we have not yet developed the infrastructure to cope robustly with a sudden decline in the ease of availability. When drought strikes, all we can think to do is conserve and cut back on our use, but the moment the rains return we go right back to doing as we always have - trusting that fate had turned favorable, and doing nothing to change.
In lieu of permanent austerity that would be tantamount to conceding the very idea of progress, there is only one way forward: Stop praying for rain, and start getting water where there is plenty to be had. The surface of the Earth is overwhelmingly comprised of water - so much of it that the vast majority of terrestrial biomass lives beneath it. If every human being on Earth were chained together at the bottom of the ocean, we would not even raise the sea level a single meter. All that stops us is the relative convenience of having our water delivered by the sky courtesy of deferred solar energy, and the much higher cost of desalinizing and transporting sea water over land.
But guess what becomes feasible with cheap, sustainable energy? Mankind will be able to transition from its "divine welfare" water supplies to desalinized seawater, and will find it practical to transport it any distance with pipelines to wherever needed. The implications of such a capability cannot be overstated: Every square inch of land human beings chose to turn into a grassland, or a vineyard, or a golf course, or a city, would become that, and it would be completely sustainable. Some pieces of desert could be left intact for their beauty and unique grandeur, but in all likelihood the Sahara, the Gobi, the Mojave, the Atacama, would be swept away under tsunamis of life. Who really wants a desert, when you get right down to it? There would be complaints, but they would largely be ignored.
J. Albedo Neutrality
A point has been made in other green diaries that even with radical reductions in greenhouse gas emissions, human technology would still be pumping out massive amounts of heat into the atmosphere resulting in net increases in atmospheric temperature. This is a crucial point if the human species intends to chart a path of indefinite growth, since even a 100% sustainable energy infrastructure would still, if the question of heat is not addressed, result in catastrophe. Although there are many unknowns and vast complexities, the ultimate picture of unchecked heating is an Earth turned into a storm planet with worldwide cloud cover, perpetual hurricanes, and the gradual erosion of landmasses to below sea level - although that could take millennia.
Of course, were that to happen, human activity would long since have ceased, and solar flux would be insufficient to maintain high enough temperatures to keep it that way. Earth would, over eons, return to some recognizable form once erosive forces could no longer overwhelm continental uplift. We, however, would not even be fossils at that point - there would be no trace we ever existed.
However, there is a way to avoid this entirely: Albedo neutrality. The reason the Earth is heating is that we are releasing millions of years of solar energy stored in the form of fossil fuels over just a few decades, which thermally is having the same effect as increasing the power of the Sun by a small degree. This changes, however, with the development of a fully sustainable, mainly solar energy infrastructure: Most of our power would come directly from immediate solar flux, and the only thermal difference between a human civilization functioning on solar power and a planet without any civilization is net albedo (reflectivity).
Without industrial activity, the temperature of the Earth's atmosphere is determined by the reflective/absorptive profile of its natural atmospheric gases/aerosols, oceans, and land surfaces. As described above, some energy is reflected back into space by the atmosphere, some is absorbed by the atmosphere resulting in wind, some is absorbed by water causing evaporation, some is absorbed by the land and re-radiated at IR wavelengths, and some is reflected by the land (especially ice). Previous ice ages have seen runaway cooling when expanded ice caps reflected so much sunlight back into space that not enough was re-radiated in IR to be trapped by greenhouse gases - it stayed at higher wavelengths and left the atmosphere. Now, with shrinking ice caps, in addition to the large release of greenhouse gases and heat, runaway heating may take place even with sustainable energy.
All sustainable energy practices ultimately increase the net albedo of the Earth by absorbing and using more energy than the natural surface. Solar PV panels are the most obvious, as they are literally dark (some even black) materials that absorb a wide spectrum of solar energy. Once that energy is used, it is radiated at IR wavelengths, so this is a large spectrum of the solar flux shifted into wavelengths that are much easier for the atmosphere to retain. But there is a way around the net increase in heat: Compensate for it by increasing reflectivity. For instance, alongside a solar power field, one might build a field of mirrors that focus and direct sunlight back into space. One might even direct the energy into a laser that could most easily and efficiently penetrate the atmosphere - a scaled version of a cooling laser. We can speculate that cheap, down-scaled cooling lasers directed at the sky might be used on machinery in general in the future.
II. Approaching Kardashev I
Having reached this point, we can go no further without setting foot into the realm of pure speculation, but that is a step I am willing to take. I think my reasoning is not too difficult to follow a bit further. Let us review each of the ideas previously mentioned in passing:
1. A future where solar energy is the sole, direct power source.
2. Maximally efficient transmission of power and communications at the speed of light.
3. Most intermediate transformations and unnecessary energy losses cut out of the process.
4. Oxygen, food, water, and power all directly, efficiently, and reliably produced on a massive and infinitely scalable level.
5. Planetary albedo neutrality.
The implication of these stipulations is staggering: Mankind would no longer be part of the Earth's ecosystem. We would be a separate one - an isolated energy sink absorbing solar power and radiating just enough infrared heat to maintain a comfortable temperature, while sending the excess into space with laser cooling systems; sucking in seawater and recycling it practically forever; and bringing raw materials into our system that virtually never leave it. With our vast energy supplies, ever-greater improvements in recycling and environmental control would be possible; the relative efficiency of our energy usage would become the driving force of all economics, if not the direct future embodiment of money, right up to the that point energy becomes too cheap to restrain growth in any significant way.
Once the situation got that weird, the only growth-limiting factors would be (a)human population, which tends to expand when people see a benefit to it, and (b)raw materials (i.e., metals). While the latter seem scarce at present, due mainly to the energy cost of extracting and processing them, they would rapidly cease to be so once their absolute availability becomes the only determinant and energy becomes a trivial matter. The amount of accessible metals present in the Earth's crust is far beyond anything mankind could realistically exhaust, and there is far more than that in the rest of the solar system.
And this raises an interesting and auspicious point: Solar power not only works in space, but works even better than it does on Earth, and there is plenty of silicon out there. In fact, we would not likely be limited to the terrestrial surface either in generating power for societies on the surface, or in finding raw materials for the physical infrastructure. Space-Based Solar Power - once the basic technology had been mastered - could scale up without bound, because in space (far enough away from Earth to avoid significant tidal effects) there really is no difference between knowing how to build a 5 km solar array or a 500 km solar array.
Soviet astronomer Nikolai Kardashev had proposed a classification scheme for civilizations based on their energy output: A Type I civilization utilizes the equivalent of all the readily available energy of a planet; a Type II civilization uses all the energy of its solar system; a type III civilization uses the energy of a galaxy; he did not bother to imagine a Type IV, since it would be truly pointless to classify such a civilization. To date, mankind has merited only fractional classification, but with the achievement of sustainability the pathway to Kardashev I is direct. As, staggeringly, is the pathway to Kardashev II, but I will be merciful and spare the normals my geekish fawning at the thought of Dyson spheres - power-generating shells completely encompassing the Sun at arbitrary distances. Either way, full sustainability implies that human evolution becomes just another field of engineering.
III. Meta: A Dimly Glimpsed Infinity
Imagining so far ahead, many thoughts, images, and notions occur to me that are difficult to articulate. My somewhat bizarre intuition is that, once the economic behavior of mankind is tied directly to the Sun, the large-scale systems and movements of civilization may begin to resemble plant organisms over the far long-term. Life is, after all, fractal in nature.
With Total System Efficiency and lightspeed communication, there will come to be recognized an ideal efficiency and a point of diminishing returns where it becomes more efficient for humanity to fission down diverse and multiple pathways, possibly even to speciate. Plants do this - they do not, for the most part, prey on each other. This would, I think, involve a physical parting of ways, though just how far apart, who knows. Mankind will found not merely new species, but entirely new ecosystems will be born from our civilizations. Whatever it is we become, it will be strange and know nothing of the primordial limits we still struggle with - its only boundaries will be dynamic, ephemeral, and forever receding with time. Language, government, basic human relationships, all of these things will be fundamentally changed by this amazing transition we see occurring around us, and the truly awesome thing to realize is that there is no end to this process - either we go extinct from starting too late, or it goes on forever and we become unimaginably alien.
* The only practical exceptions to solar energy pathways are geothermal, tidal, and nuclear energy, but there are theoretical maxima to their exploitation far short of global needs, and (far more importantly) their rate of growth toward those maxima is limited by the up-front marginal costs of each new plant. While geothermal is vaguely related to solar power by the fact that it is driven by gravitational collapse, it does not create fusion and the energies released are nowhere near as abundant even directly on the terrestrial surface. [Update: zeke L reminds me that geothermal is largely due to the decay of radioactives in the Earth's core.]
** Coriolis effects also contribute to wind, but we can consider them relatives of geothermal, since they result from planetary rotation - a consequence of gravitational collapse.