It seems to be popular nowadays to maintain that photovoltaic energy is just an "extension" of fossil energy and that it will fade away soon after we run out of fossils fuels. But photovoltaics is much more than just a spinoff of fossil energy, it is a major metabolic revolution in the ecosystem, potentially able to create a "stereosphere" analogous to the "biosphere" that could last as long as the remaining lifetime of the earth's ecosystem and possibly much more. Here are some reflections of mine, not meant to be the last word on the subject, but part of an ongoing study that I am performing. You can find more on a similar subject in a paper of mine on Biophysical Economics and Resource Quality, (BERQ)
"Life is nothing but an electron looking for a place to rest," is a sentence attributed to Albert Szent-Györgyi. It is true: the basis of organic life as we know it is the result of the energy flow generated by photosynthesis. Sunlight promotes an electron to a high energy state in the molecule of chlorophyll. Then, the excited electron comes to rest when a CO2 molecule reacts with hydrogen stripped away from an H2O molecule in order to form the organic molecules that are the basis of biological organisms. That includes replacing degraded chlorophyll molecules and the chloroplasts that contain them with new ones. The cycle is called "metabolism" and it has been going on for billions of years on the earth's surface. It will keep going as long as there is sunlight to power it and there are nutrients that can be extracted from the environment.
But, if life means using light to excite an electron to a higher energy state, there follows that chlorophyll is not the only entity that can do that. In the figure at the beginning of this post, you see the solid state equivalent of a chlorophyll molecule: a silicon-based photovoltaic cell. It promotes an electron to a higher energy state; then this electron finds rest after having dissipated its potential by means of chemical reactions or physical processes. That includes using the potentials generated to manufacturing new photovoltaic cells and the related structures to replace the degraded ones. In analogy with the biological metabolism, we could call this process "solid state metabolism". Then, the similarities between the carbon-based metabolic chain and the silicon-based one are many. So much that we could coin the term "stereosphere" (from the Greek term meaning "solid.") as the solid-state equivalent of the well known "biosphere". Both the biosphere and the stereosphere use solar light as the energy potential necessary to keep the metabolic cycle going and they build-up metabolic structures using nutrients taken from the earth's surface environment.
The main nutrient for the biosphere is CO2, taken from the atmosphere, while the stereosphere consumes SiO2, taking it from the geosphere. Both metabolic chains use a variety of other nutrients: the stereosphere can reduce the oxides of metals such as aluminum, iron, and titanium, and use them as structural or functional elements in their metallic form; whereas the biosphere can only use carbon polymers. The biosphere stores information mostly in specialized carbon-based molecules called deoxyribonucleic acids (DNA). The stereosphere stores it mostly in silicon-based components called "transistors". Mechanical enactors are called "muscles" in the biosphere and are based on protein filaments that contract as a consequence of changing chemical potentials. The equivalent mechanical elements in the stereosphere are called "motors" and are based on the effects of magnetic fields on metallic elements. For each element of one of these systems, it is possible to find a functional equivalent of the other, even though their composition and mechanisms of operation are normally completely different.
A major difference in the two systems is that the biosphere is based on microscopic self-reproducing cells. The stereosphere, instead, has no recognizable cells and the smallest self-reproducing unit is something that could be defined as the "self-reproducing solar plant factory." A factory that can build not only solar plants but also new solar plant factories. Obviously, such an entity includes a variety of subsystems for mining, refining, transporting, processing, assembling, etc. and it has to be very large. Today, all these elements are embedded in the system called the "industrial system." (also definable as the "technosphere"). This system is powered, at present, mainly by fossil fuels but, in the future, it would be transformed into something fully powered by the dissipation of solar energy potentials. This is possible as long as the flow of energy generated by the system is as large or larger than the energy necessary to power the metabolic cycle. This requirement appears to be amply fulfilled by current photovoltaic technologies (and other renewable ones).
A crucial question for all metabolic processes is whether the supply of nutrients (i.e. minerals) can be maintained for a long time. About the biosphere, evidently, that's the case: the geological cycles that reform the necessary nutrients are part of the concept of "Gaia", the homeostatic system that has kept the biosphere alive for nearly four billion years. About the stereosphere, most of the necessary nutrients are abundant in the earth's crust (silicon and aluminum being the main ones) and easily recoverable and recyclable if sufficient energy is available. Of course, the stereosphere will also need other metals, several of which are rare in the earth's crust, but the same requirement has not prevented the biosphere from persisting for billions of years. The geosphere can recycle chemical elements by natural processes, provided that they are not consumed at an excessively fast rate. This is an obviously complex issue and we cannot exclude that the cost of recovering some rare element will turn out to be a fundamental obstacle to the diffusion of the stereosphere. At the same time, however, there is no evidence that this will be the case.
So, can the stereosphere expand on the earth's surface and become a large and long-lasting metabolic cycle? In principle, yes, but we should take into account a major obstacle that could prevent this evolution to occur. It is the "Allee effect" well known for the biosphere and that, by similarity, should be valid for the stereosphere as well. The idea of the Allee effect is that there exists a minimum size for a biological population that allows it to be stable and recover from perturbations. Too few individuals may not have sufficient resources and reciprocal interactions to avoid extinction after a collapse. In the case of the stereosphere, the Allee effect means that there is a minimum size for the self-reproducing solar plant factory that will allow it to be self-sustaining and long-lasting. Have we reached the "tipping point" leading to this condition? At present, it is impossible to say, but we cannot exclude that it has been reached or that it will be reached before the depletion of fossil fuels will bring the collapse of the current industrial system.
The next question is whether a self-sustaining stereosphere can coexist with the organic biosphere. According to Gause's law, well known in biology, two different species cannot coexist in the same ecological niche; normally one of the two must go extinct or be marginalized. Solid state and photosynthetic systems are in competition with each other for solar light. There follows that the stereosphere could replace the biosphere if the efficiency of solid state transduction systems were to turn out higher than that of photosynthetic systems. But this is not obvious. PV cells today appear to be more efficient than photosynthetic plants in terms of the fraction of solar energy processed but we need to consider the whole life cycle of the systems and, at present, a reliable assessment is difficult. We should take into account, anyway, that solid state creatures don't need liquid water, don't need oxygen, are not limited to local nutrients, and can exist in a much wider range of temperatures than biological ones. It means that the stereosphere can expand to areas forbidden to the biosphere: dry deserts, mountaintops, polar deserts, and more. Silicon based creatures are also scarcely affected by ionizing radiation, so they can survive in space without problems. These considerations suggest that the stereosphere may occupy areas and volumes where it is not in direct competition with the biosphere.
The characteristics of the stereosphere also allow it the capability of surviving catastrophes that may deeply damage the biosphere and that will eventually cause its extinction. For instance, the stereosphere could survive an abrupt climate change (although not a "Venus Catastrophe" of the kind reported by James Hansen). Over the long run, in any case, the earth's biosphere is destined to be sterilized by the increasing intensity of the solar irradiation over times of the order of a billion years. (and smaller for multicellular organisms). The stereosphere would not be affected by this effect and could continue existing for the five billion of years in which the sun will remain in the main sequence. Possibly, it could persist for much longer, even after the complex transformations that would lead the sun to become a white dwarf. A white dwarf could, actually power PV systems perhaps for a trillion years!
A more detailed set of considerations of mine on a related subject can be found in this article on "Biophysical Economics and Resource Quality, BERQ).
1. I am not discussing here whether the possible emergence of the stereosphere is a good or a bad thing from the viewpoint of humankind. It could give us billions of years of prosperity or lead us to rapid extinction. It seems unlikely, anyway, that humans will choose whether they want to have it or not on the basis of rational arguments while they still have the power to decide something on the matter.
2. The concept of a terrestrial metabolic system called the stereosphere is not equivalent, and probably not even similar, to the idea of the "technological singularity" which supposes a very fast increase of artificial intelligence. The "self-reproducing solar plant factory" needs not be more intelligent than a bacterium; it just needs to store a blueprint of itself and instructions about replication. Intelligence is not necessarily useful for survival, as humans may well discover to their chagrin in the near future.
3. About the possibility of a photovoltaic-powered Dyson sphere around a white dwarf, see this article by Ibrahim Semiz and Salim O˘gur.
4. The idea of "silicon-based life" was popularized perhaps for the first time by Stanley Weinbaum who proposed his "Pyramid Monster" in his short story "A Martian Odissey" published in 1933. Weinbaum's clumsy monster could not exist in the real universe, but it was a remarkable insight, nevertheless.