Introduction
Our present society, technology and welfare have in the 20th century been built up with fossil fuels as the energy source. Fossil fuels are ‘solar stocks’, stored solar energy of hundreds of millions of years. Fossil fuels can in fact be seen as nature’s brilliant technology for very effective and durable energy storage.
The fabric and nature of today’s society appear to be closely interconnected with the properties of these solar stocks. This implies that without solar stocks, without fossil fuels, society as we know it cannot be sustained.

Solar stocks in today’s society
Let us look at the characteristics of solar stocks (fossil fuels) and of today’s society. Fossil fuels are presently abundantly available, reliable, tunable to demand, compact, easily transportable and easily storable. Much of this is due to their extremely high volumetric energy density of some 35 gigajoule per cubic meter (in liquid or solid form). This extremely high value allows for a full passenger car weighing 2000 kg to travel 1000 km at 120 km/h on 50 liters of gas or diesel. It also allows for The Netherlands to acquire and store enough natural gas to last a whole winter.
These examples illustrate the characteristics and demands of today’s society: Continuity, reliability, cost and time effectiveness, and comfort for all people and businesses. Fossil fuels answer well to these demands, and the reverse appears to be true as well: Availability creates demand, and our modern demands have been developed on the basis of fossil fuel availability and characteristics. This close interconnection between today’s societal demands and fossil fuel characteristics is crucial for future energy policies.

Solar flows in today’s society
Solar flows, i.e. energy that the sun deposits on earth real time today and tomorrow, can be exploited by various means: Wind turbines, solar PV panels, solar thermal panels, hydropower and biomass. Other energy modalities, such as geothermal, tidal and nuclear, are not based on solar flows but originate from within the earth.
Present energy policies are mainly based on replacing solar stocks by solar flows: Fossil fuels are out, wind, solar, biomass and hydropower are in. Nuclear is controversial and in any case not sufficient, and geothermal and tidal are too geographically limited on a global scale. The fact that present energy policies are mainly based on solar flows is very remarkable, as the characteristics of solar flows poorly match today’s societal demands (continuity, reliability, cost and time effectiveness, and comfort for all people and businesses).

Characteristics of solar flows
Wind and sun operate anywhere on earth, but are intermittent and unpredictable on an hourly and daily basis. This very nature of wind and solar energy is incompatible with the societal demands of continuity and reliability. Hydropower and biomass on the other hand perform better on these demands, but fail on geographic limitations and environmental impact: Hydropower requires large height differences in specific geographic circumstances, and biomass costs far too much natural land space and biodiversity.
In addition, all solar flows lack the prime property of solar stocks: High volumetric energy density. A cubic meter of moving air or water, or a square meter of solar radiation, contains less than 1% of the energy contained in a cubic meter of liquid or solid fossil fuel. That is the reason why solar flows require so much space and materials to be exploited. The much-used argument that solar flows are abundant is as true as it is irrelevant. Abundance is not the critical success factor. Energy density, continuity, reliability and cost effectiveness are the critical success factors for our present society. In other words, it is not the amount of energy that counts, but what is needed to harness and utilize it. The latter is largely determined by energy density.

Scientific analysis
The Technical University of Eindhoven has correlated the energy density of our energy sources to the evolution of our civilization. Over thousands of years, we moved from human power (galley slaves and porters) to animal power (oxes and horses), wind power (galleons and windmills), hydropower (watermills and reservoirs), fossil fuels (coal, oil and gas) and nuclear power (uranium and plutonium). With each step, the energy density and thereby our societal development increased more than tenfold. A horse replaces more than ten humans, powering ploughs and carriages. A windmill replaces more than ten horses, powering land reclamation from the sea. A fossil fuel plant replaces hundreds of windmills, powering households, public transportation and industries. And one ton of nuclear fuel replaces more than 50,000 tons of fossil fuel, reducing energy waste streams to very small volumes with low environmental impact.

The essence of energy density
The intrinsic energy density of solar stocks and solar flows is a physical characteristic which we cannot influence by any technology or innovation. We can for example concentrate solar light into one spot with a magnifying glass or mirror array, but the total space required to harvest a certain amount of solar energy cannot be decreased. The mirror array of a concentrated solar power plant for example must have a similar surface area as a solar panel array producing the same amount of power. We can also concentrate the potential energy of an entire river basin into one location by means of a hydropower reservoir. But the surface area of such a reservoir must be very large, thereby destroying part of the natural basin.

Low density solar flows in a high density society

This implies that the intrinsic energy density – i.e. the original energy content per square or cubic meter – of an energy source is directly correlated to its potential to sustain our present technology-based and energy-dense society. Our cities, industries, agriculture, transportation and data centers require large amounts of power and energy in a small space, and this cannot effectively be done with low-density solar flows that require vast amounts of space (and also vast amounts of materials, often rare materials like lithium, cobalt, copper and neodymium). Solar flow exploitation – by solar panels, wind turbines, hydropower plants and biomass farms – cannot be miniaturized like electronics, because in energy the limitations are physical rather than technological. Trying to miniaturize solar flow exploitation is like constructing a perpetuum mobile or making apples fall upward: It is physically impossible, and it can be shown to be impossible. See also professor Simon Michaux’s compelling analysis on this: Assoc Prof Simon Michaux – The quantity of metals required to manufacture just one generation of… – YouTube and https://www.youtube.com/watch?v=pwmygkdoGgc

Conclusion
My first book, The Green Illusion, explains the impossibility to base today’s society on solar flows in detail, with quantitative calculations, examples and illustrations. My second book The Green Opportunity shows that reducing our energy consumption is much more compelling and effective than replacing solar stocks by increasing solar flow capacity. One may argue endlessly about this, although the facts and figures speak very much for themselves. It would in any case be very unwise not to seriously consider the scenario that today’s society can indeed not be sustained without fossil fuels. Present national and international energy policies lack this serious consideration.