In Praise of the Sun - Can Solar Energy Solve Our Problems

Clean technology, especially energy generation, has generated (pun intended) an enormous amount of interest lately. Using one simple (and admittedly simplistic) metric, VCs invested almost $5BN in green/clean tech in 2009, out of a total of just under $18BN, or over 25%. Given that most VC investments involve uncertainty whether or not a market will adopt new technologies or services, but not whether or not they will work in the first place, while clean/green is often a question as to whether or not it will even work, that is an astounding number. Further, many green/clean firms explicitly depend on government subsidies to survive, where such subsidies and policies are fickle at best. If a "weighted" approach were taken, weighing in other normal factors - the same way that sales in retail are seasonably adjusted - then the percentages would be significantly higher.

A major player in this approach is solar. Solar energy panel creators, investors, installers, etc. are endless, and it appears everyone is getting into the business. Solar looks and feels nice: it is, literally, clean, generates no polluting byproducts (except in the creation of solar panels themselves), and the Sun is available nearly everywhere on the planet. In a recent discussion, the questions as to whether or not the Sun could ever generate all or the majority of energy requirements for the human race. I decided to perform some basic research and make it available here.

In order to simplify the issue, we need to understand several issues.

First, there are actually two parts to energy: generation and location.

• Generation: This is how you turn latent (or potential) energy into actual energy that can be used. Examples include solar cells in Arizona converting the Sun's rays into electricity; a hydro-electric dam in Beaumont, Quebec, converting the flow of water (mechanical energy) into electricity; the Indian Point nuclear reactor in New York converting the energy connecting the protons and neutrons of a uranium atom into electricity; and, the classic example, an internal combustion engine in a car converting the chemical energy connecting atoms and molecules in petroleum into mechanical energy to move the car.
• Location: Location is getting the generated energy to the point of usages. If we generate energy in Beaumont, but need it in Albany, it is useless unless we can transport it from Beaumont to Albany. Similarly, we may have a wonderful solar field in Arizona, but it is useless if it cannot be somehow stored and transported to an Acela express train currently speeding from Washington, DC, to Boston.

For the sake of simplicity, we will completely ignore location. In other words, we will assume that any energy we generate - in the case of solar, almost exclusively electricity - can be properly stored for extended periods, and properly transported. Somehow, the electricity from the Texas solar field will make its way to a battery in a car that will travel from London to Manchester.

Second, the amount of solar energy hitting the earth is not the same at every location, nor is it the same at all times of day or year. For the sake of simplicity, we will assume that every location on the earth will receive 12 full hours of Sun, and equatorial (i.e. maximum) Sun at that, every single day. We are ignoring clouds, rain, wind, haze, sunrise/sunset, elevation/azimuth, north/south, and all those other pesky factors that reduce the amount of solar energy hitting a single spot, and assuming we are simply blessed with the most we can get.

Third, we are going to assume that our solar cells are 100% efficient. In other words, our magical solar cells can capture every single joule of energy that the Sun provides to a given spot. Obviously, this is a very big fiction: nothing engineered is ever 100% efficient, and current commercial solar cells technology is around 10-12%, expensive commercial technology is ~22%, some research equipment has gotten into the 40%+ range. Obviously, produced, commercially viable, high-efficiency cells are a very long way off. Nonetheless, we will ignore all of these factors and assume we are 100% efficient.

Fourth, and last, we will ignore commercial viability. We do not care if our solar fields can operate at a profit or break even, or even at a massive loss. We only care whether or not it is scientifically feasible to capture enough solar energy to power civilization's needs.

What Are Our Needs?

Surprisingly, this is a difficult question. To simplify it, I will focus solely on the United States. The US is a large, continent-wide country, with a population of ~300MM and a surface area of 9.8MM sq km. It is an advanced Western society, with high energy needs. Most countries in the world - politics notwithstanding - aim to emulate at least the United States' economic position. According to Lawrence Livermore National Laboratory, US energy usage in 2008 was a total of 99.2 quadrillion (or quad) BTUs (British Thermal Units).

How Much Sun?

How much energy is actually hidden in those rays of light (actually photons) that hit the earth? The question depends on whether we are looking at the surface of the earth, or the edge of the atmosphere, a great distance up. According to NASA and most scientists nowadays, energy making it to the edge of the atmosphere, is 1.368 kW / sq m. For every square metre of atmospheric surface, 1.368 kilowatts of energy - of all forms of radiation - bombard it. About 30% of that is reflected by the atmosphere itself, so about 958 watts of energy per square metre make it into the atmosphere. However, another 20% is lost to clouds, atmospheric obstruction, etc., so about 51% (NASA) reaches the earth's surface. Thus, with perfect 100% efficiency, no clouds in the way, no thinking about angles of the Sun and other effects, 697 W/sq M reach the earth's surface. One Watt is 1 joule / second, so, over an entire year with 31.5MM seconds, with perfect conditions, 22MM joules of energy hit a square meter. We already saw that the US used 99.2 quads of energy in 2008. Each quad BTU is 1.06 quadrillion kilojoules, so the US used 105 quadrillion kilojoules. Since each square metre produces, under perfect conditions, 22MM joules, we would need 4,776 MM square metres, or 4,776 square kilometres, of surface to power the United States. According to the CIA Factbook, the US has a surface area of 9.8MM sq km. In other words, under perfect conditions, the US would need to cover less than 1% of its surface area to supply its energy needs.

This looks very promising... until we start to account for efficiencies, weather, Sun declination, and all the other factors. Realistically speaking, the US, even with 50% efficiency solar cells, is unlikely to capture more than 5% of the solar radiation hitting its surface. The Sun is not directly overhead all day, anywhere; much of the US is northern and thus subject to much sharper angles of the Sun in the winter; large river and lake concentrations, as well as mountainous and populated areas, make placing solar cells impractical. Economics, of course, always enter the picture. Given this fact, we must multiple by 20 the surface area that would need to be covered. With perfect energy efficiency cells, the US would need to cover ~1% of its surface area. Suddenly, it is not that simple. Now, we return to dealing with today's cells. Given the maximum (and very expensive) efficiencies of 22%, with these cells, the US would need to cover 4.4% of its surface area. That is a lot of land, larger than all of California!

Summary

In sum, given today's best technology (which will change), and the weather (whose uncertainty, if the specific characteristics, will not), the US would need to cover an area the size of California to supply its energy needs, even before considering challenges of storage, transportation and cost-effectiveness. As much as I would like to be bullish about solar, I believe solar will remain a niche service, at least for the foreseeable future.