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Energy Density: The Advantage of Solar Photovoltaics

02.11.2017 — 0

In 2015, Oxford University Press published a book by Michael E. Mackay called *Solar Energy: An Introduction. *The title is full of valuable insights, one of them being the idea of energy density comparisons.

As Mackay writes, quite rightly, besides governmental subsidies and support policies, energy technology have been aided as they develop their inherent technology and become increasingly more efficient, as well as to develop the infrastructure required for widespread use.

If any given technology is to succeed in a market that has robust technologies already in place, with an infrastructure which is already developed, then some help will be required.”

**Picture credit: **https://www.amazon.com/Solar-Energy-Introduction-Michael-Mackay/dp/0199652112

In particular, research and development effort will be needed to optimize the production and use of solar energy. While in the U.S. such research was curtailed in the 1980s as a result of a policy decision, terrestrial solar energy has become the main topic after 2000 and has continued to grow. We have written about it before as well. Mackay certainly has a point when he writes that while scientific discoveries are needed, good engineering as well as manufacturing expertise must be pursued to ensure the success and integration of solar into energy industry. When it comes to engineering, however, some specific measurements need to be taken into account. One of them is energy density.

What is the power density, in terms of energy produced per unit area, of various energy technologies?

Consider biofuels first. Here one can measure energy density in terms of how much energy can be derived from each acre of crop that can be fermented into bioethanol. Corn is a very efficient crop to produce ethanol. Corn can be produced to yield 150 bushels per acre and 2.5 gallons of ethanol can be obtained from a bushel of corn. So, one can obtain 375 gallons of ethanol per acre. The energy content of ethanol is 89 MJ/gallon and if we assume there is one crop per year then the energy density is only 0.25 W/m2, quite low.

**Corn-based ethanol production. Picture credit: **https://www.e-education.psu.edu/egee439/node/673

Compare this to the areal power density supplied by the Sun. Here power density can be defined as the number of watts generated per unit area. The Sun can produce much more power per unit area, even at a conservative estimate of 500 W/m2, and dividing by two to account for day–night cycles, one has 250 W/m2. Assuming the solar device is only 10% efficient then there is 25 W/m2 available and this power is 100 times more dense than for bioethanol.

Similar measures can be artificially constructed to compare biofuel, solar, and other power generation technologies such as petroleum, natural gas, and coal. Mackay supplies the reader with the following table, where the contemporary energy consumption in a year for the USA was used to find a power and was then divided the area of the continental USA (8.08×1012 m2) to determine the power density.

**Areal power density of various energy technologies. Adopted from Mackay M.E. Solar Energy: An Introduction. Oxford University Press, 2015. P. 10.

Looking at this table, Mackay arrives at the following conclusion:

“This is an interesting comparison of power densities as none approach that of the Sun. If you are an engineer, and you were to start an energy infrastructure, which technology would you choose? Clearly the answer is solar energy since it is of order one-hundred times more dense (at today’s usage level) than the others. Even if we increased the use of petroleum, natural gas and coal by a factor of ten (a chilling thought in terms of CO2 emissions) their density is still a fraction of solar energy’s. This calculation shows that even at 10% efficiency the Sun can produce a lot of energy.”

The other obvious and well-known advantage of solar photovoltaics is that it is capable of producing the direct current (DC). Since digital devices ultimately use low voltage DC (LVDC) power, in a contemporary house full of smartphones, laptops, cameras and other appliances, AC is passed through a transformer-rectifier that converts it into DC. In that sense, photovoltaics-generated energy can be used directly at home to power digital devices at a reduced cost.

On the contrary, coal, nuclear energy and natural gas produce heat that boils water to generate high pressure steam that turns a turbine. The latter is connected to a generator that has a shaft with permanent magnets on it that rotates within wound wires, so that the rotating magnetic field produces AC within the wires. That’s electrical power is generated today for the most part. Even solar photovoltaic plants have to use inverters in order to be able to sell electricity to the grid.

However, this is clearly much less sustainable way of generating power, which means that even the World Wide Web is not a green technology after all! Solar PV could make it greener by powering not only digital devices, but also the data centers and servers to which the Internet infrastructure is tied.

In other words, it is not only because sustainability has become a buzzword in the recent years, that solar energy growth worldwide. In addition to be cost-competitive with traditional power sources, it is also a fitting engineering option in many contexts. Stay tuned to learn more and contribute to solar energy development — with Solar DAO.

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