Sunday, February 22, 2009

A new way to look at efficiency

With all the discussion of renewable energy we hear these days, we repeatedly hear how inefficient many of these energy conversion processes are. The real question is how important are these numbers and should we be looking at efficiency in a different manner?

Let us consider the case of solar power. Our very own Dr. Webber commented, in a lecture on Renewable Power, that solar has “lower efficiency than power plants”. It is safe to assume that he is referring to the ability to convert the suns radiation to electricity via photovoltaic converters or concentrated solar power systems (not solar thermal energy conversion). And of course the statement is not inaccurate; the energy out of these systems divided by the incident energy is less than many other forms of energy conversion. But of what significance is this comparison? I would argue that efficiencies of these sorts have no relevance when comparing energy conversion processes of different types. To abuse an overused phrase, it is like comparing apples and oranges. The maximum theoretical efficiency that a photovoltaic conversion process can achieve (assuming a band-gap of 1.1 eV from the sun with incident radiation of 1000 W/m^2) is 43% (Da Rosa 2005). Does this mean that we should not consider solar power as a vital part to our future energy portfolio? I think not, instead it means that we should compare PV cells to the maximum theoretical efficiency. Simply stating that the solar to electrical is not as good as a traditional method is the argument not the correct argument.

So how should we compare energy conversion processes? That is a difficult question to answer because many of the effects of energy conversion are difficult to quantify. For example we can quantify the kWh per dollar, kWh per lb CO2, or kWh per square meter, but those don’t accurately describe the entire picture. In my opinion we must attempt to compare “Gala” to “Granny Smith” apples or at least oranges to tangerines. For example when comparing a natural gas fired Rankine cycle power plant to a natural gas driven Brayton cycle plant thermodynamic efficiencies, cost, and emissions output are an accurate ways to compare the systems. Renewable energy conversion processes should be compared to other process that require the same basic inputs, for example, wind and solar energy conversion. These two energy conversion processes both require large amount of land, so doesn’t it make since to compare them on that basis. We should evaluate the power output per unit area available as compared to the traditional efficiencies of these processes.

In conclusion I hope that as we encounter quoted efficiencies of energy conversion processes that we search for a more meaningful comparisons.


Da Rosa, A.V. (2005). Fundamentals of Renewable Energy Processes. New York: Elsevier Academic Press.

Webber, M. (Speaker). (2009, February 12). Renewable Power. Austin: University of Texas at Austin.


David Wogan said...

Conversion efficiencies become very important when you look at replacing many quads of energy. The great thing about traditional fuels is they are relatively energy dense. However, energy sources from wind or the sun are less dense. Couple the lower density with low conversion efficiencies and it becomes clear that we'll need a lot of these resources to replace even a fraction of energy consumed today. You make a good point though in arguing that lower conversion efficiencies shouldn't discourage their use.

The da Rosa book is very good, by the way.

John Saville said...

I agree with you that energy densities are a very important metric by which we can measure a energy source but does it make sense to measure these denisities in the same way. For example we take the energy density of coal to be number of btu's per unit mass. In the case of burning coal all the energy in that coal is released into the energy conversion process. Then that energy is converted using whatever processes necessary to achieve the desired
end result, and overall process efficiency can be calculated. Let's comepare that to the pv solar panel. Photons from the sun hit the surface. Energy density is measured in units of incident energy per unit area. The problem here is that, depending on band gap width of semiconductor material, only a certian fraction of those incident photons are able to displace an electron in the semiconductor and generate electrical current. I would equate this to measuring the numer of units of useful energy per unit mass of material removed in a coal mining operation rather than units of energy per unit mass of the final product. If you were to use a method where you measured efficiency of a solar pannel with respect to the useful incident energy you would find that many of the solar panels in production now are really quite efficient.