Fuel efficiency isn't. I mean that in the existential sense, as in, fuel efficiency doesn't exist. Well, that's a bit strong. What I really mean is that people usually mean something quite different from "efficiency" when they talk about "fuel efficiency". Other common terms that have a similar issue are "efficient buildings", "efficient windows", and "efficient lightbulbs".
The problem with all of the aforementioned concepts lies in the definition of efficiency. Efficiency is a dimensionless quantity expressing the ratio of what you get out of a process to what you put in. It's a number with no units -- feet, meters, gallons, Joules, etc. -- attached. So, for example, a value expressed in miles per gallon can not be measure of efficiency by definition. If you have a value with the same units on top and bottom, you might have a valid efficiency -- (energy out)/(energy in), which defines energy efficiency, is what most people are aiming for when they talk about efficiency.
Many of the quantities frequently referred to as efficiency -- especially "fuel efficiency" -- are actually measures of fuel economy. The Toyota Prius is not more fuel efficient or energy efficient than most other vehicles with an internal combustion engine, but it does have higher fuel economy. The difference is not trivial. The Prius's engine is not able to get more energy out of a gallon of gasoline than a gargantuan SUV like the Ford Expedition, but it is able to travel farther on that gallon of gasoline. It does this not by being more efficient -- their energy efficiency is comparable -- it does it by being much smaller and lighter weight, by turning off the gasoline engine off when stopped at a traffic light, and by re-capturing energy that is normally lost as heat during the braking process. None of that alters its energy efficiency, but all of it increases fuel economy.
In a similar vein, it's incorrect to say that one building is more energy efficient than another. Even worse is to say "this is an efficient building." How would you measure the energy efficiency of a building? That is, what would you measure to determine the ratio of energy out to energy in for a building? You could look at the efficiency of the furnace, perhaps, but all that will tell you is how good the furnace is at extracting heat from fuel. It will tell you very little about how much energy the building will require to maintain a comfortable temperature, comfortable illumination, comfortable noise level, comfortable humidity, etc. Those are most likely the type of things you're really interested in when you speak about the "efficiency" of the building.
To determine how much energy the building will require to maintain a comfortable temperature, you'd need to look at properties such as the size and shape of the building, the mass of the building, the amount of energy going in and out through windows and skylights, the tightness of the building's envelope, the amount of insulation in walls and ceiling, the average number of people in the building, the frequency of opening and closing doors and windows, the use pattern of the building, etc. You'd also need to know the energy efficiency of the furnace, but that's pretty far down the line.
All of those variables will lead you to a measure of the building's fuel economy. It's a different fuel economy with different units from the important one for motor vehicles (miles/gallon), but it's still a measure of fuel economy.
"Okay, okay," you might say, "I understand your point that what I'm calling energy efficiency is really fuel economy, but what does it matter? I mean, if I say building A is more efficient than building B, you know what I mean, right?" No, I don't know what you mean. Do you mean building A uses less energy than building B? If so, that could be because building A is smaller, or is used less frequently, or is used differently from building B, or any number of things. Using "efficiency" to describe this situation is like claiming that bicycle A is quieter than bicycle B because bicycle A is "more purple." The two things might be linked, but there are probably many other more important factors causing the difference.
Perhaps instead you mean that building A and building B are roughly equivalent in function and number of people served, but building A uses less energy. Again, that could be due to a number of different factors. Perhaps building A is located in Hawaii and building B is located in Alaska. All other things being equal, which will require more energy to maintain a comfortable temperature?
Perhaps you mean that building A and building B are roughly equivalent in function and number of people served, their external environments are comparable, and their internal environments are kept at roughly the same level of comfort, but building A still uses less energy. Now we're talking! Except we're definitely not talking about efficiency, we're talking about fuel economy. The definition of fuel economy implies all that other stuff about making sure we're comparing apples to apples. Efficiency does not.
Oh, and back to the examples from the very beginning: when people talk about "lightbulb efficiency", they usually mean luminous efficacy, which is a measure of light output in the visible light spectrum per energy input. When they talk about "window efficiency" they probably mean some combination of about a half-dozen metrics, only three of which, visible transmittance, emissivity, and solar heat gain coefficient, could reasonably be called an efficiency. Very likely they weren't thinking about any of those properties, and were instead thinking mostly about the thermal conductivity of the window, and possibly the closed air leakage rate.
If in doubt, what should you say instead of "efficiency"? I'd suggest just stating the parameters you're actually interested in rather than trying to sum it up with an ill-used catch-all term. If you want to know which window will keep heat in better, just ask that. Don't ask about its efficiency. For motor vehicles, get used to the term fuel economy. It's the correct term, it has a much more useful and obvious meaning than efficiency, and it's easier to spell.
No comments:
Post a Comment