Welcome to my blog! As I add to this blog, you’ll find answers to your questions about energy, especially answers about your energy. Why do your bills look the way they do? What are you really paying for? How can you reduce the amount of energy you’re using without it costing you more in the long run? You can read a little about me and the goals of this blog in the official “launch” post from August 12, but I want to focus on what you want to know, so feel free to send me questions and comments.
Today’s topic is going to be a lot like the first lecture of my first thermodynamics class in college – except I promise you don’t have to be an engineering major to understand it. In fact, if you were an engineering major, you can skip this one, because I’m going to put some gut-feel perspective on some of the terms used in the energy world, perspective you already have if you’re an engineer. It’s worthwhile to lay some groundwork so the words I use don’t just go “whoosh” over your head.
First of all … what is energy? I’m sure you have some understanding of it, but what is it really? As I think about it, it’s hard to find words to define energy without getting into some circular logic. For instance:
“Energy is the capacity to do work.”
“Work is the result of applying energy to produce a useful outcome.”
See what I mean? I had to go back to my sophomore Thermodynamics textbook to look for a good definition of energy … only to find this: “Although no simple definition can be given to the general term energy, E, except that it is the capacity to produce an effect, the various forms in which it appears can be defined with precision.[1]” So I don’t feel so bad, and you shouldn’t either. Let’s just say this about that: energy, work, and power are three basic concepts that occur throughout the field of “energy”, and they must be understood in how they relate to each other in order to be deliberate and purposeful in making decisions about your energy usage. So I’ll talk about their effects on each other, more than “what they are”.
Energy appears in many forms, and all matter contains at least one form of energy. There are even some forms of energy that don’t inhabit matter, for instance, light (although you could snarkily point out that photons have characteristics of both pure matter and pure energy). When we think of solar energy, it arrives to us in the form of light, which is a form of electromagnetic radiation. It can travel through the “void” of space – with no matter whatsoever to conduct it. Some of it we can see (visible light), and some of it we can’t (infrared and ultraviolet light, and all other ranges of the electromagnetic spectrum … and that’s all I’m going to say about it).
“Work” is what happens when you take some energy and do something with it. Obvious, but I needed to say it again. Another way to say it is that work is the net effect of converting one form of energy to another. A simple example: if I lift a one-pound weight from the three-foot shelf and place it on the five-foot shelf, the net amount of work I’ve done is two foot-pounds, abbreviated ft*lbf. The net amount of energy I used to do that work is something more than two foot-pounds, because I’m not perfectly efficient. I wasted some of the energy. But at least I got something done.
You start to see something here: energy and work can both be measured in the same units. In my example, the units were foot-pounds. There are many other units for energy and work commonly used in industry and trades, and we’ll get into some more of them here – which is the main reason for this post.
Energy (and work) have units of length * force; force, in turn, has units of length * mass / time2. OK, now we’re getting technical, but stay with me. Length, mass, and time are quantities or units that can’t be broken down any further; they are “fundamental quantities”, or “base units”. So, breaking energy down to base units,
E = mass * length2 / time2 (m * l2 / t2)
This might look vaguely familiar. It took Einstein to figure out that mass can be converted directly to energy, and the equivalence between mass and energy is the second most famous equation in physics:
E = m * C2
Yep, that equation. C is the speed of light, ~186,000 miles/second, or 299,792,458 meter/second (m/s). This amazing concept was proven beyond any doubt when we began harnessing the conversion of tiny amounts of mass to energy for useful work in nuclear reactors (and of course, nuclear weapons). But wait, I said it was the second-most-famous equation … what’s number one? It’s Isaac Newton’s Law, of course:
F = m * a
Force equals mass times acceleration. This is the equation that got me going on my engineering career while I was in high school physics class. My favorite science … but I digress. You can see that F and E are different only by one fundamental quantity: length. You can break energy down to base units using Newton’s Law this way:
E = F * length = (m * a) * l = m * l * l /( t * t) = m * l2 / t2
Remember that acceleration, a, has units of length / time2. Acceleration is the rate of increase of velocity; in British units, a is expressed in ft/s2.
One more basic concept: power. Power is a “rate of doing work”, or a rate of transferring (moving) energy. A “rate” is how fast you do something – it has another time factor in it. So, back to my example, if I’m asked by my boss to move fifty-five of the one-pound weights from the 3’ shelf to the 5’ shelf every second, the power I need is 55 * 2 = 110 ft*lbf/s, “foot-pounds per second”. The thing is, it’s hard to relate to foot-pounds when you start to get that many of them. In fact, my boss is so clueless, he forgot that last week I had an OSHA-reportable injury when I proved I could do just 10 ft*lbf/s. Yeah, I’m making it up just to keep you entertained. When the folks around Newton realized that ft*lbf/s was hard to “fathom”, they dreamed up another power unit: “horsepower”. Finally, something that some of you can really connect with – especially the gear-heads. Horsepower really caught on, and we still use it regularly today. Since you asked, one “horsepower” equals 550 ft*lbf/s. So, my boss was asking me to deliver 1/5 hp! Not gonna happen.
One important form of energy I didn’t mention yet: heat. Heat is the form of energy that we measure with “temperature”. OK, not a real definition … heat is the measure of the intensity of molecular and sub-molecular motion within a substance. Yes, even if it’s sitting still, a lump of (stone, clay, wood, whatever) is moving at the molecular and sub-atomic particle level. These motions or “vibrations” are somewhat random, but not really, because each particle moves in response to the forces exerted by the particles (and fields) around it. The more intense (faster, higher amplitudes) the motion, the higher the “internal” energy, and we can measure it with a thermometer. In general, if you have two pieces of the same substance with the same mass, and they have the same temperature, they have the same internal heat energy.
Key fact about heat (and other forms of energy) – it tends to move from areas where it is more highly concentrated to areas where it is less concentrated. Heat moves from high temperature to low temperature. Electric charge moves from high voltage to low voltage. This happens if there is a path for the energy to move along. For heat, if there is any matter in between points A and B, it can move, or “conduct”, through that matter from point A to point B. Even if there is no matter between, it can move from A to B via thermal radiation IF A can “see” B – if there is a direct line-of-sight or a path of reflectors.
So, to get back on track, the real point of this post is that different industries use different terms and different measures for quantities that could be expressed in the same measures. There are many different units used for energy (work) and power, depending on the industry and on the part of the world you’re in. In the USA, we’re used to British units, but we also use Metric (SI) units. Most of the rest of the world is accustomed to SI units. SI units are “better” in that they don’t require as many silly conversion factors, but they all work just fine if you handle them right. Here is a table of units used in different parts of the “energy” industry for different forms of energy and power. This will help you make sense of your bills and any information you get from energy consultants like SimplEnergy. Or maybe not …
OK, that’s a lot to absorb – don’t try to memorize it. And it’s far from complete. Here are some important points:
- HVAC technicians will talk about BTUs and BTU/hr; electric power providers will talk about kW and kWh. These terms have an equivalency: 1 kW = 3412.2 BTU/hr; 1 kWh = 3412.2 BTU. In fact, if you run 1 kW through your electric water heater for 1 hour, you will add (almost) 3412 BTU of heat to the water in the water heater.
- So what? The definition of one BTU is “the amount of heat needed to raise the temperature of one pound of water by one degree Fahrenheit.” More-or-less. So if you have a 50-gallon water heater, and water weighs (about) 7.5 lbm per gallon, you have 50*7.5 = 375 lbm of water in the heater, and 3412 BTU will increase its temperature by 3412/375 = 9 degrees F. So … if you start from a cold water heater (60 F), and you heat it to 120 F, you will have used 60*375/3412 = 6.6 kWh of electricity. That’s about a dollar’s worth, in most markets.
- Natural Gas companies talk in a different language – they sell you “Therms” or “ccf” of gas. A “Therm” is the amount of gas needed to provide 10,000 BTU of heat. A “ccf” of gas is 100 cubic feet of gas at standard temperature and pressure (whose standard? Topic for another day …), which at typical natural gas compositions, is slightly more than one Therm (about 1.04 Therm).
- Air conditioner techs will talk to you about “tons” of refrigeration capacity. The definition of a “ton” is the amount of power required to freeze one ton (2000 lbm) of water in 24 hours. That works out to about 12,000 BTU per hour, and the industry normally rounds-off any error so they really mean 12,000 BTU/hr when they say one ton of refrigeration.
- Lots of times, you’ll see people confusing power and energy. Just today I read an article online where somebody was writing about “kW per day”. That doesn’t make sense – kW is already a rate. They could say just “kW,” or “kWh per day”. It didn’t take long to figure out they just meant kW, so I’m usually not too hard on people for that sort of mistake.
- When talking about battery technology both “kW” and “kWh” are important measures of a battery’s capacity. The kWh rating is how much total energy it can store. The kW rating is how fast it can deliver that energy to a power-hungry load. If you’re looking for batteries, make sure you get both specs. Lots of times, I’ve seen just one or the other stated, and it’s really not enough to make a good judgment about the usefulness of that battery to a specific application.
Maybe you don’t need to know any
of this right now, but we’ll come back to it when we really get started talking
about energy technology. I’ll link back to this article when I need to. You can
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[1] Faires and Simmang, Thermodynamics, 6th ed., New York: Macmillan, 1978, p. 28.