Energy, Global Warming, and Electric Bicycles

Energy Forms, Rejection and Waste

Energy may take many forms, but the most familiar are thermal and mechanical.  Energy can be released in thermal form by burning a fuel to raise the temperature of something.  An increase in temperature is another way of saying that the average speed of atoms increase in a random fashion.  This thermal energy can be converted to mechanical or kinetic form, moving all of the atoms comprising something in a uniform manner by, for example, spinning it or sending it down a road.  Thermal energy is less organized than mechanical energy, and is said to have a lower quality.  Conversion of thermal energy to mechanical energy is typically 30-40% efficient.  Mechanical energy can be converted to electrical energy with much higher efficiency, often about 90%, since these two forms are both highly organized. 

When discussing energy use, it is important to be clear about which form of energy is being described.  Powerplant engineers typically do this by adding a subscript or parenthetic (th), (m), or (e) to powerplant specifications.  For example, a 1000 Megawatt(e) powerplant may also be described as a 3000 Megawatt(th) powerplant.  The apparently lost 2000 Megawatts is said to be “rejected” power. 

It is a theoretical impossibility to convert thermal energy to mechanical form without this type of loss, which increases as the temperature of a thermal energy source decreases.    Part of the rejected power may be used to heat buildings, and this is known as cogeneration.  Energy rejected in conversion between forms is very different from energy wasted because of forgetfulness or oversight.  Energy is rejected at the powerplant because of natural laws, but energy may be wasted at the point of use because a light is left on when nobody is there to see by it.  Similar end-use waste occurs in driving a two ton car when a 60 pound electric bicycle would do the same job faster, and with much less energy.


Energy and Power Measurement

Just as distance can be measured with various units, such as centimeters, feet or fathoms, energy may be measured in a number of ways.  If a thing is heated or moved, a transfer of energy is involved.  This energy may be inferred from measurements of the things mass and temperature or distance moved over a period of time time using instruments calibrated in whatever measurement system is convenient. Various academic disciplines or trades are accustomed to seeing energy measured in different units, resulting in some confusion.  A physicist might use the Joule, or the electron-volt.  A chemist may use the calorie.  A nutritionist may use the Calorie, which is the same as 1000 chemist-style calories, or one kcal.  A heating contractor may use the BTU.  An electric utility may use the kiloWatt-hour.  A motor manufacturer might use the horsepower-hour.  A mechanical engineer might use the foot-pound.  A nuclear engineer might use atomic mass units.  An electrical engineer might use watt-seconds.  An energy macro-economist might use the Quad, shorthand for a Quadrillion BTU.  All of these units describe the same physical parameter, energy.  Conversion between them is a simple matter using Google Calculator.  A typical gallon of gasoline contains about 133 million Joules of thermal energy.  So, a gallon of gasoline may be thought of as yet another measure of energy, and miles per gallon is readily converted to miles per Joule.    

Another source of confusion results from the terms power and energy.  Power is the rate of energy use, or energy flow rate, in a system.  Given the preceding plethora of energy units, and the variety of ways time is described, power units may appear confusing indeed.  The most standard international unit of power is the Watt, which is one Joule flowing per second.  This is the unit we will use here, distinguishing electrical or mechanical from thermal forms. 

Power may be measured over a long period of time, such as a year, in which case it is an average.  Peak or instantaneous power is measured over a period which is comparable to the response time of the energy system.  Peak power is often much higher than average power.  A healthy adult, for example, is capable of producing 800 Watts of mechanical power for ten seconds, but a yearly average power of only about 33 Watts.  Numbers in the energy flow diagrams appearing here describe yearly average power.


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