25 The "per second" is alreadybuilt in to the definition of the kilowatt. Other examples of units that, like the watt, already have a "per time" built in are the knot - "our yacht's speed was ten knots!" (a knot is one nautical mile per hour); the hertz - "I could hear a buzzing at 50 hertz" (one hertz is a frequency of one cycle per second); the ampere - "the fuse blows when the current is higher than 13 amps" (not 13 amps per second); and the horsepower - "that stinking engine delivers 50 horsepower" (not 50 horsepower per second, nor 50 horsepower per hour, nor 50 horsepower per day, just 50 horsepower).
- Please, never, ever say "one kilowatt per second." There are specific, rare exceptions to this rule. If talking about a growth in demand for power, we might say "British demand is growing at one gigawatt per year." In Chapter 26 when I discuss fluctuations in wind power, I will say "one morning, the power delivered by Irish windmills fell at a rate of 84MW per hour." Please take care! Just one accidental syllable can lead to confusion: for example, your electricity meter's reading is in kilowatt-hours (kWh), not 'kilowatts-per-hour'.
I've provided a chart on p368 to help you translate between kWh per day per person and the other major units in which powers are discussed.
For our first chapter on consumption, let's study that icon of modern civilization: the car with a lone person in it.
How much power does a regular car-user consume? Once we know the conversion rates, it's simple arithmetic:
energy used _ distance travelled per day per day distance per unit of fuel x energy per unit of fuel.
For the distance travelled per day, let's use 50 km (30 miles).
For the distance per unit of fuel, also known as the economy of the car, let's use 33 miles per UK gallon (taken from an advertisement for a family car):
33 miles per imperial gallon ~ 12 km per litre.
(The symbol means "is approximately equal to.")
What about the energy per unit of fuel (also called the calorific value or energy density)? Instead of looking it up, it's fun to estimate this sort of quantity by a bit of lateral thinking. Automobile fuels (whether diesel or petrol) are all hydrocarbons; and hydrocarbons can also be found on our breakfast table, with the calorific value conveniently written on the side: roughly 8kWh per kg (figure 3.2). Since we've estimated the economy of the car in miles per unit volume of fuel, we need to express the calorific value as an energy per unit volume. To turn our fuel's "8 kWh per kg" (an energy per unit mass) into an energy per unit volume, we need to know the density of the fuel. What's the density of butter? Well, butter just floats on water, as do fuel-spills, so its density must be a little less than water's, which is 1 kg per litre. If we guess a density of 0.8 kg per litre, we obtain a calorific value of:
Rather than willfully perpetuate an inaccurate estimate, let's switch to the actual value, for petrol, of 10 kWh per litre.
distance travelled per day energy per day = —-. . .—x energy per unit of fuel distance per unit of fuel
50 km/day x 10kWh/litre
Congratulations! We've made our first estimate of consumption. I've displayed this estimate in the left-hand stack in figure 3.3. The red box's height represents 40 kWh per day per person.
Figure 3.2. Want to know the energy in car fuel? Look at the label on a pack of butter or margarine. The calorific value is 3000 kJ per 100 g, or about 8 kWh per kg.
Figure 3.3. Chapter 3's conclusion: a typical car-driver uses about 40 kWh per day.
This is the estimate for a typical car-driver driving a typical car today. Later chapters will discuss the average consumption of all the people in Britain, taking into account the fact that not everyone drives. We'll also discuss in Part II what the consumption could be, with the help of other technologies such as electric cars.
Why does the car deliver 33 miles per gallon? Where's that energy going? Could we manufacture cars that do 3300 miles per gallon? If we are interested in trying to reduce cars' consumption, we need to understand the physics behind cars' consumption. These questions are answered in the accompanying technical chapter A (p254), which provides a cartoon theory of cars' consumption. I encourage you to read the technical chapters if formulae like jinv2 don't give you medical problems.
Chapter 3's conclusion: a typical car-driver uses about 40 kWh per day. Next we need to get the sustainable-production stack going, so we have something to compare this estimate with.
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