Energy Estimations and Models

Energy Models

To support decisions about our energy systems, we must make quantitative estimates in systems that are too complex to gather complete information.

Constructing a model is the initial mental exercise we perform before a calculation. In a model we determine the relationships and assumptions.

• If we are estimating energy what is it proportional to?

• What existing data can we base our estimate on?

• What simplifying assumptions are we making?

Strategy

Making an energy estimation is a skill that is honed through practice. The following is a good checklist to use as you develop this skill.

• Main question

  • What are you trying to estimate and why is it important?

• Model

  • What model, quantities, and/or tools will provide a plausible estimate?

  • Check that you understand the basic physical steps you are modeling.

• Quantities

  • Gather the quantities you need to estimate using your model.

• Dimensions

  • Check that your model will provide the correct dimensions.

• Units

  • Check that your model will provide the correct units.

• Calculate

  • Compute your estimate paying special attention to your calculator syntax and scientific notation.

Simple Warmup Models

If you need 2 cups of sugar for 25 cookies, how much sugar for 100 cookies?

If you are traveling at an average speed of 60 miles an hour for 3 hours, how far will you travel?

More important estimations of energy quantities are more complex, but use the same basic ideas as the two models above.

Energy quantities

There are several types of energy quantities that come up frequently. How do you decide which of these quantities is relevant to your estimation and decision?

Here is a list of types of quantities and their uses

• Unit Conversions

  • convert a quantity from one unit to another, keeping the dimension the same

• Density

  • converts volumes to masses

• Efficiency

  • determine how much of one type of energy can be converted to another

• Energy Density

  • calculates the volume or mass required to generate an amount of energy

• Carbon Intensity

  • calculates the carbon released from a given amount of energy

We will cover these in more depth in the Tools section of this chapter.

Links between models, tools, and reality

Keep in mind that each of these quantities above represent a straightforward physical fact.

You already know that coal burns and that coal must be burned to create electricity. The next step in your mastery is to realize that an energy density describes this fact.

You know now that a coal-fired power plant takes heat energy and turns it into electrical energy. Knowing that an energy efficiency quantifies this fact will help you build your models.