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Science Notes: Energy Accounting and Balance

Once we understand the various transformations of energy that are possible, an energy balance can be used to track energy through a system, and is a very useful tool for determining resource use and environmental impacts. The idea is to use the First and Second laws to determine how much energy is needed at each point in the system and in what form that energy is. The accounting system keeps track of energy in, energy out, and non-useful energy versus work done, and transformations within the system. An energy balance diagram is used. Non-useful work is what is often responsible for environmental problems.

Example: We wish to determine how much coal is needed to produce 1 kWh of electricity. Assume the power plant is 33% efficient, with 85% of waste heat to cooling tower, and 15% to stack. Assume you can get 24 kJ of energy from 1 gram of coal. Note that 1 kW of electricity is equivalent to 1 KJ/s of electricity. What are the environmental issues?

Figure 11: Coal Fired Power Plant.

1 KJ/s kWh * 3600 s/hr = 3600 KJ per hour
If the power plant is 33% efficient, then need 3600 * 3 = 10,800 KJ
10,800 KJ * 1/24 KJ/gram = 450 grams coal = approximately 1 lb = 454 grams
Therefore, we need 450 grams coal for 1 KWh of electricity.

How much electrical energy do we use worldwide?

10 EJ (1990), 1 ExaJoules = 1018 Joules, or 27.8 * 1015 KWh.
A home in the US may average 500 kWh over a month.

What happens to non useful energy in this example?

0.85*7200 = 6120 KJ to cooling water (note that we use vast amounts of water to cool waste heat; this cooling water produces thermal pollution of water bodies)

Also, 0.15*7200 = 1080 KJ goes to stack as waste heat, which carries impurities in the form of air pollution.

Environmental issues for this example are nonrenewable natural resource consumption, air pollution, water pollution, and solid waste.



  ©Copyright 2003 Carnegie Mellon University
This material is based upon work supported by the National Science Foundation under Grant Number 9653194. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.