CEE 370 Environmental Engineering Principles Lecture #10 Energy - - PDF document

CEE 370 Lecture #10 10/2/2019 Lecture #10 Dave Reckhow 1

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CEE 370 Environmental Engineering Principles

Lecture #10

Energy Balances

Reading: Mihelcic & Zimmerman, Section 4.2 & 4.3

Davis & Masten, Chapter 4 Updated: 2 October 2019

Print version

Conservation of Energy

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Generic: KNO3 + C + S → K+ + {S, SO2, SO4

• 2} + {CO2, CO3
• 2} + N2

A specific example:

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David Reckhow

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Energy Balance

 First law of thermodynamics

 Energy can be neither created nor

destroyed

 But the form can certainly change

 Thermal Energy

 Characterized by

 Temperature (T) and  Specific heat capacity (cp)

T Mc H

p  



David Reckhow

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Heat Transfer

 Conduction

 Transfer of

energy without mass flux

 Convection

 Energy is carried

by molecules in bulk motion

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Energy Balances

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Common Forms of Energy

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Energy Balance

 Much like material balances

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(Change in internal plus external energy per unit time) = (energy flux in) – (energy flux out) 𝑒𝐹 𝑒𝑢 𝐹 𝐹

Example 4.9

 Heating Water: Scenario 1

 40 gal capacity, cold water is 10C  5 kW is max heating rate  Flow is 2 gal/min  Assume 100% efficiency & steady

state

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𝑒𝐹 𝑒𝑢 0 𝐹 𝐹 0 𝑛𝑑𝑈 5𝑙𝑋 𝑛𝑑𝑈

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Example 4.9 cont

 Note that c=4184J/kg°C and 1W=1J/s

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0 𝑛𝑑𝑈 5𝑙𝑋 𝑛𝑑𝑈 0 𝑛𝑑 𝑈 𝑈 5𝑙𝑋

0 2 𝑕𝑏𝑚 𝐼𝑃 𝑛𝑗𝑜 𝑦 3.785 𝑀 𝑕𝑏𝑚 𝑦 1.0 𝑙𝑕 𝑀 𝑦 4184𝐾 𝑙𝑕°𝐷 𝑈 𝑈 5000𝐾 𝑡 𝑦 60𝑡 𝑛𝑗𝑜 0 3.16𝑦10 𝐾 𝑛𝑗𝑜°𝐷 𝑈 𝑈 3.00𝑦10 𝐾 𝑛𝑗𝑜 𝑈 𝑈 3.00𝑦10 3.16𝑦10 ℃ 9.5℃

If Tin is 10°C, then T

• ut is 19.5°C

~67°F

Note error in book

Example 4.10

 How long should you wait to get a

temperature of 54°C (~129°F)?

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GHG Effect

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• What are greenhouse gases?
• What are affects of GHGs?
• Are GHGs bad?

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Solar Radiation & Climate Change

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Global Average CO2 Concentration Trend

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Why?

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Climate Change Scenarios

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Rural vs. Urban Temperatures

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Heat Distribution in Different Areas

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Why?

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Urban Shallow Layer Energy Balance

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Comparison of the American Home

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Energy Use for Small vs. Large Homes

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Sustainable Design for Temperature Regulation

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a) Thermal walls for heat transfer and dissipation b) Ventilation systems for natural heating c) Ventilation systems for natural cooling d) Overhangs to regulate effects from sun

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 To next lecture