September 28, 2013
description
Transcript of September 28, 2013
![Page 1: September 28, 2013](https://reader036.fdocuments.us/reader036/viewer/2022062521/568168f6550346895de00499/html5/thumbnails/1.jpg)
September 28, 2013
Diego VillarrealSHP – Columbia University
Thermodynamics & Energy Conversions
![Page 2: September 28, 2013](https://reader036.fdocuments.us/reader036/viewer/2022062521/568168f6550346895de00499/html5/thumbnails/2.jpg)
2
What is energy anyway?• Energy? “Capacity to do work”• Different types of energy:
○ Mechanical• Kinetic – Associated with object or fluid motion (KE = ½ mv2)• Potential – Associated with object’s position (PE=mgh)
○ Chemical energy – Energy stored in chemical bonds and released upon transformation/reaction (coal, oil, methanol)
○ Nuclear – Energy found within the atomic nucleus. Can be released by ‘breaking’ (fission) the atoms.
○ Thermal Energy – Think of it as microscopic PE & KE of an object that results in its temperature (cup of hot coffee).
○ Radiant – Energy of electromagnetic waves.○ Electrical –
![Page 3: September 28, 2013](https://reader036.fdocuments.us/reader036/viewer/2022062521/568168f6550346895de00499/html5/thumbnails/3.jpg)
3
Wasted energy• About 3/5 of the fuel energy input is
“wasted”. • Because our energy system is highly
dependent on fossil fuels this leads to extra CO2 and pollution.
• So, why is this? Is there a natural limit to how efficient we can be at our energy conversion?
• Need to turn to thermodynamics to answer this question.
![Page 4: September 28, 2013](https://reader036.fdocuments.us/reader036/viewer/2022062521/568168f6550346895de00499/html5/thumbnails/4.jpg)
4
Energy, Heat and work
• Main principles:○ Heat spontaneously flows from Hot
Cold (Always, no exception). More formal treatment of this in a bit.
○ Energy Conservation - Energy can be transformed from one form to another, but cannot be created or destroyed.
• Thermodynamics – The study of the interchangeability of heat and work. ○ Think of thermodynamics as the “economics of science”.
It will tell us how much we will have to “pay” for particular transformations and whether or not they are feasible.
○ Basic “bookkeeping”
![Page 5: September 28, 2013](https://reader036.fdocuments.us/reader036/viewer/2022062521/568168f6550346895de00499/html5/thumbnails/5.jpg)
5
Heat• Heat (Q) is the energy transferred between
a system and its surroundings (other than by work). Usually a result of a temperature difference between two objects.
• Heat is NOT a fluid and is never contained within an object; an object contains thermal energy.
• Think of ΔT as an “index” of your ability to move heat. Remember this!
![Page 6: September 28, 2013](https://reader036.fdocuments.us/reader036/viewer/2022062521/568168f6550346895de00499/html5/thumbnails/6.jpg)
6
First law of thermo• First law of thermo?
○ The first law of thermodynamics states that during any cycle that a system undergoes, the cyclic integral of the heat is equal to the cyclic integral of the work. Hence, it requires that energy be conserved during a process. However, the first law places no restrictions on direction of flow.
• That is, work done on a system plus the heat added to it is equal to the total change in energy of the system. ΔE = Won + Qto
• Work done on a system is the negative of work done by the fluid (Won=-Wby) so: Qto= ΔE + Wby
• Temperature does not tell us the amount of energy contained in a substance. However a change in temperature tells us something about the heat added (removed)
Q = mcΔT
![Page 7: September 28, 2013](https://reader036.fdocuments.us/reader036/viewer/2022062521/568168f6550346895de00499/html5/thumbnails/7.jpg)
7
Second law• Second law?○ The only processes that can occur are ones that
result in an increase in the entropy of the systems (e.g. direction matters!)
![Page 8: September 28, 2013](https://reader036.fdocuments.us/reader036/viewer/2022062521/568168f6550346895de00499/html5/thumbnails/8.jpg)
8
Heat Engines• We know from experience that work can easily be
converted to other forms of energy, but converting other forms of energy to work in not that easy.
• Converting heat to work requires the use of some special devices. These devices are called heat engines.
1. HE receive heat from a high-temp source
2. Convert part of this heat to work (usually by rotating shaft)
3. Reject the remaining waste heat to a low temperature reservoir.
4. Operate in a cycle
![Page 9: September 28, 2013](https://reader036.fdocuments.us/reader036/viewer/2022062521/568168f6550346895de00499/html5/thumbnails/9.jpg)
9
PV diagrams• Useful tool to study heat engines.
• Points ab are at constant T. Called “isotherms”.• So moving from a represents “Isothermal
Expansion”
![Page 10: September 28, 2013](https://reader036.fdocuments.us/reader036/viewer/2022062521/568168f6550346895de00499/html5/thumbnails/10.jpg)
10
Adiabatic compression• Looking at the PV diagram, what is a
necessary condition to perform isothermal compression/expansion?○Heat must be supplied or removed!
• So what happens if I insulate the compression chamber or do fast compression?○ΔQ = 0!○ Thermal energy and T must change.○ “Adiabatic” compression/expansion
![Page 11: September 28, 2013](https://reader036.fdocuments.us/reader036/viewer/2022062521/568168f6550346895de00499/html5/thumbnails/11.jpg)
11
Carnot Engine• Let’s do a device to exchange Q and W
using isotherms and adiabats.• Assumptions:○ Piston-Cylinder device○ Perfectly insulated but insulation is such
that it can be removed instantaneously to put system in contact with heat reservoir.
○No friction, no turbulence○No mechanical inefficiency losses
![Page 12: September 28, 2013](https://reader036.fdocuments.us/reader036/viewer/2022062521/568168f6550346895de00499/html5/thumbnails/12.jpg)
12
Carnot Cycle• Step 1-2: Isothermal Expansion• Step 2-3: Adiabatic Expansion• Step 3-4: Isothermal Compression• Step 4-1: Adiabatic Compression
![Page 13: September 28, 2013](https://reader036.fdocuments.us/reader036/viewer/2022062521/568168f6550346895de00499/html5/thumbnails/13.jpg)
13
Carnot Efficiency• Wnet will be area enclosed by engine cycle. • Important question: what fraction of the heat
supplied is converted to mechanical work?○ Called the efficiency!○ Efficiency = Wout/Qin
○ η = (Qin-Qout)/Qin
• For carnot engines:
![Page 14: September 28, 2013](https://reader036.fdocuments.us/reader036/viewer/2022062521/568168f6550346895de00499/html5/thumbnails/14.jpg)
14
Steam cycle• The core of a steam power
consists of four components: ○ a boiler, turbine,
condenser, and a pump. • First, fuel is burned in a
furnace/boiler where the released heat is transferred to pressurized water contained within steel tubes.
• Then, the high-pressure, high-temperature steam is delivered to a turbine.
• Steam generated in this process is expanded in a steam turbine, which drives an electric generator to produce electric power.
• Steam is later cooled down in a condenser and is pumped back to the boiler to be reheated, completing the cycle.
![Page 15: September 28, 2013](https://reader036.fdocuments.us/reader036/viewer/2022062521/568168f6550346895de00499/html5/thumbnails/15.jpg)
15
Carnot example
![Page 16: September 28, 2013](https://reader036.fdocuments.us/reader036/viewer/2022062521/568168f6550346895de00499/html5/thumbnails/16.jpg)
16
Other Carnot Examples