Download
1 / 16
160 likes | 282 Vues
Thermodynamics & Energy Conversions. Diego Villarreal SHP – Columbia University. September 28, 2013. What is energy anyway?. Energy? “Capacity to do work” Different types of energy: Mechanical Kinetic – Associated with object or fluid motion (KE = ½ mv 2 )
E N D
Thermodynamics & Energy Conversions Diego Villarreal SHP – Columbia University September 28, 2013
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 –
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.
Energy, Heat and work • 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” • 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.
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!
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
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!)
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. • HE receive heat from a high-temp source • Convert part of this heat to work (usually by rotating shaft) • Reject the remaining waste heat to a low temperature reservoir. • Operate in a cycle
PV diagrams • Useful tool to study heat engines. • Points ab are at constant T. Called “isotherms”. • So moving from a represents “Isothermal Expansion”
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
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
Carnot Cycle • Step 1-2: Isothermal Expansion • Step 2-3: Adiabatic Expansion • Step 3-4: Isothermal Compression • Step 4-1: Adiabatic Compression
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:
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.
