MEC303 Flashcards
What is cogeneration?
Where two forms of energy are generated from the same primary fuel (ie heat and power plant)
What is regeneration?
the transfer of heat from the working
fluid to some other stage in the cycle (e.g. for the heating of feedwater).
Describe the Allam cycle
process for converting fossil fuels into mechanical power, while capturing the generated carbon dioxide and water
What is the first law of thermodynamics?
The First Law of Thermodynamics states that heat is a form of energy, and thermodynamic processes are therefore subject to the principle of conservation of energy. (cannot be created or destroyed)
What is the non-flow energy equation?
Q-W=ΔU
U is the internal energy
What is an adiabatic process?
No heat is transferred to or from the system
W=ΔU
What does isochoric mean?
constant volume
W done by the system is 0 so Q=ΔU
What does isobaric mean?
Constant pressure
Define enthalpy
the amount of heat content used or released in a system at constant pressure
What are the assumptions made with the steady-flow heat equation?
- the mass flow at the inlet is constant in time and equal to the mass flow at the outlet
- the flow properties at any particular point w/in the open system do not vary with time
- any heat and work crossing the boundary does so at a uniform rate
What does a boiler do?
increase temperature at constant pressure
What does a condenser do?
decrease temperature at constant pressure
What process are the steam turbine and pump?
adiabatic (no heat transfer)
What is the second law of thermodynamics?
The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time, and is constant if and only if all processes are reversible.
What is entropy?
the degree of disorder or randomness in a system
What does isentropic mean?
constant entropy
What is the maximum temperature of the cycle governed by?
Strength of the materials
What are some issue associated with the carnot cycle?
- pumping from 4 to 1 in a mixture of liquid and vapour, water droplets can damage turbine blades
- hard to compress a two-phase liquid would need v large compressor
- achieving isothermal heat transfer is v difficult would require v large heat exchangers for a long time
How is the rankine cycle different/better than the carnot cyle?
Pros:
- state 4 is moved to saturation line so fluid is fully condensed
- more heat is being added
-the pump consumes less power
Cons:
- there is a lower average temperature for heat addition so lower efficiency
- the turbine still has to operate with a mix of wet steam
Are efficiencies of the pump higher when it is pumping a two-phase liquid?
NO
efficiencies are higher when it is pumping just a liquid or a gas
How does reheat increase efficiency?
Average heat addition temperature increases
Which cycle is actually used in a power plant?
Highly complex modified Rankine
Why does increasing the temp of heat addition increase efficiency?
As temperature rises enthalpy difference between temperature increases, therefore, you will get higher work output going from 300>200 than 200>100
as enthalpy difference is higher between higher temperatures
What is the difference between a closed feedwater heater and an open feedwater heater?
Close feedwater heater takes the steam out and passes through closed feedwater heater, therefore, y and pump are not mixed