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
What is the advantage of a closed feedwater heater?
keeps streams separate
works well for streams at different pressures
Describe the throttling process
high-pressure liquid–>small orifice, therefore, the pressure decreases and entropy increase, enthalpy remains constant
What is the utilization factor?
(net work output+process heat used)/heat supplied
The fraction of energy that is used for either process heat or power generation is called the utilization factor of the cogeneration power plant.
Describe a back pressure turbine
adjust the pressure to achieve the required heat and power ratio, optimisation of both systems together
What is a gas turbine?
an engine designed to convert the energy of a fuel into some form of useful power
Describe how a gas turbine works
fan - take air in and compresses it slightly
LPC - low-pressure compressor, temp and pressure increase
HPC- high-pressure compressor, temp and pressure increase significantly
Combustion chamber- air burnt with fuel, temp increases significantly, ideally pressure doesn’t drop in combustor but we end up getting 5% pressure drop
Hot gas mixture forces turbine blades to spin which spins compressor as the are connected vis driveshaft
How is mechanical energy converted to electrical in a gas turbine?
drive shaft is connected to a generator
the generator consists of large magnets
magnets spin which creates magnetic field causing electrons to move
What are the advantages of a gas turbine?
large power to weight ratio
long maintenance life and low-cost maintenance
quick to start up, help with surge in demand
wide variety of fuels can be used
atmospheric air is used and no coolant required
What does the area under a bryton cycle curve equal?
the work output
What is the approach temperature?
the closest that temperatures from the cooling and heating processes can be brought
What are the advantages of adding reheat to a joule cycle?
raises average input temp without reaching temp limit of turbine blades
raises the turbine outlet temperature meaning more heat can be recovered (heat exchanger)
add heat and higher temperature which improves efficiency
Why would you add reheat and intercooling to a joule cycle?
- pump work requirement decreases
- overall efficiencies increase
- less energy required to compress cool air than hot air as enthalpy difference increases with temperature
- more heat can be recycled from the turbine outlet as temperature after the second compression is lower
- average temp of heat removal reduced
What properties do we want from a working fluid?
- as high critical pressure as possible
- easily converted to gas phase
What does the carnot cycle tell us about efficiency?
maximise temp where heat is added
minimise temp where heat is rejected
Describe the combined cycle
- thermal energy is recovered from the turbine exhaust
- transferring it to the steam in a heat exchanger that serves as the boiler for the steam cycle
- usually, more than one gas turbine is needed to supply sufficient heat
- steam cycle may also involve regeneration as well as reheating
efficiencies of around 60%
If the temperature of the combustor is too high what emissions will you get?
NOs
If the temperature of the combustor is too low what emissions will you get?
CO2
What does UHC stand for?
Unburnt hydrocarbon
not fully burnt fuel
In fuel terms what does lean mean?
more air and less fuel
Define cooling capacity
the capacity of refrigeration that will freeze 1 ton of liquid water at 0 degrees in 24 hours
How is the actual refrigeration cycle different from the reversed Carnot?
- gas is vapourised completely so no 2phase fluid
- replace turbine with throttle (reduces temp and enthalpy and entropy remain the same)
known as ideal vapour compression refrigeration cycle
List some ways to improve the ideal vapour compression refrigeration cycle
- operating two or more vapour compression systems in series (called cascading) can achieve v low temps and high eff, with higher COP
- using multistage compression with regenerative cooling
- throttling refrigerant in stages
Name the factors affecting heat pump performance
- low source temps decrease capacity and efficiency
- must be sized for largest heat load required
- if too much heat is extracted from the ground it will freeze so only extract what can be replaced
Why is the only heat source needed from refrigerant room temperature?
when a refrigerant evaporates it absorbs heat at a v high rate, since it has a lower boiling point than most liquids we can easily manipulate it so the only heat source needed is room temp, the boiling point is controlled by altering pressure
Why arent heat pumps used domestically?
large area required
lower efficiencies
fluctuation in demand
What are the requirements for a refrigerant?
the substance needs to reach a temp lower than the required cooling temp to take heat away from the object to be cooled
then it needs to reach a higher temp than the temp in a medium where you want to dump the heat
and then go back to low temp required through a cycle
What does CFC lead to?
Depletion of the ozone layer
When are emissions regulated?
take-off and landing
What is the stoichiometric mixture?
an ideal fuel/air mixture in which both the fuel and oxygen in the air are completely consumed, usually specified as a ratio of mass of air to mass of fuel
What does pre mixing fuel with air lead to?
Lean premixed pre vapourised combustor LLP
better combustion stability and can burn much leaner so you can get lower NOs emissions
What could you do to get low specific fuel consumption?
- higher firing temp
- higher pressure ratios
- complex cycles
What are the parameters used by engineers to optimise cycles?
compression ratio
turbine inlet temperature
What is the process of the otto cycle 4 stroke?
works like gas turbine air in compression combustion- spark ignition of premixed fuel exhaust
How is the atkinson cycle different from the otto?
during the compression stroke the air inlet valve is still open so less air is injected into the combustion chamber( the volume of air is the same but mass is lower) therefore a lower fuel mass is injected
the exhaust stroke is also left for longer to reach lowest possible pressure
this results in the compression ratio being smaller than the expansion ratio consequently increasing efficiency
What is carbon capture and storage?
the process of capturing waste carbon dioxide usually from large point sources, such as a cement factory or biomass power plant, transporting it to a storage site and depositing it where it will not enter the atmosphere, normally underground geological formation
What are the two engineering techniques to remove Co2 directly from the air?
Direct Air Capture (DACS/DACCS) and bioenergy with CCS (BECCS)
What is the LCV?
When 1 kg of a fuel is completely burned and the products of combustions are not cooled down or the heat carried away the products of combustion is not recovered and the steam produced in this process is not condensed then the heat obtained is known as the Lower Calorific Value.
How can you raise the COP?
raising the evaporator temperature or reducing the condenser temperature
What is specific steam consumption?
The steam required per unit of power produced. It is important because the size of the power station (turbine/pumps, boilers etc.) depend on it and power
input to Rankine cycle depends on this.
Describe some differences between aerospace and industrial gas turbines
Industrial turbine:
- lower pressure ratio and therefore requires a lower fuel gas pressure
- inlet temperatures are lower and have thicker blades so are less sensitive to corrosion and fouling
- used with a wider range of fuels as they are less sensitive
- lower emissions as fewer constraints
- can-annular or fully annular spacious combustors designed for easy maintenance and fuel flexibility
Aero:
- higher pressure ratio in the compressor
- use advanced materials to save weight
- small to fit aircraft or have complex staged compressors
Describe some differences between the otto and diesel cycle
Diesel cycle has higher compression ratios 18-24:1
otto 10-13:1 this means the diesel cycle has a higher efficiency
if the cycles worked at the same compression ratio otto would be more efficient BUT diesel can reach higher compression ratios
The diesel engine doesn’t have a spark plug because of the higher compression ratios
What are some issue with diesel?
diesel has high energy content per unit mass due to its high density, this leads to:
- higher firing temp and eventually NOx
- higher particulate emissions as some components (aromatics) are diesel are difficult to burn
How can we reduce NOx emissions?
water injection - demineralized water is injected into the combustion chamber to regulate the flame temperature.
Variable combustors- passing more air thru combustor at a certain altitude (decreasing temp)
Lean burning
Staged burning- partially combust in presence of a catalyst, then fully combust
Which cycle would you choose for low CO2 emissions? otto or diesel?
diesel has higher efficiency, which leads to higher miles per gallon.
This also leads to lower CO2 emissions.
How is the Allam cycle emission-free?
natural gas is used as fuel and instead of air uses pure oxygen eliminating emissions of nitrogen oxides
CO2 sequestered
Why can the Allam cycle work at higher pressures?
The working fluid is compressed as a gas using a compressor at low pressures but then as
a dense phase supercritical fluid (‘liquid’) using a pump at high pressures. The use of a pump
means much less work is required for a given pressure rise.
How does superheat increase efficiency?
- Average heat addition temperature increases
- dryer turbine exit flow (water droplets damaged turbine)
Why do we use a throttle valve in a closed heater feedwater system?
the liquid at point6 will be at a higher pressure so needs to be put through a condenser (throttle valve)
What are the problems with using waste heat from UK power stations?
- Heat rejected from the cycle is at very low temperature
- A cycle will produce power and heat in fixed proportions that may not be compatible with the requirements of the plant
- heat must be transferred to where it is needed
Advantages of the pass-out cogeneration plant
- provides a wider range of possible heat/power ratios
- allows bleeding out of the steam partway through the turbine
- close off routes (only generate electricity or heat)
How do heat exchangers improve efficiency?
instead of wasting rejected heat, it is used to heat up working fluid so the compressor requires less work
What is effectiveness?
actual heat transfer/maximum
What does CO2 emission depend on?
ratio of carbon to fuel going into engine
What is the COP?
coefficient of performance
Desired output/ required input
Refrigerator= QL/Wout
Heat pump= Qh/Wout
What is a rich-burn-quick-quench combustor?
- 2 stage combustor
- burn fuel rich at low temp
- introduce air suddenly and burn lean
Why do we use feedwater heaters?
- reduces irreversibilities involved in steam generation and therefore improves the thermodynamic efficiency of the system
- reduces pump work
What is the difference between internal energy U and enthalpy h?
- during a reversible constant volume process, the heat added was equal to the increase in internal energy
- Whereas in a constant pressure process it is the enthalpy that increases
What are some design drivers for industrial and aviation engines?
- lowest possible emissions
- lowest possible cost
- pressure ratios <20:1
- low specific fuel consumption ( high firing temp, complex cycles)
- high reliability
Why is Atkinson cycle used for hybrids?
The Atkinson cycle is ideal for hybrids because their electric motor(s) make up for the lost low-speed output.
Advantages of open and closed feedwater heaters
The main advantage of the above-closed system is that it only needs one feed pump. The advantage
of the open system is that it does not use a throttle valve and has higher efficiency.
