Total energy systems Flashcards
How do cogeneration systems improve efficiency
Uses the rejected heat to improve efficiency; too low to produce more electricity but can be used for other applications
What is cogeneration?
Simultaneous production of thermal and electrical power from a single source, using HX to recover waste heat
What is a prime mover
Produces mechanical power from fuel combustion (turbine or RE)
How do steam turbines work?
Boiler produces high pressure steam, which is expanded through a turbine
How do gas turbines work?
Extract mechanical energy from hot combustion gas
Rank prime movers in terms of fuel flexibility
Steam
Gas
RE
Rank prime movers in terms of maintenance required
RE
Gas
Steam
Which prime mover can have the highest heat recovery?
Gas
What is a factor for selecting a prime mover?
CHP plants are expensive so should be operated as much as possible, suiting turbines. RE engines used if frequent stops required.
What cycle is used for steam turbines
Rankine cycle
What are the 4 steps in IC engines?
1.) Induction
2.) Combustion
3. Power
4.) Exhaust
ICPE
Difference between SI and CI engines
SI: Mixture ignited before minimum volume.
CI: Spontaneous ignition
What happens during power stroke?
Burning gas gets hot and expands rapidly, exerting force on piston, work is transferred from piston to crank.
CI has a more progressive power stroke.
What happens in gas turbine cycle?
Suck squeeze bang blow. Isentropic assumption.
How are CHP systems classified?
To prioritise electricity or heat. Can be classified to to prime mover
What is a bottoming cycle?
Where is it used?
Prioritises thermal energy, with waste heat used to generate electricity. Used in industrial plants with high T requirements
What are some variations on CHP systems?
Can use waste heat from the brayton cycle to drive Rankine cycle.
Gas turbine exhaust can be used as combustion air
Afterburner can be used to produce higher pressure steam.
How does a fuel cell CHP work?
Electrons from hydrogen flow round circuit to produce oxygen ions. Combination produces water and heat recovered as hot water.
More efficient than combusting hydrogen, and few moving parts.
Advantage of CHP
Eliminates most heat wastage, makes efficient use of fuel resources minimising pollution. Allows local generation, minimising transmission losses.
Disadvantages of CHP
High capital, only viable if there is heat and electricity demand, relatively complex.
What is the load factor?
Consequences of low load factor
Average rate consumption/ peak rate consumption
Equipment must be sized large to meet peak requirements. Should size towards minimum value and buy extra energy.
5 steps of feasibility evaluation.
acronym?
GETLE
1.) Gather utility data including upper and lower limits, load factors
2.)Electricity consumption analysis
3.) Thermal consumption analysis
4.) Thermal/electrical load ratios
5.) Equipment sizing
What is a good potential for thermal/electrical load ratio
Above 1.5
How to analyse electrical consumption?
1.) Add up daily to find annual consumption.
2.) Calculate peak demand for each month
3.) Calculate annual average power
4.) Calculate annual electric load factor
