Total energy systems

Question 1

How do cogeneration systems improve efficiency

Answer 1

Uses the rejected heat to improve efficiency; too low to produce more electricity but can be used for other applications

Question 2

What is cogeneration?

Answer 2

Simultaneous production of thermal and electrical power from a single source, using HX to recover waste heat

Question 3

What is a prime mover

Answer 3

Produces mechanical power from fuel combustion (turbine or RE)

Question 4

How do steam turbines work?

Answer 4

Boiler produces high pressure steam, which is expanded through a turbine

Question 5

How do gas turbines work?

Answer 5

Extract mechanical energy from hot combustion gas

Question 6

Rank prime movers in terms of fuel flexibility

Answer 6

Steam
Gas
RE

Question 7

Rank prime movers in terms of maintenance required

Answer 7

RE
Gas
Steam

Question 8

Which prime mover can have the highest heat recovery?

Answer 8

Gas

Question 9

What is a factor for selecting a prime mover?

Answer 9

CHP plants are expensive so should be operated as much as possible, suiting turbines. RE engines used if frequent stops required.

Question 10

What cycle is used for steam turbines

Answer 10

Rankine cycle

Question 11

What are the 4 steps in IC engines?

Answer 11

1.) Induction
2.) Combustion
3. Power
4.) Exhaust

ICPE

Question 12

Difference between SI and CI engines

Answer 12

SI: Mixture ignited before minimum volume.
CI: Spontaneous ignition

Question 13

What happens during power stroke?

Answer 13

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.

Question 14

What happens in gas turbine cycle?

Answer 14

Suck squeeze bang blow. Isentropic assumption.

Question 15

How are CHP systems classified?

Answer 15

To prioritise electricity or heat. Can be classified to to prime mover

Question 16

What is a bottoming cycle?
Where is it used?

Answer 16

Prioritises thermal energy, with waste heat used to generate electricity. Used in industrial plants with high T requirements

Question 17

What are some variations on CHP systems?

Answer 17

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.

Question 18

How does a fuel cell CHP work?

Answer 18

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.

Question 19

Advantage of CHP

Answer 19

Eliminates most heat wastage, makes efficient use of fuel resources minimising pollution. Allows local generation, minimising transmission losses.

Question 20

Disadvantages of CHP

Answer 20

High capital, only viable if there is heat and electricity demand, relatively complex.

Question 21

What is the load factor?
Consequences of low load factor

Answer 21

Average rate consumption/ peak rate consumption

Equipment must be sized large to meet peak requirements. Should size towards minimum value and buy extra energy.

Question 22

5 steps of feasibility evaluation.
acronym?

Answer 22

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

Question 23

What is a good potential for thermal/electrical load ratio

Answer 23

Above 1.5

Question 24

How to analyse electrical consumption?

Answer 24

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

Question 25

How to calculate thermal consumption ?

Answer 25

1.) Determine annual fuel consumption
2.) Determine boiler to steam efficiency. 78% roughly. Times
3.) Annual thermal = fuel consumption x efficiency
3.) Calculate hourly demand and therefore load factor

Question 26

What is electrically driven dispatch?

Answer 26

Prioritise electricity demand over heat.
CHP sized to meet peak annual.

Question 27

What is thermally driven dispatch?

Answer 27

CHP is sized to meet thermal demand. Top up electricity. Suited to facilities with maximum thermal coincident with electrical (unusual)

Question 28

What is peak shaving

Answer 28

Reduces peak electricity demand, requiring top ups

Question 29

Hybrid strategy

Answer 29

switching between electrical and thermal based on seasons. CHP smaller with peak shaving during peak electricity demand.