L9 - Advanced rankine cycles

Question 1

What do rankine cycles do?

Answer 1

1. Extract power from high temperature heat sources using water steam as a working fluid

Question 2

Efficiency of the pump

Answer 2

Npump = Ws/Wa = (h2s - h1)/(h2a - h1)

a = actual 
s = isentropic

Question 3

Efficiency of the turbine expander

Answer 3

Nturb = Wa/Ws = (h3 - h4a)/(h3 - h4s)

a = actual
s = isentropic

Question 4

How does isentropic efficiency affect the energy needed and energy released by pump

Answer 4

As isentropic efficiency is below 100%, more energy is required to heat thing up to saturated line. Less energy is also released at the end

Question 5

How to increase efficiency of rankine cycle

Answer 5

1. Lower the condenser pressure
2. Superheat steam to a higher temp
3. Increase boiler pressure (less heat required to reach superheat temp)
4. Expand steam in two stages and reheat between the stages (reheat rankine cycle)

Question 6

Purpose of reheat rankine cycke

Answer 6

To reduce the moisture content of the steam at the final stages of the expansion process

= increase dryness = less likely to damage steam turbine

Question 7

What are cogeneration cycles

Answer 7

Produces both power and heat (CHP)

Question 8

Regenerative ranking cycle uses

Answer 8

Extract/bleed steam from turbine instead of this steam being expanded further and producing power, used instead to heat feedwater

Can be closed or open feedwater
Closed - no mixing (heat exhanger
Open - mixing two fluid streams

Question 9

Positives of regeneration

Answer 9

1. Improve cycle efficiency (thermal efficiency)
2. Dearates feedwater
3. Controls volume of steam at final stages of expansion

Question 10

Comparison of open vs closed feedwater

Answer 10

1. Open fwh simple and inexpensive and provide efficient heat transfer
2. However open requires pump
3. Closed more complex, additional piping and heat exchanger