Lecture 7 - Joule Advanced Binary

Question 1

Why is mercury used in a cycle?

Answer 1

It has a higher critical temp and lower vapour pressure

Question 2

What is the main problem with using mercury? (Cycle components)

Answer 2

The specific volume is very large at the temp of the condenser, so a huge condenser would be required

Question 3

Where does mercury condense in a binary cycle and what is it used for?

Answer 3

Condenses in a heat exchanger, used to heat the steam in a Rankine cycle

Question 4

Why are binary systems with mercury not used?

Answer 4

Mercury is poisonous and chemically attacks many other materials

Question 5

In what processes would a back-pressure turbine be used?

Answer 5

Processes requiring heat, but much lower temps than that at which the fuel burns.
They require large amounts of electricity at the same time

Question 6

How can the temp of the condensation of the working fluid be varied?

Answer 6

Varying the back pressure of the turbine

Question 7

Why must the condenser be replaced by a heat exchanger?

Answer 7

So the process can recover heat in the working fluid for the vapour power cycle

Question 8

What does the temp at which a back-pressure turbine cycle becomes worthwhile depend on?

Answer 8

Electrical power requirements
Heat requirements
Economic cost

Question 9

What components change in nuclear power generation?

Answer 9

Boiler is swapped for nuclear reactor and heat exchanger

Question 10

What are the limiting factors of nuclear powered cycles?

Answer 10

Nuclear fuel must operate at relatively low temperatures

Another working fluid is used in the traditional part of the cycle

Question 11

What does H represent?

Answer 11

Enthalpy- specific enthalpy (h) x mass flow rate

Question 12

What is used in the primary and secondary circuits in Nuclear powered cycles, and why?

Answer 12

Primary: steam
Secondary: usually CO2
Secondary must have sat temp above sat liquid temp of steam

Question 13

What is the pinch point?

Answer 13

Where a min temp difference occurs between the two fluids, where the primary working fluid is a saturated liquid

Question 14

What do we need to read about?

Answer 14

Technical problems with nuclear power as a heat source

Question 15

What are the advantages of reheat?

Answer 15

Raises the turbine outlet temperature

Raises the average input temperature without going over the temperature limit of the turbine blades

Question 16

What is the advantage of intercooling

Answer 16

reduce the work that must be done by the compressor as the temperatures are kept low
Temperature after the second compression is lower so that more heat can be recycled from the turbine outlet

Question 17

What happens if more reheat and intercooling is added

Answer 17

approach a carnot cycle

Question 18

what is the issue with h20 as a working fluid

Answer 18

it has a low critical temperature and high critical pressure
means we have to use complex rankine cycle to utilise it

Question 19

Why must mercury be used in a binary cycle and not just a normal one

Answer 19

at low temperatures its specific volume is extremely large therefore need an enormous condensor
therefore condense at higher temperatures - 300
gives opportunity to use with other fluids in binary cycle

Question 20

What is a binary cycle

Answer 20

combination of two cycles using different working fluids - one feeding heat to the other

Question 21

What is a combined gas and vapour cycle

Answer 21

a cycle that uses a steam and gas binary cycle to maximise efficiency

Question 22

What happens in a combined cycle

Answer 22

thermal energy is recovered from the turbine exhaust
transferring it to steam in a heat exchanged - boiler for steam cycle
Usually more than one gas turbine needed to supply sufficient heat to steam cycle

Question 23

What is the advantage of a combined cycle

Answer 23

Really high efficiencies - 60%

Question 24

What is the advantage of a combined cycle

Answer 24

Really high efficiencies - 60%

Question 25

What is the advantage of joule cycle

Answer 25

closed cycle if using heat exchanger - can use different working fluids with better characteristics than air