1

Vapour compression cycle

•Refrigerator•Air conditioning plant•Heat pump

(“ ENERGY MANAGEMENT HANDBOOK” Sixth Edition, Chapter 8)

2

Refrigerator

3

Electric-motor-driven vapour compression cycle

4

Engine-driven vapour compression cycle

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Absorption heat pump

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Applications of Heat pumps

• space heating; • heating and cooling of process streams; • water heating for washing, sanitation and

cleaning; • steam production; • drying/dehumidification; • evaporation; • distillation; • concentration.

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Heat pump performance

Heat source at 0 C

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Typical COP

Table 2: Typical COP/PER for heat pumps with different drive energies.

Heat pump type [1] COP PER

MVR 10 - 30

Closed cycle, electric 3.0 - 8.0

Closed cycle, engine 1.0 - 2.0

Absorption (Type I) 1.1 - 1.8

Heat transformer (Type II) 0.45 - 0.48

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Temperature

Pressure

Heat Transformer

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Factors affecting heat pump performance

• the climate - annual heating and cooling demand and maximum peak loads;

• the temperatures of the heat source and heat distribution system;

• the auxiliary energy consumption (pumps, fans, supplementary heat for bivalent system etc.);

• the technical standard of the heat pump; • the sizing of the heat pump in relation to the

heat demand and the operating characteristics of the heat pump;

• the heat pump control system.

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How heat pumps achieve energy savings and CO2 emissions reduction

• Heat pumps and energy saving • An efficient technology • A large and worldwide potential

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Heat sources

Heat source Temperature Range (°C)

Ambient air -10 +15

Exhaust air +15 +25

Ground water +4 +10

Lake water 0 +10

Rock 0 +5

Ground 0 +10

Waste water >10

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Heat pump working fluids

• HFC-134a • R-407C • Natural working fluids

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Heat pumps in residential and commercial buildings Table 1: Typical delivery temperatures for various heat and cold distribution systems.

Application Supply temperature range (°C)

Air distribution Air heating 30 - 50

Floor heating; low temperature 30 - 45

Hydronic systems radiators 45 - 55

District heating - hot water 70 - 100

District heating District heating - hot water/steam

100 - 180

Cooled air 10 - 15

Space cooling Chilled water 5 - 15

District cooling 5 - 8

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Example of how the COP of a water-to-water heat pump varies with the

distribution/return temperature.

Heat distribution system (supply/return temperature) COP

Conventional radiators (60/50°C) 2.5

Floor heating (35/30°C) 4.0

Modern radiators (45/35°C) 3.5

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Fundamental physics for vapor compression cycles

Vapour-pressure curve

Pressure

Temperature

LIQUID

VAPOUR

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Constant-pressure change from liquid to vapour phase for a

pure substance.

LIQUID

VAPOUR

VAPOUR

(a) (b) (c)

LIQUID

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Temperature change of substance at constant pressure and

constant heat supply Temperature

saturation- temperature

Time

4

3 2

1

1-2: subcooled liquid 2: saturated liquid 2-3: saturation area 3: saturated vapour 3-4: superheated vapour

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Temperature-volume diagram showing liquid and vapour phases for different

constant pressures

Pressure: 1 MPa

Pressure: 0.1 MPa

Pressure: 10 MPa

H

L O

D

I E

M

A

Volume

Temperature

C B

G F

J K

N

CRITICAL POINT

Saturated LIQUID-line

Saturated VAPOUR-line

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Temperature-volume diagram showing liquid and

vapour phases

Saturation vapour-line vg

Subcooled liquid

constant pressure

v

T critical point

Saturation liquid-line vf

Saturation area

Superheated vapour

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Ideal vapour compression cycle

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Relations for the cycle

1 4( )working mediumLQ m h h

2 1( )working mediumcW m h h

2 3( )working mediumHQ m h h

3 4h h

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Ideal cycle

1

2

4

3

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Log P vs Enthalpy diagram

Constant Temperature

AbsolutePressure

Saturation area

Saturated liquid

Saturated vapour

Constant Entropi

Enthalpy

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Coefficient of performance

• The coefficient of performance of a refrigeration system is

• The coefficient of performance of a heat pump system is

1 4

2 1

( )

( )LL

Rc

Q q h hCOP

w h hW

2 3

2 1

( )

( )HH

HPc

Q q h hCOP

w h hW

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Efficiency of compressor

2 1

2 1

1

2

2

isentropic efficiency of compressor

=enthalpy value before compressor

=enthalpy value after compressor for an isentropic process

=enthalpy value after compressor f

sc

c

s

h h

h h

where

h

h

h

or the real process

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Sweden - Closed-cycle compression heat pump

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Example refrigerator/Heat Pump

• An refrigerator has an cooling capacity of 500 W.Evaporator pressure 2 bar(a), Condenser pressure 10 bar(a). An ideal cycle is assumed

Determine• Power input to compressor• Heat transfer rate from Condenser

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Solution

3 4

1

2

134 1341 41 4

134 2 1

134

Reading from R134a-diagram gives:

255 /

392 /

428 /

500

0.500( ) 0.00365 /

( ) (392 255)

( ) 0.00365(428 392) 0.1314 131

L

LR a R aL

c R a

RH

h h kJ kg

h kJ kg

h kJ kg

Q W

QQ m h h m kg s

h h

W m h h kW W

Q m

2 3( ) 0.00365(428 255) 0.6314 631

500 131.4 631.4 631

a

cH L

h h kW W

Q Q W W