Heat pumps: When does it make sense…

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Heat pumps: When does it make sense… ?

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PFI workshop :

Waste Heat Utilization by Application of Heat Pumps in CHP Plants

Pirmasens, 3 September 2013

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Heat pumps: When does it make sense… ?

1. Overview of heat pump types

2. When does it make sense … ?

3. Introduction to Pinch Analysis

4. Does it really make sense ?

5. Cases

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Overview of heat pump types

Intro: Heat Pump application fields

1. Compressor based heat pumps

2. Absorption heat pumps

3. Mechanical vapour recompression (MVR)

4. Thermal vapour recompression (TVR)

5. Hybrid heat pumps

6. Heat Pump Types in Development/Research Phase

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Heat Pump application fields

• Residential use (space heating, sanitary

hot water, swimming pool heating…)

• Commercial buildings: space heating or

other uses

• Industrial heat pumps: upgrading of

waste heat to useful process heat

• Agriculture: green house heating with

ground coupled heat pumps

• Public sector: integration into urban heat

networks

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1. Mechanically Driven Compressor based Heat Pump

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𝑄𝑜𝑢𝑡

𝑄𝑖𝑛

Simple Flow diagram of Mechanically Driven

Heat Pump

Qout = Qin + Wcomp = heat deliverd by heat

pump to heat sink

(heat is delivered through condenser)

Qin = waste heat duty delivered to heat pump

by heat source

(heat is accepted through evaporator)

Wcomp = work supplied to operate heat pump

compressor

Waste heat stream evaporates heat pump

working fluid at low temperature and

pressure

Compressor increases pressure of heat

pump working fluid

Heat pump working fluid condenses at high

temperature and pressure in the condenser,

providing useful heat to a process stream

Condensed working fluid is expanded back

to the evaporator

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1. Mechanically Driven Compressor based Heat Pump

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𝐶𝑂𝑃 =𝑄𝑜𝑢𝑡𝑊𝑐𝑜𝑚𝑝

Pressures and temperatures are highly dependent on heat source and heat sink (air/water/…), mass flow, refrigerant, cycle

architecture.

The Carnot cycle limits the highest achievable COP.

COP < COPc

Typical COP values between 3 and 8

𝐶𝑂𝑃 < 𝐶𝑂𝑃𝑐

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1. Compressor based Heat Pump - Layout and main parts:

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2. Absorption Heat Pump – Layout:

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The low pressure refrigerant vapor

from the evaporator absorbed into

absorbent. Heat is generated.

Solution pressurized by low-power

pump and enters the generator (or

desorber). Refrigerant is boiled

out of absorbent by supplying high

temperature heat.

An absorption cycle can use a

variety of working pairs. The

working pair is made up of a

refrigerant, typically ammonia or

water; and a solution which absorbs

the refrigerant. Other working pairs

include lithium-bromide-water;

TriDroxide-water; and Alkitrate-

water. Tridroxide and Alkitrate are

Energy Concepts patented working

pairs with specialty applications in

industry. Absorption cycles can

operate at high efficiency by utilizing

advanced cycles, using generator-

absorber heat exchange, multiple

pressures, and multiple effects.

These cycles use extensive internal

heat recovery to require less prime

fuel input to produce the same

thermal output.

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2. Absorption Heat Pump – Layout:

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Compressor Solution circuit

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2. Absorption Heat Pump – Layout:

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Type I Type II

Compared to a compression heat pump, an absorption

heat pump has:

- higher initial cost

- lower COP

+ lower primary energy use possible

+ more flexible

+ long operational life

+ high reliability

+ low maintenance

+ silent

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3. Mechanical Vapor Recompression (MVR):

• Compressor increases

pressure of waste vapor.

• Operates as a heat pump,

adding energy to the vapor.

• Open system.

• After compression of the vapor

and condensation of the

heating steam, the condensate

leaves the cycle.

• Used for low %DS

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Compressed vapor at higher temperature Condensate

Liquid

Vapor

Sucked vapor

Compressor M

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3. Mechanical Vapor Recompression (MVR):

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Reasons for using MVR:

• Low energy consumption, operating costs.

• Gentle evaporation of the product due to low temperature differences.

• Excellent partial load behavior.

Characteristics:

• water vapor pressure increase by a factor of 1.8

• higher quality materials (e.g. titanium): by a factor of up to 2.5.

increase in saturated steam temperature ~12 -18 K, up to a max. 30 K

-> performance of MVR systems is high: typical COPs 5 to 30.

1 or 2 heat exchangers eliminated (evaporator and/or condenser).

Current MVR systems work with heat source temperatures 70-80ºC,

deliver heat between 110 and 150ºC, in some cases up to 200ºC.

Water is most common 'working fluid’

Other process vapors also used (chemical industry).

Mechanical vapour recompression systems (MVRs), classified as open or semi-open heat pumps. In open systems, vapour

from an industrial process is compressed to a higher pressure and thus a higher temperature, and condensed in the same

process giving off heat. In semi-open systems, heat from the recompressed vapour is transferred to the process via a heat

exchanger. Because one or two heat exchangers are eliminated (evaporator and/or condenser) and the temperature lift is

generally small, the performance of MVR systems is high, with typical coefficients of performance (COPs) of 10 to 30.

Current MVR systems work with heat-source temperatures from 70-80ºC, and deliver heat between 110 and 150ºC, in some

cases up to 200ºC. Water is the most common 'working fluid' (i.e. recompressed process vapour), although other process

vapours are also used, notably in the (petro-) chemical industry.

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4. Thermal Vapor Recompression (TVR):

• Use energy in high-

pressure motive steam to

increase the pressure of

waste vapor using a jet-

ejector device.

• Typically used in

evaporators, working fluid

is steam.

• No moving parts, high

operational reliability.

• T: 60 – 180 ºC (lift 25ºC)

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As a way to reduce energy consumption, a thermocompressor which uses high pressure steam as a motive fluid is added to a typical steam

heated evaporator. The motive steam at approximately 10 bar enters the thermocompressor and expands as it passes through a diffuser. This

in turn entrains recycled vapor and discharges it at an intermediate pressure while providing a steam rate reduction. Steam is once again the

source of energy. Economics and product characteristics determine the suitability of thermal vapor recompression operation. This design

provides improved steam economy with less costly equipment than straight steam heated designs but requires higher pressure steam.

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5. Hybrid Heat Pump:

• Combines absorption and compression cycle.

• Developed by IFE (Norway).

• Uses water and ammonia mixture.

• Waste heat of 50 °C is used as a heat source to deliver heat up to 100 °C with a COP

of 3.

• Chilled water down to 5 °C can be obtained.

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5. Hybrid Heat Pump:

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Advantages:

Use a standard 20-bar compressor.

Two-component mixture -> heat exchanging processes have gliding temperature (heat source and heat sink) -> reduced heat exchanging losses

with high temperature lifts.

Tailor operation to heat sink and source temperature variations: change mixture component composition.

Natural working fluid (water and ammonia).

Good COP at high temperature lift and temperature glide.

Well tested components for ammonia refrigeration systems:

low cost and reduce failure risk .

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6. Heat Pump types in Development / Research Phase

• Chemical Heat Pumps / Adsorption Heat Pumps

• Magneto-caloric Heat Pumps

• Heat Pumps based on other cycles:

– Vuilleumier cycle

– Reversed Brayton cycle

– Honigmann cycle.

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Heat pumps: When does it make sense… ?

1. Overview of heat pump types

2. When does it make sense … ?

3. Introduction to Pinch Analysis

4. Does it really make sense ?

5. Cases

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When does it make sense… ?

• Example: Mechanically Driven Compressor based Heat Pump

– Cost heat from heat pump <> Cost heat from boiler

– Cost heat from boiler

𝐶𝑜𝑠𝑡_𝐻𝑒𝑎𝑡𝑏𝑜𝑖𝑙𝑒𝑟 =1

𝜂𝑏𝑜𝑖𝑙𝑒𝑟× 𝐶𝑜𝑠𝑡𝐹𝑢𝑒𝑙

– Cost heat from heat pump

𝐶𝑜𝑠𝑡_𝐻𝑒𝑎𝑡ℎ𝑒𝑎𝑡𝑝𝑢𝑚𝑝 =1

𝐶𝑂𝑃ℎ𝑒𝑎𝑡𝑝𝑢𝑚𝑝× 𝐶𝑜𝑠𝑡_𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦

– Break even 1

𝜂𝑏𝑜𝑖𝑙𝑒𝑟× 𝐶𝑜𝑠𝑡_𝐹𝑢𝑒𝑙 =

1

𝐶𝑂𝑃𝐻𝑒𝑎𝑡𝑝𝑢𝑚𝑝× 𝐶𝑜𝑠𝑡_𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦

𝐶𝑂𝑃𝐻𝑒𝑎𝑡𝑝𝑢𝑚𝑝 > 𝜂𝑏𝑜𝑖𝑙𝑒𝑟 ×𝐶𝑜𝑠𝑡_𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦

𝐶𝑜𝑠𝑡_𝐹𝑢𝑒𝑙

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When does it make sense… ?

𝐶𝑂𝑃𝐻𝑒𝑎𝑡𝑝𝑢𝑚𝑝 > 𝜂𝑏𝑜𝑖𝑙𝑒𝑟 ×𝐶𝑜𝑠𝑡_𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦

𝐶𝑜𝑠𝑡_𝐹𝑢𝑒𝑙

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BUD……

Investment

Annual operation time

Average duty

Trade off between

captital and energy saving

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Heat pumps: When does it make sense… ?

1. Overview of heat pump types

2. When does it make sense … ?

3. Introduction to Pinch Analysis

4. Does it really make sense ?

5. Cases

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Pinch analysis: What ? Why?

– Problem: An industrial plant has a production process in which heat

needs to be exchanged between hot and cold streams.

• Which heat exchanger network is optimal? How to evaluate the network?

• Error & try? Time consuming!

• Even simple and small projects can become complicated and not simple to

solve…

– Pinch analysis is a systematic method for analysing the potential of heat

integration.

• Achievable energy targets

• A good estimate of heat exchanger surface

• Find bottleneck

• Insight

• All prior to design

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Pinch analysis (composite curves)

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On this curves on the x-axis the thermal power of process stream(s) is represented.

The Y-axis indicates the corresponding temperatures.

Red curves are hot stream(s) to be cooled down. Bleu stream(s) are cold stream to

be heated up.

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Pinch analysis (composite curves)

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We can slide the bleu curve under the red one.

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Pinch analysis (composite curves)

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Heat can be transferred from the hot process stream(s) to the cold process

stream(s), if there is enough driving force (= temperature difference).

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Pinch analysis (composite curves)

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Continue the sliding process until a minimum delta T is reached.

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Pinch analysis (combining streams)

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75 50

125

In case of more process streams, combine them into composite curves.

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Pinch analysis (combining streams)

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50 100

150

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Pinch analysis (example 1)

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Dataset of example 1.

Hot streams

Stream NameSupply

Temperature

Target

TemperatureHeat Duty

dT Min

Contribk

°C °C kW °C kW/m²

H1 stroom H1 140 30 86,0 5 2

H2 stroom H2 110 60 100,0 5 2

H3

H4

H5

H6

H7

H8

Cold streams

Stream NameSupply

Temperature

Target

TemperatureHeat Duty

dT Min

Contribk

°C °C kW °C kW/m²

C1 stroom C1 30 105 90,0 5 2

C2 stroom C2 80 115 120,0 5 2

C3

C4

C5

C6

C7

C8

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Pinch analysys (example 1)

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Slide the bleu curve under the red one.

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Pinch analysys (example 1)

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Cold

Utility

Hot

Utility

80°C

90°C

70,9 kW

46,9 kW

The graph indicates now the heat which can be exchanged

between the process stream. As well as the heat which

should be delivered by a hot utility and the heat which should

be cooled away by a cold utility.

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Pinch analysys (example 1)

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How to determine the pinch point & pinch temperature (= bottle

neck)? Shift the hot streams (red curve) with half the minimum

delta T downwards and shift cold streams (bleu curve) with half

the minimum delta T upwards.

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Pinch analysis (Example 1)

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Slide the shifted bleu curve under the shifted red curve.

Where both curves touch each other, is the pinch point.

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Pinch analysis (Example 1)

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The production process can be split up in two subsystems.

Underneath the pinch temperature there is a subsystem

which doesn’t need external heat.

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Pinch analysis (Example 1)

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The subsystem above the pinch doesn’t need cooling.

There is no heat transfer across the pinch.

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Pinch analysis (Example 1)

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When hot streams above the pinch are cooled down

using a cold utility, as a result a cold stream should be

heated up using a hot utility instead of a hot process

stream and as a result the target can not be reached

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Pinch analysis (Example 1)

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When cold streams underneath the pinch are heated up

using a hotutility, as a result a hot stream should be

cooled down using a cold utility instead of cold process

stream and the target can not be reached

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Pinch analysis (Example 1)

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When cold streams underneath the pinch are heated up

using heat from a hot stream above the, as a result a hot

stream underneath the pinch should be cooled down

using a cold utility instead of cold process stream and

and a cold stream above the pinch should be heated up

using a hot utility. The energy consumption target can not

be reached

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Pinch analysis (Example 1)

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Lets make a graph representing the difference between

the hot and the cold streams for every temperature

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Pinch analysis (Example 1)

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Pinch analysis (Example 1)

Cooling water

Hot utility

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Pinch analysis (Example 2)

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Hot streams

Stream NameSupply

Temperature

Target

TemperatureHeat Duty

dT Min

Contribk

°C °C kW °C kW/m²

H1 Condensaat uit 105 35 2438,3 2,5 1,5

H2 Damp uit 1 100 99 315,0 2,5 1,5

H3 Damp uit 2 99 35 37,7 2,5 1,5

H4

H5

H6

H7

H8

Cold streams

Stream NameSupply

Temperature

Target

TemperatureHeat Duty

dT Min

Contribk

°C °C kW °C kW/m²

C1 Product in 40 103 3150 2,5 1,5

C2

C3

C4

C5

C6

C7

C8

Data set 2

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Pinch analysis (Example 2)

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Data set 2

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Pinch analysis (Example 2)

Steam

Hot water

Cooling water

Hot utility

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Pinch analysis: Conclusions

• “Pinch rules”

– Don’t cool above pinch

– Don’t heath underneath the pinch

– Don’t transfer heat over the pinch

• Heat exchanger or mixing of streams

• Violation of rules => penalty versus the minimum energy

requirements (for the corresponding DT)

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Pinch analysis: Conclusions

• Pinch analysis gives insight in

– Minimum utility level

– Pinch analysis gives insight in thermodynamic behaviour of a process

• Can also be useful for smaller processes

• Prior to design

– Calculation of minimum utilities required including utility levels

– Calculation of heat exchanger area required and # of heat exchangers

– Trade-off between Energy and Capital

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Heat pumps: When does it make sense… ?

1. Overview of heat pump types

2. When does it make sense … ?

3. Introduction to Pinch Analysis

4. Does it really make sense ?

5. Cases

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Heat pump ?

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Heat pump?

Simplified representation: Heat is transferred to a medium. Which is inside the heat

pump “pumped” to a higher temperature level. Then the heat is transferred to a

stream which has to be heated up.

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Pinch analysis (Example 1)

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When a heat pump is used underneath the pinch…

…the same amount of heat has to be cooled away

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Pinch analysis (Example 1)

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…the same amount of utility heat is still needed

When a heat pump is used above the pinch…

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Pinch analysis (Example 1)

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When a heat pump is across the

pinch…

…less utility heat is needed above

the pinch and less process heat is

cooled away underneath the pinch

and in total we really save energy !!!

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Pinch analysis (Example 1)

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Lets make a graph representing the difference between

the hot and the cold streams for every temperature

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Pinch analysis (Example 1)

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Pinch analysis (Example 1)

Cooling water

Hot utility

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Heat pump? (Example 1)

𝑄𝑡ℎ𝐶𝑂𝑃

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Heat pump? (Example 1)

Cooling

water

Hot utility

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Remark

Some fluid mixtures used as heat

transfer medium in heat pumps have

no constant temperature during

evaporation and condensation

Pinch analysis can be a

help in selecting the right

Mixture.

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Pinch analysis & Heat pumps: Conclusions

• “Pinch rules”

– Don’t use a heat pump above the pinch

– Don’t use a heat pump underneath the pinch

– Use a heatpump to transfer heat over the pinch (from underneath to above)

• Pinch analysis helps you selecting and/or design the ideal heat pump

for your process

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Heat pumps: When does it make sense… ?

1. Overview of heat pump types

2. When does it make sense … ?

3. Introduction to Pinch Analysis

4. Does it really make sense ?

5. Cases

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Case 1

• Process

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10,0 m³/h ???? 10,0 m³/h 10,1 m³/h ???? 10,1 m³/h

25,0 °C 85,0 °C 60,0 °C 20,0 °C

696,7 kW 469,1 kW

0,2 m³/h ???? 0,2 m³/h

20,0 °C 20,0 °C

0,0 kW

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10,0 m³/h ???? 10,0 m³/h 10,1 m³/h ???? 10,1 m³/h

25,0 °C 85,0 °C 60,0 °C 20,0 °C

696,7 kW 469,1 kW

0,2 m³/h ???? 0,2 m³/h

20,0 °C 20,0 °C

0,0 kW

Case 1

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Case 1

• Process

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?

Based on the Grand Composite Curve it is very

unlikely that a heat pump will keep sense…

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Case 2

• Process

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Hot streams

Stream NameSupply

Temperature

Target

TemperatureHeat Duty

dT Min

Contribk

°C °C kW °C kW/m²

H1 stroom H1 130 55 37,5 5 2

H2 stroom H2 55 50 500,0 5 2

H3 stroom H3 50 30 26,7 5 2

H4

H5

H6

H7

H8

Cold streams

Stream NameSupply

Temperature

Target

TemperatureHeat Duty

dT Min

Contribk

°C °C kW °C kW/m²

C1 stroom C1 20 50 30,0 5 2

C2 stroom C2 50 55 400,0 5 2

C3 stroom C3 55 120 43,3 5 2

C4

C5

C6

C7

C8

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Hot streams

Stream NameSupply

Temperature

Target

TemperatureHeat Duty

dT Min

Contribk

°C °C kW °C kW/m²

H1 stroom H1 130 55 37,5 5 2

H2 stroom H2 55 50 500,0 5 2

H3 stroom H3 50 30 26,7 5 2

H4

H5

H6

H7

H8

Cold streams

Stream NameSupply

Temperature

Target

TemperatureHeat Duty

dT Min

Contribk

°C °C kW °C kW/m²

C1 stroom C1 20 50 30,0 5 2

C2 stroom C2 50 55 400,0 5 2

C3 stroom C3 55 120 43,3 5 2

C4

C5

C6

C7

C8

Case 2

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!!!

Based on the Grand Composite Curve it is very

likely that a heat pump will keep sense…

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Case 3

• Process

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Hot streams

Stream NameSupply

Temperature

Target

TemperatureHeat Duty

dT Min

Contribk

°C °C kW °C kW/m²

H1 stroom H1 130 55 37,5 5 2

H2 stroom H2 55 50 500,0 5 2

H3 stroom H3 50 30 26,7 5 2

H4 stroom H4 150 105 22,5 5 2

H5 stroom H5 105 100 550,0 5 2

H6 stroom H6 100 40 80,0 5 2

H7

H8

Cold streams

Stream NameSupply

Temperature

Target

TemperatureHeat Duty

dT Min

Contribk

°C °C kW °C kW/m²

C1 stroom C1 20 50 30,0 5 2

C2 stroom C2 50 55 450,0 5 2

C3 stroom C3 55 120 43,3 5 2

C4 stroom C4 20 100 80,0 5 2

C5 stroom C5 100 105 500,0 5 2

C6 stroom C6 105 120 10,0 5 2

C7

C8

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Case 3

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?

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Case 3

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Case 3

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?

Saving for both heat pumps together

Don’t over estimate the saving !!!

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Case 3 : alternative ??

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?

Hot utility

Cold utility

Electricity production with ORC

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Case 3 : alternative ??

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Hot streams

Stream NameSupply

Temperature

Target

TemperatureHeat Duty

dT Min

Contribk

°C °C kW °C kW/m²

H1 stroom H1 130 57 36,5 2,5 2

H2 stroom H2 57 52 500,0 2,5 2

H3 stroom H3 52 30 29,3 2,5 2

H4 stroom H4 150 107 21,5 2,5 2

H5 stroom H5 107 102 550,0 2,5 2

H6 stroom H6 102 40 82,7 2,5 2

H7

H8

Cold streams

Stream NameSupply

Temperature

Target

TemperatureHeat Duty

dT Min

Contribk

°C °C kW °C kW/m²

C1 stroom C1 20 47 27,0 2,5 2

C2 stroom C2 47 52 450,0 2,5 2

C3 stroom C3 52 120 45,3 2,5 2

C4 stroom C4 20 97 77,0 2,5 2

C5 stroom C5 97 102 500,0 2,5 2

C6 stroom C6 102 120 12,0 2,5 2

C7

C8

Change

process

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Pinch analysis & Heat pumps: Conclusions

• Pinch analysis

– gives insight in thermodynamic behaviour of a process and how a heat

pump might fit in

– gives an insight in temperature levels and heat duties

– helps for selection and/or design of a heat pump

• And also

– gives insight in thermodynamic behaviour of a process in a way that

heat pumps perhaps aren’t necessary..

– shows the wasted heat

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Pinch analysis & Heat pumps: Conclusions

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Waste heat

≠

Wasted heat

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KOVIA bvba Ir. Frank Koninckx Pierstraat 417 b9

B-2840 Reet Belgium

+ 32-475/72.02.73

E-mail:

Frank.Koninckx@kovia.com K O

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