ORNL is managed by UT-Battelle

for the US Department of Energy

IEA HPP Annex 41 –

Cold Climate Heat

Pumps: Improving

Low Ambient

Temperature

Performance of Air-

Source Heat Pumps

Tandem, single-speed compressor air-source heat pump system laboratory and preliminary field test results

Bo Shen, Keith Rice, Omar Abdelaziz

Oak Ridge National Laboratory

August 19, 2015

2 Presentation_name

Contents

1. Background

2. Lab Testing

3. Field investigation

4. Summary

Target Market: Replace electric resistance heat in cold climates.

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1.1 Background

Problem Statement: • Typical ASHPs don’t work

well at low ambient temps due to very high discharge temp and pressure ratio

• Single-speed ASHP heating capacity not sufficient to match building load

• COP degrades significantly with ambient temperature

Technical Goals (Project outputs): Develop a CCHP to minimize resistance heating • COP@ 8.3°C > 4.0; HP capacity@ -25°C > 75% of rated capacity@

8.3°C. • Maximize COPs at -8.3°C and -25°C with acceptable payback period.

0%

20%

40%

60%

80%

100%

120%

-23 -13 -3 7 17 27 37 47 57 67 77 87 97 107

Bu

ild

ing

Lo

ad

Lin

e a

nd

He

ati

ng

Ca

pa

cit

y o

f T

yp

ica

l A

SH

P a

nd

Ta

rge

t C

CH

P

Ambient Temperature [F]

Target CCHP Capacity Goals

Typical ASHP (HSPF=7.5)

Building Heating Load Line, Region V, Well-Insulated Home

Building Cooling Load Line, Region V, Well-Insulated Home

-40

-30

-20

-10

0

10

20

30

40

50

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35

Ax

is T

itle

Ambient Temperature [C]

Chart Title

Series1

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1.2 Extensive System Configurations

ORNL to assist in modeling, analysis,

component sizing, and performance

verification testing

Condenser

Evaporator

Fla

sh

Ta

nk

Scro

ll M

ulti-S

tag

e

Co

mp

resso

r

Vapor injection

with flash tank

Vapor injection

with economizer

Condenser

Evaporator

Eco

no

miz

er

HX

Scro

ll M

ulti-S

tag

e

Co

mp

resso

r

Ejector cycle

Condenser

Evaporator

Tandem

Compressors

Tandem (parallel) and multi-

capacity compressor(s)

Single-stage compression concepts:

Condenser

Evaporator

Ejector

Co

mp

resso

r

Fla

sh

Ta

nk

Condenser

Evaporator

Eco

no

miz

er

HX

High Stage VS

Compressor

Low Stage

Compressor

Two-stage (series)

compression w/

inter-stage

economizing

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1.3 Compare Different Design Options

Over-capacity is the key to meet the 75% capacity goal, i.e. using tandem compressors. Vapor injection boosts HP COP at -25°C.

20%

30%

40%

50%

60%

70%

80%

90%

100%

Performance Ratio @ -13F versus Nominal @ 47F

Capacity

Integrated COP

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2.1 ‘More Cost Effective’ Configuration

Condenser

Evaporator

EXV

TXV

Filter/Dryer

Fan

Fan

4-W

ay

Valv

e

Suction Line Accumulator

Tandem

Compressors

For space

cooling

For space

heating

• Use two identical, single-speed compressors, specially optimized for heating mode , tolerate up to 138°C discharge temperature.

• Use a single compressor to match cooling load, and heating load at moderately cold temperatures, turn on both compressors at low ambient temperatures when needed.

• (Suction line accumulator+ EXV discharge temperature control) facilitates charge optimization in a wide ambient temperature range. using TXV and optimizing charge for heating mode is an alternative.

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2.2 ‘Premium’ Configuration

• Equal Tandem, Vapor Injection Compressors + Inter-stage Flash Tank + EXV Discharge Temperature Control .

Condenser

Evaporator

Fixed

orifice

TXV

Filter/Dryer

Compressor Unit

Fan

Fan

Fla

sh

Ta

nk

Sight Glass

Oil Separator

Oil

Filt

er

4-W

ay V

alv

e

Liquid Receiver

Hand Valve

Sight Glass

P T

P T

P T

P T

EXV

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2.3 Lab Measured Heating Capacities

0

10

20

30

40

50

60

70

80

-23 -13 -3 7 17 27 37 47 57 67

HP

Air

Sid

e H

eat

ing

Cap

acit

y [k

Btu

/h]

Outdoor Air Temperature [F]

2 Comps-VI

2 Comps-1 Speed

1 Comp-VI

1 Comp-1 Speed

Load_DHRmax_V

Load_DHRmin_V

0

5

10

15

20

25

-30 -20 -10 0 10 20

Hea

ting

Cap

acit

y [k

W]

Ambient Temperature [C]

Series1

0

5

10

15

20

25

-30 -20 -10 0 10 20

He

atin

g C

ap

acit

y [

kW

]

Ambient Temperature [C]

Series1

Eliminate Strip Heat in Well-Insulated Homes, Region V

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2.4 Lab Measured Heating COPs

• The ‘premium’ system with tandem VI compressors achieved 5% better COPs than the ‘more cost-effective’ single-speed compressor version at various ambients.

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

-23 -13 -3 7 17 27 37 47 57 67

HP

Air

Sid

e C

OP

[W

/W]

Outdoor Air Temperature [F]

Heating COP vs. OD Temp

1 Comp-VI

1 Comp-1 Speed

2 Comps-VI

2 Comps-1 Speed

-30

-20

-10

0

10

20

30

40

50

60

70

80

-30 -25 -20 -15 -10 -5 0 5 10 15 20

Ax

is T

itle

Ambient Temperature [C]

Chart Title

Series1

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2.5 HSPF (DHRmin = 11.7kW) – AHRI

210/240

Heating Season Ratings, Region: IV [HSPF in W/W; OD design

temperature -15°C]

‘More Cost-Effective’ ‘Premium’ Case

3.29 3.47 Based on DHRmin

3.21 3.46 Based on DHRmax

Heating Season Ratings, Region: V [HSPF in W/W; OD design temperature

-23.3°C]

‘More Cost-Effective’ ‘Premium’ Case

2.94 3.13 Based on DHRmin

2.81 2.96 Based on DHRmax

11 Presentation_name

3.1 Field Testing of a ‘More Cost-

Effective’ System

Field testing in OH-outdoor unit, at a

residential home having a design

cooling load of 3-ton (10.6 kW)

-Indoor Unit and Data Acquisition,

record data point every half minute.

Insulated compressors’ shell

12 Presentation_name

3.2 Parameters of Field Testing Unit

Parameters (heating mode)

Indoor Fin-&-Tube Coil Outdoor Fin-&-Tube Coil

Face area, ft2 (m2) 3.30 (0.307) 22.3 (2.07)

Total Tube Number 84 64

Number of rows 3 (cross counter-flow) 2 (cross counter-flow)

Number of parallel circs 9 6

Fin density, fins/ft (fins/m)

168 (551) 264 (866)

Indoor Blower (High/Low1) Outdoor Fan

Flow Rate, cfm (m3/s) 1500/1250 (0.708/0.590) 3500 (1.652)

Power [W] 322/203 300

-the indoor and outdoor heat exchangers originally used for a 5-ton (17.7 kW) heat

pump, i.e. oversized for single-compressor operation (3-ton/10.6 kW).

-Power source: 240 V, 60 HZ, single-phase.

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3.3 Field Testing from February to May,

2015 (half winter)

0%

2%

4%

6%

8%

10%

12%

Heat

Ener

gy De

liver

ed [%

J]

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

Build

ing H

eatin

g Loa

d [W

]

-Field ambient

temperature went

down to -24.4°C

(-12°F).

-Seasonal,

average, heating

COP was 3.16,

i.e. 10.8 HSPF.

-supplemental

resistance energy

use was 1.3% in

total heating

energy.

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3.4 Compressor Running Time Fractions

0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

70.0%

80.0%

90.0%

100.0%

Compressor Run Time Fractions

2 Comp Run Time Tot Comp Run Time

-The second compressor

cycled with 80% running

time, even at -25°C (having

room for more capacity).

-The second compressor

was needed at ambient

temperatures below -12°C.

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

Hea

ting

Load

and

Cap

acity

[W]

Heat Load Capacity_1Comp Capacity_2Comp

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3.5 Resistance Energy Uses

0.0%

0.5%

1.0%

1.5%

2.0%

2.5%

3.0%

3.5%

Resistance Energy Ratio per Bin

Due to temperature setback schedule

-can be eliminated by changing the control and letting the second compressor run longer to match the temperature setting point.

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3.6 Heating COPs

2

2.5

3

3.5

4

4.5

CO

P [

w/w

]

Heat Pump COPs

COP_2Comp COP_1Comp Total COP

-Heat pump COP at 47°F (8.3°C) >3.5. -Heat pump COP at -13°F (-25°C) > 2.0. * Total COP includes resistance heat, cyclic and frost/defrost losses.

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3.7 Return and Supply Air Temperatures

15

20

25

30

35

40

45

50

55

">12.8C"

"12.8to 10C"

"10 to7.2C"

"7.2 to4.4C"

"4.4 to1.7C"

"1.7 to-1.1C"

"-1.1 to-3.9C"

"-3.9 to-6.7C"

"-6.7 to-9.4C"

"-9.4 to-12.2C"

"-12.2to -

15C"

"-15 to-17.8C"

"-17.8to -

20.6C"

"-20.6to -

23.3C"

"-23.3to -

26.1C"

Air

Te

mp

era

ture

[C

]

Heat Pump Return and Supply Air Temperatures

Return Air Temp Supply Air Temp

-Maintain 30°C supply air temperature at low ambient temperatures. * Temperature setting at 20°C (68°F) with 1.1K (2R) dead band.

18 Presentation_name

3.8 Defrost Frequency and Energy

0.0%

0.2%

0.4%

0.6%

0.8%

1.0%

1.2%

1.4%

1.6%

1.8%

2.0%

"12.8to 10C"

"10 to7.2C"

"7.2 to4.4C"

"4.4 to1.7C"

"1.7 to-1.1C"

"-1.1 to-3.9C"

"-3.9 to-6.7C"

"-6.7 to-9.4C"

"-9.4 to-12.2C"

"-12.2to -

15C"

"-15 to-17.8C"

"-17.8to -

20.6C"

"-20.6to -

23.3C"

"-23.3to -

26.1C"

Defrost Time Ratio to Bin Run Time

0.0%

0.5%

1.0%

1.5%

2.0%

2.5%

Defrost Load to Bin Heat Energy Delivered

Defrost frequency and energy were minimum for CCHP -With running one compressor, frost formation was slow, due to the oversized outdoor HX. -When the two compressors were needed at low ambients, the ambient humidity level was very low.

19 Presentation_name

4. Summary

• Over-capacity capability in a heat pump is the key to match the 75% capacity goal – tandem or variable-speed compressors are required.

• The tandem single-speed compressors had sufficient capacity to eliminate resistance heat down to -25°C during the field testing in Ohio.

• Proper charge control strategy is necessary to optimize heat pump operations at low ambient temperatures.

• Defrost losses were minimum during the field investigation.