Variable-Speed Heat Pump Model for a Wide

Range of Cooling Conditions and Loads

Tea Zakula (tzakula@mit.edu)Nick GayeskiLeon GlicksmanPeter Armstrong

ASHRAE is a Registered Provider with The American Institute of Architects Continuing Education Systems. Credit earned on completion of this program will be reported to ASHRAE Records for AIA members. Certificates of Completion for non-AIA members are available on request.

This program is registered with the AIA/ASHRAE for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.

• Describe heat pump model development using a modular approach, where the three main components beeing modeled are the evaporator, compressor and condenser.• Determine for each set of the cooling rate and outside and inside temperatures there is an optimal fan and compressor speeds as well as subcooling that will result in lowest energy consumption (highest COP).• Determine if it is more efficient to run economizer mode (free cooling) depending upon oputside temperature and the fan energy use in economizer mode or total energy when running with compressor on.

Learning objectives

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NO

T

s

1

23

Is hliq_as = hliq and Pcomp_out_as = Pcomp_out?

Calculate Coefficient of Performance (COP)

YES

Call evaporator model (1)

Call compressor model (2)

Call condenser model (3)

Assume hliq_as and Pcomp_out_as

hliq

Pcomp_out

e

comp E,fan C,fan

QCOP

E E E

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Heat pump model flowchart

Heat exchanger model

ex p ex,inlet in,inletExchanged heat m c T T

ex e in ee

1 1 t 1

UA h A kA h A

h = function (flow rate, flow regime, geometry, fluid properties)

Δp = function (flow rate, flow regime, geometry, fluid properties)

hex,evaporator

Tex,evaporator

Single phase flowTwo phase flow

Evaporator

Tin,evaporator

hin,evaporator

Δpin,evaporator

CondenserTin,condenser

hin,condenser

Δpin,condenser

hex,condenser

Tex,condenser

exfunction (m , UA,geometry)

Sub-models of the evaporator (1) and condenser (3)

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Compressor model

Given: mref, compressor inlet pressure and temperature, compressor outlet pressureFind: frequency, compressor power, outlet temperature

Where:

Volumetric efficiency model:

Sub-model of the compressor (2)

......... Proposed by Armstrong

......... Proposed by Jänhig et al.

Constants C1, C2, C3, C4, C4, C5, C6

have been found using measured data.

Constants C1, C2, C3, C4, C4, C5, C6

have been found using measured data.

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RMSE = 7.35 % (pressure drop calculations included)

RMSE = 17.45 % (pressure drop calculations not included)

Pressure drop calculations included

Pressure drop calculations not included

Error propagation:Evaporator outlet state error Compressor inlet state error Compressor energy error COP error

Heat pump model validation

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Additional heat pump modules

Development of additional heat pump modules:

• brazed plate heat exchanger

• refrigerant free-cooling mode

• inverse heat pump model with frequency as an input

• parallel evaporators, condensers and compressors

• dehumidification mode

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Model evaluation

Strengths

• Modular approach

• Balance between complexity and computational speed

• Simulation options (superheating in the evaporator, subcooling in the condenser, variable heat transfer coefficients, pressure drop inside the heat pump)

• Optimization options

Weaknesses

• Simple compressor model

• Refrigerant charge has not been modeled

• Assumed ideal expansion device

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What are the optimal fan and compressor speeds and condenser subcooling for minimum power consumption if cooling rate, room temperature and outside temperature are given?

Compressor power HIGH

Fan power LOW

T

s

Compressor power LOW

Fan power HIGH

T

s

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Heat pump static optimization

Given: Qe = 2.0 kW

To

tal p

ow

er

(W)

Vz(m3/s)

Vz(m3/s)

Vz(m3/s)Vo(m3/s)

Vo(m3/s)

To

tal p

ow

er

(W)

To

tal p

ow

er

(W)

Finding the optimal evaporator (Vz opt) and condenser (Vo opt) air flows for minimum power consumption if cooling rate (Qe), room temperature and outside temperature are given.

Given: Qe = 2.4 kW

Given: Qe = 2.8 kW

Vz(m3/s) Vo(m3/s)

Given: Qe = 3.2 kW

Vo(m3/s)

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To

tal p

ow

er

(W)

Heat pump static optimization

Power consumptionPower consumptionOptimal parametersOptimal parameters

The results of the heat pump optimization for a range of cooling conditions.

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Heat pump static optimization

Toutside = 40 oC

Tzone = 30 oC

Tzone = 26 oC

Tzone = 22 oC

Tzone = 18 oC

What is optimal subcooling?

Heat pump static optimization

Toutside = 30 oC

Tzone = 30 oC

Tzone = 26 oC

Tzone = 22 oC

Tzone = 18 oC

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How does the optimal subcooling influence COP?

Heat pump static optimization

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Toutside = 30 oC

Tzone = 30 oC

Tzone = 26 oC

Tzone = 22 oC

Tzone = 18 oC

-

How does COP change with change of refrigerant?

R410A Ammonia

Heat pump static optimization

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Toutside = 30 oC

Tzone = 30 oC

Tzone = 26 oC

Tzone = 22 oC

Tzone = 18 oC

Heat pump static optimizationWhen is it more efficient to run economizer mode?

Zakula T., Armstrong P. and Norford L. 2012. Optimal Coordination of Compressor, Fan and Pump Speeds Over a Wide Range of Conditions (in preparation).

Zakula T., Gayeski N., Armstrong P. and Norford L . 2011. Variable-speed Heat Pump Model for a Wide Range of Cooling Conditions and Loads. HVAC&R Research 17(5).

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Toutside = 15 oC

Tzone = 30 oC

Tzone = 26 oC

Tzone = 22 oC

Tzone = 18 oC

Economizer mode Compressor running