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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
ASHRAE Conference, Chicago 2012Page 1/14
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
ASHRAE Conference, Chicago 2012Page 2/14
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)
ASHRAE Conference, Chicago 2012Page 3/14
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.
ASHRAE Conference, Chicago 2012Page 4/14
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
ASHRAE Conference, Chicago 2012Page 5/14
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
ASHRAE Conference, Chicago 2012Page 6/14
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
ASHRAE Conference, Chicago 2012Page 7/14
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
ASHRAE Conference, Chicago 2012Page 8/14
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)
ASHRAE Conference, Chicago 2012Page 9/14
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.
ASHRAE Conference, Chicago 2012Page 10/14
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
ASHRAE Conference, Chicago 2012Page 11/14
How does the optimal subcooling influence COP?
Heat pump static optimization
ASHRAE Conference, Chicago 2012Page 12/14
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
ASHRAE Conference, Chicago 2012Page 13/14
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).
ASHRAE Conference, Chicago 2012Page 14/14
Toutside = 15 oC
Tzone = 30 oC
Tzone = 26 oC
Tzone = 22 oC
Tzone = 18 oC
Economizer mode Compressor running
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