1. Variable-Speed Heat Pump Model for a Wide Range of Cooling Conditions and Loads Tea Zakula (tzakula@mit.edu) Nick Gayeski Leon Glicksman Peter Armstrong

2. 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 2012 Page 1/14

3. NO T s 1 2 3 Is h liq_as = h liq and P comp_out_as = P comp_out ? Calculate Coefficient of Performance (COP) YES Call evaporator model (1) Call compressor model (2) Call condenser model (3) Assume h liq_as and P comp_out_as h liq P comp_out ASHRAE Conference, Chicago 2012 Page 2/14 Heat pump model flowchart

4. Heat exchanger model h = function (flow rate, flow regime, geometry, fluid properties) Δp = function (flow rate, flow regime, geometry, fluid properties) h ex,evaporator T ex,evaporator Single phase flow Two phase flow Evaporator T in,evaporator h in,evaporator Δp in,evaporator Condenser T in,condenser h in,condenser Δp in,condenser h ex,condenser T ex,condenser Sub-models of the evaporator (1) and condenser (3) ASHRAE Conference, Chicago 2012 Page 3/14

5. Compressor model Given: m ref, compressor inlet pressure and temperature, compressor outlet pressure Find: frequency, compressor power, outlet temperature Where: Volumetric efficiency model: Sub-model of the compressor (2)......... Proposed by Armstrong......... Proposed by Jänhig et al. Constants C 1, C 2, C 3, C 4, C 4, C 5, C 6 have been found using measured data. Constants C 1, C 2, C 3, C 4, C 4, C 5, C 6 have been found using measured data. ASHRAE Conference, Chicago 2012 Page 4/14

6. 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 2012 Page 5/14

7. 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 2012 Page 6/14

8. 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 2012 Page 7/14

9. 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 2012 Page 8/14 Heat pump static optimization

10. Given: Q e = 2.0 kW Total power (W) V z (m 3 /s) V o (m 3 /s) Total power (W) Finding the optimal evaporator (V z opt ) and condenser (V o opt ) air flows for minimum power consumption if cooling rate (Q e ), room temperature and outside temperature are given. Given: Q e = 2.4 kW Given: Q e = 2.8 kW V z (m 3 /s) V o (m 3 /s) Given: Q e = 3.2 kW V o (m 3 /s) ASHRAE Conference, Chicago 2012 Page 9/14 Total power (W) Heat pump static optimization

11. Power consumption Optimal parameters The results of the heat pump optimization for a range of cooling conditions. ASHRAE Conference, Chicago 2012 Page 10/14 Heat pump static optimization T outside = 40 o C T zone = 30 o C T zone = 26 o C T zone = 22 o C T zone = 18 o C

12. What is optimal subcooling? Heat pump static optimization T outside = 30 o C T zone = 30 o C T zone = 26 o C T zone = 22 o C T zone = 18 o C ASHRAE Conference, Chicago 2012 Page 11/14

13. How does the optimal subcooling influence COP? Heat pump static optimization ASHRAE Conference, Chicago 2012 Page 12/14 T outside = 30 o C T zone = 30 o C T zone = 26 o C T zone = 22 o C T zone = 18 o C -

14. How does COP change with change of refrigerant? R410AAmmonia Heat pump static optimization ASHRAE Conference, Chicago 2012 Page 13/14 T outside = 30 o C T zone = 30 o C T zone = 26 o C T zone = 22 o C T zone = 18 o C

15. Heat pump static optimization When 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 2012 Page 14/14 T outside = 15 o C T zone = 30 o C T zone = 26 o C T zone = 22 o C T zone = 18 o C Economizer modeCompressor running