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MSHP and 2-speed ASHP expend more energy than single-speed ASHP?

asked 2020-06-15 09:45:57 -0500

y.tanaka gravatar image

updated 2020-06-16 22:12:02 -0500

Using ResStock 2.2.4, OpenStudio 2.9.0, and EnergyPlus 9.2

I'm finding that a single-speed ASHP expends less energy for heating and cooling compared to a two-speed ASHP with HIGHER SEER&HSPF and a MSHP with the same SEER and HSPF. Why is this so?

I'd imagine that MSHP's, without duct work, would be more efficient than that of an ASHP. And furthermore, a two-speed ASHP with larger SEER and HSPF values, should, by definition, expend less energy than that of a single-speed ASHP with the lower efficiencies. Is there something I'm missing in how ResStock models these types of heat pumps? I also noticed the kwh's used for the fans are noticeably larger for the single-speed ASHP -- does electricity_heating_kwh and electricity_cooling_kwh not take into account the energy expended for fans?

Details of my results and relevant information is below:

Climate Zone: IECC Zone 6

Sizing: All heat pumps' cooling and heating capacities were sized to meet unit_final.Heat_Load. This was done by altering hvac_sizing.rb

Simulation Output Report Values:

Average electricity_cooling_kwh for 100 buildings using identical weather file and seed:

  • ASHP, single-speed: 849
  • ASHP, two-speed: 1164
  • MSHP: 898

Average electricity_heating_kwh for 100 buildings using identical weather file and seed:

  • ASHP, single-speed: 9273
  • ASHP, two-speed: 10650
  • MSHP: 9571

Average electricity_cooling_fans_kwh for 100 buildings using identical weather file and seed:

  • ASHP, single-speed: 117
  • ASHP, two-speed: 26
  • MSHP: 0.9

Average electricity_heating_fans_kwh for 100 buildings using identical weather file and seed:

  • ASHP, single-speed: 1655
  • ASHP, two-speed: 884
  • MSHP: 177

Characteristics of the single-speed ASHP

seer=15.0 hspf=8.5 eer=12.5 cop=3.5 shr=0.73 fan_power_rated=0.365 fan_power_installed=0.5 min_temp=0.0 crankcase_capacity=0.02 crankcase_temp=55.0 eer_capacity_derate_1ton=1.0 eer_capacity_derate_2ton=1.0 eer_capacity_derate_3ton=1.0 eer_capacity_derate_4ton=1.0 eer_capacity_derate_5ton=1.0 cop_capacity_derate_1ton=1.0 cop_capacity_derate_2ton=1.0 cop_capacity_derate_3ton=1.0 cop_capacity_derate_4ton=1.0 cop_capacity_derate_5ton=1.0 heat_pump_capacity=autosize supplemental_efficiency=1.0 supplemental_capacity=autosize dse=NA

Characteristics of two-speed ASHP

seer=18.0 hspf=9.6 eer=13.6 eer2=12.5 cop=4.2 cop2=3.7 shr=0.71 shr2=0.725 capacity_ratio=0.72 capacity_ratio2=1.0 fan_speed_ratio_cooling=0.86 fan_speed_ratio_cooling2=1.0 fan_speed_ratio_heating=0.8 fan_speed_ratio_heating2=1.0 fan_power_rated=0.14 fan_power_installed=0.3 min_temp=0.0 crankcase_capacity=0.02 crankcase_temp=55.0 eer_capacity_derate_1ton=1.0 eer_capacity_derate_2ton=1.0 eer_capacity_derate_3ton=1.0 eer_capacity_derate_4ton=0.93 eer_capacity_derate_5ton=0.9 cop_capacity_derate_1ton=1.0 cop_capacity_derate_2ton=1.0 cop_capacity_derate_3ton=1.0 cop_capacity_derate_4ton=1.0 cop_capacity_derate_5ton=1.0 heat_pump_capacity=autosize supplemental_efficiency=1.0 supplemental_capacity=autosize dse=NA

Characteristics of MSHP

seer=15.0 min_cooling_capacity=0.4 max_cooling_capacity=1.2 shr=0.73 min_cooling_airflow_rate=200 max_cooling_airflow_rate=425 hspf=8.5 heating_capacity_offset=2300 min_heating_capacity=0.3 max_heating_capacity=1.2 min_heating_airflow_rate=200 max_heating_airflow_rate=400 cap_retention_frac=0.25 cap_retention_temp=-5 pan_heater_power=0 fan_power=0.07 is_ducted=false heat_pump_capacity=autosize supplemental_efficiency=1.0 supplemental_capacity=autosize dse=NA

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answered 2020-06-15 21:56:30 -0500

updated 2020-06-15 22:00:23 -0500

electricity_cooling_kwh and electricity_heating_kwh do not include the fan energy, so you need to add them all together to get the overall energy use:

  • ASHP, single-speed: 11,894
  • ASHP, two-speed: 12,724
  • MSHP: 10,647

So the SEER 15/HSPF 8.5 MSHP uses 10% less energy than the SEER 15/HSPF 8.5 single-speed ASHP.

The fact that the SEER 18/HSPF 9.6 two-speed ASHP uses 7% more energy than the SEER 15/HSPF 8.5 single-speed ASHP is not too surprising. We've seen that previously and investigation showed that it was caused by increased duct losses with the two-speed equipment due to longer run times. This phenomenon has been seen in field testing of variable capacity heat pumps: https://www.aceee.org/files/proceedin...

Note that you don't need to modify hvac_sizing.rb to override ACCA Manual S and size for maximum heating load. You can set heat_pump_capacity=autosize for max load. See the argument description here:

If using '#{Constants.SizingAuto}', the autosizing algorithm will use ACCA Manual S to set the heat pump capacity based on the cooling load. If using '#{Constants.SizingAutoMaxLoad}', the autosizing algorithm will override ACCA Manual S and use the maximum of the heating and cooling loads to set the heat pump capacity, based on the heating/cooling capacities under design conditions.

https://github.com/NREL/OpenStudio-Bu...

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Asked: 2020-06-15 09:45:57 -0500

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Last updated: Jun 16 '20