eQuest Latent Heat Recovery

Topic: Parametrically simulating latent heat recovery in eQuest requires specifying a number of items in coordination to obtain meaningful results.

Following is model for a simple one-system one-zone building. The enthalpy wheel is only of interest in the Seattle climate due to a tight temperature & humidity control requirement (73°F ± 2°F and 50% RH ± 3%) for the building in occasional operating modes.

• Envelope improvements including reduced infiltration from an estimated 0.1 to 0.01 CFM/SF were modeled as the first parametric run,
• Increased temperature and humidity precision as the second, and
• Heat recovery using the enthalpy exchanger as the third.

The parametric run order is important -- modeling heat recovery before increased environmental precision produces results that don't isolate heat recovery economics.

$ This example models a sensible-latent enthalpy wheel air-to-air heat
$ exchanger (EW A2A HX) as the energy recovery equipment.
"EL1 Exhibit Hall Sys" = SYSTEM
TYPE = SZRH
HEAT-SOURCE = HOT-WATER
ZONE-HEAT-SOURCE = HOT-WATER
BASEBOARD-SOURCE = NONE
SIZING-RATIO = 1
$ The following parameter may be spec'd in precision control mode (PCM)
HUMIDIFIER-TYPE = ELECTRIC
MAX-SUPPLY-T = 95
MIN-SUPPLY-T = 55
$ The following number is changed to 53 in PCM
MAX-HUMIDITY = 80
$ The following number is changed to 47 in PCM
MIN-HUMIDITY = 10
$ The following parameter results in 1 OACH at 50% turndown, see zone flow below.
MIN-OUTSIDE-AIR = 0.32
$ The following parameter is changed to FIXED-FRACTION in PCM, which
$ caps the capacity of the heat wheel at MIN-OUTSIDE-AIR, in this
$ case 32% of 50,000 or 16,000 CFM
OA-CONTROL = OA-TEMP
$ Change the following keyword to YES in heat recovery parametric run; all other
$ heat recovery parameters are ignored when set to NO for the baseline case:
RECOVER-EXHAUST = NO
$ Full OSA economizer is used when 8precision control is not required;
$ the following number is reduced to 0.5 in precision control parametric run,
$ which effectively limits the capacity of the ERV to 50% of the main unit:
FAN-SCHEDULE = "S1 Sys1 (VAVS) Fan Sch"
$ change to VFD from default, unless VFD needs justification...
FAN-CONTROL = SPEED
SUPPLY-STATIC = 3.5
RETURN-STATIC = 1.125
$ change to VFD from default, unless VFD needs justification...
FAN-EIR-FPLR = "Fan-Pwr-fPLR-w/VFD"
$ change to VFD from default, unless VFD needs justification...
RETURN-FAN-CONTR = SPEED
$ change to VFD from default, unless VFD needs justification...
RETURN-EIR-FPLR = "Fan-Pwr-fPLR-w/VFD"
REHEAT-DELTA-T = 30
HW-LOOP = "Hot Water Loop"
CHW-LOOP = "Chilled Water Loop"
$ The following parameter may be spec'd in the precision control parametric run
HUMIDIFIER-LOC = IN-AIR-HANDLER
CONTROL-ZONE = "EL1 Exhibit Hall Zn"
ERV-RECOVER-TYPE = ENTHALPY-WHEEL
$ The following six parameters are the default values, hardcoded here
$ to provide a sense of their relative magnitudes:
ERV-SENSIBLE-EFF = 0.76
ERV-SEN-FILM-RES = 0.7
ERV-SEN-FILM-EXP = 0.5
ERV-LATENT-EFF = 0.74
ERV-LAT-FILM-RES = 0.7
ERV-LAT-FILM-EXP = 0.4
$ Remaining parameters are characteristic of a sidecar enthalpy wheel:
ERV-RUN-CTRL = OA-EXHAUST-DH
ERV-RECOVER-MODE = OA-HEAT/COOL
ERV-CAP-CTRL = MODULATE-HX
ERV-FANS = SELF-CONTAINED
ERV-OA-STATIC = 0.7
ERV-EXH-STATIC = 0.5
ERV-FROST-CTRL = USE-CAP-CTRL
..
"EL1 Exhibit Hall Zn" = ZONE
TYPE = CONDITIONED
$ The following number allows the VAV system to turn-down by 50%
MIN-FLOW-RATIO = 0.5
MIN-FLOW-SCH = "EL1 Bldg Occup Sch"
ASSIGNED-FLOW = 50000
$ The following number is changed to 72 in the precision control parametric run
DESIGN-HEAT-T = 70
HEAT-TEMP-SCH = "S1 Sys1 (VAVS) Heat Sch"
$ The following number is changed to 75 in the precision control parametric run
DESIGN-COOL-T = 80
COOL-TEMP-SCH = "S1 Sys1 (VAVS) Cool Sch"
SIZING-OPTION = FROM-LOADS
SPACE = "EL1 Exhibit Hall Spc"
..

(click on the image to see a larger version)

The annual energy cost results are essentially as expected. Compared to the...

1. Baseline condition using 100% outside air economizer, the
2. Envelope improvements save a bit, the
   
3. Precision control costs additionally, and
4. Enthalpy recovery reduces energy costs attractively.

Enthalpy recovery reduces the operational energy costs of precision temperature and humidity control to less than what is required for a code-compliant 100% outside air economizer system.

This simplifies operation, allowing to the system to function in heat recovery mode all year round, under all occupancy and ambient conditions.