Sunday, November 11, 2007

Outdoor Air Load Estimator

Topic: A nifty Java-based outdoor air load calculator, courtesy of ArchEnergy Inc.

Consider this calculator, which includes American and Canadian weather tables, for reality checking energy recovery numbers calculated by energy modeling programs. Following is a sample output report:


Result summary for Calculation Number: 5
Location: BURLINGTON, Vermont
Elevation: 341 ft
Operating Hours: 0:00 o'clock until 0:00 o'clock
Hours of Operation: 24
Makeup Air Flow: 183000 cfm
Thermostat Setpoints: Heating = 68 F, Cooling = 72 F

Dehumidification was set to limit the Relative Humidity to: No Dehumidification

Heating was locked out during: --
Cooling was locked out during: --

The Lockout of Heating or Cooling systems resulted in...
Insufficient Heating during: --
Insufficient Cooling during: --

The Heating Design Load is: 18891.3 kBtu/h
The Cooling Design Load is: 4754.9 kBtu/h

Calculated Monthly loads:
Month Heating Load Cooling Load
January : 7,847,593 kBtu 0 kBtu
February : 7,329,964 kBtu 0 kBtu
March : 6,296,320 kBtu 0 kBtu
April : 3,911,545 kBtu 28,900 kBtu
May : 1,574,753 kBtu 97,140 kBtu
June : 889,352 kBtu 128,736 kBtu
July : 399,761 kBtu 382,194 kBtu
August : 532,923 kBtu 221,369 kBtu
September : 1,517,449 kBtu 33,329 kBtu
October : 2,836,548 kBtu 0 kBtu
November : 4,821,821 kBtu 0 kBtu
December : 6,956,354 kBtu 0 kBtu
Total_Year : 44,914,382 kBtu 891,668 kBtu

Supply Exhaust
Total Static Pressure: 3.5 inW 1.0 inW
Fan Type: Forward_Curved Forward_Curved
Fan Efficiency: 63.0 % 63.0 %
Motor Class: Standard Standard
Motor Efficiency: 91.0 % 89.0 %
Motor Output Power: 159.032 HP 45.438 HP
Moter Rated Input: 130.319 kW 38.071 kW
Motor Energy Consumption: 1141594 kWh 333499 kWh

Set system runtimes matching equipment operational schedule to establish an upper bound for energy recovery savings. Here the building is operating at 7x24, and if the rough cost for a therm of natural gas and a kilowatt hour of electricity are $1.20 and $0.10 respectively, then an upper bound on heat recovery savings is:

  • 1.20 $/therm * 44,914,382 kBTU / 100 kBTU per therm = $530,973 from gas, and
  • $0.10 $/kWH * 1,141,594 kWH = $114,160 from electricity,
  • for a total of $645,133 heating load cost with systems running 24 hours a day, seven days a week, for one year at constant volume.
This number is the maximum theoretical value of the savings, however it cannot be approached without 1) consistent internal heat gains -- heat gains sufficient to keep the exhaust airstream temperature elevated effectively above the outdoor air temperature, and 2) significant fan motor energy expenditures.

What typically happens in laboratory-type buildings is that while there may be a consistent heat recovery demand, the heat available to recover varies widely with occupancy. And likely there will be a number of hours in a 7 x 24 year-round operation when the outside air approaches to within 5° of the exhaust air, rendering energy recovery essentially ineffective.

Have not counted heat recovery from a cooling standpoint, as the number of annual hours where the outside air exceeds the exhaust air by more than 5° F in this climate most likely won't be very compelling.

So for how many hours per year does the exhaust air temperature exceed the outside air and by how much, on the average? Just check that 'Outside & Exhaust Air Loads Report' in the SIM file...if it actually existed, it might tell you...;-) All of the information is calculated and carried in the program, and presumably is output to the SIM file directly and indirectly in various places.

Am hoping to figure a way to pull a report like the above-mentioned together one of these days...if anyone has an Awk-based outside air loads report, even a rudimentary one as a starting point, I'd be much obliged.

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