Gas Cooling Screening Analysis Tool

This calculation tool is designed to be used for preliminary screening. Of necessity, it makes assumptions intended to simplify the process. Final decisions should be based on a detailed analysis of all relevant factors, including actual cooling load shape, local weather, part-load performance of equipment, and specific utility rates.

The values already entered in certain cells are examples, generally using national average data. Replace them with values that correspond to your own situation (for instance, if you know it, enter your actual electricity cost in $/kWh). Then click “Calculate” to determine how the simple payback compares for a natural gas cooling system and an electric system. To estimate equivalent months of full-load cooling demand, consult chiller logs or compare the maximum expected temperature for each month with the maximum expected temperature in the design month. (Expected temperatures can be determined based on weather data for your location; data can be found at the National Climatic Data Center.)

These steps will yield a peak cooling demand profile for the year. Once you have this profile, translate it into an equivalent number of months of full cooling demand by summing the values for each month and dividing by the full-load value.

Equivalent full-load hours per year for cooling can be estimated from chiller logs or cooling degree-days. A discussion about estimating cooling loads using degree-days can be found in the 2009 ASHRAE Fundamentals Handbook, Chapter 19, p. 19.17. Also, the article “Development of Equivalent Full Load Heating and Cooling Hours for GCHPs” (ASHRAE Transactions 4558 (RP-1120), 2002) contains estimates of equivalent full-load hours for some locations in the U.S.

Other values required by this calculation tool should be available from utility bills and equipment manufacturers’ data.

Go back to the natural gas chillers topic.

Cooling load
tons at full load
Equivalent months of full cooling demand
Equivalent full load hours per year
Utilities sales tax
Summer gas price
Summer peak electric demand price
Summer peak electric energy price
Net installed cost premium for gas system
Added maintenance cost for gas chiller
($0.01/ton-hour default for engines)
($0.005/ton-hour default for absorption)
Gas chiller COP1,2
(With gas higher heating value–HHV)
Incremental power for gas chiller auxiliary systems (gas chillers require greater condenser water flow—typically 5 gallons per minute per ton [gpm/ton] versus 3 gpm/ton for an electric chiller—resulting in increased power demand. Increased demand is typically about 0.10 kW/ton for single-effect absorption or 0.05 kW/ton for double-effect absorption or an engine-driven compressor.)
Electric chiller efficiency
(preferably integrated part-load value)
Electric cooling: annual operating costs
Electric system total
Gas cooling: annual operating and incremental maintenance costs
(Does not include savings from engine heat recovery, which can be as high as 15 to 20 percent of fuel input.)
Gas fuel
Gas system total
Gas cooling savings
Gas system net cost premium
Simple payback
Notes: COP = coefficient of performance; kW = kilowatt; kWh = kilowatt-hour.
1. If the efficiency data are in terms of Mcf/ton-hr, calculate COP from COP = (1/85) x (1/Mcf/ton-hour).
2. All dollar values are U.S. dollars.