Consulting engineers routinely recommend chiller technologies to clients who are seeking the lowest annual energy operating costs to manage their facilities. Most often, evaluations are made comparing gas/steam absorption machines vs. rotary/screw/centrifugal electric-driven chillers. Naturally, first cost, reliability, maintenance, and equipment preferences are important factors during their assessment. However, annual operating costs are most often the primary consideration in their final decision.

The procedure that follows demonstrates a method to determine the magnitude of annual operating cost differentials for gas and steam vs. electric.

  • Determine the tonnage and select the type of electric chiller and the type of absorption chiller to be used in the comparison.
  • Calculate the operating cost factor (OCF) based on 1,000 annual hours of the equipment running at full load: OCF = 1,000 x (tonnage) x efficiency factor (EF) from Figure 1.
  • Once the OFC is known, multiply it by the energy cost ($/therm - $/kW - $/1,000 lbs of steam) to ascertain the annual operating energy cost (AOEC): AOEC = (OCF) x (energy cost).

Energy costs for central steam plants are usually in the range $6.00 to $8.00/1,000 lb of steam. The method used to determine the energy costs can be best shown by the following example:

Assume you are paying $0.70/gal for fuel oil or $0.5/therm for natural gas (142,500 Btu/gal at $0.70 gal or 100,000 Btu at $0.5/therm.) The cost to generate 1,000 lb of steam would be:

High-pressure steam at 100 psig:
For fuel oil:
$0.70/gal x 1,000 lb x 880 Btu/lb = $6.00/1,000 lb
142,500 Btu/gal. x 0.72 boiler efficiency

For gas:
$0.50/therm x 1,000 lb x 880 Btu/lb = $6.11/1,000 lb
100,000 Btu/therm x 0.72 boiler efficiency

Low-pressure steam at 12 psig:
For fuel oil:
$0.70/gal x 1,000 lb x 949 Btu/lb = $6.48/1,000 lb
142,500 Btu/gal x 0.72 boiler efficiency

For gas:
$0.50/therm x 1,000 lb x 949 Btu/lb = $6.59/1,000 lb
100,000 Btu/therm x 0.72 boiler efficiency

Naturally, this approximate cost of $6.00/1,000 lb is just fuel energy costs to produce the 1,000 lb of steam. Chemical treatment, operating personnel, and electrical power to equipment are not included in these costs.

Energy costs for electric are computed from the monthly utility bills in which the total costs (electric kWh consumption, demand charges, and fuel surcharges) are divided by the total monthly kWh consumption:

$/kWh= Monthly kWh cost + demand charges and fuel surcharges
Total monthly kWh consumed

Energy costs for gas are also computed from the monthly utility bill in which the total of all monthly charges is divided by the number of therms consumed ($/therm).

The following example will illustrate the procedure outlined above:

  • Assume you have an existing boiler plant capable of generating 100 psig at an energy cost of $6.00/1,000 lb or 12 psig steam at an energy cost of $6.48/1,000 lb.
  • Neglecting for this example first cost, equipment footprint size, maintenance, reliability and equipment preferences, determine the lowest operating cost for the following 300-ton water-cooled chillers. The abbreviations are defined as: single-effect, low-pressure steam absorption (SELPSA); double-effect, high-pressure steam absorption (DEHPSA); double-effect, gas absorption (DEGA); and electric-driven centrifugal (EDC).
  • Calculate the operating cost factors: SELPSA - OCF = (1,000 hr) x (300 tons) x (18 lb/ton) = 5,400,000 lb of steam. DEHPSA - OCF = (1,000 hr) x (300 tons) x (13.7 lb/ton) = 4,110,000 lb of steam. DEGA - OCF = (1,000 hr) x (300 tons) x (0.12 therms/ton) = 36,000 therms. EDC - OCF = (1,000 hr) x (300 tons) x (0.58 kW tons) = 174,000 kW.
  • Determine the annual operating energy cost (AOEC) for the SELPSA and DELPSA. SELPSA: AOEC = (OCF) x (energy cost)
    AOEC = (5,400,000) x ($6.48/1,000 lb)
    AOEC = $34,992
    DEHPSA: AOEC = (4,110,000) x ($6.00/1,000 lb)
    AOEC = $24,660
  • Determine the energy cost for gas and electric that would equal the annual operating cost of the DEHPSA.
    Electrical energy cost: AOEC = $24,660 = $ 0.14/kW OCF 174,000 kW
    Gas energy cost:AOEC = $24,660 = $0.68/Therm OCF 36,000 therms

This procedure enables one to quickly select the chiller technology that meets the energy code and also calculates the approximate magnitude of the chiller's annual operating cost.

In this example, the DEHPSA would have the lowest operating energy cost if the EDC utility kWh cost for electric exceeded $0.14/kWh. Conversely, the gas energy cost exceeds $0.68/therm for the DEGA. ES

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