Estimate the heating and cooling energy and cost savings incurred by converting a 20,000-cfm single duct reheat system to a VAV system in an office building located in Chicago, Illinois. For the CV system, the return air is measured to be 76oF and the cold duct air temperature is set to be 58oF.

Question

Estimate the heating and cooling energy and cost savings incurred by converting a 20,000-cfm single duct reheat system to a VAV system in an office building located in Chicago, Illinois. For the CV system, the return air is measured to be 76oF and the cold duct air temperature is set to be 58oF. The HVAC system is operated 80 hours per week. For the analysis, consider only the sensible loads. The cost of fuel is $5/MMBtu and the boiler efficiency is 80 percent. The cost of electricity is $0.05/kWh and the chiller efficiency is 0.9 kW/ton. For Chicago, the number of weeks is 30 for the heating season (winter) and 21 for the cooling season (summer).

Accepted Answer

First, fuel savings are determined. These fuel savings are the results of the heating load reduction incurred from the CV to VAV conversion of the 20,000-cfm air-handling unit. Assuming that the average heating load X is 50 percent over the entire year, the annual fuel savings are attributed to the energy saved through the reduction of the air volume (by a fraction of 1 – X). This air volume has to be heated from: the cold air temperature [latex]T_{return}[/latex] = 8˚ to the return air temperature [latex]T_{return}[/latex] = 76˚ Fduring the entire year (52 weeks):

[latex]ΔFU_{H}=\frac { \dot{m}_{a}.c_{p}.(1-X)(T_{return}-T_{cold})N_{h,yr}}{η_{b}}[/latex]

[latex]ΔFU_{H}=\frac {20,000 cfm*(1.08Btu/cfmhr.°F)*(1-0.5)*(76°F-58°F)*(52*80hr)]}{(0.80)*(10^{6}Btu/MMBtu)} [/latex]

(b) The electricity savings are incurred from the cooling load reduction. Again, assuming that the average cooling load X is 50 percent over the cooling season, the annual cooling energy savings are estimated based on the energy saved through the reduction of the supplied air volume (by a fraction of 1 – X). This air volume has to be cooled from: the return air temperature [latex]T_{return}[/latex] = 76˚F to the cold air temperature [latex]T_{cold}[/latex] = 58˚F during the cooling season (21 weeks):

[latex]ΔkWh_{C} =\frac { \dot{m}_{a}.c_{p}.(1-X)(T_{return}-T_{cold})N_{h,C}}{12,000Btu/ton.hr}.kW/ton[/latex]

thus:

[latex]ΔkWh_{C} =\frac {20,000 cfm*(1.08Btu/cfmhr.°F)*(1-0.5)*(76°F-58°F)*(21*80hr}{12,000Btu/ton.hr} *0.90kW/ton[/latex]

or

[latex]ΔkWh_{C} = 24,494 kWh/yr[/latex]

The electricity cost savings amounts to $1,225/yr. Therefore, savings of $6,279 are achieved for both heating and cooling energy costs as a result of the CV to VAV conversion. Additional savings can be obtained from the reduction in fan power as illustrated in Example 7.2.

Question 7.5: Estimate the heating and cooling energy and cost savings inc...

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