HVAC Energy Evaluation: Direct Evaporative Cooling vs. Air Conditioning in a Warehouse
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Should the new warehouse use air conditioning or direct evaporative cooling?
It is a common enough question for designers and owners of new warehouses, distribution centers, and manufacturing buildings located in hot, dry climates. Both systems are common, economical, and easy enough to operate and each have their pros and cons.
In many cases, the decision for which system is made according to the use of the building. For example:
There is a huge makeup air requirement due to the exhaust from the manufacturing process, making evaporative cooling the obvious choice.
The humidity requirements of the stored product make evaporative cooling prohibitive.
Workers in this warehouse district often actively seek the more comfortable environment of air conditioning; ensuring a stable labor force requires the use of air conditioning.
In these cases, the decision between air conditioning (AC) and direct evaporative cooling (Evap) is already clear. In many other cases, it is a legitimately difficult decision. Knowing the annual utility costs of each system option would help the design team and owner make the right selection for their project.
To help answer this question, we created an energy model of a theoretical warehouse.
Figure 1: Image of the simulated warehouse energy model Building Characteristics [1]
A 560,000 ft2 (1400’ x 400’) warehouse in Phoenix, AZ
50’ wall heights
R-30 above deck roof insulation
Concrete tilt up walls, insulated with R-13 at 15’ AFF
30 dock doors on either side of the warehouse
LED lighting (0.35 W/ ft2)
Minor equipment energy (i.e. forklift charging, no manufacturing equipment or large scale robotics/conveyers)
5 day (M-F) per week operation – two x 8 hour shifts (6am – 10pm)
[1] These insulation levels are more common on air-conditioned buildings. An evaporatively cooled building will often have less insulation, though this not significantly change energy use.
Air Conditioning System Details
28, 20-ton RTUs (no heating considered)
Variable speed fans, manual OA dampers
12.5 EER cooling efficiency
0.08 CFM//ft2 in ventilation
Fans cycle on as needed to maintain load [2]
Cooling setpoint of 85°F
[2] Ventilation code requires constant fan operation. In practice, however, this is relatively rare.
Evaporative Cooling System Details
Cooling setpoint of 82°F that equipment runs as needed [3] to maintain, or as low as possible.
During hot, humid days, the Evap equipment may not be able to maintain setpoint.
The lower setpoint from AC is needed to account for comfort differences.
3 CFM/ft2 supply flow needed – 0.1” ESP (~3HP per 20,000 CFM)
Gravity Relief Dampers
30% bleed rate (for water usage calculation)
[3] Ventilation code would require some constant fan operation. In practice, however, this is relatively rare.
Utility Rates
The utility rate environment is an important design consideration! The likely utility rates from the two primary electric utility companies in the Phoenix, AZ area are accounted for in the simulation, along with the water rates for the City of Phoenix. The specific rate levels can be seen below.
APS E-32 Medium Electric Rate
Figure 4: City of Phoenix Water RatesSimulation Results
The simulation results are shown in Table 1 below and the subsequent sections.
Table 1: Simulation Results
Energy Differences
Figure 5 below indicates that the AC option uses slightly more energy – this is mainly attributable to the compressor energy use, which is nonexistent in the Evap option.
Due to the higher levels of airflow (about 6X!), the Evap option has much higher fan energy use. This fan usage offsets the higher cooling usage of the HVAC option.
Note that these results concern energy use only, not costs. The utility cost of the systems, including water, are addressed next.
Figure 5: Annual Energy Use of the Evap vs. AC systems.
Annual Costs – SRP Pricing
Including water costs, the Evap case is expected to use upwards of $25,000 more in utility costs than the AC option in the SRP rate environment.
SRP currently has smaller demand charges than APS and has the lower overall utility cost for this building’s operation between the two electric companies.
Figure 6: Annual Utility Costs of the Evap vs. AC systems, using SRP rates.Annual Costs – Current APS Pricing
Including water costs, the Evap case is expected to use upwards of $26,000 more in utility costs than the AC option in the APS rate environment.
Peak demand changes make up a higher percentage of the utility bill than SRP; it is also the higher overall cost.
Figure 7: Annual Utility Costs of the Evap vs. AC systems, using APS pricing. Future Considerations
Purchasing decisions should also consider future costs. Both energy and water costs should be expected to increase. Due to the impact of rooftop solar on the utility grid, it’s reasonable to predict that peak demand rates will increase at a higher rate than energy costs.
The table and figures show potential energy costs in 10 years from now (2031) using 1% annual water and energy increases, and 1.7% peak demand rate increases. Inflation is not considered.
Under these assumptions, the AC building still projects as having the lowest utility costs.
Figure 8: Future Utility Costs of the Evap vs. AC systems, using SRP pricing.
Figure 9: Future Utility Costs of the Evap vs. AC systems, using APS pricing.What it Means
The results of this study indicate the simulated warehouse with air conditioning is expected to have lower utility bills than one with direct evaporative cooling, both now and in the future. With this information at hand, owners and design teams can weigh this information along with the other factors that might affect their decision process, such as first cost and utility incentives, maintenance requirements and costs, and employee comfort.
What about your building?
Are you designing a warehouse building? Do you have this dilemma, but with a different location, utility rate structure, operation schedule, or systems in mind?
For a nominal cost, the energy model used for this study can be adapted to your project!
We can update this energy model to reflect the specific details of your project. This will give you insight for an approximate utility cost outcome!
If you would like the exact building simulated as well, a custom study and pricing can be arranged. We are always happy to help.
About Kraemer Consulting Engineers
Kraemer Consulting Engineers, PLLC (KCE) is a consulting engineering firm specializing in HVAC, electrical, and fire-life safety design, engineering, and management since 1994. KCE’s philosophy is to provide innovative and cost effective designs which embody the “best solution” for a project as viewed from “multiple” perspectives, not just an “engineering” perspective. Most often, the best solutions are also the simplest and simplicity is a key to making our designs easy to operate and maintain.
Visit our website at www.kraemereng.com for additional detailed information such as who we are, our mission, our services, our design approach, and our distinguishing features. Kraemer Consulting Engineers would welcome a chance to discuss your next project.
About G2 Energy Solutions
G2 Energy Solutions (G2) is an energy engineering consulting firm whose purpose is to serve the design team community and their projects. G2’s team of engineers and energy analysts bring their extensive experience and deep knowledge of energy modeling, codes and regulations, and incentive programs to benefit each project. The standards of G2’s work reflects the same values all great engineering and architecture teams must: provide exceptional quality, timeliness, and value. Learn more at www.g2energysolutions.com.
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