The Weather Report: December 2002
Here are the ground rules: The model consists of a 50,000-cfm air-handling system with a minimum outdoor air volume setting of 10% of the total airflow or 5,000 cfm. The system operates 24/7. The overall cooling efficiency is 1.25 kW/ton. The electric rate is $0.075/kWh. The space temperature is maintained at 75 degrees F and the supply air temperature setpoint is 55 degrees. The wet bulb temperature threshold to activate the waterside economizer is 40 degrees. Also, there is a latent heat (moisture) gain from 200 people located in the space served by this system.
For a system with no economizer, the model simply calculates the sensible and latent cooling costs each hour from the mixed entering and leaving air condition's with the outside air dampers at minimum position at all times. For a wet bulb economizer, these values are set to zero whenever the outside air wet bulb temperature is below 40 degrees. For a dry bulb economizer, the values are set to zero when the outdoor air dry bulb temperature is less than 55 degrees, which would simulate the economizer dampers modulating maintaining setpoint in lieu of mechanical cooling. When the outdoor air temperature is above 55 degrees, the dampers are at the minimum outdoor air position.
For an enthalpy-controlled economizer, the cooling values are set to zero below 55 degrees outside air temperature. But when the outdoor air enthalpy is less than return enthalpy and the outdoor air dewpoint is above 55 degrees, the outdoor air dampers are 100% open. The outside air damper is also 100% open when the dewpoint is below 55 degrees and the outside air temperature is between 55 degrees and 75 degrees.
Note that on Figure 2 that without any economizer, the cooling costs differ only slightly from city to city because the air is 90% recirculated with only a 10% impact from outdoor air. The data also shows that regardless of climate, the enthalpy economizer offers the most cooling cost reduction, followed by the dry bulb economizer and finally, the waterside economizer. This order makes sense because there are always fewer available waterside economizer hours (below 40 degrees wb) than there are dry bulb economizer hours (below 55 degrees db). The gap widens even more if you consider the pumping and cooling tower fan energy costs required to operate a waterside economizer.
Both waterside and dry bulb economizers provide 100% of the required cooling whenever the outdoor air conditions are below 40 degrees wb or 55 degrees db. Enthalpy economizers, on the other hand, provide all of the free cooling offered by the dry bulb economizer plus additional (partial) cooling when outdoor air conditions permit. Enthalpy economizer savings can be substantial if there are a large number of hours where the outdoor air enthalpy is less than the return air enthalpy at dry bulb temperatures that are greater than the desired setpoint (55 degrees). When this condition occurs, a dry bulb economizer would drive the dampers to the minimum outdoor air position because the outdoor dry bulb is higher than 55 degrees. An enthalpy economizer would open the dampers to the 100% full outside air position to take advantage of the lower outdoor air enthalpy. The mechanical cooling system then has less work to do to achieve setpoint.
To illustrate, the results from the model using the hourly data from Seattle's mild climate show us that an enthalpy economizer will result in very significant cooling energy cost savings (about $55,000 annually in our example) because there are a large number of hours in Seattle that meet this condition. Conversely, the feasibility of a waterside economizer in Seattle could be questionable primarily because of the mild climate and resulting deficiency of wet bulb hours below 40 degrees.
Obviously, in general the results in Figure 2 show that all economizer systems located in the colder climates will offer more cooling energy cost savings. However the data also shows that applying enthalpy economizer controls in Atlanta or even Phoenix could be worthwhile if the installation can be accomplished with relatively low cost.
EDITOR'S NOTE: The images associated with this article do not transfer to the Internet. To review the figures, please refer to the print version of this issue.