Weather Report: November 2003
Well, if a traditional waterside economizer system just isn't in the budget, there could be another option. I like to refer to it as the "poor man's economizer" or PME. Basically, the PME uses AHU chilled water coils to reduce the chilled water temperature in the loop, essentially using the chilled water coil as a "dry cooler" to provide free cooling. And when the AHU requires preheat, there can be additional cost savings associated with preheat reduction.
Figure 3 illustrates one method to achieve this with some example values. Essentially, it's just a matter of adding some control valves, sensors, and programming. In the economizer cooling mode, the 60°F chilled water return bypasses the normal source of chilled water (chiller plant or purchased chilled water). The chilled water is cooled by the outside air to 45° at the AHUs, then directed to the economizer load chilled water coils. Note that the coils are then in series, so you need to make sure the pump can handle the additional head. Although the installation costs of this type of system would be much lower than that of the traditional waterside system, the operational cost savings can be comparable or even better in some cases.
Assume that the facility in Figure 3 is located in Cleveland, the total economizer load is 200 tons, and the total AHU airflow capacity is 150,000 cfm. Let's compare the potential savings of a PME to a traditional waterside system. For a traditional system, the number of economizer hours depends on the number of hours that the OA wet bulb temperature is less than setpoint (say 40° wb).
Similarly, the number of available PME hours depends on the number of hours that the outside air dry bulb temperature is less than the maximum temperature allowed by the load (Tmax). In other words, Tmax is limited by the desired supply air temperature. If you know you need to maintain 55°, then Tmax would be 55° less the air temperature rise caused by the economizer load.
In this example, Tmax = 55 - (200 x 12,000)/(1.08 x 150,000) = 40.2°. In Cleveland, there are 3,068 hours below 40° db for a PME, compared to 3,500 hours below 40° wb for a traditional system. Assuming 1.2 kW/ton and $0.09/kWh, the savings would be $66,269 compared to $75,600. Not bad, considering the installation cost difference.
In this case, the PME would be deactivated and the chillers would be energized whenever the outside air dry bulb temperature is above Tmax. If the AHUs are equipped with airside economizers, the dampers would continue to modulate as they normally would above that temperature. If the units are 100% outside air units with preheat coils, the PME provides supplemental preheat and the amount of additional savings can be significant. In the above example, assuming $0.75/therm and 80% heating efficiency, the annual preheat cost would be reduced from $112,000 to $57,000. Note, however, that the PME would have a negative impact on heat recovery savings, which also needs to be taken into consideration.
Is this measure for you? It depends on the amount of available chilled water coil capacity, airflow (VAV or constant volume), economizer load, occupancy schedule, and the amount of preheat savings (if any). Obviously, a detailed analysis is required, but one quick check would be to look at the cfm/load ratio. The amount of PME hours drops drastically as this ratio decreases, because Tmax and the associated hours decrease. The ratio for the above example would be 150,000/200 = 750. If you increase the load to 500 tons, the ratio is 300, Tmax = 18°, and the annual PME hours drop to 331. In that case, the PME would likely not be feasible, unless you arrange the plant to provide PME cooling to only a percentage of the load. ES
NOTE - This report is generated from raw data furnished by the National Weather Service (NWS). Normal max/min and degree day values are from the historical record provided by the National Climatic Data Center (NCDC). Normal values for VLI and economizer hours were derived from the TMY2 data set compiled by the National Renewable Energy Laboratory (NREL). ASHRAE design hours are number of hours that meet or exceed the 1997 ASHRAE Fundamentals design conditions. Airside and waterside economizer cooling hours are the quantity of hours that the outdoor air was below 55° dry bulb and below 40° wet bulb. The cooling ventilation load index (VLI) is the total (sensible + latent) energy in Ton-hrs/cfm required to maintain 55° discharge air temperature. Likewise, the heating VLI is the sensible heat energy in Therms/cfm required to maintain 55°. The humidification VLI is the amount of water in gal/cfm required to maintain 30% rh at 70° space temperature.