Building Load and Weather Profiles
The building load profiles included in this program were generated from the public domain EnergyPlusTM files developed by Pacific Northwest National Labs (PNNL) for energy analysis work in conjunction with and with oversight from ASHRAE, for the development of ASHRAE/IES 90.1-2010 Energy Standard for Buildings Except Low-Rise Residential Buildings. The web site for the PNNL files can be found at https://www.energycodes.gov/development/commercial/prototype_models. PNNL used the EnergyPlus simulation program for various building types in the seventeen different ASHRAE Standard 169 climate zones.
For purposes of the myPLV analysis, all chiller loads in the building profiles of less than 1% max plant capacity have been set to zero. This is due to an issue in some of the PNNL simulations that resulted in a large number of run hours for central chiller plants at extremely low loads. Typically these loads were composed of pump heat, when normal building and/or chiller controls are expected to inhibit chiller plant operation. Deleting these very low loads results in a more typical run time for the chiller plant, as validated by comparing the modeled to actual building profiles. It is important to note that the user entry for "City" is provided for the user’s convenience to look up the appropriate climate zone. The PNNL models use the most representative weather data to represent the entire climate zone, so that the building load profile can be generalized to the entire zone and limit the scope to a reasonable number of simulations. In addition, the building load profile is scaled to the Building Peak Load entry as specified by the user on myPLV™ worksheet. The four performance points listed on the myPLV worksheet as the “myPLV™ Test and Submittal Points” are developed by grouping the load data into (4) chiller plant load ranges - 0 to 37.5%, 37.5 to 62.5%, 62.5 to 87.5%, and 87.5 to 100%. These groupings are the bins where the center weighted test points are 25%, 50%, 75%, and 94%. Note that the 100% design point is not included in the performance weighting criteria since the top most bin of the 94% grouping includes the performance at the 100% load point.
However, the 100% design point is required in the submittal entry with the user specifying the entering condenser temperature. It is critical to specify this point since the chiller is selected to provide operation at this design point and meet the energy codes. Certified performance at this point is also critical for confidently sizing the electrical wiring, circuit ampacity, breakers and other protection devices, power factor correction, starters, controls, safeties, filling out the nameplate on the equipment, as well as for complying with UL and other agency listings and codes.
Tower Control Method selections (water-cooled only)
The myPLV™ tool develops four submittal points for the evaluated chillers. For water cooled chillers, each submittal point is specified as a % load at an entering condenser water temperature (ECWT). The myPLV™ tool uses the cooling tower design condition performance and tower control method to determine the hour by hour ECWT for the chillers in order to ultimately calculate a ton-hour weighted ECWT for each submittal point.
Note: All calculations assume that a tower cell is sequenced with each chiller. Chillers are allowed to operate up to 100% of their design capacity before an additional chiller is enabled.
Tower Control Method selections:
Full Tower Fan Flow - This selection will determine the entering condenser water temperature that results from the cooling tower running at full fan speed at all conditions unless the resulting entering condenser water temperature is less than the minimum specified by the user. If full fan power results in a temperature less than the minimum value specified, the entering condenser water temperature will be set to the minimum value specified. The tower performance is assumed to have a tower approach (Tleaving - Tambient wb) that is equal to the user entry for the cell labeled Tower Full Load Design Performance - Tower Wet-Bulb Approach (F). This tower approach linearly degrades to zero at no heat rejection (0 chiller plant load). The tower approach also changes with the outdoor wet-bulb temperature as the heat capacity of the moist air stream changes with ambient wet bulb conditions.
Fixed Temperature - This selection is typical of many installations that run the cooling towers to a constant temperature set point. This selection requires a temperature set point entry by the user. The computations assume this temperature set point value will be the entering condenser water temperature for the chillers unless the tower capacity at the specific conditions encountered cannot achieve the set point temperature. In this case the leaving cooling tower temperature will be equal to a value at full tower fan flow conditions.
Fixed Tower Approach - This selection will compute an entering condenser water temperature equal to the outdoor wet-bulb temperature value plus the tower approach entered by the user. If the cooling tower cannot achieve this temperature value, the temperature will be returned as that achieved by the full tower fan flow condition. If this method would result in a temperature value less than the minimum condenser water temperature specified, the minimum temperature set point value will be used.
Chiller Tower Optimization - This selection simulates the behavior of Trane’s Chiller Tower Optimization control strategy which results in a dynamically changing entering condenser water temperature set point as a function of chiller loading and ambient wet-bulb temperature. If the cooling tower cannot achieve the target set point value, the full tower fan flow leaving water temperature will be returned. If this method would result in a temperature value less than the minimum condenser water temperature specified, the minimum temperature set point value will be used.