Overview


The HVAC System Performance Tool (TSPR) allows users to specify multiple chillers in a loop. Multiple chillers on the same loop are common in commercial buildings, allowing plant equipment the flexibility to scale and meet loads as needed by staging chillers. This documentation will describe how the HVAC System Performance Tool interprets and simulates multiple chillers on a shared chilled water plant loop.


The TSPR web tool's Mechanical Equipment Schedule tab provides an interface for users to define the chillers within their building's HVAC systems.  The Plant Schedules Entries table provides the following inputs for defining a chiller's performance and are critical for simulating the chillers in TSPR's backend simulation engine:


  • Quantity – the number of chillers represented by the entry
  • Condenser Type – determines whether the chiller is an Air or Water cooled chiller
  • Compressor Type– the design mechanism used to compress refrigerant in the chiller.
    • Centrifugal
    • Reciprocating
    • Scroll/Screw
  • Rated Capacity – the maximum cooling output that a chiller can provide under standardized conditions. Measured in tons.
  • Rated Full Efficiency – the measure of a chiller’s efficiency when operating at its rated capacity. 
    • Available units: EER, COP, kW/ton
  • Part Load Efficiency -- the measure of a chiller’s efficiency when operating below its rated capacity. For more information, please view our documentation on Part Load Efficiency here.
    • Available units: EER, COP, kW/ton 


The HVAC System Performance Tool webtool passes these inputs into the tool's back end, OpenStudio and E+ simulation engine. The simulation engine uses the user provided inputs to define a chilled water (CHW) plan loop in EnergyPlus. The multiple-chiller modeling approach in HVAC System Performance is restricted to 3 chillers on a single CHW loop. The section below details the modeling approach followed by the tool based for various user input scenarios


Guiding Principles: Multiple Chillers


The guiding principles around combining chilled water (CHW) plant loop equipment are:


  1. The simulated plant loop will consist of no more than 3 chillers:

    1. If a user define more than 3 chillers serving a single CHW plant loop,  they will be consolidated down to three. This consolidation requires combined and weighting certain inputs. More information on how this is done can be found in the Chiller Consolidation section of this document.
  3. Only chillers with the same condenser type can be specified for a single CHW loop through the tool front end. The user interface prevents a user from defining a CHW loop with both air cooled and water cooled chillers.
  4. Chillers will be staged by chiller design capacity and relevant combinations.

    1. The simulation engine determines two classes of chillers-- a 'primary' chiller and ‘secondary’ chillers, each with weighted full and part-load efficiencies and capacities. This is based on logic found in the Chiller Consolidation section of this documentation.
    2. The default staging strategy for a plant loop with 3 chillers is as follows:
      1. First stage: the primary chiller meets the load, until it reaches 80% of its rated capacity
      2. Second stage: Once the primary chiller reaches 80% rated capacity, operation switches over to a secondary chiller until it reaches 100% of its capacity.
      3. Third stage: The tertiary chiller is loaded when 100% of capacity of the secondary chiller is reached. 
      4. Fourth stage: Once load exceeds the capacity of both secondary chillers combined, all 3 chillers are uniformly loaded.
    3. The staged capacity levels for a two-chiller system are:
      1. First stage: the primary chiller meets the load, until it reaches 80% of its rated capacity
      2. Second stage: Once the primary chiller reaches 80% rated capacity, operation switches over to a secondary chiller until it reaches 100% of its capacity.
      3. Third stage: Once load exceeds the capacity of the secondary chiller, both chillers are uniformly loaded.




Chiller Consolidation


When a chilled water loop consists of greater than 3 chillers, the HVAC System Performance Tool simulation engine consolidates the portfolio of chillers into two categories-- a single, lower capacity, primary chiller used for part load scenarios, and two identical larger chillers.


Consolidation is broken down for different scenarios below. Please see the Example section to see this consolidation with


Greater than 3 chillers, 2 compressor types


  • Consolidate compressor type to either Centrifugal or  Positive Displacement
    • Scroll/screw = Positive Displacement
    • Reciprocating = Positive Displacement
    • Centrifugal = Centrifugal
  • Add up capacities of each compressor type. The sum of each chiller type’s capacities will be used to help determine the ‘primary’ chiller.
  • The compressor type group with the smaller total rated capacity will be consolidated into a ‘primary’ chiller. The larger group will get consolidated into two chillers of equal capacity.
  • Primary chiller:
    • Full and part load efficiency is equal to the capacity weighted average of each chiller's efficiency in the smaller chiller compressor type group.
    • As-designed capacity is equal to the capacity weighted average of each chiller's capacity in both chiller groups.
  • Secondary chillers:
    • Full and part load efficiency is equal to the capacity weighted average of each chiller's efficiency in the larger chiller type group.
    • As-designed capacity is equal to the capacity weighted average of each chiller's capacity in the larger chiller type group divided by 2.
  • Simulated capacity is then assigned based on a ratio of each chiller's as-designed capacity versus the chilled water loop's total design capacity times the autosized capacity of the chilled water loop.


Greater than 3 chillers, 1 compressor type


  • Primary chiller:
    • The chiller with the highest full load efficiency becomes the primary chiller. If more than one chiller matches the highest full load efficiency, the one with the lowest capacity becomes the primary chiller.
  • Two secondary chillers:
    • Full and part load efficiency is equal to the capacity weighted average of each chiller's efficiency in the larger chiller type group.
    • Capacity is equal to the sum of the capacity of each chiller amongst the remaining chillers divided by 2.
  • Simulated capacity is then assigned based on a ratio of each chiller's as-designed capacity versus the chilled water loop's total design capacity times the autosized capacity of the chilled water loop.


Less than or equal to 3 chillers, any number of compressor types


  • Every chiller capacity and compressor type are equal to their as designed specifications, no consolidation needed.
  • Simulated capacity is then assigned based on a ratio of each chiller's as designed capacity versus the total design capacity times the autosized capacity of the chilled water loop.
  • Primary chiller:
    • This is the smallest with the lowest capacity. If more than one are equal, it becomes the one with the highest efficiency. If same size and efficiency, the chiller with displacement is selected as the smallest.
  • Remainder of chillers are not consolidated.
  • Simulated capacity is then assigned based on a ratio of each chiller's as-designed capacity versus the chilled water loop's total design capacity times the autosized capacity of the chilled water loop.


Example:


Four chillers, one scroll/screw, one reciprocating, and two centrifugal:


Inputs

Consider the following set of chillers on a common chilled water loop, with the following full-load and rated capacity values:

  • Scroll/Screw chiller: 3.517 COP, Rated Capacity: 250 tons
  • Reciprocating: 3.0 COP, Rated Capacity: 250 tons
  • Centrifugal: 3.517 COP, Rated Capacity: 500 tons
  • Centrifugal: 4.517 COP, Rated Capacity: 200 tons


Determining Primary vs. Secondary Compressor Type

Determining the sum of the two compressor types as-designed capacities:

  • Positive Displacement = Scroll/screw chiller (250 tons) + reciprocating chiller (250 tons) = 500 tons
  • Centrifugal = Centrifugal 1 chiller (500 tons) + centrifugal chiller 2 (200 tons) = 700 tons

The positive displacement compressor type chillers have the lower total as-designed capacity. The ‘primary’ chiller will therefore be a displacement chiller.

Determining Full-Load Efficiency and As-Designed Capacities

The efficiency for this ‘primary’ chiller will be the capacity weighted average of the two displacement chillers.

COPprimary = 3.517 * (250/500) + 3.0 * (250/500) = ~3.26 COP


Likewise, the as-designed capacity is the sum of the two positive displacement chiller capacities.

As-Designed Capacityprimary = (250 + 250) = 500 tons

 

The two secondary chillers will therefore be centrifugal. The efficiency for this ‘secondary’ chillers will be the capacity weighted average of the two centrifugal chillers.

COPsecondary = 3.517 * (500/700) + 4.517 * (200/700) = ~3.80 COP

The as-designed secondary chiller capacities for the two secondary chillers are equal to the total capacity of the centrifugal chillers divided by two.

As-Designed Capacitysecondary = (500 + 200)/2 = 350 tons x 2 chillers


Determining Simulation Capacities

The simulation engine will then run a sizing run for a chilled water loop served by three chillers with the capacities and compressor types listed above. Let’s say this simulation run resulted in an autosized capacity of 1000 tons. The final simulation run will assign the following capacities to these three chillers based on their capacity weighted averages.

  • Simulated Capacityprimary, displacement = (250 + 250)/ (250+250+500+200) * 1000 tons = ~417 tons
  • Simulated Capacitysecondary, centrifugal = (500 + 200)/ (250+250+500+200) * 1000 tons /2 chillers = ~287 tons each

 

Staging Strategy

 

The capacity ranges for the final chiller staging are as follows:

  • Primary chiller:
    1. Up to 80% of capacity = 0.8 * 417 = 0 to 333.6 tons
  • Secondary chiller:
    1. 80% of capacity to capacity of secondary chiller = 336 to 287 tons
      1. Since this range does not make sense given the capacities, this stage will never fire. Alternative configurations with larger secondary chillers will allow for this configuration, but the combination used in this example does not.
  • Both secondary chillers:
    1. 80% of capacity to capacity of both secondary chillers together = 336 to 573 tons
  • All three chillers:
    1. If greater than both secondary chillers, all three chillers will come online = > 573 tons