EnergyPlus Sizing Calculations
This section outlines the sizing methodology applied in EnergyPlus, which encompasses the following steps:
Conducting a zone-by-zone heat balance load and air-flow calculation for various design days.
Calculating design heating and cooling capacities, as well as fluid flow rates, at the zone, system, and plant levels.
Employing modular, component-specific sizing algorithms for each HVAC element.
Providing options to monitor initial sizes' performance across multiple design days, enabling adjustments and subsequent plant-level recalculations.
Of paramount importance to the user is that EnergyPlus anchors HVAC sizing calculations on their inputs for each Zone, System (such as air handling units), and Plant loop (e.g., chilled water loop).
HVAC Zone and System Sizing Routine
The following high-level summary overviews the key steps in the EnergyPlus sizing routine for the zones and HVAC systems.
Retrieve user sizing inputs for the Zone, systems, and plant.
Initiate Zone design calculations. Iterate over all sizing periods, typically by each day, including the following steps:
2.1. Initialize zone design load and flow rate sequences.
2.2. Within each hour of the day, and within each time step in a zone, perform heat balance calculations, including an HVAC simulation. The ideal zonal system is employed for design load and flow rate calculations.
2.3. Save the results of the ideal zonal system calculations in the design load and flow rate sequences.
2.4. At the end of each day, calculate peaks and moving averages from the zone design sequences for each design day.
2.5. Calculate peak heating and cooling loads and flow rates for each zone over all sizing periods, which include design days and periods from the weather file, if specified. Save the corresponding design load and flow rate sequences for use in system design calculations.
Initiate System (i.e. AHU) design calculations.
3.1. Obtain the necessary input for system design calculations, including information on zone and central system inputs required to determine air loop connectivity.
3.2. Loop over all sizing periods (typically by each day), including the following steps:
3.2.1. Initialise system design load and flow rate sequences.
3.2.2. Within each hour of the day and within each zone time step, obtain outside conditions and save the results of system design calculations in the system design load and flow rate sequences.
3.2.3. At the end of each day, calculate peaks and moving averages from the system design sequences for each sizing period.
3.2.4. Calculate peak heating and cooling loads and flow rates for each system over all sizing periods, which include design days and periods from the weather file, if specified. Save the corresponding design load and flow rate sequences for use in component sizing calculations.
Coincident Plant Sizing Routine
In the EnergyPlus sizing process, the Coincident sizing option for Plant Loops is now considered. If the user has chosen Coincident sizing for any plant loop, the following steps occur:
Establish the data logging setup to monitor operation during HVAC Sizing Simulations. This involves selecting specific variables for recording, such as system node states or load output variables.
Iterate over the specified number of Sizing Passes (default 5). This set of sizing periods, run as HVAC Sizing Simulations, can iterate up to a maximum limit on the number of passes.
Conduct complete HVAC Sizing Calculations for zones, systems, plant, and components for each day, looping over all the sizing periods by each day.
Record zone and HVAC data for further analysis.
Retrieve log data and execute the coincident plant sizing algorithm, as detailed below.
Repeat system and component sizing methods.
The coincident plant sizing algorithm can be summarised as follows:
Identify the maximum mass flow rate over all sizing periods, along with the coinciding return temperature and load. Record the sizing period and timestep. This data is logged for the plant loop supply side inlet node.
Determine the maximum load and the corresponding mass flow and return temperature. Record the sizing period and timestep. For heating or steam plant loops, the load is associated with the Plant Supply Side Heating Demand Rate output variable, and for cooling or condenser plant loops, it's associated with the Plant Supply Side Cooling Demand Rate output variable.
Calculate a maximum design flow rate based on the maximum load from step 2, the specified temperature difference in the Plant Sizing user input, and the specific heat of the plant fluid at 5°C.
Compare the flow rate from step 1 to the flow rate from step 3 and choose the higher value.
Apply a sizing factor to the flow rate from step 4 as desired, with the user having different options for sizing factors.
Compare the flow rate from step 5 to the current plant loop flow rate and calculate a normalized change. If the change is significant (exceeding a threshold of 0.005), adjust the size result for the plant loop, triggering resizing calculations for both the plant system and component levels. This may lead to another Sizing Pass. If the change is not significant, the sizes remain unchanged.
This method significantly impacts plant loop sizes. For instance, a hot water plant might be about half the size compared to the noncoincident sizing approach. The algorithm is particularly effective in increasing plant flow rates above pump sizes, though it can also reduce flow rates. It's not uncommon for sizes to fluctuate across multiple Sizing Passes, as the algorithm may switch between coincident flow and coincident demand to find a size that meets the conditions. Users can review detailed reports in the EIO file for insights into each algorithm's execution.
The use of coincident plant sizing can lead to more precise and efficient sizing of plant loops, especially in systems with variable load profiles and complex requirements.
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