Cooling

HVAC tab in model data - Simple HVAC

You can enter details on the cooling system including the maximum capacity (at zone level).

Note: for all systems, cooling setpoint temperatures are defined on the Activity tab.

Cooled

If the zone is to be actively cooled then check this checkbox. When it is checked the rest of the items described on this page become available.

Cooling system

This item is shown with a green background as it is used in Cooling design calculations only. The data is normally used to group zones together that are served by the same AHU. This allows the cooling capacity of the AHU to be calculated on the Cooling design screen.

By default DesignBuilder provides a single Cooling system called Default and all zones are allocated to it. You can create new Cooling systems and select them in the same way that any other component such as a glazing system would be created and selected. Cooling systems have no data other than a name.

Cooling capacity

The Cooling capacity can either be entered by hand or can be Autosized using the Cooling Design Calculations. If, immediately prior to a Simulation the Cooling capacity has not been entered (neither by hand nor from a previous Cooling Design Calculation) then a Cooling design autosizing simulation is started to calculate the capacity. The Plant sizing building model option controls the way this works.

You can change cooling capacities by hand if you wish but you should bear in mind that, by default, if Model Options change, all heating and cooling capacities are reset to zero. Set the Plant sizing model option to '2-Manual' to avoid this happening.

Supply Air Conditions

When using Simple HVAC  you can specify the cooling supply air conditions of the air.

Minimum supply air delivery temperature

The minimum dry-bulb air temperature of the air supplied for cooling the zone (in °C or °F).

Minimum supply air humidity ratio

The minimum humidity ratio (kg of water per kg of dry air) of the cool supply air. The default is 0.0077 kg-H2O/kg-air which corresponds to a 10C (50F) dew point. You should be careful to enter a value that is a valid condition, i.e. it is under the 100% saturation line on the Psychrometric Chart.

Cooling limit type

There are 3 ways that EnergyPlus ideal loads systems can limit zone cooling capacity (for simulations only):

• 1-Limit flow rate - means that the cooling supply air flow rate will be limited to a flow rate value calculated based on the autosizing model option settings. When using the 3-Autosize HVAC sizing method, the flow rate limit is calculated by EnergyPlus sizing calculations. Otherwise if the DesignBuilder sizing method is used, the flow rate is calculated based on the required cooling capacity calculated by DesignBuilder and the Minimum supply air delivery temperature (above).
• 2-Limit capacity - means that the sensible cooling capacity will be limited to a capacity value calculated based on the autosizing model option settings. When using the 3-Autosize HVAC sizing method the capacity is calculated by EnergyPlus sizing calculations. Otherwise if the DesignBuilder sizing method is used the flow rate is calculated based on the required cooling capacity calculated by DesignBuilder and the Minimum supply air delivery temperature (above).
• 3-Limit flow rate and capacity - means that both flow rate and capacity will be limited to the above values.

Technical description

 

EnergyPlus uses the following 4 settings to define the maximum cooling input for ideal loads systems:

 

• Minimum Cooling Supply Air Temperature

• Maximum Cooling Air Flow Rate,

• Maximum Total Cooling Capacity

• Cooling Limit Type

 

When using the default 3-Autosize HVAC sizing Model option the "Minimum Cooling Supply Air Temperature" and "Cooling Limit Type" IDF fields are set using corresponding data from the HVAC tab, and the "Maximum Cooling Air Flow Rate" and "Maximum Total Cooling Capacity" fields are calculated by EnergyPlus before the simulation in the design sizing calculations.

 

Alternatively, when using either of the 2-Manual or 4-Autosize when not set HVAC sizing Model options, the "Maximum Cooling Supply Air Temperature", "Maximum Sensible Cooling Capacity" and "Cooling Limit Type" IDF fields are set using corresponding data from the HVAC tab, and DesignBuilder calculates the corresponding "Maximum Cooling Air Flow Rate" based on the user-defined Cooling capacity and Minimum supply air delivery temperature data. In this case all of the 4 related IDF fields are specified in the IDF by DesignBuilder.

 

Or when the 1-Adequate HVAC sizing Model option is selected, DesignBuilder sets the EnergyPlus "Cooling Limit Type" IDF field to "NoLimit" which means that the cooling system can meet any load. In this case the other 3 fields are not needed and so are not provided in the IDF.

 

In all of the above cases, apart from when the 1-Adequate HVAC sizing Model option is selected, EnergyPlus holds values for each of the 4 settings used to define the maximum cooling input for ideal loads systems, and the way these are used in the simulation is defined by the Cooling limit type per the above descriptions.

Note: In practice this setting should not make a noticeable difference to the thermal heat balance of the zone during cooling since the maximum cooling delivery to the zone is already defined before the simulation and this setting simply tells EnergyPlus exactly how to apply the limit.

Cooling Plant

Cooling fuel

Select the type of fuel used to generate the cooling energy - choose from:

You should select 1-Electricity from grid you wish the results displayed in DesignBuilder to agree with those calculated by EnergyPlus directly for District cooling and displayed on the Summary tab of the Simulation screen.

Cooling system CoP

The cooling system coefficient of performance is used to calculate the fuel consumption required to meet cooling demand. The value represents the total seasonal efficiency of the entire cooling system and should include the effect of all energy consumption associated with building cooling such as fan and pump energy, chiller inefficiency, control equipment etc.

Cooling system CoP data is specified by zone so you can model different cooling efficiencies in each zone. But generally you should access this data from the building level and allow zones to take on building default.

Note: When comparing the cooling fuel consumption results with the data displayed on the Summary tab, it is important to realise that DesignBuilder applies a fuel type and a CoP to the idealised cooling systems used in EnergyPlus when using Simple HVAC. On the other hand EnergyPlus displays the unprocessed loads as District cooling in the tables on the Summary tab. This is because the idealised systems in EnergyPlus are considered to be met directly without use of HVAC equipment through a district cooling type system.

Operation

Cooling operation schedule

This schedule data is used in conjunction with the cooling setpoint temperatures on the Activity tab to define the time-varying cooling setpoint schedule in the zone. The operation schedule defines the times when full and setback setpoints should be met and the setpoint data on the Activity tab define the actual setpoint values. See Defining setpoint temperature schedules for more on this.

Note: the zone cooling operation schedule is also used in Detailed HVAC with the default 1-Simple HVAC Detailed HVAC Activity data setting.