Air Temperature Distribution - Dynamic Gradient

HVAC tab in model data

The Dynamic Gradient Air Distribution model data allows you to model stratification within a zone by setting up a temperature gradient which varies dynamically depending on:

• Outside temperature.
• Inside temperature.
• Inside-outside temperature difference.
• Heating load.
• Cooling load.

Gradient interpolation mode

This field specifics how the program will vary between the two gradients. Select one of the following keywords to choose the simulation data used to scale: ‘Outdoor Environment Drybulb Temperature’, ‘Zone Drybulb Temperature’, ‘Delta Outdoor and Zone Temperature’, ‘Sensible Cooling Load’, and ‘Sensible Heating Load’. These are explained in detail below. All of these options have several things in common. They are essentially hard-coded. There is no support for a general method. The interpolation scheme is based on some variable that might reasonably be expected to correlate with gradient changes. This variable’s current value is used to continually adjust the value of the vertical gradient for room air temperature.

• 1-Outside temperature. This option directs the program to interpolate between upper and lower values of the vertical gradient based on the outdoor air temperature. If the outdoor temperature exceeds the upper limit set under the Upper Conditions header, then the gradient entered in the Upper Conditions > Temperature gradient field is used. Similarly if the outdoor air temperature is below the value set in the Lower Conditions > Temperature field, then the gradient entered in the Lower Conditions > Temperature gradient is used. For outdoor temperatures that lie between the upper and lower bounds, the gradient is determined by linear interpolation between the two.
• 2-Inside temperature. This option directs the program to interpolate between upper and lower values of the vertical gradient based on the mean zone air temperature. If the mean zone air temperature exceeds the upper limit set under the Upper Conditions header, then the gradient entered in the Upper Conditions > Temperature gradient field is used. Similarly if the mean zone air temperature is below the value set in theLower Conditions > Temperature field, then the gradient entered in the Lower Conditions > Temperature gradient is used. For mean zone air temperatures that lie between the upper and lower bounds, the gradient is determined by linear interpolation between the two.
• 3-Inside outside delta T. This option directs the program to interpolate between upper and lower values of the vertical gradient based on the difference between the outdoor environment and the mean zone air temperature. If the temperature difference exceeds the upper limit set under the Upper Conditions header, then the gradient entered in the Upper Conditions > Temperature gradient field is used. Similarly if the temperature difference is below the Lower Conditions > Temperature field, then the gradient entered in the Lower Conditions > Temperature gradient is used. For temperature differences that lie between the upper and lower bounds, the gradient is determined by linear interpolation between the two.
• 4-Sensible cooling load. This option directs the program to interpolate between upper and lower values of the vertical gradient based on the sensible cooling load. If the cooling load exceeds the upper limit set under the Upper Conditions header, then the gradient entered in the Upper Conditions > Temperature gradient field is used. Similarly if the cooling load is below the value set in the Lower Conditions > Heat rate field, then the gradient entered in the Lower Conditions > Temperature gradient is used. For cooling loads that lie between the upper and lower bounds, the gradient is determined by linear interpolation between the two.
•  5-Sensible heating load. This option directs the program to interpolate between upper and lower values of the vertical gradient based on the sensible heating load. If the heating load exceeds the upper limit set under the Upper Conditions header, then the gradient entered in the Upper Conditions > Temperature gradient field is used. Similarly if the heating load is below the value set in the Lower Conditions > Heat rate field, then the gradient entered in the Lower Conditions > Temperature gradient is used. For heating loads that lie between the upper and lower bounds, the gradient is determined by linear interpolation between the two.

Upper Conditions

Temperature

This field is used to enter the upper bound on temperature values (in °C or °F). It is required for the interpolation modes based on temperature.

Heat rate

This field is used to enter the upper bound on heat rate values. It is required for the interpolation modes based on load. Both heating and cooling loads are entered as positive numbers (in W).

Temperature gradient

This field specifies the gradient, or slope, of temperature changes in the vertical direction (in ºC/m or °F/ft) when the selected temperature or heat load is at or above the upper condition.

Lower Conditions

Temperature

This field is used to enter the lower bound on temperature values (in °C or °F). It is required for the interpolation modes based on temperature.

Heat rate

This field is used to enter the lower bound on heat rate values. It is required for the interpolation modes based on load. Both heating and cooling loads are entered as positive numbers (in W).

Temperature gradient

This field specifies the gradient, or slope, of temperature changes in the vertical direction (in ºC/m or °F/ft) when the selected temperature or heat load is at or below the lower condition.

Operation.

Operation schedule

The operation schedule defines the times when air temperature distribution is to be calculated. When the value of the schedule is 1, the distribution calculations take place, when the value is 0, the zone air temperature is fully mixed.

Heights

Thermostat  height

Thermostat height specifies the distance above the floor where the thermostat for HVAC control is situated. This height is used by the model to determine the thermostat temperature relative to the mean air temperature by applying the gradient.

Return air height

Return air height specifies the distance above the floor where the air leaves the zone and returns to the air system. This height is used by the model to determine the return air temperature relative to the mean air temperature by applying the gradient.

Range of applicability

The air temperature distribution within the zone is used as follows:

1. To calculate boundary conditions for conduction through surfaces (walls, roofs, glazing etc).
2. To include the effects of extracting relatively warmer air at the ceiling level when using Detailed HVAC.
3. To include the vertical position of the thermostat sensor for HVAC control.

How it works

Changing the Distribution mode to 2-Dynamic gradient reveals the air temperature distribution data. By default the Interpolation mode is 2-Inside-outside DeltaT. This means that the temperature gradient within the space is modelled as varying according to the inside-outside temperature difference. The Upper conditions define the most extreme temperature gradient, typically under design conditions. By default, the upper temperature difference is 10 °C and the corresponding temperature gradient is 4 °C/m. This means that when the temperature difference between inside and outside is 10 °C or more the internal temperature gradient is 4 °C/m. The Lower conditions data shows that when the temperature difference between inside and outside is zero there is no temperature gradient, i.e. the air is fully mixed. EnergyPlus calculates the temperature gradient for other inside-outside temperature differences by linear interpolation. The calculation is similar for the other interpolation modes but is instead based on outside temperature, inside temperature, heating or cooling load.

Limitations

The air temperature distribution mechanism does not include:

1. The effects of extracting relatively warmer air at the ceiling level when using Scheduled and Calculated natural ventilation.
2. The vertical position of the sensor for ventilation control.