Chilled Water Storage - Mixed

ThermalStorage:ChilledWater:Mixed

Used in:

Supply side of Chilled Water Loops

The Chilled water storage mixed object analytically solves the differential equation governing the energy balance of the water tank. The chilled water is “used” by drawing from the “Use Side” of the water tank. The tank is indirectly charged by circulating cold water through the “Source Side” of the water tank. Control is based on cycling flow through the source side. When the tank temperature rises above a “cut-in” temperature, source side flow is requested. Source side flow will continue until the tank is cooled to below the tank set point or “cut-out” temperature.

For heat gains from the ambient environment, the ambient air temperature can be taken from a schedule, a zone, or the exterior. When used with a zone, the skin gains are removed from the zone heat balance as negative internal heat gains.

General

Name

The name of the Chilled water storage tank.

Type

Select the type of Chilled water storage tank from the list:

1-Mixed,
2-Stratified.

For a discussion of the applications, advantages and disadvantages of the two water heater types see the Chilled Water Storage overview topic.

Nominal cooling capacity

The Nominal cooling capacity describes the typical cooling capacity that the chilled water tank will provide (in W or Btu/h). Since this is a passive device, the actual cooling capacity depends on water temperatures and flow rates. However, this field is used to describe the chilled water tank’s nominal cooling capacity for supervisory control where plant operation schemes require a cooling capacity to model how equipment is dispatched.

Tank recovery time

This field is used to autosize the Source Side Design Flow Rate. The field is used if the Source side design flow rate is set to Autosize and the water tank’s source side is on the demand side of a plant loop. This is the time, in hours, that the chilled water tank is to be indirectly cooled from 14.4ºC to 9.0ºC using the Design loop exit temperature of in the associated Plant loop.

Tank volume

The volume of the storage tank (in m3 or gal).

Temperature Settings

Setpoint temperature schedule

Select the schedule which specifies the chilled water temperature setpoint (in °C). This setting is also known as the “cut-out” temperature.

Deadband temperature difference

The delta temperature difference (in Delta °C or Delta °F) between the setpoint and the “cut-in” temperature at which the storage tank will request cooling. In other words, the “cut-in” temperature is Setpoint + Deadband.

Minimum temperature limit

The temperature (in °C or °F) at which the tank water becomes too cold. No source side flow is allowed when the tank temperature is below this limit. The minimum temperature must be lower than the setpoint temperature at all times.

Ambient Heat Transfer Settings

Ambient temperature indicator

The Ambient temperature indicator specifies how the ambient air temperature will be indicated. Select from:

1-Schedule where the Ambient temperature schedule provides the values for ambient temperature.
2-Zone, where the zone air temperature of the zone specified in the Ambient temperature zone provides the ambient temperature, or,
3-Outdoors, where the outdoor dry-bulb air temperature provides the ambient temperature.

Ambient temperature schedule

Select schedule which specifies the ambient air temperature around the tank for skin gains. This field is only used if the 1-Schedule Ambient temperature indicator is used.

Ambient temperature zone

Select the zone specifying the ambient air temperature around the tank for skin gains. This field is only used if the 2-Zone Ambient temperature indicator is used.

Heat gain coefficient from ambient temperature

The gain coefficient from the ambient air temperature (in W/K or Btu/h-°F) . This coefficient is often referred to as the UA for the overall tank thermal performance with respect to heat gains from the tank’s skin.

Use Side Settings

Use side heat transfer effectiveness

This field specifies the heat transfer effectiveness between the use side water and the tank water. If the effectiveness is set to 1 then complete heat transfer occurs, simulating perfect mixing of the use side water and the tank water. If the effectiveness is lower, then the use side outlet water temperature will not be as cold as the tank water, simulating a heat exchanger.

Use side availability schedule

Select the availability schedule for the use side of the water tank. If the schedule’s value is 0.0, then the use side is not available and flow will not be requested. If the schedule’s value is not equal to 0.0 (usually 1 is used), the use side is available.

Use side design flow rate

This autosizable setting is used to specify the design flow rate through the use side of the chilled water tank (in m3/s or gal/min). For autosized tanks, sizing results are reported in the EIO file.

Source Side Settings

Source side heat transfer effectiveness

This field specifies the heat transfer effectiveness between the source side water and the tank water. If the effectiveness is set to 1 then complete heat transfer occurs, simulating perfect mixing of the source side water and the tank water. If the effectiveness is lower, then the source side outlet water temperature will not be as cold as the tank water, simulating a heat exchanger.

Source side availability schedule

This field contains the name of an availability schedule for the source side of the water tank. If the schedule’s value is 0.0, then the source side is not available and flow will not be requested. If the schedule’s value is not equal to 0.0 (usually 1 is used), the source side is available.

Source side design flow rate

This autosizable setting is used to specify the design flow rate through the source side of the chilled water tank (in m3/s or gal/min). Sizing results are reported in the EIO file.