Ice Thermal Storage

ThermalStorage:Ice:Simple ThermalStorage:Ice:Detailed

Used in:   Chilled water loop, supply side

Ice thermal storage systems allow use of off-peak electricity to "charge" a storage tank of ice or other fluid and use that as a source of cooling to help reduce peak loads during periods of cooling demand. Two types of ice thermal storage systems can be modelled in DesignBuilder, simple and detailed, the latter option providing options to control the charge/discharge performance using curves.

General

Name

This alpha field contains the identifying name for the ice storage tank.

Type

Two types of ice thermal storage systems can be modelled in DesignBuilder:

• 1-Simple, which is based on a simple simulation of an ice storage tank with a fixed capacity. The tank is charged or frozen in an ice-on-coil configuration where ice builds up on the outside of the tubes carrying the brine or glycol solution from the chiller. There are two discharge (melt) options, internal or external. Internal melt uses the same fluid tubes for charging and discharging. External melt uses a separate fluid path for discharge such that the outer layers of ice melt first. The ice storage model includes an implied 3-way valve to control the amount of charge/discharge based on the incoming water temperature and the outlet node setpoint temperature. The storage tank is assumed to be fully charged (full of ice) at the beginning of each environment. The tank is then allowed to charge and discharge during the warmup days of the environment. The tank is best controlled using the 4-Component Setpoint plant operation scheme which requires that a setpoint be placed by a Setpoint manager on the ice storage Plant Outlet Node.
  
  
• 2-Detailed, which can simulate ice storage tanks more closely based on specific manufacturers’ published performance data. This is possible through the use of curve fits to simulate the performance of the ice storage unit during charging and discharging. In this implementation, both charging and discharging are a function of the fraction charged/discharged as well as the log mean temperature difference across the storage unit.

All parameters below apply to the 2-Detailed option apart from Ice storage type below. The 1-Simple option only requires Name, Ice storage type and Capacity.

Ice storage type

This setting applies only to the 1-Simple type of Ice thermal storage tank and specifies the type of ice storage tank to be modelled. There are two options:

• 1-Ice on coil internal models ice-on-coil, internal melt.
• 2-Ice on coil external models ice-on-coil, external melt.

Capacity

Enter the maximum amount of latent thermal storage available in the ice storage system in GJ (Giga is 10⁹) or ton-hrs. The removal or addition of sensible energy from the tank is not modelled, so the tank is always assumed to be at the freezing temperature of the storage material.

Tank loss coefficient

This setting defines the loss of ice stored during a particular hour. This field is dimensionless (per hour). It is not multiplied by any temperature difference between the tank and the environment in which it might be located.

Freezing temperature of storage medium

This parameter defines the freezing/melting temperature of the ice storage medium (in °C or °F). For most tanks, this is simply 0.0°C (the default value). However, some tanks may use salts or other materials to change the freezing temperature. This can be changed using this parameter.

Thaw process indicator

This input assists in more accurate modelling of the charging process by defining how the thawing of ice takes place. There are two options for this input:

• 1-Outside melt where the ice melts from the outside, leaving ice still on the charging surface when charging begins.
• 2-Inside melt where the the charging process starts with a bare coil or no ice left on the charging surface even though there is still ice stored in the tank. An example of such a system is sometimes referred to as an ice-on-coil inside melt system.

For systems that have a charging process that does not vary significantly with fraction charged you can safely ignore this input by accepting the default value. The default value is 1-Outside melt.

Operation

Availability schedule

Select the schedule that determines whether or not the ice storage system is available during a particular time period. This allows the system to be turned off during a particular season. A value of less than or equal to zero indicates that the ice storage system is not available. Any value greater than zero indicates that the system is available.

Curve Timestep

Timestep of the curve data

This field defines what timestep was used to produce the curve fits named in the previous inputs.

Note: This parameter is important because the curve fit is non-dimensional, so the data used to develop the curve fits were based on a specific length of time. In many cases, this is probably one hour or 1.0. The units for this parameter are hours.

Charging Settings

Charging curve object type

The detailed ice storage model uses the performance curves to model performance. Currently, the only two allowed curve fit types for the detailed ice storage model are:

• 1-Quadratic Linear and
• 2-Cubic Linear curves.

More information on this curve can be found in the section on Curves.

Charging curve

Select the curve to be used to model the charging process of the detailed ice storage system.

Parasitic electric load during charging

The amount of parasitic electric consumption (for controls or other miscellaneous electric consumption associate with the ice storage unit itself) during the charge phase. This parameter is dimensionless and gets multiplied by the current load on the tank.

Discharging Settings

Discharging curve object type

The detailed ice storage model uses the performance curves to model performance. Currently, the only two allowed curve fit types for the detailed ice storage model are:

• 1-Quadratic Linear and
• 2-Cubic Linear curves.

More information on this curve can be found in the section on Curves.

Discharging curve

Select the curve to be used to model the discharging process of the detailed ice storage system.

Parasitic electric load during discharging

The amount of parasitic electric consumption (for controls or other miscellaneous electric consumption associate with the ice storage unit itself) during the discharge phase. This parameter is dimensionless and gets multiplied by the current load on the tank.

Parallel or Series Configuration with Chiller

Ice thermal storage can be either in parallel or in series to the chiller depending on the modelling requirements.

The parallel case should normally include two chillers, with one being the main chiller to operate during peak cooling time and the other being the chiller used to charge the tank during specified off peak time as long as conditions are favourable. The main chiller works in lower priority to the ice thermal storage and tops up cooling demand once ice thermal storage reaches its maximum capacity during discharging. Generally the charging chiller can be thought of as a "dummy" component because it does not physically exist in the real building and the main and charging chillers actually represent one single chiller.

For the series case, the chiller can be either upstream or downstream of the ice thermal storage. In both cases the ice tank can help reduce peak chiller demand:

• Chiller upstream: chiller works first with ice thermal storage to top up when needed and chiller size can be reduced, and
• Chiller downstream: ice thermal storage will operate first with chiller to top up when required.

Note: for upstream chillers, the capacity of the chiller is an especially important setting. When the chiller has a high capacity, it can meet the full cooling demand and the ice thermal storage does not need to operate. In this case, to see the effect of ice thermal storage, the chiller capacity must be relatively low. However the chiller size must be high enough to allow the chiller to deliver low enough outlet CHW temperature to charge the ice thermal storage tank.

Controls and Chilled Water Plant Loop Operation Scheme Settings

The chilled water loop operation schemes include two Scheme operation type options that are suitable for modelling ice thermal storage objects:

• 4-Component Setpoint, requires various setpoint managers, temperature schedules, etc. to allow the system to operate. As complex definitions are possible for this scheme, it allows the possibility of more accurate and flexible modelling, for instance, setting hourly variation of setpoint temperatures at the outlet of each piece of equipment. The screenshot below shows how to use Setpoint managers to control an upstream chiller with ice thermal storage series configuration with the scheme operation type set to 4-Component Setpoint.

• 5-Energy Thermal Storage this scheme operation type internally creates the setpoint managers required by the equipment and so allows a simple and convenient approach by eliminating the need for both setpoint manager and schedule for each piece of equipment. The only data required is a charging setpoint temperature and a discharging setpoint temperature. The screenshots below shows how to control a downstream chiller with ice thermal storage series configuration with the scheme operation type set to 5-Energy Thermal Storage.