WaterHeater:Stratified
WaterHeater:Sizing
Used in:
Supply side of DHW Loops
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WaterHeater:Stratified WaterHeater:Sizing
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Used in:
Supply side of DHW Loops |
The 2-Stratified water heater simulates a stratified, multi-node water tank. The model divides the water tank into multiple nodes of equal volume. The nodes are coupled by vertical conduction effects, internode fluid flow, and temperature inversion mixing. The water heater simultaneously solves the differential equations governing the energy balances on the nodes using a numerical method. The system timestep is divided into many small substeps that allow the simulation to capture events that occur on a very short time scale. This approach allows ambient losses and parasitic loads to be divided into on-cycle and off-cycle effects and accounted for in detail.
For losses to the ambient environment, the ambient air temperature can be taken from a schedule, a zone, or the exterior. When used with a zone, a fraction of the skin losses can be added to the zone heat balance as internal heat gains.
The Stratified water heater allows two heating elements to be simulated. The two elements can cycle on and off to maintain the node temperature within the deadband. The Heater priority control setting determines how the heaters work together. There are two priority control options: 1-Master Slave or 2-Simultaneous. In the Master slave option, the main Heater is the master and the auxiliary heater is the slave. That is, both heaters are not allowed to turn on at the same time. If the thermostats ask for heat from both heaters, only the main heater will turn on. Once the main heater has met the set point, it turns off and the auxiliary heater can turn on, if necessary. In the Simultaneous priority control option, both heaters can turn on and off independently. Autosizing is available for only for the main Heater.
The name of the Water heater.
Select the type of water heater from the list:
For a discussion of the applications, advantages and disadvantages of the two water heater types see the Water heater overview topic.
The volume of the storage tank (in m3 or ft3). Although this field is allowed to go down to zero, even so-called "tankless" water heaters have some volume of water that is maintained around the heating elements or in the heat exchanger, typically around 0.00379 m3 (1 gallon).
This field is autosizable based on the Sizing data, however we are aware of some issues with the EnergPlus autosizing method for this field and so we recommend that the tank volume is hard set wherever possible.
The height of the tank (in m or in).
For the 2-Horizontal cylinder tank shape (see below) the height of the tank is the measure in the axial direction, i.e., the height if you were to stand the cylinder up on its end.
This field is autosizable based on the Sizing data, however we are aware of some issues with the EnergPlus autosizing method for this field and so we recommend that the tank height is hard set wherever possible.
The tank shape determines the size and skin losses of the stratified nodes. There are two options:
Check this option if the water heater is to be supplied with heat from a Hot water loop. In this case inlet and outlet connections are included on the bottom of the water heater allowing the connection to be made to the demand side of the Hot water plant loop.
This field is only required when the External heating plant option is checked, i.e. when the water heater is connected to the demand side of a plant loop. It is used to provide a design parameter for autosizing design flow rates The recovery time is expressed in hours. This is the time that the entire volume of the tank can be heated from 14.4ºC to 57.2ºC (58ºF to 135ºF) with an inlet temperature defined as the exit temperature in the associated Plant Sizing object. The default is 1.5 hours. The calculation is based on log-mean temperature difference (LMTD) and includes the heat transfer effectiveness factor entered above.
The temperature (in °C or °F) at which the tank water becomes dangerously hot and is vented through boiling or an automatic safety. The tank temperature will never exceed the maximum. Any extra heat added to the tank is immediately vented.
Also, when Source side flow control mode is set to 3-StorageTank the controls always request flow unless the tank temperature is equal to or higher than the maximum limit provided here.
Note: The Maximum temperature limit must be greater than the setpoint temperature at all times.
Select the schedule specifying the hot water temperature set point (in °C) for the main heater. Also known as the “cut-out” temperature.
The delta temperature difference (Δ°C or Δ°F) between the set point and the “cut-in” temperature at which the main heater will turn on. In other words, the “cut-in” temperature is Set Point – Deadband.
The Ambient temperature indicator specifies how the temperature of the air surrounding the Water heater is to be defined. These options are available:
Select the schedule specifying the ambient air temperature around the tank for skin losses. This field is only required if Ambient temperature indicator is 1-Schedule.
The building zone specifying the ambient air temperature around the tank for skin losses. This field is only required if Ambient temperature indicator is 2-Zone.
The loss coefficient (in W/K or Btu/h-F) to the ambient air temperature. Often this coefficient is identical to the "UA" for skin losses. If the loss effects of the flue are being modelled in the Off-Cycle Loss Coefficient, than this field would have a different value accounting only for the skin losses.
The loss coefficient (in W/K or Btu/h-F) to the ambient air temperature. Often this coefficient is identical to the "UA" for skin losses. However, it can also be used to model the loss effects of the flue in a combustion water heater, in addition to the skin losses.
If the Ambient temperature indicator is 2-Zone, this field adds the specified fraction of the on-cycle losses to the zone heat balance as an internal gain.
If the Ambient temperature indicator is 2-Zone, this field adds the specified fraction of the off-cycle losses to the zone heat balance as an internal gain.
If the Ambient temperature indicator is 2-Zone, this field adds the specified fraction of the skin losses to the zone heat balance as an internal gain.
If the water heater includes an internal heating element then checking this checkbox provides access to the element settings.
The type of fuel used for heating. The fuel type can be one of:
11-Other fuel 1 and 12-Other fuel 2 can be used for representing biofuels such as biomass and biogas for example.
The thermal conversion efficiency from fuel energy to heat energy for the heater element or burner. This is not the same as the overall efficiency of the water heater.
The heat rate (in W) supplied to the water for the main heater, probably the same as the “nominal” capacity. For residential electric water heaters, heating elements are usually 4500 W. This field is autosizable.
The height of the main heating element in the tank (in m or in).
If a second "auxiliary" heater is to be included in the water heater then this checkbox should be checked.
The heater priority control determines how the main heater the auxiliary heater work together. Note these heaters are called Heater 1 and Heater 2 in the EnergyPlus documentation. There are two options:
The heat rate (in W) supplied to the water for the auxiliary heater.
The height of the auxiliary heating element in the tank (in m or in).
Select the schedule specifying the hot water temperature set point (in °C) for the auxiliary heater. Also known as the “cut-out” temperature.
The delta temperature difference (Δ°C or Δ°F) between the set point and the “cut-in” temperature at which the auxiliary heater will turn on. In other words, the “cut-in” temperature is Set Point – Deadband.
On-cycle parasitics include parts of the water heater that consume fuel when the heater is on, for example, an induction fan, or stand-by electronic control circuits. The fuel consumption rate (in W or Btu/h) is the total fuel that is consumed by all of the on-cycle parasitics.
The type of fuel used by the on-cycle parasitics. The fuel type can be
The fuel type can be the same or different from the Heater Fuel Type.
The fraction of on-cycle parasitic fuel energy that is converted to heat energy that ends up in the tank water. For example, an induction fan might (maybe) deliver a small fraction of its energy to the tank water for a value of 0.05. Electronic control circuits, on the other hand, do not add any heat to the tank and should be 0.
The height (in m or in) where any on-cycle parasitic heat gains are added to the tank.
Off-cycle parasitics include parts of the water heater that consume fuel when the heater is off, for example, a pilot light, or stand-by electronic control circuits. The fuel consumption rate (in W or Btu/h) is strictly the total fuel that is consumed by all of the off-cycle parasitics.
The type of fuel used by the off-cycle parasitics. The fuel type can be one of
The fuel type can be the same or different from the Heater fuel type.
The fraction of off-cycle parasitic fuel energy that is converted to heat energy that ends up in the tank water. For example, a pilot light would deliver most of its heat to the tank water, as long as the thermal conversion efficiency must be taken into account, so perhaps 0.80 is reasonable. Electronic control circuits, on the other hand, do not add any heat to the tank and should be 0.
The height (in m or in) where any off-cycle parasitic heat gains are added to the tank.
The inlet mode of entering fluid from the use and source sides. There are two options:
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 hot as the tank water, simulating a heat exchanger.
This field is optional and is used to specify the design flow rate through the Use Side of the water heater. The volumetric design flow rate is specified in m3/s or gal/min. The field is needed when the Use side is connected to a plant loop. The field can be autosized. Sizing results are reported in the EIO file.
The height of the use side inlet to the tank (in m or in). The default value of 0 means the bottom of the tank. The inlet height cannot be higher than the tank height.
The height of the use side outlet from the tank (in m or in). If autocalculate, the inlet defaults to the top of the tank. The outlet height cannot be higher than the tank height.
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 hot as the tank water, simulating a heat exchanger.
This field is optional and is used to specify the design flow rate through the Source Side of the water heater. The volumetric design flow rate is specified in m3/s or gal/min. The field is needed when the Source Side is connected to a plant loop. The field can be autosized. Sizing results are reported in the EIO file.
This field is optional and is used to provide control over the logic used by the source side of the water heater to request flow. There are three choices for different modes:
This field is required when using the 2-IndirectHeatAlternateSetpoint Source side flow control mode (above) and is used to provide a schedule with alternate setpoints. The Schedule specifies the hot water temperature setpoint (in °C) to use as the “cut-out” temperature for control logic at the source side.
The height of the source side inlet to the tank (in m or in). If autocalculate, the inlet defaults to the top of the tank. The inlet height cannot be higher than the tank height.
The height of the source side outlet from the tank (in m or in). The outlet height cannot be higher than the tank height.
An additional destratification conductivity (in W/m-K or Btu-in/h-ft2-°F) is added to the fluid conductivity of water (0.6 W/m-K) to account for vertical conduction effects along the inside of the tank wall, and perhaps other vertical components such as the flue, the cold water inlet pipe (dip tube), and the anode rod.
The number of stratified nodes in the tank. There must be at least one node. The maximum number of nodes is 12.
An additional loss coefficient (in W/K or Btu/h-°F)] added to the skin losses for a given node to account for thermal shorting due to pipe penetrations, water heater feet, and any other loss effects.
The Water heater sizing data is entered on the Sizing tab.
This field describes the method to be used for sizing the water heater. There are six choices:
This field provides the time, in hours, that the tank’s volume can sustain a peak draw. It is used to size the tank’s volume which is the simple product of peak draw volume flow rate and the draw time. There is no assurance that the water will be at the desired temperature for the entire draw. This field is only used if the Design mode is 1-Peak draw. The water heater must be connected to a full plant loop and be on the supply side.
This field provides the time, in hours, that tank’s heater needs to recover the volume of the tank. The temperatures used to define recovery are a starting temperature of 14.4ºC (58ºF) and a final temperature of 57.2ºC (135ºF). Time for tank recovery data is only required if the Design mode is 1-Peak draw.
This field is used in case the water heater is indirectly heated by its source side connections and they are also autosized. The units are m3 or ft3. Because of the complexity of such a water heater and the timing for when sizing calculation happen inside EnergyPlus, the Source side connection flow rates need to be reported before the tank’s volume can be sized to meet Peak draw. This input field is used to provide a nominal tank volume to use temporarily while the flow connections are sized. This field is only required if the Design mode is 1-Peak draw and the water heater has autosized plant connections on the demand side.
This field is used to enter the number of bedrooms in the model. It is only required if the Design mode is 2-Residential HUD-FHA minimum.
This field is used to enter the number of bathrooms in the model. It is only required if the Design mode is 2-Residential HUD-FHA minimum.
This field is used to enter the tank’s storage volume on per-person basis. The units are m3/person or ft3/person. The storage capacity per person is only required if the Design mode is 3-Per person.
This field is used to enter the recovery capacity per person in units of m3/person-hr or ft3/person-h. This is the volume of water the heater can recover in one hour per person. Recovery is heating water from a starting temperature of 14.4ºC (58ºF) to a final temperature of 57.2ºC (135ºF). The Recovery capacity per person is only required if the Design mode is 3-Per person.
This field is used to enter the tank’s storage volume on a per-floor-area basis. The units are m3 /m2 or ft3/ft2. The Recovery capacity per person is only required if the Design mode is 3-Per person.
This field is used to enter the recovery capacity per floor area in units of m3/h-m2 or ft3/h-ft2. This is the volume water the heater can recover in an hour per floor area. Recovery is heating water from a starting temperature of 14.4ºC (58ºF) to a final temperature of 57.2ºC (135ºF). The Recovery capacity per floor area is only required if the Design mode is 4-Per floor area
This field is used to enter the number of Units for use in sizing on per-Unit basis with the next two fields. The Number of units data only required if the Design mode is 5-Per unit. This can be used to account for any arbitrary item such as lodging rooms, desks, water fixtures, restrooms, etc.
This field is used to enter the tanks’ storage volume on per-Unit basis. The units are m3/h or ft3/h. The number of Units is entered in the previous field. The Storage capacity per unit is only required if the Design mode is 5-Per unit.
This field is used to enter the recover capacity per Unit in units of m3/h or ft3/h. This is the volume of water the heater can recover in an hour per Unit. Recovery is heating water from a starting temperature of 14.4ºC (58ºF) to a final temperature of 57.2ºC (135ºF). This field is only required if the Design mode is 5-Per unit.
This field is used to enter the tank’s storage volume on per-solar-collector-area basis. The units are m3/m2 or f3/m2. The Recovery capacity per unit is only required if the Design mode is 6-Per solar collector area.