HeatStorageStratified

Use

The HeatStorageStratified models sensitive heat storage (fluids without phase transition) with all heat flows and losses in an insulated or non-insulated cylindrical container. Typical applications for the HeatStorageStratified are the buffer storage of heat from generators for a more even operation of the generators, the mixture of fluid flows of different temperatures or the temporal shift of renewable heat generation to times with a heat demand. This model is an older version of the HeatStorage, only use this model due to legacy reasons.

Parameters and Connectors

▶TStorage[n] returns the temperatures of all storage layers within the HeatStorageStratified. ▶FlowIn1, ▶ReturnOut1, ▶FlowOut4, ▶ReturnIn4 are the storage connections for charging or discharging the HeatStorageStratified. To activate the storage connections (▶FlowIn1, ▶ReturnOut1, ▶FlowOut4, ▶ReturnIn4, ...), the parameters (use1, use2, ...) must be set to true in one of the tabs of the pipes.

The most important technical parameters must be specified for dimensioning the HeatStorageStratified. Based on the assumption that the HeatStorageStratified is a cylindrical store, the parameters storage volume (VStorage), the storage diameter (dStorage) and the heat loss rate of the insulation (QlossRate) are specified for the geometric dimensioning. The storage volume (VStorage) is required as a parameter for the capacity of the HeatStorageStratified, while the storage diameter (dStorage) together with the heat loss rate (QlossRate) are a measure of the heat losses to the environment.

The parameters in the Simulation Parameters group (see the figure) are used for more precise control of the simulation initialization or the model calculations. TupInit and TlowInit determine the start temperatures for the upper and lower storage tank temperature at the start of the simulation. If these temperatures are not known at the time of modeling , these parameters can be left at the default values. In this case, it should only be noted that the model requires a longer period of time to settle and provide more precise information. The parameter n determines the number of temperature layers within the memory and can be set as required (number 1 is the bottom layer and number n is the top layer). For a faster simulation, this parameter should have a value between 3-10. The HeatStorageStratified uses a constant temperature parameter TAmbient, which is used to calculate the heat loss of the storage to its environment.

For the SOC calculation the maximum storage temperature TMax that can be reached has to be defined (e.g. temperature of storage during fully charged). The parameters of the fluid within the heat storage are set under the Medium tab. This requires the parameters average specific heat capacity of the medium during operation (cpMed), average density of the medium during operation (rhoMed), the heat transfer coefficient between the heat storage temperature layers (alphaMedStatic) and the density of the medium as a function of the fluid temperature (rhoTable). The material values can be taken from standard books or other sources. Only one medium can be used at a time as it is not possible to mix different fluids (the default setting is water). For the setting of alphaMedStatic, the default value can be used for liquids with similar material characteristics to water. If the expected behavior of the liquid differs, this value must be adjusted accordingly. Higher values for alphaMedStatic mean more heat is exchanged between the storage layers and the temperature differences between the storage layers decrease.

As soon as use1 is activated, the inlet and outlet temperatures of the connections can be set indirectly via iFlow1 and iReturn1. These parameters specify the storage layers in which the connections are attached. The higher the number of these parameters, the higher the connection is attached to the storage tank ( maximum is the number of connections n) and the higher is the temperature of the fluid.

Model Background

Heat Transfer

The heat transfers in the HeatStorageStratified model are calculated for each layer of the storage tank and use the equations from the following figure.

Legend for the heat transfers in the figure:

• (1) Heat losses through the insulation

• (2) Heat transmission between neighboring heating medium layers

• (3) Buoyant force of heating medium due to temperature-dependent density differences - requires the temperature-dependent heating medium density definition (supplied via a table in the input parameters)

• (4) Heat extraction via volume flow input - the correspondent volume flow output does not extract heat directly (output volume flow has storage temperature level), but colder heat medium simultaneously flows back into the tank (i.e. heat extraction).

• (4) Heat supply or extraction via heat exchangers

The temperature change of the thermal layer of the storage tank results from the heat balance of the above-mentioned heat transfers and is calculated for all thermal layers of the storage tank in each time step.