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Stratified Refrigerated and Heated Water Thermal Energy Storage Systems

 Theory

      DIRRECKTOR has had the unfettered opportunity to observe the operations of a large number of Stratified Water TES Systems. The knowledge gained from this experience yields a professional, first hand understanding of the theory and practice of water TES Systems.

      The Mechanisms that operate in a Stratified Water TES System are related to both the Physical Properties of Fresh Water and the Hydraulic Characteristics of the Stratification inducing Pipework.

      The two physical properties of water that are of special interest to the TES Design Engineer are:

      1.         The Density as it varies with Temperature.

      2.         The Kinematic Viscosity as it varies with Temperature.

      These two properties provide the basic mechanism for successfully stratifying water of different temperature within a single containment vessel.  They also indicate to the designer the location within the vessel where the warm and cool fluids should be stored.  It seems intuitively obvious that the cool liquid should be stored below the warm liquid with in the vessel, and of course this is the case.  The density difference between two liquids at different temperature creates buoyancy forces where the warm liquid is literally floated on top of the cool liquid. The relatively large difference in Kinematic Viscosity of liquids separated only by a few degrees in temperature suppresses any mixing of the two fluids, due to flow disturbances and free convection at the vessel walls.

      The strength of the buoyancy forces in keeping the two fluid volumes separated is strong. This point is illustrated by a typical start up procedure that the writer uses on warm start chill down cycles.

      A new system that is being started for the first time usually contains water that is substantially above the desired inlet temperature for the chillers.  The chilling of the warm water needs special attention from the commissioning team with respect to demand limiting the chillers, to avoid overloading.  The writer favors minimizing the operational period with high inlet water to the chiller, as such a portion of the tank, usually less than 20% is chilled in the typical charging mode of operation.  The tank water volume is then rolled over by reversing to the discharge mode. The removal of cold from the bottom of the tank and inserting it in the top creates strong mixing forces as the cold water sinks through the warm water back to bottom of the tank.  The process progresses with remarkable speed and a thoroughness of mixing in the warm region but shows little penetration into the cold region. The vessel temperatures sensors recording the event and confirm this fact.

      The two properties of water of interest both vary in the direction of increased temperature that makes water the ideal candidate for TES Systems.  That is density and Kinematic Viscosity both decrease with increased temperature above 4.0 Deg C.

Practice

      Background

      The Design of Stratified water TES Systems that successfully keep the warm and cool water volumes separated by a minimum volume of mixed water is a serious concern.  The performance of thermal energy storage vessels is defined as the Factor of Merit.  The term is a measure of the Thermodynamic Availability of the fluid in the Tank, to the system that it serves.

      A typical example is used to illustrate Thermodynamic Availability;

      Presume that an air handling system has been installed such that it can properly process the air using an inlet refrigerated water temperature of 6.0 Deg C.  Water inlet temperatures above this level create operational difficulties for the air system and are thus to be avoided.  The 6.0 Deg C inlet water temperatures provide the required Thermodynamic Availability for the air handlers to properly operate.  It has been determined, through experience, that temperatures below 6.0 Deg C also provides Thermodynamically Available cooling to the air unit.

      Further, presume that a thermal Energy storage vessel has a thermodynamic Availability of 90 Percent, that is, the internal pipework design for stratification induction, is such that at the beginning of the discharge cycle, ten percent of the tank volume is raised above 6.0 Deg C as the pipe work system seeks to establish the thermo-cline.  This wasted volume is a parasitic energy consumer with no benefit to the system, and must be reprocessed during each charge cycle.

      The Factor of Merit is a function of both the Stratification header design and the aspect ratio of the storage vessel.

      Presume that a stratification header can create a thermo cline during discharge that is 1 meter thick. Further, consider this one-meter thick thermo cline as a percentage of the tank height.  A one meter high tank would have a factor of merit of zero percent, while a 100 meter high tank would have a factor of merit of ninety nine percent.  Large diameter tanks with small height to diameter ratios are inherently less efficient than small diameter high vessels

           The processing of the variable temperature water in the thermo cline presents some operational challenges for the process                      equipment and its control system.  The thicker the thermo cline the longer the challenge continues.


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