Thermal Energy Storage (TES) systems store thermal energy for use at a later time when it is more cost-effective or necessary to generate energy. With TES tanks, the operating cost and required capacity of the cooling and heating plants are reduced. This increases efficiency and reduces capital costs.
The thermal stratification observed inside the TES tank has a major influence on the thermal performance of HVAC chiller systems. The most effective way of avoiding de-stratification in a thermal energy storage tank is through avoiding water jets entering the tank.
Typically, the TES tank inlets & outlets are designed using Octagonal slotted pipe type or radial disk type diffusers to avoid the destratification & to promote effective stratification in the thermal energy storage tanks.
Mechartes have capability to carryout the design of the Thermal energy storage tanks and in this case study we will detail the mechanical and structural design of one of the TES tanks.
The objective of the project is to carry out Mechanical and structural design for the Thermal energy storage tank to design the internals and the tank.
CFD Analysis is carried out for diffuser design calculations and validation of diffuser design for the thermal energy storage tank. As part of project scope, the radial disc diffusers were designed & validated using the transient CFD analysis for both charging & discharging modes of operation to demonstrate the thermal stratification inside the TES tank and to achieve the FOM (Figure of merit) as per client requirement.
The structural design calculations include the following.
I. Shell and roof plate thickness calculation
II. Central column calculation and structural member requirement fixed cone roof
III. Bottom plate and annular plate calculation
IV. Stiffener ring calculation
V. Requirement for nozzles as per EN standards.
VI. Anchorage calculation and Anchor chair calculation
VII. Structural calculations for internal pipes and diffusers
VIII. Structural calculations for Stairs and platforms.
The radial disc diffusers for the TES tank were designed based on the input parameters by considering the limiting values of Froude Number, Reynolds number and other considerations.
The design flow rate during the discharging mode & charging mode was considered for designing the TES tank diffuser system. The TES tank diffuser system was designed with radial disc diffusers located at top as well as of radial disc diffusers located at bottom to enable to distribution of incoming flow to the complete diameter of the tank.
To maintain the low velocities from the diffuser piping, the central pipe as well as branch pipes connected to the top diffusers & bottom diffusers were selected to limit the maximum velocity entering into the diffuser from the pipe to be less than 1 m/s as the higher velocity of flow in the pipe would transfer the same high velocities into the diffuser.
The series of small scale CFD analysis were carried out by considering the geometry of diffuser only with varying the inlet pipe diameters, inlet pipe lengths, diffuser heights & internal baffle arrangements to achieve the lowest possible velocity (or Re) with close to uniform velocity distribution at outlet of the diffuser for the given tank size & maximum design flow rate condition.
Based on the small-scale results appropriate design was selected and a full scale Transient CFD analysis was carried out for both charging and discharging mode for the operation duration. The CFD results of velocity & temperature profiles inside the tank along with thermocline thickness results & FOM were presented from the full scale CFD analysis.
Hot water thermal energy storage tank is designed as per guidelines given in standard EN-144-15-2205.
Material used for construction of shell, roof, bottom plate, bottom annular plate, roof structural members, compression ring, stiffeners, stairs and platform structural material etc. were provided.
Fabrication drawings for tank construction were provided.