SIMULATION MODELS FOR SINGLE AND TWO-STAGE CHARGING OF STRATIFIED THERMAL ENERGY STORAGE

The current practice of charging thermal energy storage (TES) tank is by using
electric chillers. One of the main reasons is that the temperature limitation of the
absorption chillers which might lead to freezing the refrigerant. This was the reason
the absorption chillers at co-generated district cooling plants are not being utilized to
charge the TES tank. This research focuses on the development of models
incorporating absorption chiller to complement electric chiller to charge a stratified
TES tank of a co-generated district cooling plant. The models were developed using
two approaches, namely temperature distribution analysis approach and heat transfer
approach. For the case of temperature distribution analysis, a function was selected to
represent its profile. Functional relationship of the temperature distribution was used
to formulate thermocline thickness, thermocline limit points, temperature transition
point and limit capacity criteria. Using temperature distribution function, simulation
model was then developed based on an open charging system. For the heat transfer
approach, the models were developed as a close system by integrating the TES tank
and chiller equipments. For the TES tank, one-dimensional flow conductiveconvection
analysis was used, while the chillers utilized energy balance analysis. For
both approaches two types of models, namely single stage and two-stage models were
developed. The single stage model is limited to using electric chiller to charge the
TES tank. While the two-stage models incorporate both the absorption and electric
chillers with the absorption and the electric chillers function in sequence to charge the
TES tank. Validation was performed on the single stage charging for both approaches.
Results show similarities of temperature distribution values of R2 greater than 0.98
and parameters deviation lower than 6%. From statistical test acceptance analysis for
the single stage model, t-computed has highest value of 0.035, which is lower than
critical value of 2.145 from the t-distribution table. This indicates that both models
were statistically acceptable. Comparisons of the single and two-stage models
between the two approaches were also performed using simulation case studies.
Results imply that the models are capable of predicting charging characteristic, with
deviations lower than 4% for the charging durations and below 2% for the cumulative
cooling capacity. Findings from simulations of the two-stage models indicate that the
absorption chillers could be used to charge the TES in combination with electric
chillers.