Stratified Chilled Water Storage Tank Research Articles

Globally optimal control of hybrid chilled water plants integrated with small-scale thermal energy storage for energy-efficient operation

The integration of thermal energy storage in chilled water systems is an effective way to improve energy efficiency and is essential for achieving carbon emission reduction. However, the commonly used large-scale thermal energy storage needs significantly larger space, which hinders the wide application of thermal storage in large number of existing buildings. Unlike previous studies, this study integrated a small-scale stratified chilled water storage tank into chilled water plants and proposed a global optimal control strategy to enhance the overall system energy performance. The proposed strategy determines the optimal settings of stratified chilled water storage tank charging/discharging flow rate, chilled water supply temperature, and the number of chillers in order to minimize the daily energy consumption of the chilled water plants under varying load conditions. The stratified chilled water storage tank was modelled as a “virtual chiller” to quantify the energy consumption related to the charging/discharging. Multiple charging/discharging cycles were controlled for optimal chiller loading. The proposed control strategy was evaluated in a simulated complex central chilled water plant. The results show that the proposed optimal control strategy can save the daily energy consumption of the central chilled water plant by 4.35–7.67%, 2.10–3.90%, and 2.30–5.15% in three typical weather conditions.

Effect of Diffuser Height on Thermocline in Stratified Chilled Water Storage Tank

Chilled water energy storage using thermal stratification technique currently used in the vast area because it contributes to reducing energy consumption and refrigeration capacity as well as its maintenance, operating and capital costs are low. In this paper, experimental tests were carried out on a small-scale vertical cylindrical storage tank equipped with an elbow-type conventional diffuser at inlet heights of 20, 170, 320 and 470mm for flow charging rates from 1.5-7.5l/min. in order to obtain a good thermal separation. The degree of stratification was estimated by means of temperature distributions and performance metrics, which involve thermocline thickness, the half-cycle figure of merit and equivalent lost tank height. The results show that the decrease in diffuser height above the tank floor tends to the steep thermocline or satisfactory thermal separation, the stratification and thermal performance were obtained at diffuser height of 20 mm within the limiting volume flow rates 1.5-4.5 l/min. better than those at volume flow rates ranging from 5.5-7.5l/min. and much better than at diffuser heights of 170, 320 and 470mm for various flow rates.

Modeling Hybrid Nuclear Systems With Chilled-Water Storage

Air-conditioning loads during the warmer months of the year are large contributors to an increase in the daily peak electrical demand. Traditionally, utility companies boost output to meet daily cooling load spikes, often using expensive and polluting fossil fuel plants to match the demand. Likewise, heating, ventilation, and air conditioning (HVAC) system components must be sized to meet these peak cooling loads. However, the use of a properly sized stratified chilled-water storage system in conjunction with conventional HVAC system components can shift daily energy peaks from cooling loads to off-peak hours. This process is examined in light of the recent development of small modular nuclear reactors (SMRs). In this study, primary components of an air-conditioning system with a stratified chilled-water storage tank were modeled in FORTRAN 95. A basic chiller operation criterion was employed. Simulation results confirmed earlier work that the air-conditioning system with thermal energy storage (TES) capabilities not only reduced daily peaks in energy demand due to facility cooling loads but also shifted the energy demand from on-peak to off-peak hours, thereby creating a more flattened total electricity demand profile. Thus, coupling chilled-water storage-supplemented HVAC systems to SMRs is appealing because of the decrease in necessary reactor power cycling, and subsequently reduced associated thermal stresses in reactor system materials, to meet daily fluctuations in cooling demand. Also, such a system can be used as a thermal sink during reactor transients or a buffer due to renewable intermittency in a nuclear hybrid energy system (NHES).

MPC-based optimal scheduling of grid-connected low energy buildings with thermal energy storages

The mismatch between the energy demand and energy supply is one of the major problems in low or zero energy buildings with distributed power generations. The policy of time-sensitive electricity pricing provides a possibility to improve the energy efficiency of the building energy systems. A model predictive control (MPC)-based strategy using nonlinear programming (NLP) algorithm is proposed to optimize the scheduling of the energy systems under day-ahead electricity pricing. Evaluations are conducted using a reference building based on the Hong Kong Zero Carbon Building. A stratified chilled water storage tank is introduced as the thermal energy storage, which makes possible to optimize the scheduling of the building energy systems. The distributed power generation in the building consists of a combined cooling and power system and a photovoltaic (PV) system. Two types of grid-connections (i.e., selling electricity to grid is allowed/forbidden) are considered. Results show that the proposed optimal scheduling strategy can achieve significant reductions in carbon dioxide emission, primary energy consumptions and operation cost. Sensitivity analysis shows uncertainties in the inputs do not affect the performance of the proposed method significantly.

Tank size and operating strategy optimization of a stratified chilled water storage system

Abstract In the downtown area of Austin, Texas, United States, it is planned to build a new naturally stratified chilled water storage tank and share it among four separated chilled water plants in order to reduce the utility billing cost. Each plant is charged with a typical time-of-use utility rate including energy charge and demand charge. This paper presents the method of determining the optimal tank size as well as corresponding optimal operating strategies for this project. A simplified thermal energy storage plus four plants model is built based on some assumptions. Three conventional control strategies (full storage, chiller priority, and storage priority) with limitations on the maximum number of chillers running during the off-peak and on-peak periods are simulated. The results show that a 3.5 million gallon (13,249 m 3 ) tank has the shortest simple payback time and the projected total capital cost is within the budget. Full storage strategy is selected for the summer months and storage-priority strategy is selected for the winter months. The annual billing cost savings are estimated at $907,231 and the simple payback time is 12.5 years.

Constant flow rate charging characteristics of a full-scale stratified chilled water storage tank with double-ring slotted pipe diffusers

Temperature distributions formed during charging of a 14 m (46 ft) water depth, 7 m (23 ft) radius stratified chilled water storage tank with double ring octagonal slotted pipe diffusers were recorded and analyzed. Prior to the beginning of each test, the tank was fully discharged and at a relatively uniform temperature. To the extent possible, inlet flow rate and temperature were held constant during each test. Six tests were performed for flow rates varying from 50% to 95% of design. Thermal performance was quantified using thermocline thickness, half-cycle figure of merit (FoM 1/2) and equivalent lost tank height (ELH). The thermal performance for all tested charge flow rates was good by current standards (FoM 1/2 > 90%, ELH < 1 m in all but one case). Thermocline thickness for low and medium flow rates was relatively small, on the order of 1 m (3 ft). Erroneously large thermocline thickness measurements were obtained in the high flow rate case because of the effect of inlet temperature variation combined with the definition used. Significant bulk temperature increase observed below the thermocline in all cases was most likely due to large-scale convection cells driven by the inlet flow. Efforts to improve the performance of diffusers should focus on suppressing this circulation.

Parametric Study of Single-Pipe Diffusers in Stratified Chilled Water Storage Tanks (RP-1185)

A parametric study was performed of the charging thermal performance of a full-scale pipe diffuser in a single cylindrical stratified chilled water storage tank by applying factorial experimental theory to the results of simulations performed with a validated computational fluid dynamics (CFD) model. Dimensional parameters having the potential to influence charging inlet performance were identified and formed into dimensionless groups using the method of repeating variables. Parameters included: the inlet Richardson number based on inlet slot width (Ri l ), inlet Reynolds number (Re i ), ratio of inlet width to diffuser height (l/hi ), ratio of inlet diffuser height to tank radius (hi/RW ), and ratio of diffuser radius to tank radius (RD/RW ). Thermal performance was measured in terms of equivalent lost tank height (ELH). A full 2k factorial experiment of thirty-two simulations was performed and analyzed. Parameter ranges were: 0.05-2 for Ri l , 500–5000 for Re i , 0.1–1 for l/hi , 0.005–0.05 for hi/RW , and 0.707–0.866 for RD/RW . Within these ranges, Ri l , l/hi , and hi/RW were found to be of first-order significance, while Re i and RD/RW were not. Two-factor interactions involving Ri l , l/hi , and hi/RW were also significant. Regression models of equivalent lost tank height as functions of Ri l , l/hi , and hi/RW were developed. The predictive capabilities of the regression models were tested against the results of five additional CFD simulations having parameter values different from the 2 k factorial experiment cases. On average, regression models predicted factorial experiment data to within 10% with maximum error of 30% to 60%, depending on the model.

Measured and Modeled Charging of a Stratified Chilled Water Thermal Storage Tank with Slotted Pipe Diffusers

As part of ASHRAE Research Project 1185, field data from the constant flow rate charging of a stratified chilled water storage tank with double-ring octagonal slotted-pipe diffusers serving a university chilled water system were compared with results of a transient axisymmetric computational fluid dynamics (CFD) model. Charge processes at flows near design flow rate and 50% of design were modeled. Laminar and turbulent simulations were performed for a range of slot dimensions. Performance of the model was assessed by direct comparison of temperature profiles and by comparison of common thermal performance metrics, including thermocline thickness, lost capacity, and equivalent lost height. The inlet Richardson number based on slot hydraulic radius was found to have the strongest effect on model results. The model Richardson number giving best agreement with the lost height of the field data for near design flow rate was roughly an order of magnitude greater than the Richardson number of the actual diffuser. It is conjectured that this is due to inherent differences between the flow produced by multiple three-dimensional jets in the actual diffuser and the continuous slot of the axisymmetric model. The effect of turbulence varied from negligible to moderate as the Richardson number decreased.

Parametric Study of Charging Inlet Diffuser Performance in Stratified Chilled Water Storage Tanks with Radial Diffusers: Part 2–Dimensional Analysis, Parametric Simulations and Simplified Model Development

A parametric study of the thermal performance during charging of a full-scale radial inlet diffuser in a cylindrical stratified chilled water storage tank was performed by applying factorial experimental theory to the results of simulations performed with a validated computational fluid dynamics (CFD) model. Part 1 describes the development and validation of the CFD model. Part 2 summarizes the results of the parametric study. Dimensional storage tank and inlet dif-fuser parameters having the potential to influence inlet thermal performance were identified, then formed into dimensionless groups using Buckingham Pi analysis. These included the inlet Richardson number (Ri), inlet Reynolds number (Re i ), ratio of diffuser radius to tank radius (RD /Rw ) and ratio of diffuser radius to diffuser inlet height (RD/hi). Thermal performance was measured in terms of thermocline thickness and equivalent lost tank height. Sixteen simulations comprising a full 2 k factorial experiment were completed and analyzed. Parameter ranges considered were as follows: Ri from 1.0 to 11.1, Re i from 1,000 to 12,000, RD /Rw from 0.2 to 0.4 and RD /hi from 5 to 10. Within these ranges, Ri, RD /Rw and RD /hi were found to be of first-order significance while Re i was not. Regression models of thermal performance metrics as functions of Ri, RD /Rw and RD /hi that are sufficiently simple to be useful for design were developed. These models agreed well with CFD simulations from which they were derived and with field data.

Parametric Study of Charging Inlet Diffuser Performance in Stratified Chilled Water Storage Tanks with Radial Diffusers: Part 2–Dimensional Analysis, Parametric Simulations and Simplified Model Development

A parametric study of the thermal performance during charging of a full-scale radial inlet diffuser in a cylindrical stratified chilled water storage tank was performed by applying factorial experimental theory to the results of simulations performed with a validated computational fluid dynamics (CFD) model. Part 1 describes the development and validation of the CFD model. Part 2 summarizes the results of the parametric study. Dimensional storage tank and inlet diffuser parameters having the potential to influence inlet thermal performance were identified, then formed into dimensionless groups using Buckingham Pi analysis. These included the inlet Richardson number (Ri), inlet Reynolds number (Re i ), ratio of diffuser radius to tank radius (RD /Rw ) and ratio of diffuser radius to diffuser inlet height (RD /hi ). Thermal performance was measured in terms of thermocline thickness and equivalent lost tank height. Sixteen simulations comprising a full 2k factorial experiment were completed and analyzed. Parameter ranges considered were as follows: Ri from 1.0 to 11.1, Re i from 1,000 to 12,000, RD /Rw from 0.2 to 0.4 and RD /hi from 5 to 10. Within these ranges, Ri, RD /Rw and RD /hi were found to be of first-order significance while Rei was not. Regression models of thermal performance metrics as functions of Ri, RD /Rw and RD /hi that are sufficiently simple to be useful for design were developed. These models agreed well with CFD simulations from which they were derived and with field data.

Parametric Study of Charging Inlet Diffuser Performance in Stratified Chilled Water Storage Tanks with Radial Diffusers: Part 1–Model Development and Validation

A computational fluid dynamics (CFD) model has been developed to simulate the flow and heat transfer near a lower radial inlet diffuser in a cylindrical stratified chilled water storage tank during charging. The model was used to perform parametric simulations of inlet diffuser performance in full-scale tanks during thermocline formation, from which first-order correlation relating thermal performance to tank and diffuser design parameters were derived. Part 1 describes the development of the CFD model and its validation. Part 2 reports the results of the parametric study. The two-dimensional, transient model was implemented in a commercial finite element code. The model has been validated with field-measured data from two full-scale tanks. Laminar analysis predicted field-measured temperature profiles well, even under ostensibly transitional conditions, provided that variations in inlet temperature during the field tests were accurately modeled. These results suggest that turbulence has a secondary effect on the development of thermoclines within the range of parameters considered and that laminar models may be useful for modeling performance of full-scale tanks over a wide range of inlet parameters. The fully developed temperature profile was found to be insensitive to the velocity distribution assumed at the inlet.

Parametric studies on thermally stratified chilled water storage systems

An analysis of the stratification decay in thermally stratified vertical cylindrical cool storage systems is presented using a one dimensional conjugate heat conduction model. The degree of thermal stratification depends upon the length to diameter ratio, wall thickness to length ratio, the thermo-physical properties of the material of the storage tank, the type and thickness of the insulation and the design of the admission system for both cold and warm water. A parametric study of the stratified chilled water storage tanks in charging, discharging and stagnation modes of operation is made. The thermoclines degrade due to the heat transfer from the ambient, thermal diffusion in the storage tank, axial wall conduction and mixing due to admission of the fluid in the storage tank during charging and discharging. The degree of thermal stratification in storage tanks is expressed in terms of either heat capacity (thermal capacitance ratio) or modified Biot Number. A mixing parameter accounts for the effect of mixing on thermal stratification in both charge and discharge cycles.

Thermal Performance of Stratified Chilled Water Storage Tanks

The mechanisms which contribute to a loss of capacity in stratified storage tanks are usually grouped into heat transfer through the tank walls, conduction across the thermocline, and the flow dynamics of the charge and discharge process. In this paper, we use analytical solutions of the unsteady one-dimensional energy equation to show that the flow dynamics are generally orders of magnitude more important than the other factors. Heat transfer through the tank walls constitutes a loss which is less than a couple percent of the tank capacity for reasonably sized, underground or above-ground, insulated tanks. The importance of convective fluid motion relative to one-dimensional conduction across the thermocline is quantified with a single parameter. Values for this parameter can be estimated from measurements of in-tank temperature profiles or outlet temperature profiles. The model solutions are also used to calculate an estimate of the maximum first law efficiency for stratified storage. Published data shows current storage tanks generally operate at efficiencies of 50 to 80%. The efficiency of a storage-based cooling system is shown to be the product of storage efficiency and chiller efficiency. Storage tank losses must therefore be offset by improvements in chiller efficiency if these systems are to compare favorably with conventional cooling systems on the basis of energy efficiency.