Chilled Water Storage Research Articles

Adaptive Operation Strategy of a District Cooling System with Chilled Water Storage and Its Validations by OpenModelica Modeling and Simulations

Developing operation strategies for district cooling systems with chilled water storage is challenging due to uncertain fluctuations of cooling demand in actual operations. To address this issue, this paper developed an adaptive operation strategy and performed its validations by modeling and simulating a commercial cooling system in Shanghai using OpenModelica. Firstly, the originally designed operation strategy of the cooling system was evaluated by simulation but was found unable to meet the statistically averaged ideal cooling requirements due to the early exhaustion of stored chilled water at about 5:30 PM. Then, to build foundations for adaptive operation strategy development, a newly designed operation strategy was established by increasing the operation time of base load chillers in the valley and flat electricity price periods. The new strategy proved numerically sustainable in satisfying the ideal cooling demand. Moreover, to realize the strategy’s adaptability to actual cooling load fluctuations, an adaptive operation strategy was developed by tracking the target stored chilled water mass curve that was calculated by implementing the newly designed strategy. The simulation results verify that the adaptive operation strategy enables good adaptability to representative cooling load fluctuation cases by automatically and periodically adjusting the operation status of base load chillers. The adaptive operation strategy was then further widely numerically tested in hundreds of simulation cases with different cooling load variations. The time-lagging problem resulting in strategy failures was found in numerical tests and was addressed by slightly modifying the adaptive strategy. Results indicate that the adaptive operation strategy enables adaptability to deal with cooling demand fluctuations as well as allowing low cooling supply economic costs and power grid-friendly characteristics. This study provides theoretical support to strategy design and validations for district cooling system operations.

Characterization and prediction of demand response potential of air conditioning systems integrated with chilled water storage

Chilled water storage offers a cost-effective and convenient solution for load flexibility of air-conditioning systems. However, its impacts on system flexibility and energy efficiency have not been comprehensively explored. In this paper, demand response (DR) potential delivered by chilled water storage was explored and predicted through both experimental and simulation studies. The initial experimental investigation focused on the DR potential at various cold charging and discharging proportions of chilled water storage. Subsequently, a virtual platform, validated through experimental data, was utilized to analyze load flexibility and energy efficiency of the system across different baseline loads, storage characteristics, and heat pump characteristics. Prediction models for DR potential in different cold charging and discharging scenarios were further developed based on the simulation results. The experimental and simulation results demonstrated effective load manage performance through adjusting charging and discharging proportions, even though the flexibility may sacrifice energy efficiency in cold discharging scenarios. In addition, the developed prediction models confirmed their feasibility with the coefficient of determination (R2) exceeding 0.80 in charging scenarios and 0.90 in discharging scenarios. The analysis also revealed significant benefits from storing supply water at lower temperatures and selecting heat pumps with partial-load efficiency curves characterized by higher Pearson Correlation Coefficients.

Design of the Cooling Water System Upgrade for 42 T Resistive Magnet at the CHMFL

Abstract To meet the operation requirements of the 42 T resistive magnet which will be built by the Chinese High Magnetic Field Laboratory (CHMFL) in the future, the cooling water system needs to be upgraded accordingly. The cooling water system will improve its performance based on the existing cooling system. This article will make a detailed analysis of the system upgrading and transformation plan. In terms of refrigeration, to meet the requirements of larger refrigeration temperature difference, a new centrifugal chiller with large temperature difference will be added in series with the existing unit of refrigeration. With the increase in heat load, the system will connect a new plate heat exchanger in parallel on the basis of the original heat exchange capacity. However, the circulating cooling water flowrate of the 42 T resistive magnet is expected to exceed 1200 m3/h, the purification flowrate of the deionized water also needs to be upgraded, and the purification ratio is expected to reach more than 6%. In addition, the control system will be also combined with the new equipment and pipelines to realize a variety of chilled water storage and supply modes to better meet the operation of the new-built magnet.

Exploiting district cooling network and urban building energy modeling for large-scale integrated energy conservation analyses

Research effort in analyzing the energy consumption of buildings in districts or cities is increasing as new paradigms of distributed energy production and sharing are spreading. In this work, the Urban Building Energy Model of the Quad, an area of the University of California Davis campus, is presented, validated, and analyzed for possible actions to reduce the district cooling energy consumption. Energy conservation measures involve buildings' air handling units and district chilled water generation. To investigate this system, a district cooling networks module has been developed in EUReCA, the Urban Building Energy Modeling tool used for the analyses. The model has been validated with energy demand and temperature data from 2018 to 2020, resulting in a district cooling demand deviation lower than 7% for 2018 and 2019. Normalized Root Mean Square Error is lower than 35% for each building, proving the reliability at the hourly time scale. Systems energy reduction actions like heat recovery units' installation and heuristic control of the air supply cooling temperature are beneficial in reducing the cooling demand, and they allow a more efficient discharging of the chilled water storage, reducing the average electricity price by load shifting during peak demand.

Predictive control of power demand peak regulation based on deep reinforcement learning

As urbanization continues to accelerate, effectively managing peak electricity demand becomes increasingly critical to avoid power outages and system overloads that can negatively impact both buildings and power systems. To tackle this challenge, we propose a novel model-free predictive control method called “Dynamic Dual Predictive Control-Deep Deterministic Policy Gradient (D2PC-DDPG)" based on a deep reinforcement learning framework. Our method employs the Deep Forest-Deep Q-Network (DF-DQN) model to predict electricity demand across multiple buildings, and based on the output of the DF-DQN model, applies the Deep Deterministic Policy Gradient (DDPG) algorithm to optimize coordinated control of energy storage systems, including hot and chilled water storage tanks in multiple buildings. Experimental results show that our proposed DF-DQN model outperforms other traditional machine learning, deep learning, and reinforcement learning methods in terms of prediction accuracy, such as mean absolute error (MAE), mean absolute percentage error (MAPE), and root mean square error (RMSE). Moreover, our D2PC-DDPG method achieves superior control performance and peak load reduction compared to other reinforcement learning methods and an RBC-based control method. Specifically, our method successfully reduced peak load by 27.1% and 21.4% over a two-week period in the same regions. To demonstrate the generalizability of our D2PC-DDPG method, we tested it in five different regions and compared its performance with an RBC-based control method. The results showed that our method achieved an average reduction of 16.6%, 7%, 9.2%, and 11% for ramping, 1-load_factor, average_daily_peak, and peak_demand, respectively. These findings demonstrate the effectiveness and practicality of our proposed method in addressing critical energy management issues in various urban environments.

Transient study of thermal stratification of full-scale chilled water storage tank during optimum discharge condition

Thermal stratification of full-scale Chilled Water Storage Tanks (5855 m3) with 18 m tank diameter, and 23 m water depth during discharge mode and optimum condition was studied. The experimental and numerical analyse of stratified thermal storage tank in full-scale dimension and discharge mode has been studied. A 3D and 2D numerical model was performed. The case simplified to divide into two parts, the first part was to simulate flow in pipes and radial diffuser, and the second part included simulation in the tank according to the part one results. The optimum operation condition which found from many tests of tank operation conditions according to high thermal stratification and low thermal mixing inside the tank. Validation test with experimental data obtained from the present work and literature. The results included to analyse the thermal stratification and the flow characteristics during the discharging mode. Also, thermal performance parameters such as discharge efficiency, Richardson number, stratification efficiency, thermocline thickness, the average temperature in the tank, and discharge temperature were presented. The important result of the above analysis was the optimum stratification phenomena in the tank with low thermal mixing. The contribution of this study was the numerical modelling and simulation setting to capture the experimental results of these phenomena in the full-scale thermal storage tank, moreover to specify the optimum condition of thermal stratification of this tank dimension, diffuser type, and flow rate.

A Review on Performance of Solar Powered Air Conditioning System

Abstract: Solar energy converts the renewable energy to increase growth. The development of energy in to generated worldwide. It in the most easy to construct the process of solar air Conditioning systems. The different energy areinvolved into the solar air conditioning to the decreasing current Sources Then using high oil and concern environmental effects have been Controlling. The most comfortable Process of the solar energy system. It we implemented nowadays, increase in progress. we are air Conditioning systems are using in. every building, malls,Colleges industries, flats, etc. solar power air conditioning system is the hot issue to study building energy Consumer. I Then we indoor and outdoor comfort of air Conditioning while be cold storage system. To increasing the efficiency of air Conditioning intensity of the cooling system. The transient thermal efficiency we can using the storage system to maintain the temperature to indoor,The solar system must be using the different. Parabolic collector from the high accommodation,The basic analysis, to investment of Solar air conditioning system are using 3-phase accumulator. The solar energy using of high rate of low grade energy to high grade of energy are Converting them. It will high efficient process could be panels, collectors, will cool the High temperature can be assumed to use the chilled water storage tank. can be provided due to reduce the system of on and off options. There are improving solar system.It will high efficient process could be panels , collectors, will cool the High temperature can be assumed to use the chilled water storage tank. can be provided due to reduce the system of on and off options. There are improving solar system.it is common for the storage chilling water can be utilized, that can to be operated solar energy.

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.

Numerical Simulation of Ammonia-Water Solution Based Heat Absorber

This article provides an insight into the numerical simulation of an ammonia-water solution-based plate heat absorber, which is a crucial part of an absorption chiller. These chillers could be used as part of a stratified chilled-water storage tank configuration for coupling to a small modular reactor. The common approach utilized in the process of designing the chiller is to call the absorber a "black box" without considering the processes inside. It follows that the resulting design must be highly oversized to ensure reliable operation of the system. This article focuses on the obstacles that needs to be overcome in order to simulate the processes inside the absorber, such as stream breakup into separate droplets or absorption of ammonia. Unfortunately, we do not have any experimental data we could use to validate our simulations and thus this article should be looked at as a "proof of concept" and a possible guideline for simulations of similar phenomenon.

Optimizing single-stage double-effect LiBr-H2O absorption chiller in micro-trigeneration system using an atom search optimization algorithm

Wastes produced from several processes are focused on by many scientific researchers in order to reuse them, achieve energy savings, and reduce cost and environmental pollution. Recently, many organizations have utilized a micro-trigeneration system as a primary energy source, as it produces cooling, heat, and power in a combined effect. Various scholars designed numerous trigeneration systems in recent years. However, it is challenging to attain energy-efficient operation. Therefore, this current research focuses on developing an optimal micro combined cooling, heating, and power generation (MCCHP) to improve energy efficiency and reduce cost. The major components of the trigeneration system are the diesel engine, heat exchanger, boiler, absorption chiller, and chilled water storage system. In order to improve the cooling output of the proposed system, a single-stage, double-effect absorption chiller (LiBr-H2O absorption chiller) is utilized with the atom search optimization (ASO) algorithm. The variables of the condenser in the absorption chiller are given as input for optimization. The results show that the proposed system has a cooling capacity of 16 kW. Compared with the existing algorithms, it is efficient with 98% temperature minimization and cost-effectiveness. This micro-trigeneration device can be used in hospitals and the food production industry and is cost-effective.

Feasibility analysis and feature comparison of cold thermal energy storage for off-grid PV air-conditioned buildings in the tropics

Cold thermal energy storage (CTES) is a cost-efficient storage approach for PV powered air-conditioning systems in tropical buildings. However, the feasibility and performance of different CTESs, including chilled water storage, ice storage, PCM cooling storage, and building thermal storage, are still unclear for off-grid PV air-conditioned buildings.In this study, an off-grid PV system with battery and CTES is proposed, including the operation strategy to manage the energy-storage process. To implement all CTES-types, a co-simulation model, which integrates the system model with the ARX (autoregressive with exogenous input) cooling-load model, is constructed to enable the interaction between supply- and demand-side. Based on the minimization of net present cost and the constraint of a certain required solar fraction, an optimization method is developed to find the optimal system-configuration. Moreover, an office, a hotel, and a residence are selected for a case study.The results indicate that CTES is feasible for higher solar fraction. The feasibility ranking of building type is (from high to low): residence, hotel, and office. In the most suitable case (residence), the maximum reduction of system costs (compared to battery-only system) is 49.76%, 41.77%, 44.31%, and 22.78%, respectively, for chilled-water storage, ice storage, PCM cooling storage, and building thermal storage. These values are 22.48%, 12.00%, 17.42%, 2.60% for the hotel, and 19.26%, 11.57%, 13.64%, 4.06% for the office. In addition, the sensitivity analysis shows that the increasing cost of both PV-array and CTES-device affects the feasibility of CTES negatively, while the increasing battery cost is a positive factor. Compared to the cost of the PV-array and the CTES-device, the battery cost represents the most significant effect-factor for feasibility.

Optimal Scheduling for a District Cooling System with Chilled Water Storage: Comparative Assessment of Vapour Compression and Absorption Refrigeration Cycles

District cooling (DC) systems are able to exploit locally available energy in order to supply cooling to local end-users through an integrated DC network. The present work aims at obtaining the critical decisions in the performance of a DC network at the operating level. End-users can be of different type (e.g., residential or commercial) with different demand profiles and an optimal strategy needs to be employed in order to satisfy the demand and minimise energy usage. In published literature the vast majority of the studies employ simplified mathematical models in order to describe the thermodynamic cycle used and only a handful of studies take into account the actual dynamics of the system transition from one state to the other as well as the time delay attributed to the cooling medium transfer through the network. The critical decisions involve the operating conditions and optimal allocation for the cooing of existing units in a suitable time interval, which form the basis for the satisfaction of requirements in cooling under demand variability. Although reduced order models of the process of ABR cycle are used, the time delay of the cooling medium transfer through the pipes is considered through the employment of validated first order plus time delay models. The proposed optimisation framework enables the identification of demand driven, highly performing solutions through the direct consideration of all interacting factors over a pre-defined time horizon. Optimal results include the definition of the optimal allocation of produced cooling effect in such way that cooling demand is always satisfied.

Optimization design of the octagonal water diffuser with uniform flow orifice plate

Energy storage technology has been widely used in many fields, mainly due to the continuous development of social economy, resulting in increasing power consumption and peak valley load difference. This technology can play the role of adjusting the electric network load. Among them, water storage technology is the most representative. Naturally stratified technology is the most economical and effective method in chilled water storage technique and its core is the design of water diffuser. Octagonal water diffuser with the uniform flow orifice plate is a more effective water distribution mode in the naturally stratified chilled water storage system. In order to get the higher chilled water storage efficiency, the fluid dynamics software was used to make an optimization research on its parameters. Based on the scheme of octagonal diffuser and uniform flow orifice plate, the simulation analysis of the outlet mode of water distribution pipe and the opening parameters of orifice plate were carried out. According to the numerical simulations on the different volume water storage tanks, the comprehensive analysis showed that the diffusion performance of circular opening was better than slot opening, and the thickness of thermocline was thinner when the opening ratio of the orifice plate was 20% than the case of 30% and 45%. In addition, it was pointed out that the porous media model could almost replace the two-dimensional actual opening model under the low opening rate.

Discussion and Commentary on “Robust scheduling of multi-chiller system with chilled-water storage under hourly electricity pricing” in “Energy and Buildings” 218 (2020) 110058

A discussion of the “Robust scheduling of multi-chiller system with chilled-water storage under hourly electricity pricing” by Milad Sadat-Mohammadi et al. “Energy and Buildings” 218 (2020) 110058 is presented in this brief communication. A robust strategy for scheduling multi-chiller systems with chilled-water storage is provided in the mentioned work. The authors have discussed and presented the results for the optimal chiller loading considering the cooling load uncertainty in risk-neutral, risk-taker, and risk-averse strategies based on robust optimization method. RO takes a worst-case strategy to avoid ambiguity in probability distributions. Their proposed model (i.e., RO method), on the other hand, never can model risk-taking strategy in modeling the uncertainty of parameters that is cooling load in this study. Info-gap decision theory (IGDT) has a different formulation, which is an approach that deals with risk-neutral, risk-taker, and risk-averse strategies, which is formulated in this commentary. A clarification of the problem formulation and proper modeling of the problem are offered in this regard to attain the correct modeling of uncertain cooling load in energy systems. Also, based on the proposed formulation of the robust optimization approach, the assumption and simulation results achieved by their suggested model are analyzed and discussed.

Experimental study on the thermal performance of a novel physically separated chilled water storage tank

Naturally stratified chilled-water storage is widely used to offset the peak electrical demand caused by air-conditioning, but the mixing between cold and warm water greatly influences its thermal performance, especially for a low aspect ratio tank. In this study, a novel physically separated chilled water storage tank is proposed to take place traditional membrane tanks. The thermal performance of this novel tank is evaluated by static and dynamic experiments. The temperature distribution, outlet temperature, figure of merit (FOM1/2/FOM), and percent cold recoverable (PCR) are analyzed when the cold water temperature is approximately 2 °C and 4 °C, and the temperature difference ranges from 7.91 to 13.86 °C. The results demonstrate the excellent thermal performance of the novel tank under a large temperature difference. Compared with conventional temperature conditions, the stability of the outlet temperature under low-temperature conditions is better during the discharging process with the dimensionless temperature increasing by 10% at the end of the process. The full-cycle FOM increases significantly and the PCR decreases slightly with increasing temperature difference. When the FOM reaches its maximum of 93.2%, the PCR exceeds 91%. Therefore, this research work exhibits great application potential of this novel tank due to its ability to improve the thermal performance for a low aspect ratio tank, which provides a new direction for chilled water storage tank design.

A Flow Rate Dependent 1D Model for Thermally Stratified Hot-Water Energy Storage

Stratified tank models are used to simulate thermal storage in applications such as residential or commercial hot-water storage tanks, chilled-water storage tanks, and solar thermal systems. The energy efficiency of these applications relates to the system components and the level of stratification maintained during various flow events in the tank. One-dimensional (1D) models are used in building energy simulations because of the short computation time but often do not include flow-rate dependent mixing. The accuracy of 1D models for plug flow, plug flow with axial conduction, and two convection eddy-diffusivity models were compared with experimental data sets for discharging a 50-gal residential tank and recharging the tank with hot water from an external hot-water source. A minimum and maximum relationship for the eddy diffusivity factor were found at Re <2100 and >10,000 for recirculation of hot water to the top of the tank and vertical tubes inletting cold water at the bottom. The root mean square error decreased from >4 °C to near 2 °C when considering flow-based mixing models during heating, while the exponential decay of the eddy diffusion results in a root mean square error reduction of 1 °C for cone-shaped diffusers that begin to relaminarize flow at the inlet.

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.

Global sensitivity and uncertainty analysis of the levelised cost of storage (LCOS) for solar-PV-powered cooling

In this paper, we investigate the sensitivity of the parameters affecting the levelised cost of storage (LCOS) of Lithium ion (Li-Ion) battery storage and chilled water storage for on-grid PV-powered cooling applications, namely an office and a hotel building. Therefore, we carry out multi-objective optimisations to minimise the LCOS and maximise the self-sufficiency based on TRNSYS simulation models. For selected optimal solutions, we perform Morris and Sobol global sensitivity as well as uncertainty analyses. The results show that the chilled water storage yields up to 8 ctUSD/kWhel lower LCOS than the battery storage at the same self-sufficiency; however, the battery storage can increase the self-sufficiency more. The dominant parameter affecting the LCOS is the storage lifetime for the battery. Careful design of the dedicated chiller plant to charge the storage is important for the system configuration with chilled water storage. Because of the wide range of possible battery lifetimes, the 90% confidence interval for the LCOS of battery storage is 2.6 times larger than the one for chilled water storage given the input parameter ranges in this study. The future trend indicates that battery storage is expected to become more competitive due to technological advancements and favourable price developments.

Optimal planning of distributed hydrogen-based multi-energy systems

As a clean and renewable energy, hydrogen has attracted increasing attention for the replacement of fossil fuels because it is an emerging way to address the uncertainties of the renewable energy. Besides, coordination of the energy storage units, such as hydrogen storage unit, hot water storage unit and chilled water storage unit, could improve energy efficiency and reduce system cost. Thus, the optimal planning of a distributed hydrogen-based multi-energy system is very important to build a hydrogen-based distributed energy system in the demand side. This paper focuses on the system planning problem, which is formulated as a mixed-integer linear programming problem and the objective is to minimize annual capital and operation expenditure of the system. The problem is solved by a commercial solver. The case studies are performed under different energy demand profiles and solar radiations obtained by EnergyPlus in twelve typical cities around the world. It is found that multi-energy storage units in the systems can significantly reduce system capital expenditure and operating expenses. Hot water storage unit enjoys the best benefits with an average system cost reduction rate being 64.6% with the planning horizon (8760 h). And the DHME system is environmental friendly, which can even drop over 100% carbon emission in high solar radiation regions compared with the conventional electricity-driven energy system. Furthermore, with optimistic hydrogen price target of USA Department of Energy, the developed DHME system will maximum reduce 60.0% OPEX compared with the conventional electricity-driven energy system.

Calibrated simulation study for efficient sizing and operating strategies for the thermal storage integrated air conditioning system

ABSTRACT The chilled water thermal energy storage is a proven technology that allows dissociation of cooling from the concurrent demand for cooling within a building. However, its applicability to office buildings, which operate at daytime only, and the effect of various parameters on its sizing and operation are still not explored in detail. The key objective of this study is the demonstration of the impact of new sizing and operating strategies on energy-saving potential while implementing chilled water storage with an air conditioning system. To accomplish the objectives, using experimental data, the simulation model was calibrated and further used for the analysis of the impact of the considered variables on the storage integrated system operating strategies.