Central Chilled Water System Research Articles

Numerical analysis of rack-based data center cooling with rear door heat exchanger (RDHx): Interrelationship between thermal performance and energy efficiency

This study presents a comprehensive numerical analysis of thermal performance and energy efficiency, IT operating environment, and thermal performance of the RDHx system, a rack-based cooling solution for high-density IT power using air-cooled equipment. A 150 kW IT module and RDHx cooling system were designed and evaluated, and CFD numerical analysis confirmed that RDHx maintains a stable IT environment within ASHRAE standards at a supply air temperature of 24 °C. Additionally, the chilled water supply temperature for the secondary CDU can be increased to 14 °C, and for the primary chiller to 12 °C. A central chilled water system for a 30 MW reference data center was designed and analyzed, showing increased efficiency with a 12 °C supply temperature and varying condenser water return temperatures in winter, resulting in an annual PUEcooling range of 1.25–1.33, with an average of 1.27. The annual energy consumption distribution for rack-based cooling is approximately 46 % from chillers, 21 % from fans, and 33 % from pumps. These results provide crucial insights into data center cooling and support the application of the RDHx system in high-density data centers, with future validation involving a field mock-up test of a 150 kW IT module.

Adaptive control algorithm with a retraining technique to predict the optimal amount of chilled water in a data center cooling system

We developed control algorithms based on one of three artificial-intelligence-based retraining techniques (sliding window, vector adaptation, and vector augmentation) to provide the optimal indoor temperature and save on the energy expenditure for cooling in data centers. The artificial neural network prediction model predicts the computer room air handler supply air temperature of a central chilled water system and is added to the control algorithm. The proposed algorithm can determine the optimal chilled water flow rate required to cool the server to the set temperature by using the predicted computer room air handler supply air temperature. We developed a control algorithm embedded in an artificial neural network predictive model that includes three retraining techniques. Afterward, we compared the control performance and verified its adaptability by using computer simulation. When using the algorithm with sliding window control, the root mean-squared error between the set temperature and the control temperature was 0.08 °C, the maximum error was 0.81 °C, and the cooling load was 21,026.27 kWh. The accuracy, stability, and energy-saving ability of the sliding window control algorithm were higher those of the other two algorithms, and its superior adaptability and scalability under changing environmental conditions were demonstrated.

Model-free control method based on reinforcement learning for building cooling water systems: Validation by measured data-based simulation

In the domain of optimal control for building HVAC systems, the performance of model-based control has been widely investigated and validated. However, the performance of model-based control highly depends on an accurate system performance model and sufficient sensors, which are difficult to obtain for certain buildings. To tackle this problem, a model-free optimal control method based on reinforcement learning is proposed to control the building cooling water system. In the proposed method, the wet bulb temperature and system cooling load are taken as the states, the frequencies of fans and pumps are the actions, and the reward is the system COP (i.e., the comprehensive COP of chillers, cooling water pumps, and cooling towers). The proposed method is based on Q-learning. Validated with the measured data from a real central chilled water system, a three-month measured data-based simulation is conducted under the supervision of four types of controllers: basic controller, local feedback controller, model-based controller, and the proposed model-free controller. Compared with the basic controller, the model-free controller can conserve 11% of the system energy in the first applied cooling season, which is greater than that of the local feedback controller (7%) but less than that of the model-based controller (14%). Moreover, the energy saving rate of the model-free controller could reach 12% in the second applied cooling season, after which the energy saving rate gets stabilized. Although the energy conservation performance of the model-free controller is inferior to that of the model-based controller, the model-free controller requires less a priori knowledge and sensors, which makes it promising for application in buildings for which the lack of accurate system performance models or sensors is an obstacle. Moreover, the results suggest that for a central chilled water system with a designed peak cooling load close to 2000 kW, three months of learning during the cooling season is sufficient to develop a good model-free controller with an acceptable performance.

Modeling and optimization for the smart operation of a central chilled water system in a high-rise building

Besides meeting the cooling demands of each terminal space in a high-rise building, the smart operation of a central chilled water system is also aiming at saving more electricity. Hence, it is the key issue to distribute proper amount of chilled water to each room according to its cooling load variations, i.e. achieve hydraulic balance in the chilled water network. To save electricity consumption of both chillers and circulation pumps in operation, in this paper, a supervisory optimization model is proposed for a general chilled water system. An improved node method is also presented to simulate the hydraulic behavior of the general chilled water pipe network, which is particularly adapted for a high-rise building with faster computation speed and fine accuracy. Then the proposed model is validated in a real high-rise hotel in Shanghai. In the case study, comparing with conventional operation method (On/Off or PID control), the proposed smart operation method can save 15.4-29.3% of electricity for the chilled water system in the typical days of cooling season.

데이터센터의 외기도입 냉방시스템 적용에 따른 경제성 평가

Many studies are being conducted with the aim of reducing the energy consumption in data centers, which are one of the highest consumers of energy. The use of an air-side economizer system that uses external air during intermediate and winter seasons is being considered for reducing the energy consumption of air conditioners. In this study, using the energy simulation, we evaluated the energy performance of a central chilled water cooling system and air-side economizer system in domestic data centers. Further, the cost-effectiveness of the air-side economizer was analyzed through Life-Cycle Cost Analysis. The results showed that with the use of air-side economizer systems, the energy costs increased as the applied filter grade increased; however, unlike existing central chilled water systems, it would break even within 2 years.

Performance analysis of an ejector cooling system with a conventional chilled water system

The central chilled water system servicing one of the academic buildings at the KFUPM campus located in Dhahran has been modeled numerically to combine with the ejector cooling system. This ejector system cools the water which is in turn utilized to cool the refrigerant in the chiller condenser and hence, the function of the cooling tower in a conventional chilled water system is eliminated by an ejector cooling system. By replacing the cooling tower of the chilled water system with an ejector cooling system, constant water inlet temperature for the chiller condenser is assured, regardless of the ambient relative humidity and this effect is analyzed in detail. Results indicate that this combined system is quite advantageous at conditions of high ambient relative humidity when the cooling capacity of the cooling tower is greatly diminished but the performance of the ejector cooling system remains unaffected.

Performance analysis of a chilled water system with various pumping schemes

The central chilled water system servicing one of the academic buildings in King Fahd University of Petroleum & Minerals (KFUPM) campus located in Dhahran is numerically modeled in detail with a special focus on the pumping scheme. The software Engineering Equation Solver (EES) is used for the modeling and simulation of the system. The bin data along with the weather profile is utilized to calculate the annual energy consumption of the system for three cities in Saudi Arabia. It is found for Dhahran that by replacing the existing single loop constant pumping scheme by the constant primary and variable secondary scheme, an 8% saving in the total annual energy consumption is achieved, which is increased to 13% by adopting the single loop variable primary scheme. Similarly, for the climatic conditions of Riyadh and Jeddah reductions in annual energy consumption by approximately 7% are obtained by adopting the constant primary and variable secondary scheme. However, if the single loop variable pumping scheme is adopted, the energy savings for Riyadh and Jeddah increases to 12% and 11%, respectively.

An entransy dissipation-based optimization principle for building central chilled water systems

The recently developed entransy theory is introduced in this paper to tackle the heat transfer processes in building central chilled water systems so as to improve their energy efficiency. We first divide the irreversible heat transfer processes into four categories: (1) air mixing processes; (2) heat transfer processes between chilled water and air; (3) chilled water mixing processes; and (4) heat transfer processes between chilled water and refrigerant. The formulas of entransy dissipation rates for each irreversible process are derived, and then the total entransy dissipation rate in the whole chilled water systems is obtained, which connects the geometrical structures of each heat exchanger and the operating parameters of each fluid directly to the demands of users and the supply of refrigerating unit. Based on the formula of entransy dissipation rate together with the conditional extremum method in mathematics, two optimization equation groups are deduced theoretically. Simultaneously solving such equation groups will easily find the optimal central chilled water system with the highest energy efficiency. Finally, a simple building central chilled water system with two users is taken as an example to illustrate the applications of the newly proposed optimization principle.

An optimal control strategy for complex building central chilled water systems for practical and real-time applications

This paper presents an optimal control strategy for online control of central chilled water systems in complex building air-conditioning systems to enhance their energy efficiency. The optimal control strategy is formulated using a systematic approach by considering the system level and subsystem level characteristics and interactions among the overall system. The requirements and constraints of practical applications are also carefully considered during the development of this strategy. This optimal control strategy consists of the model-based performance predictor (i.e., simplified models), cost estimator (i.e., cost function), optimization technique, supervisory strategy and a number of local control strategies. The local control strategies are used to ensure the robust operation and keep track of control settings considering the dynamic characteristics of the local process environment. The performance of this strategy is tested and evaluated in a simulated virtual environment representing the complex central chilling system in a super high-rise building by comparing it with that of other control strategies. The results showed that this strategy is more energy efficient and cost effective than the other strategies for online applications. This strategy is being implemented in the super high-rise building under study for field application and validation.

Variable Water Flow Pumping for Central Chilled Water Systems

A new variable water (VW) flow system for chilled water system application is presented. Design, operational, and control issues are discussed. Analytical models of pump power and evaporative temperature are developed. The VW systems consume considerably less pump and compressor energy than the primary secondary (PS) systems under partial load conditions for most existing chillers. The VW system consumes less compressor energy since it results in higher refrigerant temperature in the evaporator. The VW systems also have the decoupling capability of the primary and secondary pumping systems.