Chilled Water Temperature Research Articles

Enhanced characteristic equation method for single-stage absorption heat pumps

The method of characteristic equations aims to describe the part-load behavior of sorption heat pumps and chillers as well as heat transformers as simply as possible but yet still accurately. Based on an approach by T. Furukawa for heat transformers, the method was further developed by F. Ziegler and others for absorption heat pumps and generalized for application in multistage processes. Clever simplifications were made, to represent the cooling capacity Q˙E of absorption chillers with a characteristic temperature difference ΔΔt as a simple linear equation Q˙E=sE·(ΔΔt−ΔΔtmin,E). In ΔΔt, the average hot, cooling, and chilled water temperatures are combined, and the slope and loss parameters, sE and ΔΔtmin,E are constant. However, in practical applications of this established method, inconsistencies arise. For example, the calculated slope parameter sE does not match the slope when plotting simulated or measured cooling capacities against ΔΔt. Furthermore, the loss parameter ΔΔtmin,E is actually not constant.In this work a more precise formulation of the characteristic equation is derived which takes into account that the solution entering the absorber and desorber is generally superheated or subcooled. By means of a domain-wise heat transfer calculation, these effects can be implemented into the method and explain the above mentioned inconsistencies. The new formulation allows for explicit consideration of different heat exchanger designs and cooling water configurations. No iterations or regression analyses are required. Thus, the calculation method can be easily implemented into industrial controllers e.g. for the model predictive control of absorption chillers and heat pumps.

An energy-environment coupled simulation framework for multi-scale and multi-facet evaluation of data center

This paper focuses on analyzing the thermal environment and optimizing energy consumption in data centers, which has largely omitted from previous studies. The thermal environmental of data centers is simulated and analyzed by utilizing the established “server-rack-room” multi-scale heat transfer numerical model. Based on this, the coupling simulation model of thermal environment and energy consumption in data centers is proposed to explore the relationship between them, and the corresponding optimization strategy is put forward. The energy consumption simulated by energy-environment coupled model and non-coupled model can reach a discrepancy of over 30 %, which indicates that the thermal environment impacts the power consumption of the data center significantly. Besides, the effect of several operational parameters of air conditioning system on the thermal environment and energy consumption of data center is analyzed. Through the particle swarm optimization algorithm, the optimal system parameters, which meet the requirements of thermal environment and lead to the lowest energy consumption, are found for typical cities in different climatic regions. The recommended settings include the supply air temperature of about 24 ∼ 26 ℃, the air supply volume of 8 m3/s, and the temperature difference of about 3.5 ℃ between the supply air temperature and the supply chilled water temperature. Under the optimized parameter setting, the highest temperature of server decreases to below 71 ℃, and the energy saving rate is more than 6 %.

Optimizing active chilled beam systems in the South Korean Climate: Comparative analysis of cooling energy and thermal comfort by set-point temperature

In this study, the feasibility of implementing the active chilled beam system (ACBS) in South Korea has been analyzed, and an appropriate control strategy has been proposed. While ACBS is increasingly adopted in North America and Europe, its spread in South Korea has been slow due to different climatic characteristics and a lack of diverse understanding and analysis. Furthermore, research discussing the effects of key system parameters on ACBS is notably scarce. Therefore, this study aims to assess the impact of air handling unit discharge air temperature (AHU DAT) and ACBS entering chilled water temperature (ACBS ECWT) on ACBS and propose a control strategy suitable for the South Korean climate. The applicability of ACBS, compared with existing HVAC systems using this control strategy, has been evaluated. The study incorporates two methodologies: firstly, the impact of control variables has been assessed, and a suitable control strategy has been proposed through a sensitivity analysis of these variables. Secondly, the feasibility of applying ACBS, in comparison with conventional HVAC systems, has been evaluated from the perspectives of energy and thermal comfort. It has been found that system parameters, tailored for the South Korean climate, can maintain a level of thermal comfort like existing HVAC systems while achieving up to 18% in energy savings. Overall, this research not only demonstrates the potential applicability of ACBS in hot and humid climates like South Korea but also emphasizes the need for controlling key system parameters

Energy Consumption Optimization for the Cold Source System of a Hospital in Shanghai-Part I: Analysis of Operating Characteristics and the Control Strategies of the Chillers

Background: Hospitals account for the most proportion of energy consumption in the public building sector. Chillers usually account for most of the overall energy consumption of the cold source system. Objective: To solve the problem of chillers' large energy consumption problem, novel technologies were developed, and achievements were patented. Methods: The operating characteristics influencing factors of the magnetic suspension centrifugal chiller (MSCC) and variable frequency screw chiller (VFSC) of a hospital in Shanghai were analyzed and discussed by actual measurements. Then, based on the operating characteristics of the chiller obtained from the analysis of the measured data, the cooling capacity was classified by the K-Means clustering method to obtain the startup strategy of the chillers. Results: The effects of the supply chilled water temperature, the supply cooling water temperature and variable cooling water flow rate on the maximum cooling capacity and coefficient of performance (COP) of both chillers were explored. The load distribution scheme was discussed based on the chillers' startup strategy. Conclusion: The average part load ratio operation scheme was the preferred chiller distribution scheme. A chiller's maximum allowable part load ratio should not exceed 80% during the low-load operation period and should not be less than 60% during the conventional operation period. It provided a reference for optimizing the chiller operation strategy to reduce system energy consumption.

Research on Energy-Saving Control Strategies for Single-Effect Absorption Refrigeration Systems

The automatic control device is a critical component of absorption refrigeration systems. Its functional enhancement can reduce operating costs, improve energy efficiency, and ensure long-term stable unit operation. Given that absorption refrigeration systems operate under various dynamic conditions, the rational design of control strategies is particularly important. This study analyzes the influence of changes in the cooling water and heat source water flow rates on the outlet temperature of chilled water in the unit based on the open-loop response characteristics of absorption refrigeration systems. It proposes a dual-loop energy-saving control strategy for single-effect hot water lithium bromide absorption refrigeration systems based on the setpoint comprehensive optimization algorithm. Considering the multiple variables, strong coupling, large inertia, long time delay, and nonlinear characteristics of absorption refrigeration systems, as well as the difficulties in modeling these systems, this study applies a model-free adaptive control algorithm to the system’s control. It derives both SISO and MIMO model-free control algorithms with time-delay components. Through simulations comparing MFAC, improved MFAC, and traditional PID control, the dual-loop energy-saving control strategy is demonstrated to effectively reduce system heat consumption by approximately 20%, decrease power consumption by about 10%, and enhance the system’s SCOP by approximately 19.3%.

Thermal performance study of PCM embedded radiant cooling ceiling assisted by infrared-transparent silicon wafers

Phase change materials embedded radiant chilled ceiling (PCM-RCC) systems are great for cutting peak power demand and boosting energy efficiency, but they might not work as well in really humid areas due to condensation issues. Infrared-transparent silicon wafer-assisted PCM-RCC (ISW-assisted PCM-RCC) is proposed to improve the cooling capacity in this study. A heat transfer model of ISW-assisted PCM-RCC is established and its reliability is verified. Effects of thermal conductivity, phase change temperature, and control strategy on the thermal performance of ISW-assisted PCM-RCC are discussed by using the model. The simulation results show that the thermal conductivity of PCM is recommended to be above 0.6 W/(m·K), and the temperature difference between the lower limit of the selected PCM phase transition temperature range and the chilled water temperature should not be less than 1℃. The maximum cooling power of the system is more than 80 W/m2, 46 % higher than the non-condensing cooling capacity of traditional RCC at 70 % relative humidity conditions. Interestingly, the prolonged cold energy storage strategy at night should be avoided which is not friendly to the energy efficiency of the system. In short, the combination with ISW is an effective means to increase the cooling capacity of the PCM-RCC.

Numerical investigation of surface temperature uniformity and cooling performance for ceiling radiant cooling panel with an air layer

The ceiling radiant cooling panel (CRCP) with air layer based on serpentine shaped flow channel was proposed to solve condensation risk, however, the performance of CRCP was weakened owing to the serpentine shaped flow channel with high-pressure drop. Hence, the parallel shaped type CRCP with air layer was proposed in this work. The effects of both structural and operational parameters on surface temperature distribution and the cooling performance were evaluated. Results showed the cooling capacity was increased with the increase of chilled water velocity and tube diameter, while decreased with the increase in the air layer thickness, cooling panel thickness, tube spacing and chilled water temperature. Considering that condensation risk, it is recommended to set tube spacing, tube diameter, air layer thickness and cooling panel thickness above 125 mm, 10 mm, 12 mm and 1.5 mm, respectively. In the structure parameters of the CRCP, the cooling panel thickness exhibits the maximum impact on cooling capacity with relational grade of 0.87, and the air layer thickness has the maximum impact on surface temperature uniformity with relational grade of 0.82. The findings could serve as guideline for better practical applications of parallel shaped type CRCP with an air layer in different scenarios.

Heat wave resilience in open Spaces: A case study of a Self-Sufficient cooling shelter

Climate change intensifies urban uninhabitability due to increasingly frequent heat waves. To alleviate it, climatic shelters emerge as an initiative for the research community, which should deepen the integration of technologies that use natural resources for cooling. This work aims to evaluate a novel climatic shelter by characterizing its thermal behavior through an experimental study at a bus stop. It has the capacity to alleviate the thermal stress of its occupants thanks to cooling modules. The results allow to determine the capacity of the natural water-cooling system and the level of thermal storage, allowing to quantify the impact on the citizen by means of the COMFA comfort index. It is corroborated that the chilled water temperature reduces the surface temperature by 15 °C. The installation reduces overheating caused by solar radiation by 75 % compared to traditional shelters. It also demonstrates energy and electrical self-sufficiency even during high intensity heat waves. The shelter maintains daytime water temperature between 18 and 25 °C, even in heat waves exceeding 7 days and 45 °C air temperature. The COMFA index has been reduced to values between 70 and 120 W/m2 in 90 % of the worst weather conditions in Csa climates by Köppen Geiger, for the months of July and August 2023.

Performance Analysis of Lake Water Cooling Coupled with a Waste Heat Recovery System in the Data Center

Data centers (DCs) require continuous cooling throughout the year and produce a large amount of low-grade waste heat. Free cooling and waste heat recovery techniques are promising approaches to reduce DC energy consumption. Although previous studies have explored diverse waste heat utilization strategies, there is a significant gap in combining waste heat recovery with lake water cooling in DCs. Therefore, this study proposed a system integrating lake water cooling with waste heat recovery for DCs. To evaluate the energy-saving performance of the suggested system, the influence of waste heat recovery locations and volumes has been investigated. An analysis of the improvement in system parameters is also conducted. The study’s findings highlight that targeted recovery of waste heat from sources like chilled water or air in server rooms can significantly reduce the cooling energy demand of the system. The results show that recovering heat from the return air of IT equipment can yield a remarkable power usage effectiveness (PUE) and coefficient of performance (COP) of 1.19 and 10.17, and the energy consumption of the cooling system is reduced to 10.06%. Moreover, the outcomes reveal the potential for substantial energy savings of up to 26.05% within the proposed system by setting the chilled water and air supply temperatures to 16 and 20 °C, respectively.

A practical real-time control strategy for high-performance building HVAC operation concerning potential pandemic outbreaks in post-pandemic era

The lessons learned from the COVID-19 pandemic emphasize the need for building HVAC systems to be adaptable and prepared to adjust their operation for infection risk mitigation for future pandemics. Most guidelines suggest increasing outdoor air fraction in HVAC systems during pandemics. However, this approach is not energy-efficient due to the conditioning of additional outdoor air, and many practical systems face limitations in introducing more outdoor air than normal due to capacity or design constraints. Therefore, there is an urgent need for an alternative solution for HVAC system operation during pandemics that is energy-efficient, can effectively mitigate infection risks and does not require extensive retrofits to existing systems. To meet this need, this study proposes a practical real-time control strategy for high-performance HVAC operation in response to potential pandemic outbreaks. First, a supply air temperature (SAT) reset scheme for infection risk mitigation is proposed based on Mamdani fuzzy inference theory. Then, a coordinated control scheme is proposed to coordinate SAT reset with chilled water temperature adjustment for energy-efficiency enhancement. This strategy does not require system equipment retrofits but only changes in control logic to transition to pandemic operation. The effectiveness of the proposed strategy is validated in a high-fidelity Modelica-based simulation environment under various weather conditions. Results show that, compared to normal operation, the proposed strategy can significantly reduce new infection cases by 32∼46 % without increasing energy use, and may even achieve slight energy savings of 9∼13 %, while maintaining expected occupant comfort.

Experimental study on exhaust internal circulation desiccant wheel evaporative cooling chiller

Evaporative cooling chillers reduce the water temperature by absorbing heat during the evaporation of water without the need for compressor assistance or refrigerants. It is an energy-saving and environmentally friendly cooling technology but it needs necessary improvements in the applicable environmental area and refrigeration performance due to ambient air wet-bulb temperature limitations. To obtain chilled water below the dew-point temperature of the ambient air, an exhaust internal circulation desiccant wheel evaporative cooling chiller is proposed and experimentally investigated. A combination of a heat pipe exchanger, a desiccant wheel, an air cooler and an air heater are applied to reduce the wet-bulb temperature of the ambient air before it passes through the direct evaporative cooler. At the same time, the low-temperature and low-humidity exhaust air from the direct evaporative cooler is used to circulate to improve chiller performance. The experimental results show that the average chilled water temperature of the proposed chiller is 10.2 ℃ lower than that of the direct evaporative cooling chiller under five typical environmental conditions. The wet-bulb efficiency of the proposed chiller is the highest at high temperatures and high humidities (36 ℃ and 80% RH), which is 6 times greater than that of direct evaporative cooling chillers. Therefore, the proposed chiller expands the application range of evaporative cooling technology, which is especially suitable for humid areas with low-grade heat utilization such as solar energy.

Direct/indirect optimal use of shallow geothermal energy in floor radiant cooling system coupled with air-handling system

Shallow geothermal energy is used as the cold source of floor radiant cooling systems (RFCSs). A physical simulation test bench was built to study the cooling and dehumidification characteristics of an RFCS to evaluate its feasibility. Three operation modes involving direct-supply chilled water, direct-supply chilled-water coupling air-handling system (AHS), and indirect cascade utilization were evaluated. Results showed that the room could be cooled rapidly in the direct-supply chilled-water mode. When the direct-supply chilled-water temperature was lower than 16 °C, condensation occurred on the floor surface. The AHS effectively avoided condensation, and the maximum humidity ratio difference between the air inlet and outlet was 13.61 g/kg. However, the temperature difference between the chilled water and the floor surface was reduced. Under the cascade utilization mode, the cooling efficiency of the system improved, and the indoor temperature and relative humidity were maintained at 20.6–28.1 °C and 57.0–76.1 %, respectively. This study provides valuable insights into the direct application of geothermal energy in RFCSs.

Study of water-side free cooling technology for power lithium battery plant

Power lithium battery production process needs to consume a lot of energy. In this paper, an energy consumption analysis of a representative company was conducted, the energy consumption of the facilities that provide support for the process is higher than the energy consumption of the process equipment itself, which greatly affects the cost of battery manufacturing. Air-conditioning system as the most important energy-consuming system of the facilities, accounting for 25% to 45% of the total electricity consumption. The energy consumption of the cooling source system dominates, even in winter its energy consumption can reach 14% of the total energy consumption. On this basis, this paper analyzes the chilled-water-use characteristics of the air-conditioning terminal in winter through theoretical analysis, the chilled water temperature is therefore optimized according to the real-time outdoor humidity. The chilled water temperature is linearly increased after the humidity ratio fall below 7.5g/kg(a), reaching a maximum value of 12.1°C at 6g/kg(a). In addition, the cooling tower free cooling technology based on the optimized chilled water temperature is innovatively proposed to fully explore the energy-saving potential of the technology to achieve the energy saving of the cooling source system. TRNSYS simulation results show that after optimizing the chilled water temperature, the time period for cooling tower individual cooling under winter operating condition is extended by 60%, and cooling source system energy saving rate can reach 49.0% and the cost saving rate 48.0%, both of which are higher than 15% compared to the un-optimized chilled water temperature.

Effects of Root Zone Temperature of Hydroponic Lettuce Affects Plant Growth, Nutrient Uptake, and Vitamin A Content

Water temperature can affect plant growth and quality in hydroponic production. Lettuce ‘Antonet’, ‘Waldman’s Dark Green’, ‘Parris Island’, ‘Jericho’, and ‘Rex’ were grown using the nutrient film technique with chilled (water temperature set at 21.1 °C) or ambient water. Data were collected on plant growth, foliar nutrient content, and vitamin A content. ‘Jericho’ had the greatest shoot fresh weight but was only significantly different from ‘Antonet’, which had the lowest shoot fresh weight but the greatest vitamin A content. SPAD was greatest in ‘Paris Island’ and was significantly greater in chilled water over ambient for ‘Antonet’. Plants grown in ambient water had greater number of leaves and root dry weight, whereas SPAD was greatest with chilled water. Greater nutrient values were observed in ‘Rex’, ‘Jericho’, and ‘Waldman’s Dark Green’ in chilled water, whereas no nutrient differences were observed in ‘Antonet’ and ‘Parris Island’.

Development of Virtual Sensor Based on LSTM-Autoencoder to Detect Faults in Supply Chilled Water Temperature Sensor

Supply chilled water temperature (SCWT) is an important variable for the efficient and stable operation of heating, ventilation, and air conditioning (HVAC) systems. A precisely measured value ensured by the continuous reliability of the temperature sensor is essential for optimal control of an HVAC system because temperature sensor faults can affect the chiller operation and waste energy. Therefore, temperature sensor fault-detection strategies are imperative for maintaining a comfortable indoor thermal environment and ensuring the efficient and stable operation of HVAC systems. This study proposes a fault-detection method for an SCWT sensor using a virtual sensor based on a long short-term memory-autoencoder. The fault-detection performance is evaluated considering a case study under various sensor fault scenarios to evaluate changes in indoor thermal comfort and energy consumption after correcting sensor faults detected by the virtual sensor. The results verify excellent fault-detection performance in various fault scenarios (F-1 scores ranging from 0.9350 to 1.000). After correcting the SCWT fault, indoor thermal comfort is steadily maintained without additional energy consumption (indoor set-point temperature unmet hour reduced by a maximum of 105.7 hours, and energy consumption decreased by up to 1.8%).

Comparing temperature fluctuation attenuation capabilities and feasible parameter regions of indirect- and direct-contact heat exchangers for an ultra-high precision constant-temperature chilled water system: An experimental and Modelica-based study

Constant temperature and humidity rooms have widespread applications in high-end manufacturing and scientific research. Conventional wisdom primarily focuses on controlling the air temperature at the terminals, i.e., the supplied outlets. For achieving ultra-high precision (e.g., ±0.1 °C), it becomes essential to pre-control the temperature of the chilled water of the air-conditioning system. Nevertheless, there is still limited understanding of this matter. This paper investigated two common temperature fluctuation attenuators for the chilled water system: indirect- and direct-contact heat exchangers, e.g., a plate heat exchanger (PHE) and a water mixing pump (WMP), respectively, as follows. First, Modelica models of the two systems were established and validated by an experimental set-up capable of switching between the PHE and WMP loops. Second, an orthogonal analysis was conducted to evaluate the attenuation effects of four key factors. Results indicated that both attenuators can provide the required constant temperature precision with nuanced differences. The MWP system demonstrated faster settling time, primarily affected by valve adjustment accuracy. In contrast, PHE exhibited superior fluctuation control, with valve adjustment accuracy and input chilled water temperature fluctuation amplitude as critical factors. Finally, feasible parameter regions for the two types of attenuators under ultra-high precision targets were delineated and analyzed.

THE SIGNIFICANCE OF PROPER COOLING TOWER WATER TREATMENT: A CASE STUDY

This paper presents the result of a study of the effect of cooling tower system in terms of performance and cost of maintenance. The study was conducted in a two-storey shopping complex building. Observation on site was held to obtain data on full load amp (FLA) and chilled water temperature (CWT) before and after servicing cooling tower unit. Energy of cooling tower were evaluated by analysis of FLA and CWT in percentage value. The condition on the system and component were also observed to differentiate the condition before and after treatment. The result obtained showed that cooling system performance after the treatment was increased.

Parametric analysis of chiller plant energy consumption in a tropical climate

The approach that could reduce the energy consumption of water-cooled chillers in office buildings was investigated through experimental and simulation methods. The chiller was modelled in TRNSYS and validated using physical measurements from an operational water-cooled water chiller and fan coil system. The validated model was used to analyse the energy consumption of the water chiller-fan coil system under a high chilled water temperature setpoint (CWTS) in a representative commercial office building. Subsequently, indoor thermal comfort was evaluated using the PMV-PPD model. Finally, the design of experiment (DOE) was employed using a statistical two-level non-randomized factorial design in Minitab to study the effects of high CWTS, number of rows, and tube diameter on the heat and mass transfer performance of the fan coil unit. The results showed that the CWTS can be increased by various degrees from 10 up to 18 °C for energy efficiency for a commercial office building in the tropics. This increase in CWTS would result in a daily energy saving potential of about 5% of the chiller as compared to the existing operational settings without any extra cost. Conversely, the daily energy consumption by the fan coil would increase by about 5.5% by this increment in the CWTS. It was determined that the chiller system can provide comfort even when the CWTS is increased to 14 °C. The DOE analysis showed that under the condition of high CWTS (14 °C), energy consumption that is less than the current energy consumption may be expected from the fan coil system when the number of rows increased from 3 to 6 and the tube diameter increased from 7 to 9 mm.

Model Predictive Control for Energy Optimization of HVAC Systems Using EnergyPlus and ACO Algorithm

The deployment of model-predictive control (MPC) for a building’s energy system is a challenging task due to high computational and modeling costs. In this study, an MPC controller based on EnergyPlus and MATLAB is developed, and its performance is evaluated through a case study in terms of energy savings, optimality of solutions, and computational time. The MPC determines the optimal setpoint trajectories of supply air temperature and chilled water temperature in a simulated office building. A comparison between MPC and rule-based control (RBC) strategies for three test days showed that the MPC achieved 49.7% daily peak load reduction and 17.6% building energy savings, which were doubled compared to RBC. The MPC optimization problem was solved multiple times using the Ant Colony Optimization (ACO) algorithm with different starting points. Results showed that ACO consistently delivered high-quality optimized control sequences, yielding less than a 1% difference in energy savings between the worst and best solutions across all three test days. Moreover, the computational time for solving the MPC problem and obtaining nearly optimal control sequences for a three-hour prediction horizon was observed to be around 22 min. Notably, reasonably good solutions were attained within 15 min by the ACO algorithm.

Analysis of the Influence of Chilled Water Temperature Setpoint on Thermal Comfort and Energy Consumption

This study experimentally analysed the influence of chilled water temperature setpoints on thermal comfort conditions and energy consumption of an office building. Three chilled water temperature setpoints (10 °C, 12.5 °C, and 14 °C) were studied. The indoor environment variables (temperature and relative humidity) which are considered indicators of thermal comfort were recorded with data loggers for three consecutive days for each chilled water temperature setpoint (CWTS). Similarly, energy consumption was used as a metric to determine the system’s efficiency. The predicted percentage of dissatisfied (PPD) and predicted mean vote (PMV) indicators were computed and analysed using the Thermal Comfort/ASHRAE 55-2020 modelling tool from the centre for the built environment (CBE). Additionally, computational fluid dynamics (CFD) modelling and analysis were performed using ANSYS Fluent to study the indoor environment conditions of the office at the different chilled water temperature setpoints. A comparison between the calculated, measured, and predicted satisfaction of occupants was done. The results obtained when varying the chilled water temperature setpoints reveal that increasing the chilled water temperature setpoint (by 12%) reduces energy consumption per hour by 2% without compromising thermal comfort. The study demonstrated that the CWTS could be reset between 14 and 15 °C to reduce energy consumption and maintain thermal comfort. Moreover, the CFD model can be used to compare the indoor environmental characteristics of chiller systems.