Supply Chilled Water Temperature Research Articles

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.

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%).

Balanced comparative assessment of thermal performance and energy efficiency for three cooling solutions in data centers

As the demand for data centers rises, efficient cooling systems become increasingly vital to ensure uninterrupted operations and adaptability to IT service changes. The research focuses on quantitatively comparing the thermal performance and energy efficiency of cooling systems capable of handling high-density IT power. It aims to provide objective criteria for selecting suitable cooling solutions, including an assessment of three 150 kW-class IT and facility modules, a novel contribution not extensively explored in previous studies. The study evaluates room-based, row-based, and rack-based cooling options based on the configured ITE power density, ensuring that all three solutions meet ASHRAE's recommended IT operating environment through CFD analysis. The research reveals energy efficiency improvements, with PUEcooling decreasing from 1.33 for room-based cooling to 1.28 for rack-based cooling. These improvements are attributed to reduced fan power by CRAC/H type and decreased primary plant energy consumption by increasing COP through the supply chilled water temperature. The findings offer valuable insights for designing cooling solutions tailored to ITE power density and considering factors such as space requirements, thermal performance, energy efficiency, and cost for new hyperscale data centers. Overall, this research constitutes a significant academic contribution in the field of data center cooling solutions.

Study on Effects of Operating Parameters on a Water-Cooled Loop Thermosyphon System under Partial Server Utilization

During the operation of a data center, servers are gradually installed in racks, causing most racks to work under a low heating load for a long time and affecting the cooling efficiency of the loop thermosyphon system (LTS). Thus, the effects of operating parameters on the thermal performance should be investigated. In this study, a water-cooled LTS was experimentally investigated under different airflow rates and heating loads. The results show that the additional liquid refrigerant reduced the heat transfer performance and aggravated a drop in cooling capacity when the airflow rate and heating load were decreased. To further reveal the effects of the operating parameters on the thermal performance and cooling efficiency, a steady-state distributed-parameter model was developed and validated based on the experimental data. The results show that the excessive cooling capacity was reduced by decreasing the airflow rate according to the upper limit of the server exhaust air temperature under partial server utilization. The excessive cooling capacity was reduced by 14.5–52.1% under 5–56.5% server utilization. To further reduce the excessive cooling capacity while ensuring thermal security, the water side operating parameters (including the supply chilled water temperature and water flow rate) were adjusted according to the upper limit of the rack’s average outlet air temperature, which reduced the excessive cooling capacity by more than 23.8% under partial server utilization.

Comparative study on different energy-saving plans using water-side economizer to retrofit the computer room air conditioning system

An energy efficient, environmentally friendly and economical retrofit plan of traditional computer room air conditioning system is necessary to reduce energy consumption of data centers. In this study, two energy-saving retrofit plans, a computer room air handler system with water-side economizer (Plan 1) and a loop thermosyphon system with water-side economizer (Plan 2), are compared with an air-cooled computer room air conditioning system (existing cooling system) in a data center. To analyze and compare the energy efficiency, environmental and economic potential, energy models of the existing cooling system and retrofit plans are established. Under different set points (including supply chilled water temperature and supply air temperature) and 2.08–6 kW average electric power of racks, the free cooling periods of Plan 1 and Plan 2 reach 3878–5867 h and 4541–7661 h, respectively, which lead to the annual energy consumption of Plan 1 and Plan 2 decreases by 27.6–47.3% and 53.4–63.6%, respectively. Due to the lower annual energy consumption of main equipment (including chillers, chilled water pumps, cooling water pumps and terminal equipment), Power Usage Effectiveness of Plan 2 is 0.049–0.090 lower than that of Plan 1, and Global Coefficient of Performance of Plan 2 is 23.3–67.3% higher than that of Plan 1. Furthermore, the annual CO2 emissions of Plan 1 and Plan 2 are 337.3–557.1 tons and 599.8–1106.9 tons less than those of the existing cooling system at different operating stages respectively. Finally, the dynamic payback periods of Plan 1 and Plan 2 are 4.11–5.42 years and 1.64–3.57 years.

Experimental and numerical studies of heat and mass transfer performance and design optimization of Fan-coil with high supply chilled water temperature in Air-Conditioning system

Raising chilled water temperature contributes to improving the energy efficiency of the water-chiller in air-conditioning (AC) system. However, the heat and mass transfer performance of fan-coil would be effected by high supply chilled water temperature (SCWT). This paper experimentally investigated the changing evolution of both cooling and dehumidity capacity of fan-coil in AC system, indicating the capacity of 3 rows fan-coil declines when SCWT rose from 9 ℃ to 12 ℃, which cannot satisfy the requirements of indoor comfort. Besides, experiment results show that increasing rows number of fan-coil is an effective way to solve the capacity decline issue. In order to further analyze the effects of fan-coil specification on its performance, the heat and mass transfer numerical model of fan-coil was proposed. The simulated results agreed with the experimental data well. The numerical analysis showing that when SCWT was 9, 10, 11, 12 ℃, the average increase of heat exchange was 561.50, 88.50, 332.73 W for rows number increasing from 4 to 6, diameter enlarging from 7 mm to 9 mm, tube spacing enlarging from 21 mm to 28.66 mm. The efficiency index was improved with smaller diameter and larger tube spacing. In order to ensure the optimal performance of fan-coil, the largest efficiency index was achieved. In this case, the corresponding SCWT was 10 ℃, which improved the COP of water-chiller by 10.8%. This work provides the design of fan-coil with high SCWT in AC system.

An Experimentally Validated Model of a Cross-Flow Sub-Wet Bulb Evaporative Chiller

Abstract A simplified model to predict the performance of a novel sub-wet bulb evaporative chiller (SWEC) for producing chilled water is presented. The SWEC design uses a daisy-chained arrangement of cross-flow heat exchangers (HXs) with evaporative media located in between the heat exchangers to chill water below the outdoor wet bulb temperature (WB) in either a one-pass or two-pass arrangement. Sub-models, based on the effectiveness-number of transfer units (ɛ-NTU) method, for the heat exchanger and evaporative media are coupled together to form the SWEC model. The model is validated using field data from a SWEC that is designed with a single water pass. The model results are observed to match the SWEC experimental data within a mean average error (MAE) of 0.74% in supply chilled water temperature and 5.6% in chiller capacity over a range of outdoor air dry bulb and wet bulb temperatures, inlet water temperatures, and water flowrates. The validated model is then used in a parametric study of geometrical and thermofluidic variables. The model is also used to predict the performance of the SWEC in a typical hot and dry weather over a summer week.

Load allocation improvement for chiller system in an institutional building using logistic regression

Chiller systems are an essential part of central air conditioning systems to provide cooling energy in buildings. This study discusses how to improve the load allocation of chillers to reduce the system electricity consumption. An analysis of comprehensive operating data from an existing system with 5 chillers of 2 different capacities identified 101 types of component combination. The variation of system coefficient of performance depended significantly on the chiller part load ratio, followed by the temperature of cooling water leaving the condensers and the type of component combination. Chiller sequencing took place for only 30.1% of the total operating time. There was 61.73% of the total operating time in underload conditions with an average part load ratio of 0.65. Implementing chiller sequencing at all times increased the average part load ratio to 0.78 with a 15.81% reduction in the electricity consumption. Based on the results of multi-nominal logistic regression, the underload conditions could be predicted by an increase in the temperature of supply chilled water and the temperature of cooling water entering the condensers. On the other hand, an increase in the temperature of returned chilled water and the temperature of cooling water leaving the condensers reflected a higher chance of overload conditions. Controlling system components with the trend of significant temperature variables was proposed to enhance chiller sequencing.

Experimentally-determined characteristics of radiant systems for office buildings

Radiant heating and cooling systems have significant energy-saving potential and are gaining popularity in commercial buildings. The main aim of the experimental study reported here was to characterize the behavior of radiant cooling systems in a typical office environment, including the effect of ceiling fans on stratification, the variation in comfort conditions from perimeter to core, control on operative temperature vs. air temperature and the effect of carpet on cooling capacity. The goal was to limit both the first cost and the perceived risk associated with such systems. Two types of radiant systems, the radiant ceiling panel (RCP) system and the radiant slab (RS) system, were investigated. The experiments were carried out in one of the test cells that constitute the FLEXLAB test facility at the Lawrence Berkeley National Laboratory in March and April 2016. In total, ten test cases (five for RCP and five for RS) were performed, covering a range of operational conditions. In cooling mode, the air temperature stratification is relatively small in the RCP, with a maximum value of 1.6 K. The observed stratification effect was significantly greater in the RS, twice as much as that in the RCP. The maximum increase in dry bulb temperature in the perimeter zone due to solar radiation was 1.2 K for RCP and 0.9 K for RS – too small to have a significant impact on thermal comfort. The use of ceiling fans was able to reduce any excess stratification and provide better indoor comfort, if required. The use of thin carpet requires a 1 K lower supply chilled water temperature to compensate for the added thermal resistance, somewhat reducing the opportunities for water-side free cooling and increasing the risk of condensation. In both systems, the difference between the room operative temperature and the room air temperature is small when the cooling loads are met by the radiant systems. This makes it possible to use the air temperature to control the radiant systems in lieu of the operative temperature, reducing both first cost and maintenance costs.

Energy Consumption Comparison of Large-scale Air-conditioning System With Different Temperature of Chilled-Water

In order to reduce the energy consumption of the Large-scale Air-conditioning system, this paper studies the influence on system energy consumption of different supply chilled water temperature, and the optimum temperature of supply chilled water was obtained. By through established the relation equation for the total energy consumption of chilled water supply temperature, the variation rules of the total energy consumption of the Large-scale air-conditioning with the chilled water supply temperature change were analysed. As a result, the optimum temperature of supply chilled water was 8.3153 ℃ when the total energy consumption is the minimum of the system under a certain loading.

Analysis of centrifugal chillers with oil-free magnetic bearings for enhancing building energy performance

This article examines the actual operating performance of centrifugal chillers with oil-free magnetic bearings (hereafter oil-free chillers) to analyze energy saving potentials for commercial buildings. A chiller system retrofit for a shopping arcade involved replacing three existing constant speed water-cooled centrifugal chillers by three oil-free centrifugal chillers of the same capacity. The coefficient of performance was calculated based on operating data logged at 1-h intervals for over 1 year before and after replacement. Oil-free chillers with variable speed control operated at a higher coefficient of performance under part load conditions, reducing the total electricity consumption of the shopping arcade by 9.6%. If the average technical efficiency in data envelopment analysis was improved to 1 from 0.6 in the existing case, the average system coefficient of performance could increase from 6.2 to 10.3. The improvement involved controlling the temperature of supply chilled water at a lower boundary curve and operating an equal percentage of capacity among chillers. It is economically viable to design an all-variable speed chiller system with oil-free chillers instead of the conventional constant speed system for a reference office building in a subtropical climate.

Adding a Bypass to Ensure the Chemical Plant Production

<p>The chilling output of the chiller fluctuated according to the chilling load. When it dropped down below a certain value, the temperature of the supply chilled water flowing out of the chiller would drop down too much. And this would result the chiller being shut down abnormally, because of its low temperature protection. Any this shutdown of the chiller endangered the production continuity and security, and caused a certain economic losses. A bypass, which was from the outlet of the chiller’s condenser to the chilled water filter inlet, was added to the chilling system. Through this bypass, some cooling water was introduced from the cooling water circulation into the chilled water circulation. Then the temperature of the supply chilled water flowing out of the chiller was promoted. With experiments, and by the aid of thermodynamics analysis, the amount of introduced cooling water, which could be introduced to promote the supply chilled water temperature for maintaining the chiller running normally, was researched. Ultimately, a certain amount of introduced cooling water, at which the chiller would no longer stop abnormally at any chilling load, was determined. And the energy lose caused by the introduced cooling water was less than 5 percent the rated output of the chiller. Compared with the chiller rated output, this energy lose was so small that it could be neglected.</p>

Performance Evaluation of Oil-free Chillers for Building Energy Performance Improvement

This paper investigates how to improve the energy performance of central air-conditioning systems by using oil-free chillers. Three conventional water-cooled centrifugal chillers of equal capacity were replaced by three oil-free chillers of the same capacity in an existing system. Operating data for performance evaluation were logged at 1-hr intervals for over one year before and after the replacement. The superior coefficient of performance (COP) of oil-free chillers with variable speed control brought an energy saving of 9.6% in the total electricity consumption of a shopping arcade when they operated for a wide range of system cooling demands. Results of data envelopment analysis gave an average technical efficiency of 0.6 which ascertains an opportunity for performance enhancement. The system COP can be further improved by tightening the control in the temperature of supply chilled water and implementing even load sharing among chillers to meet the overall system capacity.

Comparative study on the energy performance of chiller system in an institutional building with stochastic frontier analysis

This study demonstrates how to compare the energy performance of an existing air-cooled chiller with and without mist precooling of condenser air, considering different operating conditions between the two situations. A comprehensive set of operating data was gathered for each situation for over 4 months. Using stochastic frontier analysis, technical efficiency (TE) for the coefficient of performance (COP)—output—was evaluated for each set of operating conditions—inputs—involving weather and controlled operating variables. TE had an average of 0.64 and a standard deviation of 0.14 under mist precooling, while the corresponding values were 0.56 and 0.18 in the absence of mist precooling. A positive correlation was identified between the COP and TE. Based on the frequency distribution of TE, mist precooling enabled the chiller to attain higher performance more frequently under standardized operating conditions. Pearson correlations among the variables ascertain that TE has the highest correlation with condensing temperature, followed by the temperature of supply chilled water, so their precise control would give performance enhancement. TE can provide a basis of benchmarking chiller performance at different operating conditions and help explain the extent of improvement brought from the weather responsive control.

Theoretical and Experimental Study of Air Conditioning of the Building by the Floor Cooling System

The study is focused on the air conditioning of the building by the floor cooling system. The floor cooling system works on the same principle as the heating floor principle. But the cooling floor absorbs calories instead of emitting them. However for the air conditioning of the building, the water running from the distribution network must be at a temperature lower than of the air room temperature. This study is going to allow us to know how the thermal energy will behave on the floor and on the room environment once submitted to the thermal excitements. The used cover parameters and its nature, the thickness of the floor, the number of mesh and the debit of water circulating in pipes are taken into main consideration. In order to reduce the number of equations and to facilitate the system of equations to be solved easily, we have used the thermal electric analogy for the modeling of the constituent elements of the building and the cooling floor system. In order to verify and validate the theoretical results, experimental work in the laboratory of air conditioning technology at the technical University of Applied Sciences THM (Giessen). The supply chilled water temperature and the return chilled water temperature of the hydraulic module 2, the air room temperature and the floor surface temperature is achieved. Besides, the out flow of hydraulic module 2 as well as the evolution of the air speed in the experimental chamber is pointed out.

Thermal comfort and IAQ analysis of a decentralized DOAS system coupled with radiant cooling for the tropics

The deployment of low exergy concepts in buildings, which promotes high temperature cooling HVAC systems introduces alternative solutions in the tropical climate. This study evaluates the performance of a decentralized dedicated outdoor air system combined with a radiant cooling system (decentralized DOAS-RCS) in terms of occupant thermal comfort and indoor air quality for the tropical context. Different sets of operational scenarios (experiments) have been conducted in the BubbleZERO laboratory to realize the impact of system related parameters like ventilation rate and supply chilled water temperature on thermal comfort and indoor air quality. The results show that supply chilled water temperature and space cooling load have strong impacts respectively on the capacity of decentralized units and cooling panel, which consequently influence indoor air condition. Indoor air was predicted to be in comfort range (−0.2 < PMV < 0.2) only at specific periods of the day and an automatic control was required to modulate the system under various indoor and outdoor conditions. Main challenges of implementing DDOAS coupled with radiant cooling in the tropics include the condensation risk on the radiant panels, non-uniformity of panel surface temperature and low air movement inside the space.

Effects and Performance of a Demand Response Strategy for Active and Passive Building Cold Storage

Demand response (DR) is a set of demand-side activities to reduce or shift electricity use to improve the electric grid reliability. Existing demand response programmes and tariffs that utilities or independent system operators (ISO) often provide great incentives to consider the use of sophisticated building operation and control strategies that reduce electricity use during occasional or emergency events. This paper presents the investigations on the power demand alternation potential in commercial buildings with cold storage systems aiming to provide effective power information for grid operation and optimization. A control strategy is developed to estimate the demand-shifting and demand-shedding potentials of buildings with comprehensive utilization of passive and active storages on the premise of certain capacity of chillers shut down at the beginning of emergency event. Compared with conventional exclusive passive or active storage control strategies, the developed strategy can provide a rapid response to electrical grid and an accurate estimation of power demand reduction in advance. Besides, it can also relieve ability degradation of handling latent load through reducing temperature of supply chilled water when chillers overriding. In addition, to reduce negative effects on the electric grid, rebound avoidance is considered in implementation of this strategy that allows HVAC system to slowly ramp up to limit power usage rise after DR event. The required rebound duration is also calculated as part of effective information sent to a grid.

Energy and economic performance analysis of an open cycle solar desiccant dehumidification air-conditioning system for application in Hong Kong

In this article, a transient simulation model and the EnergyPlus were used to study the energy performance and economical feasibility for integrating a solar liquid desiccant dehumidification system with a conventional vapor compression air-conditioning system for the weather condition of Hong Kong. The vapor compression system capacity in the solar assisted air-conditioning system can be reduced to 19 kW from original 28 kW of a conventional air-conditioning system as a case study due to the solar desiccant cooling. The economical performance of the solar desiccant dehumidification system is compared with that of the conventional air-conditioning system. The results show that the energy saving potentials due to incorporation of the solar desiccant dehumidification system in a traditional air-conditioning system is significant for the hot wet weather in Hong Kong due to higher COP resulted from higher supply chilled water temperature from chiller plants. The annual operation energy savings for the hybrid system is 6760 kWh and the payback period of the hybrid system is around 7 years. The study shows that the solar assisted air-conditioning is a viable technology for utilizations in subtropical areas.

Energy evaluation of optimal control strategies for central VWV chiller systems

Under various conditions, the actual load of the heating, ventilation and air conditioning (HVAC) systems is less than it is originally designed in most operation periods. To save energy and to optimize the controls for chilling systems, the performance of variable water volume (VWV) systems and characteristics of control systems are analyzed, and three strategies are presented and tested based on simulation in this paper. Energy evaluation for the three strategies shows that they can save energy to some extent, and there is potential remained. To minimize the energy consumption of chilling system, the setpoints of controls of supply chilled water temperature and supply head of secondary pump should be optimized simultaneously.