Supply Air Temperature Research Articles

Performance evaluation of a passive air conditioning module integrating radiative sky cooling and indirect evaporative cooling

Amidst escalating global warming challenges, the demand for sustainable, carbon-free cooling technologies in buildings has become pressing. While radiative cooling (RC) and indirect evaporative cooling (IEC) are carbon–neutral, their efficacy is constrained by a limited cooling capacity. This study introduces an innovative passive air conditioning module that integrates RC and IEC to boost efficiency. The RC-IEC module unifies the air-conditioning process in both the RC and dry channels of the IEC unit, ensuring air cooling with a constant humidity ratio. A portion of the IEC dry channel outlet air is redirected to the wet channel to augment the efficiency of the IEC process. The outlet air from the wet channel is directed to the secondary RC air channel for cold energy recovery instead of being directly exhausted. The developed mathematical model, validated with experimental data, demonstrates that the RC-IEC system lowers the supply air temperature by 0.88 °C compared to the IEC-only system. This corresponds to an 11.91 % and 9.94 % improvement in dew-point effectiveness and cooling energy gain, respectively. A parametric study indicates the system's suitability for hot, arid climates, with optimal performance at a wet channel air mass fraction of 0.31. The analysis of the annual cooling performance of the RC-IEC system across diverse climates, spanning from Athens to Singapore, spotlights Dubai's system as the most efficient, achieving a remarkable annual gain of 1081.09 MJ. The study emphasizes the need for tailored strategies, such as advanced air dehumidification technologies, to optimize cooling in humid regions like Singapore.

A multi-setpoint cooling control approach for air-cooled data centers using the deep Q-network algorithm

Cooling systems provide a safe thermal environment for the reliable operation of IT equipment in data centers (DCs) while generating significant energy consumption. Therefore, to achieve energy savings in cooling system control under dynamic thermal distribution in DCs, this paper proposes a multi-setpoint cooling control approach based on deep reinforcement learning (DRL). Firstly, a thermal model based on the XGBoost algorithm is constructed to precisely evaluate the thermal distribution in the rack room to guide real-time cooling control. Secondly, a multi-set point cooling control approach based on the deep Q-network algorithm (DQN-MSP) is designed to finely regulate the supply air temperature of each air conditioner by capturing the thermal fluctuations to ensure the dynamic balance of cooling supply and demand. Finally, we adopt the extended CloudSimPy simulation tool and the real workload trace of the PlanetLab system to evaluate the effectiveness and performance of the proposed approach. The simulation results show that the proposed control solution effectively reduces the cooling energy consumption by over 2.4% by raising the average air supply temperature of the air conditioner while satisfying the thermal constraints.

Optimization of Air Supply Conditions for Large Space Radiant Air Conditioning Based on Response Surface Analysis

In large spaces, the comfort control of radiant air conditioning systems still has deficiencies. Mainly reliant on traditional parameter control methods, the interaction effects of diverse operating conditions are not taken into account, and the control results are still in need of optimization. To improve indoor air conditioning comfort control and address gaps in existing research, the response surface analysis method combined with numerical simulation is applied to determine the optimal air supply conditions for large-space radiant air conditioning. With the real model as a reference, simulations are carried out in Airpak, selecting four factors: supply air temperature, supply air humidity, air supply volume, and air supply angle for the response surface design. Discussions on interaction effects are carried out based on the simulated results of various scheme designs, determining eight groups of air supply schemes with the highest comfort. The research concludes that the response surface analysis method, considering the interactive effects of impacting factors, can lead to the strategy of high comfort and reasonable air supply conditions. Additionally, a set of recommendations are proposed for future practical application and promotion.

Climatic applicability of indirect evaporative cooling strategies for data centers in China

Modern digital developments have propelled data center (DC) to prominence, with the cooling requirement being an important driver of its energy consumption. This study explores the climate application potential of indirect evaporative cooling (IEC) technology, which utilizes water evaporation for cooling. The IEC technology is considered as a promising avenue for energy-efficient DC cooling system. This study reveals essential insights for optimizing DC cooling performance by analyzing different IEC operation modes, including dry mode, wet mode, and cascading hybrid mode. The selection of operation mode is closely related to climatic conditions, DC working conditions, and IEC cooling efficiency. Higher DC supply air temperatures extend dry mode cooling hours and decrease cascading hybrid mode hours. It enhances energy savings through better utilization of natural free cooling resources. Elevating IEC cooling efficiency expands dry mode cooling hours while reducing reliance on mechanical cooling, contributing to improved power usage effectiveness (PUE) and energy conservation. The analysis is based on a typical data center and considers representative cities in different climate zones of China, emphasizing the importance of reducing the PUE and energy consumption of the DC by adjusting the DC supply air temperature and improving the IEC cooling efficiency. This work provides a useful reference for customizing DC cooling strategies according to various climate differences, optimizing energy efficiency and promoting sustainable development.

Experimental investigation on thermal performance of self-service cold storage cabinet based on the orthogonal test

Self-service cold storage cabinets (SCSCs) are widely used in the delivery end of cold chain logistics for fresh agricultural products, to maintain the freshness of the products in a low temperature environment. Based on the three-factor and four-level orthogonal test, this paper first analyzed the impact of pure geometric structure factors on the thermal performance of SCSC. The priority of the geometric structure factors of the SCSC on the thermal performance is determined by means of four evaluation metrics. Then, two operating factors including air supply velocity and temperature are added to the consideration, and a five-factor and four-level orthogonal test is conducted based on the aforementioned five factors. The priority and optimal level of various factors were determined based on the test results. By comparing the results of two sets of orthogonal tests, it is found that the optimal level of the structure is affected by the change of air supply conditions. After optimizing the SCSC, the temperature range, temperature inhomogeneity coefficient, and velocity inhomogeneity coefficient decreased by 1.316 ℃, 21.16%, and 2.18% respectively, while the average temperature increased by 2.028 ℃, and the power consumption decreased by 4.81% compared to the original level of factors (test plan 1).

Analysis of ineffective heating in intermittently heated office buildings in hot summer and cold winter areas

ABSTRACT To reduce room heating energy consumption and improve energy utilization efficiency, this paper proposed a method for evaluating the energy efficiency of office heating based on ‘ineffective heat supply’ and ‘ineffective heat supply ratio’ and established an energy consumption analysis model. Using the office in hot summer and cold winter areas as an example, the heating energy consumption and energy utilization efficiency were simulated. The results indicate that better thermal comfort is achieved in the working area of Air Conditioning Lower Installation (ACLI) compared to Air Conditioning Upper Installation (ACUI). The ineffective heat supply ratio of the room increases with the rise of air supply temperature and decreases with the increase of air supply speed. As the supply air temperature increases from 25°C to 50°C, the ineffective heat supply ratios for ACUI and ACLI increase by 35.9% and 14.9%, respectively. The use of low-temperature heat medium is more energy-efficient than high-temperature heat medium. When the air supply speed increases from 0.8 to 3.2 m/s, its ineffective heat supply ratio decreases by 44.6% and 7.8%, respectively. Air speed change significantly impacts ACUI's ineffective heat supply ratio, with ACLI more energy efficient at lower speeds.

Overheating and Air Velocities in Modern Office Buildings During Heating Season

Proper design and operation of buildings are expected to result in optimal thermal comfort and energy performance at the same time. If occupants are not satisfied with thermal conditions, corrective actions by building managers and maintenance staff may lead to elevated room temperatures with evident energy penalties. Because of complicated technical systems and control logic, it is worth studying how well the design intent has been realised in new office buildings. In this study, thermal comfort was analysed by measurements of draught, room, and supply air temperature as well as with occupant questionnaire surveys in five modern office buildings. Both short‐ and long‐term measurements were conducted to demonstrate problems in the operation and to find potential solutions for improvement. The results revealed an issue of excessive overheating during the heating season despite generally low air velocities. Radiant ceiling panels had the lowest velocities in both summer and winter, while buildings with active chilled beams showed the potential to meet Category II air velocity and temperature requirements. The building with thermally activated building systems experienced the most overheating during the heating season. Occupants were satisfied with the heating season temperatures of 23°C–25°C that can be attributed to lighter clothing (0.7 clo) instead of the standard 1.0 clo. Ventilation supply air and indoor temperature analyses indicate that elevated setpoints have been used to compensate for draught, resulting in overheating. As a measure of improvement to avoid overheating, we propose control curves for room temperature based on the outdoor running mean temperature and for supply air temperature based on the extract air temperature.

Direct numerical simulation of contaminant removal in presence of underfloor air distribution system

Indoor contaminant removal over 0.5 ≤ FrT ≤ 5.0, 0.5 ≤ N ≤ 5.0, and 50 ≤ Re ≤ 500 was investigated numerically, wherein FrT refers to the Froude number, N refers to the buoyancy ratio, and Re refers to the Reynolds number. As demonstrated, the ventilation effectiveness increased with increasing contaminant source intensity and air supply intensity at a constant air temperature, indicating that increase the fresh air can effectively eliminate contaminants in this case. At high air supply temperatures, the heat retention time and contaminant transport was extremely short, and the fresh air induced by strong natural convection floating lift was rapidly discharged. Additioanlly, the air supply intensity had significant effects on contaminant removal. Quantification of the ventilation effectiveness under the combined effects of air supply intensity, air supply temperature and contaminant source intensity was determined based on the results of direct numerical simulations.

Performance analysis of an air compression refrigeration system for data center cooling

An air compression refrigeration performance calculation model applied to data center cooling was established, which is a reverse Brayton cycle system, the operation performance was studied by numerical simulation and the influence of supply air temperature, ambient temperature, pressure ratio, data center room temperature was analysed. The results showed that when the pressure ratio of the air compressor is 1.3, the system obtains the maximum cooling COP; heat exchanger efficiency have a significant effect on the cooling capacity and cooling COP, the efficiency of the heat exchanger increases from 0.6 to 0.8, and the increase in cooling capacity is 4.84 kJ/kg, cooling COP increased by 23.2%; Compared to conventional vapour compression/loop heat pipe natural cooling systems, in cold regions, the annual energy efficiency of air compression refrigeration can reach to 13.85.

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.

Experimental analysis of air-multiple pcm heat exchanger in evaporative cooling systems for supply air temperature stabilization

The free cooling system is a highly efficient solution that effectively regulates indoor temperature when the ambient air temperature fluctuates within the thermal comfort range. Compared to traditional cooling systems, the evaporative cooling system excels in cost-effectiveness and has exceptional efficiency in hot and dry climates. However, the system's performance is significantly influenced by the environment, leading to fluctuations in the outlet temperatures of the evaporative cooler. A novel approach to integrating thermal energy storage with an evaporative cooling system has been studied to achieve free cooling and stabilize the system's supply air temperature to address this challenge. The evaporative cooler's design, construction, and experimental testing have been carried out in highly challenging hot and dry environments. The utilization of phase change materials (RT21HC and RT25HC) as a thermal energy storage system has been implemented. Incorporating phase change materials has notably improved the stability of the outlet air temperature from the evaporative cooling system. Furthermore, the outlet air temperature from the system exhibited consistent fluctuations within the range of 21–25 °C, requiring a significant amount of energy absorption and release to exceed these temperature thresholds. The findings demonstrate that the proposed system effectively reduces and shifts peak load to off-peak hours. Moreover, thermal energy storage can deliver free cooling during the spring and autumn. However, integrating the PAHX into an evaporative cooling system becomes crucial in regions characterized by extremely hot summer seasons. The maximum deviation between numerical and experimental results is 4 %.

The extreme high cooling capacity thermoelectric cooler optimal design for kilowatts scale thermoelectric air-conditioner of high-speed railway carriage

In recent 20 years, the rapidly development of thermoelectric materials promotes thermoelectric s application in large scale cooling area, i.e. air-conditioner. To provide quiet working environment for the driver of high-speed railway, a kilowatt scale air-conditioner was designed. Firstly, a novel thermoelectric cooler with maximum cooling capacity of 450 W was designed with 199 thermoelectric elements, and the cooling performance were investigated under different cold and hot side heat exchange capacity. Second, the hot and cold side heat exchangers were optimized adopting genetic algorithm. Finally, a high coefficient of performance (COP) thermoelectric air conditioning (TEAC) system was designed for high-speed railway locomotive. The TEAC system comprising 16 TECs achieved a cooling capacity of 2342 W with a COP of 0.5 in cooling mode, and meets one 1HP TEAC requirements. The temperature field inside the locomotive was uniform and exhibited a linear correlation with the supplied air temperature. This research demonstrates the utility of high-power TEAC in high-speed railway carriages, providing valuable insights for the application of TEAC technology.

Experimental evaluation of direct-to-chip cold plate liquid cooling for high-heat-density data centers

Owing to the dramatic increase in IT power density and energy consumption, the data center (DC) sector has started adopting thermally- and energy-efficient liquid cooling methods. This study examines a single-phase direct-to-chip liquid cooling approach for three high-heat-density racks, utilizing two liquid-to-air (L2A) cooled coolant distribution units (CDUs) and a combined total heat load of 128 kW. An experimental setup was developed to test different types of CDUs, cooling loops, and thermal testing vehicles (TTVs) for different operating conditions. IR images and the collected data were used to investigate the effect of air recirculation between cold and hot aisle containments on the CDU’s performance and stability of supply air temperature (SAT). Three different types of cooling loops (X, Y, and Z) were characterized thermally and hydraulically. Results show that Type Y has the lowest cold plate thermal resistance and pressure drop, among others. In a later test that included a single rack at a heat load of 53 kW and a single CDU, the heat capture ratio for fluid was found to be 94%. Experiments show that using blanking panels on the back of the racks limits hot air recirculation and maintains a steady SAT in the cold aisle. Finally, the CDU performance was evaluated at a high heat load for the three racks at 128 kW, and the average cooling capacity of the units is 58.6 kW, and the effectiveness values for CDU 1 and CDU 2 are 0.83 and 0.82, respectively.

Optimal supply air temperature with respect to data center operational stability and energy efficiency in a row-based cooling system under fault conditions

This is the first study of data center row-based cooling strategies under fault conditions. An optimal supply air temperature (SAT) for row-based cooling is proposed to ensure energy efficiency and operational reliability. The appropriate SAT for emergency operation is determined by evaluating how long the allowable IT equipment's inlet air temperature (IAT) can be maintained by only running the fan of the computer room air handling (CRAH) unit when the chilled water supply is interrupted. Three cases with SAT values of 20 °C, 22 °C, and 24 °C are evaluated in a numerical analysis, and the maintenance potential of the IAT is assessed by analyzing 600 s of transients from the time of cooling failure. In all cases, the IAT is almost the same as the SAT under normal cooling, but in cooling failure, the time to reach downtime beyond the allowable IAT is 246s in Case 1, 133s in Case 2, and 85s in Case 3. Response strategies to cooling failure include increasing uninterruptible power supply (UPS) capacity and applying a thermal buffer tank. The study concludes that it is reasonable to set the SAT of row-based cooling to 22 °C or lower and to install a thermal buffer tank in the chilled water system for effective downtime delay. These results are based on a rack power density of 7 kW/rack.

Addressing personalized thermal comfort in residential settings: A novel dual-supply vent air conditioner

A new approach was proposed for meeting diverse thermal comfort demands of different occupants by using a dual-supply vent air conditioner with independent control of the air supply parameters at each vent to establish distinct thermal environments. The feasibility of this method was tested in a living room environment, and numerical simulation was conducted to explore the differences in thermal environment at various locations under different air supply temperatures, velocities, directions, and vent positions. The results indicated that, within the targeted air supply zone, the maximum temperature difference could reach around 2.5 °C. Considering the Cooling Effect of air velocity, the maximum Equivalent Temperature difference was around 4.5 °C, which could address the differences in common personalized thermal comfort preference. Utilizing the differential air supply velocities of different vents was an effective means of achieving diverse thermal environments, although attention should be paid to local thermal discomfort caused by drafts. This approach may offer a new perspective in addressing the differing thermal comfort preferences among occupants within the same residential room, additionally providing innovative insights for improving traditional air conditioning systems.

Experimental and simulation study on an air conditioning system cascade driven by waste heat using multicomponent solution

The cascade utilization of waste heat for air conditioning system is still considerably lacking in the literature, especially for waste heat below 80.0 °C. In this paper, the experimental and simulation studies are carried out on an air conditioning system that integrates the absorption refrigeration subsystem (ARS) with the liquid desiccant dehumidification subsystem (LDDS). The waste heat at 80 °C is first used to drive the ARS to produce chilled water for air cooling, and then drive the LDDS for air dehumidification to realize heat cascade utilization. Firstly, the experimental study is conducted to compare CaBr2–LiCl–H2O with LiBr–H2O in the ARS and CaBr2–CaCl2–H2O with LiCl–H2O in the LDDS. The results show that the multicomponent solutions perform comparably to the single-component solutions. Then, a mathematical model is established to simulate the entire system using the appropriate multicomponent solution for each subsystem. It is found that the COP of the system reaches 0.77 at a driving temperature of 80.0 °C, with the supply air temperature of 23.0 °C and the humidity ratio of 10.5 g/kg. The waste heat can be recovered down to 61.0 °C, achieving a heat recovery depth of 19.0 °C. This system is expected to have significant potential in the applications due to its efficient heat utilization capability and low-cost multicomponent solutions.

Performance of an innovative personalized ventilation mode based on air attachment under non-isothermal air supply conditions

In view of the shortcomings of traditional personalized ventilation method, an innovative mode of personalized ventilation was explored in this study. To investigate the performance of the personalized ventilation mode, velocity field measurements were performed to examine the impact of the supply air temperature on the airflow distributions, and a tracer gas system was used to test the fresh-air ratio in the breathing zone. The results show that the critical supply air velocity increases as a function of the supply air temperature and that the position height of the air attachment formation is a decreasing function of the supply air temperature. The fresh-air ratio in the breathing zone was significantly correlated with the formation and height of the attachment. Furthermore, the higher the position of the air attachment formation, the greater the breathable fresh-air ratio. The breathable fresh-air ratios were 83.3 %, 82.3 % and 84.1 % at supply air velocities of 0.3 m/s, 0.4 m/s and 0.5 m/s, respectively, and a supply air temperature of 17 °C. Compared with the traditional air supply method, the ventilation effectiveness of the the air dress personalized ventilation(ADPV) method in this study is highest. Irritation in the eyes was weaker than that in other areas. Irritation in the lips and nose was an increasing function of the supply air velocity, and no obvious connection with the supply air temperature. Furthermore, the supply air velocity and temperature significantly influenced the sensation in the upper body; however, no obvious influence was observed on the sensation in the lower body. It was verified that the ADPV method was able to satisfy the requirements of human comfort and the breathable air quality simultaneously.

Effect of drying air supply temperature and internal heat exchanger on performance of a novel closed-loop transcritical CO2 air source heat pump drying system

A novel closed-loop transcritical CO2 air-source heat pump drying (HPD) system with a relatively simple structure, higher drying temperature, and internal heat exchanger (IHX) for the drying of medical equipment was proposed and built. With or without the IHX in the system, the effects of different drying air supply temperatures on the performance of transcritical CO2 HPD system were studied experimentally. The results showed that the maximum drying air supply temperature (DAST) in the designed system could reach 72.9 °C. As the DAST raised, whether with or without the IHX in the system, the coefficient of heating performance (COPh), coefficient of system performance (COPsys), and specific moisture extraction rate (SMER) decreased gradually and moisture extraction rate (MER) increased continuously. When the DAST was 70 °C, COPh, COPsys, MER and SMER were respectively 3.68, 6.07, 3.27 kg/h and 1.68 kg/(kW·h) in the HPD system without the IHX, and were respectively 4.00, 6.89, 3.67 kg/h and 1.97 kg/(kW·h) in the HPD system with the IHX. In contrast, the compressor discharge pressure decreased 4.26 %–5.99 %, COPh increased 1.39 %–8.65 %, COPsys increased 1.89 %–13.57 %, MER increased 4.68 %–14.01 %, SMER increased 6.93 %–17.18 % in the HPD system with the IHX.

Model-based data center cooling controls comparative co-design

This article presents a comparative simulation-based control logic design process. It uses the Control Description Language (CDL) and the ASHRAE Guideline 36 high-performing building control sequences with the Modelica Buildings Library (MBL) to demonstrate a comparative analysis of two control designs for a data center chilled water plant. Details include a description of the closed-loop plant and control design methodology, including sizing and parameterization, base and alternative (Guideline 36) control logic with software implementation structure, and outline of the simulation experimentation process. The selected control designs are paired with comparable chilled water plant configurations. The models include a chiller, a water-side economizer, and an evaporative cooling tower. The plant provides cooling at 27ºC zone supply air temperature to a data center in Sacramento, CA. The comparative simulation results examined the impacts of a selected control logic detail, and present an example model-based design application. Overall, the simulation results showed a 25% annual and a 18% summer energy use reduction for alternative controls. This shows that simulation-based control logic design performance evaluation can improve energy efficiency and resilience aspects of system controls at large.

Thermal performance research on a novel coupled heating system combined solar air heater with ventilation PCM wall

Solar heating technology is increasingly recognized as a prominent option for clean building heating. However, challenges such as intermittent thermal energy supply and supply–demand mismatch hinder its widespread adoption. To address this issue, this paper introduces a novel coupled heating system that combines a solar air heater with a ventilation Phase Change Material (PCM) wall. A rural residential house in Tianjin was chosen as an experimental platform and comparative experiments was conducted to compare the advantages of the ventilation PCM wall with the conventional wall. Additionally, numerical simulations were utilized to address the non-uniform heat transfer in the direction of the Heat Transfer Fluid (HTF) flow by employing cascaded PCM. The experimental results demonstrated that the coupled heating system exhibited high thermal storage and release efficiency, significantly improving indoor thermal comfort. Under optimal operating conditions (air supply temperature: 35-45℃, air supply speed: 3–4 m/s), the system achieved rapid thermal storage within 4 h, reaching a maximum thermal storage efficiency of up to 87.6 %. The test room maintained a nighttime temperature of 15.7 °C, exhibiting temperatures 6.3–10 °C higher than the control room. Numerical results revealed that the cascaded PCM enhanced the thermal storage rate, particularly at the first 2.8 h of the thermal storage period, contributing to the overall efficiency and flexibility of the system. This research provides theoretical guidance for the optimal design and practical application of the ventilation PCM wall.