This paper introduces a new measure for individual thermal comfort, inspired by the current standards for population thermal comfort, and a statistical model allowing us to imitate individuals’ thermal comfort preferences. Our approach is based on the observation that an individual has a temperature range around his or her desired temperature point in which he or she is comfortable with the surrounding thermal environment. The crucial parameters of our statistical model, which represents the thermal characteristic of individuals of building occupants, have been assumed to be normally distributed random variables so that the thermal comfort preferences of different individuals can be generated for the further simulation purposes. When aggregated to a population’s general thermal comfort parameters, the variables of these distributions have been adjusted in such a way as to bring very close consistency with the current standards, which define the criteria for acceptable thermal conditions of human occupancy in a built environment.

The micro-environment in this study refers to the air space and environment around a person that directly impacts their thermal sensation. This study aims at evaluating the performance of a newly developed microenvironmental control system (mu X) designed to cool the occupants locally for thermal comfort when the temperature in the ambient unoccupied space is raised from 23.9 degrees C to 26.1 degrees C in summer to reduce the HVAC cooling load. The mu X was tested first with a 20-segment thermal manikin wearing summer clothing in a full-scale stainless-steel chamber and then with human participants in a climate chamber. Results show that the heat loss by the manikin increased with the distance between the mu X air supply diffuser and the manikin and decreased with the clothing insulation. Changing the air delivery angle from 0 degrees to 10 degrees from the horizontal direction resulted in additional heat loss from the manikin. The heat loss from the manikin was found to be positively correlated with the supply air flow rate, but negatively correlated with the supply air temperature. However, the overall cooling efficiency dramatically increased with the supply air temperature. Overall, both the manikin test and the human participant test showed that the mu X was able to cool the occupant in a room of expanded temperature set-point, and the Clothing Independent Thermal Comfort Model gave a consistent prediction with the human participant test. However, slight thermal discomfort was reported when the mu X was used due to the effect of clothing, season, metabolic rate and local draught.

Micro-environment control system has been proved to be able to provide local thermal comfort control. It also has the potential to save building energy by relaxing the control on the unoccupied space. This work aims at designing an efficient heating delivering device (HDD) to locally warm the person to restore thermal comfort when the unoccupied space temperature is reduced from 70.0 degrees F to 66.0 degrees F to save the energy. A three-stage approach was developed for designing the HDD including the initial stage of developing by CFD simulation, the second stage of manikin test verification and final stage of human subject test confirmation. The results of the first two stages were presented and discussed in this work. It was shown that convective heating was not efficient enough unless a confinement box was used to hold the hot air around the legs and feet. A more ergonomically friendly design of using a foot warmer was finally proved to be a better solution for individually heating by both simulation and experiments since it can heat the occupant sufficiently. The ability of using CFD to predict heat loss from the clothed human body and its limitation was also investigated. It was found that with proper clothing insulation, the CFD can give a reasonable prediction, although simulating the clothing as a layer of certain thermal resistance might be problematic regarding the structure and permeability of the clothing material. A more appropriate method for simulating the clothing was needed for better prediction in the future.

An experimentally validated finite element model (FEM) was developed to analyze the design parameters of a latent heat storage device (LHSD) for a micro environmental control system (mu X). The mu X provides local cooling to an office worker in a room whose thermostat setpoint has been elevated from 23.9 degrees C (75 degrees F) to 26.1 degrees C (79 degrees F) in order to reduce heating, ventilation, and air conditioning (HVAC) energy consumption. For this application, the LHSD is designed to provide >= 50 W of cooling for a full, 8.5 h workday to restore thermal comfort in the warm, 26.1 degrees C room. The LHSD comprises several parallel slabs of encased phase change material (PCM) with interposed airflow channels. The airflow rate is selected to obtain >= 50 W of cooling at the end of the 8.5 h operation. The LHSD exhibits a decreasing cooling rate over the 8.5 h period when a constant airflow is passed through it, indicating that more cooling is supplied during the day than the minimum 50 W required for thermal comfort. The parametric analysis explores the effects of PCM thermal conductivity, slab thickness, air channel width, and number of slabs on LHSD performance. Parametric cases are compared against each other on the basis of their required PCM mass and energy consumption.

Aisle containment systems reduce temperature nonuniformities in traditional air-cooled data centers and pave the way for higher cooling air temperature and more efficient operation of the cooling system and wider application of economizers. However, enclosing the aisle does not guarantee the lowest possible cooling infrastructure power. Enclosed air (EA) configurations require computer room air handler (CRAH) fans to provide the entire rack air flowthrough the perforated tiles, as well as the leakage flow that inherently exists in data centers. In this paper, we propose the installation of induction bypass (BP) fans beneath the cold aisle (CA) perforated tiles that draw air from the plenum, placing the latter under a slightly depressed pressure. In doing so, warm air from the hot aisle is drawn into the plenum, where it is mixed with a smaller amount of cold air from the CRAH. As a result, the CRAH fans operate at lower speeds and consume less power, but the air passing through the CRAH is cooled to a lower temperature so that the air supplied to the enclosed CA remains at or slightly below the server redline temperature (~27 °C). Detailed optimization based on hour-by-hour annual energy simulations for seven U.S. cities has shown that the proposed induced CRAH BP scheme can reduce combined chiller and air-moving power consumption substantially in EA data centers with or without an airside economizer.

Aisle containment separates hot and cold aisles in air-cooled data centers to prevent hot air recirculation into cold aisle and decrease temperature nonuniformity among the servers. Uniform server inlet temperatures allow the cooling system to operate more efficiently at a higher evaporator temperature. Lower cooling power consumption can be achieved by optimizing the combined power consumption of the chillers and the computer room air handlers (CRAHs) fans. CRAH bypass (BP) is proposed, in which additional fans at the tiles induce a fraction of tile flow from the room into the plenum through low-resistance ports or leakage paths, thus decreasing the airflow passing through the high pressure resistance of the CRAH heat exchangers and filters and associated fan power usage. A further advantage of the proposed induced CRAH BP is the elimination of leakage as a result of reversing the room-plenum pressure difference. However, in order to keep the enclosed-aisle temperature below acceptable limits (typically ≤27 °C), the chiller needs to operate at a lower, less efficient temperature. This paper presents the experimental verification of a flow network model (FNM) in a data center test cell demonstrating the proposed induced CRAH BP concept. Exercise of the verified FNM in conjunction with a thermodynamic model of the cooling infrastructure (TDM) confirms that there is an optimum BP fraction that minimizes the combined chiller and CRAH fan power consumption.

Micro-environment is hereby defined as the air space and environment around a person that directly impacts his/her thermal sensation. Most existing HVAC systems condition the air of the entire room including the unoccupied space, which leaves a big potential to save energy. This study aims at evaluating the performance of three existing air terminal devices (ATDs) to locally remove enough heat from the micro-environment to manage the thermal balance so as not to sacrifice thermal comfort when the ambient unoccupied space temperature is increased by 2.2 degrees C from 23.9 degrees C to 26.1 degrees C in the summer to reduce the external cooling load. A computational fluid dynamics (CFD) model was developed, validated by full-scale chamber tests and applied to evaluate different configurations of the ATDs for local cooling. Results show that the predicted performance agreed well with the measurements, and the selected ATD, with only 50 W cooling power, was always able to remove a sufficient amount of heat from the micro-environment in a room of raised temperature, when the manikin was moved inside a semicircle movement range. The cooling performance of the jet was increased more by increasing the supply air flow rate than reducing the supply temperature and was highly dependent on the shooting angle. The heat flux from the manikin surface is very sensitive to the surface temperature and furniture placement, and proper specification of the surface temperature is crucial for the CFD simulation to match the measured results. (C) 2017 Elsevier Ltd. All rights reserved.

Two time-dependent mathematical and numerical models with different levels of complexity and fidelity were developed to investigate the melting of a phase change material (PCM) configured as a number of aluminum-encased, PCM-filled slabs with embedded micro-channel aluminum tubes, and with parallel air-flow passages interposed between the slabs. Melting was first analyzed with the COMSOL Multiphysics® finite-element model (FEM) in a 2-D domain representing a full-size slab. The melting process is simulated via the apparent heat capacity method. The model captures the effect of natural convection in the PCM melt as well as the conjugate heat transfer through the aluminum tubes. A fast-executing quasi 2-D reduced-order model (ROM) was developed for repetitive design optimization studies. The ROM relies on a time-dependent 1-D closed-form solution of the heat conduction equation in a melting PCM, coupled with variations of the air temperature and heat transfer coefficient. Consequently, the FEM results were employed to develop corrections to the ROM. The corrected ROM was then utilized to study the melting process in a multi-slab thermal storage device that is designed to freeze the PCM at night and release 500 W-h of cooling over a span of ∼10 h during the day.

Aisle containments in data centers help provide uniform server inlet air temperatures. This allows the cooling system to run at a higher evaporator temperature and more efficiently. On the other hand, CRAH units run at higher speeds to ascertain that racks receive sufficient air flow. Since CRAH fan power already constitutes an important component of data center power use, such increases in the fan power can overshadow the energy savings due to more efficient chiller operation. CRAH bypass configuration is proposed to achieve optimum operating condition for enclosed aisle data centers. This configuration utilizes fan-assisted perforated floor tiles to induce a fraction of tile flow from the room through bypass ports or leakage paths and help decreasing the amount of air flow passing through the large flow resistances of CRAH units. Experimental results show that there is an optimum operating condition for the specific data center test cell that is designed to represent an enclosed aisle data center utilizing the proposed CRAH bypass configuration. Here, the flow characteristics of major system components and experimental measurements have been used to calibrate a flow network model (FNM) for the design optimization and trade-off analysis of the proposed system. Calibrated FNM along with a thermodynamic model (TM) of the cooling infrastructure provides an estimate of the energy use at various fractions of CRAH bypass air and chilled water temperatures. This study introduces the design of the experimental setup for testing CRAH bypass configuration for enclosed aisles and for calibrating models to predict the cooling infrastructure energy saving potential of the proposed technique.

Temperature non-uniformities in traditional data centers can be eliminated or at least reduced by utilizing containment systems. As all servers receive the same inlet air temperature in a contained configuration, the cooling system can be operated more efficiently at a higher temperature, which also increases the potential for free cooling through various economizer modes. However, enclosed aisle configurations require computer room air handler (CRAH) fans to operate at a higher speed and provide entire rack air flow through the perforated tiles, unlike open aisle data centers that can make up a fraction of server air from the data center air space. Hence, the traditional enclosed aisle configuration is likely to consume more fan power. This study confirms that enclosing the aisle does not guarantee optimum cooling infrastructure power in air cooled data centers. Proposed CRAH bypass configuration for enclosed aisle data centers provides a fraction of the tile airflow rate through a set of bypass fans while CRAH fans operate at lower speeds. These low-lift fans operate across a pressure difference between the room and plenum, which is significantly less than the flow resistance of CRAH units. Meanwhile, CRAH fans operate at lower speeds and consume less energy. Accordingly, a certain bypass air fraction with respect to total rack air flow rate leads to a minimum cooling infrastructure power for a particular configuration. This study investigates energy savings potential of the enclosed aisle data centers with CRAH bypass configuration utilizing a calibrated flow network model for estimating the energy consumption of air movers as well as a thermodynamic modeling tool to evaluate the off-design performance of major components of data center cooling infrastructure. Hour-by-hour annual energy simulations complement the energy assessment for 7 U.S. cities considering indirect air side economizer operation.

Lumped capacitance models have been introduced to study transient thermal response of data centers. Chilled water interruption of a Computer Room Air Handling (CRAH) unit is one of several failure scenarios of data center cooling infrastructures. In such a scenario, predicting the transient thermal response of the CRAH unit depends requires the determination of the CRAH lumped capacitance model parameters: the thermal capacitance (thermal mass) and the time constant. In this paper, we propose an experimental methodology to extract sufficient information for the lumped capacitance modeling of CRAH units. The method requires measurements of inlet and exit air temperature, air flow rate and CRAH fan power. If the chilled water supply to a CRAH unit is intentionally interrupted in a data center with multiple redundant CRAH units, sufficient information to estimate the CRAH lumped capacitance parameters can be obtained without disturbing the data center operation.

Transient CFD analysis of data centers requires appropriate representations of the transient thermal characteristics of servers. Thermal conductance and thermal capacitance are two determining characteristics for the response of servers under unsteady conditions. Previous studies proposed tests that require detailed temperature and thermal capacitance measurements for each of the server component, requiring access to individual components inside the server. In this paper, we propose a method for obtaining the transient thermal characteristics of a server from server inlet and outlet temperatures under transient operating conditions.

Current trends in data center cooling infrastructure design drive toward raising the coolant temperature to as high a level as equipment reliability allows in order to reduce the energy consumed by the cooling infrastructure and, consequently, improve (i.e., decrease) the power utilization effectiveness (PUE). However, as currently applied, PUE would appear to have deteriorated if measures are taken to reduce the energy consumption of the servers themselves. Because the temperature-dependent leakage power dissipation of modern deep-submicron computer chips constitutes a large fraction of the total power dissipation of these chips, the power consumption of such chips can be reduced by operating them at lower temperature. Not only does low-temperature operation reduce leakage power dissipation, but also it increases reliability and opens the potential for increasing the chip clock speed. However, these benefits may not lead to a reduction in the overall power consumption of the system of the servers and their associated cooling infrastructure if the leakage power reductions are more than offset by an increase in the power consumption of the low -temperature cooling infrastructure. In this paper we present a preliminary analytical study of a power and refrigeration cascade system (PARCS) that has the potential to realize the benefits of chip low-temperature operation while decreasing the overall power consumption of the servers and their cooling infrastructure. We describe a parametric conceptual model of the combined servers and PARCS as a function of the coolant temperature and show that the optimum coolant temperature of servers equipped with 45, 32 and 22 nm chips could be substantially lower than the 27°C recommended by ASHRAE.

An experimentally validated computational fluid dynamics (CFD) model was enhanced to predict the concentrations of ozone, and the products of its reactions with skin oils in the breathing zone of a person seated in an office cubicle with and without personal ventilation. A series of increasingly realistic hypothetical reactions were studied. The results show that the concentration distribution is not well mixed with noteworthy variations in the species distributions at different locations in the room, the personal micro-environment and the breathing zone. The assumption of well mixed conditions can lead to significant over or under prediction of inhalation exposure. The results also show that PV can remove oxidation products from the breathing zone effectively; however, PV can potentially increase the ozone concentration levels in the breathing zone if installed without an ozone filter.

Transient thermal events in air-cooled data centers may lead to undesirable operating conditions such as the formation of hot spots and associated degradation of equipment reliability. These transients may be caused by cooling equipment failures, server load changes, or other time-dependent scenarios in data center operations. This paper introduces a fast-executing hybrid computational fluid dynamics (CFD)/Lumped-Capacitance model for predicting server inlet temperatures resulting from common transient events such as server shutdown, partial or total chilled water interruption, or partial or total failure of the computer room air handlers (CRAH). The model uses initial steady-state CFD or experimental data in combination with several lumped-capacitance models of the various thermal masses in the data center, including the servers, the room enclosure, the CRAHs and the underfloor plenum. The inclusion of these thermal capacitances and their associated thermal conductance was found to be an important contributor to the overall transient response of the data center air-space. The model predictions have been compared with experimental data obtained in a three-rack data-center test cell and found to agree well with the experimental measurements. Examples of the application of the model to more realistic data center configurations are also given.