Advances in the application of passive down-draft evaporative cooling technology in the cooling of buildings

A passive down-draft evaporative cooling (PDEC) tower is a component that is designed to capture the wind at the top of a tower and cool the outdoor air using water evaporation. While several different types of this particular system exist, PDEC tower with spray was studied in that it is flexible, efficient, and prevalent. Sustainable feasibility has been known as the main benefit of PDEC towers, leading to significant energy savings, improvement of indoor environmental quality, and reduction of environmental cost. In contrast, PDEC towers have not been considered in some circumstances as an alternative to conventional air conditioning systems due to strong climatic dependency, insufficient cooling capacity, and huge water consumption when they could be successfully integrated. In addition, suitable methods that can resolve problems associated with PDEC towers and improve the performance of this particular system have not been presented in the literature. This study was thus designed to present the solutions that overcome these problems with PDEC towers so that they can be widely used in many types of buildings and climatic regions. Computational process modeling was carried out to understand fundamentals of down-draft evaporative cooling processes. This study developed a computational model using a commercial CFD code FLUENT, and this model was validated against experimental data. The model then explained physical phenomena occurring within the effective area of PDEC towers, so that conditions of the air were accurately predicted in different weather conditions as well as PDEC tower conditions. In addition, parametric study with this computational model defined critical factors that significantly impact the cooling performance of PDEC towers, and the importance of various factors. As a result, a practical design guide to droplet size and tower height was presented, which is applicable to most circumstances where PDEC towers could be integrated. Regression analysis using general-purpose statistical software Minitab was then followed to formulate mathematical models that provide accurate conditions of PDEC air flows. Two dependent variables, temperature and velocity, were considered. An individual sample was created by the computational model developed in this study. Correlation analysis determined independent variables that have significant relations with each dependent variable. A preliminary sampling process collected reasonable numbers of samples dealing with wide ranges of weather conditions and PDEC tower conditions. Additional samples were added using forward sampling methods so that minimum number of samples, which explain certain relations between dependent and independent variables at the lowest cost, can be appropriately determined. As a result, linear relations between dependent and independent variables were found and mathematical forms of regression equations obtained was presented. Dynamic simulations, using a whole building energy simulation program EnergyPlus, employing new mathematical models developed in this study were performed to investigate actual impacts of PDEC towers in various situations. A short-term simulation analysis demonstrated problems with current PDEC towers operation, as well as impacts to indoor thermal environment. Various alternatives to typical PDEC towers operation were analyzed, so that water flow control in conjunction with primary cooling system was determined as a reliable solution that overcomes those problems defined in this study. In addition, energy performance and various impacts to indoor thermal environment were analyzed in a long-term simulation. Consequently, PDEC towers are considered as a feasible component in various types of buildings and climates. The findings using the methods in this study demonstrate that typical PDEC towers are inefficient in energy performance and indoor thermal environment. The cooling performance of PDEC towers should thus be properly controlled to be an energy-efficient system. In addition, PDEC towers can be considered as a secondary cooling system that meets a portion of cooling loads in a space in order to accomplish low-energy goals as well as a comfortable indoor thermal environment. Furthermore, the performance of PDEC towers is strongly dependent on each critical parameter described in this study. Efforts should thus be made to find the most efficient design conditions for main parameters corresponding to the local environment. Moreover, PDEC towers are viable in various climates rather than a hot-dry climate, achieving almost the same level of energy savings with lower water consumption

Quantification of Heat Flows Through Building Entrance Doors on a Winter Day

The current methods for the prediction of the heat flow through building entrance doors have been used for years while involved with a certain degree of uncertainty. This paper describes an experimental method that accurately quantified such heat flow. The experiment monitored local environmental parameters in a college building on a cold winter day. The results of the experiment demonstrated that the experimental method is viable to accurately estimate the heat flow throughout the entrance doors and the presence of a vestibule moderated heat losses from the conditioned area. The results will be used for validating the existing methods

Significance of Parameters Affecting the Performance of a Passive Down-Draft Evaporative Cooling (PDEC) Tower with a Spray System

PDEC towers with spray systems are known to achieve substantial energy savings. Various parameters such as the wet-bulb depression, the tower height, and the wind speed have been known to be key factors affecting the performance of the system. To date, the significance of these parameters and other important factors have not been adequately treated in the literature. There also has been a lack of models that can successfully investigate potential benefits of the system under various conditions where this particular system could be applicable. To address these critical issues, this study performed a parametric analysis by using a FLUENT model validated against experimental data. It demonstrated the significance of individual parameters including water droplet sizes. As a result, practical design guidelines for important system parameters were presented. A statistical analysis was then used to formulate analytic models that account for all of the relationships found in this study between the parameters and the supply air conditions of the system. Two regression equations were formulated for predicting supply air temperature and velocity

무선 이동 애드 혹 네트워크에서 안정적인 브로드캐스트/유니캐스트 서비스를 위한 라우팅 프로토콜 설계

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2016. 2. 박세웅.A mobile ad-hoc network (MANET) is a dynamic, self-organizing network that is composed of numerous mobile devices scattered in a particular area. Each device is equipped with a wireless transceiver for physical-layer communication. In MANET, data traffic should traverse intermediate nodes between the source and the destination due to the limited communication range of devices. In this dissertation, we develop routing schemes for reliable and scalable broadcast/unicast communication service in MANETs. First, we develop efficient broadcast protocol, named ST-BCAST, that exploits collision resilient tone-signals and employs receiver triggered forwarding decision / cancellation mechanism. It reliably disseminates a packet over MANET without any topological information. We verify the reliability and efficiency of ST-BCAST through logical analysis and NS-3 based simulations. Second, we investigate two well-known classes of routing mechanisms for unicast service in MANET: hop-by-hop routing and gradient routing. We evaluate their performance under realistic MANET environments with unreliable links and node mobility. Based on the understanding of their behaviors, we propose a practical gradient forwarding architecture (E-GRAD) that includes on-demand cost update and SNR-based cost calculation. We demonstrate that the performance of E-GRAD is closed to that of the ideal routing scheme with global information. Last, we consider wireless multi-hop access networks with a single gateway, e.g., sensor networks and smart-phone based disaster recovery networks, and design novel gradient routing schemes for uplink/downlink unicast services. In our proposed scheme, every node can efficiently calculate the routing cost to the gateway by relaying a tone signal across subcarriers, where a cost value (e.g., hop count to the gateway) is pre-assigned to each subcarrier. The cost calculation is initiated by the gateway and is computed as the signal propagates to the network boundary. For uplink gradient routing, the cost can be used directly, and for downlink gradient routing, the cost is used in conjunction with uplink transmission history. We verify through NS-3 simulations that our proposed single-gateway routing scheme provides reliable uplink and downlink traffic, and substantially reduces the routing overhead by successfully exploiting OFDM signals.1 Introduction 1 1.1 Background 1 1.2 Contributions and outline 3 2 ST-BCAST: An Efficient Broadcast Protocol using Subcarrier-level Tone-signals 6 2.1 Introduction 6 2.2 System model 10 2.3 ST-BCAST 12 2.3.1 Control signals 12 2.3.2 Forwarding state lists 13 2.3.3 Receiver-triggered forwarding decision and cancellation 15 2.3.4 Operation example 17 2.3.5 Mode switching mechanism for multiple source case 19 2.3.6 Physical layer aspects for RTF and FR transmission and detection 20 2.3.7 Duration of RTF and FR 21 2.4 Reliability of ST-BCAST 21 2.5 Feasibility of tone signal detection 25 2.6 Simulation results 28 2.6.1 Implementation 28 2.6.2 Broadcast schemes in comparison 29 2.6.3 Performance metrics 30 2.6.4 Simulation environments 31 2.6.5 Results - single source 33 2.6.6 Results - multiple sources 38 2.7 Related Work 40 2.8 Summary 43 3 E-GRAD: Revisiting Gradient Routing Protocol for Reliable Unicast in MANET 44 3.1 Introduction 44 3.2 Forwarding mechanisms for unicast transmission 47 3.2.1 Hop-by-hop routing 47 3.2.2 Gradient routing 49 3.3 Reliability analysis of hop-by-hop routing and gradient routing 51 3.3.1 Impact of errors in link quality estimation 52 3.3.2 Impact of node mobility 54 3.4 E-GRAD: A practical greedy routing architecture for MANET 57 3.4.1 On-demand flooding-based cost update protocol 57 3.4.2 SNR-based cost allocation 61 3.4.3 Miscellaneous for efficient forwarding cancellation 64 3.5 Simulation results 65 3.5.1 System model 65 3.5.2 Schemes in comparison 68 3.5.3 Performance metrics 70 3.5.4 Simulation environments 70 3.5.5 Results 71 3.6 Related work 84 3.7 Summary 85 4 Access-GRAD: A Gradient Routing Protocol for Uplink and Downlink Unicast in Wireless Multi-hop Access Networks 87 4.1 Introduction 87 4.2 System model 89 4.3 Access-GRAD 90 4.3.1 Tone-signal based cost update 90 4.3.2 History based gradient routing for downlink transmission 91 4.4 Performance evaluation 95 4.4.1 Uplink performance 98 4.4.2 Downlink performance 101 4.5 Summary 103 5 Conclusion 104 5.1 Research contributions 104 5.2 Future research directions 106 Bibliography 107 국문초록 117Docto

Competence of a Spray Passive Down-draft Evaporative Cooling (PDEC) System for Space Cooling

A spray PDEC system has been relatively less considered than other passive cooling strategies as one of the viable low-energy solutions in the cooling of buildings while having a great potential in energy savings. This study is intended to evaluate the capability of a spray PDEC system for space cooling. It comprises four simulation scenarios to see the system response and influence of indoor thermal environment when a spray PDEC system is adopted as a primary cooling system in two different climates. The simulation results show that a spray PDEC system causes a substantial variation in the indoor thermal environment and sensible cooling rates while substantiating significant energy savings

Analysis of the system response of a spray passive downdraft evaporative cooling system

A spray Passive Downdraft Evaporative Cooling (PDEC) system achieves great savings for space cooling and improves indoor environmental quality by supplying a large amount of fresh outdoor air. As previous studies heavily focused on the energy saving capability of a spray PDEC system due to lack of methods for a detailed analysis, the influence of cool humid supply air from a spray PDEC system in a space in buildings has not been comprehensively studied. This study is intended to evaluate the competence of a spray PDEC system as a primary cooling system in typical spaces in a primary school building by developing a method using building energy simulation tool. It runs one-day simulations in a hot dry climate and a warm moderate climate in order to distinctly examine the benefits and the areas that should be advanced. It analyses how a spray PDEC system responds to the space cooling loads and spaces conditioned by the system thermally behave. The results of the simulations affirm that a spray PDEC system is capable of conditioning building spaces in the two climates, indicating some areas to be improved

Performance Control of a Spray Passive Down-Draft Evaporative Cooling System

A spray passive down-draft evaporative cooling system has been regarded as a low-energy cooling system that leads significant energy savings in the cooling of buildings. While the energy saving capability of the system has been proven, the ability to control a comfortable indoor environment is still inadequate due to strong climatic dependency. This study seeks viable solutions to advance the control competence of the system by mitigating critical problems of the system to be a reliable cooling application in the cooling of buildings. It proposes potential control strategies for the system and alternative operations. It develops a control algorithm for the proposed control strategies and implements the algorithm in EnergyPlus. A simulation analysis follows to examine the functionality of each proposed control strategy and alternative operations. The results of the simulations ascertain that a spray PDEC system with a water flow control performs better. In addition, a spray PDEC system contributes most when it operates as a secondary cooling system to abate space cooling loads and to maintain a steady thermal environment by reducing 62.1% electricity for space cooling and 47.9% water consumption in a warm-moderate climate

Comparative insight of pesticide transformations between river and wetland systems

The widespread use of pesticides threatens the environment and ecosystems. Despite the positive effects of plant protection products, pesticides also have unexpected negative effects on nontarget organisms. The microbial biodegradation of pesticides is one of the major pathways for reducing their risks at aquatic ecosystems. The objective of this study was to compare the biodegradability of pesticides in simulated wetland and river systems. Parallel experiments were conducted with 17 pesticides based on the OECD 309 guidelines. A comprehensive analytical method, such as target screening combined with suspect and non-target screening, was performed to evaluate the biodegradation via identification of transformation products (TPs) using LC-HRMS. As evidence of biodegradation, we identified 97 TPs for 15 pesticides. Metolachlor and dimethenamid had 23 and 16 TPs, respectively, including Phase II glutathione conjugates. The analysis of 16S rRNA sequences for microbials characterized operational taxonomic units. Rheinheimera and Flavobacterium, which have the potential for glutathione S-transferase, were dominant in wetland systems. Estimation of toxicity, biodegradability, and hydrophobicity using QSAR prediction indicated lower environmental risks of detected TPs. We conclude that the wetland system is more favorable for pesticide degradation and risk mitigation mainly attributed to the abundance and variety of the microbial communities

A Reliable and Scalable Broadcast Protocol for Wireless Multi-Hop Networks Using Subcarrier-Level Tone-Signals

In this paper, we propose a scalable broadcast protocol, named Subcarrier-level Tone-signal based Broadcast (ST-BCAST), that disseminates a packet over OFDM-based wireless multi-hop networks in an efficient and reliable manner. Exploiting collision-resilient tone-signals and receiver-triggered forwarding decision/cancellation, ST-BCAST achieves both high packet delivery ratio and low communication overhead without using any topological information, thereby providing scalability to the network size. Under a mild assumption, ST-BCAST satisfies two sufficient conditions for reliable broadcasting: first-hop delivery condition and successful relay condition. We verify the feasibility of tone-signal generation and detection through experiments using Universal Software Radio Peripheral (USRP) devices, and show through NS-3 simulations that ST-BCAST significantly outperforms the state-of-the-art broadcast schemes in terms of packet delivery ratio and communication overhead

Address-free contention in wireless access networks with common control channel for throughput improvement

In wireless local area network (WLAN) with common control channel (CCC), where the spectrum bandwidth is divided into two separate channels for access control and data transmission, concurrent transmissions of control and data frames can improve the spectrum efficiency by pipelining data transmissions. However, the state-of-the-art MAC schemes cannot fully exploit the channel separation due to large control overhead and receiver contention problem, where the former causes bandwidth erosion reducing the data transmission rate and the latter prevents the pipelining when two consecutive transmissions involve a common node as the receiver. In this paper, we focus on the two problems, and develop a novel CCC-based MAC scheme with address-free contention resolution for wireless access networks. Through both analysis and simulations, we show that the proposed scheme minimizes the control overhead and significantly improves the spectrum efficiency: It achieves as twice throughput as the conventional CCC-based MAC and 30% average performance gain over IEEE 802.11 MAC.close0