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Title: Study of operation dynamics of building with radiant system
Authors: Hu, Rong
Degree: Ph.D.
Issue Date: 2017
Abstract: Radiant cooling systems are now widely used throughout the world because they are regarded as suitable mechanisms with numerous advantages in indoor environmental control and energy efficiency. The smaller flow rate required in an accompanied air-conditioning system reduces ductwork dimension, fan size, and energy consumption. The high water temperature in the hydronic system for cooling provides opportunities to directly apply natural low-grade energy sources. Nevertheless, several problems have not been resolved. First, a method that assesses the cooling load for a room with a radiant system has yet to be finalized, and the cooling capacity of a specific category of radiant systems is not fully understood. Both of these issues not only depend on the configuration of radiant system, supply water temperature, and their specific environment, but also relate to the operation strategy. Second, managing radiant system operations remains a concern for many engineers. Several engineers believe that the continuous operation can keep the system running in a steady state and avoid a large amount of transient energy input when the system starts. Therefore, this thesis investigates the characteristics of radiant systems in dynamic operations through simulation and on-site measurement. It also searches for optimal strategies for different categories of radiant systems, including capillary mats embedded surface cooling system (ESCS) and Polybutylene pipe thermally activated building system (TABS). A typical office building is first assumed for simulation, whose thermal properties must meet the national building regulation and local building energy efficiency standard. The effects of climate conditions and thermal mass on system performances are also considered, and the weather data on both cooling design days and typical days in June in the cities of Beijing and Nanjing are selected. The thermal mass can be varied by changing the concrete layer thickness in the external wall of the assumed building. Six operation strategies are involved in this study, which are separated into two groups according to the operation with or without a thermostat. Finally, three strategies without thermostats are implemented in an existing building.
Simulation results reveal significant differences in the system performance between the interior and perimeter zones. The cooling surface reduces the temperature at the inactive surface through radiation heat transfer in the interior zone, which has adiabatic outside faces. Some of the heat gain can be conserved in the inactive surface instead of being removed by the cycling water in the hydronic system. The radiation heat gains, including the solar gain, largely increase in the perimeter zone, whose external construction faces the outdoor environment. Most of the heat gains are absorbed by the active surface through direct or indirect radiation heat transfer because obvious temperature differences exist between the active and inactive surfaces. Therefore, the gap between the mean radiant temperature and mean air temperature is narrower than the environment dominated by a convective air system only. However, the conduction heat gain at the inactive surface increases. The ESCS can mostly maintain the environment at a set point during the occupied period, and the cooling load is basically consistent with the heat extraction on the active surface in the operation with a thermostat. Although the peak cooling load of terminal system is higher than the heat extraction in the perimeter zone by 6% to 13%, the energy and economic savings targets can be achieved because of the lower air flow rate for ventilation. The room temperature in the zone with TABS cannot be maintained at a constant value when the heat gain fluctuates frequently, and it is equal to or slightly less than the design criteria. The cooling load of the hydronic system avoids the heat extraction at the active surface in the operation with a thermostat. Thus, an improved operation is attempted, in which the radiant system is free from the thermostat instruction, starts at midnight, and then runs continuously. The results reveals that at least 9% cooling demand during occupied period can be conserved in the thermal mass in advance, and the peak cooling load of terminal system reduces 30% above, compared to the performance of an equivalent convective air system. The electricity consumption and the corresponding cost are expected to be saved by 5% and 19%, respectively. The effects of the thermal mass on the system performances are limited in the building whose external construction is properly insulated, but the thermal mass is still necessary to prevent the inside face temperature from frequently fluctuating. Finally, the three operation strategies are implemented in an existing building with TABS. Results basically validate the characteristics of TABS in dynamic operations and the feasibility of the proposed strategy. Finally, the author provides several suggestions for radiant system design and recommends the content of future work based on several issues that have yet to be addressed.
Subjects: Hong Kong Polytechnic University -- Dissertations
Air conditioning
Buildings -- Energy conservation
Pages: xxx, 253 pages : color illustrations
Appears in Collections:Thesis

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