A hydronic radiant system is a type of the space thermal environment control system using a heated or cooled surface of building components such as floor, wall, and ceiling. The systems have been reported to have advantages in improving occupants??? thermal comfort and reducing heating and cooling energy consumption. Although the system has advantages for the thermal comfort of occupants and energy saving potential, it also has drawbacks to be overcome. The most important disadvantages of the conventional radiant system are that it is hard to control the ventilation requirements and the system capacity for cooling operation decreases due to surface condensation in most humid climate conditions. Several incorporation models, which combine the hydronic radiant system and the forced all-air system, have been implemented into actual buildings and have been used to evaluate the system performance. However, there are no attempts to develop a new radiant system that controls the fresh air for the space without additional components such as heating and cooling coils. This study developed a new radiant system having a concentric tube heat exchanger in a radiant panel and evaluated the system characteristics. The concentric tube heat exchanger allows two fluids, air and water, to flow in the same direction. The outdoor air for the space ventilation requirement passes through an inner tube of the heat exchanger and exchanges heat with an outer water tube of the heat exchanger. The primary heat transfer medium, water, flows through the outer tube exchanging heat with the radiant panel and the air tube. At the outlets of the heat exchanger, both fluids have an identical temperature. The air is delivered into the space directly and the water returns into the plant side. The system configuration enables simultaneous satisfaction of the space thermal loads and ventilation loads without additional components for conditioning the outdoor air. Moreover, the outdoor air can circulate through the air tube during the nighttime in the summer season to damp the heat sink source for daytime cooling loads. The flexibility of using the heat transfer medium, air or water, is recognized in the naming of the hybrid heat source radiant (HHSR) system.This conceptual idea has been evaluated by a numerical analysis model based on computational fluid dynamics (CFD). Comparing with the result of a typical hydronic radiant system and a hydronic radiant system incorporated with a convective forced-air system under steady state analysis with the same conditions, the proposed system can also provide an acceptable local thermal environment in terms of the vertical temperature difference, the floor surface temperature, and the percentage of discomfort due to draft. A transient energy simulation model of the proposed system is also developed for a whole building energy simulation program, EnergyPlus. Three different analysis models have been studied for the transient model to investigate the heat transfer characteristics of the concentric tube heat exchanger with a non-adiabatic outer shell condition. A modified effectiveness and number of transfer unit (??-NTU) method, which considered the heat capacity properties of the radiant panel and the heat mediums, is selected and the semi-numerical analysis model is interpreted as a program module for the whole building energy and indoor thermal environment simulation program. The water outlet temperature and space mean air temperature of both models of the CFD and EnergyPlus are in agreement with each other in the acceptable tolerance under the given conditions.Finally, the case study is conducted to investigate the system capability and characteristics under 16 different climate conditions in the United States for two different building types, a medium size office and residential units. The building types and climate conditions are based on the presentation of the Department of Energy in the U.S. for the standardized energy simulation models for commercial and residential buildings and the representative locations for 16 different climates. The hourly heating and cooling energy consumption and thermal environment condition for each building type in the 16 climate zones are predicted for three different radiant systems: a typical hydronic radiant system, an outdoor air control system incorporated with the typical radiant system, and the HHSR system. From the annual energy consumption simulation, it was shown that the system performance of the HHSR system allows the system to be considered as a viable HVAC system compared with the conventional radiant system types for the office building type. The HHSR system provides acceptable indoor thermal comfort indices in terms of Predicted Mean Vote (PMV) and has energy saving potential for heating and cooling operating in the most locations. The configuration of the proposed system also effectively prevents system interruption due to surface condensation during cooling periods in humid climates. In addition, the system expected an additional day time cooling energy saving, when passive cooling using outdoor air ventilation is applied in the night time.Comparing the case of residential units, the results are different from those of the office building case. Although the HHSR system can provide similar indoor thermal conditions to other conventional radiant systems, the heating and cooling energy saving potential depends on the climate condition. By the internal heat gain condition of the residential units, the proposed system does not expect as substantial energy saving benefits as the office building in a particular climate condition.This study also found that some limitations should be considered when the proposed system is implemented and operated in buildings. For the HHSR system, the ventilation air temperature depends on the water outlet temperature, so the water flow rate control scheme impacts the air temperature. Therefore the overall system performance might be changed by the water flow rate logic. Another important consideration is water condensation in the air tube. Although the proposed system can prevent surface condensation on the radiant panel, the water condensation is predicted in the air tube in most humid locations. Internal condensation should be carefully controlled for to assure the hygiene of ventilation air.Future works that should be done based on this study include the experimental validation of the proposed system, the development of a practical tube connection and split methods between the fluids (air and water) supply/delivery side and the concentric tube heat exchanger, and a lifecycle assessment (LCA) of the HHSR system from the manufacturing stage to the demolition stage.
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