Evaluating the performance of passive chilled beams with respect to energy efficiency and thermal comfort
Existing modeling approaches for passive chilled beams determined from tests on individual chilled beams in a laboratory are not adequate for assessing overall energy usage and occupant comfort within building simulation programs. In addition, design guidelines for passive chilled beam systems are needed for identifying appropriate applications and optimal configurations. This thesis includes (i) extensive experimental studies for characterizing the performance of passive chilled beams, in both laboratory settings and in field studies, (ii) development of passive chilled beam performance prediction models, (iii) integration of these models into building simulation models/tools and (iv) use of building simulation for overall assessment of different passive chilled beam system configurations in different climates in order to provide guidelines for appropriate applications. Experiments were conducted with a single passive chilled beam in a laboratory setting and with multiple passive chilled beams installed in a real occupied office space. Based on the experimental results, models that can predict total cooling capacity and chilled surface temperature of passive chilled beams were developed. These models use essential operating conditions of the system and thermal conditions in the environment as inputs and are able to predict the energy and thermal comfort performances of the passive chilled beam system when integrated into a system simulation. The validity of using a model developed from laboratory tests on a single passive chilled beam in a system simulation for spaces with multiple chilled beams was evaluated. Comparison of laboratory and field measurements indicates that the conventional method of predicting total cooling capacity of a passive chilled beam from laboratory measurements underestimates its performance when installed in a system. These differences could have an important impact on system sizing and commissioning. Side-by-side field measurements were conducted to compare energy and comfort performance of a passive chilled beam system against constant and variable air volume systems for nearly identical office spaces. While maintaining very similar thermal comfort levels in the two offices, the passive chilled beam system led to a 57% reduction in electric energy compared to the constant air volume system. However, the variable air volume (VAV) system consumed 21% less energy compared to the passive chilled beam system during the field measurements. This is mostly because of the current configuration of the passive chilled beam system which represents the worst case scenario in terms of system configuration. The parallel air system used in the field measurement is a typical air system including the outdoor air and return air damper system. As a starting point followed by various configurations assessment with computer simulations, the return air damper was closed during the entire field measurements of the passive chilled beam system. In order to consider more realistic energy savings compared to VAV systems, alternative passive chilled beam configurations were evaluated using a system simulation model that was validated with the available measurements. The integrated simulation tool was developed and validated for the case study office space and was then used to perform comprehensive comparisons of alternative passive chilled beam and conventional systems in order to evaluate savings potential in various climatic zones. While maintaining the same thermal environments in spaces, the best passive chilled beam configuration provided electrical energy savings up to 24% for hot and humid climates and up to 35% savings for hot and dry climates compared to a variable air volume system. The radiation cooling effects of passive chilled beams were also analyzed through experiments and simulations. Both experiments and computer simulations revealed that the effect of the radiation cooling of passive chilled beams is not significant in terms of energy savings and thermal comfort improvement. Based on simulation results covering various passive chilled beam system configurations and climatic zones, the percentage of radiation cooling energy relative to total passive chilled beam cooling energy varied between 7 to 15%.
Tzempelikos, Purdue University.
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