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Variable Refrigerant Flow (VRF) heat pumps are often regarded as energy efficient air-conditioning systems which offer electricity savings as well as reduction in peak electric demand while providing improved individual zone setpoint control. One of the key advantages of VRF systems is minimal duct losses which provide significant reduction in energy use and duct space. However, there is limited data available to show their actual performance in the field. Since VRF systems are increasingly gaining market share in the US, it is highly desirable to have more actual field performance data of these systems. An effort was made in this direction to monitor VRF system performance over an extended period of time in a US national lab test facility. Due to increasing demand by the energy modeling community, an empirical model to simulate VRF systems was implemented in the building simulation program EnergyPlus. This paper presents the comparison of energy consumption as measured in the national lab and as predicted by the program. For increased accuracy in the comparison, a customized weather file was created by using measured outdoor temperature and relative humidity at the test facility. Other inputs to the model included building construction, VRF system model based on lab measured performance, occupancy of the building, lighting/plug loads, and thermostat set-points etc. Infiltration model inputs were adjusted in the beginning to tune the computer model and then subsequent field measurements were compared to the simulation results. Differences between the computer model results and actual field measurements are discussed. The computer generated VRF performance closely resembled the field measurements. [ABSTRACT FROM AUTHOR]

Increasing loop chilled water return and supply temperature difference or delta-T is an effective way to improve the chilled water system efficiency and save chiller plant operating costs. In some chiller plants, the delta-T can be expected to fall to about one-half to two-thirds of design at low loads due to various causes, such as coil load, air entering and leaving temperatures, and chilled water supply temperature. Computational simulation is a useful and flexible tool to study cooling coil delta-T characteristics under various conditions. This paper simulated the cooling coil delta-T profiles with different geometric configurations and various waterside and airside conditions using an effectiveness-NTU model. The waterside heat convection coefficient was modeled with a combined correlation. The simulation results show that the existing waterside correlations generated significantly different delta-T profiles. With variant geometric configurations, the coil delta-T at full load may be higher, equal, or lower than that at part load. The complexity of cooling coil delta-T characteristics indicates that it is hard to make conclusions about the coil delta-T change with cooling loads. [ABSTRACT FROM AUTHOR]