ENERGY SAVINGS AND THERMAL COMFORT OF SEPARATE SENSIBLE AND LATENT COOLING AIR-CONDITIONING SYSTEMS

Abstract

Conventional air conditioning (AC) systems have limited control of sensible cooling and latent cooling capacities; therefore additional energy-consuming devices, i.e. electric heaters, are often used to reheat the conditioned air in order to provide thermal comfort for the building occupants. Separate sensible and latent cooling (SSLC) AC systems are capable of providing better control of cooling at no extra overload in the form of energy input. Moreover, because of a higher coefficient of performance (COP) in the sensible cooling cycle, the SSLC technology reduces total energy input to vapor compression systems (VCS), and makes AC systems more energy efficient. This dissertation explores and compares two main methods for implementing the SSLC concept: cycle options for SSLC systems and methods of indoor heat transfer. One of these options consists of two independent VCS, and the other consists of one VCS removing sensible load only and one solid desiccant wheel (DW) regenerated with the waste heat from the VCS. The objectives of the system option study are to understand the reasons behind energy savings and explore the best possible configurations of SSLC systems in different summer outdoor conditions. The simulation results of the first kind of SSLC system show that the energy savings come from a reduced compressor power input of the sensible cycle. Under wide ranging ambient conditions, the amount of energy savings varies from 22% to 50% over conventional system energy input. However, such a system has limited independence of varying sensible to latent load ratio and the extra cost of an internal heat exchanger. The integration of VCS and DW overcomes these limitations. An experimental setup was constructed in an environmental chamber to test the performance of the second kind of SSLC system using carbon dioxide as refrigerant. The experimental results show only a 7% improvement by using SSLC systems, and two negative factors hindering SSLC systems from achieving more energy savings were later identified. As a result, the application of divided heat exchangers is proposed as a solution to address one of the issues. An optimal SSLC system, which incorporates the application of divided heat exchangers, an enthalpy wheel and other energy-saving methods, was modeled and demonstrated a doubling of the COP as compared to a conventional AC system. The second method crucial to implementing SSLC is a so called "low ΔT indoor heat exchanger" which is being introduced as an improved sensible heat exchanger design for the successful implementation of SSLC system concept. Its capability of providing both radiative heat transfer and convective heat transfer leads to better thermal comfort to occupants. Compared to the baseline fan-coil unit, the low ΔT indoor heat exchanger creates better thermal comfort in terms of reducing temperature stratification from head to feet by 0.8 K and providing higher operative temperature at the foot level in winter. Numerical models were developed to simulate the operative temperature field created by the low ΔT indoor heat exchanger. The model had only an average deviation of 0.4 K compared to the experimental data. The air temperature simulation in the model was later replaced by the proper orthogonal decomposition (POD) method. The POD method provides simulation results almost identical to CFD simulation (maximum deviation of 0.1 K), and moreover reduces the computation time from 24 hours to only minutes.