Separate Sensible and Latent Cooling

Abstract

Conventional air-conditioning (AC) systems have a limited control of balancing between sensible 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 balancing between sensible and latent cooling capacities 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 chapter explores and compares two main methods of 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 for 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 a wide range of ambient conditions, the amount of energy savings varies from 22 to 50 % over conventional system. However, such a system has a limited capability of varying sensible to latent cooling capacity ratio and needs the extra cost of an internal heat exchanger. The integration of VCS and DW enables to overcome 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. Later, two negative factors hindering the SSLC system from achieving more energy savings were 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, which is crucial to implementing the SSLC concept, is the application of radiative heat transfer. A radiant heat exchanger is introduced as an improved sensible heat exchanger design. Its capability of providing both radiative and convective heat transfers leads to better thermal comfort to occupants. Compared to the baseline fan coil unit, the radiant indoor heat exchanger provides better thermal comfort by reducing temperature stratification from head to feet by 0.8 K and providing higher operative temperature (OT) at the foot level in winter. Numerical models were developed to simulate the OT field created by the radiant 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) at a reduced computation time from 24 h to only minutes.