Energy Performance Evaluation of Hollow Fiber Membrane-based Latent Heat Exchanger for Ventilation

Abstract

Recently, net-zero energy buildings have led to an increase in the energy efficiency of buildings by insulating external walls and enhancing airtightness. Highly insulated buildings have significantly reduced the sensible load, while airtight buildings should be well-ventilated to maintain indoor air quality and occupants’ comfort; therefore, energy efficient mechanical ventilation system is essential. Energy recovery ventilators (ERVs) using enthalpy exchanger to recover wasted heat and moisture from exhausted room air have widely used for building ventilation systems. Commercial ERVs exhibits from 60 to 80% of sensible effectiveness, while exhibits from 25 to 60 % of latent effectiveness; therefore, the demands for ERV core with higher latent heat exchange performance are increasing in hot humid climates. In this thesis, the latent heat exchanger using hollow fiber membrane (M-LHX) was suggested. Hollow fiber membrane module, which consists of a number of moisture permeable micro-thin fiber tubes, could selectively absorb and permeate the water vapor in air, and could achieve higher air-to-membrane contacting area per module volume compared with flat-sheet structure; thereby it can be inferred that an air-to-air hollow fiber module could have application potential as a latent heat exchanger, which can be used for humid climate zones. In chapter 2, the design guidance of M-LHX suitable for building ventilation unit is described in terms of membrane material and the physical size of membrane module. The membrane material showing the highest water permeance with mechanical stability was chosen. A series of experiments were conducted to investigate the effect of each design factor on latent cooling performance. Finally, a non-dimensional design index for M-LHX was derived, and suggested the target value of design index which can attain enhanced latent heat exchange performance compared to the conventional ERV core. In chapter 3, a prototype M-LHX was constructed, and the investigation on operating characteristics of M-LHX in various operating conditions is explained. The experiment results showed that M-LHX presented better latent heat exchange performance than the conventional ERV core, particularly in cooling seasons. In addition, it was observed that M-LHX has superior latent heat exchange performance in cooling seasons, while the sensible heat exchange performance is superior to latent heat exchange performance in heating seasons owing to the asymmetric structure of hollow fiber membrane used in this research. The characteristics of pressure loss from supply air and exhaust air side were analyzed as well. The results showed that the pressure loss from supply air side is much higher than that of the exhaust air side because the cross-sectional area of supply air side (fiber tube side) is smaller than that of the exhaust air side (fiber shell side). In chapter 4, the derivation of empirical correlations to predict the operating performance of the M-LHX, based on the experimental design method is explained. Each correlation was derived by considering the performance test conditions of ERV addressed by AHRI. Furthermore, the empirical correlations for predicting pressure loss of supply air and exhaust air side for M-LHX were also developed to predict the fan working load of M-LHX. Using the measured data, the empirical correlations were derived based on a statistical analysis using response surface methodology (RSM), and the developed correlations were verified based on the measured values through additional experimental data. Finally, the configuration of M-LHX assisted energy recovery ventilation unit was suggested, and its energy saving potentials compared with the conventional ERV were evaluated via detailed energy simulations. As a result of the significant latent load reduction during the cooling seasons, the proposed ventilation unit was able to remove 30% more ventilation load annually than the conventional ERV, and it also helped to reduce annual energy consumption by 10.6% despite using 15.4% more fan energy.