Vapor Adsorption Transient Test Facility for Dehumidification and Desorption Studies

The steady state performance testing of industrial-scale energy wheels requires large-scale and advanced instrumentation to analyze large volumes of data. In order to address the feasibility of laboratory-scale studies, experimental modelling and data simulation have been successfully performed by means of the transient and cyclic testing of a heat exchanger within an energy wheel setup in a parallel-flow air stream configuration. However, major challenges have been encountered in terms of predicting the effectiveness of a counter-flow energy wheel configuration in different operating conditions via the use of a transient test setup in a parallel-flow configuration. In the present study, we report the modification of a transient test facility intended to facilitate the more accurate simulation of a full-scale energy wheel operation in a small-scale test facility. A new test section was designed to: (1) enable tests in both counter-flow and parallel-flow configurations; (2) afford automated cyclic testing and achieve the reliable simulation of the energy wheels dehumidification/regeneration cycles; and (3) enhance the accuracy and reduce the uncertainty of the relative humidity (RH) measurements through utilization of the bag sampling method. The latter method is shown to yield greater accuracy with regard to the RH in non-isothermal operating conditions, as well as to reduce the data processing required for the estimation of latent effectiveness.

Bio-desiccants; Energy/heat wheel; Latent effectiveness; Starch particles; Water vapor

In North America, there is increasing demand for greater energy efficiency due to the rising costs of both heating and cooling in residential and industrial buildings alike. Based on population growth and climate change projections, energy consumption is forecast to increase (ca. 30%) over the course of this century. For this reason, heating, ventilation, and air-conditioning (HVAC) systems that incorporate air-to-air energy exchangers (AAEEs) have been designed to provide significant energy savings between the intake and exhaust air streams for buildings with an external environment. A key feature of these AAEEs is the energy wheel, which employs a rotating metallic wheel with a desiccant coating to recover moisture and latent heat from the two air streams during the exchange between indoor and outdoor air supplies.

In order to overcome the challenges associated with the infrastructure and operational costs of research concerning large-scale industrial energy wheels, an alternative approach has been developed (Abe et al., 2006a; Abe et al., 2006b; Shang & Besant, 2009a; Shang & Besant, 2009b; Fathieh et al., 2015). Fathieh et al. (2015) demonstrated the general utility of the transient test facility at the University of Saskatchewan, as well as its utility in terms of the simulation of key parameters for large-scale AAEEs. Transient tests that use a small-scale heat exchanger were performed to predict the latent effectiveness of an energy wheel with the same matrix geometry, materials, and coating pattern in order to overcome the need for a full-scale wheel. The full-scale testing protocol involves the removal and replacement of the desiccant coating, the re-sealing of the casing, external driving systems, and air ducts. By contrast, the small-scale transient test device circumvents these requirements. In particular, the transient dehumidification and regeneration measured using the small-scale test facility revealed that the latent effectiveness of large-scale energy wheels was accurately predicted with comparable geometry and coating (Fathieh et al., 2016). The latent effectiveness of the systems varied depending on the angular speeds, air flow rates, and the level/type of coating on the metal substrate of the exchanger.

Our group has demonstrated a significant improvement in the latent effectiveness of the exchanger according to the nature of the desiccant coating, as shown by the significant differences between silica gel particles with variable textural properties and high-amylose starch biopolymers (Fathieh et al., 2015; Fathieh et al., 2016; Dehabadi et al., 2017; Hossain et al., 2018). The higher rate of sorption and the increased uptake capacity of high-amylose starch (HAS) per unit mass have been shown to exceed those of silica gel materials. Further studies have found that the regeneration process for a starch-coated energy wheel is effective, which has led to the further investigation of both HAS (Fathieh et al., 2015; Dehabadi et al., 2017) and carnation starch (Vaccaria hispanica, syn. Saponaria vaccaria) (Hossain et al., 2018) as promising desiccant materials for full-scale AAEEs.

This study focused on the modification of a transient test facility with the aim of achieving the accurate simulation of a full-scale energy wheel operation in a small-scale test environment. A test section was designed and fabricated in order to enable a CF configuration and automated cyclic tests so as to afford the reliable simulation of the energy wheel, as well as to improve the experimental accuracy of the relative humidity measurements. The estimated latent effectiveness of the starch-coated exchanger indicated that not only is the new test facility capable of reproducing the previous data, but it can also be used to simulate and estimate the performance of an energy wheel in a laboratory-scale facility. The transient and cyclic experiments also showed a good level of agreement in terms of the latent effectiveness. This further proved that the transient characteristics obtained from a single step-change test can be used to estimate the performance of an exchanger for adsorption/desorption cycles of various durations. It was additionally found that the starch desiccant particles showed promising durability over a 16-month period. Thus, the material could represent a promising desiccant coating for energy wheel applications. Moreover, it is expected that the test facility is capable of providing results capable of accounting for higher rotation frequencies (rpms) after incorporating vacuum bag sampling. Therefore, the upgraded design of this experimental test facility should contribute to further promising advances in the research and development of HVAC systems and industrial AAEEs that reflect typical industrial conditions.

The Government of Saskatchewan (Ministry of Agriculture), through the Agriculture Development Fund (Project #20160266), is gratefully acknowledged for supporting this research. The support provided by Daniel Vessey and Blair Cole (College of Engineering Machine Shop, University of Saskatchewan) in terms of the fabrication of the new test facility is also greatly appreciated.

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Abe, O.O., Simonson, C.I., Besant, R.W., Shang, W., 2006b. Effectiveness of Energy Wheels from Transient Measurements. Part II: Results and Verification. International Journal of Heat and Mass Transfer, Volume 49(1-2), pp. 63–77

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