Assessment of Temperature Swing Adsorbent Pre-cooler

Evaporative cooling technologies such as precoolers and indirect evaporative chillers have the potential to greatly reduce energy use in hot and dry climates. However, they require the use of water. This report describes a feasibility study into use of certain adsorbent materials to capture moisture from atmospheric air at night time and use it to precool a condenser evaporatively during daytime. The key objectives are to (a) Perform a literature search of various potential adsorbent materials and to identify the ones that would work best for most of the climate zones in CA (b) Develop a heat and mass exchanger (HMX) model for the selected adsorbent from the literature search (c) Perform a parametric variation study on the performance of the HMX for a climate zone in the SCE territory, and (d) Assess the feasibility of the selected HMX design in different climate zones (CZ) in CA. Assessment included energy savings and the associated cost savings over a cooling season and water savings compared to traditional evaporative precooler. Among the materials found in open literature, MIL-101 was chosen as the adsorbent for this application based on its high water affinity and the ability to swing between charge and discharge states within temperatures and partial pressures relevant to hot and dry climates. A HMX design based on a honeycomb-type network of parallel passages or channels resembling a flow straightener is proposed. The HMX is placed in a ducted configuration with a condenser bypass at nighttime to decrease fan power during adsorption. It is assumed that the walls of the HMX would be comprised of the adsorbent material. A simplified mass transfer model is developed with correlations from literature for mass transfer. The spatially one-dimensional model is stepped in time by using timevarying input information of outdoor air conditions from the Typical Meterological Year (TMY) 3 database. Outputs of the model include exit air temperature and state of water mass loading in the adsorbent at different spatial locations within the HMX at a given time instant. The model outputs are used to calculate an effectiveness and fan power consumption. Furthermore, the performance of the HMX as an evaporative pre-cooler is assessed by coupling it to a A/C unit and estimating the energy and cost savings associated with use of the pre-cooler. A parametric analysis of the effect of geometrical variations on the HMX performance was performed with the model. Varied parameters included mass of the adsorbent, packing density of the adsorbent, wall thickness of the HMX and the flow passage dimensions. Results indicate that, for the chosen parameters and over the varied range, an adsorbent mass of around 50 kg, a packing density of 300 kg/m3 , flow passage dimensions of 5 cm side, and a channel wall thickness of 2 mm results in the best energy savings per unit adsorbent mass. 5 percent in CZs 10 and 13 and around 3 percent in CZs 8, 9, 14-16. However, in the coastal zones, the HMX performs poorly, due to the selection of the adsorbent specific to hot and dry climates. The advantages of the pre-cooler include those typical of an evaporative pre-cooler, including reduction in electrical energy over a cooling season, peak load during hot summer days, and ability to facilitate demandresponse. Advantages over traditional evaporative pre-coolers include elimination of water use and related maintenance. While the technology is promising, significant advances in high-capacity adsorbents specific to the CZs, as well as in fabrication methods for the HMX, are needed. Additionally, a 10-15 fold decrease in the cost of high-capacity adsorbents is needed to enable adoption of the technology. It is recommended that further work be performed in the materials development and fabrication methods to enable such cost reductions.