Residential Applications of Dew Point Fluid Cooler

A viability analysis of replacing a residential Heating Ventilation and Air Conditioning Unit’s (HVACs) vapor-compression unit with a sub-wet bulb evaporative chiller (SWEC) is presented in this report. The SWEC is used to chill water to below the wet bulb temperature of outdoor air, and chilled water is subsequently used to cool air in the fan-coil heat exchanger of a residential air handler (AH). The particular geometry of the SWEC under consideration is a daisy-chained configuration of cross-flow heat exchangers with evaporative media sandwiched in between. The objective of this project is to size the SWEC, air handler, and thermal storage that can meet the cooling load requirements of a single family home. Specific tasks are to: - Determine the fluidic and geometrical parameters of the SWEC and air handler that will be required to meet cooling demand of a single family residential home in the 16 California climate zones, - Determine if thermal cold storage is required to expand applicability of the SWEC to climate zones where cooling loads are not met, and - Determine the size of rainwater storage that will be required to meet 50 percent of the SWEC water needs; and if typical annual rainfall will be sufficient to provide this water. To size the cooling system for the modeled single family home and estimate its performance, an integrated model of the SWEC and an air handler (SWEC + AH) was developed. Prior to the development of the integrated model, sub-models for the SWEC and AH were developed. The SWEC sub-model was validated using previously collected experimental data from a prototype. A parametric analysis was performed to determine the effect of various geometrical and flow parameters of the AH on the SWEC + AH model. Once the SWEC + AH model was complete, cold storage strategies were investigated. Furthermore, each of the 16 California climate zones were modeled to determine the performance of the SWEC + AH system. The results for each of these analyses are described below. The SWEC sub-model is based on analytical modeling of one-quarter of the daisy-chained system. The model includes a cross-flow heat exchanger and two passes of water flow through an evaporative medium. The experimentally measured exit temperature of the water of the SWEC was compared with that of the modeled exit temperature for fixed dry bulb and wet bulb temperatures, and air flow rate. While the model and experimental trends agreed favorably, the modeled exit temperature was found to be 6-10 percent lower than the experimentally measured exit temperature, indicating that the SWEC theoretically performs better than the first generation prototype used to collect the experimental data. Possible causes for the discrepancy include experimental uncertainties in temperature measurements, non-uniform distribution of air and water flows in the evaporative media, and heat losses. After the SWEC sub-model was validated, a sub-model for the AH was developed based on cross-flow heat exchanger analysis. A parametric study was then performed to determine the geometrical variables of the AH needed to transfer the load from the building to the SWEC+AH system. Parameters studied included fin pitch, AH thickness, transverse pitch, and tube diameter. The results of the parameter study indicated that the AH size in the SWEC + AH coupled model needed to be increased to account for the higher inlet temperature of the coolant when compared with a traditional refrigerant unit. From the parameter study, an AH geometry was selected to be used in the SWEC + AH model for climate zone analysis. Upon completion of the parametric study, the implementation of two cold storage strategies, both involving use of the SWEC at nighttime to provide chilled water, were explored. The first strategy involved an open-loop wherein the SWEC and AH were decoupled from each other during peak cooling hours. During peak cooling when the SWEC cannot meet the load, chilled water is provided for the AH from a cold storage tank and flows into a hot storage tank. The stored hot water is then cooled by the SWEC during the nighttime period and stored in the cold storage tank for the next day. The second strategy is identical to the first during non-peak cooling and nighttime modes. At times of peak load, water from the SWEC enters the last row of the AH water tubes and then exits out the fourth row. Water from the cold storage then enters the row after the third row and exits the first row. Both methods have advantages and disadvantages and therefore both were investigated for this project. Both cold storage strategies increased the capacity by approximately 20 percent over the baseline performance. Drawbacks of the cold storage strategies include increased cost and space requirements as well as a reduction in COP since the SWEC will need to run for several hours at night to replenish the cold water supply. It was also determined that increasing the number of passes of water coils has the same effect as incorporation of cold storage and therefore should be implemented instead of a storage system in the California climate zone modeling. In order to determine the feasibility of using a SWEC + AH system for cooling in a residential single family home within California, modeling was performed for the 16 California climate zones. Building loads were determined with EnergyPlus data and the SWEC + AH model was used to compute the cooling power, COP, Energy Efficiency Ratio (EER), water-use efficiency, and potential rain water collected for each climate zone. Results indicate that the seasonally-averaged Coefficient of performance (COP) and EER of the SWEC+AH cooling system is greater than 15 and 50, respectively, for most climate zones. Analysis of the water-use efficiency was used to predict in which climate zones the SWEC will be most efficient as well as the plausibility of wide-spread implementation of this system from a wateruse perspective. Water-use efficiency and total water used by the SWEC for each climate zone was found to be under 2.5 gallons/ton-hr of cooler. Based on the 30-year typical meteorological year (TMY) data rainwater storage can provide for more than 50 percent of the water-use by the SWEC in all but two climate zones. Individual climate zone modeling results indicate that the SWEC+AH system was able to meet cooling demand for a majority of the hours during the hottest week of the year for two-thirds of the climate zones. Although there were hours where the SWEC+AH system was not able to meet the cooling load, pre-cooling could reduce this unmet need. Based on these promising modeling results, further evaluation of this technology in terms of a coupled cooling system and building model, as well as field demonstrations, is recommended.