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Evaluation of chemical looping heat pump cycle.

Author(s) : KIM J.

Type of monograph: Doctoral thesis

Summary

Air conditioning, space heating, and refrigeration account for approximately 40% of the electricity usage in the U.S. residential and commercial building sector. To improve energy utilization and reduce energy consumption in space conditioning applications, advanced heat pumping technologies are needed. The chemical looping heat pump (CLHP) is a promising thermodynamic cycle that has shown the potential to achieve a cooling coefficient of performance (COPc) increase of over 20% relative to conventional vapor compression (VC) systems.

The overarching goal of this study is to evaluate the chemical looping heat pump concept for residential applications, including thermodynamic potential, as well as technical and economic feasibility before developing and deploying a pilot scale system. The evaluation process includes advanced thermodynamic modeling for better assessments of working fluids and systems, techno-economic analysis for initial cost assessment of the scaled-up system, and small-scale experiments for proof-of-concept.

A working fluid screening process was developed to identify suitable working substance pairs for CLHP systems. The key metrics for evaluating the working fluids are associated with the possibility of phase change after a chemical reaction, reversible cell potential and power consumption, and cooling capacity of the system. Such metrics were applied to several fluids to assess their suitability. It was found that isopropanol/acetone working substances showed the highest cooling capability for a given power consumption. Even though this approach was applied to particular organic fluids (e.g., alcohols and ketones), this analysis can be generalized to other single-component fluids, multi-component fluids, and several chemical designs.

A modeling framework to estimate operating cost, capital cost, and levelized cost of energy was developed to enable a direct early-stage comparison of a CLHP with conventional VC systems. The models were helpful in understanding the influence of key factors such as efficiency, unit utilization (annual cooling and heating delivered, kWht/yr), and price of electricity ($/kWhe) with the goal of determining target markets for initial CLHP products. The LCOE of CLHP could be less than that of VC in the case of high utilization (≥  20,000 kWht) with high performance improvements (COPCLHP/COPVC = 1.3) even though the capital cost of the CLHP is nearly 1.5-2 times higher than VC.

The key process of a CLHP cycle, which is electrochemically driven phase transformation, was experimentally demonstrated based on the advanced test rig and electrochemical cell. A polymer electrolyte membrane flow cell with a self-fabricated membrane electrode assembly and flow channels was employed to drive the reaction. The breakdown voltage analysis indicates that ohmic and mass transfer overpotentials account for more than 90% irreversibilities of the reactions. In addition, the results showed the possibility of phase transition of 20-30% at current density of ~0.003 A/cm2 and the cell voltage of 0.025 V. The extent of a chemical reaction can be further improved by increasing the current and reducing the flow rate.

A semi-empirical cycle model was leveraged to predict realistic system performance. The model includes an electrochemical cell model with other component models in a CLHP cycle. The Second law efficiency was 50% of the Carnot limit with a cooling capacity of 2.24 mW (cooling density of 1.6 W/m2) at sink temperature of 40 °C  and source temperature of 23 °C. The cause for the precipitous drop in COPc with increasing current density was overpotential, which requires further research on the optimization of membrane and catalytic materials as well as a geometry of flow channels to minimize the losses. Higher efficiency can theoretically be achieved at an elevated fluid temperature as long as an electrochemical cell can achieve a greater degree of conversion.

There are several challenges that should be reconciled in a future operational device and cycle at scale. Additional research on both material- and system-level performance is indispensable to meet practical size requirements. Nevertheless, this study is intriguing in terms of the possibility of developing a high efficiency device with the ability to use more environmentally friendly working fluids. Broadly, this CLHP research can contribute to accelerating the development of the newly emerging field, which is thermal systems coupled with electrochemical processes, that can maximize system efficiency using low-GWP fluids.

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Pages: 219 p.

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