DEVELOPMENT OF MODELING AND OPTIMIZATION METHODOLOGIES TO FACILITATE TRANSITION OF HEAT PUMP AND REFRIGERATION SYSTEMS TO LOWER GWP REFRIGERANTS

Abstract

In light of Kigali Amendment, Fluorinated Gas (F-Gas) regulations, and the United States Environmental Protection Agency (EPA) SNAP program calling for the phase out of hydrofluorocarbon (HFC) refrigerants which have an extremely high global warming potential (GWP), it is necessary to transition to lower GWP alternatives. For example, natural refrigerants such as hydrocarbons (HC, e.g., propane (R290), isobutane (R600a), etc.) and carbon dioxide (CO2) have gained traction as attractive low GWP alternatives for a wide range of residential and commercial heating, ventilation, air-conditioning and refrigeration (HVAC&R) applications which have historically depended on high-GWP refrigerants such as R134a, R410A, R22, and R404A. Moreover, researchers worldwide have also shown immense interest in utilizing refrigerant blends to achieve the required performance whilst minimizing environmental impact. One such area is supermarket refrigeration systems, which often employ multi-stage cascade cycle configurations due to large temperature lifts, making single-stage vapor compression (VC) systems impractical. A key challenge in this field is selecting environmentally-friendly refrigerants, as many common low-temperature refrigerants have high GWP, high ozone depletion potential (ODP), and/or are flammable. The use of HC refrigerants is also highly regulated due to their flammable nature, hence necessitating high-performance heat exchangers (HXs) with significant size, weight, cost, and refrigerant charge reductions compared to current state-of-the-art to comply with flammable refrigerant charge limits.Decarbonization of buildings is essential to reducing greenhouse gas emissions, as space and water heating account for a substantial share of energy-related emissions. Transition to heat pump technologies is central to this effort, offering a low-emission alternative to fossil fuel and electric resistance heating. They can substantially reduce greenhouse gas emissions from space and water heating when powered by renewable or low-carbon electricity, making them a key technology for building electrification and climate mitigation goals. This thesis focuses on the development of modeling and optimization methodologies to facilitate the transition of heat pump and refrigeration systems to lower GWP refrigerants and is applied to three different case studies: (i) refrigerant blend optimization and selection for a supermarket refrigeration system, (ii) a water source heat pump system, and (iii) low-charge air-to-refrigerant HXs for a residential air-conditioning system. In the first part of the thesis, an approach was developed to design optimal refrigerant blends to serve as lower-GWP alternatives to conventional high-GWP refrigerants while also enhancing system efficiency for a two-stage cascade refrigeration system. Significant improvements in COP by up to 49% and decreases in GWP (< 50) were observed when the optimal blends were incorporated into the system. In the second part of the thesis, experimental validation of a water source heat pump system model utilizing R32 as the refrigerant and a brazed plate heat exchanger (BPHX) as the condenser was conducted for a steady-state VC system simulation platform to demonstrate the platform’s capability to accurately model and simulate VC systems with lower-GWP refrigerants and plate heat exchangers. In the final part, a comprehensive system-level optimization methodology was developed for air-to-refrigerant tube-fin and finless non-round tube HXs which is capable of designing optimal HX pairs (i.e., condenser and evaporator) with minimal refrigerant charge to replace the baseline HXs in an air-to-R410A air-conditioning system to facilitate the transition to lower GWP refrigerants R32 and R290. System-level simulations with the optimal HX pairs suggest that the optimal HX pairs have comparable thermal-hydraulic performance to the baseline system with significant reduction in HX-level charge (> 70%) , thereby supporting the adoption of lower-GWP natural refrigerants such as R290.