Conference Agenda

ANSI/ASHRAE Standard 34, Designation and Safety Classification of Refrigerants prescribes the Refrigerant Classification System, refrigerant concentration limits (RCL) and lower flammable limits (LFL) which are used in applying ASHRAE Standard 15. A2L refrigerants and blends are weakly flammable, require a high energy ignition source and are mildly energetic when combustion occurs. For some A2L systems they are very close to the transition point where they become A1. It is known that pressure (high and low) can have a significant effect on flammability limits. The heights of major populated cities around the world above sea level ranges from Milan (Italy) at 122m to Denver (Colorado) at 1609m. The pressure at these elevations will be used to explore if the flammable range (LFL and UFL) is moved to the extent that the refrigerant/blend is no longer flammable.

Due to the serious concerns about global warming, manufacturers have started phasing out high Global Warming Potential (GWP) refrigerants in commercial refrigeration equipment (e.g. R134a). As a replacement, propane (R290) is an environmental-friendly refrigerant for commercial refrigeration equipment because its GWP is only 3. However, propane is a flammable refrigerant, which is classified as Class A3 refrigerants per ASHRAE Standard, so safety is a very important consideration when such propane-based equipment is designed and deployed in buildings. In the event of a refrigerant leak, the flammability of the refrigerant depends on the refrigerant’s local concentration, which is highly affected by the indoor air environment including temperature and air flow. In this study, a computational fluid dynamics (CFD) model is developed to investigate the refrigerant leak from a commercial refrigeration equipment. The CFD model can visualize the flammable zones due the leaking. Moreover, the numerical model can also assist in the deployment of the commercial refrigeration equipment as well as the design of ventilation systems to keep safe of buildings.

The transformation of heat supply in urban areas from fossil based to renewable is a key factor to reduce the CO₂ emissions. Every existing district network has its own characteristics in heat supply temperature (>90°C), capacity, age, heat generation technology, and options to change the volume flow to keep the heat supply costs low. Decentralized non-used heat sources in urban areas can be implemented with multi-source heat pumps. These sources could be cold water networks, sewage up to server farms and low- and mid-temperature district heating networks. The challenge for multi-source heat pumps are the different temperature levels of each source.

To investigate different multi-source scenarios for heat pump technology a fluid screening has been the foundation for a test rig design. The focus will be hydrocarbon refrigerants and mixtures with a capacity of 5 to 50 kW. The source temperatures can be varied between -10 °C and 50 °C, the sink temperatures between 30 °C and 85 °C. The temperature difference between inlet and outlet of the heat source/heat sink is adjustable between 5 and 25 K. The focus of the system is the compression technology: A scroll compressor and two reciprocating compressors are installed and can be tested separately or in a booster option with an intercooling. A comparison and interaction between refrigerant mixtures, compressor technology, source/sink temperature and temperature glide should be enabled.

Two key challenges to transitioning heating processes to electricity are improving the efficiency of heat pump systems, especially at high temperature lift conditions, and using low-charge sustainable refrigerant. A modular cold-climate heat pump system (ccHP) with thermal storage, which can efficiently, cost-effectively, and flexibly serve space heating, space cooling, and water heating has been designed. This heat pump system consists of a factory-charged propane (R290) modular outdoor unit, and end-use modules selected and tailored to the climate and connected to a secondary glycol loop. These modules are an auxiliary thermal energy storage (TES), a hot water tank (WT), and an indoor air handling unit (AHU). They can be installed using low-cost, low-tech equipment with low-skill labor.

One innovation of this system lies in the development of a multiple segment heat exchanger (MSHX) composed of a de-superheater, a condenser, and a sub-cooler section in one component. The heat pump system works under different operating modes, including typical heating and cooling modes, but also modes that use different sections of the MSHX to charge or discharge the thermal storage, or heat domestic hot water. This paper describes the models of this new MSHX and of the ccHP. Once designed, the proposed system is first compared to a typical heat pump, then we simulated this ccHP model, including the TES and AHU modules, under various operating conditions and modes. It shows an increase in efficiency and lowers the peak demand by using thermal storage.

The increasing importance of heat pumps as a climate- and resource-friendly option for heating buildings and considering the political phase-down of high GWP refrigerants (EU F-Gas Regulation No. 517/2014) increases the focus on natural refrigerants. In particular, using R-290 in domestic heat pumps is becoming increasingly important due to its lower environmental impact (GWP₁₀₀ = 3) and high efficiency. In addition to the potential, using this refrigerant also poses challenges regarding the system in general and especially the compressor, which must be considered.

This study focuses on methods and approaches for identifying and investigating challenges when using R-290 as a refrigerant in domestic heat pumps. In addition to looking at the system as a whole, the importance of the compressor for this refrigerant is classified. This study aims to identify and analyze possible challenges for the compressor and the system in general and to present methods for investigating these points. This includes the consideration of degradation phenomena of compressors that were operated in heat pumps and identifying potential causes.

Trifluoroethylene (HFO-1123) emerges as a promising candidate for next-generation refrigerants in refrigeration and air-conditioning applications, attributable to its extremely low global warming potential (GWP) of less than one and zero ozone depletion potential (ODP). However, HFO-1123 is known to cause a thermal decomposition called disproportionation reaction. Inhibitors like R290 are added to suppress the reaction propagation. Considering the high flammability of R290, which has a laminar burning velocity (LBV) of approximately 38 cm/s, it is crucial to determine the appropriate composition of binary mixtures that can limit the LBV to below ten cm/s. Achieving this lower flammability level, as classified by ASHRAE as 2L, is essential for developing next-generation refrigerants. Typically, substances rich in hydrogen (H) atoms like R290 show high flammability, while substances rich in fluorine (F) atoms like fluorocarbons show low flammability. Existing studies have focused on the H/F balance of the mixture and proposed various mixing rules for predicting the LBV of refrigerant combinations. However, the applicability of these mixing rules to mixtures with significant differences in LBV like HFO-1123 and R290 remains unverified. The following mixing rules are selected as the mixing rules for comparison: Linear correlation which is a weighted average by mass/mole fraction; Le Chatelier’s law which is used for LFL prediction in flammability studies; Oxford correlation, an extension of Le Chatelier’s law concerning combustion heat of each component. This study experimentally investigated the LBVs of binary mixtures of strongly flammable R290 and weakly flammable R32, R1234yf, or HFO-1123. Experimental results indicated a significant deviation in the LBV of binary mixtures from the prediction by proposed mixing rules. This phenomenon was notably evident in binary mixtures of substances rich in H and substances rich in F. The deviation was significant when the H/F ratio of the mixture was approximately one. The discrepancy between the experimental results and mixing rule prediction was attributed to an additional exothermic reaction between combustion products. This finding revealed complex interactions in such mixtures not reflected in the existing mixing rules. Additionally, this study examines the correlation between LBV, product composition, and adiabatic temperature, offering new insights into refrigerant mixture developments.