Session: 02-07: Sustainable Buildings, Communities, and Cities

Paper Number: 125884

125884 - Improved Performance of Transcritical R744 Gas Coolers For Efficient Heat Pumps in the Northeastern US Winter Markets 

Abstract: 

Transcritical R744 systems have been typically used in refrigeration setups. While mildly used in heating applications, it has seen limited use in western hemisphere markets. Use cases have been most efficient in more moderate temperature regions. With the implementation of NYC local law 97, which aims to decarbonize the building sector, it has become more apparent that the need for this technology has increased. With buildings accounting for about two thirds of greenhouse gas emissions, this law aims at reducing building emissions by 40% by the year 2030 and by 80% by 2050. NYC is expected to see an increase in population (approximately 5% by 2030) and therefore the number of multifamily residential buildings will continue to grow to meet this demand. R744 was chosen as the working fluid due to its low global warming potential (GWP) of 0 and ozone depletion potential (ODP) of 1. Typically used refrigerants such as R410A or R134 have GWP ratings above 1,400. This is a significant reduction and can lead to a more environmentally friendly outcome.

The goal of our project is to analyze complete system performance as an extension of gas cooler efficiency, while investigating suitable evaporators and compressors which can make heat pumps viable as a winter heating alternative. Currently, research is being performed for heating mode and then eventually cooling mode.  In the gas cooler, it is optimal to have transcritical R744 operate in a single stage process where the pinch point occurs across the expansion valve. Avoiding having this pinch point within the gas cooler increases efficiency. It is quite important to avoid this phase change in the middle of the gas cooler, as this tends to illustrate the worst performance. Many different variations of heat exchangers are possible but only a few are practical for commercial manufacture. Microchannel, louvered fins, circular fins, and plate fin designs, to name a few, are being investigated.

The aim of increasing efficiency and improving compactness of the gas cooler for fit into the indoor unit becomes important as we try to make it suitable for use in multifamily residential buildings. Design conditions being held constant are working pressure (10MPa), inlet temperature and refrigerant flow rate. Currently, with simple design models such as circular fin on circular tube, the standard design results in about 65 feet of tube and an NTU in the range of 1.1 - 1.5. These values vary slightly as the number of inlets and the banks are manipulated. The greatest improvement comes from changing the type of fin design that is applied to the heat exchanger. The NTU increases by a factor of 2-3.5 as the fin design changes to plate, microchannel and louvered fins. This factor is proportional to the sizing of the machine, and so, the heat exchanger becomes smaller as the value of its NTU increases.

Additional byproducts of this increase in efficiency are that the gas cooler inlet temperature decreases. There becomes less load on the compression stage and makes the heat pump operation quite a bit safer. This will aid in gaining certification for residential use as the operating characteristics are also more stable. For commercial setups in multifamily buildings, the COP must be monitored and kept at a reasonable range (about 2.5 COP).

Our developed python models are being used with commercial software to verify these results under variable heating load conditions for a mid-size multi-family building in NYC. The heat pump system is being modeled using proprietary code for the heat pump system coupled to EnergyPlus software with different gas cooler variations for constant and then variable loads. This will be used case to optimize the system performance of the heat exchangers and compressor for partial loads, in a hybrid Blackbox setup. Vendor data and thermodynamics analysis are used to simulate the compressor's operation.  Future studies will include lab testing as a final verification of results.

Presenting Author: David Garraway The City College of New York

Presenting Author Biography: I am a 2nd year PhD student in mechanical engineering at The City College of New York. I currently focus on HVAC and sustainable energy systems.

Authors:

David Garraway The City College of New York
S M Abdur Rob The City College of New York
Geoffrey Turbeville The City College of New York
Samuel Chussid The City College of New York
Prathap Ramamurthy The City College of New York
Jorge Gonzalez-Cruz University at Albany