Session: 02-01 Heat Pump and Building Waste Heat Utilization

Paper Number: 107045

107045 - Development of Electrified Transcritical R744 Heat Pump Systems for Northeastern Winter Markets 

The building sector is a prime contributor of energy consumption and the emission of Green House Gases (GHGs) and is therefore a good starting place to address the path of decarbonization. The northeastern market of New York City (NYC) recently passed a bill to decarbonize the building sector by 2050, starting in 2027 (aka Local Law 97). With a housing sector of one million buildings mainly consisting of 1-4 family homes, responsible for 52.9 million metric tons of carbon dioxide in 2014 alone, this is a model city to accelerate our sustainability efforts. Fossil fuels used in both space heating and cooling account for 60% of GHG emissions from buildings and 30% of citywide GHG emissions. For NYC approximately 6,000 buildings have adopted cleaner forms of energy, as of 2017. It is further expected that the population of New York City will expand by 100,000 buildings and over 9 million occupants by 2050.

This need for electrification in HVAC systems lends us to dive more into the use of heat pumps (HPs). While moving away from fuel powered systems is a big step forward, it is also as important to consider the refrigerant that is used by these machines. Heat pumps commonly use a variant of hydrofluorocarbons such as R410A or R134 which have high global warming potentials and ozone depletion potentials. Our proposal is to use a natural refrigerant such as carbon dioxide (R744), which has values of 0 and 1, respectively. However, there are challenges in adapting R744 HP technologies in cold climates in the mainstream market namely high-pressure operations that places it in the supercritical zone (>1,069psia), having no obvious distinction between gaseous and liquid states. As such, R744 will have to run in a transcritical cycle where the fluid will be continuously cooled due to being superheated.

The main objective of our project is to develop a commercially feasible R744 HP system for the multi-family building sector for cold climates. However, we foresee various challenges along the way.  In the process of bringing our project to fruition, we plan to address the efficiency for space heating for colder climates as those of the Northeast, while being affordable and attractive to customers with existing gas or oil powered systems. The HP market has been long optimized for milder climates where cooling is the bigger concern. This makes the conversion to electric HPs a major challenge. Our strategy aims to focus on optimizing the system to run on transcritical R744, this primarily means a revamp of various components particularly the gas cooler. Gas coolers have been a challenge in terms of optimization with lower COPs than that of the more commonly used refrigerants. There are many design variations which have been investigated. Currently our research is conducting assessment of the viability of a staggered tube design which allows for more efficient heat transfer between the R744 and air. This is quite important as the refrigerant operates at a much higher temperature (about 220-250F) than seen in hydrofluorocarbon systems. Lowering the outlet temperature on the gas cooler side is a big step in providing a suitable design to be used in both residential and commercial buildings.

A Blackbox model has been designed which allows for the simulation of different design options for optimization of the gas cooler design. A 2.5-ton capacity gas cooler with a COP above 2.7 has been designed in being built for lab testing conditions under simulated outdoor winter conditions. Design data and initial results will be shared in the upcoming technical paper. Additionally, work is being considered to make this system reversible so that we can also provide cooling to the building during the hot summer months, thereby making this a complete system to replace traditional heating and cooling methods.

Presenting Author: David Garraway The City College of New York

Presenting Author Biography: I am an international student from Trinidad and Tobago, currently pursuing a PhD in Mechanical Engineering at The City College of New York.