Session: 30-08 Heat Pumps

Paper Number: 122716

122716 - Integration a Rotary Pressure Exchanger in Transcritical CO2 Refrigeration System: Modeling and Annual Energy Saving Estimation 

This paper highlights application of a rotary gas pressure exchanger (PX) to increase efficiency of transcritical CO2 refrigeration systems. It uses high pressure discharge of the condenser/gas cooler as a motive fluid to compress the low-pressure flash gas with minimal external energy input, thus reducing the power consumption of the main compressor. PX simultaneously expands the high-pressure supercritical/liquid CO2 into a two-phase CO2 ready for heat absorption, thus also emulating the functionality of a typical high-pressure expansion valve. This paper discusses multiple CO2 refrigeration system architectures utilizing PX along with multiple techno-economical scenarios. Annualized work savings are presented for various geographical locations with wide variety of temperature patterns in North America and USA. Field data from a few installations is presented which illustrates PX can save + 30% power at temperatures of + 40⁰ C. Therefore presence of the PX can allow the system to run more frequently at extreme temperatures, and lower the probability of shutting down the store on hot summer days during heat waves. One unit of PX can roughly add about 5.4 m³/hr or 20% capacity increase for a typical 150 kW store.

An architectures utilizing application of PX as a mechanical subcooler is presented. A part of the main gas cooler flow is bypassed and expanded and used for exchanging heat and subcooling the flow entering the high pressure inlet port. The bypass flow that gets heated is sent to the low pressure inlet port. Exit flow from low-pressure and high-pressure streams ends up in the receiver after the high-pressure flow rejects heat through an auxiliary gas cooler. System modeling involves calculation of thermodynamic states at various nodes of the systems satisfying mass and energy balances, along with estimation of various key parameters such as flow-rates, compressor work, receiver quality, etc. Assumptions associated with modeling an ideal PX are presented, which include constraints on mass flow arising from internal physics of the pressure exchanger and density ratio between low-pressure inlet and high-pressure inlet ports. System of equations is solved for PX based system as well as baseline valve (HPV) based system to illustrate increase in Coefficient of Performance (COP) and reduction in compressor power due to inclusion of PX in the CO2 refrigeration system. Key parameters such as quality in receiver, flow rates in the system are studied.

Multiple sensitivity studies are conducted to assess the effect of key parameters. Energy savings due to PX increase with increasing ambient temperature, until the flow capacity is reached, after which the energy savings reach an asymptote. Sensitivity to store size is also studied to understand the right combination of flow capacity, number of PX units for a given store size and give geography. Effect of adding PX on increase in compressor capacity is also quantified, which enables the rack to operate at higher temperatures during extreme events such as heat waves. Other parameters such as higher proportion of low-temperature load, can lead to overall higher savings, but a lower amount of percentage saving. Effect of annual temperature profiles across multiple regions in USA and Europe is studied to highlight the effect of weather patterns on energy savings. Effect of time spent at different temperature buckets and its relative contribution on energy savings enabled by PX is highlighted.

This paper illustrates the significance of adding PX into a CO2 based refrigeration system, and provides sensitivity studies of energy savings with various system parameters and geographical locations.

Presenting Author: Neelesh Sarawate Energy Recovery Inc

Presenting Author Biography: Dr. Neelesh Sarawate is R&D Manager for Systems Modeling team in Energy Recovery, Inc. Prior to that, Dr. Sarawate worked in GE Global Research for 13 years. Dr. Sarawate’s research areas span across Turbomachinery Efficiency & Durability improvements for Aerospace & Energy Systems, Sealing technologies, Advanced Materials, CO2 Cycles, System Modeling and Machine Learning. Prior to starting professional career, he graduated with Ph.D. in Mechanical Engineering from The Ohio State University. Dr. Sarawate has authored over 25 journal and conference publications, and has filed over 50 patents, with 25+ granted patents.

Authors:

Neelesh Sarawate Energy Recovery Inc
Arnav Deshmukh Energy Recovery Inc
Azam Thatte Energy Recovery Inc