Session: 30-07 Heat Pumps

Submission Number: 178345

Ocean Source Trans-Critical CO2 Heat Pump Cycle Using Rotary Gas Pressure Exchanger for District Heating 

Heat pumps with CO2 as the working fluid provide unique advantages. First, it provides significant thermodynamic advantage over natural gas fired boilers in terms of energy consumed per unit heat supplied. Secondly, it facilitates transition away from global warming hydrofluorocarbon (HFC) refrigerants as well as from environmentally harmful hydro-fluoro-olefin (HFO) refrigerants towards natural refrigerants with ultra-low global warming potential. Ocean water as thermal source for CO2 heat pump provides further benefits in low temperature climates by allowing higher evaporator pressures and thus increasing cycle efficiency. Thermal inertia of the ocean also helps keep evaporating pressures within a narrow range.  However due to non-polar nature of CO2 molecule, the saturation pressure and thus the heat rejection pressure for CO2 is much higher compared to that for HFCs, resulting in higher compression power requirement per unit heat delivered in CO2 heat pumps, thus reducing its energy efficiency (COP) compared to HFC based cycles. Further, the load return temperature has a significant influence on the amount of flash gas produced after expansion and hence COP drops at high load return temperatures. Exergy and second law analysis for the CO2 heat pump cycle indicates largest fraction of exergy destroyed during the throttling process over a high differential pressure between heat source and heat sink. To increase the exergetic efficiency of the cycle, this paper presents some novel cycles using rotary gas pressure exchanger (PXG), which allows expansion work recovery using direct fluid-to-fluid contact acoustic pressure exchange between high pressure supercritical CO2 at the exit of heat sink and low pressure gaseous CO2 from flash tank or from the exit of the heat source. PXG facilitates both, the compression of low pressure gaseous CO2 and expansion of high pressure supercritical CO2 using acoustic waves generated in a compact high speed axially ducted rotary machine. Thus PXG can compress up to 30% of the total system mass flow without consuming any external mechanical or electrical energy but through the expansion work recovery. The paper presents the cycle analyses for various architecture comparing efficiency improvement provided by PXG in a 10 MW scale district heating application using ocean water as the low temperature thermal source. Second law analysis and the increase in exergetic efficiency of the cycle enabled by PXG will be presented. The paper will also present the experimental data on trans-critical compression-expansion performance achieved by PXG at conditions relevant to district heating. Test results demonstrate more than 95% pressure recovery coefficient and up to 20% mass boost ratio. Effect of monthly variation in ocean temperature on cycle efficiency and on reduction in carbon footprint will be presented. Lastly, the techno-economic analysis and the de-carbonization potential of PXG integrated trans-critical CO2 heat pump systems for large scale district heat in Europe and USA will be presented.

Presenting Author: Azam Thatte Energy Recovery, Inc.

Presenting Author Biography: Dr. Azam Thatte is the Chief Scientist at Energy Recovery, Inc. where he leads the fundamental research in developing novel sustainable energy systems like Trans-Critical CO2 Refrigeration & Heat Pumps, Supercritical CO2 Power Cycles and Turbomachinery, Thermal Energy Storage, Geothermal Power Generation and Seawater Reverse Osmosis Desalination. Dr. Thatte is the inventor of the revolutionary PXG technology (Trans-Critical Rotary Pressure Exchanger), which recently won the Innovation of the Year award from ATMO for fundamentally transforming the low global warming refrigeration systems and improving their efficiency significantly. Previously Dr. Thatte was a lead scientist at GE’s Research Labs, where he led development of the next generation aircraft engines like LEAP and GE 9X and several gas turbine technologies. As the P.I. on U.S. Dept. of Energy’s PREDICTS program, Dr. Thatte led the development of the first megawatt scale supercritical CO2 turbine in the world. As a part of the NASA team, Dr. Thatte has also made the first discovery of an organic molecule outside our solar system. He is the author of more than 40 journal and peer reviewed conference publications and has more than 30 U.S. and international patents to his name. Dr. Thatte is the recipient of the Young Scientist award from STLE, Paul Cook Innovation award from Energy Recovery, Inc. and an invention medal from GE. Dr. Thatte received his Ph. D from Georgia Tech and was an invited scholar at MIT.

Authors:

Azam Thatte Energy Recovery, Inc.
Choon Tan Massachusetts Institute of Technology