U.S.-based Electric Power Research Institute (EPRI) has found that replacing a traditional hydronic air-conditioning system with a low-charge ammonia (R717) chiller and CO2 (R744) convection loop can reduce costs, energy use and greenhouse gas emissions.
EPRI partnered with utility company Southern California Edison (SCE) and Japanese HVAC&R manufacturer Mayekawa to develop a prototype and test the concept. Results from the study – “Development and Evaluation of Ammonia Vapor Compression Coupled to a CO2 Convection Loop” – were presented by Ron Domitrovic, Program Manager at EPRI at the 14th IEA Heat Pump Conference in Chicago, May 15–18.
According to the study, the concept was proven to be technically feasible, with the “workability of the system” being successfully demonstrated.
Modeled on supermarket systems
Driven by increasing restrictions on synthetic refrigerants, EPRI’s research focused on how natural refrigerants – particularly ammonia and CO2 – could be used in commercial air-conditioning applications.
“We were looking specifically into replacing chillers that use traditional refrigerants and [water loop systems] in large buildings for air-conditioning,” explained Domitrovic. “The idea was to mimic what’s done sometimes in supermarket systems where you might have an ammonia chiller coupled to a CO2 distributions system.”
“The idea was to mimic what’s done sometimes in supermarket systems where you might have an ammonia chiller coupled to a CO2 distributions system.”
Ron Domitrovic, EPRI
He noted that for use in this application, the project team was looking at elevated pressure, and temperatures for the CO2 loop as such systems are generally used for much lower temperatures.
First-of-a-kind system
The prototype air-conditioning system, which was developed and tested in EPRI’s laboratory in Knoxville, Tennessee, comprises a 28kW (8TR) Mayekawa ammonia chiller for vapor compression, a CO2 convection loop and three indoor remote air handling units (AHUs).
According to Domitrovic, the off-the-shelf chiller was adapted with a heat exchanger to ensure compatibility with chilling CO2 rather than water.
As the system is the first of its kind, not all components were not readily available, he explained.
“There were no appropriate expansion valves, so we used needle valves instead,” he said. “We also had limited confidence in the ability to pump CO2 in these conditions, but we found a [low-temperature] pump that we thought was also appropriate for the higher temperature and pressure.”
Special coils were also made for the AHUs to suit air-conditioning with CO2, he added.
From a safety perspective, the low refrigerant charge – less than 130g of ammonia per kW of cooling capacity (1lb/TR) – combined with the fact the chiller was installed outdoors helps to limit the risk of an ammonia leak, said Domitrovic.
The CO2 loop operates at 31–38bar (450–550psi), and techniques were used to ensure the system could withstand much higher pressures, he added.
“It’s a relatively simple system and relatively easy to install,” said Domitrovic.
Lower costs
Compared to a traditional chiller and water loop system, the use of an ammonia-based chiller and CO2 convection loop offers multiple benefits, said Domitrovic during his presentation. Benefits include reduced costs, increased efficiency, lower GWP and improved flexibility.
Potential costs savings come from reduced spending on piping, installation and energy, all of which are made possible thanks to the higher heat capacity of phase-changing CO2 compared to water, he explained.
Reduced pumping requirements results in a fivefold reduction in energy use, he added.
While the analysis assumes little difference between an ammonia or HFC-based chiller in terms of capital cost per unit of installed capacity, the researchers have said that low-charge ammonia offers a cheaper alternative to high-GWP refrigerants.
“The smaller size of piping required for CO2 convection offers potential for material and installation cost savings that could help to offset any capital premium for this new type of equipment,” said the research team.
“The smaller size of piping required for CO2 convection offers potential for material and installation cost savings that could help to offset any capital premium for this new type of equipment.”
EPRI
Efficiency gains come from the high efficiency of ammonia and the high heat capacity of CO2, which can transfer ten times more heat than water can.
According to Domitrovic, a COP of 2–3 was achieved with little system optimization. This offers the possibility of substantially improving the efficiency of the system in the future, he added.
While the system was only designed and tested for cooling, the research team has said that the concept can be extended to include heating with additional considerations, such as accommodating high-pressure CO2 for heat convection.
