An improvement in secondary loop technology is key to implementing hydrocarbon-based air-conditioning/heat pump systems in electric vehicles (EVs), according to Stefan Elbel, a Professor and Researcher at Technische Universität (Technical University) Berlin and CEO of U.S.-based Creative Thermal Solutions.
Secondary loop systems are primarily used in large-capacity refrigeration applications, and Elbel noted that the inclusion of glycol as a heat transfer fluid poses a challenge due to its low specific heat and high viscosity at low temperatures. This makes them less efficient because of the low heat transfer between fluids.
Elbel, a researcher of sustainable heating and cooling systems, shared his findings during a presentation on transport market trends at the ATMOsphere (ATMO) Europe Summit 2023. The conference took place September 19–20 in Brussels and was organized by ATMOsphere, publisher of Hydrocarbons21.com.
To address the issues posed by the secondary loop system, Elbel highlighted the potential of an adapted three-fluid heat exchanger, a device that regulates the temperature of engine coolant, transmission fluid and air in cars.
The adapted design integrates a refrigerant with specialized heat transfer fluids – ethylene glycol/water and CO2 (R744)-diluted triacetin – for varying temperature ranges. Triacetin is used to enhance cooling performance because of its lower viscosity at low temperatures, although diluted triacetin still has a low specific heat capacity.
“Addressing the issue of temperature glide, understanding and managing the temperature difference in the refrigeration cycle is vital for the efficiency of these new systems,” said Elbel.
He referred to Japanese OEM Mayekawa’s NewTon as a system that uses a refrigerant (ammonia/R717) coupled with CO2 as a high heat transfer fluid. “This combination ensures a consistent temperature and maximizes the system efficiency with its two-phase heat transfer coefficient.”
Additional options for transportation
Along with secondary loop systems, Elbel also spoke about the potential of the ECOOLTEC transportation refrigeration system from German OEM ECOOLTEC Grosskopf.
“The ECOOLTEC system employs propylene [R1270] externally and CO2 internally in a two-phase heat transfer fluid,” said Elbel. “The key here is thermosyphon technology, which operates efficiently without a pump.”
In a thermosyphon system, a fluid circulates due to the natural convection caused by temperature differences. When the fluid at the bottom of the system is heated, it becomes less dense and rises. As it cools at the system’s top, it becomes denser and sinks. This cycle creates a continuous flow and transfers heat to colder areas without the need for a mechanical pump.
Elbel said he also foresees cryogenic nitrogen storage tanks, nitrogen liquefaction and insulation being used to provide cooling in large shipping applications in the near future.
In addition, PCM (phase change material)-based systems, using materials like paraffin, eutectic mixtures and salts for cooling or heating, can be charged or discharged without having an onboard R290 or R717 refrigeration system, said Elbel. “That makes the transport refrigeration system advantageous, reducing the complexity.”
Elbel also addressed the potential of hydrocarbons like propane and butane (R600) in the refrigeration sector more broadly.
“Hydrocarbons indeed pose a challenge due to their flammability,” said Elbel. “However, the other side of the coin reveals their advantages. The components are relatively inexpensive, and the systems resemble those used for chemical alternatives. From this perspective, transitioning to hydrocarbons is somewhat easier.”
“Addressing the issue of temperature glide, understanding and managing the temperature difference in the refrigeration cycle is vital for the efficiency of these new systems.”Stefan Elbel, a Professor and Researcher at Technische Universität (Technical University) Berlin and CEO of U.S.-based Creative Thermal Solutions