A new “virtual refrigerant charge sensor" saves energy and servicing costs by indicating when air conditioners are low on refrigerant, preventing the units from working overtime. Engineers at Purdue University developed the sensing technique, which is particularly practical for automotive air conditioners.
Maintaining the proper charge, or amount of refrigerant in a system, saves energy because air conditioners low on refrigerant must operate longer to achieve the same degree of cooling as properly charged units. Automotive air conditioners tend to leak refrigerant more than other types of units.
“Not only does the energy efficiency go down, but you also reduce the lifetime of the unit because it has to work harder, causing parts to wear out faster," said James Braun, a professor of Mechanical Engineering. “It's also very time consuming and costly to have a technician check the refrigerant and charge it up to specification. To accurately learn how much charge is in the system, you have to remove all of the refrigerant and weigh it, a procedure that requires a vacuum pump and is quite time consuming."
The new alternative works by using sensors to monitor the temperature of refrigerant at various points along the tubing in an air-conditioning unit. The technique is easy-to-use, with the sensors simply attached to the outside of the tubing.
Researchers tested the system on various types of air conditioners running on conventional refrigerants, including R-22 and the more environmentally friendly R-410A, which is replacing R-22 in the latest units. The research has been funded by the California Energy Commission through its Public Interest Energy Research (PIER) program.
Another research project at Purdue is nearing completion and has involved a more extensive evaluation of the virtual charge sensor. A software algorithm interprets temperature-sensor data to estimate the amount of refrigerant in the system. Four sensors are attached to tubing running into and out of components called heat exchangers.
In air conditioning and refrigeration systems, liquid refrigerant evaporates in a heat exchanger called an evaporator, cooling the air. The refrigerant vapor turns back into a liquid in another heat exchanger called a condenser. During these evaporation and condensing steps, the refrigerant undergoes dramatic temperature changes.
Automotive air conditioning units equipped with the new refrigerant-charge system could activate a warning light on a car's dashboard, and technicians servicing home air conditioners could simply plug a personal digital assistant into the unit to read the refrigerant-charge information.
The project, also funded through California's PIER program, has been led by graduate student Woohyun Kim. Purdue has applied for a patent on the technique.