The Toyota Land Cruiser may be thought of as just a rugged hunk of off-roading SUV. But the 2008 model has a Denso air-conditioning system with the first automotive use of ejector-cycle technology, an advancement that provides a low-cost way to improve energy efficiency.
The ejector is a no-moving-parts pumping device that uses energy otherwise wasted by the expansion valve. As liquid refrigerant from the high-pressure side of the system passes through the expansion valve and vaporizes in the low-pressure side, that high-pressure energy is simply dissipated as the refrigerant expands and absorbs heat in the evaporator. If recovered, some of this high-pressure refrigerant energy could be used to reduce work by the compressor that produced the pressure in the first place, which would reduce overall energy consumption. Or it could perform a new function, as it does in the Land Cruiser: provide two temperature zones with a continuous refrigerant flow. The Land Cruiser has an optional refrigerator, or “cooler box,” which operates at lower temperatures than the vehicle A/C.
Various piston, scroll, and screw-type recovery devices called expanders have been designed and demonstrated to recover energy wasted as refrigerant depressurizes during the A/C operating cycle. They have even been tried in commercial refrigeration, but their cost-benefit equation to date has not been sufficiently favorable for automotive application.
Different than an expander, the ejector is a uniquely shaped pipe that also produces a pumping action, vs. energy-wasting vortices that form when refrigerant vaporizes as it comes out of a conventional expansion valve.
Denso also is developing an automotive A/C system with an ejector to replace the expansion valve and provide a pumping assist to reduce the energy use of the compressor. At present, the ejector also is used to reduce compressor work in some Japanese commercial refrigeration systems.
In the Land Cruiser, however, a special expansion valve is installed at the inlet of the ejector, but it just reduces pressure and regulates the mass flow of high-pressure liquid refrigerant directly into the ejector. The refrigerant flows through the ejector and exits the ejector’s nozzle as a high-speed jet spray, which creates a low-pressure area at the nozzle outlet. The low-pressure area draws refrigerant vapor from the evaporator outlet of the refrigerator.
The ejector spray next produces a mixing action with the vapor flow from the evaporator outlet of the refrigerator, in the straight pipe section of the ejector, where the pressure of the mixing refrigerant begins to rise. The flow of mixed refrigerant continues from the straight pipe into the diffuser, a tapered-wider section of the ejector pipe. Here the velocity of the mixed refrigerant slows, but the pressure continues to increase. The mixed refrigerant then flows into and through the passenger cabin evaporator, absorbs heat for cabin cooling, and returns to the compressor.
Establishing dual temperature zones could also be done with “reheat” (in which both zones are cooled to the lowest required temperature, with one of the zones, the cabin, being reheated to a desired higher temperature), but that would waste energy vs. an ejector. Yes, reheat has been the primary method for cabin comfort adjustment for many years, and is the basis for multi-zone cabin A/C.
However, computer-controlled changes in compressor displacement and special cycling strategies for fixed-displacement compressors are coming into wide use to adjust A/C cooling to match the temperature load in the cabin. These engineering changes increase A/C efficiency by reducing use of reheat. Further, with the ejector, temperatures in the Land Cruiser refrigerator may be maintained at as low a level as desired, independent of A/C cooling loads.
Denso has not announced an anticipated energy-efficiency improvement from its ejector system. The amount of compressor work expended in producing high-pressure refrigerant in automotive A/C systems, which theoretically is available for recovery at the expansion valve, has been estimated at 21% by Sanden. This company, a leading compressor manufacturer, is working to develop a low-cost expander.
The Land Cruiser also improves A/C efficiency with what is called a “sub-cool accelerator.” It is a pipe-within-a-pipe heat exchanger that uses the still-cool vapor from the evaporator outlet to assist cooling of refrigerant flowing from the condenser, and is being installed on many Denso systems.
A/C systems use an estimated 5% of the gasoline consumed by U.S. motor vehicles, according to the U.S. EPA, and making major reductions is the objective of engineering groups formed by SAE International and manufacturers worldwide.