Interestingly enough, though, is the fact that the greatest impact doesn't come from the refrigerant entering the atmosphere. It comes from the tailpipe emissions and extra fuel burned as a result of powering the A/C system.
You already might have seen some of the changes made in response to the need for greater efficiency. Smaller compressors and smaller system charges take less energy to circulate. Variable displacement compressors can be continuously altered to pump only the refrigerant needed to maintain cabin comfort. Electric compressors reduce the energy load on the engine compared to belt driven compressors. And this is just the start of things to come.
The Ejector Cycle System Evaporator
Think of a conventional expansion valve system for a moment. The compressor works hard to increase the refrigerant pressure and then passes it along to the expansion valve. The valve meters that refrigerant out as a low pressure vapor in the evaporator. All the energy required to push the refrigerant is lost in vaporization.
The DENSO ECS (Ejector Cycle System) evaporator is designed to recover some of that lost energy."The new air conditioning system reduces the compressor's power consumption by up to about 25 percent, compared to conventional car air conditioning systems," says Akio Shikamura with DENSO's Thermal Systems Business Group. DENSO first applied ejector technology in 2003, using the system for truck refrigeration units and other large applications. But it wasn't until recently that the ejector could be made small enough for automotive use.
The ECS evaporator is actually two evaporators in one. One is the primary evaporator and the other is the secondary, or suction, evaporator. Refrigerant flow is the same as a conventional system up to the expansion valve, called a flow control valve in the DENSO system. Here, though, is where differences start.The ECS expansion valve meters refrigerant flow, but does not cause the refrigerant to vaporize. The metered flow then passes by a capillary tube that allows refrigerant to enter the secondary evaporator. The capillary tube further reduces refrigerant flow, but what causes the refrigerant flow is the really interesting part.
The ejector itself is shaped something like a rifle scope, with a fixed orifice at the inlet side. Pressurized refrigerant hits the orifice and its velocity is significantly increased. If the speed of the refrigerant entering the orifice was that of a man walking, then the speed of the refrigerant leaving the orifice would be the same as a jet plane. This creates an area of low pressure in the orifice outlet zone that is used to draw the refrigerant through the capillary and the suction evaporator. The low flow, low pressure through the suction evaporator results in a large temperature drop and this half of the design is the cold half.
The outlet end of the ejector feeds the primary evaporator, much the same as a conventional design. However, there is a mix of refrigerant from the ejector and what already has been drawn through the suction evaporator core. The outlet of the ejector widens, slowing down refrigerant flow and increasing the refrigerant pressure entering the primary evaporator.
Because the pressure entering the primary evap is higher than a normal evaporator core, the temperature required to vaporize also is higher and less heat is absorbed from the surrounding air. The system depends on the combination of a really cold suction core mated to the primary core to provide the cooling requested by the occupants.
The advantage comes from that higher than normal discharge pressure. The pressure differential between what is entering the compressor and what the compressor is putting out is less, requiring less energy to recompress.This design is a first in that it incorporates all the elements needed for an ejector cycle system in a package that can fit in an HVAC case. It is being used in the 2010 Toyota Prius for its efficiency improvement. But it isn't the first time the ejector cycle concept has been used in an automotive application. The 2008 and up Toyota Land Cruiser and Lexus RX570 are offered with an optional console refrigerator that uses a dedicated suction evaporator core, with the primary core located normally in the dash. An ejector meters refrigerant flow between the two in a similar manner. And while the Prius currently is the only model using the unique ECS Evaporator, I suspect it won't be alone for long.
A/C Without a Compressor?
Thermoelectrics is a solid state phenomenon that occurs when a temperature difference is applied to a semiconductor, which then generates electricity. Conversely, if a current is applied to the conductor, it can pump heat in either direction depending on the direction of current flow.
The ability of these devices to create electricity using wasted engine heat could be a real contributor to improving fuel economy, and according to John Fairbanks, technology development manager with the U.S. Department of Energy, we can expect to see thermoelectric generators in use on production cars from BMW, Ford and GM as soon as the 2012 model year.
Currently, though, more than 4 million vehicles use thermoelectric heating/cooling units in their seats, and some models use them to keep beverages hot or cold. Now imagine using similar units in the dash and overhead as well. This concept, called zonal air conditioning, is designed to control the comfort of each individual passenger, separately from the rest. How often do you see a car being occupied by more than one or two people?According to Fairbanks, it would only take about 630 watts of power to cool one occupant, compared to the 3,500 to 4,500 watts it takes to drive a conventional A/C system to do the same thing. The use of thermoelectrics is, he adds, one solution especially appropriate to cabin comfort in plug-in hybrids, electric and fuel cell vehicles.
There are several challenges to implementing such a concept, not the least of which is the cost of the materials used in the manufacture of thermoelectric conductors and the efficiency of the conductors themselves. The Department of Energy's Vehicle Technologies Program is partnering with the National Science Foundation to develop higher efficiency materials. In September 2009, the program announced that Ford and GM had been awarded funds to be used in the development specifically of automotive thermoelectric HVAC.Adsorptive Cooling
Like most modern technology, this is an idea that has been around for a while, just not practical until today. First discovered by Michael Faraday in 1824, adsorptive cooling is a process that works similarly to conventional A/C. The difference is in how the refrigerant is moved through the system.In place of the mechanical compressor, two sorption generators are used. Faraday noticed during his experiments that certain gases would stick to certain materials at low temperatures and then be released at higher temperatures. One generator is cooled to ambient temperature, causing the vaporized refrigerant leaving the evaporator to stick to its material coating. When the first generator is saturated, valves are switched and the second generator takes over. In the meantime, the first generator is heated, releasing the refrigerant and sending it to the condenser.
The heat needed is taken directly from the wasted heat energy leaving the exhaust. Because nearly 40 percent of the energy created in the combustion process escapes this way, powering the system isn't a problem. The problem is the size of the generators that would be needed to provide sufficient cooling — about the size of a small sedan.
Recently, though, a team led by Professor Bob Critoph, who is with the School of Engineering at the University of Warwick, has developed a breakthrough in sorption generator efficiency, allowing them to get the same work out of a generator only 6x6x6 inches in size. They use coolant tubes only 0.3 mm in diameter passing a methanol refrigerant through an activated charcoal absorbent so porous that one gram has a surface area of 3.5 acres. The generators take no mechanical energy away from the engine, and that means increased fuel economy and reduced emissions.
The topic of global warming is a heated one, with vocal advocates on both sides. Regardless of how you personally feel about this issue, I'm sure you'd agree that any step we can take to reduce our use of the world's resources and leave our planet a little cleaner for our kids and grandkids is a good thing. These new technologies will help us do just that, and keep us comfortable in the process.
