Designing a Heat Exchanger with Four Fluid Streams
“Essentially, we were able to design a heat and mass exchanger with four fluid streams coming into thermal and mass transfer contact,” Kozubal said. “We did this in a manner such that none of these streams became mixed with another.” This was no simple task, and it was the ability to use membranes to contain the liquid desiccant that enabled such a design.
“It wasn’t until advances in membrane technology and careful thermodynamic modeling and design that Eric was able to come up with a method to cheaply and efficiently build such an air conditioner,” Judkoff said. There is a ticklish problem called droplet carryover, in which some of the corrosive desiccant gets entrained in the air. That air gets into the duct work and corrodes it. It can also corrode metal fan blades, and in rare cases, structural steel. DEVAP’s membrane technology solves this problem.
Once the air is sufficiently dried out, a clever indirect evaporative heat exchanger design allows it to be cooled down enough to cool a building. What comes out is air as dry and cool as the air in Colorado on a nice fall day.
Air conditioning currently accounts for 15% of all electricity use in the United States, and can be as much as 70% of use during hot summer days. DEVAP’s first iteration will be for the commercial market; later, it is expected to enter the residential realm.
NREL enlisted two companies, AIL Research and Synapse, as partners to build prototypes. The final R&D 100 award winning device incorporated ideas from each.
“We knew we couldn’t just slap on any indirect evaporative cooler off the shelf,” Kozubal said. “We needed an evaporative cooler that could reduce temperature below the wet-bulb temperature, minimize water usage, and purge air.” And they needed to maintain a size and weight similar to conventional rooftop air conditioners for the entire DEVAP package.
Cooling Air with Heat
To cool the air that will enter the building (called the supply air) from the DEVAP system without adding moisture, Kozubal developed a counterflow indirect design. A small amount of the dry supply air is bled off and fed through an adjacent air channel flowing in the opposite direction to the supply air. That channel has wetted surfaces that cool this “bled-off” air via evaporation. The cold, wet channel then cools the adjacent dry supply air via conduction through the channel walls. The supply air is cooled to about 55 degrees Fahrenheit while remaining dry.
To complete the cycle, the liquid desiccant must be dried out to remove the water it absorbed. To accomplish this, heat is added to the desiccant in another specially designed heat and mass exchanger called a regenerator. The heat drives the water out of the desiccant into an air stream that carries away the moisture and vents it to the outside.
“This air conditioner works by adding heat!” Kozubal said. “We can use natural gas, solar heat, or waste heat from many industrial processes to drive an air conditioner.”
“I can foresee a time when this approach replaces most air conditioning in the world,” Judkoff said.
DEVAP is an Energy Miser, Too
The other huge advantage of DEVAP is that it is an energy miser. A traditional commercial A/C uses 25% of its energy removing humidity, 75% dropping the temperature.
By contrast, DEVAP only uses energy for that first step — removing humidity. The second step is achieved simply by adding a little water.
“As a cooling process, evaporative cooling is incredibly efficient,” Judkoff said. “The fact that we have to put a little energy into drying the air is more than made up for out of the efficiency of evaporative cooling. Especially compared to typical A/C, where you have to use electricity to compress the working fluid.”
Numerous Advantages over Typical A/C
According to Kozubal, aside from large energy savings, DEVAP has several other advantages over conventional cooling, including:
- There is no need for environmentally damaging working fluids such as the chlorofluorocarbons, hydrochlorofluorocarbons, or hydrofluorocarbons used in vapor compression systems.
- The working fluids in DEVAP are environmentally benign: water and a strong salt solution for the desiccant.
- DEVAP allows independent control of temperature and humidity, something that is not possible with conventional A/C unless an expensive overcooling and reheating process is employed.
- There is no need for a compressor or large amounts of expensive copper coils.
- DEVAP contains fewer moving parts in the form of simple low-pressure pumps and fans.
- Vapor compression A/C has been incrementally improved for over 100 years, so there are few low-cost energy improvements left for that technology.
- As efficient as DEVAP already is, there is lots of “thermodynamic room” for cost-effective efficiency improvements.
A typical air conditioner cools the air and dehumidifies it all at once, but not in a controlled way. The limit to how much drying can be achieved depends on how much water condenses on the evaporator coils as the air passes through.
This so-called “wet-bulb limit” is the reason typical evaporative coolers either can’t cool things down enough or can’t create a truly comfortable space when there is a lot of heat and humidity in the air.
By contrast, DEVAP can provide cooling in any climate. The first stage wrings out all the moisture in the air. Doing so lowers the effective temperature limit the indirect evaporative cooler can achieve. It has a wet-bulb effectiveness of 125% — a huge boon compared to most current technology that has tried to get as close as it can to 100%.
“Developing a brand-new thermodynamic cycle and an apparatus to accomplish this process was difficult,” Kozubal said. Conceptualizing this new device required a lot of ingenuity and breakthroughs in materials to become a reality.
“It took a lot of pondering. I spent a lot of time looking up at the ceiling.”