An electric vehicle (EV) can use as much as 30% or 40% of its battery charge just to heat or cool the passenger cabin. That means coming up way short if you are driving 100 mi (161 km) in a Minnesota winter or an Arizona summer and want to stay comfortable on the trip.
Planners at the Advanced Research Projects Agency-Energy (ARPA-e) who are tasked with promoting the adoption of EVs in the U.S. know that EVs will remain range-challenged for the foreseeable future, so they have been funding development of “transformational energy ideas” to attempt to remedy this abiding shortcoming. Several R&D projects are under way that could deliver a separate cabin climate-control system that would maximize EV driving ranges because they would not need to eat into mileage.
Several of these projects are targeting development of a “thermal battery”—a heat supply and storage unit—that could replace conventional HVAC systems in EVs so users can run the air-conditioning and heating systems independently of battery power. These thermal batteries would be recharged overnight in garages with electrical heaters about 5000 times during a vehicle’s lifetime.
If the new technology turns out to be sufficiently energy-efficient, compact, lightweight, and low-cost, it may eventually allow electrochemical EV battery packs to be downsized, reducing their substantial size, weight, and cost. Hybrids and even future conventional cars and trucks could also benefit from thermal batteries, which might enable the use of smaller engines.
Thermal battery options
“A few years ago ARPA-e funded different technology options for the thermal battery, including phase-change materials, new refrigerants, adsorption of water and other liquids, and metal hydrides,” said Zak Fang, professor of metallurgical engineering and powder metallurgy specialist at the University of Utah. (See: “Adsorption-based thermal batteries could help boost EV range by 40%,” http://articles.sae.org/12376/)
The Utah-led research team, which includes material chemists as well as metallurgical, thermomechanical, and automotive engineers, is nearing the middle of a three-year $2.67-million research project. The project's resulting thermal battery technology is to be tested by General Motors engineers.
Fang heads the research team along with Utah professor Kent Udell, a thermomechanical engineer who leads the design and prototyping efforts, while John Vajo of HRL Laboratories in Malibu, CA, is fine-tuning the advanced metal-hydride materials that release and absorb heat when they react with hydrogen gas. Vajo has long studied metal-hydride chemistry at HRL, though in years past predominantly as a potential fuel-storage system for hydrogen fuel cells.
More inherent energy in metal hydrides
“Broadly speaking, metal hydrides have an advantage over the others because the exo- and endothermic chemical reactions they undergo contain substantially more energy than those, say, in phase-change or adsorption processes,” Fang noted. “Considerable heat is released or absorbed in a reversible way, so theoretically we have a step up if we can make it work as hoped.”
Metal hydrides store the heat associated with the formation enthalpy of the metal-to-hydrogen atomic bonds; the heat is released when the bonds form and heat is stored when the bonds break.
He added that using metal hydrides for thermal applications is “not a new concept. It had been studied for use in heat pumps and upgrading [the temperature of] waste heat, for example. The main thing is that it can provide the energy density both the weight and volume that’s needed for this purpose.”
Fang’s group has built and run a small-scale “cell-level” prototype of the device that featured two beds of powder metals to react with hydrogen gas.
“The prototype operated as expected in lab tests,” he said. “We demonstrated the target temperatures for heating and cooling, the range, and cycled it successfully.”
Now the researchers are searching for a pair of metal-hydride materials that operate with a suitable differential vapor pressure to release and absorb hydrogen at the right temperatures for heating and cooling internal cabin air. These hot and cold beds could be coupled together and packaged to work seamlessly and operate as one unit, he said.
Finding materials with sufficient energy density and adequate thermodynamic and kinetic properties is challenging, though. The process is one of engineering design, modeling, optimization (maximization of efficiency), fabrication and laboratory short cycle testing, long cycle testing of prototypes, and validation of the model systems.
“It’s a work-in-progress” that involves the synthesis of carefully selected and matched pairs of metal-hydride materials, characterization of their physical and chemical properties, and tests of their capacity to perform well over an extended period of time, Fang explained.
“We need a custom design that has to satisfy multiple considerations,” such as the ability to deal successfully with relatively extreme ambient temperatures. “It has to work at -10º” and remain safe when users recharge them with heaters. “It can’t run at too high a temperature,” he said.
The team is concentrating on magnesium-based hydride materials for the hot bed and transition metal alloys for the cold bed, Fang reported. Another consideration in dealing with these metal powders is heat transfer. The powders feature the high-surface areas that speed the hydrogen reaction along, but thermoconductivity within the individual powder grains is relatively poor, so the researchers are investigating ways to supply heat pathways out of the materials so it may be harvested effectively and efficiently.
“Next year we’re hoping the latest prototype systems will be tested in a GM simulation/testing lab, though it probably won’t be ready to put in a car; it could be testing in a simulation rig,” Fang said.
“We’re looking at other chemical reactions beyond metal hydrides,” he added, but declined to specify them.
The metal-hydride thermal battery concept could have merit for solar energy applications as well, said Fang’s research colleague Udell, who was quoted in Continuum, a University of Utah publication: “You’ve got to figure out some way of storing energy so you can get past those cloudy, dark winter days.”
Energy storage is a key to being able to make the transition to renewable energy, and thermal energy storage is just as important as electrical energy storage. “There’s been a lot of money put into electric batteries, but not as much put into the idea of storing energy as heat,” he said.