Today’s electric vehicle (EV) batteries can provide only enough power to propel them 100 mi (160 km) or so on a single charge, a shortcoming that often leaves little left over for cooling or warming the passengers. Climate-control systems can reduce an EV’s range by as much as 30%, especially in the summer. And while “range anxiety” continues to put off many potential EV buyers, the prospect of being forced at times to minimize the use of the HVAC system certainly doesn’t help either.
Several research and development projects funded by the Advanced Research Projects Agency for Energy (ARPA-e) are under way to create novel heating and cooling technologies that could lessen the power drain of HVAC systems on already overtaxed EV battery packs. Some of these efforts aim to create “hot-cold systems” based on “thermal batteries” that exploit enhanced thermo-adsorptive effects to replace electromechanical vapor-compression refrigeration technology.
One of these systems takes advantage of new high-capacity adsorbents that can store great quantities of refrigerant in a small space. The coolant is then used for either heating or cooling as required as it cycles through the system. This project is being conducted by a university/industry team of researchers at MIT, the University of California at Berkeley, the University of Texas at Austin, and engineers at Ford. The group received $2.7 million in support a year and a half ago to demonstrate a thermal adsorption-based climate control system capable of delivering both heating and cooling for EVs with minimal use of the electrochemical battery bank.
“A thermal battery charges and discharges much like an electrical battery, except that it provides a temperature difference instead of a voltage difference,” said Evelyn Wang, a professor of mechanical engineering at MIT, who is leading the work. “Our focus is developing a thermal battery with enough energy density to run the HVAC systems in EVs and so help overcome existing range limitations.”
In addition to storing enough energy, the new system must be compact and lightweight enough to fit in electrical vehicles, she said, noting that her team “was working closely with Ford on meeting the EV packaging constraints.” The plan is to test the technology in a Ford Focus EV after the first prototype hot-cold device is completed, which is expected in six to nine months.
“So far, we’ve finalized the design and have started building a prototype that demonstrates the energy density we need,” Wang said. “Next we’ll need to scale up materials production.”
The program is targeting a 2.5-kW device with 2.5 kWh of cooling capacity and 4.5 kWh of heating capacity. The unit should weigh about 35 kg (77 lb) and occupy a volume of 30 L (1.1 ft3). If successful, the technology could potentially extend EV driving range by 30-40%. Such a system—if sufficiently effective—could also work in hybrids and conventional internal-combustion-engine powered cars, not to mention buildings.
Passive heat pump
In the thermal battery system, said Shankar Narayanan, an MIT postdoctoral associate, water from a reservoir “is pumped across a valve into a low-pressure vessel, during which it evaporates and absorbs heat.” This evaporative cooling can be used to lower the temperature of air that is sent into the passenger cabin.
The water vapor is then exposed to a special adsorbent—a high-surface-area material that is entirely shot through with many tiny pores. Each pore has been engineered to have an extremely strong “hydrophilicity,” or affinity for bonding to water molecules, he said. The high-capacity material pulls the vapor out of the container while keeping the pressure low so more water can be pumped in and evaporated.
Heat is released as the material adsorbs the water molecules, Narayanan continued. This heat can then either be used to warm the passenger compartment or, if not needed, extracted by a heat exchanger and dumped into the atmosphere using a radiator.
Unlike a conventional vapor condensation system, the new device should use little electricity because it transports the vaporized refrigerant water from the evaporator to the adsorbent bed place passively—that is, without needing external means of vapor transport or compression, he said. The bed would operate at about 80°C (176°F), while the evaporator would run at 0 or -5°C (32 or 23°F).
At some point the adsorbent will become saturated with water, so the thermal battery system would need to be recharged when the electrochemical battery is charged. Regenerating the adsorbent means heating it to above 200°C (392°F) for several hours, which forces it to release the water. The vapor, desorbed from the bed, then moves on to the condenser, where gas turns to liquid and the water is collected in the reservoir for subsequent cycles.
New adsorbent materials
“The basic concept of the adsorption cycle has been used in large industrial systems, but ours works in a different manner,” Wang said. Existing systems have low energy-conversion efficiencies and are bulky and heavy. New, much better-performing materials are needed to make for a thermal battery that is sufficiently powerful, compact, and lightweight.
Another obstacle to packaging is that most processes require separate containers for evaporating and condensing the coolant, but the team’s streamlined design employs a single container for both purposes. Its monolithic, one-piece condenser/evaporator saves space and brings the vapor flow close to the condensing surface. To provide maximum packing and water storage, layers of adsorbent are mounted on concentric fin tubes that fit around the evaporator/condenser unit.
The MIT researchers have developed a new material that features “really high adsorption capacities” by modifying zeolite, a stable, readily available aluminosilicate material that is microporous. By chemically removing part of the framework of silicon atoms in the zeolite, they have tailored its pore size and boosted its affinity for water, creating a material with strong hybrid physi/chemisorption capabilities. On the down side, it requires 200°C heat to release the water during regeneration.
Because the highly porous mineral does not conduct heat well, the researchers have added thermally conductive carbon-based binder materials to provide highways for heat to escape the bed more rapidly.
The ARPA-e-supported project is also studying so-called designer molecules that could have extremely strong adsorption capacities. Chemistry professor Omar M. Yaghi and his colleagues at UC Berkeley specialize in metal organic framework (MOF) molecules, whose physical and chemical properties can be systematically altered by varying the composition.
“The nice thing about MOFs is that you have a lot of ability to use different linkers to connect up metal clusters,” Narayanan said. Their Tinkertoy-like arrangement “provides lots of flexibility so you can easily change pore size.”
MOFs also could have lower regeneration temperatures, which could enhance cycling efficiency.
But the designer molecules pose their own challenges. Making stable MOFs that resist reacting with water is an issue, and production scalability and costs are less than clear. With further progress in both areas, MOF adsorbents could enable even higher-density thermal energy storage than the modified zeolites.