Speaking of Hybrid Electric Vehicles...
Hybrid Electric Vehicle Program - Components
Thermal Management of HEV Components
Just as conventional gasoline engines require a cooling system (water/glycol circulating through the engine and radiator), HEVs need proper thermal management of the power unit and energy storage unit for optimum performance and durability. The type of thermal management system required will depend on the type of power unit and energy storage unit(s) selected. In many cases, waste heat from these components can be used for cabin air and other heating needs.
Batteries
Many of the current candidates for power-dense HEV batteries, such as lead-acid, have significant cycle-life problems (most propulsion lead-acid batteries last only about two years). The life can be significantly shortened by allowing the battery to get too hot. For lead-acid batteries, this degradation in cycle life can start as low as 50C (122F). Unregulated battery packs can easily reach 80C (176F) during periods of sustained discharge or rapid charging. On the other hand, batteries in winter climates are exposed to low temperatures which reduce power and energy capacity. Experiments indicate that even at 50 F, only 80% of the battery capacity is available compared to full capacity at 77 F. Battery controllers and power inverters also may need active cooling to maintain performance. A major challenge of using batteries with hybrid vehicles is to identify fundamental and practical solutions for maintaining the battery related equipment within an appropriate operable temperature range over a wide range of ambient driving conditions. NREL has demonstrated the usefulness of variable conductance insulation (VCI) in battery thermal management.
Power Units
Fuel cells offer highly efficient and fuel-flexible power systems with low to zero emissions for future HEV designs. There are a variety of thermal issues to be addressed in the development and application of fuel cells for hybrid vehicles. For example, solid oxide fuel cells potentially offer very high efficiencies and lower cost than PEM or phosphoric acid cells, but run hotter (600C to 1000C). Isolation of this heat from the rest of the vehicle is important not only for improved efficiency, but also passenger safety. Gas turbine engines, particularly ceramic turbines, also operate at high temperatures (up to 1400C), yet have components which must be kept much cooler. Reducing the warmup time of both fuel cells and turbines via thermal management is important to achieve quick power and minimal emissions. More standard power units such as small diesel or spark-ignition engines also need proper cooling.
Exhaust Systems
60% to 80% of emissions in an auto's typical driving cycle comes from "cold start" emissions, that is, pollutants that are emitted before the catalytic converter is hot enough to begin catalyzing combustion products. NREL has shown that its patented variable conductance insulation (VCI) and phase-change heat storage material, can be used to keep the catalyst hot for more than 17 hours, yet allow heat to flow during peak engine loads to prevent the converter from overheating. This would allow 95% of all auto trips to begin with a hot catalyst and little or no cold start emissions. This is particularly important with many HEVs, since their power unit may cycle on and off during a trip.
Fuel System
As emissions standards tighten and exhaust control technologies improve, the issue of evaporative emissions becomes increasingly important. Thermal management of fuel tanks is one approach to reducing these emissions.
Waste Heat Utilization
Heat recovered from any of the above sources can be used in a variety of ways. For winter driving, heat recovery from HV sources such as the power unit exhaust, propulsion motors, batteries, and power inverter can significantly improve cabin warmup. Because of their small power units, hybrid vehicles generally cannot supply enough heat to the cabin via the conventional coolant-to-air heat exchanger. Waste heat can also be converted into electricity via thermophotovoltaic devices.
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The following companies are working with GM on developing Thermal Management Systems for the HEV Program:
DELPHI Harrison - Lockport, NY |
