Chanwoo Park

Dynamic Thermal Model of Li-Ion Battery for Predictive Behavior in Hybrid and Fuel Cell Vehicles

Li-Ion battery is attractive for HEVs and FCEVs because of its high power density and lack of memory effect. However, high battery temperatures during operation result in a short battery lifespan and degraded performance.To address this issue, battery manufacturers and OEMs have used different pre-set cooling strategies. Unlike the pre-set cooling strategy this thermal model forecasts battery temperatures, allows a better usage of the battery system, responds to battery power demand and maintains battery temperature limits. This paper discusses the real-time control of the battery cooling including battery stress analysis. The authors present a dynamic thermal model for the Li-Ion battery system using the finite-volume method and discuss transient battery thermal characteristics and real-time battery cooling control under various battery duty cycles. Validation results of the model are presented in this paper.

Evaporation-Combustion Affected by In-Cylinder, Reciprocating Porous Regenerator

An existing in-cylinder thermal regeneration concept for Diesel engines is examined for the roles of the porous insert motion and the fuel injection strategies on the fuel evaporation and combustion and on the engine efficiency. While the heated air emanating from the insert enhances fuel evaporation resulting in a superadiabatic combustion process (thus increasing thermal efficiency), the corresponding increase in the thermal NO x is undesirable. A two-gas-zone and a single-step reaction model are used with a Lagrangian droplet tracking model that allows for filtration by the insert. A thermal efficiency of 53 percent is predicted, compared to 43 percent of the conventional Diesel engines. The optimal regenerative cooling stroke occurs close to the peak flame temperature, thus increasing the superadiabatic flame temperature and the peak pressure, while decreasing the expansion stroke pressure and the pressure drop through the insert. During the regenerative heating stroke, the heated air enhances the droplet evaporation, resulting in a more uniform, premixed combustion and a higher peak pressure, thus a larger mechanical work.

Dynamic behavior of heat and hydrogen transfer in a metal hydride cooling system

An experimental study has been carried out to investigate transient transport processes of hydrogen and heat between two coupled reactors in a metal hydride cooling system. This problem is of particular interest in the design of metal hydride thermal energy conversion systems, such as refrigerators, heat pumps, and thermal storage systems. A pair of hydride reactors are designed to extend the contact surface between hydride materials and flowing hydrogen for fast kinetics. Dynamic correlations of pressure-temperature and temperature-concentration are investigated in a typical operational condition. The amount of hydrogen gas transferrable between the paired metal hydrides is measured and the optimum value of the charged hydrogen amount is found for the maximum hydrogen transfer.

Combustion-Thermoelectric Tube

In direct combustion-thermoelectric energy conversion, direct fuel injection and reciprocation of the air flowing in a solid matrix are combined with the solid-gas interfacial heat transfer and the solid conduction to allow for obtaining superadiabatic temperatures at the hot junctions. While the solid conductivity is necessary, the relatively large thermal conductivity of the available high-temperature thermoelectric materials (e.g., Si-Ge alloys) results in a large conduction loss from the hot junctions and deteriorates the performance. Here, a combustion-thermoelectric tube is introduced and analyzed. Radially averaged temperatures are used for the fluid and solid phases. A combination of external cooling of the cold junctions, and direct injection of the fuel, has been used to increase the energy conversion efficiency for low thermal conductivity, high-melting temperature thermoelectric materials, The parametric study (geometry, flow, stoichiometry, materials) shows that with the current high figure of merit, high temperature Si 0.7 Ge 0.3 properties, a conversion efficiency of about 11 percent is achievable. With lower thermal conductivities for these high-temperature materials, efficiencies about 25 percent appear possible. This places this energy conversion in line with the other high efficiency, direct, electric power generation methods.

Performance Evaluation of a Pump-Assisted, Capillary Two-Phase Cooling Loop

A hybrid (pump-assisted and capillary) two-phase loop (HTPL) is experimentally investigated to characterize its thermal performance under stepwise heat input conditions. An integration of mechanical pumping with capillary pumping is achieved by using planar evaporator(s) and a two-loop design separating liquid and vapor flows. The evaporator(s) use a sintered copper grooved wick bonded with a liquid screen artery. No active flow control of the mechanical pumping is required because of the autonomous capillary pumping due to the self-adjusting liquid menisci to variable heat inputs of the evaporators. Unlike other active two-phase cooling systems using liquid spray and microchannels, the HTPL facilitates a passive phase separation of liquid from vapor in the evaporator using capillary action, which results in a lower flow resistance of the single-phase flows than two-phase mixed flows in fluid transport lines. In this work, a newly developed planar form-factor evaporator with a boiling heat transfer area of 135.3 cm 2 is used aiming for the power electronics with large rectangular-shaped heat sources. This paper presents the experimental results of the HTPLs with a single evaporator handling a single heat source and dual evaporators handling two separate heat sources, while using distilled water as the working fluid for both cases. For the single evaporator system, the temperature results show that the HTPL does not create a big temperature upset under a stepwise heat load with sudden power increases and decreases. The evaporator thermal resistance is measured to be as low as 0.5 K cm 2 /W for the maximum heat load of 4.0 kW. A cold-start behavior characterized by a big temperature fluctuation was observed at the low heat inputs around 500 W. The HTPL with dual evaporators shows a strong interaction between the evaporators under an asymmetric heat load of the total maximum heat input of 6.5 kW, where each evaporator follows a different heat input schedule. The temperatures of the dual-evaporator system follow the profile of the total heat input, while the individual heat inputs determine the relative level of the temperatures of the evaporators.

Two-phase flow cooling for vehicle thermal management

Armys next generation vehicles require more electric and electronic devices with increasing power density for improved multi-functionality. The increasing waste heat from these devices will present great challenges to the capabilities of conventional air/liquid cooling systems in cooling multiple, high heat flux sources dispersed over the entire vehicle. In this paper, a high performance hybrid loop thermal bus technology for vehicle thermal management is presented. The technology combines the robust operation of pumped two-phase flow cooling with the simplicity of capillary flow management. The test results show the hybrid loop thermal bus can manage multiple high heat flux heat sources during the startup and transient heat input operation with no flow control.

Reciprocating Battery Cooling for Hybrid and Fuel Cell Vehicles

Traction batteries for hybrid and fuel cell vehicles must maintain temperatures within operational limits for longer battery lifetime and better performance. The uneven battery temperature due to improper heat transfer during discharging/charging could accumulate battery degradation on hot cells resulting in early failure of the battery pack. Current battery systems use a unidirectional coolant flow for battery thermal management. However, due to the nature of the cooling method, the unidirectional cooling systems are prone to show a largest temperature differential ΔTs between the battery cells at fixed flow boundaries, although sophisticated thermal/fluid designs are implemented to make the battery temperature uniform. Here, an innovative battery cooling method ([1]) using a reciprocating cooling flow is proposed. The reciprocating cooling system switches the coolant flow direction periodically by a valve-fan mechanism. By switching the flow direction periodically and thus the cold and hot boundaries of the battery cooling system, the battery cell temperatures are regulated with a very small fluctuation and the temperature differential ΔTs is drastically reduced. In hybrid electric vehicle and fuel cell vehicle applications, the cooling improvement using the new concept would set battery cooling system free of auxiliary air-conditioning system. Parametric study shows that using the reciprocating cooling system for a Li-Ion battery system, an optimum reciprocating period to minimize temperature differential ΔTs and maximum battery temperature Ts,max exists.Copyright

Hybrid Loop Cooling of High Heat Flux Components

*† ‡ This paper discusses the development of an advanced hybrid loop technology that incorporates elements from both passive and active loop technologies. The result is a simple yet high performance cooling technology that can be used to remove high heat fluxes from large heat input areas. Operating principles and test results of prototype hybrid loops are discussed. Prototype hybrid loops have been demonstrated to remove heat fluxes in excess of 350W/cm 2

Vapor Compression Hybrid Two‐Phase Loop Technology for Lunar Surface Applications

NASAs vision for Space Exploration that would return humans to the Moon by 2020 in preparation for human explorations of Mars. This requires innovative technical advances. The lunar mission requires a temperature‐lift (heat pump) technology to reject waste heat to hot lunar surface (heat sink) environments during lunar daytime. The lunar outpost and Lunar Surface Access Module (LSAM) to operate anywhere during the hot lunar daytime require a high performance and energy‐efficient, yet reliable refrigeration technology. A vapor compressor‐driven hybrid two‐phase loop was developed for such high temperature‐lift applications. The vapor compression loop used an advanced porous wick evaporator capable of gravity‐insensitive capillary phase separation and excess liquid management to achieve high temperature‐lift, large‐area, isothermal and high heat flux cooling capability and efficient compression. The high temperature lift will allow the lunar surface systems use compact radiators by increased heat rejection...

Spacecraft Thermal Management Using Advanced Hybrid Two‐Phase Loop Technology

The paper discusses an advanced hybrid two‐phase loop technology for spacecraft thermal management. The hybrid loop integrates active mechanical pumping with passive capillary pumping promising a reliable yet high performance cooling system. The advanced evaporator design using porous wick structures was developed for the hybrid loop to enhance boiling heat transfer by passive phase separation. The prototype testing using various hybrid loops and components demonstrated that the hybrid loop was capable of removing high heat fluxes from multiple heat sources with large surface areas up to 135 cm2. Because of the passive capillary phase separation, the hybrid loop operation doesn’t require any active flow control of excess liquid in the evaporator, even at highly transient and asymmetrical heat inputs. These performance results represent significant improvements over state‐of‐the‐art heat pipes, loop heat pipes and evaporative spray cooling devices in terms of performance, robustness and simplicity.