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The National Renewable Energy Laboratory (NREL) has initiated projects to investigate the benefits and design challenges of using heat pipe/two-phase flow technologies to provide sub-system cooling and thermal management in future advanced vehicles, hybrid electric vehicles, and heavy-duty vehicles. Projects initially focused on vehicle instrument panel (IP) heat pipe cooling and passenger seat thermal management, but will also investigate engine cooling, electric motor cooling, and battery cooling in the future. Experimental results have demonstrated IP surface temperature reductions of 20°C-30°C during maximum solar intensity environments of 525-800 W/m2 (typical of Golden, CO from January to April) compared to uncooled conditions. The heat pipe cooling effect in the IP also reduced windshield temperatures by 9°C-12°C compared to the non-cooled configuration in April 2001 testing.

The energy used to air condition an automobile has a significant effect on vehicle fuel economy and tailpipe emissions. If a small reduction in energy use can be applied to many vehicles, the impact on national fuel consumption could be significant. The SCO3 is a new emissions test conducted with the air conditioner (A/C) operating that is part of the Supplemental Federal Test Procedure (SFTP). With the 100% phase-in of the SFTP in 2004 for passenger cars and light light-duty trucks, there is additional motivation to reduce the size of the A/C system. The U.S. Department of Energy's National Renewable Energy Laboratory (NREL) is investigating ways to reduce the amount of energy consumed for automobile climate control. If the peak soak temperature in an automobile can be reduced, the power consumed by the air conditioner may be decreased while passenger comfort is maintained or enhanced. Solar reflective glass is one way to reduce the peak soak temperature.

This paper describes the need for dynamic (transient) simulation of automotive air conditioning systems, the reasons why such simulations are challenging, and the applicability of a general purpose off-the-shelf thermohydraulic analyzer to answer such challenges. An overview of modeling methods for the basic components are presented, along with relevant approximations and their effect on speed and accuracy of the results.

The National Renewable Energy Laboratory (NREL) has developed a transient air conditioning (A/C) system model using SINDA/FLUINT analysis software. It captures all the relevant physics of transient A/C system performance, including two-phase flow effects in the evaporator and condenser, system mass effects, air side heat transfer on the condenser/evaporator, vehicle speed effects, temperature-dependent properties, and integration with a simplified cabin thermal model. It has demonstrated robust and powerful system design optimization capabilities. Single-variable and multiple variable design optimizations have been performed and are presented. Various system performance parameters can be optimized, including system COP, cabin cool-down time, and system heat load capacity. This work presents this new transient A/C system analysis and optimization tool and shows some high-level system design conclusions reached to date.

An AMTEC cell optimization study investigated various cell design performance tradeoffs for an AMTEC cell operating in a 4-GPHS (General Purpose Heat Source) ARPS using 16 AMTEC cells per system. The design objective was to generate 141 watts at beginning-of-mission (BOM), 112 watts at 6-year end-of-mission (EOM), and 99 watts at 14-year EOM from the 4-GPHS/16-cell system at a system voltage of 28 volts (cell voltage 3.5 volts). Cell performance predictions on a system-level compared the effect of BASE tube number, BASE tube sizing, electrode performance parameters, thermal shield design, and condenser emissivity on cell-level and system- level performance. The selected reference cell design is 2 inches diameter, 4 inches length using electrodes characterized by a B=120 A-K1/2/m2-Pa and G=10, one cylindrical and 21 conical thermal radiation shields, and eight BASE tubes having a 0.40 inch diameter and 1.0 inch active length.

The alkali metal thermal to electric converter (AMTEC) technology has grown tremendously over the past 10 years. Innovations such as wick pumping, remote condensing and multi-tube cells have advanced the technology substantially. New innovations have been identified that will further advance the technology. The internal selfheat pipe (ISHP), for example, is predicted to increase the efficiency by a full 3 points. Proof-of-principle tests have been completed, and prototype testing is about to begin. This paper will present the design concept and available test data for this new AMTEC innovation.

An Advanced Radioisotope Power System (ARPS) in the 150 watt class is currently under development for space applications. The ARPS is a potential power source for potential NASA missions to Europa in 2003 and Pluto/Kuiper in 2004. The ARPS system, which has been described in recent literature (Hendricks, 1997), will utilize a multi-tube Alkali Metal Thermal to Electric Converter (AMTEC) cell design. Designated EPX-1, the cell is designed to survive ground handling and launch stresses and provide reliable power for the duration of the missions. The EPX-1 AMTEC cell is based on the PX series of demonstration cells (Borkowski, 1997, and Sievers, 1998). The planned operating temperatures and life requirements for these missions have made an all refractory metal design a requirement. Several other design enhancements have been implemented to meet specific mission requirements and improve predicted cell reliability.