Jason Lustbader

Advanced thermoelectric power system investigations for light-duty and heavy duty applications. II

For pt. I see ibid., p.381-6, 2002. Part II of this two-part paper leverages off the findings in Part I describing the mathematical basis and system modeling approach used in thermoelectric power generation (TEPG) investigations for waste heat recovery in light-duty passenger (LDP) and heavy-duty (HD) vehicles. The TEPG system model has been used to: (1) investigate the behavior and interdependence of important thermal and TEPG design parameters, and (2) compare potential TEPG system power output for a variety of thermal conditions in LDP and HD vehicles. Integrated system modeling and analyses have been performed for: (1) LDP conditions of T/sub exh/ = 700/spl deg/C (973 K) and m/spl dot/h = 0.01, 0.02, and 0.03 kg/sec, and (2) HD conditions of T/sub exh/ = 512/spl deg/C (785 K) and m/spl dot//sub h/ = 0.2, 0.3, and 0.4 kg/sec. Analysis results, TEPG design parameter behavior, thermoelectric (TE) material effects, and interdependence of critical thermal/TE system design parameters are discussed. Interaction of heat exchanger performance and TEPG device performance creates critical system impacts and performance dependencies, which maximize TEPG system power outputs and create preferred heat exchanger and TEPG performance regimes. Part II demonstrates the integrated system analysis approach to heat exchanger/TEPG system performance, allowing NREL to simultaneously quantify these critical system design effects in LDP and HD vehicles. HD vehicle analysis results also indicate that 5-6 kW of electrical energy production is possible using HD vehicle exhaust waste heat.

Reduction in Vehicle Temperatures and Fuel Use from Cabin Ventilation, Solar-Reflective Paint, and a New Solar-Reflective Glazing

An analysis to determine the impact of reducing the thermal load on a vehicle using solar-reflective paint and glazing.

Evaluation of Advanced Automotive Seats to Improve Thermal Comfort and Fuel Economy

Automotive ancillary loads have a significant impact on the fuel economy of both conventional and advanced vehicles. Improving the delivery methods for conditioned air is an effective way to increase thermal comfort at little energy cost, resulting in reduced airconditioning needs and fuel use. Automotive seats are well suited for effective delivery of conditioned air due to their large contact area with and close proximity to the occupants. Normally a seat acts as a thermal insulator, increasing skin temperatures and reducing evaporative cooling of sweat. Ventilating a seat has low energy costs and eliminates this insulating effect while increasing evaporative cooling. The U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) has applied a combination of experimental testing and modeling to quantify improved thermal comfort and potential fuel savings by using a ventilated seat. The thermal comfort improvement can be used to reduce the A/C heat capacity by 4%, resulting in a predicted A/C fuel use reduction of 2.8% on an EPA highway cycle and 4.5% on an EPA city cycle. This is a 0.3%-0.5% reduction in total vehicle fuel use when the A/C system is on; while modest for an individual car, the potential fuel savings is significant on a national level.

CoolCalc: A Long-Haul Truck Thermal Load Estimation Tool

In the United States, intercity long-haul trucks idle approximately 1,800 hrs annually for sleeper cab hotel loads, consuming 838 million gallons of diesel fuel per year. The objective of the CoolCab project is to work closely with industry to design efficient thermal management systems for long-haul trucks that keep the cab comfortable with minimized engine idling. Truck engine idling is primarily done to heat or cool the cab/sleeper, keep the fuel warm in cold weather, and keep the engine warm for cold temperature startup. Reducing the thermal load on the cab/sleeper will decrease air conditioning system requirements, improve efficiency, and help reduce fuel use. CoolCalc is an easy-to-use, simplified, physics-based HVAC load estimation tool that requires no meshing, has flexible geometry, excludes unnecessary detail, and is less time-intensive than more detailed computer-aided engineering modeling approaches. It is intended for rapid trade-off studies, technology impact estimation, and preliminary HVAC sizing design and to complement more detailed and expensive CAE tools by exploring and identifying regions of interest in the design space. This paper describes the CoolCalc tool, provides outdoor long-haul truck thermal testing results, shows validation using these test results, and discusses future applications of the tool.

Application of Sleeper Cab Thermal Management Technologies to Reduce Idle Climate Control Loads in Long-Haul Trucks

Each intercity long-haul truck in the U.S. idles approximately 1,800 hrs per year, primarily for sleeper cab hotel loads. Including workday idling, over 2 billion gallons of fuel are used annually for truck idling. NRELs CoolCab project works closely with industry to design efficient thermal management systems for long-haul trucks that keep the cab comfortable with minimized engine idling and fuel use. The impact of thermal load reduction technologies on idle reduction systems were characterized by conducting thermal soak tests, overall heat transfer tests, and 10-hour rest period A/C tests. Technologies evaluated include advanced insulation packages, a solar reflective film applied to the vehicles opaque exterior surfaces, a truck featuring both film and insulation, and a battery-powered A/C system. Opportunities were identified to reduce heating and cooling loads for long-haul truck idling by 36% and 34%, respectively, which yielded a 23% reduction in battery pack capacity of the idle-reduction system. Data were also collected for development and validation of a CoolCalc HVAC truck cab model. CoolCalc is an easy-to-use, simplified, physics-based HVAC load estimation tool that requires no meshing, has flexible geometry, excludes unnecessary detail, and is less time-intensive than more detailed computer-aided engineering modeling approaches.

Impact of Paint Color on Rest Period Climate Control Loads in Long-Haul Trucks

Cab climate conditioning is one of the primary reasons for operating the main engine in a long-haul truck during driver rest periods. In the United States, sleeper cab trucks use approximately 667 million gallons of fuel annually for rest period idling. The U.S. Department of Energy’s National Renewable Energy Laboratory’s (NREL) CoolCab Project works closely with industry to design efficient thermal management systems for long-haul trucks that minimize engine idling and fuel use while maintaining occupant comfort. Heat transfer to the vehicle interior from opaque exterior surfaces is one of the major heat pathways that contribute to air conditioning loads during long-haul truck daytime rest period idling. To quantify the impact of paint color and the opportunity for advanced paints, NREL collaborated with Volvo Group North America, PPG Industries, and Dometic Environmental Corporation. Initial screening simulations using CoolCalc, NREL’s rapid HVAC load estimation tool, showed promising air-conditioning load reductions due to paint color selection. Tests conducted at NREL’s Vehicle Testing and Integration Facility using long-haul truck cab sections, “test bucks,” showed a 31.1% of maximum possible reduction in rise over ambient temperature and a 20.8% reduction in daily electric air conditioning energy use by switching from black to white paint. Additionally, changing from blue to an advanced color-matched solar reflective blue paint resulted in a 7.3% reduction in daily electric air conditioning energy use for weather conditions tested in Colorado. National-level modeling results using weather data from major U.S. cities indicated that the increase in heating loads due to lighter paint colors is much smaller than the reduction in cooling loads.

MATLAB/Simulink Framework for Modeling Complex Coolant Flow Configurations of Advanced Automotive Thermal Management Systems

The National Renewable Energy Laboratorys (NRELs) CoolSim MATLAB/Simulink modeling framework was extended by including a newly developed coolant loop solution method aimed at reducing the simulation effort for arbitrarily complex thermal management systems. The new approach does not require the user to identify specific coolant loops and their flow. The user only needs to connect the fluid network elements in a manner consistent with the desired schematic. Using the new solution method, a model of NRELs advanced combined coolant loop system for electric vehicles was created that reflected the test system architecture. This system was built using components provided by the MAHLE Group and included both air conditioning and heat pump modes. Validation with test bench data and verification with the previous solution method were performed for 10 operating points spanning a range of ambient temperatures between -2 degrees C and 43 degrees C. The largest root mean square difference between pressure, temperature, energy and mass flow rate data and simulation results was less than 7%.

Novel power electronics three-dimensional heat exchanger

Electric-drive systems, which include electric machines and power electronics, are a key enabling technology to meet increasing automotive fuel economy standards, improve energy security, address environmental concerns, and support economic development. Enabling cost-effective electric-drive systems requires reductions in inverter power semiconductor area, which increases challenges associated with heat removal. In this paper, we demonstrate an integrated approach to the design of thermal management systems for power semiconductors that matches the passive thermal resistance of the packaging with the active convective cooling performance of the heat exchanger. The heat exchanger concept builds on existing semiconductor thermal management improvements described in literature and patents, which include improved bonded interface materials, direct cooling of the semiconductor packages, and double-sided cooling. The key difference in the described concept is the achievement of high heat transfer performance with less aggressive cooling techniques by optimizing the passive and active heat transfer paths. An extruded aluminum design was selected because of its lower tooling cost, higher performance, and scalability in comparison to cast aluminum. Results demonstrated a 102% heat flux improvement and a package heat density improvement over 30%, which achieved the thermal performance targets.

Characteristics of low reynolds number steady air jet impingement heat transfer over vertical flat surfaces

In this paper, heat transfer characteristics of single-slot steady-impinging air jets on a 25.4 mm × 25.4 mm vertical surface were experimentally investigated. The experiments were conducted with four different nozzles (length × width: 4 mm × 1 mm, 8 mm × 1 mm, 12 mm × 1 mm, and 15 mm × 1 mm). The parameters varied in the testing were Reynolds number (Re) (100 - 2,000) and dimensionless nozzle-to-plate spacing (H/Dh = 5, 10, 15, and 20). Correlations for average Nusselt numbers (Nu) were developed that accurately predict experimental data. The heat transfer coefficient over a vertical surface increases with increasing Re. For a small nozzle-to-plate spacing (H/Dh = 5), the average Nu correlation is not only a function of Re but also a function of nozzle length. For large nozzle-to-plate spacing (H/Dh ≥ 10) and nozzle length larger than 8 mm, the heat transfer coefficient is insensitive to H/Dh and nozzle length. A subset of this data was then compared to synthetic jet data in a separate study.

Total Thermal Management of Battery Electric Vehicles (BEVs)

The key hurdles to achieving wide consumer acceptance of battery electric vehicles (BEVs) are weather-dependent drive range, higher cost, and limited battery life. These translate into a strong need to reduce a significant energy drain and resulting drive range loss due to auxiliary electrical loads the predominant of which is the cabin thermal management load. Studies have shown that thermal subsystem loads can reduce the drive range by as much as 45% under ambient temperatures below −10 °C. Often, cabin heating relies purely on positive temperature coefficient (PTC) resistive heating, contributing to a significant range loss. Reducing this range loss may improve consumer acceptance of BEVs. The authors present a unified thermal management system (UTEMPRA) that satisfies diverse thermal and design needs of the auxiliary loads in BEVs. Demonstrated on a 2015 Fiat 500e BEV, this system integrates a semi-hermetic refrigeration loop with a coolant network and serves three functions: (1) heating and/or cooling vehicle traction components (battery, power electronics, and motor) (2) heating and cooling of the cabin, and (3) waste energy harvesting and re-use. The modes of operation allow a heat pump and air conditioning system to function without reversing the refrigeration cycle to improve thermal efficiency. The refrigeration loop consists of an electric compressor, a thermal expansion valve, a coolant-cooled condenser, and a chiller, the latter two exchanging heat with hot and cold coolant streams that may be directed to various components of the thermal system. The coolant-based heat distribution is adaptable and saves significant amounts of refrigerant per vehicle. Also, a coolant-based system reduces refrigerant emissions by requiring fewer refrigerant pipe joints. The authors present bench-level test data and simulation analysis and describe a preliminary control scheme for this system.