Idling of heavy-duty truck engines causes significant costs and emissions, yet electrical energy for driver comfort functions such as climate control and personal-use electronics is required. Fuel-cell auxiliary power units (APUs) will bring significant advantages by reducing idling costs, according to researchers from AVL List, Eberspächer Climate Control Systems, Topsoe Fuel Cell, Volvo, and Forschungszentrum Jülich. The companies are collaborating for the European Commission-funded DESTA project that is exploring solid-oxide fuel cell (SOFC) technology for such applications.
The idling time of Class 8 trucks in the U.S. is in the range of 1500 to 2500 h per year, resulting in fuel costs of $5000 to $9000 per year. Using the SOFC APUs developed in DESTA, fuel consumption can be reduced by about 85%. Compared to commercially available, diesel-driven APUs with an internal-combustion engine (ICE), fuel consumption can be reduced by about 45%. Hence, SOFC APUs promise a very convincing business case to truck owners and operators with very short amortization periods (around 1 to 1.5 years).
AVL and Eberspächer have been developing SOFC APU technology for around 10 years, and in the past year tremendous progress has been achieved with complete stand-alone prototype systems performing extremely well. System tests on ultra-low sulfur diesel (ULSD) proved electrical efficiencies of around 30% and net power outputs of about 3 kW.
In the current DESTA phase, six APU systems (three from AVL, three from Eberspächer) have been thoroughly tested and compared. After this benchmark test, the superior features of both systems will be merged into an optimized DESTA APU, which will be employed in 2014 on board a U.S.-market Volvo Class 8 demonstration truck.
The Eberspächer SOFC APU
In the reformer, diesel and ambient air are converted into a synthesis gas, containing hydrogen and carbon monoxide, by means of catalytical partial oxidation. Electrical power is generated in the SOFC due to the chemical reaction of gaseous fuel and air/oxygen. This process runs with a theoretical efficiency of more than 35% at temperatures of more than 800°C (1472°F). An unembodied startup burner module consisting of a startup burner and heat exchanger provides the heating of the fuel cell.
As the fuel cell does not convert all the gaseous fuel, an off-gas burner uses the remaining hydrogen and carbon monoxide containing ingredients of the fuel-cell exhaust gas. This waste heat conditions the cathode air for the fuel cell. It could also be used for auxiliary heating purposes in the vehicle cabin. A part of the unconverted fuel-cell exhaust gas is passed back into the reformer to cool and to improve the process, similar to exhaust gas recirculation (EGR) in combustion engines.
In comparison to an ICE APU, emissions are up to 60 times lower, and dangerous soot particles are not detectable at all.
Within the first tests with an autarkic fuel-cell system for the DESTA project, an electrical gross output of 2.1 kW and an electrical net output of 1.7 kW at a diesel input of 8 kW have been demonstrated. The system was operated with sulfur-free, synthetic diesel.
SOFC APU performance was tested at diesel fuel inputs of 8 kW, 6 kW, and 5 kWtherm. The corresponding electrical gross power, electrical net power, and the electrical net efficiency were calculated. The efficiency ranges between 18% and 22%, depending on the diesel input into the reformer.
For a weekly APU load profile, the APU is activated by the truck driver and provides electrical power for cooling, heating, refrigerator, lights, radio, PC, and TV. The daily cycle, including two short breaks and a long overnight run, is typically repeated five times a week. The system needs a diesel startup burner at the beginning of the weekly cycle. A complete shutdown only occurs at the end of the weekly cycle.
THE AVL SOFC APU
The first AVL SOFC APU Gen I was demonstrated in end of 2011. With this system, the first successful tests with sulfur-free diesel fuel were performed. Around 1 kW of electrical power and about 23% electrical efficiency were reached.
The system is characterized by an auto-thermal diesel reforming process, a hot anode recirculation system, highly efficient radial blowers for media supply, and a small and light packaging approach. The hot anode recirculation system is a similar process to EGR in ICEs. Exhaust gas of the SOFC anode is recirculated into the fuel processor, which improves the reforming reaction because product water (from electrochemical reactions in the fuel cell) is recirculated back into the reformer. The water addition to the fuel reformer suppresses carbon formation and reduces the operating temperature, both leading to an extension of useful reformer product life. For this recirculation process a hot gas anode blower is required. This component was developed by AVL and is capable of supplying hydrogen-containing gases up to 600°C (1112°F).
AVL has since made Gen I.I available, which is the first AVL APU system that can be integrated into vehicles. It is equipped with all necessary power electronics and an electrical anode oxidation protection system (EAP). This EAP system is needed to protect the Ni-based fuel-cell anode from oxidation when no fuel supply is possible (startup and shutdown). The Gen I.I system has also undergone significant optimization toward mechanical stability (for vibration and shock loads). The complete system was analyzed and improved by CFD and FE methods.
With Gen I.I, a significant performance improvement has been achieved. Based on a one-stack system, 2.2 kW of gross power and 1.8 kW of net electrical power have been reached. The electrical net efficiency at 2 kW was around 30%. The noise of the system (at 1-m distance) was without any acoustic measures under all operating conditions below 55 dB(A).
To reach the 3-kW power target, the system will be equipped (starting with Gen II, revealed earlier this year) with two stacks, which actually might lead to a 4-kW system. With a two-stack system, the electrical efficiency is expected to be higher because the blowers—the biggest system internal auxiliary-power consumers—will be operated in their desired operating window.
Gen II of the SOFC APU system is about 20% smaller than before and is expected to bring significant performance improvements.
In parallel to the DESTA project, AVL has already performed a vehicle demonstration of its Gen I.I system on a military vehicle. This demo was successful and the vehicle battery pack was continuously charged with about 1 kW (one-stack system) of electrical power. Together with license partner HyRef Power BV, AVL has defined a product development plan to introduce commercial military 3- to 5-kW APUs to the defense market starting in 2016.
SOFC stack development
Topsoe Fuel Cell and its parent company Haldor Topsøe have been engaged in the development of SOFC technology since 1989. The stacks are based on planar anode supported cells with metallic interconnects. The stack design used for APU applications has a side air manifold and internal fuel manifold and is integrated into a stack module.
This module includes a cast casing containing:
• A high-temperature compression system that holds the stack itself in place, ensures mechanical integrity, and secures tightness of the gaskets inside the module
• A flat interface allowing for bolting onto the system, either as a single stack or in a twin/flat configuration
• Electrical isolation of both terminals of the stack itself, so that the casing can be connected to vehicle ground and allowing for galvanic isolation of the high voltage part of the system
• Power outlet feed through
• Voltage probe feed through, connecting to some of the interconnects in the stack. These allow for diagnostics during development.
The airflow to the stack also functions as a purge flow around the stack, ensuring that any leaking fuel is picked up by the airflow and fed to the burner. Hence, the design with external air eliminates any safety concern related to possible leaks.
To verify that the stacks are fit for the application including a weekly full cool-down and heat-up, an accelerated test method has been developed so that a full thermal cycle can be completed in less than 4 h. With this test, it has been shown that the stacks can withstand more than 100 thermal cycles without any damage. Testing is ongoing with emphasis on increasing the number of load cycles and thermal cycles verified.
To accept the sulfur levels of ULSD, the anodes have been modified. These modifications have been tested on stacks with 10-25 cells. The anode modification has been implemented and verified in some of the full-size stacks used for developing the two APU systems.
Integrating systems in a truck can be challenging, and the environmental conditions for chassis-mounted equipment are demanding. The operation temperature could be both high and low due to heat-producing equipment nearby or due to weather conditions. Driving under mild conditions will also expose the APU to tough vibrations from road input and the engine.
The quality and temperature of the inlet air must also be considered. Most likely a snorkel solution will be used to ensure the air is as clean as possible. The APU must also withstand the supply voltage levels present in a typical heavy-duty truck caused by, e.g., cranking the engine or load dumps due to switching off inductive loads.
The size of the APU is critical for truck integration. The vertical size is limited by the trailer/cab and the required ground clearance, and the lateral size is limited by the chassis and fairings. The longitudinal dimension must also be kept at a minimum since a wide APU will reduce the available space for diesel tanks and result in lower acceptance by the customers.
The intended installation position for the APU in the DESTA project is on the left-hand side of the chassis frame. This position has the advantage of being close to the vehicle batteries as well as the diesel tank, which will minimize the length of power cables and fuel hoses. In the DESTA project, the left-side tool compartment will be used for the dc/dc converter and controller ECU (electronic control unit) due to its short distance to the other system components and the protected environment.
The role of the APU in a vehicle can be generalized to maintaining a nominal state of charge for the vehicle battery. The main task for the vehicle battery is to crank the engine, but it is also used as an energy buffer for auxiliaries.
To be able to use an APU in a heavy-duty truck, the truck needs additional components:
• A control panel needs to be installed in the living part of the truck’s cabin to enable remote start, set-points adjustment, monitoring, alarming, and shutdown of the APU
• A dc/dc converter is needed to convert the output voltage from the APU to a voltage suitable for charging the vehicle battery and a SOC sensor measuring the state of charge of the vehicle battery (if this is not already available on the vehicle)
• A system controller is needed that controls the APU and dc/dc converter with the target of keeping the battery SOC on a predefined level.
Since the APU is an optional component, all components need to be compatible with the existing vehicle architecture.
The system controller can be implemented as a stand-alone ECU or as a part of another vehicle system ECU. The controller will normally use standard communication interfaces such as CAN to interface with the APU, the dc/dc converter, and the SOC sensor, as well as other available sensors and actuators. An example of other useful sensor information could be the current power consumption of auxiliaries in the cabin. An example of other relevant actuators could be power relays that could be used to isolate the APU from the vehicle electrical system in case of an error.
Due to the dynamic nature of the vehicle battery and the APU, as well as a potentially irregular power demand from vehicle auxiliaries, the task of keeping the battery SOC level on a suitable level can be challenging. To be able to develop, calibrate, and test the control algorithms for the system controller, a system model needs to be developed.
Within the DESTA project, the SOFC APU systems from Eberspächer and AVL have been significantly improved and thoroughly tested over thousands of hours. Topsoe Fuel Cell has improved SOFC stack technology, especially toward sulfur tolerance and thermal cycling, which makes competitive products possible. Volvo has prepared the vehicle integration by developing a dc/dc converter and an energy management system.
Based on recent accomplishments, the DESTA partners are confident that the truck demonstration in 2014 will prove successful. AVL and Eberspächer are planning a commercial U.S. market introduction of SOFC APU systems in 2016/17.
This article is based on SAE International technical paper 2013-01-2470 written by Juergen Rechberger of AVL List GmbH, Andreas Kaupert of Eberspächer Climate Control Systems, Christoffer Graae Greisen of Topsoe Fuel Cell, Jonas Hagerskans of Volvo Group Truck Technology, and Ludger Blum of Forschungszentrum Jülich.