Automotive Hybrid Electric Systems
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The CEC automotive hybrid electric power system is based on a mostly-forgotten, well-proven thermodynamic cycle which, with modern materials, aerodynamics, combustion and heat transfer design, will silently, with ultra low levels of emission, generate electric power to propel hybrid electric vehicles including commercial vans and passenger cars at competitive costs with significantly higher thermal efficiencies, substantially higher reliability, longer life and lower maintenance costs than other power generating systems being projected. It consists of an externally-fired closed Brayton cycle turbo-generator with a basic power core that is adjustable to serve applications with maximum power requirements ranging from 30 to 130 kW. A second, slightly larger power core could serve a power range from 150 to 300 kW. Each power core can be trimmed with inserts to meet specific applications and be matched with modularly sized heat transfer elements in proportion to the maximum output required. This approach drastically reduces the manufacturing and servicing costs of a fleet of vehicles equipped with such power plants. The most valuable attributes of the system are that it suffers virtually no part load efficiency loss from full power to as low as 8% of maximum power, and that it inherently operates quietly with no discernable vibration, requiring no lubrication nor scheduled maintenance. |
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System Performance:
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For hybrid electric propulsion of a commercial or passenger vehicle, the power unit is sized below maximum propulsion needs, with the difference in power made up with electricity drawn from a storage system. For the Diesel generator, shown in blue, the engine is turned off at lower loads where it is inefficient and the substitute power drawn from storage is later recharged at a power level where the Diesel is more efficient. The opposite is true for the fuel cell, shown in green for a unit equipped with a Tier 2 gasoline reformer, which wants to be on at low loads but wants assistance from storage at higher levels of demand to remain at high efficiency as far as possible. The flat efficiency curve shown in red of the closed Brayton cycle makes it much less storage-dependent, except at very low or negative (dynamic braking) loads. |
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Based on a range of analytical test drives with different size vehicles, ranging in weight from 3,000 to 5,000 lb and with various drag and rolling resistance coefficients, the closed Brayton cycle improves upon the fuel economies of the Diesel and of the fuel cell by 6 to 26 percentage points. Finally, since the closed Brayton cycle creates its own operating atmosphere, it is flat rated to 10,000 ft altitude, whereas turbocharged Diesels and pressurized fuel cells suffer losses in maximum power output of about 1kW for each 1,000 ft rise in elevation.
Comparison with DOE Specific Goals
The air next enters the low pressure turbine which drives the low pressure compressor and direct shaft-mounted, high speed, wild-frequency alternator, in the course of which its pressure is further reduced and temperature decreased9 before passing on to the recuperator. In the recuperator much of the heat in the air is transferred to the cooler air leaving the high pressure compressor10 and the remaining heat is then removed in the heat sink heat exchanger to bring the temperature down to near ambient before repeating the cycle1. A blower drives ambient cooling air through the intercooler, heat sink heat exchanger, and alternator stator. In most passenger car applications, some of the heat in the loop should be used to drive an absorption chiller to provide air conditioning for the interior. The external combustor is charged with a highly diluted mixture of fuel and air that has been partially heated by a combustion air recuperator before entering the combustor. The exhaust gas from the combustor is cooled by the combustion air recuperator before being discharged overboard. The control system consists of a fill and drain unit that either adds small amounts of air under pressure into the low-pressure compressor inlet to increase the pressure in the loop and therefore, the air density, or to discharge air from the high-pressure compressor outlet to ambient to reduce the density of the air in the loop. As the density increases, the power output from the generator increases and as the density is decreased the output declines. A secondary thermostatic control regulates the fuel flow to maintain the turbine inlet temperatures constant at 1,800ºF. Once the density drops to its lowest point, which corresponds to about an 8% full load condition, further reduction in power output requires the fuel flow to be diminished to reduce the turbine inlet temperatures below 1,800ºF. Once the turbine inlet temperatures reach 700°F, there is no electrical output. Turbo MachineryThe core of the system consists of the two turbo compressors and a recuperator. The turbo compressors selected for the proposed program are virtually identical to two units developed for an air pressurization system used in Ballard Generating System’s 250 kW, stationary, PEM fuel cell. These fuel cells were designed for service lives of ten years and Ballard subjected 15 sets of fuel cells to international Beta test trials in which each unit was required to complete a full year’s uninterrupted operation without failure. Turbo compressors returned for teardown inspection after about 9,000 hours in service showed almost no deterioration. Low- and high-pressure spools were in excellent condition, except for the normal buildup of varnish in the bearing seals, which would not have inhibited continued operation. There is a video clip of detail parts of the disassembled units - [scroll down to the photo of two men inspecting them and click on the photo to initiate the video] The system core was demonstrated in laboratory tests using variable high pressure turbine backpressure to simulate the turbo generator load, and screens on the low pressure compressor inlet to verify sub-atmospheric performance. The demonstration was attended by third-party, qualified professionals and, unrehearsed as this test turned out to be, the results verified the performance levels used in the system calculations for this presentation. RecuperatorThe high-temperature, counter-flow, plate-fin recuperator is similar to many units developed for automotive and other ground power units. The proposed recuperator is significantly simpler and will be lower in cost than the sophisticated recuperator used in the Capstone Microturbine which is an all-welded assembly. In our concept we are using a modular approach to heat transfer designs which allows the use of standard designs tailored to a specific application only by changing the stack height of the heat exchanger to match the power requirements. To attest to the durability of micro gas turbine recuperators, there are growing numbers of Capstone turbines with more than 30,000 hours of service that have not had recuperator problems. Lower Temperature Heat ExchangersThe intercooler and heat sink heat exchangers are of customary aluminum, cross-flow, plate-fin, brazed design very similar in performance to like units employed in many military and civilian applications on aircraft and in high performance vehicles, including turbocharger after-coolers in race car engines. Combustor and combustion Heat ExchangerThe ceramic combustor is technically no more sophisticated than domestic fuel oil furnace and gas heater combustors. However, while it uses established techniques, the combustion heat exchanger requires additional development. Because of the high temperatures associated with the combustion system, ceramic heat transfer elements have been selected. The major design problem with a ceramic heat exchanger for this application is the limited tensile strength of suitable materials. Creative Energy Concepts has a proprietary ceramic design that overcomes this obstacle by taking the high pressure loads in compression. The ceramics have more than adequate compressive strength. This design is unique and modular, yielding a standard design for a wide range of applications. |
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Physical Design Aspects:
The conical housing upstream of the combustor contains the combustion air recuperator and fuel / air mixer leading to the combustor. The combustion exhaust out of the annulus is used to heat the incoming combustion air in the recuperator.
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The system is designed around a power core consisting of the combustor, combustion heat exchanger and the two stages of turbo compressor. The combustion system is actually contained within the cylindrically shaped annular combustion heat exchanger as depicted in the drawing on the left.
As shown in the cross sectional diagram, hot gas from the internal combustor is directed through a plenum at the end of the cylinder into two cylindrical annular heat exchangers in series which heat the air in the closed loop before entering the low pressure and high pressure turbo compressor turbines which are mounted on plenum chambers built into the core assembly. The combustor and combustor heat exchanger is fabricated from Silicone Carbide. In production, the rotors of the turbochargers as well as turbine inlet nozzles and compressor diffusers will be made of Silicone Nitride. The specific weight of ceramics is about the same as aluminum, which besides reducing the weight of the system has the advantage of reducing the inertia of the rotors.
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The intention is to use the same power core to meet the power requirements of a wide range of applications. In order to fit all commercial vehicle needs, two power cores will be required. The smaller of the two is illustrated in this description. The 80 kW design presented can easily be adapted to a large range of vehicles with maximum power demands from 30 kW and 130 kW. This is accomplished by modifying the length of the magnet shaft of the low pressure spool and the alternator stator to match the max power required without changing the aerodynamics or housings of the turbo compressor. The high pressure turbo compressor turbine nozzle area needs to be adjusted to match the desired pressure ratio. |
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The power core to cover the output range up to 300 kW will employ a turbo compressor that was developed as the low pressure stage of a turbo-compound diesel research application for the U.S. Army. The high pressure stage will be the low pressure stage of the smaller power core with the alternator removed. The compressor for the military project shown in the photograph had the distinction of exceeding 80% efficiency at a pressure ration of 5 to1.
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System Heat Transfer Components:Unlike the power core, the recuperator, intercooler and heat sink heat exchanger all have to be sized for specific applications. To do so as painlessly as possible, the heat transfer elements are designed to be modularly adjustable. The heat exchanger core stack heights are directly proportional the system airflow. In this manner it is easy match heat exchanger sizes with the system flow. Combustion Heat ExchangerThe combustor design is sized to accommodate the maximum power of 130 kW. The combustion heat exchangers are wrapped around the combustion chamber to place the high pressure elements in compression. The circumference of the heat exchanger is the maximum stack height of the heat exchanger. If a smaller stack height is required an empty shell is placed where the stack has been reduced to maintain structural integrity. RecuperatorThe high temperature steel recuperator is of high-density, plate-fin, counter-flow design. It is rectangular in shape and for an 80 kW system has dimensions of 10” flow length, 20”stack height and 6”width. |
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The aluminum intercooler is also of plate-fin design, but uses a cross-flow configuration. For an 80 kW application its dimensions are 20” hot flow length, 4” cold flow length and 15” stack height. A typical example of such an intercooler previously used by the company in a high performance vehicle engine application is shown on the left. Heat Sink Heat ExchangerThe heat sink heat exchanger is also made of aluminum with the same heat transfer configuration as the intercooler. For 80 kW its dimensions are 15” hot flow length, 4” cold flow length and 20” stack height. The proposed ATD program contains an element to evaluate the possibility of building the “generator” part of an absorption chiller into this sink heat exchanger. This would involve incorporating into its heat transfer core a liquid section to boil off the water in the mixture of Lithium Bromide and water arriving from the chiller evaporator. The Lithium Bromide is returned to the evaporator and the water vapor is ducted to the chiller’s condenser. The purpose of the chilled water from the evaporator would be to: • cool the payload and personnel in the combat vehicle; • reduce the infrared signature of any heat leaving the vehicle; and • decrease engine power loss in extreme heat by cooling the inlet air to the closed loop. By using heat that would otherwise be rejected to power the cooling system instead of using some of the power being generated, the overall energy efficiency of the vehicle is improved. |
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Alternator: The permanent magnet alternator in the production version will likely be integral with the low pressure turbo compressor. Its magnetic media is housed within the rotor as illustrated in the earlier rotor drawing. The stator with the windings is attached to the compressor housing and is cooled by air from cooling air blower. In the Phase 1 prototype tests the alternator will be driven by a separate turbine. Conditioning of the high, wild frequency power output from the alternator is done separately by solid state rectifiers and inverters.
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Comparison With Other On-going Research Alternatives.The closed Brayton cycle competes principally with the latest designs of direct injected, turbocharged, Diesel engines and PEM automotive fuel cells. Both alternatives have been subject to intensive research efforts by the automotive industry in partnership with the Department of Energy. Other alternatives, including spark ignition internal combustion engines and simple cycle recuperated gas turbines fundamentally operate at efficiency levels too low to be competitive in future passenger vehicle applications, although they have been used in experimental vehicles.
Storage System Objectives:While great improvements have been made in electrical storage system technology, they are still heavy in comparison with the specific energy of gasoline and diesel fuel. Hence, the objective clearly has to be to manage the state of charge (SOC) to minimize the size of the storage system. One of the challenges is that no SOC management system can anticipate on-coming drive conditions, which denies it the luxury of postponing dissipation or replenishment opportunities. Basically, the SOC has to be maintained as close to its mid point as possible.
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Efficiency ComparisonDiesel engines in hybrid electric power systems take advantage of the electrical storage system to satisfy the lower power demands in order for the Diesel to operate mostly at high output power levels where it is more efficient. Closed Brayton cycle generators and fuel reformer equipped PEM fuel cells share the advantage of reaching peak efficiencies at much lower part loads and accordingly employ different control strategies. Whereas the Diesel is turned off at low load levels, the closed Brayton and fuel cell alternatives remain at “hot idle” until significant dynamic break recovery occurs when they too can be shut down. But, whereas the closed Brayton cycle has a flat efficiency curve, the fuel cell characteristics militates in favor of using fuel cells at power levels below 50% of full capacity, as much as possible. Simulation of Possible Drive Conditions:To test the extent of this challenge, operational simulations were used:
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Topographic conditions for this LA commute are shown on the graph along with assumed velocities
Topographic details and assumed speeds of this commuter ride were as shown in the graph to the left. The elevation varied from 5,400 to 7,200 ft. As a consequence, the maximum power of the Diesel and Fuel cell dropped from 60 to 52 kW while the closed Brayton cycle maintained its rating of 60 kW.
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LA Commuter Cycle:The course follows surface roads in Chatsworth to Topanga Boulevard, south to I-101, east to I-405, over the Sepulveda Pass, south to Wilshire Boulevard and on surface streets to an address in Westwood. This cycle was selected for its challenging elevation changes at high speeds for the most part, except for infamously heavy traffic portions on steep slopes.
Colorado Springs Commuter CycleThe second commuter cycle involves a drive from a Colorado Springs address in the foothills to a place of work on the southwest edge of Denver, CO. The route follows surface roads around the Garden of the Gods to I-25 north to Castle Rock, northwest on US 85 to Highway 470 and 75 to a foothill destination. The purpose of this driving cycle was to explore the effects of high altitude and large elevation changes at highway speeds in combination with warm (95ºF) ambient temperatures. The route covered 66.4 miles in an hour and seven minutes.
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Assumed Parameter |
Dimension |
Value |
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CD = Drag Coefficient |
Coeff. |
0.330 |
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μ = Rolling Resistance Coefficient |
Coeff. |
0.011 |
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A = Frontal Area |
Sq ft |
23.20 |
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W = Vehicle Gross Weight |
lb |
3,500 |
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M = Mass = W/g |
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108.78 |
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Power Generator Capacity |
kW |
60.00 |
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Percent recovery - deceleration energy |
% |
70% |
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Turnaround storage efficiency |
% |
92% |
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Vehicle accessory electric load |
kW |
2.50 |
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Traction Motor Efficiency |
% |
94% |
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Power Conditioning Efficiency |
% |
96% |
The assumed characteristics of the passenger car for this evaluation were similar to those of a 2002 model Ford Taurus, as listed in the table. Even though this presentation is centered on a power rating of 80 kW, the simulations indicated a choice of a 60 kW generator. However, the dynamics of sudden accelerations in traffic could bias the ultimate maximum power choice toward 80 kW.
Overall fuel economy results were as shown in this next table. They show the CBC leading the Diesel in fuel economy by 5 to 16 and the fuel cell by 15 to25 percentage points.
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Drive Cycle |
LA Commute |
COS Commute |
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Power System |
MPG |
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Closed Brayton Cycle |
54.06 |
57.05 |
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Direct Injection Turbo Diesel |
51.32 |
49.01 |
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PEM Fuel cell with reformer |
43.06 |
49.77 |
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State of ChargeHowever, the fuel consumption performance of each of the three types of power generator depends in large measure on the management strategy of the State of Charge (“SOC”) of the vehicle’s electric storage system. For the Closed Brayton Cycle (“CBC”) the generator was left on at 8% of power even if demand dropped below that level, except when the demand turned negative. Then the generator was switched off. Excess demand over maximum generator output was drawn from storage. The rate of recharging the storage from the generator was also controlled to minimize the size and density of charge of the vehicle batteries. The CGC and fuel cell could be set to mild charge rates.
The blue curve on the left shows the same results from the Diesel generator. The Diesel was turned off whenever demand dropped below 15% of full load and the recharge rate had to be made more aggressive to accommodate the greater amount of charge restoration needed in the available time. Unlike the CGC which is flat-rated, the Diesel and fuel cell outputs were diminished from 60 to 55.7 kW by the elevation of the terrain above sea level and an ambient temperature of 85ºF. This added to the amount of power cycled through the storage system where a turnaround loss of about 8% is incurred.
The chart in green shows the performance of the reformer equipped fuel cell, which was kept on-line down to 10% of maximum output, except during negative loads. It was apparent that the fuel cell was out of its element because of the reformer and would only be competitive in performance running on pure hydrogen fuel.
The consequence of shutting the Diesel off so frequently is reflected in the plot of SOC throughout the drive cycle. In order to approach the fuel efficiency of the CBC, the Diesel drew much more electricity from storage for most of the drive. The drive ends in slow traffic on downhill slopes causing the SOC to build up quickly for the CBC and fuel cell. Since they can be turned off too, it would take a minor refinement of the SOC control system to avoid such build ups.
However, in this drive cycle it would not have penalized the vehicle because the battery capacity was sized by charge and discharge intensity rather than holding capacity. As shown in the graph, charge rates reached 61 kW and discharge rates 91 kW. Had NiMH batteries with a specific power of 900 W/kg been used, 220 lb of batteries would have been required. To accommodate a peak charge density of 61 kW 150 lb of batteries would have suffice. The simple solution would have been to avoid the peak demand spike by accelerating less vigorously near the summit of the Sepulveda Pass in order to stay within the limits of a 150 lb of storage system. Had Lithium Ion batteries which a specific power ranting of 400 W/kg been used, the required storage weight would have amounted to 330 lb. To keep these weights in context, it should be remembered that the whole CBC power generator only weighs 220 lb. |
Regulatory Drive Cycles
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