Abstract:
A thermal-to-electrical conversion system converts internal combustion engine exhaust heat into useful electrical energy in a hybrid vehicle system. The converted energy is sent to an energy-storage control system via a DC-DC converter. High power-density storage medium (e.g. ultracapacitors) is used in the designated “short-cycle” hybrid design to maximize transfer efficiency and capacity while minimizing weight. An upconversion circuit within the control system allows high use of the storage capacity. Employed in a lightweight vehicle platform, the system addresses the most significant barriers to achieving high transportation efficiency-mass and combustion energy loss to waste heat.

Description:
[0001]    The application is a continuation in part of Bartilson U.S. patent application Ser. No. #1,168,314 filed Mar. 14, 2007, which is a continuation in part of Bartilson U.S. patent application Ser. No. #11/613,014 filed Dec. 19, 2006. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to an apparatus and method to efficiently extract waste thermal heat from a heat engine powering a vehicle with a hybrid drivetrain. 
       BACKGROUND OF THE INVENTION 
       [0003]    The opportunity afforded by improving the thermal efficiency of internal combustion engines (ICE) (commonly 28% for gasoline, 34% for diesel engines) is well understood, however long-standing research and development efforts have not produced marked technical and/or market impacts. Turbo-charging and ceramic insulation of combustion chamber components have made the most significant impact, however, cost and the extent of efficiency improvement (˜10%), has not led a to significant reduced fuel consumption on a functional work unit basis, and accordingly not impacted the per capita basis. Exhaust gas turbines with mechanically-connected generators have been presented as alternative, however, cost and efficiency have similarly pre-empted commercialization. Thermal-to-Electric devices and systems have also been presented to capture the 
         [0004]    Laid open US Pat App. US 2004/0100149 describes topologies for multiple energy sources, including UCs, and accommodates reverse power flow from the utility being driven (case of regenerative braking for a transportation vehicle). In the described topologies, all power is continuously directed through a power converter module, with inherent losses and limitations per device sizing. 
         [0005]    U.S. Pat. No. 7,004,273 discusses a bank of ultracapacitors directly bussed to an engine-driven generator with a control management unit bringing the engine on and off to maintain the state-of-charge of the ultracapacitors. This approach does not address the inefficient ultracapacitor capacity utilization issue, resulting in extensive burden/cycling of the engine and/or significant oversizing of the ultracapacitor bank. 
         [0006]    U.S. Pat. No. 7,109,686 describes the use of braking resistor and switch structure to assist in charging and discharging an ultracapacitor bank and to protect the ultracapacitor from excessive pre-charge current. A DC-DC converter is referenced as expensive, and its use is referenced only as an alternative method to pre-charge the ultracapacitor bank. While low in cost the use of the braking resistor diverts energy, thereby wasting said energy. 
         [0007]    A solution which could extract more of an ultracapacitor&#39;s capacity would greatly assist in reducing wasted capacity and enable an all-ultracapacitor storage solution for a lightweight vehicle. Augmentation with thermal-to-electric recovery of waste heat furthers this potential. 
       2. SUMMARY OF THE INVENTION 
       [0008]    Large-scale improvement in the transfer of combustion energy into useful work has heretofore remained elusive. The apparatus and method of this invention contributes significant advancements over the background art as demonstrated in the following objects. 
         [0009]    An object of the invention is to extract a significant amount of waste energy available from the ICE through higher device (&gt;15%) and transfer system efficiency, and to perform this extraction down to a much lower temperature (&lt;200° C.). 
         [0010]    Another object of this invention is to provide an electrical storage system which accepts high and variable rates of charge transfer, exhibits high efficiency, high reliability over many cycles, and high capacity utilization. 
         [0011]    Another object of this invention is to effect a low-complexity, “no moving parts”, solid-state transfer system, exhibiting low cost, and high reliability. 
         [0012]    Another object of the invention is to create a low-mass design, thereby incurring minimal added weight burden to a high efficiency transportation vehicle. 
         [0013]    Another object of the invention is to reduce or eliminate the requirement for noise abatement from the exhaust system. 
         [0014]    Additional objects and advantages of the invention are advanced by the following description and may also be learned from the practice of the invention. 
         [0015]    In brief summary, the foregoing objects are achieved by an apparatus which comprises: a low-mass hybrid drivetrain having an ICE operating at a peak efficiency point, with exhaust stream directed through a compact heat exchanger housing high efficiency thermoelectric elements in a planar array, with electrical output of the array directed into a wide input span DC-DC converter serving power to a high power density storage system with high capacity utilization circuitry. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The appended drawings depict specific embodiments of the described invention as to render understanding of the invention, however, they are not to be considered limiting of the scope of the invention. The drawings illustrate only typical, and presently understood, best use mode of the invention: 
           [0017]      FIG. 1  is a schematic of a hybrid vehicle drivetrain and control system 
           [0018]      FIG. 2  is an example of a brushless DC motor drive 
           [0019]      FIG. 3  is a schematic of the ESS control system with heat recovery input 
           [0020]      FIG. 4  is a schematic of the comparator circuit of the ESS control system 
           [0021]      FIG. 5  is a crossectional view of the waste heat-to-electric conversion system 
           [0022]      FIG. 6  illustrates the fin construction of the conversion system and the stacking of heat exchange layers into a complete unit 
           [0023]      FIG. 7  illustrates the thermal-to-electric conversion module construction 
           [0024]      FIG. 8  is a graph of vehicle mass versus fuel efficiency 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    The present invention embodies a serial hybrid drivetrain for a lightweight vehicle consisting of an ICE and directly-coupled generator serving power to electric motors that provide rotational power to the vehicle&#39;s wheels, with said ICE having its exhaust directed through a heat exchanger housing modules which transform exhaust heat into electricity which is made available to the ESS system, and said generator alternately or in parallel providing power to restore energy to the ESS, with said ESS system configured to provide power directly to the said electric motors, with said ESS having a single energy storage medium of high power density (&gt;1000 W/kg), and having a high cycle life (&gt;500,000 cycles to a deteriorated state of 80% capacity recharge level), and accepting charge rates of the maximum of charge rates from the ICE/generator and/or regenerative braking from the electric motors, and the sizing and charge/discharge efficiency of said ESS system such that excess storage and corresponding wasted “carrying” energy is minimized, and said serial hybrid drivetrain operated by a compatible control system which operates the engine at its peak efficiency point ( FIG. 1 ). 
         [0026]    Under the stated conditions of low vehicle mass, high storage power density and minimized energy storage, higher vehicle fuel efficiency is obtained, with a resulting shorter duration, higher-frequency of on and off cycling (herein labeled “short cycling”) of the ICE and depletion/replenishment of the ESS than is obtained with current low power density storage mediums (batteries for example) and their combinations with high density mediums. Recovery of thermal energy from the exhaust stream furthers fuel efficiency, as well as increasing the total power output capacity. 
         [0027]    In the preferred embodiment of the present invention high efficiency (&gt;95% efficiency units are currently available) brushless DC motors (also referred to as AC synchronous motors driven at variable frequency via DC-AC motor controller) drive the wheels. As displayed in  FIG. 1 , the motors may be located in the wheel housing (called a wheel-motor) thus eliminating mechanical drive shaft and differential components. Operated via motor controller having “4-quadrant” control, the brushless DC motors and controllers permit the capture of regenerative braking energy to flow back to the ESS. Alternate motor and mechanical drive configurations are displayed in  FIGS. 5 ,  6 , and  7 . While incurring additional cost, the option of driving of all vehicle wheels with motors enables higher capture of regenerative braking energy. 
         [0028]    In the preferred embodiment of the present invention (reference  FIG. 1 ), an ICE  1  drives a high efficiency generator  3  via a shaft  2  which delivers DC power to the UC storage  5  via electrical connections  4 . Sequencing (on/off, durations) of the ICE  1  is controlled by the drive control system  15  via connection  36 . The ESS control system  7  controls power flow between the UCS  5  via connection  6  and the loads (4 quadrant brushless DC motor controllers  9 ) via connection  8 . A micro controller within the brushless DC motor controller creates PWM (pulse width modulated) signals to gate drivers which in turn pulse high current transistors that in turn provide the multi-phased, pulsed power to the DC brushless motors  11  and  12  via connection  10  ( FIG. 2 ). Control of the brushless DC motor controllers is provided via connection  14  to the Drive Control Unit  15 . Under braking signal from the vehicle driver, the drive frequency is decreased below that of the rotating motors  11  and  12 , inducing reverse current flow which is then converted back to DC power and fed onto the UCS  5 . 
         [0029]    As illustrated in  FIG. 3 , a Comparator Circuit  20  monitors the high voltage side of the bus via connector  21 . When the feed bus voltage falls beneath a threshold value the Comparator  20  releases the inhibit signal line  22  inducing the on-state of the DC-DC Boost Converter  30 . Bus Connections  28 ,  29  supply power to the DC-DC Converter  30  which is connected to common terminals of the Contactor  27 . Contactor  27  is in a normally-closed condition when the UCS bus  6  voltage is above the low-voltage threshold previously described. Under low voltage conditions, Comparator  20  releases an inhibit release signal via connection line  23  to a regulated supply  24  which energizes the coil of Contactor  27 , thus directing all power flow from the UCS  5  to the DC-DC Converter  30  for up-conversion to the requisite output voltage required by the load. Connection lines  8  (from the Contractor  27 ) direct power to the common load bus under the NC state of Contactor  27 , whereas power is directed via connection lines  8  from the DC-DC Converter output under the case of low UCS bus voltage. A delay corresponding to the known response time of the DC-DC converter ensures that opening of Contactor  27  connection to the output bus lines  6  does not cause momentary sagging of the ESS output voltage (alternative control designs may equivalently serve to ensure smooth operation of the ESS output bus during switching). A Regulated Power Supply  17  is connected to the battery bus via connection lines  18 , to provide controlled, logic level power supply to the Comparator  20 . Regulated Power Supply  24  provides a controlled DC level to operate the coil in Contactor  27 , also supplied by the battery  36 . Power sourced from the waste heat recovery “generator”  49  by summation of thermoelectric (or thermionic) modules  50  is delivered to DC-DC Converter  47  via connecting lines  45 ,  46 . Power is then given to the UCS bus  6  via connecting lines  48 ,  49  for system use (storage or load-serving). Converter  47  serves to convert and condition “generator”  49  input voltage, as well as to isolate “generator”  49  from UCS bus  6  (prevent reverse power flow). Starting of the ICE  1  is accomplished by an electric starter motor  32 . A 12 VDC battery  36  has as its sole function to provide power to the starter motor  32 , and control circuits within the ESS control system  7  and Drive Control Unit  15 . The battery  32  capacity is maintained by a separately regulated output from the generator  3  via connection lines  34 . 
         [0030]      FIG. 4  displays the Comparator Circuit  20  wherein connection line  19  receives regulated power from Regulated Power Supply  17 , delivering controlled logic-level voltage as reference input to the low-side of the Comparator  37  via the Reference Regulator  31 . Connection line  21  provides the voltage sense input to the Comparator  37  from UCS bus  6 . When the difference between the Comparator  37  inputs falls beneath the threshold value, connection line  38  energizes Power Transistor  41  sending the inhibit release signal to the DC-DC Converter  30  via connection line  22 . Connection line  38  also activates the Delay  39  which activates Power Transistor  40  (after its prescribed delay, or similar control function) which, via connection line  23  enables Regulated Power Supply  24 , which then powers the coil in the Contactor  27  via connection line  26 . 
         [0031]    In an example case of a 48V system bus, a single Maxwell BMOD00165-E048 UltraCapacitor provides 50 Watt-hr of capacity, a discharge rate maximum of 93 kW and maximum short circuit current of 4800 Amps. A Curtis Albright SW1000 A DC contactor affords 1000 A expected maximum application requirements, and consumes 30 W of coil power when activated. American Power Design&#39;s H30 DC-DC converter supplies the Contactor coil power over the wide range of input voltage. A Dallas Semiconductor Max5090A affords regulated 3.3 V power to the National Semiconductor LM139 comparator over an input voltage range of 6.5 to 76V. A Logic-level, 5V signal from LM139 drives power transistor VN02NSP by STMicroelectronics, enabling the DC-DC converter  24  and the delay  26 . A typical 48V brushless DC motor having 95% efficiency and a controller of similar efficiency exhibits a functional input range of 40 to 50 VDC and exhibits combined efficiency of 90%. When applied to single unit of the described UC, the UC output would be below the useable range of the motor controller in 1 second, when drawing at a 500 amp rate. Utilizing the prescribed apparatus of this invention and using available high-efficiency DC-DC converter technology having a 2:1 input range from 20-40 Volts, 500 amps could be then drawn for 5 additional seconds, an increase of 500%. The use of the upconversion device in only the lower voltage or state-of-charge condition averts the upconversion loss when not required. When such a system is in regenerative mode, the vehicle application controller signals the Comparator Circuit, closing the Contactor enabling reverse energy flow to the UCS. This all-UC based design permits full-rate regenerative braking energy flow from the wheel motor-generators, where other designs either waste the excess energy in heat, or require additional storage. In operational cases where the UCS is currently at a full SOC, additional UCS storage would also be required when regenerative braking energy becomes available, however, this is accommodated in design with a larger UCS system in the prescribed apparatus, and is more efficient in energy transfer and weight than a combined battery system, translating to higher vehicle efficiency. 
         [0032]    In the preferred embodiment of the invention, the waste heat-to-electric conversion system  49  shown in  FIGS. 1 and 5  comprising hot fluid and cold fluid passages formed by plates  61  and alternating and fastened “fins”  63  which mechanically interconnect the plates  61  and provide extended heat transfer surface area, heat-to-electric modules (thermoelectric or thermionic, hereafter referred to as thermoelectric modules) which are held in thermal contact with hot and cold side plates in a stacked arrangement by mechanical fastening such as bolts  61  or other mechanical fastening means, insulation  64  to reduce heat transfer other than through the thermoelectric modules  51 , and insulation  61  to reduce heat transfer from the outer channel to the ambient environment, generates electricity. Hot exhaust stream flow is ducted into, and out of, the conversion system  49  by manifolds  45  and  46  respectively. The circuitous exhaust stream flow path in the conversion system  49  also serves to dissipate exhaust stream audible noise (similar to a muffler). Fin stock is commonly obtained in bent form via large area forming dies and assembled with the plates to form a completed structure or subassembly as shown in  FIG. 6 . Coolant flow is ducted into the conversion system  49  by manifolds  47  and  48 . 
         [0033]    For the given case of the present invention, a 160 mile-per-gallon, 338 kg vehicle having an 18 hp (13.4 kW) diesel engine has an exhaust temperature is 400° C. Ducted air, having a maximum temperature of 70° C. is used as the coolant, and is extracted from the vehicle exterior to maintain the cold side temperature of the thermoelectric modules. For corrosion resistance, carbon steel is the selected construction material for exhaust stream-side plate and fin material, having a mean thermal conductivity of 47 W/m°K. Thermal analysis and geometrical optimization of compact heat exchangers are provided in heat transfer texts (reference  1 ) as: 
         [0000]    
       
      
       Q=U×A×LMTD  
      
     
         [0000]    Where Q=total heat exchanged
 
U=overall heat transfer coefficient
 
A=exchanger surface area
 
LMTD=log mean temperature difference
 
         [0000]    
       
         
           
             LMTD 
             = 
             
               
                 ( 
                 
                   
                     GTD 
                     - 
                     LTD 
                   
                   
                     ln 
                      
                     
                       ( 
                       
                         GTD 
                         / 
                         LTD 
                       
                       ) 
                     
                   
                 
                 ) 
               
               × 
               CF 
             
           
         
       
     
       Where: 
       [0034]    GTD=larger of (T 1 −t 2 ) or (T 2 −t 1 )
 
LTD=smaller of (T 2 ×t 1 ) or (T 1 −t 2 )
 
And fin geometries optimized by:
 
         [0000]        h =( J//t ) 0.66 ×(0.323)×( D×V×L/Dv ) 0.5   ×D×Cp×J    
       Where: 
       [0035]    h=Heat transfer coefficient
 
J=Velocity of the air past the fin
 
D=Air density
 
L=Length of the cooled surface in the direction of air flow
 
D v =Dynamic viscosity
 
t=Thickness of the boundary layer
 
C p =specific heat of the cooling air
 
         [0036]    For the given condition, and in a counter-flow configuration (hot and cold fluids flow in opposing directions) an LMTD of 208° C. is obtained giving rise to 0.2 square meter of transfer area required using 1 millimeter thick plate material. Design considerations of exhaust back pressure (impact on ICE performance), fluid pumping power, and thermoelectric module area impact final design of channel size and number of channels. In the presented case, thermoelectric module size requirements drive the areal dimensions, per the following prescription. 
         [0037]    Heat-to-Electricity conversion is performed by thermo-electric modules  51  as displayed in  FIG. 7  comprising individual thermoelectric devices  54  which are exposed to hot and cold surfaces on opposite sides through electrically-isolating and thermally conducting substrates  52 . The individual thermoelectric devices  54  are electrically-interconnected by a conductive pattern  53  applied to substrates  52 . An interconnection system  55  connects the multiplicity of modules  51  and carries the summary power to the ESS control system  7  ( FIGS. 1 ,  3 ). 
         [0038]    Illustrated in  FIG. 7 , advanced thermal diodes (reference U.S. Pat. No. 6,396,191) providing thermal conversion efficiency of 20% are of 2 square millimeters in area, and deliver 10 amps per a 2 diode stack, developing 80 mV when subjected to the stated thermal difference. A 12V module of dimension 30 mm×30 mm, delivers 120 Watts via a series connection of 150 diode stacks. 32 total modules connected serially in groups of 4 to provide 48V output and the groups then connected in parallel, provide extraction of 3.75 kW from the exhaust waste heat stream from the stated engine. Aluminum nitride (sintered) of 1 mm thickness is the substrate material (thermally conductive, electrically insulating). Construction consisted of substrate metallization and patterning, solder pre-form, diode placement and reflow. The modules are set into metallized (for oxidation control) locating recesses, in an array of 4×4 in the carbon steel plates, thus requiring 2 layers of modules as shown in  FIG. 5 . Refractory fiber high temperature insulation (thermal conductivity of 0.7 BTU-in. /Hr. Ft2° F. in sheet form by Cotronics) surrounds the perimeter of modules to avert heat transfer other than through the thermoelectric modules. 
         [0039]    Herein, the invention has been described with reference to specific embodiments, however, modifications and variations of the invention may be constructed without departing from the scope of the invention as it is defined in the following claims.