Abstract:
A Combined Heat and Power System (“CHPS”) includes a solid oxide fuel cell system and a vapor compression cycle heat pump. The CHPS improves the overall efficiency of a CHP system with respect to conversion of fuel energy to usable heat and electrical energy without need for an accessory burner-heat exchanger system. The compressor motor of the heat pump is powered by a portion of the electricity generated by the SOFC, and the thermal output of the heat pump is increased by abstraction of heat from the SOFC exhaust. This integration allows for novel and complementary operation of each type of system, with the benefits of improved overall fuel efficiency for the improved CHP system.

Description:
[0001]    This invention was made with United States Government support under Government Contract/Purchase Order No. DE-FC26-02NT41246. The Government has certain rights in this invention. 
     
    
       [0002]    The present invention relates to fuel cells; more particularly, to an Auxiliary Power Unit (APU) including a solid oxide fuel cell (SOFC) system; and most particularly, to a Combined Heat and Power System (CHPS) for producing electric power and heating through combination of an SOFC system and a vapor-compression-cycle heat pump (VCCHP). 
       BACKGROUND OF THE INVENTION 
       [0003]    Solid Oxide Fuel Cell systems are high-efficiency generators of electric power from a variety of fuels including Natural Gas, Liquified Petroleum Gas (LPG), Ethanol, and other hydrocarbon and non-hydrocarbon fuels. Due to the high operating temperature of an SOFC (700-900° C.), the tail pipe exhaust is generally also at a high temperature. A known state-of-the-art integration of SOFC systems is as part of a Combined Heat and Power System (CHPS). Prior art CHP systems use the electrical output of the SOFC system directly, and also utilize the energy leaving the SOFC system in the form of hot exhaust for heating air or water for space heating or for heating water for domestic usage (showers, etc). 
         [0004]    No fuel cell system is 100% efficient, so there will always be heat leaving in the SOFC exhaust. For a typical 1 kW electrical service demand (e.g., a small residence), the heating or thermal needs are typically in the range of 5-10 kW. If the SOFC system has a reasonably good electrical efficiency, for example 33%, the heat output for 1 kW net electric output is 2 kW. Since 2 kW is much less thermal energy than desired, auxiliary direct-fueled condensing or non-condensing burner-heat exchangers are commonly used to make up the difference. The best of these are 80-90% efficient in converting fuel to electric and thermal energy. 
         [0005]    What is needed in the art is an improved CHP system with increased overall fuel efficiency. 
         [0006]    It is a principal object of the present invention to increase the fuel efficiency of a CHP system. 
       SUMMARY OF THE INVENTION 
       [0007]    Briefly described, the invention seeks to improve the overall efficiency of a CHP system with respect to conversion of fuel energy to usable heat and electrical energy. In addition, a method to flexibly close the gap between thermal energy available vs. thermal energy demand is presented without the need for an accessory burner-heat exchanger system. The invention is directed to an improved CHP system which combines a VCCHP system with an SOFC system, both of which are well known in the art, for application as a combined CHP system wherein the compressor motor of a heat pump is powered by a portion of the electricity generated by the SOFC, and wherein the thermal output of the heat pump is increased by abstraction of heat from the SOFC exhaust. This integration allows for novel and complementary operation of each type of system, with the benefits of improved overall fuel efficiency for the improved CHP system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0009]      FIG. 1  is a schematic drawing of a first exemplary CHP system in accordance with the invention; 
           [0010]      FIG. 2  is a diagram of a second exemplary CHP system wherein the evaporator section of a VCCHP is interfaced with system process air; 
           [0011]      FIG. 3  is a diagram of a third CHP system embodiment wherein the evaporator section of a VCCHP is interfaced only with SOFC exhaust which is tempered by mixing with intake system process air; 
           [0012]      FIG. 4  is a table showing total percent efficiency of a CHP system in accordance with the invention as a function of electric demand and compressor power; and 
           [0013]      FIG. 5  is a table showing kW thermal output of a CHP system in accordance with the invention as a function of electric demand and compressor power. 
       
    
    
       [0014]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate currently preferred embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    Referring to  FIG. 1 , a schematic drawing of a first exemplary CHP system embodiment  10  in accordance with the invention is shown. A solid oxide fuel cell system  12  as is well known in the fuel cell arts is provided with a supply of fuel  14  and air  16 . Fuel  14  is typically a hydrocarbon fuel conventionally available in liquid or gaseous form such as an alkane or alcohol. It is also known to fuel an SOFC directly with ammonia, obviating the need for a reformer. SOFC  12  provides electric power  18  and also emits a hot exhaust  20  that is directed through one side of a heat exchanger  22 , creating a partially-cooled exhaust  24  that may be discharged to atmosphere  26 . 
         [0016]    A VCCHP system  28  includes conventionally a compressor  30  driven by an electric motor  32 ; a heat exchanger condenser  34 ; an expansion valve  36 ; and a heat exchanger evaporator  38 . VCCHP system  28  may be of a prior art type with a suitable refrigerant (first working medium). As used herein, a “working” medium is a fluid medium recirculated in a closed loop. The first working medium is pumped as a gas through a first side of heat exchanger/condenser  34  wherein the medium is condensed to a heated liquid wherein the heat of vaporization is recovered. A second thermal carrier fluid medium  41  is pumped by a recirculation pump  35  through the second side of heat exchanger/condenser  34 , abstracting heat from the hot first working medium, and thence through a customer application  37  requiring heated fluid  39 , for example, hot air, hot water, or hot refrigerant. The second fluid medium may be in a closed system wherein heat is extracted therefrom in customer application  37  and the medium is then returned  43  for reheating; or application  37  may consume the heated second working medium, in which case fresh cold medium is supplied to recirculation pump  35 . Advantageously, the second fluid medium is also passed through the second side of heat exchanger  22  wherein the second fluid medium is further heated by abstraction of waste heat from hot SOFC exhaust  20 . The thermal output of VCCHP system  28  is thus augmented by heat from exhaust  20  in accordance with the invention, and thus the thermal efficiency of the overall CHP system is substantially increased. 
         [0017]    In addition, a portion or all  40  the balance of the SOFC exhaust heat  24  may be channeled to the evaporator  38  of the heat pump system, thus providing an additional temperature sink for the remainder of the SOFC heat before final exhaust  26 ′. This provides additional efficiency, and/or expanded operating range for lower outdoor or low temperature reservoir temperatures. 
         [0018]    In operation, electric compressor motor  32  is driven by a portion of the electric power  18  of SOFC system  12 . The heat pump system drives evaporator  38  to a temperature below the temperature of a low temperature reservoir  52  or SOFC exhaust  40 , causing heat to flow to the first working medium. The compression process condenses the first working medium and increases the temperature thereof to a temperature above a high temperature reservoir. The condensed high temperature medium then passes through condenser  34  which has heat exchange with a high temperature reservoir defining a thermal carrier medium, for example, space heating air or coolant or water for circulation heating in customer application  37 . A separate water loop may be channeled through the condenser to handle domestic water needs (showers, drinking, etc.). In this way, heat from both the low temperature reservoir  52 , 40  and electric power  18  are channeled to the high temperature reservoir (coolant, water, or air  39 ). The amount of heat transferred from the low temperature reservoir to the high temperature reservoir is a function of the amount of compression power (assuming non-limiting cases in heat exchangers etc.). 
         [0019]    For a heat pump system, a coefficient of performance (COP) is defined as the heat output to the high temperature reservoir divided by the heat, or work, driven into the refrigerant by the compressor. COPs for good heat pump systems are typically between 2 and 3. This means that 2 to 3 times the electric power (minus motor losses) driven to the compressor is driven to the high temperature reservoir (air, coolant, or water). This is a primary efficiency improvement for the utilization of fuel power to heat power. 
         [0020]    Heat exchanger  22  may be separate from VCCHP system  28  or may be an integrated heat exchanger with condenser/exchanger  34  used to transfer the SOFC exhaust heat to the same heating air, coolant, or heating water. Since the SOFC exhaust  20  is at a higher temperature than the condenser  34  of the heat pump system, additional heat flows from the exhaust of the SOFC to the coolant. Where constant massflow of coolant or air is desired at a prescribed temperature, the heat pump compressor may be driven at variable speed to adjust the heating load depending on demand or operating conditions. By this method, a novel simple control may be obtained for either constant temperature or constant massflow heating needs under variable demand or environmental conditions. 
         [0021]    Note that with the addition of a compressor reversing valve (not shown but well known in the prior art) in the refrigerant loop of VCCHP  28 , the heat pump may be reversed, and condenser  34  becomes the evaporator, and evaporator  38  becomes the condenser. Heat exchanger  22  is bypassed, and the hot SOFC exhaust is not used with the VCCHP  28 ; SOFC electrical output  18 , however, continues to power the pump compressor. In this way, electrical air conditioning, or water or coolant chilling, may be provided during warm months where heating is not needed. This provides additional features to the customer not provided by any prior art CHP system, all of which are limited to electric power generation and heating. 
         [0022]      FIG. 2  shows an exemplary second embodiment  110  of an SOFC Heat Pump CHP system in accordance with the invention. A key feature is the integration of the heat exchanger for evaporator  138  with the process air inlet and exhaust streams  152 , 154 , respectively, of the device. Thus, separate heat exchanger  22  ( FIG. 1 ) is omitted, and all heat from SOFC exhaust  20  is entered into the heat pump through extraction by evaporator  138 . 
         [0023]    An SOFC system normally intakes both process air and auxiliary cooling air (cabinet, electronic, and space cooling) from an external source and vents the hot exhaust to a suitable outside air space. In this embodiment, the evaporator also draws heat out of the process air  152  coming into the system. This low temperature air is used for cooling and SOFC system operation. The lower temperature process air intake  152  improves the efficiency of the SOFC air pumps and blowers as well as improving the cooling of onboard electronics and other devices. The heat entering evaporator  138  from this stream becomes available to the application at the condenser  134  through the heat pump system operation. The hot system exhaust stream  20  also travels through evaporator  138  giving additional heat input to the heat pump process (analogous to stream  40  in  FIG. 1 ). This integration allows for access to the low temperature heat source in the outside air without having to place an evaporator outside of the system or appliance boundary  160 , or directly outdoors. This integration also improves system cooling and allows for efficient use of system exhaust heat. The specifics of the ducting and heat exchanger technology are not critical to the invention, but use of concentric inlet and outlet ducts and multi-pass heat exchangers enhances the functionality and performance and are obvious to those skilled in the art. 
         [0024]    Referring to  FIG. 3 , an exemplary third embodiment  210  in accordance with the invention is generally similar to first embodiment  110  but outside air  16  bypasses evaporator  238  and passes directly to SOFC  12  as process air and cooling air. Additionally, air intake plenum  252  includes a diversion plenum  270  connecting the outside air  16  with hot SOFC exhaust  20  such that a portion of the intake air may be diverted ahead of SOFC  12  and mixed with the SOFC exhaust in a mixing zone  272  to adjust the temperature of heating gas being passed through evaporator  238 . Exemplary steady-state operating temperatures are provided for various locations in system  210 . 
         [0025]    System efficiencies and thermal outputs of a combined SOFC and VCCHP CHP system in accordance with the invention are shown in  FIGS. 4 and 5  for variable electric demand. Note that the first row of these tables, wherein compressor input is 0 kW, represents a prior art CHP system wherein the heat in the system exhaust is recovered via an auxiliary burner-heat exchanger. Thus it is seen that for the typical prior art CHP case without a heat pump, thermal output is low at low electric loads, and is insufficient to meet high thermal demand at any electrical load; hence the need for the supplemental burner. In the present invention, the thermal demand may be met with high efficiency at low to moderate electrical demand through use of a VCCHP. This is a primary advantage of the invention. 
         [0026]    While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.