Abstract:
An absorption system powered by low temperature heat for producing at least one of refrigeration and power is disclosed, wherein a low-pressure drop heat reclaimer  1  reclaims heat from the source into a heating agent, which in turn supplies heat to the absorption cycle desorber  5  via internal coils  7.  The extra temperature differential normally present in closed cycle heating systems is avoided by using the absorption working fluid as the heating agent, in an integrated system.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to methods of efficiently applying low temperature heat to absorption refrigeration cycles and absorption power cycles. In conventional absorption cycles, high temperature heat is applied to a high-pressure desorber or generator, where high-pressure vapor is desorbed from the absorbent solution. When the resulting vapor is pure refrigerant, as with LiBr-H 2 O absorption cycles, no further treatment is necessary. When the resulting vapor has appreciable absorbent content, as with NH 3 -H 2 O absorption cycles, it is necessary to distill, analyze, or rectify the vapor to higher refrigerant purity by contacting it with lower temperature absorbent. That distillation may be done either adiabatically or diabatically. The external heat addition portion of the desorber is customarily termed the generator, and the distillation portion may have internal heat addition.  
           [0002]    When the external heat source is at relatively low temperature, for example only modestly above the generator temperature, and when it has a temperature glide, then very little of the heat content of the source can be effectively transferred to the generator using conventional techniques. Consider for example a combustion exhaust stream at 270° C., and an absorption cycle generator at 170° C. Given a 30° C. minimum temperature difference for heat transfer, it is only possible to cool the heat source from 270° C. to 200° C. by transferring heat to the generator. This is only on the order of 30% of the available heat content of that source.  
           [0003]    Two other possible problems arise when supplying low temperature waste heat such as combustion exhaust gas to an absorption cycle. With one approach, the combustion exhaust directly contacts the heat transfer surface of the generator. However, there are usually stringent limitations on the allowable pressure drop of the exhaust gas. For example, the backpressure for a combustion turbine is typically specified at no more than six to ten inches water column. The generator which satisfies both this criterion and also the specialized mass transfer criteria of the absorbent solution will be very large and costly. That is, the transfer geometry necessary for effective desorption is very different from that necessary for low Δp extraction of heat from combustion gas. Alternatively a closed cycle heat transfer fluid can be circulated between the heat source and the generator, such that the geometry of each heat exchanger is free to be optimized for the respective requirements. This has the disadvantage that two separate heat exchanger temperature differentials are interposed between the waste heat and the absorbent solution in the generator. For example, the heat transfer fluid must be heated to well above the generator peak temperature. If water is the heat transfer fluid, it will have to be at a much higher pressure than the generator.  
           [0004]    There are a variety of hydrocarbon-fueled prime movers which exhaust a combustion gas, including gas turbines, microturbines, reciprocating engines, and fuel cells. Depending upon the prime mover, the exhaust temperature varies from 200° C. to 550° C. There is increasing need and desire to convert that exhaust heat to useful purpose, such as cooling, refrigeration, shaft power, or electricity. It is one objective of the present invention to convert greater fractions of waste heat to useful purpose than has heretofore been possible. It is another objective to avoid the prior art disadvantages of applying waste heat to absorption cycles, i.e., the high backpressure associated with direct contact heat transfer, and the high temperature differentials associated with pump-around loops. That is, there is a need for a method of transferring heat from a low temperature sensible heat source to an absorption cycle which avoids the Δp and ΔT and high pressure penalties associated with traditional methods, while achieving greater utilization of the heat source, i.e., more useful result.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    The above and other useful objects are achieved by apparatus wherein thermal energy is converted into at least one of refrigeration, cooling, and shaft power comprising:  
           [0006]    a) an absorbent solution comprised of sorbate plus absorbent;  
           [0007]    b) a desorber comprised of:  
           [0008]    i) an entry port for sorbate-rich liquid absorbent;  
           [0009]    ii) a means for separating said sorbate-rich absorbent into sorbate vapor and sorbate-lean absorbent;  
           [0010]    iii) an exit port for said sorbate vapor; and  
           [0011]    iv) an internal heat exchanger which has an entry port in communication with said sorbate-lean absorbent;  
           [0012]    c) an external heat exchanger which is in thermal contact with said thermal energy;  
           [0013]    d) a first flowpath from an exit port of said internal heat exchanger to said external heat exchanger; and  
           [0014]    e) a second flowpath from said external heat exchanger to said desorber;  
           [0015]    and also by process comprising:  
           [0016]    a) circulating an absorbent solution successively through absorbing and desorbing steps;  
           [0017]    b) desorbing the absorbent solution into high-pressure sorbate vapor and heated strong absorbent by heating it;  
           [0018]    c) using the heated strong absorbent as the heating agent in step b);  
           [0019]    d) reheating said heating agent by thermally contacting it with said thermal energy; and  
           [0020]    e) combining said reheated heating agent with said heated strong absorbent.  
           [0021]    The greater utilization of the thermal energy in the waste heat or other low temperature heat source is accomplished by applying it to a heat transfer agent, and then applying the heat transfer agent heat to at least part of a distillation step,( when present) which is at lower temperature, and/or by applying it to an intermediate-pressure desorber which is at lower temperature. Either or both of these steps further reduce the heat transfer agent temperature to below the high-pressure generator temperature, and in turn make it possible to reclaim lower temperature heat from the heat source. With this technique, the heat transfer agent can be routinely cooled to approximately 80° C. or lower, which means the combustion gas can be cooled to approximately 100° C. or lower. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0022]    [0022]FIG. 1 depicts one embodiment of the integrated heating system constituent parts and their arrangement.  
         [0023]    [0023]FIG. 2 depicts a two-pressure single-effect absorption cycle with co-current mass exchangers which produces cooling from low temperature waste heat using the integrated heating system.  
         [0024]    [0024]FIG. 3 depicts a three-pressure absorption cycle for a volatile absorbent such as NH 3 -H 2 O which is adapted to produce shaft power from waste heat using an integrated heating system.  
         [0025]    [0025]FIG. 4 depicts a two-pressure absorption cycle adapted to produce both power and cooling from combustion turbine exhaust via an integrated heating system.  
         [0026]    [0026]FIG. 5 depicts a three-pressure absorption refrigeration cycle powered by low temperature heat via an integrated heating system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    Referring to FIG. 1, a low temperature sensible heat stream such as combustion exhaust gas is supplied to heat reclaimer  1  through inlet  2 , where it contacts the external heat exchanger  3 . Pump  4  circulates a heat transfer fluid through heat exchanger  3 , in direction overall counter-current to the flow direction of the exhaust gas. By having the heat reclaimer  1  vertically oriented as shown, any condensate formed on the cooler bottom coils drains away, and also the coils can be adapted to be self-draining should pump  4  fail, thus preventing over-pressurization. The heated heat transfer fluid exits reclaimer  1  preferably as a two-phase mixture and is routed to desorber  5 , where phase separation occurs. The resulting liquid phase comprised of both liquid from the reclaimer and also sorbate-lean absorbent solution (i.e. “weak” absorbent) from the remainder of the desorber, is routed through pipe  6  into internal heat exchanger  7  which supplies heat to colder portions of the desorber, for example, by means of a succession of vertically stacked diabatic trays  49 . The hot vapor also traverses up through the desorber, on the other side of internal heat exchanger  7 . The purified vapor exits the generator through pipe  8  and is routed to the remainder portion of the absorption cycle  9 . The heat transfer fluid exits the internal heat exchanger  7  and desorber  5  through pipe  10 , and is split at splitter  12 , with part going via pressure letdown valve  13  to the absorption step in portion  9 , and the remainder to pump  4  for recycle to reclaimer  1 . The high-pressure vapor from pipe  8  is converted in portion  9  to a low-pressure vapor, via a condenser and evaporator so as to produce cooling, and/or via a work expander to produce shaft power. The resulting low-pressure vapor and absorbent from pipe  10  are subsequently recombined in portion  9  and pumped back to the entry port for sorbate-rich absorbent of desorber  5  via pipe  11 . The heat exchanger in reclaimer  1  can be comprised of concentric tube coils, pancake tube coils, or any other known geometry, e.g., fin tubes, folded plates, or others such as those used for steam cycle economizers. Particularly pertinent are the steaming type of economizers which ordinarily produce a two-phase mixture. With ammonia-water cycles, the heat transfer fluid will usually be nearly pure water, and the pressure will be essentially the generator pressure, since the two fluids combine at the generator. With LiBr-H 2 O absorption cycles, the circulating heat transfer fluid will be concentrated LiBr solution.  
         [0028]    By integrating the heat transfer fluid directly into the absorption cycle, the advantage is retained that the reclaimer can be optimized for the necessary low pressure drop, and yet there is no additional temperature differential penalty because the heating fluid temperature never increases to appreciably above the hottest generator temperature. Since most of the heating duty in the heat reclaimer is sensible heating of the heating agent, the temperature difference between the heating agent and the combustion exhaust can be relatively constant, resulting in highly efficient heat exchange, i.e., avoiding the pinch temperature associated with constant temperature boilers.  
         [0029]    In FIG. 2 and succeeding figures, objects with similar descriptions are afforded the same number in each sequence, e.g., object  201  of FIG. 2 is described similarly as object  101  of FIG. 1.  
         [0030]    Referring to FIG. 2, low temperature sensible heat is supplied to heat reclaimer  201  via entry port  202 . Pump  204  circulates heat transfer agent through reclaimer  201  counter-currently to the exhaust flow direction. Two-phase heat transfer agent is then routed to the hot end of generator  205  (also called a desorber). Vapor is withdrawn via pipe  208 , and hot liquid is supplied to an internal heat exchanger in generator  205  via pipe  206 . That liquid exits at pipe  210 , is split at splitter  212 , with part being recycled via pump  204 , and the remainder supplied to low-pressure absorber  217  via pressure letdown valve  213 . High-pressure vapor in pipe  208  is condensed in condenser  214 , subcooled in subcooler  215 , reduced in pressure in pressure letdown  219 , and evaporated in evaporator  216 . The resulting low-pressure vapor is absorber into sorbate-lean (“strong”) absorbent  217 , which is cooled by coolant  220 , and the resulting sorbate-rich (“weak”) absorbent is pumped by pump  218  back to desorber  205 . The various exchanges may be shell and tube, coil in shell, or other known types.  
         [0031]    Referring to FIG. 3, waste heat enters reclaimer  301  through entry port  302 . Heat transfer fluid is counter-currently circulated through steaming economizer  303  via pump  304 , and thence to the bottom of desorber column  305 , where phase separation occurs. The liquid phase enters internal heating coils  307  via inlet pipe  306 . Part of the llliquid phase is split off at splitter  312  and routed to pressure letdown  313  via solution heat exchanger  326 . The remainder heats the colder top end of column  305 , then supplies lower temperature heat to intermediate pressure desorber  323 , and then is recycled by pump  304 . Desorber vapor in pipe  308  is superheated in superheater  321  by counter-current heat exchange with the source heat, in parallel with exchanger  303 . Then the superheated vapor is work-expanded in expander  322 . The resulting low-pressure vapor is absorbed in low-pressure absorber  317  into the strong absorbent from letdown  313 , while absorption heat is removed  
         [0032]    By integrating the heat transfer fluid directly into the absorption cycle, the advantage is retained that the reclaimer can be optimized for the necessary low pressure drop, and yet there is no additional temperature differential penalty because the heating fluid temperature never increases to appreciably above the hottest generator temperature. Since most of the heating duty in the heat reclaimer is sensible heating of the heating agent, the temperature difference between the heating agent and the combustion exhaust can be relatively constant, resulting in highly efficient heat exchange, i.e., avoiding the pinch temperature associated with constant temperature boilers.  
         [0033]    In FIG. 2 and succeeding figures, objects with similar descriptions are afforded the same number in each sequence, e.g., object  201  of FIG. 2 is described similarly as object  101  of FIG. 1.  
         [0034]    Referring to FIG. 2, low temperature sensible heat is supplied to heat reclaimer  201  via entry port  202 . Pump  204  circulates heat transfer agent through reclaimer  201  counter-currently to the exhaust flow direction. Two-phase heat transfer agent is then routed to the hot end of generator  205  (also called a desorber). Vapor is withdrawn via pipe  208 , and hot liquid is supplied to an internal heat exchanger in generator  205  via pipe  206 . That liquid exits at pipe  210 , is split at splitter  212 , with part being recycled via pump  204 , and the remainder supplied to low-pressure absorber  217  via pressure letdown valve  213 . High-pressure vapor in pipe  208  is condensed in condenser  214 , subcooled in subcooler  215 , reduced in pressure in pressure letdown  219 , and evaporated in evaporator  216 . The resulting low-pressure vapor is absorber into sorbate-lean (“strong”) absorbent  217 , which is cooled by coolant  220 , and the resulting sorbate-rich (“weak”) absorbent is pumped by pump  218  back to desorber  205 . The various exchanges may be shell and tube, coil in shell, or other known types.  
         [0035]    Referring to FIG. 3, waste heat enters reclaimer  301  through entry port  302 . Heat transfer fluid is counter-currently circulated through steaming economizer  303  via pump  304 , and thence to the bottom of desorber column  305 , where phase separation occurs. The liquid phase enters internal heating coils  307  via inlet pipe  306 . Part of the llliquid phase is split off at splitter  312  and routed to pressure letdown  313  via solution heat exchanger  326 . The remainder heats the colder top end of column  305 , then supplies lower temperature heat to intermediate pressure desorber  323 , and then is recycled by pump  304 . Desorber vapor in pipe  308  is superheated in superheater  321  by counter-current heat exchange with the source heat, in parallel with exchanger  303 . Then the superheated vapor is work-expanded in expander  322 . The resulting low-pressure vapor is absorbed in low-pressure absorber  317  into the strong absorbent from letdown  313 , while absorption heat is removed by cooling heat transfer stream  320 . The resulting absorbent is pumped to intermediate-pressure in pump  318 , then split into a feed to intermediate-pressure desorber  323  and to intermediate-pressure absorber  324 . Vapor from intermediate-pressure desorber  323  is separated at separator  327  and then absorbed in intermediate-pressure absorber  324 . Pump  325  pumps the resulting weak absorbent back to high pressure for re-entry into column  307 . The FIG. 3 cycle incorporates both counter-current mass exchange columns ( 305  and  317 ) and co-current mass exchangers ( 323  and  324 ). Branch pump  328  improves the linearity of the temperature glide in column  307 .  
         [0036]    Referring to FIG. 4, a two-pressure absorption cycle for a volatile absorbent such as aqua ammonia is depicted, adapted to be powered by combustion turbine waste heat, and further adapted to co-produce both shaft power and also refrigeration, for cooling the turbine inlet air or other cooling loads. Air compressor  451  is supplied air through filter  452  and cooling coil  453 . The compressed air supports combustion in combustor  454 , and the resulting hot pressurized combustion gas is work-expanded in turbine  455 . The combustion exhaust is ducted through exhaust duct  456  to optional heat recovery steam generator (HRSG)  457 , and thence to heat reclaiming section  401 , comprised of heating agent heater  403 , superheater  421 , and HRSG economizer  458 . The heating agent is supplied to the sump of column  405  where it phase separates. The liquid fraction enters internal exchanger  407  through entry port  406 , and part is split off at splitter  412 , and sent to letdown valve  413 , thence to low-pressure absorber column  417 . Low-pressure vapor from turbine  422 , evaporator  416 , and inlet cooler  453  is absorbed in low-pressure absorber  417 , with the colder portion of the heat of absorption removed by cooling stream  420 , and the warmer portion by high-pressure GAX (generator absorber heat eXchange) desorption coil  459 , from which the two-phase mixture is routed to a mid-height of column  405 . Part of the pumped weak absorbent from pump  418  is routed to GAX coil  459 , through split control valve  460 , and the remainder is routed through split controller  461  to solution-cooled rectifier  462 , and then sprayed into the top portion of column  405 . Pump  404  circulates the heating agent. The vapor split between turbine  422  and coolers  416  and  453  is controlled by valves  463  and  464 , respectively. As shown, those two vapors can be of differing purity, governed by the height of column  405  from which they are withdrawn. It is desirable to send quite high purity vapor to condenser  414 , for example at least 95% purity ammonia.  
         [0037]    Referring to FIG. 5, low temperature heat supplied to reclaimer  501  heats heating agent in fin coils  503 . Then the two-phase heating agent is routed to the sump region of desorption column  505 , where the phases separate. The liquid phase enters entry port  506  of internal heat exchanger  507 , a succession of coils on vertically stacked vapor-liquid contact trays  549 . High-pressure vapor from column  505  is condensed in condenser  514 , subcooled in subcooler  515 , expanded in pressure letdown  519 , and evaporated in evaporator  516 , thus producing refrigeration and low-pressure vapor. That vapor is absorbed into the strong absorbent from splitter  512  and pressure letdown  513 , in low-pressure absorber column  517 . Column  517  has three sets of cooling coils, in top to bottom (hot to cold) order: High-pressure GAX desorption coil  559  (shown as occupying two trays  548 ); intermediate-pressure GAX desorption coil  547 , (shown as a occupying single tray  546 ); and the bottom coils for external cooling agent  520 , shown as occupying two trays  545 . The absorbent from low-pressure absorber  517  is pumped to intermediate-pressure by pump  518 , then split by valves  544  and  543  into feeds to an intermediate pressure GAX absorber  547  and the intermediate-pressure absorber  524 . The weak absorbent (water with high ammonia content) from intermediate-pressure absorber  524  is pumped to high pressure by pump  525 , and split into two streams by valves  542  and  541 ; the former stream being supplied sequentially to solution-cooled rectifier coil  540  and then to high-pressure GAX desorber coil  559 , and finally to column  505  as two-phase; and the latter directly injected into column  505 . Branch pump  528  supplies a mid-height of column  505 , thereby providing a more linear temperature glide in that column.  
         [0038]    The three pressure cycles have similarity to prior art disclosures such as U.S. Pat. No. 5,097,676. The diabatic counter-current columns such as the desorber (distillation column) and low-pressure absorber (reverse distillation column) may be any known geometry. One preferred geometry is the diabatic multi-tray design with contact coils, such as disclosed in U.S. Pat. No. 5,798,086. Particularly preferred are those diabatic trays with same-direction liquid flow and minimal vapor mixing, as disclosed in International Publication No. WO 00/10696, dated Mar. 2, 2000.