SUPERCRITICAL CARBON DIOXIDE REGENERATIVE BRAYTON CYCLE WITH MULTIPLE RECUPERATORS AND AUXILIARY COMPRESSORS

Abstract

Method for producing energy by means of a supercritical carbon dioxide (sCO2) regenerative Brayton cycle with N recuperators in series and N or N1 auxiliary compressors, where N3. By using a higher number of recuperators in series and an auxiliary compressor for each recuperator, the heat recovery process is improved and thus the performance of the cycle compared to the cycles of the state-of-the-art.

Claims

1. Method for producing energy by means of a supercritical carbon dioxide (sCO.sub.2) regenerative Brayton cycle with N recuperators in series and N or N1 auxiliary compressors, where N3, the method comprising a main compression process, and further: a. expanding a stream of supercritical CO.sub.2 (sCO.sub.2) in a turbine to a pressure between 3 MPa and 10 MPa to generate a sCO2 stream RHI.sub.N, for generating some mechanical or electrical energy; b. cooling the sCO.sub.2 stream RHI.sub.N in N recuperators, this cooling step comprising the following steps c to e: c. cooling the sCO.sub.2 stream RHI.sub.N to a stream RHO.sub.N by heating a stream RCI.sub.N to generate a stream RCO.sub.N in the recuperator number N, stream RHI.sub.N1 corresponds to the stream RHO.sub.N; d. cooling the sCO.sub.2 stream RHI.sub.N1 to generate a stream RHO.sub.N1 in recuperator number N1 by heating a stream RCI.sub.N1 to generate a stream RCO.sub.N1, wherein stream RHO.sub.N1 is split into two streams: a stream RHI.sub.N2 and a stream ACI.sub.N1, compressing stream ACI.sub.N1 in an auxiliary compressor N1 to, preferably, the pressure of stream RCO.sub.N1 thereby generating a stream ACO.sub.N1, mixing stream ACO.sub.N1 with stream RCO.sub.N1, thereby obtaining the mixture stream RCI.sub.N, and sending stream RHI.sub.N2 to recuperator N2; e. If N>3, repeating step d) for the recuperators number N2 to recuperator number 2; f. cooling the sCO.sub.2 stream RHI.sub.1 to generate a stream RHO.sub.1 in recuperator number 1 by heating a stream RCI.sub.1 to generate a stream RCO.sub.1, wherein stream RHO.sub.1 is split into two streams: a stream CI and a stream ACI.sub.1, compressing stream ACI.sub.1 in an auxiliary compressor 1 to, preferably, the pressure of stream RCO.sub.1 thereby generating a stream ACO.sub.1, mixing stream ACO.sub.1 with stream RCO.sub.1, thereby obtaining the mixture stream RCI.sub.2, and sending stream CI to the cooler to generate a stream MCI, and sending the stream MCI to the main compression process, performing an intercooling stage within the main compression process, thereby generating the stream RCI.sub.1 at the exit of the main compression process; wherein N is calculated as i1, for the value of i corresponding to the number of the iteration for which T.sub.out,iT.sub.RHI.sub.Npinch; is fulfilled, considering in iteration i=1 T.sub.in,1 is defined as the temperature of the main compression process inlet, P.sub.in and P.sub.out are defined as the main compression process inlet pressure and outlet pressure and T.sub.out,1 is defined as the outlet temperature of main compression process, and by using following relations for iterations i>1: $\begin{array}{cc}{h}_{\mathrm{in},i}=f\left(T_{\mathrm{in},i},P_{\mathrm{in}}\right)& \left(1\right)\end{array}$ $\begin{array}{cc}{s}_{\mathrm{in},i}=f\left(T_{\mathrm{in},i},P_{\mathrm{in}}\right)& \left(2\right)\end{array}$ $\begin{array}{cc}{h}_{\mathrm{out},s,i}=f\left(P_{\mathrm{out}},s_{\mathrm{in},i}\right)& \left(3\right)\end{array}$ $\begin{array}{cc}{h}_{\mathrm{out},i}=h_{\mathrm{in},i}+\frac{h_{\mathrm{out},s,i}-h_{\mathrm{in},i}}{{}_{s,C,i-1}}& \left(4\right)\end{array}$ $\begin{array}{cc}{T}_{\mathrm{out},i}=f\left(P_{\mathrm{out}},h_{\mathrm{out},i}\right)& \left(5\right)\end{array}$ and calculating T.sub.in,i=T.sub.out,i-1+pinch.sub.i1 at the beginning of each iteration for i>1, a number i of iterations until T.sub.out,iT.sub.RHI.sub.Npinch; is fulfilled, where T.sub.in,i stands for the temperature of (i1).sup.th auxiliary compressor inlet, h.sub.in,i stands for the specific enthalpy of (i1).sup.th auxiliary compressor inlet, h.sub.in,i stands for the specific entropy of (i1).sup.th auxiliary compressor inlet, h.sub.out,s,i stands for the specific enthalpy of the outlet of the (i1).sup.th auxiliary compressor for an adiabatic and isentropic compression, h.sub.out,i stands for the specific enthalpy of stream leaving the (i1).sup.th auxiliary compressor and T.sub.out,i stands for the outlet temperature of (i1).sup.th auxiliary compressor, i being defined as the number of iterations, starting with i=1 for the main compression process and pinch.sub.i being defined as the minimum temperature difference between the cold stream and hot stream of the i.sup.th recuperator.

Description

DESCRIPTION OF THE DRAWINGS

[0043] To complement the description which is being made and for the purpose of aiding to better understand the features of the invention according to a preferred practical embodiment thereof, a set of drawings is attached as an integral part of said description, in which the following has been depicted with an illustrative and non-limiting manner:

[0044] FIG. 1Schematic diagram of the multiple recompression cycle with N recuperators.

[0045] FIG. 2Schematic diagram of the state-of-the-art recompression cycle.

[0046] FIG. 3TemperatureThermal Power Exchange diagram of the heat recovery process within the two recuperators of the state-of-the-art recompression cycle for a high temperature heat source.

[0047] FIG. 4EMBODIMENT 1, schematic diagram of the multiple recompression cycle with four recuperators.

[0048] FIG. 5TemperatureThermal Power Exchange diagram of the heat recovery process within the four recuperators of the multiple recompression cycle for a high temperature heat source.

[0049] FIG. 6EMBODIMENT 2, schematic diagram of the multiple recompression cycle with four recuperators and one intercooling stage.

[0050] FIG. 7TemperatureThermal Power Exchange diagram of the heat recovery process within the four recuperators of the multiple recompression cycle including one intercooling stage for a high temperature heat source.

[0051] FIG. 8EMBODIMENT 3, schematic diagram of the multiple recompression cycle with three recuperators.

[0052] FIG. 9TemperatureThermal Power Exchange diagram of the heat recovery process within the three recuperators of the multiple recompression cycle for a medium-temperature heat source.

[0053] FIG. 10EMBODIMENT 4, schematic diagram of the multiple recompression cycle with three recuperators and three auxiliary compressors.

[0054] FIG. 11TemperatureThermal Power Exchange diagram of the heat recovery process within the three recuperators of the multiple recompression cycle for a low-temperature heat source.

[0055] FIG. 12EMBODIMENT 5, schematic diagram of the multiple recompression cycle with three recuperators.

[0056] FIG. 13TemperatureThermal Power Exchange diagram of the heat recovery process within the three recuperators of the multiple recompression cycle for a high-temperature heat source and a cold sink that allows the CO2 to be cooled to temperatures below its critical temperature.

PREFERRED EMBODIMENT OF THE INVENTION

[0057] As has been set forth, the invention comprises combinations of several elements which have synergistic effects on the improvement of the energy efficiency and on the use of different heat source temperature ranges. Five embodiments are described below, without these examples being a limitation to the possibilities of combination and application of the inventive concepts described above.

[0058] FIG. 4 shows a highly regenerative Brayton cycle with multiple recuperators and auxiliary compressors driven by a high-temperature heat source stream.

[0059] The cycle depicted in said FIG. 4 is a preferred embodiment of the invention for electric generation by means of a heat source available at high temperature. This preferred embodiment, has four recuperators and three auxiliary compressors. It must be noted that, from now on, when making reference to the total sCO.sub.2 mass flow rate, total sCO.sub.2 mass flow being expanded in the turbine is being referred.

[0060] In view of said FIG. 4, the high temperature heat source permits to heat up the sCO.sub.2 stream leaving the recuperator 4 (stream 14) up to 680 C. at 20 MPa (stream 15). The stream 15 is expanded in the turbine to 548 C. and about 7.5 MPa (stream 16).

[0061] Stream 16 enters the hot side of recuperator 4 and is cooled down to 428.5 C. (stream 19) by means of heating stream 10 from 422 C. to 537 C. (stream 14). Stream 19 is then cooled down in the recuperator 3 to 308 C. (stream 20) by heating stream 7 from 301.5 C. to 421.5 C. (stream 8). Auxiliary compressor 3 compresses the 6.4% of the total sCO.sub.2 mass flow rate from about 7.5 MPa and 308 C. to about 20 MPa and 429 C. (stream 9). Stream 9 is mixed with stream 8 to obtain the stream 10 mentioned above. The 93.6% of total sCO.sub.2 mass flow rate goes to the hot side inlet of recuperator 2 at about 7.5 MPa and 308 C. (stream 21).

[0062] Stream 21 is then cooled down in the recuperator 2 to 191.5 C. (stream 22) by heating stream 4 from 185.5 C. to 302 C. (stream 5). Auxiliary compressor 2 compresses the 13.2% of the total sCO.sub.2 mass flow rate from about 7.5 MPa and 191.5 C. to about MPa and 299 C. (stream 6). Stream 6 is mixed with stream 5 to obtain stream 7. The 80.4% of the total sCO.sub.2 mass flow rate goes to the hot side inlet of recuperator 1 at about 7.5 MPa and 191.5 C. (stream 23).

[0063] Stream 23 is then cooled down in the recuperator 1 to 90 C. (stream 24) by heating stream 1 from 85 C. to 187 C. (stream 2). Auxiliary compressor 1 compresses the 28.8% of the total sCO.sub.2 mass flow rate from about 7.5 MPa and 90 C. to about 20 MPa and 183 C. (stream 3). Stream 3 is mixed with stream 2 to obtain stream 4. The 51.6% of the total sCO.sub.2 mass flow rate goes to the cooler at about 7.5 MPa and 90 C. (stream 25).

[0064] Stream 25 is cooled in the cooler from about 90 C. to about 32 C. (stream 26). Stream 26 is compressed in the main compressor from about 32 C. and 7.5 MPa to about 85 C. and 20 MPa (stream 1).

[0065] This embodiment allows achieving increases up to 3.8 points with respect to the state-of-the-art recompression cycle working with equipment with identical isentropic efficiencies and effectiveness. Said FIG. 4 shows a preferred embodiment for the exploitation of a heat source at high temperature.

[0066] On the other hand, according to a second embodiment, FIG. 6 shows a multiple recompression cycle that uses a high temperature heat source with an intermediate cooling stage in the main compression process.

[0067] The cycle depicted in said FIG. 6 is a preferred embodiment of the invention for electric generation by means of a heat source available at high temperature. This preferred embodiment, has four recuperators, three auxiliary compressors and one intercooling stage in the main compression process.

[0068] In view of said FIG. 6, the high temperature heat source permits to heat up the sCO.sub.2 stream leaving the recuperator 4 (stream 14) up to 680 C. at 20 MPa (stream 15). The stream 15 is expanded in the turbine to 548 C. and about 7.5 MPa (stream 16).

[0069] Stream 16 enters the hot side of recuperator 4 and is cooled down to 398 C. (stream 19) by means of heating stream 10 from 390 C. to 534 C. (stream 14). Stream 19 is then cooled down in the recuperator 3 to 264 C. (stream 20) by heating stream 7 from 257 C. to 390.5 C. (stream 8). Auxiliary compressor 3 compresses the 8.2% of the total sCO.sub.2 mass flow rate from about 7.5 MPa and 264 C. to about 20 MPa and 380 C. (stream 9). Stream 9 is mixed with stream 8 to obtain stream 10. The 91.8% of the total sCO.sub.2 mass flow rate goes to the hot side inlet of recuperator 2 at about 7.5 MPa and 264 C. (stream 21).

[0070] Stream 21 is then cooled down in the recuperator 2 to 150 C. (stream 22) by heating stream 4 from 144 C. to 258 C. (stream 5). Auxiliary compressor 2 compresses the 18.4% of the total sCO.sub.2 mass flow rate from about 7.5 MPa and 150 C. to about 20 MPa and 252 C. (stream 6). Stream 6 is mixed with stream 5 to obtain stream 7. The 73.4% of the total sCO.sub.2 mass flow rate goes to the hot side inlet of recuperator 1 at about 7.5 MPa and 150 C. (stream 23).

[0071] Stream 23 is then cooled down in the recuperator 1 to 56 C. (stream 24) by heating stream 1 from 52 C. to 146 C. (stream 2). Auxiliary compressor 1 compresses the 29.3% of the total sCO.sub.2 mass flow rate from about 7.5 MPa and 56 C. to about 20 MPa and 140 C. (stream 3). Stream 3 is mixed with stream 2 to obtain stream 4. The 44.1% of the total sCO.sub.2 mass flow rate goes to the cooler about 7.5 MPa and 56 C. (stream 25).

[0072] Stream 25 is cooled in the cooler from about 56 C. to about 32 C. (stream 26). Stream 26 is compressed in the main compressor 1 from about 32 C. and 7.5 MPa to about 59 C. and 12.25 MPa (stream 27). Stream 27 is cooled to about 40 C. in the intercooler to obtain stream 28. Stream 28 is compressed in main compressor 2 to about 20 MPa and 52 C. (Stream 1).

[0073] This embodiment allows achieving increases up to 4.6 points with respect to the state-of-the-art recompression cycle without intercooling working with equipment with identical isentropic efficiencies and effectiveness. Note that this embodiment allows achieving increases up to 5.4 points with respect to the state-of-the-art recompression cycle with intercooling working with equipment with identical efficiencies and effectiveness and an identical intercooling stage. FIG. 6 shows a preferred embodiment for the exploitation of a heat source at high temperature.

[0074] Likewise, according to a third preferred embodiment, depicted in FIG. 8 there is a multiple recompression cycle using three recuperators and two auxiliary compressors. The cycle depicted in said FIG. 8 is a preferred embodiment of the invention for electric generation by means of a heat source available at medium temperature.

[0075] In view of said FIG. 8, the medium temperature heat source permits to heat up the sCO.sub.2 stream leaving the recuperator 3 (stream 14) up to 377 C. at 17 MPa (stream 15). The stream 15 is expanded in the turbine to 289 C. and about 7.5 MPa (stream 16).

[0076] Stream 16 enters the hot side of recuperator 3 and is cooled down to 240 C. (stream 20) by means of heating stream 7 from 238 C. to 282 C. (stream 14). Stream 20 is then cooled down in the recuperator 2 to 160 C. (stream 22) by heating stream 4 from 156 C. to 236 C. (stream 5). Auxiliary compressor 2 compresses the 16.3% of the total sCO.sub.2 mass flow rate from about 7.5 MPa and 160 C. to about 17 MPa and 246 C. (stream 6). Stream 6 is mixed with stream 5 to obtain stream 7. The 83.7% of the total sCO.sub.2 mass flow rate goes to the hot side inlet of recuperator 1 at about 7.5 MPa and 160 C. (stream 23).

[0077] Stream 23 is then cooled down in the recuperator 1 to 80 C. (stream 24) by heating stream 1 from 76 C. to 156.5 C. (stream 2). Auxiliary compressor 1 compresses the 32.5% of the total sCO.sub.2 mass flow rate from about 7.5 MPa and 80 C. to about 17 MPa and 155.5 C. (stream 3). Stream 3 is mixed with stream 2 to obtain stream 4. The 51.2% of the total sCO.sub.2 mass flow rate goes to the cooler at about 7.5 MPa and 80 C. (stream 25).

[0078] Stream 25 is cooled in the cooler from about 80 C. to about 32 C. (stream 26). Stream 26 is compressed in the main compressor from about 32 C. and 7.5 MPa to about 76 C. and 17 MPa (stream 1).

[0079] This embodiment allows achieving increases up to 0.94 points with respect to the state-of-the-art water-steam regenerative Rankine cycle. Said FIG. 8 shows a preferred embodiment for the exploitation of a heat source at medium temperature. In this case, the cold outlet temperature of the heat source stream is fixed by the solar field. The selected turbine inlet pressure permits to work with the Heat Transfer Fluid entering the Heat Transfer Fluid Heat Exchanger at about 390 C. (stream HS.sub.1) and leaving this exchanger at about 295 C. (stream HS.sub.2).

[0080] Besides, according to a fourth preferred embodiment depicted in FIG. 10, there is a multiple recompression cycle using three recuperators and three auxiliary compressors. The cycle depicted in said FIG. 10 is a preferred embodiment of the invention for electric generation by means of a heat source available at low temperature.

[0081] In view of said FIG. 10, the low temperature heat source permits to heat up the sCO.sub.2 stream leaving the recuperator 3 (stream 14) up to 85 C. at 8.6 MPa (stream 15). The stream 15 is expanded in the turbine to 73.7 C. and about 7.5 MPa (stream 16).

[0082] Stream 16 enters the hot side of recuperator 3 and is cooled down to 61.9 C. (stream 20) by means of heating stream 7 from 61.35 C. to 73.05 C. (stream 8). Auxiliary compressor 3 compresses the 15% of the total sCO.sub.2 mass flow rate from about 7.5 MPa and 61.9 C. to about 8.6 MPa and 73.4 C. (stream 9). Stream 9 is mixed with stream 8 to obtain the stream 14 at 73.1 C. and 8.6 MPa. The 85% of the total sCO.sub.2 mass flow rate goes to the hot side inlet of recuperator 2 at about 7.5 MPa and 61.9 C. (stream 21).

[0083] Stream 21 is then cooled down in the recuperator 2 to 50.4 C. (stream 22) by heating stream 4 from 49.9 C. to 61.3 C. (stream 5). Auxiliary compressor 2 compresses the 20.4% of the total sCO.sub.2 mass flow rate from about 7.5 MPa and 50.4 C. to about 8.6 MPa and 61.5 C. (stream 6). Stream 6 is mixed with stream 5 to obtain stream 7. The 64.6% of the total sCO.sub.2 mass flow rate goes to the hot side inlet of recuperator 1 at about 7.5 MPa and 50.4 C. (stream 23).

[0084] Stream 23 is then cooled down in the recuperator 1 to 39.8 C. (stream 24) by heating stream 1 from 39.4 C. to 49.8 C. (stream 2). Auxiliary compressor 1 compresses the 33.6% of the total sCO.sub.2 mass flow rate from about 7.5 MPa and 39.8 C. to about 8.6 MPa and 50.0 C. (stream 3). Stream 3 is mixed with stream 2 to obtain the stream 4. The 31.0% of the total sCO.sub.2 mass flow rate goes to the cooler at about 7.5 MPa and 39.8 C. (stream 25).

[0085] Stream 25 is cooled in the cooler from about 39.8 C. to about 32 C. (stream 26). Stream 26 is compressed in the main compressor from about 32 C. and 7.5 MPa to about 39.4 C. and 8.6 MPa (stream 1).

[0086] This embodiment allows achieving increases up to 2.1 points with respect to the state-of-the-art ORC cycles. Said FIG. 10 shows a preferred embodiment for the exploitation of a heat source at low temperature being the cold outlet temperature of the heat source stream (stream HS.sub.2) fixed by the heat source stream cooling requirements. In this embodiment, the selection of 8.6 MPa as the turbine inlet pressure leads to a particular case where there are as many recuperators as auxiliary compressors.

[0087] Finally, according to a fifth preferred embodiment, depicted in FIG. 12 there is a multiple recompression cycle using three recuperators and two auxiliary compressors. The cycle depicted in said FIG. 12 is a preferred embodiment of the invention for electric generation by means of a high-temperature heat source and a cold sink that allows the CO.sub.2 to be cooled to temperatures below its critical temperature. This configuration makes it possible to take advantage of hot sources in the form of mass flows or hot streams that must be cooled about 240 C. by expanding the sCO.sub.2 from MPa to subcritical pressures of 5.3 MPa.

[0088] In view of said FIG. 12, the high temperature heat source permits to heat up the sCO.sub.2 stream leaving the recuperator 3 (stream 14) up to 680 C. at 35 MPa (stream 15). The stream 15 is expanded in the turbine to 437 C. and about 5.3 MPa (stream 16).

[0089] Stream 16 enters the hot side of recuperator 3 and is cooled down to 391 C. (stream 20) by means of heating stream 7 from 389 C. to 430 C. (stream 14). Stream 20 is then cooled down in the recuperator 2 to 200 C. (stream 22) by heating stream 4 from 189 C. to 381 C. (stream 5). Auxiliary compressor 2 compresses the 20.3% of the total sCO.sub.2 mass flow rate from about 5.3 MPa and 200 C. to about 35 MPa and 420 C. (stream 6). Stream 6 is mixed with stream 5 to obtain stream 7. The 79.7% of the total sCO.sub.2 mass flow rate goes to the hot side inlet of recuperator 1 at about 5.3 MPa and 200 C. (stream 23).

[0090] Stream 23 is then cooled down in the recuperator 1 to 32 C. (stream 24) by heating stream 1 from 26.5 C. to 186 C. (stream 2). Auxiliary compressor 1 compresses the 25.6% of the total sCO.sub.2 mass flow rate from about 5.3 MPa and 32 C. to about 35 MPa and 197.5 C. (stream 3). Stream 3 is mixed with stream 2 to obtain stream 4. The 54.1% of the total sCO.sub.2 mass flow rate goes to the cooler at about 5.3 MPa and 32 C. (stream 25).

[0091] Stream 25 is cooled in the cooler from about 32 C. to about 5 C. (stream 26). Stream 26 is compressed in the main compressor from about 5 C. and 5.3 MPa to about 26.5 C. and 35 MPa (stream 1).

[0092] This embodiment allows achieving increases up to 1.2 points with respect to the state-of-the-art recompression cycle working with equipment with identical isentropic efficiencies and effectiveness. Said FIG. 12 shows a preferred embodiment for the exploitation of a heat source at high temperature and a cold sink that allows the CO2 to be cooled to temperatures below its critical temperature. In this case, the outlet temperature of the hot stream or thermal fluid that works as a heat source would be set at about 460 C. The selected turbine inlet pressure permits to work with the Heat Transfer Fluid entering the Heat Transfer Fluid Heat Exchanger at about 700 C. (stream HS.sub.1) and leaving this exchanger at about 460 C. (stream HS.sub.2).