Supercritical CO2 recompression Brayton power cycle for hybrid concentrating solar and biomass power plant

The supercritical CO2 Brayton cycle has great potential in various renewable energy systems. The supercritical CO2 recompression Brayton power cycle for hybrid concentrating solar and biomass power plant is proposed and analyzed in this paper. The supercritical CO2 is heated by both the molten salt and flue gas from the biomass boiler. The inlet temperature of the turbine in the supercritical CO2 recompression Brayton power cycle for hybrid power plant rises to 620 °C. The waste heat from the system is recovered by the steam turbine to improve energy utilization efficiency. The efficiency of the supercritical CO2 recompression Brayton power cycle increases from 0.382 to 0.41. The efficiency of the steam cycle is 0.46, and the efficiency of the combined cycle is 0.427.


Introduction
The supercritical carbon dioxide (sCO2) Brayton cycle drives the turbine with the working fluid CO2 [1].Owing to the properties of CO2, like nontoxicity, nonflammability, low critical parameters, the sCO2 Brayton cycle has a higher efficiency and a simpler configuration than the conventional steam Rankine cycle, and has great potential in various energy systems, like low-grade heat recovery, concentrating solar power (CSP) and fossil fuel power [2][3][4].Among these, the solar thermal power (STP) system integrated with molten salt thermal energy storage (MSTES) device and sCO2 Brayton cycle exhibit flexible output ability and high system efficiency and could play a vital role in future low-carbon energy [5][6][7].However, due to the degradation temperature of molten salt (MS) and the intermittent nature of solar irradiation, the STP system integrated with MSTES and sCO2 Brayton cycle has difficulties in achieving higher solar-to-electric efficiency [8,9].
Hybridization of renewable energy sources provides a promising solution for the intermittency issues and improvement of the solar thermal energy system [10][11][12][13].Especially, given its relatively steady and reliable output, biomass energy is an attractive option coupling with solar thermal energy.Sahoo et al. [14] study a hybrid biomass-solar power system.In this system, solar thermal energy is used to heat the heat transfer oil and make saturated steam.The inlet temperature of the turbine and energy efficiency rise from 300 ℃ to 600 ℃ and from 17.29% to 27.08%, respectively.Anvari et al. [15] found that by adding the solar unit to the biomass power unit, there is a 30% increase in generated power and a 22% decrease in CO2 emission.Jensen et al. [16] present the design and performance of the hybrid concentrated solar power-biomass plant.The heat provided by a solar collector and biomass boilers is used to generate power using an organic Rankine cycle system, while the waste heat for the local heating.The steam was first generated by the thermal oil and then superheated by the biomass boilers, achieving a higher conversion efficiency.Middelhoff et al. [17] study a hybrid concentrated solar biomass plant involving the biomass boiler and concentrated solar power.The steam can be generated by both the concentrated solar power and biomass boiler.Therefore, the hybrid biomass-solar power generation system performs well in stable power output with high efficiency, especially in raising the steam parameter.However, there are few studies on the hybrid biomass-solar power generation system with sCO2 Brayton cycle, and the optimization for the hybrid power system is less understood.
The performance of a hybrid power system depends on the configuration optimization of the system and the complementary of different energy sources [18][19][20][21].Wang et al. [22] evaluate the performance of a solar-nuclear power system using the sCO2 Brayton cycle.Under the condition of insufficient solar energy, the power of the system is only provided by the nuclear power.The mass flow distribution and temperatures of sCO2 should be changed to match the variation of the system output power.Alenezi et al. [23] analyze the hybrid sCO2 Brayton power cycle using solar power and natural gas.The heat sources from solar power and oxy-combustion works and heats the hybrid cycle solely.Yang et al. [24] study the load-matching performance of a solar thermal power system with sCO2 Brayton cycle in a solar-wind hybrid system under load-following mode and stable output mode.The results prove that solar thermal power can provide stable base load for the hybrid system and cooperate well with wind plants.However, there is lack of design for sCO2 Brayton cycle driven by hybrid biomass-solar power.
Therefore, this paper proposes a sCO2 recompression Brayton power (RBP) cycle for hybrid concentrating solar and biomass power plant.The thermomechanical analysis of the sCO2 RBP cycle is conducted.The efficiency of sCO2 RBP cycle for hybrid power plant is compared with that of the sCO2 RBP cycle for CSP plants.

sCO2 recompression Brayton power cycle for concentrating solar power plant
The conventional sCO2 recuperative Brayton power cycle consists of adiabatic compression, isobaric heating, adiabatic expansion, and isobaric cooling.The isobaric heating and isobaric cooling process can be connected by the recuperator, and a simplified reheating cycle is formed, which recover the heat and increases the efficiency of the sCO2 recuperative Brayton power cycle.Compared to the conventional sCO2 recuperative Brayton power cycle, the sCO2 RBP cycle introduces a parallel adiabatic compression process and splits the recuperator into two units, i.e., high-temperature recuperator (HTR) and low-temperature recuperator (LTR).The sCO2 RBP cycle for CSP plant is shown in Figure 1.The sCO2 is generated in the main heater (MHE) by the MS, which contains thermal energy from the CSP.The sCO2 then expands in the turbine.The low-pressure CO2 output from the turbine will heat the high-pressure sCO2 in HTR and LTR.After this, the CO2 will be split into two streams according to the given split ratio.The stream with a higher flow rate enters the precooler (PC), and is in the main compressor (MC), before being heated in the LTR.The other CO2 is directly compressed in the auxiliary compressor (AC).Finally, the CO2 streams from the HTR and the LTR mix before the HTR and then are heated in the MHE.
The T-s diagram of sCO2 RBP cycle is shown in Figure 2. sCO2 is heated up in the MHE by the MS (from state 6 to state 7).Then, sCO2 expands in the turbine from state 7 to state 8. Then the sCO2 is cooled in HTR (from state 8 to state 9), and LTR (from state 9 to state 10) sequentially.Exiting from LTR, the sCO2 splits into two streams.The stream with a higher flow rate is cooled in the precooler (from state 10 to state 1), and is compressed in the MC (from state 1 to state 2), followed by heated in the LTR (from state 2 to state 5).Another stream is compressed in the AC (from state 10 to state 3).The two streams mix before the HTR (from state 5 to state 4 and from state 3 to state 4) and then is reheated in the HTR (from state 4 to state 6), and enter the MHE at state 6 again.To analyze the performance of the sCO2 RBP cycle, the efficiency of the cycle is: where, ηt is the efficiency of the sCO2 RBP cycle, Wtur is the energy output of the turbine, Wcom is the energy consumption of the compressor, QH is the heat exchange capacity of MHE, Wcom1 is the energy consumption of the AC, m1 is mass flow rate of stream with lower flowrate, m is mass flow rate of stream with higher flowrate, Wcom2 is energy consumption of the MC, hi is the specific enthalpy at each state point in Figure 2, r is the split ratio.The parameters and constraints of the sCO2 RBP cycle are shown in Table 1.To reduce the energy consumption of the compressor and avoid phase change of CO2, the inlet pressure and temperature a of the MC are 8.47 MPa and 34.5 ℃, respectively.The inlet pressure and temperature of the turbine are from 18 to 50 MPa and 550 to 700℃, respectively.An in-house code is used to evaluate the thermodynamic characteristics of the sCO2 power cycle.The effects of the inlet pressure and temperature of the turbine on the performance of the sCO2 power cycle are shown in Figure 3.For the same inlet pressure of the turbine, the efficiency of the sCO2 power cycle increases with the increasing inlet temperature of the turbine.For the same inlet temperature of the turbine, the efficiency of the sCO2 power cycle rises firstly and then drops with the increasing turbine inlet pressure.As the turbine inlet pressure increases, the compressor operates off optimum condition, and the rise in energy consumption of the compressor is larger than the increase in the energy output of the turbine.Therefore, with the turbine inlet pressure of above 26 MPa, the enhancement of cycle efficiency by increasing inlet pressure of turbine is weak, and, it is better to increase the turbine inlet temperature from 550 ℃ to 620 ℃.To address the intermittency issue and limitation of degradation temperature of MS, the biomass boiler (BB) is integrated after the MHE.The inter temperature of the turbine can be increased from 550 ℃ to 620 ℃.The waste heat from the BB is utilized by the small steam turbine (ST).Therefore, the efficiency and the economic characteristics of the sCO2 RBP cycle can be improved.Figure 4 and 5 illustrates the schematic diagram and T-s diagram of the sCO2 RBP cycle for hybrid power plant.Different from the sCO2 RBP cycle for CSP plant, sCO2 is first heated up in the MS heat exchanger (HX) (from state 6 to state 11), and then heated in MHE by the flue gas (from state 11 to state 7).Then, sCO2 expands in the turbine (from state 7 to state 8).Then the sCO2 is cooled in HTR (from state 8 to state 9).Exiting from HTR, the sCO2 splits into two streams.The stream with a lower flow rate (stream 1) heats the steam in preheat 1(PH1) (from state 9 to state 12).The stream with a higher flow rate (stream 2) is cooled in LTR (from state 9 to state 10).Exiting from LTR, the sCO2stream 2 also splits into two streams (stream 3 and 4).The stream with a higher flow rate (stream 3, i.e. main stream) mixes with stream 1 and then is cooled in the preheater 2(PH2) (from state 10 to state 13) and the PC (from state 13 to state 1), and then is compressed in the MC (from state 1 to state 2), followed by heated in the LTR (from state 2 to state 5).All the streams mix before the HTR (from state 5 to state 4 and from state 3 to state 4) and then is reheated in the HTR (from state 4 to state 6), and finally enter the MS heat exchanger at state 6 again.After the MHE, the flue gas heats the steam in the steam generator and the air in the air preheater.The pressure of water from the cooling tower is raised by the pump firstly.Then the water is heated by the sCO2in the PH1 and the PH2 (state 14-15-12-16).After going through the water pump, the water is heated by the flue gas in the steam generator, and the steam is generated.The heat in the steam transits to mechanical energy in the steam turbine (from state 17 to state 18).The steam condenses in the cooling tower (CT) finally.To analyze the performance of the sCO2 RBP cycle for hybrid power plant, the efficiency of the cycle is: where m1, m2, and m are mass flow rates of preheater 1, AC and main stream, respectively, Wcom2 is energy consumption of MC, hi is the specific enthalpy of CO2, ri is the split ratio.The steam cycle efficiency of utilizing the waste heat from the BB can be calculated as w = ℎ 18 − ℎ 17 ℎ 17 − ℎ 16 (18) where ηw is the steam cycle efficiency, Wtur, w is the energy output of the ST, QH is the heat exchange capacity of the steam generator, mw is mass flow rate of main steam stream, and hi is the specific enthalpy.The thermal efficiency of sCO2 RBP cycle for hybrid power plant can be calculated as follows where, QH is the heat input from the MT and BB, ηw is the efficiency of the recompression cycle.Qw is the input waste heat from the BB.The parameters and constraints of the sCO2 RBP cycle for hybrid power plant is shown in Table 2.  5, the efficiency of sCO2 RBP cycle for hybrid power plant is 41%.The T-s diagram of the steam cycle for hybrid power plant is shown in Figure 6.The efficiency of the steam cycle for hybrid power plant is 46%.The efficiency of the combined cycle can reach 42.1%.

Conclusion
The supercritical CO2 recompression Brayton power cycle for hybrid concentrating solar and biomass power plant is proposed and analyzed in this paper.The results can be concluded as follows: (1) Compared with the solar supercritical CO2 recompression Brayton power cycle, the turbine inlet temperature in the supercritical CO2 recompression Brayton power cycle for hybrid concentrating solar and biomass power plant rises from 550 ℃ to 620 ℃, and the efficiency of the power cycle increases from 0.382 0.41.
(2) The waste heat from the biomass boiler can be recovered by the steam turbine.The waste heat from the supercritical CO2 can be recovered in the preheater 1 and the preheater 2. The efficiencies of the steam cycle and the combined cycle are 0.46 and 0.427, respectively, which is better than the original ssupercritical CO2 recompression Brayton power cycle.

Figure 4 .
Figure 4. Schematic diagram of sCO2 RBP cycle for hybrid power plant.

1 Figure 5 .
Figure 5. T-s diagram of sCO2 RBP cycle for hybrid power plant.

Figure 6 .
Figure 6.T-s diagram of the steam cycle for hybrid power plant.

Table 1 .
Effects of the inlet pressure and temperature of the 20MW turbine on the efficiency of the sCO2 power cycle.Parameters and constraints of the sCO2 RBP cycle for CSP plant.

Table 2 .
Parameters and constraints of the sCO2 RBP cycle for hybrid power plants.