Abstract
The supercritical CO2 cycle is one of the next generation’s power generation cycle. Especially, the transcritical CO2 Rankine cycle (TCRC) is a suitable candidate for dispersed generation systems with small-scale solar thermal applications. Compared to other cycle studies applied in other fields such as nuclear energy, there are limited reports on renewable energy fields. Therefore, in this study, the TCRC for a small-scale solar thermal heat source is investigated by thermodynamic and experimental methods. The experimental facility was built with approximately 12 kW of thermal capacity and commissioned to evaluate its performance using a case study. The maximum temperature of the cycle is the primary optimization variable of the experiment, and it has a significant impact on the cycle thermal efficiency. Based on the experimental data, the trends of the cycle thermal efficiency and generated power are simulated assuming that the TCRC is operating with real insolation during the daytime. As a result of the simulation, maximum efficiencies of 6.41 and 6.03% are obtained from maximum solar radiation amounts of 758 and 674 W/m2 in May and June, respectively. At that time, the amounts of power generated were 726 W and 626 W, respectively.
1. Introduction
As global demand for energy transition grows, many countries are attempting to reduce their reliance on fossil fuels and increase the proportion of electricity generated by renewable energy sources. In line with global trends, the Korean government is promoting an energy transition policy in the power generation sector, with a target of increasing the proportion of renewable energy generation from 7.6% in 2017 to 20% by 2030 and 35% by 2040 [1]. This policy demonstrates the government’s willingness to develop new dispersed generation systems based on renewable energy sources such as solar thermal, photovoltaic, and wind power rather than existing centralized power generation facilities like fossil fuel thermal power plants. However, the energy transition roadmap focuses solely on photovoltaics and wind power, with inadequate research and development on solar thermal power generation.
Various types of concentrated solar power (CSP) plants are being developed globally and classified based on their concentrating method. There are relatively small and independent reflector types, such as the parabolic dish (PD) and the parabolic trough (PT). On the other hand, there is also the solar power tower (SPT) method, in which numerous reflectors are installed on a very large site to focus sunlight on one tower. The 392 MW Ivanpah solar power plant in California, for example, is the most well-known SPT-type plant, with a very large landscape with 175,000 mirrors [2]. Furthermore, Niu et al. [3, 4] investigated a small-scale solar thermal power cycle in Japan using an evacuated tube-type solar collector with a thermal capacity of only 5.4 kW. Unlike large-scale power plants that require large land areas, the study is aimed at developing a solar thermal power cycle suitable for dispersed generation technology. The same studies on small-scale solar thermal technology should be conducted in countries where the land area is very limited for large plant sites. However, in comparison to other renewable energy sources, solar thermal generation technologies have not been actively researched and are currently limited to water-heating systems for households.
The supercritical CO2 (sCO2) cycle was first proposed by Feher [5], of which the working fluid is operated above the critical pressure. As a working fluid in power-generation cycles, CO2 offers numerous advantages. CO2 is a nontoxic gas with no risk of explosion or corrosion. Its critical temperature and pressure conditions are 31°C and 7.4 MPa, respectively, which are significantly lower than those of other working fluids. As a result, sCO2 can be used in both large heat sources and dispersed generation facilities with small heat sources, such as solar thermal. Because the sCO2 cycle operates in the high-pressure supercritical region, CO2 has a higher energy density and lower volumetric flow than other working fluids. This improves the performance of the sCO2 cycle while also reducing its size. Therefore, a CO2 turbine and compressor can be manufactured 10 times smaller than a conventional steam turbine.
The Rankine cycle outperforms the Brayton cycle at low turbine inlet temperatures (TIT) [6]. Thus, the Rankine cycle is a suitable power conversion system for small-scale solar thermal energy as a heat source. The transcritical CO2 Rankine cycle (TCRC), which includes both the supercritical and subcritical regions of CO2, has been studied to take advantage of both the sCO2 and Rankine cycles. TCRC has been studied by several researchers to design a power conversion system with a relatively low-temperature range for renewable energy or waste heat recovery systems. Choi [7] designed a TCRC for a 6800 TEU container ship that generated 383 kW of power by utilizing waste heat from cooling jackets and scavenging air. The TCRC design case was applied to a heat source below 100°C, and the maximum cycle efficiency was 9.27%. Zhang et al. [8, 9] proposed a solar CO2 Rankine system as a power-generation cycle that used an evacuated tube-type solar collector and CO2 as the working fluid. Despite the relatively low operating temperature, experimental studies have confirmed a reasonable cycle efficiency of 8.78–9.45%, indicating that small-scale solar thermal energy collected by evacuated tubes is a viable power source. Choi et al. [10] also designed and built a 12 kW TCRC experimental facility and used theoretical analysis to determine the optimal operating conditions. They expect to achieve a cycle efficiency of 6.98% at a TIT of 70°C. Furthermore, the efficiency change was analyzed in relation to the increase in TIT. However, no experiments were carried out to validate the findings of their theoretical analysis. As mentioned earlier, several studies on TCRC have been conducted; however, the number of studies on TCRC for small-scale solar thermal technology remains insufficient. More studies are required to improve the cycle efficiency by optimizing the operating conditions and layouts.
In this study, TCRC, which is competitive in the low-temperature range, was investigated for application in dispersed generation using a small-scale solar thermal heat source. The experimental data were generated with an optimization variable using a small-scale sCO2 experimental facility. Furthermore, the cycle performance was simulated using real insolation data by creating a model from experimental data that represents the cycle efficiency and amount of generated power.
2. TCRC Design
2.1. System Description
A transcritical cycle is a closed cycle in which the working fluid operates in both the subcritical and supercritical regions. The compression process for the TCRC was carried out in the subcritical region using a liquid pump. As a result, the compression work is significantly less than that of the Brayton cycle using a compressor in the gas region. In addition, because the target thermal capacity of the heat source in this study is quite low, the TCRC is considered a suitable system for a power generation cycle with a low TIT range.
Figure 1 depicts a conceptual diagram of the TCRC. A pump was used to pressurize liquid CO2 to the maximum pressure at points 1 and 2. The liquid CO2 in the solar collector changes to a supercritical state via an isobaric heating process at points 2 and 3. At point 3, the working fluid, which has reached its maximum pressure and temperature, rotates the turbine, and the phase of CO2 becomes a gas state at point 4. During this process, electricity is generated by a generator connected to a turbine with the same shaft. Finally, CO2 passes through the cooler at points 4 to 1 and returns to a liquid state via the isobaric cooling process. Based on the enthalpy difference at each point, the energy stream of each component was calculated using the following equation:
In the above equations, is the mass flow rate of the working fluid, is the heat added from the solar collector to the working fluid, is the work done by the turbine to the outside, is the work used to operate the pump, and is the heat withdrawn through the cooler. is the enthalpy (kJ/kg) per unit mass at point .
Calculation of the cycle thermal efficiency (th) of the system using the above equations is shown in the following:
2.2. Experimental Facility
An experimental facility was designed and built to study the power generation process based on the TCRC design described above [10]. Figure 2 shows a schematic of all the components of the experimental facility, including the data acquisition system. There are some constraints while manufacturing the experimental facility, so components such as the solar collector and turbine were implemented differently than the TCRC concept shown in Figure 2. Because TIT is an important optimization variable for parametric studies, the solar collector was replaced with an electric heater so that the heat rate could be easily changed. The electric heater (CAS, CAST-X 3000) has a heat rate ranging from 1.0 to 24.6 kW. In addition, a proportional-integral-derivative (PID) controller was used to prevent rapid temperature rise and sudden expansion, as well as to ensure that it did not exceed the maximum operating temperature range. CO2 turbines are currently in the development stage, with no commercialized products. As a result, the amount of electricity generated by real turbine operations could not be confirmed. As an alternative, the turbine was replaced with a gas regulator, which caused the pressure to drop as if the working fluid was passing through the real turbine, allowing the cycle to run. Because the gas regulator’s maximum operating temperature was initially limited to 74°C, verifying the cycle characteristics at high temperatures was impossible. To improve this, a new regulator (Drasta, DR80) with an operating temperature of up to 250°C was installed.
In the schematic diagram in Figure 2, the , , and in Equations (1), (3), and (4) are the supplied heat of the electric heater (), power consumption of the pump (), and withdrawn heat from the cooler (), respectively. Each term is calculated using and the enthalpy difference. was measured using a Coriolis flow meter (Korea Flow Meter; KMS-2000). In addition, as the turbine is replaced by a regulator (RV5 in Figure 3), it is assumed that the turbine is operating with an isentropic efficiency of 80%. Point is the output node of the virtual turbine, which operates based on the pressure difference between points 3 and 4. The work performed by the virtual turbine is , which is calculated using the following equation:
The cycle thermal efficiency () of the experimental facility was obtained using the following equation:
In addition to the abovementioned components, the experimental facility’s CO2 injection unit consisted of two CO2 gas cylinders and a gas booster. The target pressure and mass flow conditions were achieved using a liquid pump, and the CO2 was cooled and liquefied using a cooler. The gas booster and pump were both air-driven reciprocating models with model numbers DLE5-15-NN-C and G35D-CO2, respectively. An air compressor delivers compressed air at a pressure of 8 bar to the air-driven components. A reciprocating pump, unlike a centrifugal pump, has a self-priming function that allows it to pump CO2 even when gas and liquid fluids coexist in the pump during start-up. A shell-and-tube type heat exchanger was built and used as a cooler, rejecting CO2 heat with chilled water at 4°C from the chiller. Many other studies on the sCO2 cycle use a printed circuit heat exchanger (PCHE) as a heat rejection component to reduce the cycle’s footprint [11]. However, the objective of this study was not to evaluate the performance of the heat exchanger but to analyze the overall efficiency of the cycle using a well-known shell-and-tube type heat exchanger. In the future, the heat exchanger’s performance and additional methods for reusing the heat rejected by the cooler can be investigated.
Minor improvements to the experimental facility are made following the initial design and manufacturing stages, as shown in Figure 2. Further, another CO2 gas cylinder and precooling line were installed to reduce the injection time in half. Sensors such as pressure, temperature, and flow meters were connected to the data acquisition system, and real-time data were collected at each stage of the cycle. Table 1 summarizes the information on sensors used in the experimental facility. Figure 3 shows a laboratory-built experimental facility.
2.3. Reference Heat Source for Solar Thermal
Based on actual insolation data from Pohang, Korea, the available range of solar thermal capacity in the TCRC was determined. Figure 4 shows a sample of the measured insolation and atmospheric temperature in May. The maximum insolation during the day was approximately 758 W/m2 on a 1-hour average around noon. The TCRC is expected to generate electricity between the hours of 10 a.m. and 4 p.m. when insolation is relatively high. Table 2 displays the 1-hour average insolation data for May and June 2019 as reference data. Further, changes in the cycle efficiency and generated power are presented based on these data.
An evacuated tube-type solar collector was adopted in the TCRC design to supply solar thermal energy. According to the findings of Zhang et al. [8], who experimentally analyzed the heat collection efficiency of evacuated tubes, an average heat collection efficiency of 70% can be obtained during the day, even when the mass flow rate of CO2 is less than 0.02 kg/s. Assuming the heat collection efficiency and effective area of the evacuated tubes are 70% and 22.7 m2, respectively, approximately 12 kW of heat can be supplied to the TCRC at a fixed mass flow rate of 0.04 kg/s.
3. Experiment
3.1. Procedure of Experiment
The experimental procedure is divided into three steps, as shown in Figure 5. Step 1 was the start-up stage, Step 2 was the normal operation stage, and Step 3 was the shut-down stage. Several critical considerations for each stage of the experiment are described below, as the procedure can affect not only the cycle’s operating conditions but also the experiment’s results.
During the start-up stage, it is crucial to inject the desired amount of pure CO2 into the cycle loop. Before injecting CO2, the air inside the cycle loop was removed with a vacuum pump, and thus, the internal pressure of the cycle was lowered to 7 kPa. Because residual air was still present, an additional process was added to vent it as much as possible by repeating the process of injecting and discharging CO2 into and out of the cycle loop. In addition, the weight of the gas cylinder before and after the injection was measured using a scale to determine the amount of CO2 injected into the cycle. The CO2 injection process is divided into primary and secondary injection steps. For the primary injection, CO2 is injected through cylinder 1 until the cycle pressure is balanced with the discharge pressure of the gas cylinder, which was directly connected to the cycle loop, as shown in Figure 3. The CO2 was then injected at a high pressure of approximately 13.8 MPa using the gas booster connected to cylinder 2. Simultaneously, the injected CO2 is cooled by a cooler, enabling the CO2 to liquefy quickly. The internal pressure of the cycle increases as the amount of CO2 injected increases, but it stagnates for a long time near the saturation pressure (about 3.9 MPa at 4.3°C) during the liquefaction process. Because a liquid pump is used for compression in TCRC, normal operation can begin once the working fluid has been completely liquefied. When liquefaction was completed and the pressure in the cycle loop began to rise again, CO2 injection was immediately stopped to maintain the lowest possible initial pressure. In other words, the pressure rise point was the injection’s stop point. This is because the initial pressure of the cycle changes depending on the amount of CO2 injected into the cycle, and the pressure range of the experiment can be increased in the normal operation stage by minimizing the CO2 injection.
An experiment was conducted during normal operation by adjusting the heater, pump, and RV5 to achieve the desired operating conditions. When starting the pump, all valves (needle valve, RV5) for controlling the CO2 flow rate should be opened, and the air pressure for driving pump should be gradually increased. T3 is an optimization variable that is influenced by the heater output. When the temperature and pressure at each point reached the desired values, data were collected while the flow was held steady for a predetermined amount of time.
CO2 in the loop is discharged during the shutdown stage after the pump and heater devices have been turned off. The pressure of CO2 in the loop is much higher than atmospheric pressure, and the temperature rapidly decreases during discharge due to the Joule-Thomson effect. As a result, the venting valve must be precisely adjusted. When the discharge is completed, the data logging is stopped, and the data is analyzed. In steps 1, 2, and 3, the phenomena inside the loop during the experiment were analyzed by comparing log data recording events with real-time sensors. By comparing the pressure and temperature data measured at each point of the cycle, the relationship between the other variables and the optimization variables was identified. Using the enthalpy values, the thermal efficiency of the cycle was calculated. The REFPROP property package provided by NIST was used in this process to obtain properties such as enthalpy.
3.2. Operating Condition
The primary objective of this experiment was to identify changes in the operating conditions of the TCRC due to the changes in insolation and to analyze cycle efficiency and generated power. Choi et al. [10] found the optimal operating conditions for the TCRC experimental facility, where T3 is 70°C and P3 is 12.1 MPa, as summarized in Table 3. The optimal conditions that they proposed are a single temperature and pressure point at which the highest cycle efficiency of the TCRC could be obtained. It could be meaningful for thermal or nuclear power plants, which generate electricity continuously at a single operating point, but in solar power generation, it is impossible to maintain steady operating conditions. Instead, the temperature and pressure inside the cycle may change depending on the amount of insolation during the day, and the power generation efficiency and amount of electricity may also change accordingly. Therefore, this experiment was conducted mainly to identify the trend of operating conditions of TCRC according to the TIT change.
Initially, it was investigated whether the previous researcher’s optimal operating condition at T3 of 70°C could be implemented in the experimental facility. The cycle efficiency was calculated from experimental results and compared to the design values. Secondly, when the T3 was increased from 60 to 120°C to represent a change in insolation, the changes in cycle efficiency and generated power were investigated using theoretical and experimental methods. Before the experiment, the maximum allowable temperature of the facility’s components was raised from 74°C to 120°C. Furthermore, because the P3 value was dependent on T3, the relationship between P3 and T3 was analyzed. Finally, the differences between the theoretical and experimental values were examined, and improved design conditions were proposed.
Table 4 summarizes the variables and fixed conditions set in the experiment. Based on the guidelines of the CO2 injection procedure, the amount of CO2 injected into the cycle loop was 25.8 kg. Choi et al. [10] in a previous study designed the CO2 mass flow rate to be 0.068 kg/s under the conditions listed in Table 3; however, the target mass flow was not achieved in the experiment. It was assumed that there were two regulating valves after the pump, whose flow coefficients were too small to achieve the target mass flow rate under the target pressure conditions. As a result, the experiment was carried out with a mass flow rate set to 0.040 kg/s. At that time, the discharge pressure of the air compressor was set to 0.8 MPa, and the driving air was supplied to the pump at a pressure of 0.2 MPa by the regulator. The cooler’s coolant temperature and mass flow rate were set to 4°C and 0.764 kg/s, respectively, which was the maximum cooling performance of the cooler. The atmospheric temperature of the laboratory was around 19°C during the experiment.
3.3. Experiment Results
3.3.1. Case Study of °C
Following the experimental procedure, a case study of °C was conducted to implement the operating conditions listed in Table 3, and the experimental results are summarized in Table 5. In terms of differences between the design and experimental data, the minimum pressure of the cycle at the design stage was 5.1 MPa, whereas it was 5.85 MPa in the experimental data. This was due to the dramatic volume expansion of CO2 caused by phase changes to the supercritical state and the fact that CO2 saturation pressure at the cooler was higher in the experiment than in the design value. The reason for increasing the saturation pressure was that the desired heat duty exceeded the maximum capacity of the cooler at the conditions of T3 of 70°C and a mass flow rate of 0.040 kg/s. It was also reflected in the value, which was 5.8°C higher than the design condition. Meanwhile, T1, T2, and T3 were similarly implemented within about a 1°C difference.
Figure 6 shows a T-S diagram of the cycle with a comparison between the design and experimental data. The TCRC power generation process is a 1-2-3- process, and the internal area of the diagram confirms the difference in the cycle thermal efficiency between the design and experiment. Using Equation (7) to calculate the cycle thermal efficiency, the design and experiment yielded efficiencies of 6.98% and 4.97%, respectively, indicating a 2% lower efficiency in the experiment compared to the design. This was due to the fact that the P3 data in the experiment were higher than the design value, while T3 was the same in both the design and experiment. Because CO2 enthalpy in the low-temperature region varies sensitively with pressure, it was 9.8 (kJ/kg) less than the design value as P3 increased by 0.45 MPa. Furthermore, the mass flow rate of CO2 was 0.040 kg/s, which was lower than the original design value of 0.068 kg/s, resulting in differences in the amount of heat supplied and power generated. The heat supplied in the design and experiment was 12.4 kW and 7.0 kW, and the power generated was 864 W and 350 W, respectively. In conclusion, the designed thermal capacity and amount of power generated were greater than the actual capacity of the experimental facility.
3.3.2. Parametric Study
A theoretical and experimental parametric study was conducted with T3 as the optimization variable to examine the change in cycle efficiency due to the change in heat added from the solar collector. While the theoretical analysis was performed using the TIT (T3) temperature range of 60–180°C, the experimental study data of 60–120°C were collected due to the operating temperature limitation.
Figure 7 shows a comparison of design and experimental cycle efficiencies, which both increased as T3 increased; however, the experimental efficiency was about 2% lower than the theoretical efficiency, and the difference of the two efficiencies gradually increased. In the theoretical analysis, the maximum and minimum pressures were fixed at 12.1 MPa and 5.1 MPa, respectively, and the minimum temperature was fixed at 11.8°C, as shown in Figures 8(a) and 8(b). In other words, the cooling capacity of the cooler was assumed to be large enough to maintain the minimum temperature and pressure at a constant level. In the experiment, on the other hand, the saturation temperature and pressure of CO2 inside the cooler were changed according to the change in T3, affecting the overall cycle pressure. Thus, the T1, T2, P1, and P4 values in the experiment are proportionally increased according to T3. Consequently, the maximum efficiency of the experiment was limited to 6.35% at 120°C. In contrast to the other points, P3 decreased at an intermediate temperature rather than increasing linearly. This was because of the phenomenon where the density of CO2 rapidly decreases within a specific temperature range in the supercritical region. However, it is expected to show a linearly increasing trend with an increase in T3 after passing through a specific temperature range.
(a)
(b)
The variations in temperature and pressure conditions at points 1 and 2, which were considered fixed values in the theoretical analysis, will be important characteristics of the TCRC in the actual solar thermal generation plants. In other words, even if the TCRC is initially operated in optimal operating conditions, the temperature and pressure of the cycle will inevitably change as the insolation changes with time. Consequently, the influence of the heat input on the temperature and pressure conditions at each point was identified as a follow-up study based on the above experimental results. Moreover, a model is presented in Section 4.1 to track the cycle efficiency and power generation of TCRC by reflecting experimental results in which the temperature and pressure change according to T3.
Figure 8(b) also shows that the increase rate of T4 becomes steep from T3 of 80°C or higher. This is because, at T3 of 60–70°C, point is in a two-phase state (liquid and gas) of CO2, whereas at T3 of 80°C or higher, the superheated vapor state is maintained without phase change. Subsequently, during the turbine design stage, this criterion can be used to determine whether the working fluid is condensed when passing through the turbine.
A previous study [10] proposed an advanced design in which a recuperator exchanges heat between high-temperature fluid at point and low-temperature fluid at point 2. According to the study, installing a recuperator under T3 conditions of 100°C or higher can improve the cycle thermal efficiency, and the maximum increase in cycle thermal efficiency is expected to be about 4% at T3 conditions of 180°C. From the experimental data of , as shown in Figure 8(b), the recuperator can be introduced from T3 at 85°C or higher case. These experimental results will be useful in determining optimal operating conditions for a recuperator in the future.
4. Modeling and Simulation
4.1. Modeling of TCRC
The minimum temperature (T1) and the minimum and maximum pressures (, ) were fixed constants during the design stage; however, the experiment revealed that the values also changed as T3 changed. Because there was a significant difference between the design and experimental data, a TCRC model that could adequately explain the experimental data was built by considering temperature and pressure variations into account. At first, using the experimental data from the parametric study, the changes in T1, P1, and P3 as a function of T3 were modeled through polynomial regression analysis, as shown in Figure 9. Simple linear regression equations assuming a linear change were obtained for T1 and P1. For P3, the following 3rd-order polynomial regression equation was obtained: In the order of T1, P1, and P3, the coefficients of determination (-square) of the polynomial regression equations were 0.97, 0.97, and 0.93, respectively. The TCRC model was developed using these equations in the following process: (i)(Point 1) Set the minimum temperature and pressure conditions of the cycle by applying the regression equations shown in Figures 9(a) and 9(b)(ii)(Point 2) In the pumping process, the enthalpy increased slightly, along with the isentropic efficiency 90% curve. T2 can be obtained from the property package using the information on the enthalpy change. In the existing design, a pump with 100% efficiency was assumed. However, experimental data confirmed that the efficiency of the piston pump currently in use was only 90% on average(iii)(Point 3) The T3 is the set value by the heater, and the P3 is determined by the Figure 9(c) regression equation(iv)(Point ) The was calculated from the enthalpy drop at points 3 and by assuming an expansion process with a turbine of 80% isentropic efficiency. was calibrated to equal the pressure at point 1
(a)
(b)
(c)
Figure 10 compares the cycle efficiency determined by the TCRC model to that of the design and experimental data. When compared to the design value, the TCRC model closely matches the experimental data. According to the model, the cycle efficiency seems to converge to around 6% even if the T3 is raised to 180°C. However, the model cannot be used outside the experiment range, i.e., above 120°C of T3. In the future, additional experiments with T3 at 120–180°C will be necessary to expand the available range of the model. Therefore, this model was only used in the T3 range of 60–120°C. Furthermore, the experimental results at T3 of 120°C confirmed that the phase of the working fluid does not change to a gas state at point after passing through the turbine and remains in a supercritical state. Thus, under a high T3 condition of 120°C or higher, increasing the pressure ratio of the turbine above the current level will improve the cycle efficiency because the increase in power generated by the turbine is expected to be greater than the increase in power consumption of the pump. Subsequently, an additional experiment should be conducted to observe the change in cycle efficiency as a result of the change in P3 in the future. Figure 11 shows a graph that compares the temperatures at point as obtained from the design, experiment, and model. Similar to the efficiency graph, the TCRC model follows the T4 data better than the design values. It is because the model was made from the experimental data and reflects what happens in the experiment, i.e., the temperature and pressure of each point change according to the T3 variable. at the experiment was about 5–15°C higher than that at the design. It also shows that the amount of work that can be obtained from the turbine will be less than expected at design.
4.2. Simulation of TCRC with Insolation Data
The cycle simulation was conducted by feeding real insolation data from Pohang into the TCRC model. The simulation lets us predict the cycle efficiency and the amount of generated power obtained when the TCRC is applied for actual power generation with a solar collector. In the simulation, the cases in which the operating temperature of T3 is beyond the model’s lower and upper limit temperatures, i.e., 60 and 120°C, were excluded for reliability.
Figure 12 shows the plotted hourly data on the insolation in May and June 2019 and the cycle thermal efficiency of the TCRC. The T3 values at 4 p.m. in May and 10 a.m. and 4 p.m. in June were outside the TCRC model temperature range. The maximum insolations of 758 and 674 W/m2 were obtained at 12 : 00 noon in May and 13 : 00 in June, respectively, and the cycle efficiencies were 6.41 and 6.03%, respectively, which were the highest values during the day. Of course, the TCRC was most efficient during the middle of the day when solar heat could be collected the most. Figure 13 shows the net power of the cycle under the same conditions. Similarly, the TCRC can generate maximum power during the maximum insolation periods, with up to 726 and 626 W produced in May and June, respectively. Interestingly, the up and down of the cycle efficiency during the day was much lower than that of insolation. Under the current operating conditions, a cycle efficiency of 5–6.5% can be obtained without significant change above a certain level of insolation (approximately 500 W/m2). Accordingly, the power generated in the cycle changes similarly to the change in the insolation data. If the pressure ratio of the turbine increases by improving the facility’s cooling ability, the cycle efficiency and the power generated at the T3 of over 100°C could be enhanced.
5. Conclusion
In this study, the TCRC for small-scale solar thermal applications was investigated as a new power generation technology. Initially, the optimal design conditions were compared to experimental data at the T3 of 70°C. The cycle efficiency in the experiment was 4.97%, which was approximately 2% lower than that of the design. Secondly, a parametric study was conducted with the optimization variable T3, and it was confirmed that increasing T3 improved the cycle efficiency in both the design and experiment results. However, it is bounded to about 6% in the experiment because the operating conditions of T1, P1, and P3 change and move away from the optimal points as T3 increases. As this phenomenon inevitably occurs in real applications, a model representing the experimental data was developed. A simulation was carried out with the model by applying actual insolation data in May and June. As a result, the maximum efficiencies were 6.41% and 6.03%. and the amounts of power generated were 726 W and 626 W, respectively. More research on the subject is necessary in future works. Further experiments should be conducted with a wider range of T3 and the maximum pressure variable. In addition, a recuperator can be added to the experiment facility to improve TCRC cycle efficiency.
Abbreviations
| CSP: | Concentrated solar power |
| PCHE: | Printed circuit heat exchanger |
| PD: | Parabolic dish |
| PID: | Proportional integral derivative |
| PT: | Parabolic trough |
| sCO2: | Supercritical CO2 |
| SPT: | Solar power tower |
| TCRC: | Transcritical CO2 Rankine cycle |
| TIT: | Turbine inlet temperature. |
Data Availability
Data is available on request.
Conflicts of Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Following are the results of a study on the “Leaders in INdustry-university Cooperation 3.0” Project, supported by the Ministry of Education and National Research Foundation of Korea. This work was supported by the Industrial Technology Innovation Program (Development of data-based energy efficiency optimization technology applicable to 70 ton Electric-Arc-Furnace in Steel making process, RS-2022-00155555) funded by the Ministry of Trade, Industry & Energy (MOTIE, South Korea).