تجزیه و تحلیل عملکرد از یک سیستم خورشیدی ترکیبی تولید برق انتشار CO2 نزدیک به صفر

A novel solar hybrid power generation system with near zero CO2 emission (ZE-SOLRGT) has been proposed in the previous work, which is based on a GRAZ-like cycle integrating methane–steam reforming, solar-driven steam generation and CO2 capture. Solar heat assistance increases power output and reduces fossil fuel consumption. Besides near zero CO2 emission with oxy-fuel combustion and cascade recuperation of turbine exhaust heat, the system is featured with indirect upgrading of low-mid temperature solar heat and its high efficiency heat-to-power conversion. A performance analysis of ZE-SOLRGT cycle has been carried out using ASPEN PLUS code to explore the effects of key parameters on system performances. It is concluded that ∼54% exergy efficiency can be attained with ∼100% CO2 capture. The net solar-to-electricity efficiency can reach up to 34.7% in the base case. Steam-to-methane molar ratio of 2–3 is suitable for system performance improvement. High system efficiency can be obtained as the HPT pressure ratio is in the range of 15–18. The system integration achieves the complementary utilization of fossil fuel and solar heat, as well as their high-efficiency conversion into electricity.

Solar thermal power generation is considered an efficient way to use concentrated solar radiation. However, due to its relatively low intensity, intermittent availability and uneven distribution of solar radiation, solar thermal power generation which uses solar energy as the exclusive or main input is generally costly and with low efficiencies. Therefore it is strategically desirable to develop hybrid solar/fossil processes and systems which use multiple heat sources at different temperature levels, for example in a way that low/mid-temperature solar thermal energy are used when they are relatively inexpensive, and higher temperature fossil fuel energy resources are integrated according to their cost to raise the energy efficiency. Hybrid system offers a solution for saving depletable fossil fuel and increasing the solar heat-to-power conversion efficiency simultaneously. An earlier such hybrid system was proposed by Lior and co-workers [1] and [2], named SSPRE (solar steam powered Rankine Engine). Solar heat about ∼100 °C collected by low-cost flat-plate solar collector provides the latent heat of steam generation and accounts for nearly 80% of the total system input, and fossil fuel is added to boost the steam temperature up to 600 °C for a higher efficiency (18%) power generation in a Rankine power cycle. The solar thermal aided power generation (SAPG) proposed by Hu et al. uses solar heat to replace some bleed steam in the regenerative Rankine power cycle for feedwater heating, attaining either additional power generation or reduced fuel consumption [3]. Besides its application in Rankine power systems, solar heat integration has also been introduced to gas turbine system and combined cycle power systems. Depending on its temperature level, solar thermal energy into combined cycle systems can principally be integrated with either the gas turbine topping cycle (for air preheating prior to the combustor) or into the steam turbine bottoming cycle (for steam generation) [4], [5], [6] and [7]. Based on the solar aided steam generation with parabolic trough technology, the integrated solar combined cycle systems (ISCCS) were proposed and demonstrated with different projects in Egypt, Iran, Germany and many other countries. It was concluded that solar hybridization in combined cycle systems has a high potential for cost reduction as compared with other solar hybrid power systems [4] and [7]. Besides solar thermal conversion, another technical option exists which converts the collected solar heat into chemical energy in an endothermic process using fossil fuels as chemical reactants and solar energy as the source or process heat [8], [9] and [10]. The absorbed solar heat is stored chemically as the incremental heating value of the produced solar fuel, and will further covert into electricity when the solar fuel is used in advanced power systems. Solar thermochemical power generation system is thus composed of solar fuel production and power generation sectors, and methanol and methane are the commonly used fossil fuel for the solar thermochemical power generation. Methanol–steam reforming and methanol decomposition can achieve over 90% conversion into H2-rich syngas at around 250 °C. Hong, Jin and co-workers proposed a combined cycle system that integrates mid-temperature solar heat driven methanol decomposition [11]. Via heating the endothermic decomposition reaction, solar thermal energy is upgraded to chemical energy of the produced syngas. The net solar-to-electricity efficiency can reach 35% with the solar thermal share of 18%, and the CO2 emission is 310 g/kW h without regard for turbine blade cooling requirements and CO2 capture. The same authors also proposed a solar hybrid combined cycle system integrating methanol-fuel chemical-looping combustion and low temperature solar thermal energy [12], in which Solar-driven endothermic reduction of Fe2O3 by methanol is carried out. The CO2 can be easily separated from vapor by condensation, with no extra energy penalty. Methane steam reforming is highly endothermic, and higher temperature favors methane conversion. Tamme et al. proposed and analyzed a high temperature solar methane reforming process (>1000 °C) and power generation with the produced solar fuel in a combined cycle system. [13]. Compared with a conventional gas–steam combined cycle system, the introduced solar heat may substitute about 30% fossil fuel and lead to equal percent CO2 emission reduction. To avoid the high cost and low collector efficiency associated with the high temperature solar heat collection, and to enable low/mid-temperature solar heat (below ∼400 °C) to achieve its high-efficiency heat-power conversion, Zhang and co-workers proposed a solar-assisted chemically recuperated gas turbine system (SOLRGT) with solar heat indirect upgrading [14], [15] and [16]. Rather than driving the endothermic reforming reaction directly, the solar thermal energy collected at ∼220 °C is used for steam generation and thus first transformed into the latent heat of vapor supplied to a reformer, and then via the reforming reactions to the produced syngas chemical exergy. This is a two-step conversion, a combination of thermal integration and thermo-chemical conversion process. The resulting syngas is burned to provide high temperature working fluid to a gas turbine. The solar-driven steam production helps to improve both the chemical and thermal recuperation of the system. Researching result shows that overall exergy efficiency could be about 5.6%-points higher than a compared intercooled chemically recuperated gas turbine cycle (IC-CRGT) without solar assistance. Since 80% of the total energy input is provided by methane, 342.7 g/kW h CO2 emission still reaches. In order to reduce CO2 emission, the authors proposed a low CO2 emission hybrid solar combined cycle (LEHSOLCC) power system [17] with solar-driven methane–steam membrane reforming and pre-combustion decarbonization. With 91% CO2 capture ratio, the system attains a net exergy efficiency of 58% at design point. The solar driven membrane reformer calls for further R&D efforts. Based on the combination of oxy-fuel combustion and the concept of SOLRGT cycle, an improved system configuration with CO2 capture (named ZE-SOLRGT) was proposed in [16], which employs water as the main working fluid. It is a solar assisted oxy-fuel combustion system, integrating with solar-driven steam generation, steam reforming of methane and CO2 capture. The power subsystem configured is similar as a Graz-like cycle [18], [19] and [20] to benefit from the combination of a high temperature Brayton topping cycle with a high-pressure ratio Rankine like bottoming cycle. As compared with other solar hybrid power plant concepts including steam Rankine power systems or integrated solar combined cycle systems (ISCCS), the ZE-SOLRGT system offers the significant advantage of high conversion efficiency of mid-temperature solar heat to electricity. The combination of solar thermal integration and thermochemical conversion enables solar heat collected at a relatively low temperature (and thus at lower cost) to be upgraded to high quality chemical exergy, and be added to the power system at the highest temperature attainable in the topping cycle. The increased efficiency results in reduced solar field size and thus less investment cost for the solar block. In addition, exergy destruction in the conversion from chemical into thermal energy is lower in syngas combustion than that in the unreformed methane fuel, as discussed in [16], this creates a gain from the combustion of syngas as compared with the direct combustion of methane. Oxy-fuel combustion, which based on the close-to-stoichiometric combustion between fuel and enriched oxygen in the environment of recycled flue gas, is employed for CO2 capture. It allows a large fraction of steam latent heat in the flue gas to be recovered internally which otherwise would be dumped to the environment. At the same time, the oxygen separation is energy consuming and may cause an efficiency penalty up to 7–10%. A comparison study was conducted in [21] between ZE-SOLRGT and gas/steam combined system with post-combustion decarbonization (CC-PC). It is found that solar heat in the ZE-SOLRGT system contributes an additional 18% to the total energy input leading to a much higher power output by 28.8% as compared with that in the CC-PC system, resulting to a thermal efficiency of 51% with nearly 100% CO2 capture. For the combined cycle system, the thermal efficiency drops from 55% to 48% taking into account the energy penalty for 90% CO2 capture ratio. In this paper, based on the previous study, a parametric sensitivity analysis of ZE-SOLRGT cycle is performed with ASPEN PLUS code [22] to further investigate thermodynamic performances of this type of systems. In the base case with steam-to-carbon molar ratio of 2 and nearly ∼100% CO2 capture, the ZE-SOLRGT system attains a net exergy efficiency of ∼54% and net solar-to-electricity efficiency of ∼35%. Sensitivity analysis results show that steam-to-methane molar ratio of 2–3 is suitable for system performance improvement. High system efficiency can be obtained as the high-pressure gas turbine (HPT) pressure ratio is in the range of 15–18. This novel solar-hybrid combined cycle not only offers a new approach to highly efficient conversion of low/middle temperature solar thermal energy, but also provides a possibility of CO2 mitigation with low energy penalty

A novel near zero CO2 emission hybrid solar-assisted power system with oxy-fuel combustion (ZE-SOLRGT) has been analyzed. The power generation part is arranged as the Graz-like cycle, which combines a high temperature Brayton-like topping cycle and a high pressure ratio Rankine-like bottoming cycle with steam as the main working fluid. The mid-temperature solar heat around 350 °C is added to generate steam, and thus first transformed into the latent heat of vapor. Then, a proportion of it is converted to the produced syngas chemical exergy via the reforming reaction, and finally accomplishes its high-efficiency heat-power conversion in an advanced power generation system. The system integration fulfills the complementary utilization of multiple energy resources and their high-efficiency conversion into electricity. Based on the same NG and O2 inputs, the system is simulated and compared with a GRAZ-like cycle without solar thermal energy input. The results show that the mid-temperature solar heat contributes to 15.1% additional power output, and a net solar-to-electricity efficiency of 34.7% can be achieved for the ZE-SOLRGT cycle in the base case. With the parametric optimization, net exergy efficiency for the hybrid system can be increased by about 5–8%-points as compared with that in the reference GRAZ system. CO2 capture with oxy-fuel combustion technology demands 10.4% of the overall power output for O2 production, imposing a penalty of 6.3%-points on the system exergy efficiency. The parametric sensitivity analysis results indicate that steam-to-carbon molar ratio of 2–3 might be suitable considering the compromise between system power generation and thermal efficiency. High system efficiencies can be obtained as the HPT pressure ratio is in the range of 15–18 and the split fraction of HPT flue gas of ∼0.43.