تجزیه و تحلیل عملکرد از یک سیستم تولید ترکیبی با استفاده از انرژی خورشیدی و فن آوری SOFC

Due to the increasing future energy demands and global warming, the renewable alternative energy sources and the efficient power systems have been getting importance over the last few decades. Among the renewable energy technologies, the solar energy coupling with fuel cell technology will be the promising possibilities for the future green energy solutions. Fuel cell cogeneration is an auspicious technology that can potentially reduce the energy consumption and environmental impact associated with serving building electrical and thermal demands. In this study, performance assessment of a co-generation system is presented to deliver electrical and thermal energy using the solar energy and the reversible solid oxide fuel cell. A mathematical model of the co-generation system is developed. To illustrate the performance, the system is considered in three operation modes: a solar-solid oxide fuel cell (SOFC) mode, which is low solar radiation time when the solar photovoltaic (PV) and SOFC are used for electric and heat load supply; a solar-solid oxide steam electrolyzer (SOSE) mode, which is high solar radiation time when PV is used for power supply to the electrical load and to the steam electrolyzer to generate hydrogen (H2); and a SOFC mode, which is the power and heat generation mode of reversible SOFC using the storage H2 at night time. Also the effects of solar radiation on the system performances and the effects of temperature on RSOFC are analyzed. In this study, 100 kW electric loads are considered and analyzed for the power and heat generation in those three modes to evaluate the performances of the system. This study is also revealed the combined heat and power (CHP) efficiency of the system. The overall system efficiency achieved for the solar-SOFC mode is 23%, for the solar-SOSE mode is 20% and for the SOFC mode is 83.6%. Besides, the only electricity generation efficiency for the solar-SOFC mode is 15%, for the solar-SOSE mode is 14% and for the SOFC mode is 44.28%. An economic analysis is presented based on the annual electricity generation from the system and the system has shown the good economic viability in this study with a unit cost of energy (COE) about 0.068 $/kW h.

The World Bank and International Energy Agency reported that the world will require twice installation capacity over the next 40 years for the new-electrical power to meet the anticipated demands. In an another estimation the World Business Council reported that for the Sustainable Development, 40% of the world primary energy will be used for cooling, heating and providing power. Most of this energy is from electricity which is generated at centralized power stations; where at present up to 70% of available energy already lost. Although, the finite sources like natural gas, coal, and unprocessed oil, are the major sources of energy those are supplying large portion of energy on this planet, but the increasing rate of populations and energy demands are growing faster than the energy generation. Hence to meet the climbing energy demands the world cannot depend only on the limited conventional sources [1] and [2]. This paper presents a complete renewable based sustainable cogeneration system to produce combined electricity and thermal energy using hybrid solar energy and solid oxide fuel cell technology. The importance of solar energy, solid oxide fuel cell as well as cogeneration system is described in next subchapters. 1.1. Solar energy The solar energy is an unlimited source of energy which is originated from the sun. When the light and heat from the sun are used directly without changing the form, then the technology refers as a direct or passive technology of solar energy and when it used by converting the form of energy, that is called indirect or active technology of solar energy. The photovoltaic technology is the renowned indirect way and the solar thermal system is the direct way to harvest the abundant energy [2] and [3]. Approximately 60% of total emitted energy from the sun reaches the surface of earth. Considering 10% conversion efficiency of 10%, about 0.1% of this energy can generate 3000 GW power; which is four times larger than the world’s total generation capacity. Among the renewable sources solar energy is the most clean and amicable for the environment. As a consequence, it is getting more concentration to play an important contributor in electricity generation system [4] and [5]. Although, the solar energy system is still more expensive than the conventional energy system, but the solar energy system cost reduces progressively due to the improvement of modern and reliable PV technologies. The solar energy cost has dropped over the last few decades in such a way that the solar module cost was around US$27,000/kW in 1982, US$4,000/kW in 2006 and the solar-PV installation cost was approximately US$16,000/kW in 1992, US$6,000/kW in 2008. Regardless of, the acceptance of solar energy and R&D works have been tremendously increasing because of the worldwide supportive movements and policies implemented by the governments [2]. In this study, the solar PV is used for water steam electrolysis and electrical loads. The parabolic trough solar collectors (PTSC) are used for supplying high temperature water steam to produce hydrogen. The PTSC is chosen for this study because it is the most established technology among the solar thermal technologies [6]. 1.2. Solid oxide fuel cell The hydrogen production by the endothermic electrochemical reactions of water can be possible in reverse fuel cell operation. If the required electrical and heat input could be provided by the non-fossil fuel, CO2 emission free sources (like, solar, wind, hydro, biomass, and geothermal) the sustainable H2 production by water electrolysis would be more promising in economical and cleanliness point of view [7]. The main advantage of H2 production at high temperature is, significantly low electrical energy required to electrolyze the water compared to the low temperature system. The total energy requirements for H2 production are less sensitive of the operating temperature; as a consequence high temperature fuel cell offers more opportunities to use the industrial waste heat [8] and [9]. The fuel cell technologies are getting importance for global energy supplies instant of centralized power plants in a small to large scale power generation because, it is more environmental friendly as well as higher efficient compared to the fossil fuel based power plant. Among the fuel cell technologies the solid oxide fuel cell (SOFC) has been recognized as a promising clean energy technology which produces electricity by the chemical reactions of fuel and oxygen at higher efficiency (45–65%). The various range of fuel utilization makes the SOFC more attractive. The gaseous hydrogen, natural gas, products of coal gasification can be used as a fuel of SOFC. It becomes possible for the high operating temperature (600–1000 °C), which helps internal fuel reforming [10], [11], [12] and [13]. Additionally, the SOFC produces steam at high temperature that can be harnessed for further uses such as combined cycle or space and domestic water heating. This hybrid operation of SOFC can raise the overall system efficiency above 80% [14] and [15]. The production of H2 as well as electricity by a single solid oxide fuel cell makes it economically sound. Some novel studies have also been done on dual mode operations of SOFC. For example, Ni et al. [16] have presented a theoretical model of SOFC in dual mode operations and developed successfully. Jie Guan et al. [17] have developed a high performance reversible SOFC. They have tested 10 RSOFC stacks over 1000 h alternating the modes. The project was successful for producing hydrogen and electricity with high efficiency. Recently lots of studies have been done to improve the performance of RSOFC such as Rao et al. [18] have proposed a co-doped BaZrO3 (BZC-x) samples of a single phase air electrode for reversible solid oxide cells and found the polarization resistance promisingly lower. Nguyen et al. [19] have built a two-cell planar stack in the Jülich F-design with solid oxide cells and demonstrated the reversible operation between fuel cell and electrolysis modes. They have found that the mixed-conducting oxygen electrodes LSCF were presented as good candidates for reversible oxygen electrodes in high temperature electrolysis cells. Zhang et al. [20] have developed a designed apparatus for testing of single solid oxide cells in both fuel cell and electrolysis modes of operation. Laguna-Bercero et al. [21] have presented an electrochemical performance of LSCF and LSM/YSZ composites as oxygen electrodes for RSOFC. Both LSCF and LSM/YSZ were shown as good applicants as reversible oxygen electrodes using Scandia stabilized zirconia based cells. He et al. [22] have studied on a RSOFC, where the RSOFCs with thin proton conducting electrolyte films of BaCe0.5Zr0.3Y0.2O3-δ were fabricated and their electro-performance was characterized with various reacting atmospheres. 1.3. Co-generation system Co-generation system is not a new concept which came in the 1880s from industrial plants when as a primary energy source in industry was steam. Before 20th century, main electricity generation system was coal fired boiler and steam generator based and the exhaust steam from this system was used for industrial heating applications. The co-generation system gained more attention just after the oil crisis in 1973 because of the lower fuel consumption and environment pollution. In addition, the co-generation system can provide both electricity and thermal energy using a single source of fuel with high efficiency. The efficiency of co-generation system is over 80%, where the average efficiency of a conventional fossil fuel system is 30–35%. Consequently the generation cost becomes lower in cogeneration system. Because of these advantages, today many countries like Europe, USA, Canada and Japan are taking leading contribution to establish cogeneration system not only in industrial but also residential sector. Now-a-days, it can provide electricity and heat for small to large scale applications, such for hospital, office building, hotel, and single or multifamily residential buildings [23]. The renewable energy is doing a great contribution in a cogeneration system to provide green energy solution around the world. Among the renewable energy sources the contribution of solar energy is more noticeable than the other sources. Various technologies and studies have been proposed utilizing the solar energy in a cogeneration system. Rheinländer and Lippke [24], Pearce [25], Prengle et al. [26], Moustafa et al. [27], Mcdonald [28], Mittelman et al. [29], Qiu and Hayden [30], have considered solar energy in their study to establish the cogeneration system. The solid oxide fuel cell has been implemented and investigated in a cogeneration system by the many researchers not only in large power system but also in a building integrated system over the last few decades. For example, Zink et al. [31] have studied on a building integrated CHP system and funded the superiority of SOFC to supply electricity and heat according to the economic and environmental analysis. Naimaster and Sleiti [32] have presented a medium level cost economical SOFC based CHP system for an office building. Wakui et al. [33] carried out a study on 0.7 kW SOFC-CHP system with a plug-in hybrid electric vehicle. Han Xu et al. [34] have developed a 1 kW residential CHP system considering planar counter-flow SOFC. Lee and Strand [35] have analyzed on the modeling algorithm for the simulation of SOFC cogeneration system and parametric studies carried out to investigate the effect of each cell parameter on system performance. Verda and Calí Quaglia [36] have modeled a distributed power generation and cogeneration system and investigated possible improvements of SOFC to increase the plant performance. Rokni [11] presented a hybrid system with SOFC and steam turbine where the cyclic efficiency of the system has been improved considerably higher than the conventional system. This study proposed and investigated a new concept of a cogeneration system for green energy supply. The system differs from others in such a way that the solar energy in both direct and indirect forms is used and stored as a H2 gas instead of battery bank for continuous power and heat supply using reversible solid oxide fuel cell technology. This study illustrates the performance of the proposed model considering the three modes of operation through important output parameters. These parameters are H2 generation efficiency, energy efficiency, net electrical power, electrical to heating ratio and the unit cost of energy. The investigation considers the effect of changing different operating variables on these parameters. The variables are the solar radiation, the operating temperature of RSOFC for both modes and the H2 utilization of SOFC.

A mathematical model has been developed to simulate the electrical power and the heat energy generation in a co-generation system. The model has been conducted by considering the solar energy in thermal and photovoltaic form with the reversible solid oxide fuel cell (RSOFC) to produce the H2 at higher solar radiation time as well as to generate the electrical power and the heat energy at night time utilizing the storage H2. The cogeneration system model has been analysed with considering three modes of operation: solar-SOSE, SOFC and solar-SOFC. The effects of temperature on the RSOFC and the effects of solar radiation on the solar-PV and the PTSC have been discussed taking different parameters value from literature. After that, considering 100 kW of electric loads, the performances and the economic analysis of the system according to the operation modes have been presented. The following findings can be concluded from the analysis: i. The SOFC mode provides the highest net electrical as well as overall energy efficiency compared to other modes. The main reason behind this efficiency increase is lower number of components is used in SOFC mode. The lowest efficiency was found at solar-SOSE mode because of inefficient solar-PV, although the efficiency of H2 production is high. Another cause of low efficiency of the system at solar-SOSE and solar-SOFC mode is the solar radiation intensity (W/m2) on the PV modules and the collectors’ surface, since the radiation is considered as the input energy of the system in order to calculate the efficiency. ii. The CHP efficiencies for the SOFC, the solar-SOSE and the solar-SOFC mode are 83.6%, 20% and 23% respectively. On the other hand the only electrical efficiency for the SOFC, the solar-SOSE and the solar-SOFC are 44.28%, 14% and 15% respectively. iii. Additionally, the system provides heat energy at high temperature that can be harnessed for further uses such as for combined cycle or, for space and domestic water heating. It will make the system more efficient and economical. In this study, the heat to power ratio has been found 0.917 at SOFC mode and 1.09 at solar-SOFC mode. The more heat energy can be joined from the PTSC at day time. iv. The temperature changes, instantaneously affected the most part of RSOFC. The RSOFC system with higher operating temperature for the both H2 generation in electrolyzer mode and the power and heat generation in fuel cell mode is more beneficial than lower operating temperature system. v. The solar radiation changes are highly reflected on the H2 production in solar and H2 mode because the H2 production rate relies on the electrical input that is provided by the PV and the thermal energy input that is provided by the PTSC. The power generation of SOFC at solar-SOFC mode is also changed with solar radiation. vi. Annually 2920 h in electrolyzer mode and 2815.2 h in fuel cell mode of operation have been considered for the system performance and the economic analysis, where the annual cost of electricity is found 0.068 $/kW h.