Introduction of an efficient small-scale freshwater-power generation cycle (SOFC–GT–MED), simulation, parametric study and economic assessment

Simulation and economic evaluation of small-scale SOFC-GT-MED

In this chapter, a comprehensive thermodynamic modeling of a hybrid solid oxide fuel cell-gas turbine (SOFC-GT) is conducted. A heat recovery steam generator is used to produce saturated water for the heating purpose. This saturated hot water can be used in an absorption chiller system to meet the cooling load of the system. In order to model the hybrid system, chemical and electrochemical analyses of SOFCs and other components are carried out through energy and exergy analyses. The results of a hybrid system are compared to a gas turbine power generation system in order to investigate the effect of fuel cell on the system performance. Based on the model results, exergy efficiency of a hybrid SOFC-GT is higher than the one for conventional gas turbine and steam generator cycle. To enhance the understanding of the results in this study, a complete parametric study is performed and the results are presented. The results of this study, show that an increase in fuel cell stack temperature and compressor pressure ratio increases the efficiency; however an increase in fuel cell current density and gas turbine inlet temperature (GTIT) decreases the efficiency. In addition, an increase in HRSG steam pressure and a decrease in HRSG pinch point temperature results in an increase in system exergy efficiency.

Energy and exergy analysis of internal reforming solid oxide fuel cell–gas turbine hybrid system

In this paper, a small scale biomass gasification based solid oxide fuel cell/gas turbine (SOFC/GT) combined heat and power (CHP) plant is investigated by means of both conventional and advanced exergy and exergoeconomic analysis. A one-dimensional model of an internal reforming planner SOFC is employed to account for the temperature gradient within the fuel cell solid structure, which is maintained at the maximum allowable temperature gradient (150 K) under different operating conditions. Two main parameters of the gasification process, namely, air-to-steam ratio and modified equivalence ratio, are investigated, and the key parameters of the cycle exergy and exergoeconomic study are analyzed. Moreover, a multi-objective optimization procedure is applied to determine the unavoidable gasifier conditions required for the advanced exergy analysis of the system. The results of the conventional exergy and exergoeconomic analysis reveal that the highest rate of exergy destruction occurs in the gasifier, followed by the afterburner (AB) with 41.87% and 21.98%, respectively. Also, the lowest exergoeconomic factor is related to AB by 5.34%, followed by heat recovery steam generator (HRSG), gasifier, air compressor, and SOFC, which implies that the priority is to improve these components to reduce the exergy destruction cost rate. The results obtained from the advanced exergy and exergoeconomic analysis indicate that the most of the total exergy destruction rate is unavoidably in the CHP plant. The AB shows the least improvement potential in terms of reduction of the exergy destruction by almost 2% avoidable part, followed by Heat Exchanger 3 (H.X.3), gasifier, and SOFC duo to their lowest avoidable exergy destruction parts of almost 5%, 10% and 13%f respectively. Furthermore, the unavoidable part of the investment cost rate for all the components of the cogeneration plant is larger than the avoidable part, which means that it is difficult to reduce the investment cost rate of the system components. Meanwhile, the endogenous/exogenous analysis shows that the exergy destruction is completely endogenous for all components of the integrated plant, except for HRSG, GT, and HX1. Compressors and turbines have the highest potential to reduce endogenous exergy destruction. This is due to their higher avoidable endogenous exergy destruction. Reducing the investment cost rate seems difficult, as the main investment cost rate was found to be an unavoidable endogenous part for all system components. Finally, some results obtained from the advanced analysis approach are the opposite to those of the conventional method. This fact emphasizes that the results of conventional exergy analysis alone are insufficient and unreliable. For example, based on the advanced analysis perspective, the gas turbine and H.X.2 by 8.9% and 8.46% modified exergoeconomic factor, respectively, should be considered for reducing investment cost rate, while the conventional method gives opposite results.

Energy cost contributes a large portion of the overall cost of desalinated water. Improving the energy efficiency of desalination plants is therefore a primary design goal. However, accurately evaluating and comparing the energy consumption of desalination plants that use different forms and grades of energy is difficult, especially for power–water coproduction systems in which primary energy (PE) consumption leads to both salable electricity and potable water. The power plant converts PE into grades of thermal energy and electricity usable by the desalination plant. To fully capture the thermodynamic and economic cost of energy, and to fairly compare desalination systems that use different grades of input energy, we must compare energy consumption not at the point where energy enters the desalination plant itself, but as PE consumption entering the power plant. This paper investigates a variety of metrics for comparing the energy and exergy consumption attributable to desalination in coproduction plants. Previous results have shown that reverse osmosis (RO) is approximately twice as efficient as multiple effect distillation (MED) on a PE basis. We then compare the PE consumption of MED and RO from a thermoeconomic perspective. The entropy generation at the RO membrane and in the MED effects are derived in similar terms, which enables a comparison of the overall heat transfer coefficient in an MED system to the permeability of an RO membrane. RO outperforms MED in energy efficiency because of a balance of material costs, transport coefficients, and cost of energy.

With high efficiency and very low emissions, fuel cells have been one of the choices of research in current energy development. The Solid Oxide Fuel Cell (SOFC) is a high temperature type fuel cell. It has the characteristic of very high operating temperature 1,027°C (1,300K). The SOFC has the main advantage of very high performance efficiency (over 50%), but also has very high exhaust temperature. Current studies point out that the combination of SOFC and Gas Turbine (GT) can produce efficiency more than 60%. The exhaust temperature of this hybrid power system can be as high as 227–327°C (500–600K). With this waste heat utilized, we can further improve the overall efficiency of the system. A simulation program of SOFC/GT system and the introduction of the concept of Combined Cooling, Heating, and Power System (CCHP) have been used in this study. The waste heat of SOFC/GT hybrid power generation system was used as the heat source to drive an Absorption Refrigeration System (ARS) for cooling. This waste heat enables the SOFC/GT to generate electricity in the system while providing additional cooling and heating capacity. Therefore, we have a combined CCHP system developed using three major modules which are SOFC, GT, and ARS modules. The SOFC module was verified by our test data. The GT and SOFC/GT modules were compared to a commercial code and literature data. Both the single- and double-effect ARS modules were verified with available literature results. Finally, the CCHP analysis simulation system, which combines SOFC, GT, and ARS, has been completed. With this CCHP configuration system, the fuel usability of the system by our definition could be above 100%, especially for the double effect ARS. This analysis system has demonstrated to be a useful tool for future CCHP designs with SOFC/GT systems.

Analysis and performance assessment of a new solar-based multigeneration system integrated with ammonia fuel cell and solid oxide fuel cell-gas turbine combined cycle

Heat recovery steam generators are important equipment at district cooling plants. The capability of heat recovery steam generators in generating steam influences the steam absorption chiller's performance. The steam generation capability of the heat recovery steam generators in turn is linked to the values of pinch point and approach point. Hence, a study on the pinch point and approach point for the heat recovery steam generators would be useful in understanding the effects of varying pinch point and approach point values to the heat recovery steam generators' performance. In relation to this subject, a parametric study on the heat recovery steam generators was done. The study covered the effects of the pinch point and approach point on the following: mass flow rate of steam generated; exhaust heat temperature leaving the heat recovery steam generators; and the efficiencies of the heat recovery steam generators. The first law of thermodynamics was used for the analysis. Four scenarios were covered in the study: the effects of the pinch point and approach point on steam generation; the effects of the pinch point and approach point on the exhaust heat temperature leaving the heat recovery steam generators; the effects of the pinch point and approach point on the efficiency of the heat recovery steam generators; and the effects of the exhaust heat temperature of the gas turbine on the mass flow rate of steam. Operating data at Universiti Teknologi PETRONAS gas district cooling plant were used to validate the model. The results from the first scenario indicated that higher pinch point and approach point led to a decrease in the steam being generated. For the second scenario, the increase in pinch point and approach point resulted in higher exhaust heat temperature leaving the heat recovery steam generators. Meanwhile, for the third scenario, it was noted that there was only a minimal variation of the efficiency of the heat recovery steam generator when the pinch point and approach point were increased. The findings of the fourth scenario indicated that with higher gas turbine exhaust heat temperatures, there was an increase in steam being generated. Therefore, the findings could be useful for the plant to set the operating parameters for operating heat recovery steam generators. © Universiti Malaysia Pahang.

An integrated solid-oxide fuel cell-micro gas turbine system with biomass gasification is investigated based on energy and exergy. The system consists of a biomass gasification system, a solid-oxide fuel cell (SOFC), a micro gas turbine (MGT), and a heat recovery steam generator (HRSG). Various parameters are determined for the integrated system, including syngas molar fraction, and heat input to the gasifier. Moreover, exergy flows and exergy destruction rates of major components of the system are calculated. The maximum energy and exergy efficiencies of the gasification system are 65.7 and 84.8 %, respectively. These values for the SOFC-MGT cycle with biomass gasification are reported as 58.3 and 69.6 %. The variations of syngas molar fraction and mass flow rate, gasifier exergy destruction, and CO2 emissions with the steam to carbon (SC) ratio are investigated. The results show that there is an optimum value for the SC ratio at which the syngas mass flow rates and the gasification energy efficiency reach a maximum. The CO2 emission, which is an important factor for the sustainability of the system, increases by 19.4 % as the value of the SC ratio increases. Increasing the gasification temperature, from 800 to 1000 °C, reduces the energy and exergy efficiencies of the total integrated system by 7.90 %, mainly by increasing the required heat input to the gasifier.

Water is a scarce natural resource fundamental for human life. Power plant architects, engineers, and power utilities owners must do everything within their hands and technical capabilities to decrease the usage of water in power plants. This paper illustrates the research carried out by Pöyry Switzerland to reduce the water consumption on power and desalination combined cycle power plants, on which there are gas turbine evaporative cooling systems in operation. The present study analyzed the potential re-utilization and integration of the heat recovery steam generator (HRSG) blowdown into the evaporative cooling system. Relatively clean demineralized water, coming from the HRSG blowdown, is routed to a large water tank, where it is blended with distillate water to achieve the required water quality, before being used on the gas turbine evaporative cooling system. To prove the feasibility of the HRSG blowdown recycling concept, the Ras Al Khair Power and Desalination Plant owned and operated by the Saline Water Conversion Corporation (SWCC), located in the Eastern Province of the Kingdom of Saudi Arabia, was used as case study. Nevertheless, it is important to mention that the principles and methodology presented on this paper are applicable to every power and desalination combined cycle power plant making use of evaporative cooling. Sea water desalination is the primary source for potable water production on Saudi Arabia, with secondary sources being surface water and groundwater extracted from deep wells and aquifers. Saving water is of utmost importance for power plants located in locations where water is scarce, and as such, this paper aims to demonstrate that it is possible to decrease the water consumption of power and desalination combined cycle plants, on which evaporative cooling is used as gas turbine power booster, without having to curtail power production. The outcome of the study indicates that during the summer season, recycling the HRSG water blowdown into the gas turbine evaporative cooling systems would result on the internal water consumption for the gas turbine evaporative coolers decreasing by 545 ton/day, or 23.79%, compared with the original plant design which does not contemplate blowdown re-use. Using evaporative cooling results on an overall gain of 186 MW, or 10.27%, on gross power output, while CO2 emissions decrease by 46.8 ton CO2/h, which represents a 13.8% reduction compared with the case on which the evaporative cooling system is not in operation. A brief cost analysis demonstrated that implementation of the changes would result in a negligible increase of the operational expenses (OPEX) of the plant, i.e., implementation of the suggested modification has an unnoticeable impact on the cost of electricity (CoE). The payback of the project, due to limited operating hours on evaporative cooling every year, is of 12 years for a 30 year plant lifetime, while 2.22 M USD of extra-revenue on potable water sales are generated as a result of implementing the proposed solution. Although in principle this value is modest, the effect of government subsidies on water tariffs as well as political and strategic cost of water is not included on the calculations. In conclusion, the study results indicate that water recycling, and reduction of plant's water footprint for power and desalination combined cycle plants using evaporative cooling, is not only technically possible but commercially feasible.

Multi-level modeling of SOFC–gas turbine hybrid system

The paper presents the exergy analysis of a natural gas fuelled energy conversion process consisting of a hybrid solid oxide fuel cell coupled with a gas turbine. The fuel is partly processed in a reformer and then undergoes complete reforming in an internal reforming planar SOFC stack (IRSOFC). The syngas fuels in turn a standard gas turbine cycle that drives the fuel compressor and generates excess shaft power. Extensive heat recovery is enforced both in the Gas Turbine and between the topping SOFC and the bottoming GT. Two different configurations have been simulated and compared on an exergy basis: in the first one, the steam needed to support the external and the internal reforming reactions is completely supplied by an external Heat Recovery Steam Generator (HRSG), while in the second one that steam is mainly obtained by recirculating part of the steam-rich anode outlet stream. The thermodynamic model of the fuel cell system has been developed and implemented into the library of a modular object-oriented Process Simulator, Camel-Pro®; then, by means of this simulator, the exergetic performance of the two alternative configurations has been analyzed. A detailed analysis of the exergy destruction at component level is presented, to better assess the distribution of irreversibilities along the process and to gain useful design insight.

This paper presents a conceptual model of a 200-kWe Solid Oxide Fuel Cell – Gas Turbine (SOFC-GT) plant operating on biomass derived synthetic gas. The plant also has a Heat Recovery Steam Generator (HRSG) integrated at the GT exhaust, producing low-pressure process steam. The SOFC has been considered to be of tubular cell construction and internally reforming type. The reaction kinetics of the reforming and shifting reactions have been integrated with the cell electrochemical model. Simulated thermodynamic performance of the modeled plant in terms of electrical power, efficiency and process heat is presented and analyzed. Influences of variations in operating pressure and gas composition, on the plant performance are also reported.

Thermo-environmental and economic comparison of three different arrangements of solid oxide fuel cell-gas turbine (SOFC-GT) hybrid systems

Implementing multiple-effect distillation and reverse osmosis thermal coupling to improve desalination process performance in combined water and power plants