Question

Consider two 50 MW power generating plants. One plant is fueled by combustion of coal, the other by solar energy. Using the concept of exergy, explain why the ecological impacts of the solar power process are lower than the coal fired process. Use words and diagrams in your answer

Answer 1

Exergy Analysis

Exergy can be defined as ‘work potential’, meaning the maximum theoretical work that can be obtained from a system when its state is brought to the reference or ‘’dead state’ (under standard atmospheric conditions). The main purpose of exergy analysis is to identify where exergy is destroyed. This destruction of exergy in a process is proportional to the entropy generation in it, which accounts for the inefficiencies due to irreversibility. Exergy analysis helps in identifying the process of irreversibility leading to losses in useful work potential and thus pinpointing the areas where improvement can be sought.

Exergy analysis of coal-fired power plant:

In India, coal found to be the most important and abundant fossil fuel and about 80% of the coal produced consume by India’s electricity sector. Also, this is of particular relevance as coal-fired power stations form the backbone of the Indian power generation sector. Mostly, coal-fired power plants in India operate on subcritical steam parameters with the exception of a few plants that use supercritical steam parameters. Most of the coal-fired power plants have efficiencies less than 35% by using indigenous high ash coal. Now a day, efforts are taking place to bring in highly efficient super critical technology in the country for thermal power plants. EXergy analysis of coal-fired power plant is done using mass and energy balance equation are attatched. Mathematical models for economic and exergoeconomic analysis of coal-fired power plant can be used. There exist a number of research papers concerning energetic and exergetic performances of coal-fired thermal power plants in the literature. For instance, thermal, metallurgical and chemical analyses of power plants can be achieved using exergy analysis. The performance of thermal power plant can be easily defined by the help of exergy as it enables the locations, types and true magnitudes of wastes and losses . Exergy analyses have been utilized as a tool for the assessment of energy quantity as well as quality in coal-fired power plant using operational data at different conditions. It was concluded that the exergy lose in the boiler can be easily reduced by providing preheating air at boiler entrance and reducing fuel to air ratio in a power plant. The relationship between thermal power plant efficiency and the rotary air preheater total process irreversibility was proposed by using exergy analysis. proposed operation and maintenance decisions based on exergy analysis for a 500 MWe steam turbine power plant . Conducted exergy analyses in a large-scale ultra-supercritical coal-fired power plant. In ammonia-water based Rankine and regenerative Rankine power generation cycle exergy analysis has been compared based on the second law of thermodynamics. In an organic Rankine cycle and in micro-organic Rankine heat engines, the exergy topological method was used to present a quantitative estimation of the exergy destroyed using different working fluids, carried out their research on the simulation and exergy analysis of a 600 MWe and 800 MWe Oxycombustion pulverized coal-fired power plant. The fundamentals of exergy analysis method along with minimization of entropy generation and its applicability for the system optimization were reviewed with the help of some examples. Exergy analysis of raw juice production and the steampower units of the sugar production plant successfully assesses the true thermodynamic efficiency of chemical processes. In thermal power plants the optimization of the first and second reheat pressures provided using the energy efficiency and exergy balance]. Multi-objective optimization techniques can be used for searching the decision space frontier in a single run in supercritical coal-fired plants. A system simulation calculation model has been carried out to explore the exergy destruction along with pollutant emission characteristics of the plant. In a pulverized coal-fired power plant, the effects of different operating conditions and parameters on the performance of each individual component of the plant using second law analysis have been observed as well as a study based on thermoeconomics has been proposed for the cost formation of the plant. conducted exergy and techno-economic analyses for the optimization of a double reheat system in an ultra-supercritical power plant. A systematic correlation derived for capital cost and exergy loss, also it was suggested that devices in plant approximately conform to a particular ratio value which reflects the appropriate trade-off between exergy losses and capital costs based on exergy analysis . So, thermoeconomics is a promising tool for the diagnosis of complex energy systems. Exergy efficiency analysis through irreversibility also helped out to reduce thermal irreversibility of the Kalina cycle using ammonia-water mixture as the working substance. Also, in a high temperature Kalina cycle system, using the exergy efficiency, the performance of the cycle can be assessed. Modi and Haglind investigated the benefit of using Kalina cycle for a direct steam generation, central receiver solar thermal power plant with high steam temperature and pressure. Also, the thermodynamic performance of Kalina cycle was compared with a simple Rankine cycle using exergy efficiency of the plant. Singh and Kaushik optimized Kalina cycle couple with a coal-fired power plant using the energy and exergy analysis. In recent years, a number of researchers have focused their research on both the energy and exergy analysis of coal-fired power plant having different capacity. In a steam power plant, energy and exergy of the boiler was analyzed. Vandani, Bidi, and Ahmadi, performed boiler blowdown heat recovery using energy and exergy analyses in steam power plants. Energy and exergy efficiencies of Rankine cycle reheat steam power plant evaluated at different operating conditions. In a coal-fired supercritical thermal power plant, an energetic and exergetic analysis was performed at different load conditions. Energy and exergy analysis based on the thermal efficiency, exergy efficiency, exergetic performance criterion, exergy destruction and net specific work output for irreversible single reheat Rankine cycle and the double reheat Rankine cycle was proposed by Gonca. In Turkey, nine coalfired power plant performance analyses and their comparison were performed based on energetic and exergetic methods at design conditions, which help the designer to locate and evaluate the process inefficiencies. Energy and exergy analysis of the Kalina cycle system using an ammonia-water mixture was analyzed by Nasruddin. Gupta and Kaushik carried out an energy and exergy analysis of a proposed conceptual direct steam generation solar-thermal power plant. It was concluded that the condenser has maximum energy loss followed by solar collector field. A comparison between coal-fired and nuclear steam power plants based on energy and exergy analyses, identified areas with potential for improvement in plant performance.

Exergy analysis for solar power plants:

Picture attached  shows a medium-temperature solar thermal power plant with solar collector, heat exchanger, turbine, condenser, regenerator and pump, its main components and roughly the main energy and exergy flows of the plant. This diagram shows where the main energy and exergy losses occur in the process, and also whether exergy is destroyed from irreversibility or whether it is emitted as waste, often waste heat, to the environment. In the energy flow diagram energy is always conserved, the waste heat carries the largest amount of energy into the environment, far more than is extracted as work in the turbine. However, in the exergy flow diagram the temperature of the waste heat is close to ambient so the exergy becomes much less than the energy. In the solar collectors the energy efficiency is assumed to be about 55%. This depends on type of collector, average temperature difference between absorber and environment, and saturation temperature in the boiler. The exergy efficiency of the concentrated medium solar thermal and hybrid PV/thermal as an example. The produced heat is used for hot water and/or space heating. In the solar PV systems, the energy and exergy efficiencies are almost the same or 15% and 16% since for solar radiation exergy is 93% of the energy and for the outflow of electricity both energy and exergy is identical. In a PV cell solar radiation is directly transferred to electricity by means of photons of light exciting electrons into a higher energy state to act as carriers of an electric current. The low energy efficiency of a PV cell is partly due to physical limitations in the photo-electric conversion, and the energy losses are mainly due to this that instead becomes heat radiation to the environment. A solar thermal converter has an energy efficiency of about 75%, however the exergy efficiency is very low or 10% because the temperature of the heat is close to ambient and thus of low exergy. In the case of hybrid PV/thermal systems, the energy and exergy efficiency is about 66% and 16% respectively.

Life Cycle Analysis (LCA)

To estimate the total exergy used in a process, it is necessary to take all the different inflows of exergy to the process into account. This type of budgeting is often termed Exergy Analysis.It evaluates the total exergy use by summing the contributions from all the individual inputs, in a more or less detailed description of the production chain.Life Cycle Analysis or Assessment (LCA) is common to analyze.In this method we distinguish between renewable and nonrenewable resources. The total exergy use over time is also considered. These kinds of analyses are of importance in order to develop sustainable supply systems of exergy in society. The exergy flow through a supply system over time, such as a power plant, usually consists of three separate stages. At first, during the construction stage (0 ? t ? t _start) exergy is used to build a plant and put it into operation. The exergy is spent, of which some is accumulated or stored in materials, e.g. in metals, as well as exergy used for transportation etcetera. Secondly the system need to be maintained during time of operation (t_start ? t ? t _stop), and finally the cleaning up and disposal stage during destruction stage (t_stop ? t ? t_life). Eventually, some material, i.e. stored exergy, can be recycled. These time periods are analogous to the three steps of the life cycle of a product in an LCA. The exergy input used for construction, maintenance and destruction are called indirect exergy Eindirect and it is assumed that this originates from non-renewable resources. By using recycled material in the production stage, the indirect exergy may be considerably reduced. If exergy is recovered by recycling in the destruction stage, this is accounted for as an additional product of the system, E_rec. When a power plant is put into operation, it starts to deliver a product, e.g. electricity with exergy power E_pr , by converting the direct exergy power input into demanded energy forms.The direct exergy is a non-renewable resource, e.g. fossil fuel and in the direct exergy is a renewable resource, e.g. solar radiation.

  In the first case the system is not sustainable,since the system use exergy originating from a non-sustainable resource and it will never reach a situation where the total exergy input will be paid back, simply because the situation is powered by a depletion of resources, i.e. Epr+Erec<Ein+Eindirect. In the second case,at time t= tpayback.The produced exergy that originates from a natural flow has compensated for the indirect exergy input. In a solar thermal plant the energy payback time is much shorter than the exergy payback time since the exergy of the output of heat is much lower than its energy value. This may lead to false assumptions in the evaluation of the sustainability of renewable energy systems. From Figure we see the advantage of LCEA when applied to systems based on non-renewable and/or renewable energy resources. Solar energy systems producing electricity have less exergy back time than system for heat production. The exergy payback time of PV/T system is much longer than the energy payback time or about 4 times longer. This indicates that LCEA gives a completely different view of these systems that is of essential importance in scientific evaluations. Among the three solar thermal power plants the PV/T plant has the shortest energy payback time or 3.5 years. However, by applying exergy the 4.2 kWp stand-alone PV plant has the shortest payback time or 7 years, since the product in this case has a higher exergy value, i.e., more of electricity. This system is also the larger of the two pure PV systems. LCEA is shown to be advantages in the study of solar based energy systems and is recommended as a suitable tool for the design and evaluation of renewable energy systems.