EXERGETIC ANALYSIS OF SOLAR AIDED COAL FIRED (210MW) THERMAL POWER PLANT

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1 International Journal of Advances in Thermal Sciences and Engineering Volume 2 Number 2 July-December 2011, pp EXERGETIC ANALYSIS OF SOLAR AIDED COAL FIRED (210MW) THERMAL POWER PLANT V. Siva Reddy *#, S. C. Kaushik *, S. K. Tyagi ** ABSTRACT: This article deals with comparative energetic and exergetic analysis for evaluation of coal fired thermal power plant and solar thermal aided coal fired thermal power plant. In this analysis assuming 8h/day solar energy is available, remaining 16 hrs, SAFWH can be supplied by solar thermal storage for 8hr duration. An instantaneous increase in power generation capacity of about 35% is observed by substituting turbine bleed streams to all the low pressure and high pressure feedwater heaters with SAFWH. Coal consumption 3.5% is increased for reheat of the steam by replacement of high pressure feed water heaters with SAFWH. Energetic efficiencies of solar aided coal fired thermal power plant appear high as compared to solar alone thermal power plant and low as compared to coal fired thermal power plants. Furthermore, for a SAFWH, it is found that the land area requirement is 3.3 hectare /MW for large scale solar thermal storage system running the plant for 24hrs. Keywords: Thermal power plant, Energetic efficiency, Exergetic efficiency, Solar aided feed water heating, Linear Fresnel Reflecting Solar Concentrator. 1. INTRODUCTION The total power generation capacity of India is MW. On that coal fired thermal power plant have the generation capacity of MW, and major domination of total power generation [1]. But fossil fuels scarcity and global warming problems associated with the power plant exhaust (CO 2 ), it is desired that solar energy utilization in the power plant is one preferable option to fulfill the system demand. The exergy method of analysis is based on the second law of thermodynamics and the concept of irreversible production of entropy. The exergetic performance analysis not only determines magnitudes, location and causes of irreversibilities in the plants, but also provides more meaningful assessment of plant individual components efficiency [2]. Gupta and Kaushik [3] have carried out a case study of a typical 50kW solar thermal power plant and a 220MW coal fired thermal power plant. Reddy et. al [4], analyzed the solar concentrator aided coal-fired power plants to establish their techno-economic viability of subcritical and supercritical power generation in India. Negi et. al [5] presented optical designs and performance characteristics of a Linear Fresnel Reflector Solar Concentrator (LFRC) with a flat vertical absorber by two different approaches with a variation in the width and equal width of the constituent mirror elements. The present work has been undertaken for exergetic analysis of a Solar aided coal fired (210MW) thermal power plant using LFRSC. 2. DESCRIPTION OF COAL FIRED THERMAL POWER PLANT Detailed flow diagram of coal fired thermal power plant is shown in Figure 1. The symbols identifying the streams and state point properties are described in Table 1 & process descriptions for each component is summarized. The boiler of the case-study power plant produces kg/s of live steam at 149 bar and 811K. There is a single reheat at 811K. A continuous mass flow diagram of the power plant modeled in * Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi , India, ( # Corresponding author: vundelaap@gmail.com) ** Sardar Swaran Singh National Institute of Renewable Energy, Jalandhar-Kapurthala Road, Wadala Kalan, Kapurthala (Punjab) India.

2 86 International Journal of Advances in Thermal Sciences and Engineering this study includes the main components such as high, intermediate and low pressure turbines (HPT, IPT and LPT), a boiler, condensate extract pump (CEP), Boiler feed water pump (BFP), a dearetor, a generator (G), a condenser (C), low and high pressure feed water heaters (LPH and HPH). 3. EXERGETIC ANALYSIS In an open flow system there are three types of energy transfer across the control surface namely work transfer, heat transfer, and energy associated with mass transfer and/or flow energy. The first law of thermodynamics or energy conservation equation for the steady flow process of an open system is given by: 2 2 ci co Σ Qk + m hi + + gzi = m ho + + gzo + W 2 2 Where Q k heat transfer to system from source at temperature T k, and W is the net work developed by the system. The other notations C is the bulk velocity of the working fluid, Z, is the altitude of the stream above the sea level, g is the specific gravitational force. The Energetic efficiency η I of a system and/or system component is defined as the ratio of energy output to the energy input to system/ component i.e. Desired output energy η I = Input engery supplied Exergy is a generic term that defines the maximum possible work potential of a system from a stream of matter and/or heat interaction; with respect to the state of the environment being used as the datum state. The Exergy (Ψ Q ) of heat transfer (Q) from the control surface at temperature (T) is determined from maximum rate of conversion of thermal energy to work W max. is given by: To Ψ Q = Wmax = Q 1 T Exergy of steady flow stream of matter is the sum of kinetic, potential and physical exergy. The kinetic and potential energy are almost equivalent to exergy. The physical specific exergy Ψ i and Ψ o depends on initial state of matter and environmental state. Exergetic analysis is a method that uses the conservation of mass and degradation of the quality of energy along with the entropy generation in the analysis design and improvement of energy systems. Exergetic analysis is a useful method; to complement but not to replace energy analysis. The exergy flow for steady flow process of an open system is given by T Σ 1 o Q k + Σin m Ψ i = Ψ W + Σout m Ψ o + I Tk 2 0 C h = h + + gz 2 I = T S o gen 0 0 ( 0 ) o ( o ) Ψ = m h h T s s

3 Exergetic Analysis of Solar Aided Coal Fired (210MW) Thermal Power Plant 87 Figure 1: Simplified Schematic View of the Coal Fired Thermal Power Plant Where Ψ i and Ψ o are exergy associated with mass inflow and outflows are respectively, Ψ W is useful work done on/by system, I is irreversibility of process and h 0 is the methalpy as summation of specific enthalpy, kinetic energy and potential energy. The exergetic or second law efficiency is defined as Actual thermal efficiency Exergy output Exergy loss η II = = = 1 maximum possible () reversible thermalefficiency Exergy input Exergy input 4. RESULTS AND DISCUSSION OF COAL FIRED THERMAL POWER PLANT The following parameters have been used during the coal fired thermal power plant analysis: Atmospheric condition is taken as 299K and 1.0 bar, 20% excess air for complete combustion, inlet condition for highpressure steam turbine is taken as 137 bar and 812K, condenser pressure is assumed to be 0.08 bar, heat rejection in the condenser is treated as a energy loss, and generator efficiency is 98%. A computational model was developed for carrying out the energetic and exergetic analysis of the system using Engineering Equation Solver (EES) software [7]. M ass flow, enthalpy and entropy, exergetic power and energetic power of water/steam at each thermodynamic state point are represented in Figure 1 corresponding to design conditions of coal fired thermal power plant as shown in Table 1. Boiler has large difference in exergetic power input and outputs as compared to energetic power input and output. Exergetic power input and output in boiler are kw and kw respectively. Energetic power input and output in boiler are kw and kw respectively. Energetic and exergetic power losses of different components in the power plant have been shown in Figure 2. Among the all components boiler have major exergetic power loss kw as compared to energetic power loss kw. In condenser energetic power loss is high kw as compared to the exergetic power loss kw. But in the turbines and pumps energetic and exergetic power losses are negligible. In regenerative heat exchangers (HPH1, HPH2 LPH1, LPH2 and LPH3) have exergetic power loss. Fig. 3. shows energetic and exergetic efficiency of coal fired thermal power plant. Among the all heat transfer components, exergetic efficiency is lower than the energetic efficiency. In electric power generation

4 88 International Journal of Advances in Thermal Sciences and Engineering Table 1 Stream Data for 210 MW Steam Power Cycle (At Turbine Inlet Pressure 150bar and 812K) S. ID Fluid Mass Tempe- Pressure Sp. Sp. Energetic Exergetic flow rature (bar) Enthalpy Entropy power Power (kg/s) (K) (kj/kg ) (kj/kg K) (kw) (kw) 1 steam steam steam steam steam steam steam steam steam steam water water water water water water water water water water water water water water water water water water water and consuming components, exergetic efficiency is higher than the energetic efficiency. Over all plant energetic efficiency is 34.19% and exergetic efficiency is 31.81%. It is evident from exergetic analysis that major exergetic power loss is in the boiler. So increasing the capacity of thermal power plant by increasing fuel input itself is inefficient. Further analysis has been done upon exergetic analysis of solar aided feedwater heating system 4. RESULTS AND DISCUSSION OF SOLAR (LFRSC) AIDED COAL FIRED THERMAL POWER PLANT The coal fired thermal power plants with feed water cycle consists of two series of heaters as LP & HP Heaters (Figure 1). From the energy conservation point of view, the solar concentrator aided feed water heating process in existing power plant can be accomplished by replacing both LPH and HPH heaters

p o w e r p l a n t w e r e : A v e r a g e s o l a r b e a m r a d i a t i o n o f 4 0 0 W / m Exergetic Analysis of Solar Aided Coal Fired (210MW) Thermal Power Plant 89 Figure 2: Energetic and

5 p o w e r p l a n t w e r e : A v e r a g e s o l a r b e a m r a d i a t i o n o f W / m Exergetic Analysis of Solar Aided Coal Fired (210MW) Thermal Power Plant 89 Figure 2: Energetic and Exergetic Power Loss in Coal Fired Thermal Power Plant Components with SAFWH. The non-extracted steam will expand in the respective turbine stages and finally will be exhausted to the condenser. The following fixed parameters are considered for the analysis. Width and length of the Linear Fresnel Reflector Solar Concentrator (LFRSC) glass is 0.5m & 500m and focal distance is 14m, total number of reflectors in single LFRSC is 40, concentration ratio is 48; average mass flow rate (m f ) is kg/s, Therminol VP-1 as the heat transfer fluid. Assumptions made to design the solar aided coal fired thermal 2 available in 8 hr/day. Remaining 16 hr backup from solar thermal storage facility. The results indicate that replacement of LPH and HPH by SAFWH can gives the exergetic efficiency more than the coal fired thermal power plant. Table 2 shows a summary of the results. In HPH and LPH replacement by SAFWH, the extra output is 74635kW with the solar thermal power input of kw. Total power plant energetic efficiency is 32.47% and exergetic Figure 3: Energetic and Exergetic Efficiency in Coal Fired Thermal Power Plant Components

6 90 International Journal of Advances in Thermal Sciences and Engineering efficiency is 35.73%. The land area required for 24hr operation of solar thermal power input is around 244 hectare. Solar aided exergetic efficiency is found to increase from 32.47% to 35.73%, while energetic efficiency is decreased from 34.19% to 31.81%, because the solar thermal conversion efficiency is high (65.52%). Table 2 Various Results of Solar Aided Feed Water Heating Options in Coal Fired Thermal Power Plant Solar aided Feedwater heating options Without Solar Aided With Solar Aided Key parameters Energetic Exergetic Energetic Exergetic Solar heat input (kw) Net fuel power input (kw) Condensate extract pump(cep) (kw) Boiler feed water pump(bfp) (kw) High pressure turbine(hpt) (KW) Low pressure turbine(lpt) Net unit capacity (KW) Solar power generation (KW) Net efficiency (%) Total active aperture area (m 2 ) Collector field required land area (hectare) CONCLUSION The exergetic analysis shows that boiler subsystem is main source of exergetic power loss in coal fired thermal power plant. The exergetic power loss in condenser is less. Analysis of solar aided feedwater heating (SAFWH) option in the coal fired thermal power plant, shows that an instantaneous increase in power generation capacity of about 35% is observed by substituting turbine bleed streams to all the low pressure and high pressure feedwater heaters with SAFWH. Coal consumption of 3.5% is increased for reheat of the steam by replacement of high pressure feed water heaters with SAFWH. With the SAFWH in coal fired thermal power plant, energetic efficiency decreases from 34.19% to 31.81% and exergetic efficiency increases from 32.47% to 35.73%. The net power generation capacity is increased from kw to kw with a land area of 244 hectare. REFERENCES [1] Indian Electricity Scenario. Available from Accessed on April 10, [2] Kaushik SC, Siva Reddy V, Tyagi SK. Energy and Exergy Analysis of Thermal Power Plants: A Review. Renewable and Sustainable Energy Reviews, 15, (2011), [3] Gupta M K, Kaushik S C., Exergetic Utilization of Solar Energy for Feed Water Preheating in a Conventional Thermal Power Plant. International Journal of Energy Research, 33, (2009), [4] Reddy K. S., Suresha MVJJ, Ajit K. K., 4-E (Energy, Exergy, Environment, and Economic) Analysis of Solar Thermal Aided Coal-fired Power Plants. Energy for Sustainable Development, 14, (2010), [5] Negi B. S., Kandpal T. C., Mathur S. S., Designs and Performance Characteristics of a Linear Fresnel Reflector Solar Concentrator with a Flat Vertical Absorber. Solar & wind Technology, 7, (1990), [6] Negi B. S., Mathur S. S., Kandpal T. C., Optical and Thermal Performance Evaluation of a Linear Fresnel Reflector Solar Concentrator I. Solar & wind Technology, 6, (1989), [7] Klein SA, Alvarado F. Engineering Equation Solver, Version F Chart Software, Middleton, WI

Feedwater Heaters (FWH)

Feedwater Heaters (FWH) A practical Regeneration process in steam power plants is accomplished by extracting or bleeding, steam from the turbine at various points. This steam, which could have produced