CHAPTER 4 FEASIBILITY ANALYSIS OF PV-WIND-FUEL CELL HYBRID SYSTEM FOR CAUSTIC SODA INDUSTRIES

Transcription

1 89 CHAPTER 4 FEASIBILITY ANALYSIS OF PV-WIND-FUEL CELL HYBRID SYSTEM FOR CAUSTIC SODA INDUSTRIES 4.1 INTRODUCTION In this chapter, the feasibility analysis for autonomous power supply of caustic soda plant owned by DCW Ltd, the chosen industry in India is carried out using the Hybrid Optimization Model for Electric Renewables (HOMER) software. Out of the four cases considered, the first case deals with Diesel only power system, the second integrates PV/Wind/Diesel and hydrogen technologies with battery backup and the third one without battery. A novel cost effective and environmental friendly hybrid Renewable Energy System (RES) with maximum Biomass fraction is proposed as the fourth case. Simulation and optimization of the component sizes in the four cases are accomplished using HOMER. Finally, the economics of transition from fossil fuel to Biomass power plants is illustrated with an example to analyse the amount of fuel oil that can be saved due to the transition. 4.2 SYSTEM DESCRIPTION The objective of this study is to determine the feasibility of using stand-alone solar-wind-fuel cell hybrid system as the primary source of electrical power at the industrial facility. As DC power is the major electrical input to the process of caustic soda production, the soda ash industries are selected to be the appropriate target for fuel cells and PV systems. The major soda ash industries in India are Tata Chemicals, Saurashtra Chemicals Ltd, GHCL, Nirma Ltd, Tuticorin Alkalis and DCW Ltd. DCW is the chosen industry and is situated in Tamil Nadu on the Tuticorin- Tiruchendur State highway in Arumuganeri Taluk.

2 90 The factory together with the township facilities and Salt Fields covers a total area of about 1012 hectare and it works round the clock. Salt, the raw material for caustic soda, is produced at Sahupuram site itself in the company s salt pans spread over 600 hectares. Since the manufacturing process is power intensive, a Coal based captive cogeneration plant of 50MW capacity has been commissioned in 2007 to meet its demand of electricity. The installed capacity of CS plant in DCW is about Tons Per Annum (TPA) requiring about 27MW DC power for the producing 283 Metric tonnes of caustic soda output per day. The process description of CS plant is described in the subsequent section Existing Generation System and the Proposed RE System for the CS Plant The Caustic Soda Plant of DCW is equipped with UHDE Membrane cells. Brine for Ion Exchange Membrane process (IEM process) for caustic soda manufacturing is prepared by dissolving crystal salt into the return brine from the electrolyzers, and is purified with chemicals in order to precipitate the impurities of raw salt. Chemicals, such as caustic soda (NaOH) and sodium carbonate, are added to the saturated raw brine flow. The resulting suspension is sent to the Brine Clarifier and then is separated to sediment and solution. Now, the brine is passed to the ion membrane electrolyzer unit for NaOH production. The schematic diagram of the existing power generation caustic soda plant is depicted in Figure 4.1. In the membrane cell process, the anode and the cathode are separated by an ion exchange membrane that selectively permits sodium ions but restricts the hydroxyl ions from the cathode section into the anode section. Saturated brine is fed into the anode compartment, where chlorine gas is generated and sodium ions migrate into the cathode section through the membrane. In the cathode section, hydrogen is evolved, leaving behind hydroxyl ions, which reacts with sodium ions, producing caustic soda. The reaction in membrane cells involving the electrolytic separation of sodium chloride is given

3 91 by the equation 2NaCl (aq ) + 2H 2 O 2NaOH (aq) + Cl 2 (g) + H 2 (g) (4.1) (Source: ) The caustic solution is further concentrated to form flakes at a later stage by evaporation method. The weak brine from the electrolyzer is dechlorinated and re-circulated. The hydrogen, generated from the caustic soda plant, is mainly combined with the chlorine for producing hydrochloric acid. Figure 4.1 Schematic diagram of existing power generation system for Caustic Soda Plant In the proposed PV/FC hybrid system shown in Figure 4.2, the hydrogen gas output can be used by FC to produce the DC power along with PV system to meet the load demand required by the CS plant. This eliminates the need of coal based Captive Power Plant (CPP), transformer and electrolysis rectifier in the existing system. Wind Energy Conversion Systems (WECS) is also added as the third generation source due to the huge potential available near the industrial location.

4 92 Figure 4.2 Schematic diagram of the proposed PV/Wind/FC hybrid system 4.3 METHODOLOGY This section focuses on the framework adapted for the feasibility study of large scale off-grid renewable power generation in chlor-alkali industries through the resources available in the locality and sustainable development of such RE sources. The feasibility study is carried out in three phases: Pre-feasibility analysis, HOMER analysis and Post-HOMER analysis and the framework of the feasibility study is depicted in Figure The relevant data needed for the initial assessment and sustainability analysis has been obtained from DCW Private Ltd. In the pre-feasibility study, a detailed assessment of the available resources in the chosen region, industrial load demand, costs of the chosen RE technologies and biomass fuel viability assessment have been carried out. In the HOMER phase of the study, techno economic analysis and environmental impact of the hybrid RE technology have been discussed.

5 93 Figure 4.3 Framework of the feasibility study The techno-economic analysis of biomass based system uses HOMER software package to identify the optimal hybrid system configuration. The analysis is based on Net Present Cost (NPC) and Cost of Energy (COE) which includes capital cost, fuel cost and O&M cost. Simulations are performed by HOMER to select the best configuration in order to meet the demand of the caustic soda industry. The possible GHG emission reductions with the adoption of RE sources are also analysed. The micro and macro-economic impacts that would influence the future sustainability of such RE projects have been discussed in the Post- HOMER analysis Pre-Feasibility Assessment In this section, an initial assessment of the available resources, cost and demand of the caustic soda industry is accomplished to analyze the technoeconomic viability of hybrid renewable energy systems using HOMER tool. This initial assessment is carried out outside HOMER and data is fed into the

6 94 software. HOMER simulation software provides optimized results of the hybrid system by performing thousands of simulations based on input parameters like resource inputs, primary load inputs of the chosen renewable energy sources, fuel price and costs per unit for various components of the proposed hybrid system. All the parameters are explained in the subsequent sub-sections Renewable energy resources availability Sahupuram region is enriched with enough solar and wind energy resources. The caustic soda plant produces hydrogen as its by-product which can be utilized by the fuel cells for DC power output required for the process. Hence, solar-photovoltaic, wind and fuel cells are considered as the primary sources of renewable power generation for the feasibility study. The solar energy resource data for DCW Unit at Sahupuram (8.72ºN Latitude and 78.12ºE Longitude) in Tuticorin district is taken from NASA Surface Meteorology and Solar Energy as seen in Table 4.1. From the table, it is obvious that the global solar irradiation is 5.55 KWh/m 2 on an average, which means that there is enough solar potential and considerable amount of solar energy can be obtained throughout the year. Average wind speed is the key in determining the wind energy potential at any particular location and long term (over 10 years) speed average is the most reliable data for the resource assessment. Hence, a 10-year average wind speed data is taken from the website of NASA Surface Meteorology and depicted in Table 4.1. From the table, it is evident that the average wind potential is about 4.99 m/sec and is sufficient for hybrid power generation in DCW. The Chlor-alkali industry can be considered as an important target for the fuel cell system because large amount of high purity hydrogen is produced as a by-product of caustic soda plant.

7 95 Table 4.1 Lat 8.72 Lon Average insolation Average Wind speed Monthly Averaged Insolation (KWh/m 2 /day) and Wind Speed (m/s) data for DCW Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual Average In caustic soda production, mercury was used earlier and hydrogen thus obtained was impure and had no market in caustic soda manufacturing. But, the hydrogen now obtained is pure due to the membrane cell process. DCW can make use of this pure hydrogen as fuel input to the Fuel Cell system, which is a more financially attractive option. Caustic soda production is about 283 metric tons per day and 7075 Kg of H 2 obtained as output per day can supply 5 MW Fuel Cell system approximately, as per the Nedstack product specifications. It states that 1MW Nedstack Fuel Cell system consumes 63 Kg of H 2 per hour. So, the fuel cell power system can meet 20 to 40% of DCW s electricity consumption Electrical load profile At present, the electrical demand of Sahupuram unit is met with 2 25= 50 MW coal based Captive Power Plant (CPP) on site and the surplus power is exported to the Grid. Also DCW has a 12 MW DG set as standby unit to cope up with unexpected power failures. The electrical energy consumption of the electrolysis rectifier and CS process equipment is shown in Table 4.2. About two-third of the total power generated is required for CS manufacturing and remaining one-third is consumed by other process plants in DCW Unit. In order to promote H 2 usage as clean fuel input to the fuel cells, the electrical load of the caustic soda plant is alone considered for the modeling of the RE system.

8 96 From Table 4.2, it can be seen that the total electrical energy consumption of the CS plant is about 555 MWhr/day on an average for the year As per the observations of the plant manager, daily average power consumption of the membrane plant is about MW per hour requiring 648 MWhr of energy per day for seven tons of hydrogen production. Table 4.2 Electrical energy profile for caustic soda plant of DCW Ltd Year Electrolysis rectifier (KW-hr) CS Process (Primary and Secondary brine) equipments (KW-hr) CS Auxiliary Utility Equipments (KW-hr) CS - Lighting and Welding (KW-hr) Apr/ Mar/ Jun/ Jul/ Aug/ Sep/ Oct/ Nov/ Dec/ Jan/ Feb/ Mar/ Total Component costs HOMER tool also requires economic inputs of RES components for performing the simulation. The PV panels are connected in series. The capital cost and replacement cost for a 1 kw SPV is taken as $6000 and $5000 respectively. As there is very little maintenance required for PV, only $10/year is taken for O&M costs. The lifetime of PV panels is 20 years. A de-rating factor of 90% is chosen for the PV panels to account the varying effects of temperature and dust on the panels. The investment cost, replacement cost and the O&M cost

9 97 of 1 MW biomass plant is about 1 M$, 0.8 M$ and 0.01$/h respectively. The life-time of the project is 25 years. The model constraints include maximum annual capacity shortage varying from 0% to 10%. The capital cost, replacement cost, O&M costs of a 1 kw Fuel cell system are taken as $1200, $1000, and $1.03/h respectively. Out of the RES components, the cost of PV, Wind, Battery, Biomass and Diesel generator are chosen with relevance to the literature (Tzamalis et al 2011). But, the electrolyzer price is chosen as per the cost of one ion exchange membrane electrolyzer unit in DCW which is about 1.18 M$ Biomass supply estimation The estimation of surplus biomass available in the chosen region is needed in pre-feasibility study for the implementation of biomass based projects. Prosopis juliflora, acacia, rice husk, cotton wastes are the main fuel types obtained in the nearby areas. Prosopis juliflora is to be collected within a 40 km radius of the plan in Thirunelveli area.it grows on wastelands and can be collected from the wastelands where 20,000 ha of the biomass grows in the 40 km vicinity of the plant. Rice husk: will be procured from the Rice Mill at Thirunelveli town and over 25 small rice mills in the 40 km radius of the plant. Cotton wastes are available periodically. The data regarding availability of biomass fuel types in the region around DCW was obtained from the nearby biomass power plant in Tuticorin and is shown in Table 4.3. Table 4.3 Biomass fuel availability near DCW No. Biomass Generation Consumption Surplus (tonnes/annum) (tonnes/annum) (tonnes/annum) 1 ProsopisJuliflora 240,120 63,200 1,76,920 2 Acacia 23,000 11,000 12,000 3 Field Level Residues 43,000 3,000 40,000 4 Plantation residues 15,000 8,000 7,000 5 Industrial level residues 77,000 67,000 10,000

10 98 Biomass available in the region is found to be 398,120 tons/year. The biomass consumption in the region is 152,200 tons/year. The surplus biomass is estimated by subtracting the total consumption from the biomass available and is found to be 245,920 tons/year. Other biomass residues available in the region are sunflower waste, silk cotton waste, Sesamum/Palmarosa waste, Chilly waste, Tamarind Shell, De-oiled cashew nut shell, Corriander waste, Avuri waste etc. These would be used in case of shortfall in the biomass residues listed in the table. Hence continuous supply of biomass fuel for the off-grid power generation is ensured. Biomass resource input is assumed to be 325 tons of plant wastes per day, based on its availability in 20 MW Biomass power plant commissioned by Ind-Barath Energies Thoothukkudi Limited (IBETL) in Tuticorin district. The biomass generator efficiency is assumed to be 40% in the HOMER model Techno-economic Analysis using HOMER HOMER tool requires technical inputs like Solar, Wind, Biomass resource, component sizes and economic inputs for RES components. AkzoNobel, Amsterdam, has successfully coupled 1-MW PEM power plant to its chlor-alkali process in 2011 at Solvay's chlorine plant at Lillo, near Antwerp, Belgium (Chlorine plant benefits 2012). Hence, size of the fuel cell is chosen to be fixed at 1MW and the other resources of varying sizes have been considered in the models. Four scenarios are simulated and analyzed using HOMER software based on the consideration of all possible generation source options. The schematic diagram of the scenarios enumerating the system components for each of the three cases is shown in Figure 4.4. The first case deals with Diesel only power system, the second integrates PV/Wind/Diesel and hydrogen technologies with battery backup and the third one without battery. The fourth case analyzes biomass based hybrid RE system without any conventional source of generation.

11 99 Case 1 Case 2 Figure 4.4 Case 3 Case 4 Comparison of various optimal renewable energy system configurations HOMER displays a list of configurations based on NPC. The optimized configuration is selected as a compromise between the cost and emissions in this chapter. The results of the techno-economic analysis are analysed in the subsequent section.

12 RESULTS AND DISCUSSION With the depleting reserves of fossil fuels and increasing interests for environmental sustainability concerns, the following case study focuses on renewable energy penetration in DCW. In the caustic soda plant chosen for the study, six electrolyzer units with 283 metric tons of NaOH output is considered. So, 283 x 2500 KWh/day equal to 648 MWh/day demand and 29 MW peak value is chosen as load inputs. Four different RE systems are simulated and analyzed using HOMER software. The results of the simulation in terms of the economic parameters and emissions for each of the case studies are depicted in Table 4.4. Table 4.4 Results of the simulation Economic parameters and CO 2 Emissions Parameters PV/Wind/Fuel PV/Wind/Fuel PV/Wind/ Diesel only cell/diesel with cell/diesel with Fuel cell/ battery out battery Biomass NPC($)* 5,258,145,792 1,184,524,288 1,225,249, ,477,248 LCOE ($/KW-hr)** Operating Cost($/yr) 408,433, ,820 60,314,444 23,604,618 Pollutants (Kg/yr) 1,169,426, ,143, ,958,912 75,232 *NPC-Net Present Cost; **LCOE- Levelized Cost of Energy In the first case, the diesel generator system has higher initial capital cost, higher operating cost and higher total net present cost for the whole project because no other sources are considered for back up power generation. Moreover, the ac power output has been rectified using the high cost power electronic converters to feed the DC load demand of the caustic soda plant. The system produces 275,930 MWh/yr of the total electricity with a net present cost of 5,258 M$. Also, pollutants are the highest for Diesel only system as seen in Table 5.4. The aim of the second scenario is to meet the utilization of electricity of the industrial process plant by adding 30 to 50% renewable fraction to the

13 101 existing Diesel Generator sets. 60 MW solar panels, 90 MW Wind turbine, 1 MW Fuel cell and 29 MW Diesel generator are used to produce a total of 344,854 MWhr/year to meet the consumption of DC primary load and Electrolyzer load. The renewable energy fraction accounts for 36% with the NPC being 1,184 M$. The Diesel-Renewable Mixed hydrogen System (PV, Wind, FC, EZ) without battery is analyzed in case-3. This system produces a total of 345,586 MWhr/year to meet the load requirements. The renewable energy fraction is 37.5% with the NPC being 1,225 M$ slightly more than in Case-2 because of more renewable penetration compared to the above system with battery as seen in Table 4.4, which portrays the comparative economic and emission analysis for all the four cases. The main focus of this section is the switch over of power generation options from conventional captive power plants to renewable energy based power system. As per the resource assessment, PV/ Wind/ FC energy sources with Diesel back-up form the sustainable energy options for DCW unit in Tuticorin district. In order to obtain 100% renewable fraction, the Diesel Generator set of the Case-3 hybrid system can be replaced by the Biomass system as in case-4. Biomass energy resources are essentially carbon neutral and has reduced pollutants. The total electrical production, cost, and CO 2 emissions for different combinations of PV/Wind/Fuel cell with Biomass is presented in Table 4.5. The table illustrates that the combination of 60 MW Wind, 1 MW FC and 30 MW Biomass accounts for lower NPC and LCOE but higher emissions compared to the 60 MW PV, 1 MW FC and 30 MW Biomass combination.

14 102 Table 4.5 Optimal combination of renewable energy sources Total Optimal Electrical Pollutants Combination NPC($) LCOE($/KWh) production (Kg/yr) of RES (KWh/year) Wind-FC- Biomass 285,518, ,477, PV-FC- Biomass 297,749, ,805, Biomass 262,088, ,501, The simulation results show that the system with maximum Biomass resource penetration has the least cost of 275 M$ (NPC) and lower pollutants of 75,646 Kg/year Control Strategy HOMER has been used to consider two types of control strategies. Under the load-following strategy, the generator provides only the power necessary to meet the load at that time. Lower priority tasks like charging the battery bank are left to the renewable energy sources. With the cycle-charging strategy, once the generator is operating, it uses as much power as possible to charge the batteries in addition to meeting the load. A load following control strategy has been followed in the all the four cases, as the strategy tends to be optimal in systems with a surplus of RE compared to the load.the two control techniques are simulated for Case-2 and the results are shown in Figure 4.5(a) & 4.5(b), which points that the total NPC is slightly higher for cyclic charging. Hence, load following stategy is applied for all the cases.

15 103 Figure 4.5 (a) Simulation results of the RES in Case-2 with cycle charging Figure 4.5 (b) Simulation results of the RES in Case-2 with load following Sensitivity Analysis The results of sensitivity analysis on diesel fuel price and wind speed for the Case-3 hybrid system are presented in Figure 4.6. Wind speed variations given as input in the software range from 3 to 5 m/s and diesel price from 0.9 to 1.1 $ per litre. When wind speed drops to 3 or 4 m/s, no feasible solutions are obtained as pointed by black box in Figure 4.6. This indicates that, only when the speed is above 5m/s, Wind turbines can produce reliable power to meet the load. From the base price of 0.9 $/L, when the diesel price is increased, NPC also increases linearly with fuel cost as given by the results of the sensitivity analysis.

16 104 In the sensitivity analysis, the distance of the proposed system is taken into consideration and the optimal design configuration of case-3 is assumed that it can draw power from the external grid. In Figure 4.7, it can be observed that NPC of the system is lower than the one without external grid until the break even point. As the distance of the proposed system from grid increases beyond the break even point of 20,606 kms, the NPC increases and grid connection is no longer economical as in Figure 4.7. Figure 4.6 Results of sensitivity analysis for Case-3 system Figure 4.7 Variation of NPC with distance from grid

17 POST-HOMER ANALYSIS Transition from conventional energy system to renewable energy based system will have certain economic impacts. Though a high share of renewable energy will be more cost effective than fossil fuels over the entire lifecycle of new power installations, the relatively high investment costs for renewables remains an important challenge. HOMER suggests the technical feasibility of PV-Wind-FC hybrid system and finds biomass based RE system as the best option in the near term. But, the business dimensions are not covered. Post HOMER analysis and improvements in the financial sector will thus be necessary to make use of full renewable energy potential of the region. These include optimal exploitation of the locally available resources and financial guarantees to improve investment security in the sustainable energy market. The following section analyzes the economics of transition from conventional to renewable energy based system Economics of change over to Biomass Power Generation Around the globe, there is a significant interest in using biomass for power generation, as power generation from coal continues to raise environmental concerns. Various cases have been analyzed to determine the rate at which RES replace the conventional energy sources. With enormous biomass resource availability in India, viable technologies for commercialization of electricity production from biomass are being analyzed in the present scenario (Singh & Setiawan 2013). The conversion of coal-fired power stations to biomass plants can be a cost-effective option for sustainable power generation, as the existing assets can be used for their remaining life-time. An economic analysis for converting coal based captive power plant in DCW plant to Biomass plant is described in this section. DCW has TPH boilers fired with coal and oil for steam generation. For example, the savings in cost for converting a 10 TPH oil-fired boiler to a rice husk-fired boiler is analyzed (Arvind Kumar 2012).

18 Boiler operating parameters Steam generation capacity = 10 TPH Enthalpy of steam = 660 K.cal/kg Inlet temperature of Feed water = 60ºC No. of operating days/year = 300 Efficiency of oil fired boiler = 80% Efficiency of rice husk fired boiler = 70% Assuming the cost of fuel oil as 500 $ /ton and rice husk to be 30 $ /ton, Cost of 10 T/hr rice husk boiler = $ 2,50,000/ Consumption of Fuel Oil Gross Calorific Value (G.C.V) of fuel oil = 10,000 K.cal/kg Boiler operating time per year = 24 x 300 = 7200 hrs Annual steam production = 10 Tons / hour x 7200 = 72,000 Tons / year Heat required for producing 72,000 tons of steam = 72,000x10 3 x (660-60) Heat Input = Heat Output/Boiler Efficiency = 4320 x 10 7 / 0.80 = 5400 x 10 7 K.cal. Oil consumption (G.C.V. of oil=10000 K.cal/kg) = 5400 x 10 7 /10,000 = 5400 tons Cost of fuel oil = 5400 x 500 dollars = $ 27,00,000/ Consumption of rice husk G.C.V. of rice husk = 3,200 K.cal/kg Heat requirement for producing 72,000 tons of steam = 4320 x 10 7 K.cal Heat Input = 4320 x 10 7 / 0.70 = 6171 x 10 7 K.cal. Rice Husk equivalent = 6171 x 10 7 / 3200 = tons (approximately)

19 107 Cost of rice husks ($ 30/ton) = x 30 = $ /- Fuel savings = 27,00,000 5,78,520 = $ 21,21,480/- Investment for purchase of new rice husk-fired boiler = $ 2,50,000/- Payback period = Investment / Fuel savings = 1.4 months The cost of purchasing Biomass feedstock can still be reduced, if juliflora, a biomass variety is planted in the waste land of DCW unit. Hence, the above analysis suggests that replacement of certain percentage of coal with biomass can definitely enhance India s energy security and thereby reducing the imports. With the capital subsidy from Ministry of New and Renewable Energy (MNRE), Biomass power generation will be a future solution in the aspect of cost and carbon credit. This biomass based hybrid system proves to be a possible path for agriculturally rich areas in terms of socio-economic and environmental sustainability. But this can be achieved depending on the investment capacity of the industry, incentives and information policies of the Government. 4.6 CONCLUSION In this chapter, the potential availability and feasibility of PV/Wind/ FC hybrid system for caustic soda manufacturers is analyzed. The economic viability of the proposed hybrid system in comparison with the conventional Diesel-Generator system is studied using HOMER to determine the sustainable energy options for the industries of Tuticorin district. Also, the economics of change over to Biomass Power Generation is presented with an example calculation.the following conclusions are drawn based on the simulation results: Fuel Cells are the promising key of future for caustic soda industries, when economic FCs are manufactured in India. In the current to near term, PV-FC hybrid combination is the most suitable power generating option for the soda ash industries like

20 108 DCW producing hydrogen as output in large proportion and requiring DC power for the caustic soda process. The CO 2 emissions are the least with biomass based RE system compared to that of Diesel power plants. In the economic point of view, biomass based electricity generation is cheaper compared to other generation options in the aspect of converting existing coal fired cogeneration plants into Biomass plants and high capital investment required for new solar and wind installations. The cost analysis show that the Biomass power system costs less than the cost of Fuel cell hybrid system in the present scenario. With successful technological up gradation in the future, the price of PV-FC hybrid system will drop and it will be the best option for the chlorine industries with hydrogen as the major plant output. With increasing diesel prices and specific power requirement year over year and uncertain Grid power availability, the proposed Biomass based hybrid power system is the long term solution for corporate industry and policy makers.