Sustainable Energy Technologies and Assessments 7 (2014) 6–16

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Sustainable Energy Technologies and Assessments

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Original Research Article

Techno-economic performance evaluation of grid integrated PV-biomasshybrid power generation for rice mill

http://dx.doi.org/10.1016/j.seta.2014.02.0052213-1388/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +91 9436582874; fax: +91 381 2346360.E-mail address: subhadeep_bhattacharjee@yahoo.co.in (S. Bhattacharjee).

Subhadeep Bhattacharjee ⇑, Anindita DeyDepartment of Electrical Engineering, National Institute of Technology (NIT), Agartala 799046, Tripura, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 July 2013Revised 6 February 2014Accepted 21 February 2014

Keywords:Biomass and solar energyHybrid power plantSensitivity analysisCost of energyGrid electricity conservation

The present study examines the feasibility of harnessing rice husk potential for power generation in therice mills of an Indian state Tripura in combination with the solar photovoltaic (PV) energy throughhybrid technology. The study indicates that grid-connected hybrid system including grid, PV and biomasssystem is the most feasible solution in view of the monthly average solar radiation intensity, biomassresource availability of the rice mills and the present equipment costs. The cost of generating energy fromthe hybrid configuration has been found to be 0.143 $/kWh, while the renewable fraction is found to be0.91. Sensitivity analysis is carried out to evaluate the effect of changing solar radiation level, load, elec-tricity rate and maximum annual capacity shortage. It has emerged from the study that this grid-con-nected PV-biomass hybrid power model may conserve over 90% of grid electricity utilized in thetypical rice mills of the state where supply of grid electricity has been a great concern.

� 2014 Elsevier Ltd. All rights reserved.

Introduction

There are many locations, especially in remote areas of develop-ing countries, which have no access to a reliable power supply. Pro-viding energy for a community, in a sustainable manner, nowadayshas become a more important issue as we face global warming andclimate change realities due to burning fossil fuel with depletingfossil fuel reserve, fuel cost escalation associated with conventionalenergy generation, population growth, shortage of essentialcommodities, insufficient and inefficient waste disposal facilities[1–4]. Harnessing renewable energy sources such as solar, wind,biomass, hydro which is abundantly available in nature providesan opportunity to produce energy in an environment friendlyway. These resources have enough potential to become importantsource for power generation in future because of their environ-mental, social and economic benefits in addition to public supportand government incentives [5]. The application of these sources in-clude the very small to large isolated, grid-connected and hybridpower systems. However, the foremost concern for implementa-tion of any renewable energy technology is its economic viability.The hybrid power systems exhibit higher reliability and lower costof generation than those that use only one source of energy [6].Instead of the photovoltaic (PV)-only system, the PV-hybridsystem – consisting of a photovoltaic system backed-up by an

engine-generator set – has greater reliability for electricity produc-tion. The engine generator set reduces the PV component size,while the PV system decreases the operating time of the generator,reducing its fuel consumption, operation and maintenance (O&M),and replacement costs [7]. Bagen and Billinton [8] have also shownthat the integration of wind and/or solar energy into existing dieselplants can improve the overall system reliability and significantlyreduce the system operating costs.

The analysis of stand-alone or grid-connected hybrid powersystem with different combinations using PV, wind, battery, diesel,fuel cell are found in many investigations. Besides, exploration ofhybrid power system with PV/biomass arrangement is anotherauspicious alternative for power generation. In recent years, somestudies have been conducted to explore the feasibility of PV-bio-mass hybrid power concept. Janardhan et al. [9] examine the pos-sibility of PV-biomass hybrid system for rural electrification atGanj Village in Uttar Pradesh, India. In another study, the economicconsideration of off-grid PV-biomass hybrid system for electricityproduction in remote areas has been investigated by Pradhanet al. [10]. A model for integrated biomass and solar systemequipped with load shifting and energy storage has been devel-oped by Hashim et al. [11] to satisfy the demand at the most com-petitive way. A feasibility study of PV-biomass hybrid powersystem to supply electricity to Auckland city, New Zealand hasbeen carried out by Harish Kumar [12].The study indicates thatthe proposed hybrid system for Auckland can be implemented ina cost effective and environmental friendly manner. In a different

Nomenclature

d dayCann,cap total annualized capital costCann,tot total annualized costCoperating operating costCrep replacement cost of the componentEdef total amount of deferrable load that the system serves

per yearEgrid,sales amount of energy sold to the grid per yearEprim total amount of primary load that the system serves per

yearh houri annual real interest rateN number of yearsRcomp component lifetimeRproj project lifetimeRrem remaining life of the componentRs. Rupees (Indian currency)S salvage valuet tony year$ US dollarCOE cost of energy

CRF capital recovery factorHOMER hybrid optimization model for electric renewableHP horse powerkW kilowattkWh kilowatt-hourLCC life cycle costNASA national aeronautics and space administration, USAN–E north–eastNPC net present costNREL national renewable energy laboratoryNTPC national thermal power corporationO&M operation and maintenancePV photovoltaicRen.Frac. renewable fractionRMB renminbi, the official currency of the People’s Republic

of ChinaSERC state electricity regulatory commissionTSECL Tripura state electricity corporation limitedVVN vidyutvyaparnigamYuan currency of China and the basic unit of the RMB, espe-

cially in international context

S. Bhattacharjee, A. Dey / Sustainable Energy Technologies and Assessments 7 (2014) 6–16 7

study, a PV-biomass grid connected distributed hybrid power sys-tem has been proposed by Harish Kumar [13] for South Australia.The performed feasibility study has revealed that for annuallyvarying electricity load, a grid-connected hybrid power systemcould be economical and environmental friendly viable solution.Afzal [14] shows that hybrid system containing biomass energyshould be given preference over other hybrid renewable energysystems devoid of biomass energy. Integration of biomass energysystem with wind energy or PV system is suggested to use in anypart of the world wherever biomass is available. Nixon et al. [15]assess the feasibility of hybrid solar-biomass power plants foruse in India in various applications including tri-generation, elec-tricity generation and process heat. Bhattacharjee and Dey [16] ex-plore the possibility of stand-alone biomass-PV-battery-dieselautonomous power system for supplying continuous power in ricemill. The renewable fraction and the cost of energy are found to be0.62 and 0.803 $/kWh for the proposed hybrid system of 15 kW PV,7 kW biomass gasifier and 8 kW diesel system.

The present work intends to develop a rice husk based biomass-PV grid-integrated hybrid system without battery and diesel backup for the rice mills of an Indian state Tripura, located in thenorth-eastern region of the country. A techno-economic analysishas been carried out for implementing PV/Biomass grid inte-grated-hybrid system in rice mills with a view to improving thesystem renewable fraction and declining the cost of energy furtherfor such type of systems.

Scope of the study

Every year approximately 120 Mt of paddy is produced in India.This gives around 24 Mt of rice husk [17]. The north–east (N–E)region of India is considered to be one of the hot pockets of ricegenetic resources in the world and a potential rice growing regionas compared to other parts of the country [18].

Tripura is one of the smaller states of N–E India. The economy ofTripura is predominantly agrarian. Food grains contribute 91.7% ofthe total cropped area of the state in 2000–01. Among the foodgrains, rice is the main cereal produced in the state in terms of pro-duction and cropped area [19]. There are many scattered rice mills

in the state which are operated with grid electricity. Rice husk isabundant agricultural residue in the state which has no commer-cial utilization till now. But this is a captive source of biomass forthe rice mills and may be utilized successfully in rice husk gasifier.The N–E region of India ranks lowest in terms of per capita energyconsumption in India [20]. The per capita energy consumption ofthe state Tripura is merely 163 kWh/head/y [21,22]. By virtue ofits physical location, provision of grid electricity is neither techni-cally feasible nor commercially viable in many parts of the N–Eregion as well as for the state [23]. As grid electricity is a very pre-cious commodity for the state, rice husk produced in the rice millscould play a pivotal role for power generation in the rice mills witha view to conserving a bulk amount of grid electricity. As the stateis also rich in sun shine [24], this biomass potential could be har-nessed for power generation along with the mix of solar energythrough hybrid technology to realize better power supply reliabil-ity for the rice mills. Fig. 1 reveals a glimpse of typical rice millfunctioning in the state. It shows the amount of unutilized ricehusk stack and sacs. The figure also depicts the grid electricity con-nection in the mill and usual size of the motor employed in themills.

Brief assessment of rice husk based electricity generation

Rice husk based gasifier dominates the present re-introductionof small gasifiers for engine operation coupled with generator indeveloping countries. Much of the indigenous research and devel-opment now carried out in developing countries is also concen-trated on rice husk based gasifier in view of their good prospectsfor commercialization. It is reported that rice husk gasifiers are stillin successful operating conditions at many rice-producing coun-tries like Indonesia, China and Mali [25]. As an important decen-tralized power technology, biomass gasification and powergeneration has a potential market in making use of biomass wastesin China. For a 1 MW rice hull gasification and generation system,electricity cost is about 0.27 Yuan RMB/kWh (note: 1 YuanRMB = 0.12 US dollar) in China, which is about the same as the costof a coal-fired power station, and much lower than that of dieselgeneration [26]. Sookkumnerd et al. [27] presents an economic

Stack of rice husk Motor is in operation in a rice mill

Rice mill runs with grid electricity Sac of rice husk

Fig. 1. Typical rice mill of Tripura (pictures are taken from rice mills at Ranirbajar, at the outskirt of the state capital Agartala).

8 S. Bhattacharjee, A. Dey / Sustainable Energy Technologies and Assessments 7 (2014) 6–16

model to find out the internal rate of return, on the investment insteam engines as an energy-saving technology in Thai rice mills. Asystem combining a biomass gasification power plant, a gas stor-age system and stand-by generators to stabilize a generic 40 MWwind park is proposed and evaluated with real data by Pérez-Nav-arro et al. [28]. The wind park power production model is based onreal data about power production of a Spanish wind park and aprobabilistic approach to quantify fluctuations and so, power com-pensation needs. This hybrid system can mitigate wind predictionerrors and so provide a predictable source of electricity. A simpleframework for estimation of unit cost of electricity for biomass-gasifier-based power generation systems in India has beenpresented by Tripathi et al. [29]. The results of typical numericalcalculations indicate that medium and large capacity systemscould be financially attractive for higher values of the capacity uti-lization factor. Buragohain et al. [30] attempts to highlight thetechnical and economical issues related to decentralized powergeneration in India using biomass gasification. The paper indicatesthat in the remote areas and hilly terrains of India, biomass gasifi-cation-based power generation offers a highly viable solution formeeting energy demands of small villages and hamlets, whichwould not only make them independent but will also reduce bur-den on state electricity boards. Shafie et al. [31] evaluated the lifecycle analysis of electricity derived from rice husk combustion inthe Malaysia rice mills. The characterized data from rice huskderived electricity is compared with coal and natural gas derivedelectricity. The results indicate the performance of rice huskderived-electricity is better in the aspect of environment impactparameters. Reliable and affordable electricity is one of the biggestproblems in Cambodia. Only 24% of the population has an access toelectricity while the others suffer from insufficient electricitysupply. Akgün and Luukkanenin [32] show the current situationin Cambodian electricity facilities and offer rice husk gasification

systems as an option for electricity generation. If all rice husks thatare produced annually could be used for electricity generation itcould cover the yearly electricity consumption of the country.Moreover, elimination of rice husk as a waste would be a solutionfor waste disposal and pulmonary diseases.

Methodology used

Hybrid optimization model for electric renewables (HOMER)software [33,34], the renewable energy based system optimizationtool developed by the USA national renewable energy laboratory(NREL) is used in this work for modeling, optimization and sensi-tivity analysis purpose. It is a flexible tool that models a mix of con-ventional fuels and renewable energy to determine the most costeffective configuration for each system. Input information to beprovided in HOMER includes: electrical load (primary energydemand), renewable resources (solar radiation, biomass input),component technical details/costs, constraints, controls, type ofdispatch strategy, etc.

HOMER performs hundreds or thousands of hourly simulationsto ensure the best possible matching between supply and demandin order to design the optimum system. It performs sensitivityanalysis where the value of certain parameters (sensitivity vari-able) like solar radiation, primary load, power price, capacity short-age etc. can be varied to determine their impact on the cost ofenergy for the particular system. For each of these values, 8760 val-ues are formed in the software. HOMER cannot model transientchanges which are smaller than 1 h [35]. However, hourly valueis sufficient in order to investigate the systems like the proposedone presented here.

Before installing such power generation system, economic anal-ysis is essential. Methodology for economic analysis is described inthe following segments [36–38].

S. Bhattacharjee, A. Dey / Sustainable Energy Technologies and Assessments 7 (2014) 6–16 9

Initial capital cost

It includes the cost of purchasing the equipment and installa-tion cost.

Replacement cost

The cost of replacing a component at the end of its usefullifetime. The replacement cost may differ from the initial capitalcost as only a part of the component may need replacement.

Operation and maintenance (O&M) cost

This is the sum of all the programmed expenses for lifetimeoperation and maintenance of the plant.

Salvage value

It is the value remaining in a component of the power system atthe end of the project lifetime. To calculate the salvage value ofeach component at the end of the project lifetime, HOMER usesthe following equation:

S ¼ CrepRrem=Rcomp ð1Þ

where S is the salvage value, Crep is the replacement cost of the com-ponent, Rrem represents the remaining life of the component, andRcomp is the lifetime of the component.

Life cycle cost (LCC)

The total cost of installing and operating a component or systemover a specified time span, typically many years. For financialevaluation of the project, life cycle analysis of hybrid system iscomputed as:

LCC ¼ initial capital costþ O&M costþ fuel cost

þ replacement cost� salvage value ð2Þ

Annualized cost

HOMER combines the capital, replacement, maintenance, andfuel costs, along with the salvage and any other costs or revenuesfor each component to find the component’s annualized cost. Thisis the hypothetical annual cost that if it occurred each year of theproject lifetime would yield a net present cost equivalent to thatof all the individual costs and revenues associated with that com-ponent over the project lifetime. HOMER sums the annualizedcosts of each component, along with any miscellaneous costs tofind the total annualized cost of the system. The value is an impor-tant one because HOMER uses it to calculate two principaleconomic figures of merit for the system: the total net present costand the levelized cost of energy.

Operating cost

The operating cost is the annualized value of all costs (replace-ment, fuel, operation and maintenance) and revenues other thaninitial capital cost. The following equation is used to calculatethe operating cost.

Coperating ¼ Cann;tot � Cann;cap ð3Þ

where Coperating is the operating cost, Cann,tot is the total annualizedcost and Cann,cap is the total annualized capital cost.

Net present cost (NPC)

HOMER makes economic analysis and ranks the system accord-ing to NPC. It comprises all costs and revenues that occur withinthe project lifespan which includes the initial capital cost of thesystem components, cost of any component replacements thatoccur within the project lifetime, cost of maintenance, fuel cost,and cost of purchasing power from the grid. Economic analysis isperformed by ranking the systems according to their NPC to makea reasonable comparison. HOMER assumes that all prices will riseat the same rate over the project lifetime. With that supposition,inflation can be factored out of the analysis simply by using thereal (inflation-adjusted) interest rate rather than the nominalinterest rate when discounting future cash flows to the present.The NPC is given by the following equation:

NPC ¼ Cann;tot=CRFði;RprojÞ ð4Þ

where Cann,tot is the total annualized cost, i is the annual real inter-est rate (discount rate), Rproj represents the project lifetime, and CRFis the capital recovery factor, given by the equation

CRFði;NÞ ¼ ið1þ iÞN=½ið1þ iÞ � 1� ð5Þ

where i is the annual real interest rate and N is the number of years.

Levelized cost of energy

Another important economic parameter for the system is cost ofenergy (COE) which is the average cost per kWh of useful electricalenergy produced by the system. To compute the COE, HOMER di-vides the annualized cost of producing electricity by the total use-ful electric energy production as given in the following equation.

COE ¼ Cann;tot=ðEprim þ Edef þ Egrid;salesÞ ð6Þ

where Cann,tot is the total annualized cost, Eprim and Edef are the totalamounts of primary and deferrable load respectively that the sys-tem serves per year, and Egrid,sales represents the amount of energysold to the grid per year. The denominator in Eq. (6) is an expressionof the total amount of useful energy that the system producesannually.

Renewable energy resources

Solar

No ground measurement data of solar radiation exist for thestate Tripura (22�56

0and 24�32

0north latitude and 91�09

0and

92�200

east longitude). Total area of the state is 10,491.69 sq.km.Solar radiation pattern of the state is almost same in its every part.Therefore, solar resource of capital Agartala (23�52

0north latitude

and 92�500

east longitude) is taken into account for this study.HOMER introduces the clearness index and average radiation fromthe latitude information of the site via internet from the NASA’s(national aeronautics and space administration, USA) surface solarenergy data set. Average daily solar radiation in a year is shown inFig. 2. Solar resource of Agartala is summarized in Table 1. Averagesolar radiation of the state is found to be 4–6 kWh/m2/d.

Biomass

In a typical rice mill of Tripura, available paddy in a day is440 kg. It is assumed in the study that rice husk production is22% of the paddy and immature paddy production is 3% of thepaddy [39]. Therefore, total biomass (considering rice husk andimmature paddy) available in typical rice mills of Tripurais = 440(0.22 + 0.03) = 110 kg/d or 0.11 t/d. It is also learnt during

Fig. 2. Solar resource of the state Tripura.

Table 1Monthly solar resource of the site.

Month Clearness index Average radiation (kWh/m2/d)

January 0.661 4.558February 0.635 5.075March 0.626 5.844April 0.576 6.016May 0.529 5.833June 0.393 4.395July 0.386 4.271August 0.418 4.433September 0.414 4.010October 0.581 4.857November 0.620 4.419December 0.659 4.306Average 0.526 4.834

0

4

8

12

16

00:00 - 01:00 05:00 - 06:00 10:00 - 11:00 15:00 - 16:00 20:00 - 21:00

Loa

d (

kW)

Hour

Fig. 3. Load profile.

Fig. 4. Proposed grid-coneected PV-biomass hybrid model used in the study.

10 S. Bhattacharjee, A. Dey / Sustainable Energy Technologies and Assessments 7 (2014) 6–16

visit to some of the rice mills of the state that 4–10 sac of 30 kg/sacof rice husk is produced in every 3–4 ds in the rice mills.

Load profile

HOMER uses scaled data for calculations. To create scaled data,it multiplies each of the 8760 baseline values by a common factorthat results in an annual average values. To determine the value ofthis factor, it divides the scaled annual average by the baseline an-nual average. If hourly load demand for a month is given in thesoftware, it calculates the average 24 h load profile for the wholeyear. A survey is conducted to identify energy consumption in atypical rice mill of Tripura. Generally the rice mills of Tripura areoperated with the help of 10–15 HP motor during 7 am–3 pmevery day (including Sunday also). There are also some other elec-tric appliances and the load is considered as ac in the study. A typ-ical rice mill of Tripura consumes around 92 kWh/d with a peakdemand of nearly 25 kW over a year. The usual daily load profileof the rice mill is shown in Fig. 3 which reveals that maximum loadof 14 kW occurs during the period of 1�2 pm. Load demand is low-er during night time, early morning and late afternoon.

System sizing and component modeling

Fig. 4 shows the proposed scheme as implemented in thesimulation tool. In this system, non-polluting renewable solarand biomass are chosen as the primary sources. No storage deviceis included in the system design as it is connected to grid. Workingof the system is such that it can transport power to the grid as wellas draw power from the grid whenever needed to obtain continu-ous power flow. Sizes and cost of the various components of thisproposed grid-connected PV/biomass hybrid power plant are

depicted in the following sections. Currency conversion is assumedas 1$ = Rs. 50 (i.e. 1 US dollar = 50 Indian rupees) [40] in the studywhile converting the cost of the components in Indian scenariointo US $.

Biomass generator

Cost of biomass gasifier is taken as Rs. 20,000/kW and that of anengine-generator set is Rs. 33,000/kW. So the total per kW capitalcost of gasifier-engine-generator unit comes Rs. 53,000 ($1060)[41]. Replacement cost of the whole unit is taken as $860, O&Mcost is 0.5 $/h. Unlike the lifetime inputs for most other compo-nents, the generator lifetime is specified not in years but in hoursof operation. This is because the lifetime of a generator dependsstrongly on the hours of operation, but not very strongly on itsage. The lifetime of the generator is considered here 15,000 h. Gen-erator minimum load ratio is taken 30% which is the minimumallowable load on the generator, as a percentage of its rated capac-ity. Different sizes of biomass generator considered are: 0, 5–7, 10,12, 14–16, 20, 25, 30, 35 and 40 kW. Fig. 5a shows the variation ofcost of the generator with respect to its size. The cost curve is lin-ear, i.e. when size increases cost also increases. Fig. 5b shows thevariation of efficiency of the generator with respect to its output.Depending on the value of intercept coefficient and slope of inputfuel, the efficiency curve is generated. It is clear from the curve thatwhen output increases efficiency also increases.

PV array

The per kW capital cost of the PV module is Rs. 60,000 ($1200)[42] and replacement cost is taken as $720. O&M cost for the PVarray is assumed to be zero. The lifetime of the PV arrays are takenas 20 y and no tracking system is included in the PV system. De-rating factor and ground reflectance are assumed to be 90% and

Fig. 5. Cost and efficiency curves of the hybrid components.

S. Bhattacharjee, A. Dey / Sustainable Energy Technologies and Assessments 7 (2014) 6–16 11

20%. Various sizes that are considered for optimization purposeare: 0, 2, 3 , 5, 7–10, 12,15–18, 20 and 25 kW. Fig. 5c shows thelinear variation of the cost of the PV module with respect to size.

Converter

For converter, per kW capital cost, replacement cost and O&Mcost are presumed as $720, $460 and 0$/y respectively [43]. Sizeof the converter is considered as 20 kW to satisfy the peak demand.Lifetime of the device is 15 ys and efficiency is 90%. Fig. 5d showsthe variation of cost of the converter with respect to size. Both cap-ital and replacement cost curve are found to be linear relative toconverter size.

Grid input

The grid rates are specified by choosing scheduled rates. It is as-sumed that the power price and sellback rate will stay constant.Under net metering, the sellback rate applies only to net excessgeneration. Different electricity tariff slabs are prevailing with Tri-pura state electricity corporation limited (TSECL) which is the onlyelectric utility in the state. Electricity purchase rate from grid ismoderately assumed as Rs. 4/kWh (0.08 $/kWh) [44] for this anal-ysis. Central and state electricity regulatory commissions (SERC) inIndia have announced preferential tariff for purchase of renewablepower from the project developers. During 2010–11, the projectdevelopers signed power purchase agreements with respectivestate utility/distribution companies or national thermal power cor-poration (NTPC) vidyut vyapar nigam (VVN) at a rate ranging fromRs. 10.95/kWh to Rs. 18.52/kWh, depending on the capacity of theplant [45]. Thus, sellback rate is judiciously chosen as Rs. 15/kWh(0.3 $/kWh) for this project. In the advance window of the

software, both the sell capacity and purchase capacity of the gridis specified as 5 kW.

Economics and constraints inputs

It is assumed for the hybrid model that economic parameterslike annual real interest rate is 6%, project lifetime is 25 ys andthat of constraints factor of maximum annual capacity shortageas 4%.

Results and discussion

Cost analysis

Fig. 6 shows the overall optimization results of the gird con-nected hybrid system under this study. The tabular optimizationresults in Fig. 6 have been generated by HOMER software. Eachrow in the table of Fig. 6 represents a feasible system configuration.The first four columns contain icons indicating the presence of thedifferent components, the next four columns indicate the numberof size of each component, and the next eight columns contain afew of the key simulation results: namely, the total capital costof the system, the operating cost, the total net present cost, the lev-elized cost of energy, renewable fraction, capacity shortage, annualbiomass fuel consumption, and the number of hours the generatoroperates per year.

The first row in Fig. 6 is the optimal system configuration,meaning the one with the lowest NPC. From the optimization re-sults, it is observed that the most optimum system comprises of25 kW PV, 6 kW biomass-generator unit, 20 kW converter and5 kW grid support. COE and renewable fraction are found to be0.143 $/kWh and 0.91 for this configuration. Biomass usage forthe system is 11 ton and biomass-generator operating period is

Fig. 6. Overall optimization results of the proposed gird-connected hybrid system.

Fig. 7. Cash flow summary of the project.

Table 2NPC summary of the project over one year.

Component Capital ($) Replacement ($) O&M ($) Fuel ($) Salvage ($) Total ($)

PV 30,000 5612 0 0 �3145 32,467Generator 1 6360 2418 44,218 0 �94 52,901Grid 0 0 �23,913 0 0 �23,913Converter 14,400 3839 0 0 �715 17,524System 50,760 11,869 20,305 0 �3954 78,980

Fig. 8. Cash flow details of the project.

12 S. Bhattacharjee, A. Dey / Sustainable Energy Technologies and Assessments 7 (2014) 6–16

1153 h. Initial capital cost of this configuration is $50,760,operating cost is 2208 $/y, total NPC is $78,980. To calculate NPC,it performs a cash flow analysis. Fig. 7 shows the cash flow sum-mary based on the components selected on the system. PV module

and converter share the maximum portion of the capital andreplacement cost. Biomass generator has a high impact on theoperating cost. Table 2 depicts the NPC summary of the most opti-mized hybrid model.

Table 3Annualized costs.

Component Capital ($/y) Replacement ($/y) O&M ($/y) Fuel ($/y) Salvage ($/y) Total ($/y)

PV 2347 439 0 0 �246 2540Generator 1 498 189 3459 0 �7 4138Grid 0 0 �1871 0 0 �1871Converter 1126 300 0 0 �56 1371System 3971 928 1588 0 �309 6178

Fig. 9. Monthly average electrical production rates of the system.

Table 4Performance summary of different system components.

PV array Parameter Value Unit

Rated capacity 25.0 kWMean output 4.39 kWMean output 105 kWh/dCapacity factor 17.5 %Total production 38,421 kWh/yMinimum output 0.00 kWMaximum output 24.8 kWPV penetration 115 %Hours of operation 4380 h/yLevelized cost 0.0661 $/kWh

Biomass generator Hours of operation 1153 h/yNumber of starts 487 starts/yOperational life 13.0 yCapacity factor 13.0 %Fixed generation cost 3.34 $/hMarginal generation cost 0.00 $/kWh/yElectrical production 6830 kWh/yMean electrical output 5.92 kWMin. electrical output 3.85 kWMax. electrical output 6.00 kWBio feedstock consumption 10.8 t/ySpecific fuel consumption 1.105 kg/kWhFuel energy input 11,535 kWh/yMean electrical efficiency 59.2 %

Converter Capacity 20.0 kWMean output 3.7 kWMinimum output 0.0 kWMaximum output 20.0 kWCapacity factor 18.5 %Hours of operation 4380 h/yEnergy in 36,087 kWh/yEnergy out 32,478 kWh/yLosses 3609 kWh/y

S. Bhattacharjee, A. Dey / Sustainable Energy Technologies and Assessments 7 (2014) 6–16 13

The yearly cash flow throughout the system’s lifespan is shownin Fig. 8. Cash inflows (income) appear as positive numbers, andcash outflows (expenditures) appear as negative numbers. Possiblereplacements will likely to occur mainly in the 14th, 15th and 20thyear of the system.

The annualized cost of the system components are depicted inTable 3. HOMER calculates annualized cost by first calculatingthe NPC, then multiplying it by the CRF. The initial capital costfor the whole system is 3971 $/y, the total annualized cost forthe whole system is 6178 $/y as shown in Table 3. The capital costof the biomass-generator makes up only 12% of the system’s totalcapital cost, where as almost 59% of the initial investment goes tothe PV array.

Electricity production

In the optimized grid-connected PV/biomass hybrid power sys-tem, total annual electrical energy available to meet the demand is49,159 kWh. 78% of the electricity demand is met from solar panelswith 38,421 kWh/y, while 14% and 8% of the energy requirementare supplied from biomass-generator with 6830 kWh/y and fromgrid with 3908 kWh/y. Monthly average electricity production ofthe entire system is shown in Fig. 9.

Performance of hybrid components

Performance of different components of the plant such as PV ar-ray, converter and biomass generator are summarized in Table 4.

Grid output is presented in Table 5. Electrical meter runs back-wards when surplus power available in the rice mill is sold to thegrid. At the end of the billing period (either monthly or annually)rice mill is charged for the net amount purchased (purchasesminus sales). Here ‘net grid purchases’ value is negative, meaningthat selling power to the grid is more than the power purchasedfrom the grid by the rice mill over the billing period.

Impact of component cost to the optimum system

It is presumed in the study that the costs of the system compo-nents are likely to decrease in future. In order to simulate thedeclining cost for the long term analysis; a 50% decrease in compo-nent costs has been included in calculation. When only the PV costdecreases by 50%, the total NPC and COE also decreases by 25.34%and 25.44% in the system. The initial capital cost decreases by 42%

and the operating cost decreases by 3.6%. The renewable fraction,capacity shortage, biomass usage remains same as before whilebiomass-generator operating time increases by 14 h.

In case of 50% cost decrease for biomass-generator unit, thebiomass-generator capacity increases to 7 kW from 6 kW and theinitial capital cost decreases by 5.5%. The NPC and COE decreasesby 33.86% and 66.3%. Renewable fraction increases to 0.96 from0.91, capacity shortage decreases to 0.02 from 0.03. Biomass con-sumption increases by 21 t with increase in biomass-generatoroperating duration by 1985 h.

Table 5Grid output.

Month Energy purchased Energy sold Net purchases Peak demand Energy charge Demand charge(kWh) (kWh) (kWh) (kW) ($) ($)

January 413 720 �307 5 �92 0February 243 777 �534 5 �160 0March 283 925 �643 5 �193 0April 224 1038 �814 5 �244 0May 189 1180 �991 4 �297 0June 386 737 �352 5 �106 0July 266 837 �572 5 �172 0August 367 800 �433 5 �130 0September 357 846 �489 5 �147 0October 308 858 �550 5 �165 0November 406 742 �337 5 �101 0December 467 681 �214 5 �64 0Annual average 3908 10,143 �6235 5 �1871 0

Table 6Impact of sensitivity variables.

Effect of solar radiation

Solar radiation (kWh/m2/d) Excess energy (%) COE ($/kWh) Biomass (t) Ren. Frac. Running (h)

4.834 4.75 0.143 11 0.91 11535.025 6.59 0.119 8 0.91 10577 13.4 0.070 6 0.93 7376.2 13.1 0.081 6 0.92 792

Effect of load on the system

Load (kWh/d) Excess energy (%) COE ($/kWh) Biomass (t) Ren. Frac. Running (h)

91.7 4.75 0.143 11 0.91 115392 4.71 0.144 11 0.91 116289 5.15 0.130 9 0.90 110194 4.49 0.148 11 0.91 1210

Effect of maximum annual capacity shortage

Capacity shortage (%) Excess energy (%) COE ($/kWh) Biomass (t) Ren. Frac. Running (h)

4 4.75 0.143 11 0.91 11531.4 4.54 0.177 17 0.94 11512.7 4.69 0.151 12 0.92 11533.3 4.75 0.143 11 0.91 1153

Effect of electricity rate on the system

Electricity rate ($/kWh) Excess energy (%) COE ($/kWh) Biomass (t) Ren. Frac. Running (h)

0.08 4.75 0.143 11 0.91 11530.2 4.75 0.143 11 0.91 11530.4 4.75 0.143 11 0.91 11530.8 4.75 0.143 11 0.91 11561 4.74 0.143 11 0.91 1167

14 S. Bhattacharjee, A. Dey / Sustainable Energy Technologies and Assessments 7 (2014) 6–16

When cost of both of the biomass unit and PV decreases by 50%,the biomass-generator capacity increases to 7 kW whereas the PVarray capacity remains same as before, i.e., 25 kW. In this consider-ation, the initial capital cost decreases by 53.3%, NPC decreases by84.7%, and COE decreases by 56.64%. Renewable fraction, capacityshortage, increase in biomass consumption in gasifier and genera-tor operating hour are again found to be 0.96, 0.02, 21 ton and1985 h.

Sensitivity analysis

In the sensitivity analysis process, multiple optimizations areperformed, each using a different set of input assumptions. Oneof the primary uses of sensitivity analysis is in dealing with uncer-tainty [38]. Four sensitivity variables (solar radiation, primary load,power price, and capacity shortage) are considered in this analysis.The impact of these sensitivity variables on various concerns suchas excess energy, COE, biomass utilization, renewable fraction,

biomass-generator operating duration are explained hereafter withthe help of Table 6.

Three values of load have been taken close to the scaled annualaverage value which is 91.7 kWh/d. These are 92, 89 and 94 kWh/d.It appears from that if the load decreases to 89 kWh/d, COE also de-clines and the percentage of excess energy increases as comparedto the other loading conditions.

The contribution of solar energy in the hybrid system dependson the intensity and the duration of solar radiation. Three prospec-tive solar radiation values, higher than the annual average solarradiation (4.834 kWh/m2/d in Table 1) of the site have been takeninto account to examine the performance of the hybrid powerplant under better sunny condition. These are 5.025, 7 and6.2 kWh/m2/d. When solar radiation increases, the solar energycontribution also increases. Moreover, the percentage of excess en-ergy increases with the increase of solar radiation and the COE ofthe system decreases. With the increase of solar radiation renew-able fraction increases and the biomass usage decreases which inturn cuts out the biomass-generator operating hours.

Fig. 10. Effect of capacity shortage to the total electrical production and cost of energy.

Fig. 11. Effect of electricity rate to the grid purchase and biomass-generator production.

S. Bhattacharjee, A. Dey / Sustainable Energy Technologies and Assessments 7 (2014) 6–16 15

The capacity shortage has been considered here to varybetween 0% and 4% of the load which implies that up to 4% ofthe load demand is accepted to be unsupplied. Fig. 10 shows thevariation in electricity cost and total electricalproduction in thisrange. It is observed that with an increase in capacity shortage,the system cost and electrical production are decreased about23.6% and 4.5% correspondingly. The graph points up that allocat-ing a small amount of un-served load, could considerably reducethe cost of the system and makes it more profitable.

It is apparent from the information in the said table that withthe decrease in capacity shortage, the system cost increases, which

have also been corroborated in Fig. 10. On the other hand, therenewable fraction and biomass usage augment with the decreaseof capacity shortage.

In this study, it is anticipated that electricity rate will escalate infuture. It is found from the analysis that in the event of electricityrate rising to 1 $/kWh, the optimal system includes 25 kW PV,6 kW biomass-generator, 20 kW converter with grid connection.The NPC is increased to $79,245 from $78,980 whereas the COEremain same as 0.143 $/kWh. The renewable energy fraction alsoremains same. No change has been observed in biomass consump-tion. But the biomass-generator operating time is increased by

16 S. Bhattacharjee, A. Dey / Sustainable Energy Technologies and Assessments 7 (2014) 6–16

14 h. The decline of energy purchased from the grid and the in-crease of biomass-generator energy production with rising elec-tricity rate are shown in Fig. 11. Purchased energy from the gridis decreased about 0.972% whereas 1.012 times as much raise inthe biomass energy production is observed from the Fig. 11.

Conclusion

The feasibility of utilization of rice husk, a waste product in therice mills of the N–E Indian state Tripura, in rice husk gasifier-gen-erator unit for power generation purpose in hybrid mode has beentried to be explored in this paper. Keeping in view the exploitationof this unused potential, an optimized grid-connected solar PV/bio-mass combined energy system has been developed for typical ricemill of the state where supply of electricity is always a critical is-sue. It is perceived from the study that major energy production(78%) of the optimized hybrid plant comes from PV system. Never-theless, 14% of energy of the plant is produced from its rice huskbased biomass-generator unit. It is also evident from the study thatboth PV and biomass alone cannot realize the energy requirementof the rice mill of the state. But mixing of these two availablerenewable resources through hybrid technology along with littlegrid support (8%) may easily fulfill the typical rice mill’s demandof the state. Thus it is possible to conserve over 90% grid electricityconsumption in the rice mills which could undoubtedly be a greataccomplishment as the state has been starving of electricity in itsall parts. Further, grid-integrated PV-biomass hybrid power gener-ation shows better result in terms cost of energy and renewablefraction compared to such type of investigation [16] with stand-alone operation.

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