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Techno-economic-environmental study of hybrid power supply system: A case study in Iran
Sustainable Energy Technologies and Assessments 25 (2018) 1–10
Contents lists available at ScienceDirect
Sustainable Energy Technologies and Assessments journal homepage: www.elsevier.com/locate/seta
Original article
Techno-economic-environmental study of hybrid power supply system: A case study in Iran
MARK
⁎
Mehdi Mehrpooyaa,b, , Mohammad Mohammadia, Esmaeil Ahmadia a b
Renewable Energies and Environment Department, Faculty of New Sciences and Technologies, University of Tehran, Tehran, Iran Hydrogen and Fuel Cell Laboratory, Faculty of New Sciences and Technologies, University of Tehran, Tehran, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: Hybrid energy system Techno-economic-environmental analysis HOMER PV-hydrogen
This paper presents an optimal planning model of a hybrid renewable energy system to meet a real load with a combination of photovoltaic panels (PV), diesel generators and batteries. Also, replacing the conventional energy storage system with a fuel cell is investigated. Energy Modelling and Energy Resources Assessment Lab (EMERAL), at University of Tehran, in Tehran, Iran has selected as a case study. A technical, economic and environmental assessment was conducted for optimal planning of the hybrid energy system. Finally, the impact of diesel fuel price fluctuation on the economic competitiveness of proposed hybrid renewable energy systems in providing off-grid electrical load demand is investigated. In this study simulation, optimization and modeling procedures are done by HOMER software.
Introduction Obviously, world energy consumption has rapidly increased in recent years. Depletion of fossil fuel resources and low efficiency of current energy systems are caused many concerns about providing energy in the future [1]. Furthermore, conventional energy resources, which are mainly fossil fuels, have environmental issues and their carbon emission lead to concerns about global warming problem [2,3]. These issues have led to the use of alternative energy sources such as renewable energies that have little environmental impact and have the ability to produce clean and sustainable energy [4,5]. However, these resources have periodic nature and their production capacity is not precisely predictable and is always uncertain. In order to solve this problem, it is possible to use these resources as hybrid systems with other energy sources, along with the use of energy storage systems. The hybrid energy systems have become more common due to uncertainty and the high investment cost of renewable systems. Therefore, renewable energy sources can be combined with other traditional energy sources and that will lead to the creation of hybrid renewable energy systems (HRES) [6]. A comprehensive overview of planning models, as well as operational optimization and control of HRES in remote areas, can be found in [7]. The models presented for PV-base HRES in off-grid mode have been investigated in [8] from the planning, modeling, control, and optimization aspects. Also, related information about optimization technique for optimal sizing of PV/diesel hybrid power generation systems can be found in [9]. An overview of hybrid energy
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systems consisting of photovoltaics, wind turbines and batteries from optimal planning, suitable converter design, and operational optimization aspects can be found in [10]. Also, reviews of the models presented for HRES in the micro-network content, taking into account the role of energy storage systems, can be found in [11,12]. Examples of modeling, optimization, and application of HRES can be found in [13]. In this regards, other work has been done which includes integration of the tidal power into a hybrid PV/wind/battery renewable energy system [14]. Due to the existence of various energy technologies and systems in HRES, the problem of planning and optimizing these systems is very important. Therefore, considering the problem conditions and the application of the power supply system, the choice of appropriate method and tool for optimizing the HRES should be carefully performed. An overview of existing simulation tools for evaluating, optimizing and planning the HRES has been done by Sinha and Chandel [6]. The results of this study showed that among all the available software, HOMER has been most used and applied for planning and optimization of hybrid energy systems. This has been due to the prominent features of this software. HOMER has the capability to assess the technical, economic, and environmental aspects of the energy systems, and reliability indicators can also be used for optimal scheduling of the system. HOMER also has the ability to handle a large number of system components and related variables in a reasonable time and also evaluate the system in different scenarios. On the other hand, sensitivity analysis on input data is another feature of this software [6]. In this study HOMER software
Corresponding author at: Renewable Energies and Environment Department, Faculty of New Sciences and Technologies, University of Tehran, Tehran, Iran. E-mail address: [email protected] (M. Mehrpooya).
http://dx.doi.org/10.1016/j.seta.2017.10.007 Received 8 September 2016; Received in revised form 13 October 2017; Accepted 31 October 2017 2213-1388/ © 2017 Elsevier Ltd. All rights reserved.
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shows daily load profile during a year. As can be seen, the main part of energy consumption occurs between 8 am and 8 pm. Also, the hourly AC primary load is shown in Fig. 2.
has been selected to design and evaluate hybrid energy systems. Solar energy is a clean energy [15] and due to its various advantages, such as no greenhouse gas (GHG) emission and low maintenance costs is applied in many studies [16–19]. In Off-grid applications, battery units can be used as a storage system with PV panels to reduce the uncertainty related to renewable energy resources production and increasing the system reliability. However, due to environmental concerns of the battery units, finding an alternative option for these storage systems is necessary. Energy systems that convert electricity to hydrogen can be a practical solution to this issue [20–23]. Other studies on solar and hydrogen hybrid energy systems for use in remote areas and off-grid mode can be found in [24]. In this study, four different hybrid energy systems including different combination of diesel generators, PV, battery, and hydrogen storage system for providing the electrical load of EMERAL at the University of Tehran in Tehran is investigated and analyzed. This paper also provides an evaluation of the technical, economic, and environmental aspects of replacing conventional hybrid energy systems with an HRES based on the conversion and storage of hydrogen. Due to the low price of diesel fuel in Iran, sensitivity analysis is done with respect to the future price of diesel fuel based on the average price of diesel fuel in Europe. The effects of rising diesel prices on hybrid energy system performance and costs are discussed. The proposed energy systems are simulated by HOMER software [25].
Solar energy The information of solar energy is obtained from Solar Energy and Surface Meteorology NASA [26]. Hourly and average monthly solar radiation data for the studied area are shown in Figs. 3 and 4, respectively. Average daily radiation in this area is 4.89 kWh/m2/d. System components The hybrid system studied includes various combinations of a diesel generator, PV, battery, and hydrogen storage and conversion system. The generator modeled in this study is a sample generator with the general specification of HOMER models. This model of the generator has an investment cost of 420 euros per kilowatt, yearly operation & maintenance (O & M) and replacement costs of 0.1 and 420 €/kW respectively. The cost of the generator's fuel is separate from its (O & M) costs. The investment cost for PV panels is 3000 €/kW, replacement cost and O & M costs are 0 and 64 €/kW, respectively. Amount of derating factor set to 80%, this factor reduces the production of PV by 20% in order to make an approximation to various influences such as temperature and dust and slope of the PV panels set to 35.5 degrees toward a south direction. In this study Trojan IND17-6V type of battery has been used with the following configuration: The price of each battery is 800 €/kWh. The replacement cost and O & M costs are set 800 €/kWh and 16 €/yr, respectively. The parameters of the other components of the system are presented in Table 1. By considering 20 years as the lifetime of the project, the real annual interest rate is assumed 6%. The real interest rate equals the nominal interest rate minus the inflation rate [27]. System shortage factor (CSF)
System description Load demand The electrical load is the electricity consumption of a renewable energies laboratory in Faculty of New Science and Technology of the University of Tehran. The average electrical energy consumption in the laboratory is 60 kWh/d and the peak load is estimated 8.61 kW. Fig. 1,
Fig. 1. Daily load profiles within a year.
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Fig. 2. Hourly AC primary load.
Fig. 3. Average daily radiation kWh/m2/d.
Optimization method
is a fraction of unmet load plus operating reserves that the system is not able to provide it. In this study, CSF is assumed to be 2%. HOMER uses the Net Present Cost (NPC) method for selecting the optimal combination of resources, which include investment, replacement, and maintenance costs. Table 1, shows the parameters of the components used in hybrid energy system [28].
Different criteria can be used to evaluate the performance of the hybrid energy systems. In this study, three different criteria are used for a comprehensive assessment of the proposed system. The first indicator is NPC which is the basis for optimization in HOMER. Net present cost is calculated by Eq. (1). 3
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Fig. 4. Hourly solar radiation kW/m2.
Table 1 The parameters of the system components. Component
Investment cost
Yearly O & M costs
Replacement cost
Lifetime
PV Generator Electrolyzer H2 storage Fuel cell Converter Battery
3000 €/kW 420 €/kW 3400 €/kW 600 €/kg 2700 €/kW 600 €/kW 800 €/kWh
64 €/kW 0.1 €/kW 68 €/kW 12 €/kW 0.006 €/kW 30 €/kW 16 €
0 400 €/kW 700 €/kW 0 800 €/kW 600 €/kW 800 €/kWh
20 years 15,000 h 15 years 20 years 40,000 h 15 years 12
T
NPC = I − ∑ t=1
Fig. 5. Schematic representation of diesel generator system.
CFt (1 + k )t
(1) average production cost per kWh.
where I is initial investment cost, CFt is cash flows (income or expenses) and K is the discount rate (interest rate). The second optimization factor is Levelized Cost of Energy (LCE). HOMER defines LCE value as an
LCE =
Cann,tot −Cboiler Hserved Eserved
(2)
Table 2 The search space of system components. Converter Capacity (kW)
Htank Capacity (kg)
Electrolyzer Capacity (kW)
Battery: Trojan IND17-6V (number)
Fuel Cell Capacity (kW)
Gen1 Capacity (kW)
Gen2 Capacity (kW)
PV (KW)
0 2 4 6 8 10 12 14 16
0 20 40 60 80 100 120
0 10 20 30 40 50 60 70 80
0 2 4 6 8 10 15 20 25 30 35
0 2 4 6 8 10 12 14 16 18 20
0 5
0 5
0 4 8 12 16 20 24 28 32 36 40 44 48 52
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Table 3 Optimization results for diesel generator system. Term
Quantity
Unit
LCE NPC Operating Cost Initial Cost Generator 1 Generator 1 (Hours of operation) Generator 2 Generator 2 (Hours of operation) Fuel Renewable fraction CO2 Emission
0.353 89503 7366 4200 21694 8760 2240 1676 9350 0 24622
€/kwh € €/kwh € kWh/yr hr/yr kWh/yr hr/yr L/yr – kg/yr
Fig. 7. Schematic representation of PV-diesel-battery system.
Table 4 Optimization results for PV-diesel-battery system.
where Cboiler is boiler marginal cost ($/kWh), Cann,tot is the total annualized cost of the system ($/yr), Eserved is total electrical load served (kWh/yr) and Hserved is total thermal load served (kWh/yr). Hserved = 0 because there is no thermal load in this study. The third optimization factor is the amount of carbon dioxide emission that evaluated for the proposed systems. Also, HOMER uses the following equation to calculate the output of the PV array:
PPV = fPV YPV
GT GT ,STC
(3)
where fPV is the PV derating factor, YPV stands for the rated capacity of the PV array (kW), GT is the solar radiation incident on the PV array in the current condition (kW/m2), and GT,STC refers to the incident radiation at standard test condition (1 kW/m2). As mentioned, HOMER uses NPC method to search for an optimal combination of the system components in order to provide the load demand. Table 2 shows the search space for each component.
Term
Quantity
Unit
LCE NPC Operating Cost Initial Cost Generator 1 Generator 1 (Hours of operation) PV production Fuel Renewable fraction CO2 Emission
0.316 79895 4127 32100 12656 4689 12907 4745 42 12495
€/kwh € €/kwh € kWh/yr hr/yr kWh/yr L/yr – kg/yr
Simulation and optimization results To meet demand, four different combinations of energy sources are considered as a power supply system: 1. Diesel generator system (current system)
Fig. 6. Summary of diesel generator system’s costs.
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Fig. 8. Summary of PV-battery-diesel system costs.
Fig. 9. Hourly power production of PV panels and diesel generator.
presented in the next sections based on the real price of diesel fuel in Iran (0.1 €/L).
2. PV- diesel-battery system 3. PV- battery system 4. PV- fuel-cell system Simulation and optimization results for the proposed systems are
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Fig. 10. Schematic representation of PV-battery system.
Table 5 Simulation results for PV-battery system. Term
Quantity
Unit
LCE NPC Operating Cost Initial Cost PV production Fuel Renewable fraction CO2 Emission
0.452 112977 2088 88800 32267 0 100 0
€/kwh € €/kwh € kWh/yr L/yr
Fig. 12. Schematic representation of PV-fuel cell system.
that has also been selected as the base system in this study. According to the optimization of the proposed system in Fig. 5, optimal system consists of two 5 kW diesel generators. The system has a net present cost of 89503 € and the initial investment cost of 4200 € and LCE of 0.353 €/kWh. Table 3 presents the simulation results and provides a comprehensive view of the system's performance. Fig. 6, illustrates the share of different costs of the system in total NPC. As can be seen, in the diesel generator system investment cost is less than the operating costs and the major part of the costs is related to the system’s O & M costs.
kg/yr
Diesel system A demonstration of the diesel generator based system for power supply is shown in Fig. 5. This figure shows two 5 kW diesel generators, which are considered to meet the peak demand in this system. This combination is one of the most commonly used power supply systems
Fig. 11. Monthly battery charge status of the PV-battery system.
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As expected, in this system due to elimination of diesel generator and increasing the capacity of PV panels, the investment cost is extremely high and operating cost decreases. There is no fossil fuel consumption and thus no carbon dioxide emissions in this combination. As a result, this system compared to the previous systems has better environmental performance in the term of GHG emissions. Fig. 11, shows the charge status of batteries in different months for the PV- battery system. The charging status in November and December is lower than other months because of low solar radiation in these months. Therefore batteries are frequently used for compensating PV power shortage in these months. This performance has been reversed in the warm months of the year and the batteries’ charging status is at an appropriate level without rapid fluctuation.
Table 6 Optimization results for PV-fuel cell system. Term
Quantity
Unit
LCE NPC Operating Cost Initial Cost Fuel cell production Fuel cell (Hours of operation) PV production Fuel Renewable fraction CO2 Emission
0.924 231769 4332 181600 6322 5340 64533 0 100 0
€/kwh € €/kwh € kWh/yr hr/yr kWh/yr L/yr – kg/yr
PV-diesel-battery system
PV-fuel-cell system
The combination intended for this system is shown in Fig. 7. The optimized system consists of 8 kW PV panels, a 5 kW diesel generator, 2 kW converter and 6 batteries. Table 4 shows the simulation results for this system. As shown in Table 4, by adding the combination of photovoltaic cells and batteries, a part of energy production has been assigned to this system and the amount of electricity produced by diesel generator decreases. The investment cost compared to the base case increases significantly due to the high investment cost of PV panels and battery units. But, due to the lower operating cost of PV system, the total operating cost of the system decreases. In fact, operating cost is the difference between initial investment cost and total cost, including maintenance, fuel, and the replacement cost. Fig. 8, summarize this discussion schematically. The main part of the investment cost is related to the PV panels and batteries; on the other hand, the main part of the operating cost is related to the diesel generator. Combining these features makes this system more efficient and reliable than previous system. Fig. 9, shows the hourly power production of PV panels and diesel generator during different months of the year. Considering the proper solar radiation in the study area, the PV panels’ power output is considerable (Fig. 9). Due to this proper solar radiation, a large portion of the load is provided by PV panels during summer months and in winter months, power shortage is compensated by the diesel generator.
Due to environmental and recycling issues of batteries, researchers are looking for suitable alternatives for this storage systems. One of the most promising solutions is converting electrical energy to hydrogen and generating electricity from stored hydrogen by the fuel cells [29,30]. Fuel cells are able to work with different types of fuels [31]. In addition, since the electrochemical reactions are used to produce electricity, they have no moving parts [32]. These characteristics make them reliable and noiseless [33]. In both fuel cell and battery electrical power is obtained through a chemical reaction. In a battery, chemical reactants are stored inside the battery, which are used during the reaction and then the battery must be recharged or discarded. But, the fuel cell has an outside storage for fuel [34,35]. It will continue to produce electricity as long as the fuel is supplied. The fuel cell has a smaller volume, higher energy density and more variety application than batteries. Therefore, in this study, a combination of fuel cell, electrolyzer, and hydrogen tank is used as an alternative system for battery-based energy storage systems. The schematic representation of this system is shown in Fig. 12. The optimized system includes 40 kW PV, 4 kW fuel cell, 10 kW electrolyzer and 8 kW converter and a hydrogen tank with 20 kg capacity. Table 6, presents the simulation results for this system. Batteries are removed and Load-following strategy used in this case. The capacity of PV panels increase to 40 kW in this system. The total production of electrical energy is 70,585 kWh during a year, which 91% of this power is produced by PV panels. As seen in Table 6, the optimal hydrogen-based system works entirely with renewable energy resources with zero carbon emissions and no diesel consumption. The results indicate that the proposed system works 5340 h/yr and has 380 kg/yr hydrogen consumption with 7.49 years lifetime, which means fuel cell is replaced twice during the project lifetime. Fig. 13, shows the monthly averaged energy production of the PV and fuel cell. The fuel cell is typically used during cold months with low solar radiation. During the summer months, the load is routinely
PV-battery system In this system, all of the electrical demand is provided by renewable resources. The diesel generator is removed in the proposed system (Fig. 10). According to the optimization results, the optimized system consists of 20 kW PV, 8 kW converter and 30 batteries. The PV size and the number of batteries are considerably increased. Table 5 shows the simulation results for this system.
Fig. 13. Monthly averaged energy production of PV and fuel cell.
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Fig. 14. Electricity generation of fuel cell in different months.
Sensitivity analysis and discussion
Table 7 NPC for various systems. NPC (€)
0.1 (€/L)
0.55 (€/L)
1.1 (€/L)
Diesel PV-Diesel-Battery PV-Battery PV-Fuel cell
89503 79895 112977 231769
138227 98938 112977 231769
19780 104471 112977 231769
LCE (€)
0.1 (€/L)
0.55 (€/L)
1.1 (€/L)
Diesel PV-Diesel-Battery PV-Battery PV-Fuel cell
0.353 0.316 0.452 0.924
0.543 0.390 0.452 0.924
0.780 0.412 0.452 0.924
The effects of diesel price changes on the performance of proposed systems from the standpoint of indicators such as LCE, NPC, and carbon emission are investigated in this section. Due to the government subsidies for the production and supply of the fossil fuels in Iran, the diesel fuel price is extremely lower than the developed countries. But according to the laws passed in recent years, the government plans to increase the price of energy and fuel over the next few years. Therefore the impact of the rise in prices on the HRES competitiveness is explored in this study. The diesel price is considered 1.1 €/L based on the average of European Union price [36]. In this study, three prices for diesel fuel is considered and optimization results for different systems in three mentioned cases are presented in Tables 7–9. Table 7 shows the NPC for different systems in three scenarios for diesel price. As can be seen with the current price of the diesel fuel, the diesel generator system is the most economical option for providing the offgrid electrical load. But with the rise in diesel fuel prices, the renewable-based systems become more competitive. As it can be seen, assuming diesel prices equal to half the EU average, the PV-battery is even more economical than the conventional diesel generator system. By assuming the price of diesel fuel equal to the EU average, even PV-fuel cell system, considering the environmental benefits of this system, can economically compete with the conventional diesel generator systems. Table 8, shows the electricity cost per kWh for different diesel prices. The electricity price of PV-battery system is less than conventional diesel generator systems with half of European Union diesel price, which indicates the role of the future price of fossil fuels in renewable energies penetration in the future energy systems. As can be seen from Table 9, with the current price of diesel fuel, carbon dioxide emission is reduced up to 51% by adding PV panels to the diesel generator system. But with the assumption of a price of diesel fuel equal to the EU average, the emission reduction in the PV-dieselbattery hybrid system is 91% less than the diesel system.
Table 8 LCE for proposed systems.
Table 9 Carbon dioxide emission in different scenarios. CO2 Emission (kg/yr)
0.1 (€/L)
0.55 (€/L)
1.1 (€/L)
Diesel PV-Diesel-Battery PV-Battery PV-Fuel cell
24622 12495 0 0
24622 2288 0 0
24622 2288 0 0
provided by PV panels. Fig. 14, shows the energy production of the fuel cell during every month. Batteries and fuel cells have a similar role and act as a backup system for PV panels. During the cold months of the year, as well as the late hours of the day, the sun's radiation is at its lowest level, power production of the fuel cell increases to compensate the power shortage of PV panels.
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Conclusions In this paper four different hybrid energy systems consisting of various combinations of diesel generators, PV, battery, and hydrogen storage system for providing the electrical power of a renewable energy laboratory at the University of Tehran has been investigated. Technicaleconomic and environmental aspects of replacement of a conventional system (diesel generator) with renewable hybrid systems (batteries and a fuel cell hybrid system, the electrolyzer and hydrogen storage) have been evaluated. Due to the low price of diesel fuel in Iran, the sensitivity analysis was done by considering the future price of diesel fuel based on the average price of the diesel fuel in Europe. The impact of rising diesel price on the penetration of renewable energy resources, electricity costs and the economic competitiveness of HRES with conventional diesel systems was explored. The results indicated that replacing diesel generators with battery units led to a significant increase in installed capacity of PV panels. The cost of energy production (COP) due to the high investment cost of fuel cell and electrolyzer in the PVhydrogen system is so high, but with increasing the price of diesel price, COP of the hybrid photovoltaic-fuel cell system becomes near to the cost of producing energy from conventional diesel system. Therefore, the economic competitiveness of the HRES according to the environmental advantages of this system is increased compared to conventional systems. In conclusion, due to high investment costs of renewable energy systems especially fuel cell systems and low prices of fossil fuels in Iran, the competitiveness of such systems with diesel generator systems is low and PV-diesel-battery system is strongly recommended as an economical, reliable, and clean off-grid power supply system. The development of renewable energy systems in Iran requires government support and incentive schemes to increase the economic acceptability and competitiveness of these systems in comparison with conventional energy supply systems. References [1] Mohammadi M, Noorollahi Y, Mohammadi-ivatloo B, Yousefi H. Energy hub: from a model to a concept – a review. Renewable Sustainable Energy Rev 2017;80:1512–27. 2017/12/01/. [2] Noorollahi Y, Itoi R, Yousefi H, Mohammadi M, Farhadi A. Modeling for diversifying electricity supply by maximizing renewable energy use in Ebino city southern Japan. Sustainable Cities Soc 2017;34:371–84. 2017/10/01/. [3] Mehrpooya M, Sharifzadeh MMM. A novel integration of oxy-fuel cycle, high temperature solar cycle and LNG cold recovery – energy and exergy analysis. Appl Therm Eng 2017;114(Supplement C):1090–104. 2017/03/05/. [4] Noorollahi Y, Yousefi H, Mohammadi M. Multi-criteria decision support system for wind farm site selection using GIS. Sustainable Energy Technol Assess 2016;13:38–50. 2016/02/01/. [5] Mehrpooya M, Shahsavan M, Sharifzadeh MMM. Modeling, energy and exergy analysis of solar chimney power plant-Tehran climate data case study. Energy 2016;115(Part 1):257–73. 2016/11/15/. [6] Sinha S, Chandel S. Review of software tools for hybrid renewable energy systems. Renewable Sustainable Energy Rev 2014;32:192–205. [7] Bernal-Agustín JL, Dufo-López R. Simulation and optimization of stand-alone hybrid renewable energy systems. Renewable Sustainable Energy Rev 2009;13(8):2111–8. [8] Bajpai P, Dash V. Hybrid renewable energy systems for power generation in standalone applications: a review. Renewable Sustainable Energy Rev 2012;16(5):2926–39. [9] Askarzadeh A. “Solution for sizing a PV/diesel HPGS for isolated sites,” IET Renewable Power Generation, vol. 11, no. 1, pp. 143–151. Available from: http:// digital-library.theiet.org/content/journals/10.1049/iet-rpg.2016.0319. [10] Mahesh A, Sandhu KS. Hybrid wind/photovoltaic energy system developments:
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Contents lists available at ScienceDirect
Sustainable Energy Technologies and Assessments journal homepage: www.elsevier.com/locate/seta
Original article
Techno-economic-environmental study of hybrid power supply system: A case study in Iran
MARK
⁎
Mehdi Mehrpooyaa,b, , Mohammad Mohammadia, Esmaeil Ahmadia a b
Renewable Energies and Environment Department, Faculty of New Sciences and Technologies, University of Tehran, Tehran, Iran Hydrogen and Fuel Cell Laboratory, Faculty of New Sciences and Technologies, University of Tehran, Tehran, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: Hybrid energy system Techno-economic-environmental analysis HOMER PV-hydrogen
This paper presents an optimal planning model of a hybrid renewable energy system to meet a real load with a combination of photovoltaic panels (PV), diesel generators and batteries. Also, replacing the conventional energy storage system with a fuel cell is investigated. Energy Modelling and Energy Resources Assessment Lab (EMERAL), at University of Tehran, in Tehran, Iran has selected as a case study. A technical, economic and environmental assessment was conducted for optimal planning of the hybrid energy system. Finally, the impact of diesel fuel price fluctuation on the economic competitiveness of proposed hybrid renewable energy systems in providing off-grid electrical load demand is investigated. In this study simulation, optimization and modeling procedures are done by HOMER software.
Introduction Obviously, world energy consumption has rapidly increased in recent years. Depletion of fossil fuel resources and low efficiency of current energy systems are caused many concerns about providing energy in the future [1]. Furthermore, conventional energy resources, which are mainly fossil fuels, have environmental issues and their carbon emission lead to concerns about global warming problem [2,3]. These issues have led to the use of alternative energy sources such as renewable energies that have little environmental impact and have the ability to produce clean and sustainable energy [4,5]. However, these resources have periodic nature and their production capacity is not precisely predictable and is always uncertain. In order to solve this problem, it is possible to use these resources as hybrid systems with other energy sources, along with the use of energy storage systems. The hybrid energy systems have become more common due to uncertainty and the high investment cost of renewable systems. Therefore, renewable energy sources can be combined with other traditional energy sources and that will lead to the creation of hybrid renewable energy systems (HRES) [6]. A comprehensive overview of planning models, as well as operational optimization and control of HRES in remote areas, can be found in [7]. The models presented for PV-base HRES in off-grid mode have been investigated in [8] from the planning, modeling, control, and optimization aspects. Also, related information about optimization technique for optimal sizing of PV/diesel hybrid power generation systems can be found in [9]. An overview of hybrid energy
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systems consisting of photovoltaics, wind turbines and batteries from optimal planning, suitable converter design, and operational optimization aspects can be found in [10]. Also, reviews of the models presented for HRES in the micro-network content, taking into account the role of energy storage systems, can be found in [11,12]. Examples of modeling, optimization, and application of HRES can be found in [13]. In this regards, other work has been done which includes integration of the tidal power into a hybrid PV/wind/battery renewable energy system [14]. Due to the existence of various energy technologies and systems in HRES, the problem of planning and optimizing these systems is very important. Therefore, considering the problem conditions and the application of the power supply system, the choice of appropriate method and tool for optimizing the HRES should be carefully performed. An overview of existing simulation tools for evaluating, optimizing and planning the HRES has been done by Sinha and Chandel [6]. The results of this study showed that among all the available software, HOMER has been most used and applied for planning and optimization of hybrid energy systems. This has been due to the prominent features of this software. HOMER has the capability to assess the technical, economic, and environmental aspects of the energy systems, and reliability indicators can also be used for optimal scheduling of the system. HOMER also has the ability to handle a large number of system components and related variables in a reasonable time and also evaluate the system in different scenarios. On the other hand, sensitivity analysis on input data is another feature of this software [6]. In this study HOMER software
Corresponding author at: Renewable Energies and Environment Department, Faculty of New Sciences and Technologies, University of Tehran, Tehran, Iran. E-mail address: [email protected] (M. Mehrpooya).
http://dx.doi.org/10.1016/j.seta.2017.10.007 Received 8 September 2016; Received in revised form 13 October 2017; Accepted 31 October 2017 2213-1388/ © 2017 Elsevier Ltd. All rights reserved.
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shows daily load profile during a year. As can be seen, the main part of energy consumption occurs between 8 am and 8 pm. Also, the hourly AC primary load is shown in Fig. 2.
has been selected to design and evaluate hybrid energy systems. Solar energy is a clean energy [15] and due to its various advantages, such as no greenhouse gas (GHG) emission and low maintenance costs is applied in many studies [16–19]. In Off-grid applications, battery units can be used as a storage system with PV panels to reduce the uncertainty related to renewable energy resources production and increasing the system reliability. However, due to environmental concerns of the battery units, finding an alternative option for these storage systems is necessary. Energy systems that convert electricity to hydrogen can be a practical solution to this issue [20–23]. Other studies on solar and hydrogen hybrid energy systems for use in remote areas and off-grid mode can be found in [24]. In this study, four different hybrid energy systems including different combination of diesel generators, PV, battery, and hydrogen storage system for providing the electrical load of EMERAL at the University of Tehran in Tehran is investigated and analyzed. This paper also provides an evaluation of the technical, economic, and environmental aspects of replacing conventional hybrid energy systems with an HRES based on the conversion and storage of hydrogen. Due to the low price of diesel fuel in Iran, sensitivity analysis is done with respect to the future price of diesel fuel based on the average price of diesel fuel in Europe. The effects of rising diesel prices on hybrid energy system performance and costs are discussed. The proposed energy systems are simulated by HOMER software [25].
Solar energy The information of solar energy is obtained from Solar Energy and Surface Meteorology NASA [26]. Hourly and average monthly solar radiation data for the studied area are shown in Figs. 3 and 4, respectively. Average daily radiation in this area is 4.89 kWh/m2/d. System components The hybrid system studied includes various combinations of a diesel generator, PV, battery, and hydrogen storage and conversion system. The generator modeled in this study is a sample generator with the general specification of HOMER models. This model of the generator has an investment cost of 420 euros per kilowatt, yearly operation & maintenance (O & M) and replacement costs of 0.1 and 420 €/kW respectively. The cost of the generator's fuel is separate from its (O & M) costs. The investment cost for PV panels is 3000 €/kW, replacement cost and O & M costs are 0 and 64 €/kW, respectively. Amount of derating factor set to 80%, this factor reduces the production of PV by 20% in order to make an approximation to various influences such as temperature and dust and slope of the PV panels set to 35.5 degrees toward a south direction. In this study Trojan IND17-6V type of battery has been used with the following configuration: The price of each battery is 800 €/kWh. The replacement cost and O & M costs are set 800 €/kWh and 16 €/yr, respectively. The parameters of the other components of the system are presented in Table 1. By considering 20 years as the lifetime of the project, the real annual interest rate is assumed 6%. The real interest rate equals the nominal interest rate minus the inflation rate [27]. System shortage factor (CSF)
System description Load demand The electrical load is the electricity consumption of a renewable energies laboratory in Faculty of New Science and Technology of the University of Tehran. The average electrical energy consumption in the laboratory is 60 kWh/d and the peak load is estimated 8.61 kW. Fig. 1,
Fig. 1. Daily load profiles within a year.
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Fig. 2. Hourly AC primary load.
Fig. 3. Average daily radiation kWh/m2/d.
Optimization method
is a fraction of unmet load plus operating reserves that the system is not able to provide it. In this study, CSF is assumed to be 2%. HOMER uses the Net Present Cost (NPC) method for selecting the optimal combination of resources, which include investment, replacement, and maintenance costs. Table 1, shows the parameters of the components used in hybrid energy system [28].
Different criteria can be used to evaluate the performance of the hybrid energy systems. In this study, three different criteria are used for a comprehensive assessment of the proposed system. The first indicator is NPC which is the basis for optimization in HOMER. Net present cost is calculated by Eq. (1). 3
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Fig. 4. Hourly solar radiation kW/m2.
Table 1 The parameters of the system components. Component
Investment cost
Yearly O & M costs
Replacement cost
Lifetime
PV Generator Electrolyzer H2 storage Fuel cell Converter Battery
3000 €/kW 420 €/kW 3400 €/kW 600 €/kg 2700 €/kW 600 €/kW 800 €/kWh
64 €/kW 0.1 €/kW 68 €/kW 12 €/kW 0.006 €/kW 30 €/kW 16 €
0 400 €/kW 700 €/kW 0 800 €/kW 600 €/kW 800 €/kWh
20 years 15,000 h 15 years 20 years 40,000 h 15 years 12
T
NPC = I − ∑ t=1
Fig. 5. Schematic representation of diesel generator system.
CFt (1 + k )t
(1) average production cost per kWh.
where I is initial investment cost, CFt is cash flows (income or expenses) and K is the discount rate (interest rate). The second optimization factor is Levelized Cost of Energy (LCE). HOMER defines LCE value as an
LCE =
Cann,tot −Cboiler Hserved Eserved
(2)
Table 2 The search space of system components. Converter Capacity (kW)
Htank Capacity (kg)
Electrolyzer Capacity (kW)
Battery: Trojan IND17-6V (number)
Fuel Cell Capacity (kW)
Gen1 Capacity (kW)
Gen2 Capacity (kW)
PV (KW)
0 2 4 6 8 10 12 14 16
0 20 40 60 80 100 120
0 10 20 30 40 50 60 70 80
0 2 4 6 8 10 15 20 25 30 35
0 2 4 6 8 10 12 14 16 18 20
0 5
0 5
0 4 8 12 16 20 24 28 32 36 40 44 48 52
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Table 3 Optimization results for diesel generator system. Term
Quantity
Unit
LCE NPC Operating Cost Initial Cost Generator 1 Generator 1 (Hours of operation) Generator 2 Generator 2 (Hours of operation) Fuel Renewable fraction CO2 Emission
0.353 89503 7366 4200 21694 8760 2240 1676 9350 0 24622
€/kwh € €/kwh € kWh/yr hr/yr kWh/yr hr/yr L/yr – kg/yr
Fig. 7. Schematic representation of PV-diesel-battery system.
Table 4 Optimization results for PV-diesel-battery system.
where Cboiler is boiler marginal cost ($/kWh), Cann,tot is the total annualized cost of the system ($/yr), Eserved is total electrical load served (kWh/yr) and Hserved is total thermal load served (kWh/yr). Hserved = 0 because there is no thermal load in this study. The third optimization factor is the amount of carbon dioxide emission that evaluated for the proposed systems. Also, HOMER uses the following equation to calculate the output of the PV array:
PPV = fPV YPV
GT GT ,STC
(3)
where fPV is the PV derating factor, YPV stands for the rated capacity of the PV array (kW), GT is the solar radiation incident on the PV array in the current condition (kW/m2), and GT,STC refers to the incident radiation at standard test condition (1 kW/m2). As mentioned, HOMER uses NPC method to search for an optimal combination of the system components in order to provide the load demand. Table 2 shows the search space for each component.
Term
Quantity
Unit
LCE NPC Operating Cost Initial Cost Generator 1 Generator 1 (Hours of operation) PV production Fuel Renewable fraction CO2 Emission
0.316 79895 4127 32100 12656 4689 12907 4745 42 12495
€/kwh € €/kwh € kWh/yr hr/yr kWh/yr L/yr – kg/yr
Simulation and optimization results To meet demand, four different combinations of energy sources are considered as a power supply system: 1. Diesel generator system (current system)
Fig. 6. Summary of diesel generator system’s costs.
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Fig. 8. Summary of PV-battery-diesel system costs.
Fig. 9. Hourly power production of PV panels and diesel generator.
presented in the next sections based on the real price of diesel fuel in Iran (0.1 €/L).
2. PV- diesel-battery system 3. PV- battery system 4. PV- fuel-cell system Simulation and optimization results for the proposed systems are
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Fig. 10. Schematic representation of PV-battery system.
Table 5 Simulation results for PV-battery system. Term
Quantity
Unit
LCE NPC Operating Cost Initial Cost PV production Fuel Renewable fraction CO2 Emission
0.452 112977 2088 88800 32267 0 100 0
€/kwh € €/kwh € kWh/yr L/yr
Fig. 12. Schematic representation of PV-fuel cell system.
that has also been selected as the base system in this study. According to the optimization of the proposed system in Fig. 5, optimal system consists of two 5 kW diesel generators. The system has a net present cost of 89503 € and the initial investment cost of 4200 € and LCE of 0.353 €/kWh. Table 3 presents the simulation results and provides a comprehensive view of the system's performance. Fig. 6, illustrates the share of different costs of the system in total NPC. As can be seen, in the diesel generator system investment cost is less than the operating costs and the major part of the costs is related to the system’s O & M costs.
kg/yr
Diesel system A demonstration of the diesel generator based system for power supply is shown in Fig. 5. This figure shows two 5 kW diesel generators, which are considered to meet the peak demand in this system. This combination is one of the most commonly used power supply systems
Fig. 11. Monthly battery charge status of the PV-battery system.
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As expected, in this system due to elimination of diesel generator and increasing the capacity of PV panels, the investment cost is extremely high and operating cost decreases. There is no fossil fuel consumption and thus no carbon dioxide emissions in this combination. As a result, this system compared to the previous systems has better environmental performance in the term of GHG emissions. Fig. 11, shows the charge status of batteries in different months for the PV- battery system. The charging status in November and December is lower than other months because of low solar radiation in these months. Therefore batteries are frequently used for compensating PV power shortage in these months. This performance has been reversed in the warm months of the year and the batteries’ charging status is at an appropriate level without rapid fluctuation.
Table 6 Optimization results for PV-fuel cell system. Term
Quantity
Unit
LCE NPC Operating Cost Initial Cost Fuel cell production Fuel cell (Hours of operation) PV production Fuel Renewable fraction CO2 Emission
0.924 231769 4332 181600 6322 5340 64533 0 100 0
€/kwh € €/kwh € kWh/yr hr/yr kWh/yr L/yr – kg/yr
PV-diesel-battery system
PV-fuel-cell system
The combination intended for this system is shown in Fig. 7. The optimized system consists of 8 kW PV panels, a 5 kW diesel generator, 2 kW converter and 6 batteries. Table 4 shows the simulation results for this system. As shown in Table 4, by adding the combination of photovoltaic cells and batteries, a part of energy production has been assigned to this system and the amount of electricity produced by diesel generator decreases. The investment cost compared to the base case increases significantly due to the high investment cost of PV panels and battery units. But, due to the lower operating cost of PV system, the total operating cost of the system decreases. In fact, operating cost is the difference between initial investment cost and total cost, including maintenance, fuel, and the replacement cost. Fig. 8, summarize this discussion schematically. The main part of the investment cost is related to the PV panels and batteries; on the other hand, the main part of the operating cost is related to the diesel generator. Combining these features makes this system more efficient and reliable than previous system. Fig. 9, shows the hourly power production of PV panels and diesel generator during different months of the year. Considering the proper solar radiation in the study area, the PV panels’ power output is considerable (Fig. 9). Due to this proper solar radiation, a large portion of the load is provided by PV panels during summer months and in winter months, power shortage is compensated by the diesel generator.
Due to environmental and recycling issues of batteries, researchers are looking for suitable alternatives for this storage systems. One of the most promising solutions is converting electrical energy to hydrogen and generating electricity from stored hydrogen by the fuel cells [29,30]. Fuel cells are able to work with different types of fuels [31]. In addition, since the electrochemical reactions are used to produce electricity, they have no moving parts [32]. These characteristics make them reliable and noiseless [33]. In both fuel cell and battery electrical power is obtained through a chemical reaction. In a battery, chemical reactants are stored inside the battery, which are used during the reaction and then the battery must be recharged or discarded. But, the fuel cell has an outside storage for fuel [34,35]. It will continue to produce electricity as long as the fuel is supplied. The fuel cell has a smaller volume, higher energy density and more variety application than batteries. Therefore, in this study, a combination of fuel cell, electrolyzer, and hydrogen tank is used as an alternative system for battery-based energy storage systems. The schematic representation of this system is shown in Fig. 12. The optimized system includes 40 kW PV, 4 kW fuel cell, 10 kW electrolyzer and 8 kW converter and a hydrogen tank with 20 kg capacity. Table 6, presents the simulation results for this system. Batteries are removed and Load-following strategy used in this case. The capacity of PV panels increase to 40 kW in this system. The total production of electrical energy is 70,585 kWh during a year, which 91% of this power is produced by PV panels. As seen in Table 6, the optimal hydrogen-based system works entirely with renewable energy resources with zero carbon emissions and no diesel consumption. The results indicate that the proposed system works 5340 h/yr and has 380 kg/yr hydrogen consumption with 7.49 years lifetime, which means fuel cell is replaced twice during the project lifetime. Fig. 13, shows the monthly averaged energy production of the PV and fuel cell. The fuel cell is typically used during cold months with low solar radiation. During the summer months, the load is routinely
PV-battery system In this system, all of the electrical demand is provided by renewable resources. The diesel generator is removed in the proposed system (Fig. 10). According to the optimization results, the optimized system consists of 20 kW PV, 8 kW converter and 30 batteries. The PV size and the number of batteries are considerably increased. Table 5 shows the simulation results for this system.
Fig. 13. Monthly averaged energy production of PV and fuel cell.
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Fig. 14. Electricity generation of fuel cell in different months.
Sensitivity analysis and discussion
Table 7 NPC for various systems. NPC (€)
0.1 (€/L)
0.55 (€/L)
1.1 (€/L)
Diesel PV-Diesel-Battery PV-Battery PV-Fuel cell
89503 79895 112977 231769
138227 98938 112977 231769
19780 104471 112977 231769
LCE (€)
0.1 (€/L)
0.55 (€/L)
1.1 (€/L)
Diesel PV-Diesel-Battery PV-Battery PV-Fuel cell
0.353 0.316 0.452 0.924
0.543 0.390 0.452 0.924
0.780 0.412 0.452 0.924
The effects of diesel price changes on the performance of proposed systems from the standpoint of indicators such as LCE, NPC, and carbon emission are investigated in this section. Due to the government subsidies for the production and supply of the fossil fuels in Iran, the diesel fuel price is extremely lower than the developed countries. But according to the laws passed in recent years, the government plans to increase the price of energy and fuel over the next few years. Therefore the impact of the rise in prices on the HRES competitiveness is explored in this study. The diesel price is considered 1.1 €/L based on the average of European Union price [36]. In this study, three prices for diesel fuel is considered and optimization results for different systems in three mentioned cases are presented in Tables 7–9. Table 7 shows the NPC for different systems in three scenarios for diesel price. As can be seen with the current price of the diesel fuel, the diesel generator system is the most economical option for providing the offgrid electrical load. But with the rise in diesel fuel prices, the renewable-based systems become more competitive. As it can be seen, assuming diesel prices equal to half the EU average, the PV-battery is even more economical than the conventional diesel generator system. By assuming the price of diesel fuel equal to the EU average, even PV-fuel cell system, considering the environmental benefits of this system, can economically compete with the conventional diesel generator systems. Table 8, shows the electricity cost per kWh for different diesel prices. The electricity price of PV-battery system is less than conventional diesel generator systems with half of European Union diesel price, which indicates the role of the future price of fossil fuels in renewable energies penetration in the future energy systems. As can be seen from Table 9, with the current price of diesel fuel, carbon dioxide emission is reduced up to 51% by adding PV panels to the diesel generator system. But with the assumption of a price of diesel fuel equal to the EU average, the emission reduction in the PV-dieselbattery hybrid system is 91% less than the diesel system.
Table 8 LCE for proposed systems.
Table 9 Carbon dioxide emission in different scenarios. CO2 Emission (kg/yr)
0.1 (€/L)
0.55 (€/L)
1.1 (€/L)
Diesel PV-Diesel-Battery PV-Battery PV-Fuel cell
24622 12495 0 0
24622 2288 0 0
24622 2288 0 0
provided by PV panels. Fig. 14, shows the energy production of the fuel cell during every month. Batteries and fuel cells have a similar role and act as a backup system for PV panels. During the cold months of the year, as well as the late hours of the day, the sun's radiation is at its lowest level, power production of the fuel cell increases to compensate the power shortage of PV panels.
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Conclusions In this paper four different hybrid energy systems consisting of various combinations of diesel generators, PV, battery, and hydrogen storage system for providing the electrical power of a renewable energy laboratory at the University of Tehran has been investigated. Technicaleconomic and environmental aspects of replacement of a conventional system (diesel generator) with renewable hybrid systems (batteries and a fuel cell hybrid system, the electrolyzer and hydrogen storage) have been evaluated. Due to the low price of diesel fuel in Iran, the sensitivity analysis was done by considering the future price of diesel fuel based on the average price of the diesel fuel in Europe. The impact of rising diesel price on the penetration of renewable energy resources, electricity costs and the economic competitiveness of HRES with conventional diesel systems was explored. The results indicated that replacing diesel generators with battery units led to a significant increase in installed capacity of PV panels. The cost of energy production (COP) due to the high investment cost of fuel cell and electrolyzer in the PVhydrogen system is so high, but with increasing the price of diesel price, COP of the hybrid photovoltaic-fuel cell system becomes near to the cost of producing energy from conventional diesel system. Therefore, the economic competitiveness of the HRES according to the environmental advantages of this system is increased compared to conventional systems. In conclusion, due to high investment costs of renewable energy systems especially fuel cell systems and low prices of fossil fuels in Iran, the competitiveness of such systems with diesel generator systems is low and PV-diesel-battery system is strongly recommended as an economical, reliable, and clean off-grid power supply system. The development of renewable energy systems in Iran requires government support and incentive schemes to increase the economic acceptability and competitiveness of these systems in comparison with conventional energy supply systems. References [1] Mohammadi M, Noorollahi Y, Mohammadi-ivatloo B, Yousefi H. Energy hub: from a model to a concept – a review. Renewable Sustainable Energy Rev 2017;80:1512–27. 2017/12/01/. [2] Noorollahi Y, Itoi R, Yousefi H, Mohammadi M, Farhadi A. Modeling for diversifying electricity supply by maximizing renewable energy use in Ebino city southern Japan. Sustainable Cities Soc 2017;34:371–84. 2017/10/01/. [3] Mehrpooya M, Sharifzadeh MMM. A novel integration of oxy-fuel cycle, high temperature solar cycle and LNG cold recovery – energy and exergy analysis. Appl Therm Eng 2017;114(Supplement C):1090–104. 2017/03/05/. [4] Noorollahi Y, Yousefi H, Mohammadi M. Multi-criteria decision support system for wind farm site selection using GIS. Sustainable Energy Technol Assess 2016;13:38–50. 2016/02/01/. [5] Mehrpooya M, Shahsavan M, Sharifzadeh MMM. Modeling, energy and exergy analysis of solar chimney power plant-Tehran climate data case study. Energy 2016;115(Part 1):257–73. 2016/11/15/. [6] Sinha S, Chandel S. Review of software tools for hybrid renewable energy systems. Renewable Sustainable Energy Rev 2014;32:192–205. [7] Bernal-Agustín JL, Dufo-López R. Simulation and optimization of stand-alone hybrid renewable energy systems. Renewable Sustainable Energy Rev 2009;13(8):2111–8. [8] Bajpai P, Dash V. Hybrid renewable energy systems for power generation in standalone applications: a review. Renewable Sustainable Energy Rev 2012;16(5):2926–39. [9] Askarzadeh A. “Solution for sizing a PV/diesel HPGS for isolated sites,” IET Renewable Power Generation, vol. 11, no. 1, pp. 143–151. Available from: http:// digital-library.theiet.org/content/journals/10.1049/iet-rpg.2016.0319. [10] Mahesh A, Sandhu KS. Hybrid wind/photovoltaic energy system developments:
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