Optimum Scheduling of Isolated Hybrid Renewable Energy System-A Case Study Using HOMER

This paper compares the different scheduling scenarios of hybrid renewable energy system in terms of system configuration, economy, power production, biomass availability and pollutant emission. The hybrid renewable energy system comprises the solar, wind and biomass based power generating systems. This case study is carried out for a specific geographical location (village Ramgarh) in Ambala district of Haryana state, in India. The HOMER software tool is used to carry out simulations of three different scenarios viz. Generator is on or off depending on the availability of solar and wind power; the generator is on for all time; and the generator operation is optimized. Sensitivity analysis is also carried out for different maximum annual capacity storage in relation to change in net present cost (NPC) and levelized cost of energy (LCOE). The results of the analysis indicate that as the maximum annual capacity storage increases, the net present cost and levelized cost of energy decreases. The comparison of the results is presented in a tabular form. *Corresponding author: Ravita Saraswat, School of Renewable Energy and Efficiency, National Institute of Technology, Kurukshetra, Haryana-136 119, India, Tel: 01744233208; E-mail: ravita.saraswat009@gmail.com Received October 05, 2017; Accepted October 20, 2017; Published October 29, 2017 Citation: Saraswat R, Sathans S (2017) Optimum Scheduling of Isolated Hybrid Renewable Energy System A Case Study Using HOMER. J Environ Anal Toxicol 7: 520. doi: 10.4172/2161-0525.1000520 Copyright: © 2017 Saraswat R, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Citation: Saraswat R, Sathans S (2017) Optimum Scheduling of Isolated Hybrid Renewable Energy System A Case Study Using HOMER. J Environ Anal Toxicol 7: 520. doi: 10.4172/2161-0525.1000520


Introduction
Electrical energy is a basic necessity of everyday life [1]. The world energy demand is increasing rapidly which we are not able to meet and even today 1.4 billion people are still lacking electricity [2]. Now it is high time to exploit the renewable energy resources instead of fossil fuels to produce electricity which will not have adverse effect on the climate behaviour [3]. The high participation of renewable energy sources is need of the hour not only for grid connected load, but also for stand-alone systems [4]. The use of hybrid renewable energy systems, which includes two or more energy sources having at least one of these resources as renewable, is being pursued [5]. However, there are challenges in the design of hybrid renewable energy systems due to the variability and availability of renewable energy sources which affect the performance of hybrid systems all through the year and hence the renewable sources can be considered in terms of average value and distributed over year [6].
Lot of studies have been reported on hybrid system in respect of their design optimization. Jose et al. [7] conducted a design study of hybrid system for minimizing costs and pollutants emissions. Lopez et al. [8] performed optimization of control strategies for stand-alone renewable energy system with hydrogen storage, wherein the relevant strategy developed is explained and its application to a PV-dieselbattery-hydrogen system is demonstrated. Chun et al. [9] investigated the scenario of Thailand where they examined the capital cost, net present cost and cost of energy for different types of hybrid systems.
Joseph et al. [10] proposed a microhydro PV hybrid system addressing the issue of sizing a small hydro PV hybrid system for rural electrification in developing countries. Nandi et al. [11] performed a pre-feasibility study of wind-PV-battery hybrid system in eastern southern part of Bangladesh. Lau et al. [12] conducted a performance analysis of hybrid photovoltaic/diesel energy system under Malaysian climatic conditions with a suitability study of utilizing hybrid PV/diesel energy system over stand alone diesel system.
Yadav et al. [13] presented optimal hybrid power system design using HOMER simulation platform where they simulated a wind-diesel hybrid system and presented a comparative analysis in respect of the operational and economic parameters. Kumar and Manoharan [14] analysed hybrid system economy for each climate zone of Tamil Nadu on the basis of net present cost, consumption of diesel and renewable function. Chmeil et al. [15] conducted a study of Scotland and tried alternative configuration which could work more effectively and efficiently. As can be observed from the available literature, optimization of hybrid systems thus far has focused only on technical, economical and environmental aspects. There is lot of scope for research in terms of optimal scheduling in the utilization of solar and wind energy. This paper is an attempt in that direction and this study investigates different scheduling scenarios of hybrid system, consisting of renewable sources viz. solar power, wind power, biomass and battery for storage, and provides the comparison showing the effect on Net Present Cost and Levelized Cost of Energy.
The rest of this paper is organised as follows. The profile of study area is given in section 2. Methodology is discussed in section 3, followed by the mathematical model of renewable energy sources in section 4. Section 5 discusses the load demand assessment while section 6 shows the potential of renewable energy resources. Section 7 gives the list of component details that comprise the hybrid system. Results are discussed in section 8. A sensitivity analysis is performed in section 9, whereas in section 10, the conclusion is given. covered by forest and 15% is the total cropped area [16,17]. There are 21 districts in the State grouped into four divisions. It is situated in the North Western region of India and Himachal Pradesh in its North, Uttrakhand in North East, Rajasthan in the South, U.P and Delhi in the East and Punjab in North West are the other adjoining states as shown in Figure 1. Agriculture has remained and is currently the main leading occupation for the people of the State. The agriculture sector has always been an important contributor to the Gross Domestic Product (GDP) of the State. Energy is an important factor for sustained economic growth of any state. However, Haryana has limited availability and potential of natural sources of energy like hydro, coal etc. The total power available in the state from the installed capacity at present is 10,729.04 MW which includes 3230.20 MW from state's own generating stations, 829 MW jointly from its share in central projects and independent private projects. The power available from these sources during the year 2014-15 is about 17040.3 million units.
Power demand is increasing rapidly in the state and to fulfil the same and also due to environmental concerns there is focus on the use of renewable sources of energy. With this background, this study investigates different scenarios with the available potential of the diverse renewable sources and analyses the performance in respect of optimal scheduling. For this purpose the geographical location of Ramgarh village in Ambala district is selected. The area is located at the height of 275 m from the main sea level, latitude of 30 0 21' N and longitude of 76° 52' E.

Methodology
The proposed hybrid renewable energy system, consisting of PV array, wind turbine, biomass generator and battery storage unit, was modelled considering the system parameters specific to the geographical area considered and simulation was carried out using Hybrid Optimization Model for Electrical Renewable (HOMER) [18] software platform developed by National Renewable Energy Laboratory (NREL), U.S. This software is facilitates the simulation of all possible operating conditions of a system and displays the feasible solution that meets the load requirement in an efficient and effective manner [19].

Mathematical models for renewable energy sources
The hybrid system, investigated in this study, consists of wind turbine, solar PV array, biomass generator and battery storage unit. The modelling of the renewable energy sources was carried out as explained below:

Wind turbine
The specific wind turbine, parameters of which are given in Table 1, is selected and the wind power of the same is computed by the software as per Eq. (1) [20]: Where, A=rotor swept area in m 2 ; ρ=air density in kg/m 3 ; v=wind velocity (m/s); C p =power co-efficient of wind turbine specific to the wind turbine design; β=pitch angle; λ=speed ratio.

Solar PV panel
The specific solar PV array, parameters of which are given in Table  1, is selected and the output power of the array is computed by the software as per Eq. (2) [20]: Where, Y PV =is the rated capacity of the PV array (power output under standard test condition (KW)); F PV =PV derating factor; G T =solar radiation incident on the PV array in current time step (KW/ m 2 ); G T,STC =incident radiation at standard test condition (1 KW/m 2 ); α=temperature co-efficient of power (% °C); T c =PV cell temperature in the current time step (°C); T c, STC =PV cell temperature under standard test condition (25°C).
If temperature does not affect the PV array, so that temperature coefficient of power is zero, this equation can be written as:

Biomass generator
The hourly electrical power output of biomass gasifier system depends on biomass availability and generating operating hours per day. Biomass output is represented as per the formula given under [ Where, P BMGS =hourly output of biomass generator system; CV BM =calorific value of biomass; η BMG =overall conversion efficiency of biomass generator system.
Annual energy production of a biomass gasifier based system can be estimated as:

Load demand assessment
The load demand assessment is based on the assumption that there are various types of loads to be catered such as domestic load, commercial load, agriculture load, school load and street light load etc. The assumed electrical load consumption pattern for 24 hours for each month is shown in Figure 2. It can be observed from the figure that load demand is varying over a period of 24 hours and is the maximum in the night hours.

Solar energy resource
The solar radiation data for the area under study has been downloaded from NASA website [22]. Hourly average solar radiation for each month is shown in Figure 3. It is observed from the figure that solar radiation is obtained more during the period from February to May and in the month of October whereas, less solar radiation is available during the period of June to September, and the months of November, December and January.

Wind energy resource
The wind speed data for the study area has been downloaded from NASA website [22]. The hourly wind speed (measured in m/s) data for each month is shown in Figure 4. It can be seen from the figure that the highest wind speed is obtained in the months of April, May and June.

Hybrid system component parametric details
The details of the parameters of PV module, wind turbine, biomass generator, battery, converter etc. constituting the proposed hybrid system is given in Table 1. The life period and annual interest rate of this proposed hybrid renewable energy system are assumed to be 25 years and 6% respectively.

Results and Discussion
Optimized results are obtained through simulation using the HOMER software for the following three scenarios:    (i) When Biomass generator is scheduled depending upon the solar and wind availability for each month.

Rotor
(ii) Biomass generator is forced on all time.
(iii) Biomass generator is optimized with the system requirement.

Scenario-I
In scenario-I, biomass generator is switched on or off depending upon the load requirement and availability of solar radiation and wind speed. The generator on-off time for each month is shown in Figure 5. Figures 6 and 7 show the simulated results, in terms of share of each resource in the total generated power month wise. The quantitative comparison of the three scenarios is given in Table 2 with system configuration, cost economics, power production, biomass generator utilization and emissions as the performance parameters. In scenario-I, the share of power generated by solar PV array is the highest among all sources as is evident from the Table 2 where power generated by PV, wind and biomass generator is 115,619 KWh/yr, 20,596 KWh/yr and 62,222 KWh/yr respectively.

Scenario-II
In this scenario, Generator is forced on all time irrespective of availability of solar radiation and wind speed. Figures 8 and 9 show the simulated results, in terms of share of each resource in the total generated power month wise. In scenario-II, the share of power generated by biomass generator is the highest among all sources as is evident from the Table 2 where power generated by PV, wind and biomass generator is 19,270 KWh/yr, 1,373 KWh/yr and 107, 397 KWh/yr respectively.

Scenario-III
In this scenario, the renewable energy sources which constitute the hybrid system under investigation are scheduled in optimized manner as per requirement. Figures 10 and 11 show the simulated results in terms of share of each resource in the total generated power month wise. In this scenario, the share of power generated by biomass generator is the highest among all sources as is evident from the Table  2 where power generated by PV, wind and biomass generator is 57,810 KWh/yr, 4,119 KWh/yr and 74,580 KWh/yr respectively. From Table 2, it is noticed that the number of PV panel and wind turbine units are maximum for scenario-I as compared to the other two scenarios. In scenario-I, not only the NPC and LCOE are high but the total energy production, excess electricity and unmet load in KWh/yr are also highest among all scenarios. Whereas, in scenario-II, the biomass generator fuel consumption is the highest along with the CO 2 , NO 2 , CO and hydrocarbons emissions also being the highest in scenario-II. Net present cost is one of the most important parameters for any system. However, cheaper system may not always be a preferred solution. Other parameters like biomass consumption, generating hours, pollutant emissions etc. can be considered important while selecting the best suitable system. Depending upon the priority consideration the choice of suitable scenario can be made as the potential solution.

Sensitivity analysis
Sensitivity analysis has been performed for different maximum annual capacity storage in relation to change in net present cost and levelized cost of energy. The impact of variation in maximum annual capacity storage on net present cost and levelized cost of energy is           shown in Figures 12 and 13. As is evident from the figures, net present cost and levelized cost of energy get decreased by increasing maximum annual capacity storage for all the three scenarios considered.

Conclusion
Solar and wind energy could be the potential solution to cater to the increasing load demand, scarcity of fossil fuels and green house effect. However, the main limitation of solar and wind energy is the higher cost of energy production. This problem can, to some extent, be overcome by proper scheduling and time utilization for solar and wind energy according to their availability. There are very few works reported on the scheduling concept in this area. This paper, in that scope of work, has investigated and presented the comparison of the different scheduling scenarios for hybrid renewable energy system in terms of system configuration, economy, power production, biomass availability and pollutant emission. As per the results of this study, from the economic point of view, scenario-II is considered to be most feasible as net present cost and levelized cost of energy are least in this scenario but biomass consumption is very large. From the emission point of view, scenario-I is found to be the most feasible solution as CO 2 , NO 2 , CO and hydrocarbons emissions are the least in this scenario. Sensitivity analysis has been performed w.r.t. varying maximum annual capacity storage vis-a-vis the change in net present cost and levelized cost of energy and it is observed that increase in maximum annual capacity storage results in decrease in the net present cost and levelized cost of energy.