Renewable energy based microgrid system sizing and energy management for green buildings
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The objective of this paper is to model a hybrid power system for buildings, which is technically feasible and economically optimal. With a view to promote renewable energy sources, photovoltaics and wind turbines are integrated with the grid connected building. The system is modeled and the optimal system configuration is estimated with the help of hybrid optimization model for electric renewables (HOMER). The logic is illustrated with a case study based on the practical data of a building located in southern India. This building is associated with 3.4 MWh/day priority load (peak load as 422 kW), as well as 3.3 MWh/day deferrable load (peak load as 500 kW). Sensitivity analysis is performed to deal with uncertainties such as the increase in electricity consumption and grid tariff, environmental changes, etc. From the simulation result, it is observed that the designed system is cost effective and environment friendly, which leads to 6.18 % annual cost savings and reduces CO2 emissions by 38.3 %. Sensitivity results indicate that the system is optimal and adaptable in a certain range of unanticipated variances with respect to best estimated value. Finally, an energy management strategy is developed for the optimal system to ensure reliable power during contingency and disturbances. The green and hybrid power system designed can be adaptable to any critical and large consumers of urban buildings.
KeywordsRenewable energy sources (RES) Hybrid power systems (HPS) Photovoltaic (PV) Wind turbine (WT) Low-carbon electricity Energy management Total net present cost (TNPC)
India, a dwelling place for 1.21 billion people, which is over 17 % of the world’s population, has a seemingly unquenchable thirst for energy. One blaring result of its meteoric growth is the widening gap between the energy demand and the supply; therefore, the government is paying increased focus to bridge the gap by capacity addition. In chorus, attention is being paid to that the growth should be in a sustainable manner while addressing the climatic change concerns. In 2008, with the announcement of the National Action Plan on Climate Change (NAPCC), there is a marked shift in policy to diversify the energy mix to the lower carbon intensity . Besides, low-carbon generation systems bring competition in electricity markets and are in turn improves the country’s sustenance and economy . Nowadays, the country is promoting renewable energy to augment the total power supply and to meet the rural needs either by augmenting grid supply or by off-grid supply.
The statistics show that approximately 38 % of the total electric energy production is being consumed by the industrial sector, which is playing a crucial role in the economic growth of the country. With India emerging as an IT/BPO hub, erratic power supply hampers the business process, which ultimately reflects on the country’s economy. Besides, cross subsidies, higher tariff, power outages, load shedding and inconsistent quality of power supplied by utilities are other factors that adversely affect such industries. With the demand far outstripping the electricity generation in the country, many industries are now looking at alternatives based on the guidelines and roadmaps given by various forums and government bodies for clean energy generation [3, 4]. The solution as envisioned by many organizations is to set up captive power plants for electricity generation or diesel generator set as a back-up, which is again not a clean energy.
In this framework, the hybrid and green power system uses renewable energy resources that are available at the site of use and are used to produce electricity in and around the premises of the building in an economic way. An energy management strategy is developed, which ensures uninterruptable power supply to all the priority loads connected to the building. Optimization result shows that the system is cost effective and reliable with less carbon emission. Sensitivity analysis shows that the system performance is nearly optimal in all scenarios.
1.1 Literature review
Below literature indicates some of the related works carried out in the design of low-carbon electricity systems. Various studies were done on microgrid formation for a particular combination of distributed units in a location. This paper deals with the formation of microgrid with various combinations of distributed resources to achieve a sustainable design for the low-carbon power system. Further, it compares all the combinations in terms of cost and carbon emissions to select best sized microgrid for a specific case study. Finally, optimum energy usage is achieved by the design of energy management strategy.
Reference  identified and suggested some insights for the people live in Germany and USA to choose and develop low-carbon electrical power generation systems. It also presented the advantages of low-carbon initiatives and crisis with the nuclear based generation systems briefly. Reference  presented the design of low-carbon optimum evaluation model by using multi-objective function to improve the efficiency of carbon footprint calculations. This considers dealer manufacturing capacity, green-house gas values, product components transport modes, design phase costs, as well as the product decisions made by enterprises. Reference  presented the scenarios and consideration of combined nuclear and renewable based energy systems design in Japan in 2030 to adhere the country’s policies of CO2 emission reduction. The safe and clean electricity was analyzed for “smart control” on both demand and supply sides. Reference  presented whole-systems method to appreciate the role of grid-scale electricity storage by concurrently optimizing investment into network and storage capacity, new generation. This minimizes system operation cost by considering system’s reserve and security constraints. Reference  presented an integrated methodology that considered demand responses and renewable generation as inputs for the distribution system planning to make the system as a low-carbon sustainable system. This method optimized the allocation of renewable/non-renewable units and smart metering units simultaneously to minimize total carbon emissions cost. Similar integrated model for low-carbon power generation in China was presented in . Reference  presented a comprehensive model to study low-carbon power system dispatches. The operating mechanisms and characteristics were analyzed by considering feasible power output limits, rates, and the carbon emissions. Reference  presented a generation output model to evaluate the power dispatch problems under low-carbon and economy systems development. In [13, 14, 15, 16], similar analysis was carried out for the design of low-carbon electricity systems and study on their impacts on the energy crunches for different cases.
Reference  presented the design of hierarchical control for microgrids. Energy management system was designed to achieve a reasonable balance between generation and battery usage. However, the system was developed only for island mode but not for grid tied mode. Reference  presented the dynamic behavior of a mixed system consisting of a wind farm and a diesel group supplying a load, under different disturbances. Reference  presented two droop control strategy for distributed generation in interconnected grids. Reference  presented a simulation model to investigate specific hybrid architecture, which is based on the use of DC bus, DC/AC and DC/DC converters for power sources. This increases number of converters usage and leads to high cost and poor quality. Reference  had done an optimal cost analysis of hybrid renewable system using hybrid optimization model for electric renewables (HOMER). The real time cost analysis and system optimization was done based on the load profile, solar radiation and wind speed. Reference  and  described the integration of wind power into traditional diesel-only power system, which reduced greenhouse gases and thereby global warming. The wind-diesel power plant was designed, and the operational and economic analysis were compared with traditional diesel system using HOMER. In , the distributed energy resource system was designed and simulated. The cost analysis was done by calculating net present value. Reference  presented microgrid with micro-controller based energy management unit. Hydro generators, PV panels, and diesel generators were integrated to study the generation patterns under various ecological conditions. Reference  presented a voltage and frequency control strategy for active island microgrids design. There are few literatures available on the study of economic and technical feasibility of microgrids and hybrid systems in [27, 28, 29, 30, 31, 32, 33]. However, most of these were designed for standalone systems of remote areas, where it is either difficult or impossible to extend the grid service. And these studies considered the loads in a nonspecific manner. This paper focuses on the grid-connected HPS developed for a practical case study.
2 Hybrid power based microgrid description
3 Case study description
In this case study, the considered building is an organization located at Hyderabad, India. The area enclosed by the building is about 125000 sq ft. The existing architecture of the building is that it is connected to the utility grid that serves the entire load connected. A diesel generator set was also installed to meet certain portion of the load at the time of grid outage. The new proposed architecture of the system is that the electric energy produced from the PV and WT augments the grid supply to meet the demand. A converter is connected to convert the DC power produced by the PV array to AC, as the entire load served is the AC type. A 396 V battery bank is also connected to the DC bus, which can store energy and be used as a backup energy source. The system is modeled, and the best configuration of hybrid system is estimated by using HOMER [34, 35]. The profiles of load demands and energy resources are given as follows.
3.1 Load profile
Categorized breakdown of building loads
Total energy consumption/%
3.1.1 Priority load
3.1.2 Deferrable load
3.2 Solar and wind resources
4 Hybrid power system modeling and simulation
4.1 Introduction to HOMER and system simulation
Microgrid architectures with different combinations of distributed resources are developed for the performance analysis in each hour of the year to determine its technical feasibility and life-cycle cost. Energy balance calculations are performed based on the system architecture consisting of different combinations of the components. This paper chooses PV, WT, diesel generators, battery, and converter as the elements of HPS and the best feasible configuration is determined that can adequately serve the load.
4.1.2 Optimization process
4.1.3 Sensitivity analysis
As the TNPC varies based on sensitivity variables chosen, in this analysis, multiple optimizations are performed to measure the effects of the uncertainties such as wind speed, solar radiations, grid tariffs, and fuel prices on the system performance. Hence, the HPS configuration has to be chosen to tolerate all these situations.
4.2 Microgrid system modeling
The microgrid system is formed as an interconnection of various distributed units such as PV, WT, utility grid, diesel generator, converter, and battery that are modeled as shown below. It is important to know the technical and economic detail of these units precisely to get an accurate simulation result. Besides, decisions on system control parameters, constraints, and economic inputs have an equal effect on the simulation results.
4.2.1 PV panel modeling
The range of the PV array rating is permitted to vary between 125 kW and 275 kW to choose the optimal size. The capital cost of the PV panel is 3500 $/kW and after a reasonable percentage of discount on bulk amount, the replacement cost is considered as 3400 $/kW. The subsidy provided for using solar energy was also discounted in the given capital cost. Operation and maintenance cost for the PV array is considered as 0 $. The lifetime of the selected PV array is 25 years with a derating factor of 85 %. Two-axis tracking system is established for the PV panel.
4.2.2 WT power unit modeling
Wind turbine converts kinetic energy of the wind into AC electricity according to a particular power curve. Each hour, power output of the wind turbine is calculated in a four-step process. Firstly, it determines the average wind speed for the hour at the anemometer height by referring to the wind resource data. Secondly, it calculates the corresponding wind speed at the turbine’s hub height using either the logarithmic law or the power law. Thirdly, it refers to the turbine’s power curve to calculate its power output at that wind speed assuming standard air density. Fourthly, it multiplies output power by air density ratio.
The rating of each wind turbine considered is 50 kW, AC. Range of 2 to 5 numbers of such turbines is taken as decision variable. The cost of each wind turbine is 181035 $. The subsidy provided to set up wind firm is deducted in the given capital cost. Operation and maintenance cost of the wind turbine is 4320 $/year/turbine and the total operation and maintenance cost is reduced as the number of turbines increases. The lifetime of the turbine is 25 years and the height of the turbine hub is 40 m.
4.2.3 Battery system modeling
Batteries are used to store excess power in the HPS, and to operate when the system has deficit power. Its life depends on how many times the battery is charge/ discharge per day. Battery bank is formed for 396 V DC bus. The rating of each battery is 12 V, 200 Ah, 2.4 kWh. A string of 33 such batteries are connected in series to deliver the power at a desired voltage level. The cost of each battery is 475 $ and the replacement cost is 430 $. The battery lifetime is 10 years. Value of 0, 1 and 2 strings was entered as decision variable.
4.2.4 Diesel generator modeling
Three diesel generator sets are installed in the existing system (one with a capacity of 380 kW and the second, third sets with a capacity of 1055 kW each). The capital cost is not considered as these are already installed but operation and maintenance cost of 0.002 $/hour was taken into account. Cost of the diesel is 0.9 $/liter. The lifetime of diesel generator set is 15000 operating hour and the minimum load factor is 30 %. In the decision variable, ‘0’ value was also included to obtain the most optimal unit.
4.2.5 Converter modeling
The rating of the converter (working as both rectifier and inverter) is chosen according to the total PV panel output, as the total PV panel output is converted in to AC by the converter. Hence, the size considered for converter is 125 kW to 275 kW, with a cost of 110 $/kW. The replacement cost is 100 $/kW. The lifetime of converter is 20 years with an efficiency of 95 % in both the directions.
4.2.6 Utility grid
In this model, the building is connected to the utility grid, and the building produces maximum demand of 600 kW on the utility grid. The tariff of the electricity charged by the utility is 0.065 $/kWh of energy consumption and demand rate of 5 $/kW/month . The interconnection charge paid to the utility is 8000 $.
4.2.7 System control and constraints
The operating reserve is set at 7 % of the total energy demand and 20 % of each solar and wind power output.
The minimum renewable fraction (RF) is set at 35 %, i.e. at any moment the minimum energy from RES is 35 % of the total energy consumed at that moment.
Maximum annual capacity shortage is set at 0 %.
4.2.8 System economics
5 Energy management strategy
While simulating the proposed system, it was assumed that the grid is supplying rated energy, and RES produces nominal power throughout the project lifecycle, which is not obvious. Again, while perceiving the grid in Indian context, the grid outage and load shedding occur in an unpredictable manner. In this case, the microgrid designer faces a major challenge as to how reliability can manage and coordinate with the energy system. In simple term, managing the energy system means, how smartly use the available energy from all the sources to meet the load reliably [41, 42]. A simplified energy management method is presented here by considering four operation states.
5.1 Normal operating condition
In this state, it is assumed that the grid is supplying required energy and the RES are producing energy as per the nominal rating. In this case, just like the “load following” strategy, the priority loads are served by the grid and the deferrable loads are served by the RES. Excess renewable energy is used to charge the battery.
5.2 Grid outage
This state occurs during load shedding or an occurrence of fault in the transmission or distribution lines so that the utility is unable to serve the load. Simulation result shows that the renewable sources in the system produce 38 % of total energy in a year with minimum renewable fraction of 35.6 %. Thus in this state, the energy produced from the renewable source is fed to the most important priority load (i.e. UPS load which is 32 % of total energy consumption at any time). The excess renewable energy can be fed to other priority loads in the order of lighting, elevator, etc.
5.3 Unavailability of renewable energy sources
Sometimes due to constant poor solar radiation and wind speed less than the cut-in speed of the turbine, the renewable energy sources do not respond. During such period, the grid energy can be used to serve the entire priority load and a fraction of important deferrable load.
5.4 Total blackout
6 Simulation results
Energy management results
Sensitivity analysis results
6.1 Energy management results
6.2 Optimization results
Optimization result of proposed microgrid system
Operating cost ($/year)
Comparison between existing system and microgrid
Grid-only connected system
Hybrid power based microgrid
6.3 Sensitivity analysis results
As there are several sensitivity variables on which the user has no control, several sensitivity analyses were also performed on the optimal system configuration to guarantee that the proposed system configuration is optimal, robust and adaptable in unanticipated variances with respect to best estimated value. Some examples of such variable are environment changes like solar radiation and wind speed variation, variation in the total electricity consumed, grid tariff and demand rate variation, etc.
6.3.1 Effect of solar radiation and wind speed variations
6.3.2 Effect of electricity consumption variations
6.3.3 Effect of utility grid tariff variations
6.3.4 Effect of variation of minimum renewable fraction and shortage capacity fraction
It is considered that the proposed system must able to produce 35 % minimum renewable energy of the total electric energy consumed, which is a user specified constraints and depends on system design. This value can be varied as per the user choice and causes remarkable impact on the TNPC. Similarly, one more constraint imposed during the simulation process is maximum annual capacity shortage. This parameter is set at ‘0’ as the building cannot compromise with reliability or interruption in energy supply. With this assumption, the system configuration is estimated that the system is able to supply peak load, which even occurs for a short interval.
The most economic system configuration is achieved for the combination of 600 kW utility grid contribution, 180 kW PV contribution, and 3 units WT contribution (50 kW for each, 150 kW in total), 33 batteries (1 string) and a 160 kW converter.
The levelized cost of energy of such system is achieved as 0.092 $/kWh which is less than the grid-only connected system.
Besides, the system is environment friendly and beneficial for sustainable development which leads to 6.18 % annual cost savings and reduces CO2 emissions by 38.3 %.
From sensitivity results, it was observed that the system work satisfactorily in a range of varying scenarios such as solar radiation, wind speed and increase in energy consumption.
The energy management strategy developed ensures uninterruptable and reliable supply for the priority load for smooth running of the business.
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