Long-Term Techno-Economic Analysis of Sustainable and Zero Grid Cellular Base Station

Green wireless networking has attracted considerable research attention, especially in academics and industry from economic and ecological perspectives. Promoting wireless infrastructures by exploiting green power sources has the potential to enhance sustainability and address the adverse impact of conventional power sources. A sustainable optimal standalone solar-powered model for green cellular base stations in urban locations of South Korea is proposed in this work to extend 24-hour uninterrupted power supply support to LTE cellular base stations (BSs) and take advantage of integrated storage devices. The optimal system architecture, energy management, and economic analysis are examined using the hybrid optimization model for electric renewable optimization software based on actual prevailing conditions of regions and their technical feasibility. Results showed that the proposed solar photovoltaic system can achieve significant operational expenditure savings of up to 43% and 43.58% in on- and off-grid sites, respectively, and reduce greenhouse gas emissions in the telecommunications sector. Moreover, the results of this study can provide a stronger platform for a sustainable green wireless network paradigm that can ensure energy sustainability compared with conventional technology.


I. INTRODUCTION
Rapid technological advancement has remarkably increased mobile devices/subscribers and data/multimedia applications in the last few decades. Consequently, wireless applications have progressed rapidly and significantly. Telecommunications operators have increased the number of base stations (BSs), which are considered key energy consumption sources in cellular networks and account for 57% of the total energy used, to satisfy high data demand [1], [2]. Energy consumption and operational expenditure (OPEX) have increased significantly due to the substantial growth of The associate editor coordinating the review of this manuscript and approving it for publication was Zhehan Yi .
installed BSs to meet subscriber demands [3], [4]. Therefore, network operators have applied economic energy consumption schemes to reduce the OPEX and increase profit while mitigating adverse environmental issues [5]. However, meeting the energy demand while considering both ecological issues and OPEX savings to attain energy-efficient cellular networks is a challenging task for researchers, academics, vendors, and network operators due to its anticipated financial and environmental impact in the coming years. Accordingly, a new initiative of green communication was created to shift toward energy-efficient cellular networks [1], [2]. The green communication initiative primarily focuses on increasing energy efficiency, curtailing the OPEX, and eradicating greenhouse gas (GHG) emissions. Available renewable energy sources (RESs), specifically solar energy, can be adopted for long-term, cost-effective, and reliable power supply to BSs because sunlight is free, available everywhere, and an effective alternative energy option for remote areas. However, creating a photovoltaic (PV) scheme requires a viability assessment to avoid poor power supply, particularly in BSs. Therefore, cellular operators need to consider both technical and economic factors before the implementation of solar-powered BSs.

A. RELATED WORK
A combination of various RESs or non-RESs with RESs have been proposed to overcome the limitations of a single technology. For instance, the combination of an electric grid with RESs or a single RES with adequate battery storage devices is put forward to power access networks in wireless infrastructures. Table 1 summarizes the different investigations on renewable energy-powered BSs.
The integration of diesel generator (DG) with the RES can overcome single RES-related problems. However, fuel transportation is still challenging in several sites that considerably increase the OPEX apart from toxic gas (CO 2 ) emissions. While hybrid utilization of RES with on-grid is presented to warrant a reliable power supply to the BSs. Optimal conditions, key challenges, and viable solutions are suggested to extract the maximum power from RESs to reduce the grid pressure. However, utilization of power from the electric grid, that is, conventional power production, such as burning fossil fuels, extensively generates GHGs and increases global warming. Therefore, the hybridization of various RESs is proposed. The commissioning of RES-powered BSs is based on the availability of wind profile/solar irradiation that relies on latitude, seasonal disparities, and ecological circumstances.
The desired zone for PV-powered BSs is located in the midlatitude between 30 • N and 30 • S. Notably, low latitudes are highly profitable regions for PV-based BSs. On the one hand, preferred locations for PV-based cellular networks in the world are the western coast of the USA, northern coast of South America, Mediterranean littoral, southern part of Africa, northwestern part of India, and eastern coast of Australia. On the other hand, wind energy-based BSs are commonly installed in mountainous and coastal parts. Considering these factors, wind energy-based BSs can be positioned in the northeastern coast of North America, southern area of South America, England, northern area of Europe, northwestern part of India, southern coast of China, and coastal area of Japan [6].

B. CONTRIBUTIONS
This work investigates the plausibility of using the solar PV system as the principle power source to meet the energy demand of macro LTE-BS 2/2/2 in metropolitan cities of South Korea.
South Korea is positioned at a latitude between 34 • N and 38 • N. The geographical report of these locations showed excellent potential for solar energy production with an average daily solar irradiation ranging from 2.474 kWh/m 2 to 5.622 kWh/m 2 between December and May [7]. Wind turbines are excluded from this study because South Korea demonstrates low wind potential at an average of 4 m/s [7], [8]. Moreover, a standalone solar PV system together with adequate storage devices is preferable for low-rated energy demand applications, such as cellular BSs.
Studies on solar PV systems remain unclear due to their uncertain parameters and diverse design options. Furthermore, the high complexity of solar irradiation is due to its intermittent, seasonal, and uncertain nature. The application of the HOMER model mitigates these challenges by creating the energy balance scheme for every one hour of 8,760 hours per year [9]. HOMER relates the actual electric demand of the load for every one hour with energy generation, computes energy transactions between each component of the scheme, and governs charging and discharging characteristics of batteries. Furthermore, this model determines the feasibility of the configuration on the basis of energy balance under definite conditions and installation and operating costs of the system throughout the lifespan of the entire scheme. Therefore, HOMER is used in this work to examine the techno-economic viability of the solar-powered macro LTE-BS 2/2/2.
Significant contributions of this study are summarized as follows: • Proposes and determines technical benchmarks of an optimal standalone PV system that guarantees energy autonomy.
• Obtains long-tenure energy balance for cellular networks on the basis of available solar irradiation in metropolitan cities of South Korea that warrant sustainable green wireless networks • Examines, analyzes, and evaluates the viability of a standalone PV system for maximum energy yield and economic savings that require both sustainability and cost effectiveness • Assesses the influence of adopting a standalone PV system over existing works while considering OPEX savings

C. PAPER ORGANIZATION
The remainder of this work is prepared as follows. Section II describes the proposed system. Section III presents the cost model and optimization formulation. Section IV presents the methodology and simulation configuration. Subsequently, the results and discussion are presented in Section V. Further, the economic feasibility of the proposed system is given in Section VI. Lastly, Section VII concludes the work. Fig. 1 illustrates the two subsystems of the proposed architecture: (i) the cellular macro LTE-BS 2/2/2, and (ii) the solar power subsystem (PV array, battery bank, inverter). A set of standalone solar PV panels arranged in series and parallel connections based on voltage and current ratings functions as an energy source of the proposed system. A battery bank comprising a number of small cells organized in a series/parallel fashion and accompanied by an intelligent energy management system forms the storage device for reserving surplus electricity from the PV array. Notably, the use of a storage device guarantees the desired system reliability and power quality of the generated source. Hence, excess energy stored in a battery energy storage system (BESS) can be utilized during non-sunny periods, especially at night (load shedding hours). DC bus maintains the constant voltage (e.g., 48 V for the proposed model) and supplies power to the cellular macro LTE-BS 2/2/2. A DC/AC converter is used to power to AC for the AC load (Air Conditioner). The following subsections demonstrate the architecture of the solar-powered macro LTE-BS 2/2/2 in detail.
TRXs comprise of PAs that augment the signal power coming from the BB unit, which is adopted for internal processing and coding. The detailed block diagram of macro LTE-BS 2/2/2 hardware elements is shown in Fig. 1. A detailed discussion of BS components are referred to in [26], [27].
The power consumption of the BS is expressed as follows [26], [27]: where N TRX represents the number of transmitting/receiving antennas for individual sites, that is, transceivers, and P DC PA , P DC RF , and P DC BB represent the power amplifier, radiofrequency, and BB power, respectively. Power loss factors are approximately σ DC = 6 % and σ cool = 10 % for converters and air conditioners, respectively.
Typically, the most efficient PA operating point is close to the maximum output power (P max tx ) (near saturation). Unfortunately, nonlinear effects and OFDM modulation with non-constant envelope signals force the power amplifier to operate in a more linear region (6-12 dB below saturation). This prevents adjacent channel interference due to nonlinear distortions, and therefore avoids performance degradation at the receiver. However, this high operating back-off gives rise to poor power efficiency (η PA ), which translates to an increased PA power consumption P max tx /η PA . The output power of the BS is normally influenced by the range of coverage radii (R o ) and signal propagation fading. The macro-BS VOLUME 9, 2021 transmission power is normalized to P o = 40 W with a coverage R o of 1 km to establish a simple derivation model. Furthermore, the BS output power considering coverage radii is computed using the relation P max where α defines the loss coefficient of the path. Finally, the operating power of the BS with coverage radii R is reformulated as The power utilization scale of various parts of the macro LTE-BS 2/2/2 system with 2×2 multi-input and multi-output (MIMO) antenna arrangement with three sectors is presented in Table 2.

B. SOLAR PV SUBSYSTEM
The solar PV subsystem comprises several types of equipment that generate green energy effectively for the entire system, achieves energy savings, and allows ease of dismantling for recycling.

1) PV ARRAY
The PV array consists of numerous solar cells connected in series and parallel and generates DC electric power via absorption of shortwave irradiance [28], [29]. The total annual energy of the PV array (E PV ) is calculated as [9] where PC PV represents the peak capacity of the PV array (kW); PSH represents the peak sun hours or peak solar hours and is calculated on the basis of the equivalent average daily solar irradiation; and DF PV is the derating factor of the PV array that represents the influence of dust, losses, temperature variations, and added potential issues that can reduce the output power of the panel.

2) BATTERY BANK
A battery bank or BESS stores the excess power generated by PV arrays during the day for use at night or during bad weather conditions to prevent outages. The BESS capacity of the BS merely depends on the depth of discharge (DOD), which must be evaluated before the commissioning. The DOD can be expressed as where SOC min represents the minimum state of charge. The Trojan L16P battery model considered in this study demonstrates a DOD of 70%, which indicates that 70% of the energy can be shared and 30% can be used for the critical condition. Days of autonomy (A batt ) must be computed to determine the performance of fully charged batteries, that is, the number of days fully charged batteries can supply power to the load without any influence of supplementary power sources. A batt is derived using HOMER as follows [9]: where N batt and V nom are the total quantity of battery units BESS and the nominal voltage of a single battery unit (V), respectively; Q nom represents the nominal rating of a single battery (Ah); and L prim,ave refers to the nominal rating of the average daily load of macro LTE-BS 2/2/2 load (kWh). The total cost of solar-powered BS highly depends on the cost of batteries. Therefore, the lifetime of the battery plays a crucial role. The lifetime of a battery can be predicted on the basis of its operational settings. Specifically, the DOD during every diurnal charge-discharge cycle demonstrates an important part of the battery lifetime. R batt can be computed using HOMER as follows [9]: where Q lifetime represents the lifespan throughput of an individual battery in kWh, Q thrpt represents the annual battery throughput in kWh per year, and R battf represents the battery float life in years. The number of series-connected batteries is computed between the ratio of DC busbar voltage (V b−b ) and voltage rating (B V ) of a single battery that can be expressed as The total number of parallel paths can be determined between the ratio of the total number of batteries adopted and the total number of series-connected batteries.

3) INVERTER
The output power from the PV system is the DC, which can be converted into usable 220 V AC voltage with nominal frequency using an inverter that feeds the power to the AC load, such as the cooling system of the BS. The total capacity of the inverter (C inv ) is calculated as follows [30]: where L AC represents the available maximum AC load, η inv represents the inverter efficacy, and σ sf represents the safety factor.

III. COST MODEL AND OPTIMIZATION FORMULA
The configuration of the solar-powered BS is based on the following considerations: • What are essential components involved in the complete design?
• How many components need to be used? • Size of each element Selecting energy resources from various sources is difficult due to the many technology options. The HOMER software is an effective platform established by the U.S. National Renewable Energy Laboratory to simplify the modeling process of solar-powered systems and evaluate the maximum number of possible system configurations [9]. Furthermore, the HOMER micropower optimization tool aids in obtaining the optimum PV system with low net present cost (NPC). The NPC term contains all incurred expenses and income throughout the project lifetime [9].
The total annualized cost (C TAC ) demonstrates the annual price of the complete scheme in $/year that contains the initial capital (C The net present cost (C NPC ) can also be described for the annualized value as where C NPC represents all prices that incur within the scheme lifespan but with impending cash flows cut-rate to the current discount ratio. The NPC comprises the initial capital (IC), replacement, and O & M costs. However, the total NPC value is reduced due to the salvage value, particularly at the end of the venture lifespan. Capital recovery factor (CRF) denotes the recovery factor that converts C NPC into the flow of equal annual costs over a definite period and can be calculated on the basis of the annual interest rate (i) and project lifespan (N ) as follows: where N is equal to 10 years and i is 0.5%. The salvage value (S) is calculated as where R comp represents the lifespan of the component in years, R rem represents the remaining lifespan of the component in years, and C rep represents the replacement rate of the component in $.
This study aims to decrease the total cost of NPC because the optimal scheme of a standalone PV system depends on various constraints. The objective function of NPC is defined as where E BS is the annual BS load consumption and E PV is the total annual energy of the PV array subjected to the (i) number of PV panels; (ii) derating factor, which represents the influence of dust, losses, temperature variations, and added potential issues that can distress the output power of the panel; and (iii) peak sun hours, as presented in Eq. (3). E Battery is the energy afforded by the battery bank. The Trojan L16P battery model is used in this study because its DOD is 70%. Hence, this battery can deliver 70% of its energy effectually with 30% of its energy reserved for use in the critical condition. If E PV is higher than the required BS load with losses, then the excess energy will be stored in the battery bank. If E PV is lower than the required BS load with losses, then the battery bank will compensate for the shortage in energy and the maximum energy allowed for sharing is 70% of the total energy of the battery bank.
Power shortages must be prevented in the cellular network sector. The output power of the PV system must not constantly be larger than zero because of the absence of generation at night. Thus, the constraint (13.1) guarantees that the energy generated by the PV and battery must be higher than zero. In addition, the supply, which (energy output of

IV. METHODOLOGY AND SIMULATION CONFIGURATION
The algorithm of the proposed system is simplified in the flowchart depicted in Fig. 2. The standalone solar-powered cellular system is defined as the optimization problem with an objective function of the minimum NPC subjected to different design conditions. The operation of the methodology can be categorized into the following steps. First, if the green energy (E PV ) harvested from the on-site installed solar panel can sufficiently handle the BS demand and associated losses, then energy will not be stored in the battery bank and without deficit. Second, the additional solar energy can be accumulated when E PV is higher than the required BS load. Note that total losses include battery and converter losses because devices are non-ideal. The energy conversion from the DC power to feed the AC load incurs some conversion losses and the battery charging-discharging phenomenon causes the battery loss. By comparison, the storage device supplies backup electricity when the harvested solar energy is insufficient due to abnormal conditions. Moreover, the surplus electricity should not exceed the maximum battery storage capacity and the excess electricity becomes equal to zero when energy VOLUME 9, 2021   production balances the energy consumption. By contrast, the deficit energy can be computed when the cumulative contribution of E PV and storage discharging (E disch ) are lower than the required BS demand that includes losses. However, energy deficiency is zero under the balance condition, that is, E PV + E disch can handle the total load demand for specified 54166 VOLUME 9, 2021 network settings. Note that HOMER determines every hour whether to cater to the BS load demand, including losses, by calculating excess and deficit energy values throughout the duration of one year. Hence, HOMER computes the minimum component combinations to meet the load demand and ensure zero energy shortage under each iteration. Finally, the minimum NPC is calculated on the basis of the total annualized cost and capital recovery factor among a number of iterations.
• Monthly average solar irradiation values (Fig. 3). 2) Definitions of economic value, system value, project lifespan, and interest rate, as provided in Table 3.
• Configuration of the range and sizes of components, such as PV, battery, and inverter.
• Application of the cost information for individual components, such as IC, replacement, O&M.
• Other technical restraints, such as lifetime and efficiency, as listed in Table 3.

3) Optimization:
The HOMER software performs an hourly simulation for all potential arrangements by calculating the accessible energy from the PV arrangement (E PV ), matching it with the available electric demand (E BS ) and losses (E losses ), and managing the surplus PV power during excess generation (battery charging) or producing extra energy during deficit (battery discharging). Finally, HOMER sorts all possible combinations to increase the NPC, which characterizes the lifecycle cost of the scheme.

4) Outputs:
The investing, maintenance, and auxiliary (replacement) costs and the salvage value of each component are used to calculate TAC by adding annualized costs and the economic index of individual components. TAC is used to calculate the total NPC value.

V. RESULTS AND DISCUSSION
A brief comparative study of the proposed solar-powered macro LTE-BS 2/2/2 for main metropolitan cities of South Korea is presented in Table 4. Optimal size measures, energy harvest, and economic investigation for Seoul City is discussed thoroughly in the following subsections as the case study in this work. Seoul is the capital and largest city in the country with a high population. This investigation can be extended to include other metropolitan cities with small variances in daily peak sun hours (solar irradiation).

A. ENERGY YIELD ANALYSIS
The total NPC cost of the solar-driven macro LTE-BS 2/2/2 comprising 6.0 kW-rated PV panels and 64 numbers of batteries is economical at $28,623. Batteries are connected to eight parallel strings along with a 0.1 kW inverter. The yearly energy output of the PV array is calculated using Eq. (3). The total energy output of the PV arrangement is calculated at 8,656 kWh (6 kW × 4.65 h × 0.85 × 365 days/year). The generating energy may increase to a maximum of 27%, that is, 10,993 kWh more annually due to the use of a dual-axis tracking system. This energy yield also meets losses incurred in the system, including BESS and inverter losses of approximately 787 and 42 kWh, respectively, and supplies the power to load (7,972 kWh) the BS with an annual excess energy of up to 2,192 kWh, which is 19.93% of the total energy generation. The monthly average energy generation by the PV system is illustrated in Fig. 4. The maximum and minimum energy generation occurred in February and August, respectively. Monthly disparities occur largely due to the elevation angle shift of the sun. In addition, the extended spell of rainy meteorological conditions in early summer remarkably reduces the global horizontal irradiance in August. The average hourly energy generation of the PV, BESS, and excess electricity for 12 months is presented in Fig. 5. The initial phase of August shows a low rate of energy contribution in the PV system. Therefore, the energy stored in the BESS condensed into the minimum scale, and the SOC stretched to 44%, as shown in Fig. 6. Seasonal statistics of the maximum and minimum SOC are illustrated in Fig. 7. The maximum energy influence of the BESS occurred in August due to the minimal energy contribution of the PV array.
The annual energy output and input of BESS are 4,458 and 5,245 kWh, respectively, with a battery roundtrip efficiency of 85%. Moreover, the BESS supplies power to the load for approximately 106 h during the malfunction of the PV system.
The net capacity of the inverter unit is 0.1 kW, and its efficiency is computed between the input (837 kWh) and output (795 kWh) energy annually at 95%. The total operating hours are 8,759 hours/year (24 hours × 365 days/year).

B. ECONOMIC ANALYSIS
The cash flow summary for Seoul City throughout the project lifespan is presented in Fig. 8. The breakdown of the cash flow summary is presented as follows. The BESS demonstrates a higher ratio from the capital cost compared with other components but depends on the sum of individual batteries used for the arrangement. HOMER computed the optimal number of batteries at 64 in this configuration. Reducing the total number of batteries is possible, but it reduces the load autonomy, which is a serious concern.
• Replacement costs are excluded due to the short operational lifespan of the project (10 years) and the long lifespan of the BESS, PV arrays, and inverter of 10, 25, and 15 years, respectively.
• The salvage value of each component at the end of the project lifespan must be considered. Eq. (12) is used to compute the salvage value of the PV array at $3,425, which is the highest value among all components. The salvage value of the inverter is estimated at $13. The total salvage value at the end of the venture lifespan is $3,438.
The net NPC is $28,623, that is, $25,240 (IC) + $6,821 (O&M)-$3,425 (salvage). This investigation can be extended to other schemes with system costs dependent on individual component sizes. The array of PV panels rated 6.0 kW is considered using 24 Sharp ND-250QCs modules (polycrystalline) [36]. The voltage (V pm ), current (I pm ), and power (P pm ) of the system are 29.80 V dc , 8.40 A, and 250 W, respectively. Four series and six parallel modules (24 Sharp ND-250QCs) are connected to achieve compatibility with solar control regulator (Solarcon SPT-4830) specifications [37]. The open-circuit voltage of the SCR (V SPT −4830 oc ) is 192 V dc , which is greater than that of the PV panel (153.2 V dc with four PV modules in series × V ND−250QCS oc 38.3 V dc ). The voltage rating and capacity of the single Trojan L16P battery are approximately 6 V dc and 360 Ah, respectively [35]. Sixty-four batteries are arranged in eight series and eight parallel to obtain the required ratings.  The inverter rating must be capable of handling 0.1 kW. An LS Drive/M100 model with a capacity of 0.1 kW, 12/24/48 V dc input voltage, and output voltage (220/110 V ac ) is considered in this study. An AC output power with a frequency of 50/60 Hz with pure sine waves is obtained.

VI. ECONOMIC FEASIBILITY OF THE PROPOSED SOLAR POWERED MACRO LTE-BS 2/2/2
Mobile operators primarily aim to increase their profit with reduced OPEX in cellular networks. The economic viability of the PV system with conventional energy resources is as follows: • The net energy cost spent for the macro LTE-BS 2/2/2 due to the utilization of the electrical grid for 10  • IC costs are computed by multiplying the system size of 3.5 kW with its cost of around $660/kW.
• The O & M cost (annual) of the DG is approximately $4,150 (excluding the fuel transportation cost), which includes the following: -The net maintenance cost of DG is $438/year, which is estimated using the product of DG maintenance cost ($0.05/h) with annual operational hours (8,760 h). -The total fuel cost of $3,712 is computed using the product of the diesel price ($1.04/liter) with the total diesel consumption (3,569 liter/year). This calculation is based on specific fuel consumption (0.388 liter/kWh) × annual electricity generation by the DG (9,198 kWh/year, that is, the product of DG size [3.5 kW] and its efficiency [0.3 × 24 hours × 365 days per year]). Therefore, the net O&M cost for the complete plan lifespan is estimated at $41,496.
• A cellular operator must replace the DG every three years, that is, a minimum of three DG replacements during the lifespan of the scheme. Therefore, the net replacement rate is equal to $6,930 (3 × 3.5 kW × $660/kW). The net NPC of the solar-powered macro LTE-BS 2/2/2 is approximately $28,623. Compared with conventional power sources, the total OPEX savings of 43% and 43.58% can be achieved in on-and off-grid areas, respectively, by applying the proposed solar-powered macro LTE-BS 2/2/2 scheme.

VII. CONCLUSION
A comparative analysis of the proposed solar-powered macro LTE-BS 2/2/2 for main metropolitan cities of South Korea is performed in this study to minimize the OPEX. The following key aspects are highlighted in this study: (i) optimum system architecture, (ii) energy yield analysis, (iii) implementation and technical criterion, and (iv) economic analysis. The simulation results revealed that the proposed PV-based system can meet the total demand of macro LTE-BS 2/2/2. Moreover, the BESS can supply essential power to the macro LTE-BS 2/2/2 load autonomy for 106 h, which is considered sufficient time to fix the solar array in case of malfunctions. The simulation results showed that the proposed solar-powered system can significantly reduce the OPEX. These outcomes demonstrated that the PV-based energy system can be a superior alternative for telecommunications providers.

APPENDIX
A list of abbreviations used in this paper have been shown in Table 5.
A list of symbols used in this paper have been shown in Table 6.