Techno-Economic Comparative Analysis of Renewable Energy Systems: Case Study in Zimbabwe

Fluctuations in fossil fuel prices significantly affect the economies of countries, especially oil-importing countries, hence these countries are thoroughly investigating the increase in the utilization of renewable energy resources as it is abundant and locally available in all the countries despite challenges. Renewable energy systems (RES) such as solar and wind systems offer suitable alternatives for fossil fuels and could ensure the energy security of countries in a feasible way. Zimbabwe is one of the African countries that import a significant portion of its energy needs which endanger the energy security of the country. Several studies in the literature discussed the feasibility of different standalone and hybrid RES either with or without energy storage systems to either maximize the technical feasibility or the economic feasibility; however, none of the studies considered maximizing both feasibilities at the same time. Therefore, we present a techno-economic comparison of standalone wind and solar photovoltaic (PV) in addition to hybrid PV/wind systems based on maximizing the RES fraction with levelized cost of electricity (LCOE) being less than or equal to the local grid tariff where Gwanda, Zimbabwe, is the case study. The methodology suggested in this study could increase the utilization of renewable energy resources feasibly and at the same time increase the energy security of the country by decreasing dependency on imported energy. The results indicate that the PV/wind hybrid system does not only have the best economic benefits represented by the net present value (NPV) and the payback period (PBP), but also the best technical performance; where the maximum feasible size of the hybrid system-2 MW wind and 1 MW PV-has RES fraction of 65.07%, LCOE of 0.1 USD/kWh, PBP of 3.94 years, internal rate of return of 14.04% and NPV of 3.06 × 106 USD. Having similar systems for different cities in Zimbabwe will decrease the energy bill significantly and contribute toward the energy security of the country.


Introduction
Environmental protection, energy resources conservation, and sustainable energy development are the core challenges that the world is facing nowadays [1] especially in hard times when crises have prevailed [2,3]. Moreover, given that the demand for electricity or rather energy is continuously carried out for a remote area of Sahel Zone in Cameroon by [49,50] did an assessment for another location in Cameroon as well and determined that the total energy produced annually could reduce carbon dioxide emissions by 1200 tons per year. Their calculated results and those of the Wind Atlas Analysis and Application Program (WAsP) were in good correlation [50].
The goals and objectives of the Zimbabwe National Renewable Energy Policy (NREP) included the installation of 1100 MW of renewables by 2025 and 2100 MW of renewables by 2030. Renewables in this context referred to grid-connected solar PV, grid-connected wind, small hydro and bagasse. The policy reported the provision of tax and sale of power to third-party incentives by the government and also reduced license fees for renewable energy projects. However, the policy does not outline any possible feed-in tariffs for renewable energy resources which might still make investments unattractive. Additionally, the policy still does not mention any development of stand-alone microgrids to electrify remote areas in which the rural electrification rate is only 13% [51]. The NREP also reports that the Zimbabwean government will introduce mechanisms for funding renewable energy systems as well as implementing a renewable energy technologies program that encourages Independent Power Producers (IPPs) to invest in renewable energy projects in Zimbabwe. Additionally, a fund is to be established by the Ministry of Energy to promote solar energy to address the electricity crisis.
All of the papers reviewed for this study did not compare the economic and technical feasibilities of wind, PV and PV-wind hybrid system at the same location in order to determine the most suitable system and the most profitable one. Moreover, none of the studies in the literature considered maximizing both the technical and economic feasibilities of different standalone and hybrid PV/wind systems in Zimbabwe at the same time. Therefore, this study aims to perform a techno-economic comparative study of an on-grid wind, PV and PV/wind hybrid system to determine the best RES configuration to be installed where Gwanda, Zimbabwe, is the case study knowing that this methodology can be used to determine the best configuration to install in any region in the world. The optimal RES configurations were found based on maximizing the RES fraction with levelized cost of electricity being less than or equal to the local grid tariff; such a constraint will ensure the maximum environmental benefits of the systems, increase the energy security of the country and at the same time ensures the economic feasibility of the alternative energy systems. Such methodology will increase the utilization of the local and abundant renewable energy resources in oil-importing countries such as Zimbabwe which would count toward significant cuts in the imported energy bill. Moreover, the proposed methodology is in accordance with the NREP goals and provides a pathway to achieve these goals in Zimbabwe.

Photovoltaic (PV) Energy Model
Energy production from the PV plant is affected by the ambient conditions; where the effect of the ambient temperature on the module efficiency is the only ambient condition considered in this study. The efficiency of the PV module can be estimated using Equation (1) [52].
where η PV is the photovoltaic module efficiency, η PV,R is the module reference efficiency, β R is the temperature coefficient (1/ • C), T a is the ambient temperature ( • C) which was obtained for Gwanda city using Meteonorm v7.1 software which generates a typical meteorological year (TMY) [52], T NOC is the nominal operating cell temperature ( • C), T R,NOC is the reference module temperature at nominal conditions ( • C), I T the total irradiation on a tilted surface (Wh m −2 ), I R is the reference irradiation at nominal conditions (Wh m −2 ) and T R,STC is the reference module temperature at standard test conditions ( • C). In this study, PV modules from Canadian Solar company (Guelph, Ontario, Canada) type CS6K-285M were used [53]. After estimating the global insolation on the photovoltaic module and, with the estimation of the PV module efficiency, the hourly energy generation from the photovoltaic plant can be found. Using the methodology in Duffie and Beckman [54], the global insolation was estimated which was not repeated in this study for brevity. The hourly energy generated, E P , can be estimated as, where A m is the single module area [m 2 ] and N m is the number of PV modules.

Wind Energy Model
The wind shear coefficient (α) can represent the factors that affect the wind speed at hub height like the speed at ground level, the hub height, the time (hour, day, season), the nature of the terrain and the ambient temperature. α can be taken as 1 7 if the specific data of the site is not available [55]. At hub height, the wind speed (u Z ) can be extrapolated as, where u g is the wind speed at ground level (m/s), Z is the height of the hub (m) and Z g is the ground level height (m) at which speed is measured and it is equal 10 m. The hourly wind speeds at ground level for Gwanda city were generated using Meteonorm v7.1 software. Figure 1 shows the average hourly wind speeds at 10 m in addition to the average daily global insolation on a horizontal surface in Gwanda.
After estimating the global insolation on the photovoltaic module and, with the estimation of the PV module efficiency, the hourly energy generation from the photovoltaic plant can be found. Using the methodology in Duffie and Beckman [54], the global insolation was estimated which was not repeated in this study for brevity. The hourly energy generated, E , can be estimated as, where A is the single module area [m 2 ] and N is the number of PV modules.

Wind Energy Model
The wind shear coefficient (α) can represent the factors that affect the wind speed at hub height like the speed at ground level, the hub height, the time (hour, day, season), the nature of the terrain and the ambient temperature. α can be taken as 1 7 if the specific data of the site is not available [55]. At hub height, the wind speed (u ) can be extrapolated as, where u is the wind speed at ground level (m/s), is the height of the hub (m) and Z is the ground level height (m) at which speed is measured and it is equal 10 m. The hourly wind speeds at ground level for Gwanda city were generated using Meteonorm v7.1 software. Figure 1 shows the average hourly wind speeds at 10 m in addition to the average daily global insolation on a horizontal surface in Gwanda. It is assumed that the energy generated from each wind turbine-in the case of multiple wind turbines-is the same; moreover, it is assumed that the energy production does not change during the hour. E which is the hourly energy production can be estimated using Equation (4). It is assumed that the energy generated from each wind turbine-in the case of multiple wind turbines-is the same; moreover, it is assumed that the energy production does not change during the hour. E w which is the hourly energy production can be estimated using Equation (4).
where P R is the wind turbine rated power (kW), N is the number of turbines, u C is the wind turbine cut-in speed (m/s), u F is the furling speed (m/s), K is the shape parameter of Weibull distribution and it can be calculated based on Justus' theory using Equation (5) and u R is the rated wind speed (m/s). 2 MWwind turbine (G114-2.0) from Siemens Gamesa Renewable Energy company (Vizcaya, Spain) [56] was used in this study.
where u is the mean wind speed at hub level (m/s), and σ is the standard deviation of the wind speeds sample (m/s).

Performance Assessment of the RES
The harmony between the energy generation from the RESs and the demand can be inspected using the RES Fraction (F R ) which represent the percentage of demand met by the RES. F R can be calculated as, where D R is the hourly demand met by the RES (kWh) and D is the hourly demand of Gwanda city (kWh) where the daily demand of Gwanda was obtained from [38] while the hourly load profile for Gwanda city was approximated to be like the profile of a city in Zambia [57]. Moreover, in order to inspect the autonomy of the system the demand supply fraction (DSF) which is the number of hours in which the demand is totally met by the RES in a year over the total numbers of hours in a year and it can be calculated using Equation (7).
where DSF is the demand supply fraction (%); H is the number of hours in which the demand is totally met by the RES in a year.

The Economic Assessment of the Renewable Energy Systems (RES)
The levelized cost of electricity (LCOE) is used to assess the economic feasibility of the RESs. LCOE of the energy systems can be found using Equation (8) where the effect of the mismatching between the demand and energy production is incorporated by using the demand met by the hybrid system instead of the energy generated.
where C i is the capital cost of the RES (USD), M t is the annual maintenance cost of the RES (USD), L is the lifetime of the system (years) where it is assumed that all the components will have the same lifetime and r is the annual discount rate. The economic parameters used in the analysis are shown in Table 1.

Results and Discussion
Investment in renewable energy projects became attractive due to the profitability of such projects. Moreover, the deployment of RESs helps in the mitigation of greenhouse gases (GHGs) and at the same time ensures sufficient and secure energy sources. However, the significant drawback of RESs is the intermittency of the energy production from these systems due to the nature of the renewable energy resources which causes a mismatch between the demand and the supply and also affects the economics of the RESs.
The hybridization of RESs can overcome the intermittency of the resources and increase the harmony between the supply and the demand up to a certain limit where RESs can work in a synergistic way. Moreover, hybridization increases the economic benefits gained by the RESs which makes it more attractive to invest in. As the case study of this paper, Gwanda has a significant potential of wind and solar energy where the hybridization of these two resources proves that this option is the most suitable to achieve not only the maximum economic benefits but also the maximum technical benefits represented by the RES fraction, Figure 2 shows the increase in the technical feasibility of the renewable energy systems with the increase in the PV and wind capacities. However, it can be depicted from Figure 2 that after certain capacities' threshold is reached, the technical feasibility (represented by the RES fraction and the DSF) reaches saturation due to the mismatch between the demand and the supply which increase the electricity cost as shown in Figure 3. Hence, it vital to find the optimal PV and wind capacities that achieve the maximum technical and economic benefits, Table 2 shows the economic parameters as well as technical parameters of the maximum feasible capacities of PV, wind and PV-wind hybrid systems in Gwanda. Note that in Table 2, the hybrid system does not have the highest RES fraction and DSF only, i.e., the highest technical feasibility but also it has the highest NPV and the lowest payback period (PBP) which means that the hybrid system in Gwanda achieves the maximum economic and technical benefits compared with separate systems where solar and wind resources complement each other in a synergistic way. Moreover, note that the PV system alone has the lowest RES fraction, DSF and the lowest economic benefits since it has the lowest capacity factor among the other options due to the nature of the solar resources where on average the PV meets totally the demand of Gwanda for two hours only in a day while the wind system on average meets the demand for five hours. On the other hand, the hybrid PV/wind system meets on average the demand nine hours a day. Figure 4 shows the average hourly demand of Gwanda as well as the average hourly energy generation from the maximum feasible PV, wind and PV-wind hybrid systems.
Note that in Figure 4, the hybridization between solar and wind systems achieves synergistic performance that increases the fraction of demand met by the RES and increases the autonomy of the RES system. Figure 5 shows the monthly RES fraction and demand supply fraction of the maximum feasible PV, wind and PV-wind systems in Gwanda.
it can be depicted from Figure 2 that after certain capacities' threshold is reached, the technical feasibility (represented by the RES fraction and the DSF) reaches saturation due to the mismatch between the demand and the supply which increase the electricity cost as shown in Figure 3. Hence, it vital to find the optimal PV and wind capacities that achieve the maximum technical and economic benefits, Table 2 shows the economic parameters as well as technical parameters of the maximum feasible capacities of PV, wind and PV-wind hybrid systems in Gwanda.     the lowest solar resources. The proposed system in this study had better economic and technical feasibilities compared with similar renewable energy systems either standalone PV and wind systems or hybrid PV/wind systems with and without energy storage systems at similar geographic locations in Africa, as shown in Table 3.    Note that in Figure 5 the maximum RES fraction and DSF occur in October where this month has the highest wind resources and the highest solar resources throughout the year while the lowest RES fraction and DSF occur in May and June since these months have the lowest wind resources and the lowest solar resources. The proposed system in this study had better economic and technical feasibilities compared with similar renewable energy systems either standalone PV and wind systems or hybrid PV/wind systems with and without energy storage systems at similar geographic locations in Africa, as shown in Table 3.

Conclusions
The hybridization between solar and wind systems partially solve the mismatch between the demand and the energy generation caused by the intermittency and the fluctuation of the resources and so increases the economic benefits of the RESs which makes it an attractive option to invest in. Solar and wind resources can complement each other and work in a synergistic way. Therefore, in order to prove that PV-wind hybrid system has better economics and performance compared with separate PV and wind systems, Gwanda city in Zimbabwe was the case study of this paper. The results indicate that the maximum feasible PV system in Gwanda-1.41 MW-has a RES fraction of 28.72% and NPV of 87 × 10 3 USD while the maximum feasible wind system-2 MW-has a RES fraction of 55% and NPV of 3 × 10 6 USD. On the other hand, the maximum feasible PV/wind system-2 MW wind and 1 MW PV-has RES fraction of 65.07% and NPV of 3.06 × 10 6 USD where it can be concluded that the PV-wind system did not only have the best technical performance but also the best economic benefit represented by the NPV and the PBP.