**Evaluation of the Impact of High Penetration Levels of PV Power Plants on the Capacity, Frequency and Voltage Stability of Egypt's Unified Grid**

**Hamdy M. Sultan 2,3, Ahmed A. Zaki Diab 2,\*, Oleg N. Kuznetsov 3, Ziad M. Ali 1,4,\* and Omer Abdalla 1,5**


Received: 31 December 2018; Accepted: 6 February 2019; Published: 11 February 2019

**Abstract:** In this paper, the impact of integrating photovoltaic plants (PVPs) with high penetration levels into the national utility grid of Egypt is demonstrated. Load flow analysis is used to examine the grid capacity in the case of integrating the desired PVPs and computer simulations are also used to assess the upgrading of the transmission network to increase its capacity. Furthermore, the impact of increasing the output power generated from PVPs, during normal conditions, on the static voltage stability was explored. During transient conditions of operation (three-phase short circuit and outage of a large generating station), the impact of high penetration levels of PVPs on the voltage and frequency stability has been presented. Professional DIgSILENT PowerFactory simulation package was used for implementation of all simulation studies. The results of frequency stability analysis proved that the national grid could be maintained stable even when the PVPs reached a penetration level up to 3000 MW of the total generation in Egypt. Transmission network upgrading to accommodate up to 3000 MW from the proposed PV power plants by 2025 is suggested. In addition, analysis of voltage stability manifests that the dynamic behavior of the voltage depends remarkably on the short circuit capacity of the grid at the point of integrating the PVPs.

**Keywords:** photovoltaic; voltage stability; grid capacity; penetration level; frequency stability; Egypt's national grid

#### **1. Introduction**

Renewable energy sources are recently becoming one of the most promising topics of energy systems and policies in most countries. Among the different renewable sources, photovoltaic generation plants have reached a fast growth in the last decades with capacities ranged from small residential application to large-scale grid-connected commercial projects [1]. At present, among all countries, in USA, China, and Germany medium and large-scale photovoltaic power plants (PVPs) have drawn more attention. With the rapid increase in the penetration level of such renewable sources and dispense with the conventional power plants, power systems are anticipated to face changes in their steady-state and dynamic performance. Consequently, integrating high generation from irregular PVPPs creates supplementary challenges to support the transmission networks stability, not only during normal operation but also in the occurrence of abnormal disturbances [2,3]. These abnormal conditions include different types of faults, to which a bus bar or a transmission line might be subjected (three phase to ground fault, single phase to ground fault, phase to phase fault, and etc.), tripping of a mean transmission line, outage of large conventional generating stations and heavy change in load. Accordingly, the expected operating scenarios should be directed and studied in anticipation in order to maintain the transmission network's stability and the power supply reliability throughout the days and nights.

Egypt is rich in renewable energy resources as it is one of the countries located in the solar belt region, which is most suitable for implementation of solar energy projects. The results of Atlas Egypt, depending on the average values of the last 20 years, show that the solar radiation in average ranges between 2000–3200 kWh/(m2 year) and the duration daily sunshine hours fluctuates between 9–11 hours/day, which creates good opportunities for investment in different fields of solar energy [4,5]. Global horizontal irradiation (GHI) over the territory of Egypt is presented in Figure 1. According to the annual report of the Egyptian electricity holding company (EEHC) published in June 2016, there is a national plan to provide 20% of the demand for energy from renewable sources by 2027, while in this plan wind energy provide 12%, hydroelectric 5.8% and solar energy 2.2%. The plan anticipates significant participation from the private sector, which planned to reach 67% of the desired generation in the plan of the New and Renewable Energy Authority [6,7]. Renewable Energy sector plans to implement 51.3 GW from nonconventional energy sources to the present installed capacity of the national power system. The first integration of renewable sources of energy was in 2011 from the solar thermal part of the power station built in Kuraymat in the south of Egypt with a share of 20 MW from a total capacity of 140 MW. A 10 MW solar photovoltaic plant has entered service in 2015 in the Siwa Oasis in Western Sahara [4]. At the present time, Egyptian electricity holding company is coordinating to take the administrative steps to complete the agreements, which control the process of purchase of the generated energy from the private sector's projects. The project has a total of capacity of 1000 MW distributed as follow: a wind farm in Suez Golf with an installed capacity of 250 MW, a 200 MW photovoltaic power plant in Aswan (Komombo), and a project in the west of Nile River with a total installed capacity of 550 MW from different renewable sources [5].

**Figure 1.** Global horizontal irradiation over Egypt's landscape.

The connection of renewable power plants with high degrees of generation is not a new matter in the most of the developed countries. Taking into account the reports of the International Energy Agency published in 2014 there are about 20 renewable energy power plant in the world, mainly in China and the USA, with a high level of generation of more than 100 MW. An appreciable number of vendors are taking part in the development of the technology that adapts the unusable energy from the sunlight or the wind into usable electrical energy [8]. In view of the above, both the supplier and the operator of the grid have to ensure the connection appropriateness of renewable energy sources to the electrical utility. The National Energy Control Center (NECC) and the Ministry of Electricity and Energy in Egypt issue the technical requirements, defined in the Grid Code that has to be accomplished by any renewable energy projects willing to be integrated into the grid [4,8]. To bypass costly design changes after installation, the performance of these plants is stimulated and examined to meet the applicable technical requirements at the design and pre-design stage. The special simulators are able to model both renewable sources and the electric utility grid, thus inspecting in advance the plant's performance, before the process of on-site testing during commissioning.

The impact of PVPPs with high penetrations was demonstrated and different types of power system stability have been studied [9,10]. In [9] using an equivalent model of Ontario utility grid, Eigen-values and voltage stability have been used in a comprehensive study describing the effect of low and high degrees of photovoltaic generation on the power system stability. While in [10] the impact of PV systems on the short-term stability of voltage has been performed, and the results obtained from this study showed that in the case of voltage sage and the disconnection of PV system, the short-term voltage stability has been strongly impaired. Other authors give attention on the low-voltage ride-through capability of PV power plants with high generation levels [11,12]. In brief, the large PV power plants should remain integrated and into the grid utility in instances of heavy disturbance in grid voltage, as the outage of this large power may further deteriorate voltage recovering throughout and after fault time [13]. The capability to voltage support, because of the integration of PV power plants, has been studied for a wide range of generation [14,15]. Voltage stability denotes to the power system's ability to maintain stable voltages on all buses in the system after deviation from a particular initial operating point. The state of the power system enters the region voltage instability when a disturbance or a sharp increase in the current drawn by the loads results in an unmanageable and continuous drop in the voltage at the buses of the system. Instability of voltage appears in the form of a continuous increase or decrease in the voltages at some buses in the power system [16]. The breakdown of the voltage is usually related to the demand for the reactive power of the load that has not been met due to the lack of reactive power production and transmission. The system is called unstable, if the magnitude of the voltage at one bus in the system, at least, is decreased when increasing reactive power injected into the same point.

Many authors have investigated the study of the impact of high penetration of PV generation on voltage, frequency and power [17–19]. The impact of ambient conditions (Solar radiation and temperature) on the frequency and power of large-scale PV power plant at the point of connection with the grid is conducted in [20]. Studying the techniques of integrating PVPs and WPPs with high power production into the transmission and distribution networks and their impact on the frequency of the power system has been examined [21–23].

The stability analysis of Egypt grid has been discussed in [24–26]. The analysis did not take into account the PV systems and its effect on the grid. In [24], the analysis of the Egypt grid with wind energy plant of Gabl El-Zite wind farm has been discussed. In this reference, the frequency stability did not take in the consideration. In [26], the impact of small PV plants on the stability and performance of the Egypt grid has been introduced. As a result of the complexity and change of the power system structure of Egypt, more analysis are asked to determine the voltage stability, frequency stability and the requirements to interconnect new large-scale PVPs.

Egypt's national grid is hugely extended with new cities, which are established related to the population density and with the new growing industrial areas, which demand more energy. This extension will be more and more in the future because of the government plan for industrial development. In this paper, study and analysis of the national grid of Egypt connected with planned PVPs will be presented. The Impact of Large PV Plants with respect to the capacity, frequency and voltage stability of Egypt's national utility grid is discussed in details. Egypt's national electric network has been simulated and tested using DIgSILENT software (DIgSILENT, 2017) with the connection of the suggested PVPs.

DIgSILENT has set standards and trends in power system modelling, analysis and simulation for more than 25 years. The proven advantages of the PowerFactory software are its overall functional integration, its applicability to the modelling of generation, transmission, distribution and industrial grids, and the analysis of these grids' interactions. It's considered a software and consulting company providing engineering services in the field of electrical power systems and has a particular interest in the fields of simulation and grid integration of renewable energies. Also, it will allow to for example automatically identify the over/under and loading elements in the power grid, also it can help to identifies exactly the suitable bus that can carry the new load. [27–38]. Moreover, DIgSILENT PowerFactory offers a range of load flow calculation methods, including a full AC Newton-Raphson technique (balanced and unbalanced) and a linear DC method. The enhanced non-decoupled Newton-Raphson solution technique with current or power mismatch iterations, typically yields round-off errors below 1kVA for all buses. The implemented algorithms exhibit excellent stability and convergence. Several iteration levels guarantee convergence under all conditions, with optional automatic relaxation and modification of constraints. Many authors use the DIgSILENT as a benchmark to simulate and to analyze the power system load flow problem [27–31], Newton-Raphson Load flow method modeling by using DIgSILENT is explained in [27,28]. For load flow study and grid simulation, the DIgSILENT is preferred and is recommended in a comparison between many software packages that is because it behaved as it is expected [27–37].

This paper is considered as a part of a project studying the influence of the increasing the level of renewable generation from PV and wind turbines. The first step in achieving this work was collecting the real data of the component of high voltage (220 kV) and extra high voltage (500 kV) transmission networks, the data of transformer substation and loads. These data have been obtained from the national energy control center and the annual reports of the electricity holding company. Then, the data were organized and analyzed. Depending on the parameters of system components, a complete model of the national utility grid of Egypt have been developed using DigSILENT PowerFactory platform. After that, the according to the plan of the Ministry of Electricity and Energy and our previous study for selecting the suitable locations of large-scale PV power plants over the territory of the country, four sites of installing PV systems are proposed. The capacities of the proposed PV power plants have been determined by examining the grid capacity at the points of connection with the national grid. The main part of this paper is to study the impact of the PV plants on the performance of the national utility grid. The validated model has been used to study and analyze the impact of the planned PV power plants on the voltage and frequency stability of the national power system. The future work is to consider the impact of the wind energy power plants. Moreover, the combination with facts devices to improve the overall stability of the system will be considered in the research plane.

#### **2. National Electric Grid of Egypt**

In the last ten years, the Egyptian energy system has witnessed rapid developments. New power stations have been built and extensions in the transmission network were implemented to provide electric energy to the existing loading centers as well as access to most isolated systems. A model for the electric utility grid in Egypt, which may be suitable for academic as well as research purposes, has been explored in our previous work [39]. The starting point towards achieving this objective was the electric map of the unified energy system in Egypt, published on the official website of the Ministry of Electricity and Energy (see Figure 2) [7]. Egypt is electrically connected through 500 kV and 220 kV transmission networks, which extended along the Nile River from Aswan in the south to Alexandria in the far north.

**Figure 2.** Electricity map of the unified power system in Egypt.

The national utility grid under study is modeled, simulated and assessed using DIgSILENT PowerFactory simulation software package [38]. The following elements were included in the model:


The demands for electric energy are covered, nowadays, from 38 main conventional steam power plants and at least ten of these stations were built more than 35 years ago, two hydroelectric power plants in the south (High-dam and Aswan-dam), in addition to two wind farms on the shores of Red Sea in Zaafrana and Gabl El-Zait. During modeling, the values of the resistances, inductances, capacitances, thermal limits for 500 kV and 220 kV transmission lines and data of the generating stations were obtained from NECC. The parameters of the transformers are taken in accordance with the power limits of the transmission lines. The 220 kV network is divided into six electric zones, similar to the current reality of power transmission companies in Egypt. Single line diagrams for all these regions in addition to the 500 kV grid and technical information about generating stations, substations, transmission lines and nature of loads have been discussed in details in our previous study [39]. The full model includes 218 synchronous machines, 443 transmission lines, 205 substations, 426 bus-bars, 248 transformers and 369 loads. The capacity of the generating stations which are considered in this study is presented in Table 1. Moreover, the data specification of the substations has

been listed in Table 2. As stated in the standard of electric energy transmission in Egypt the values of acceptable voltages for different voltage levels under different operating conditions are shown in Table 3 [40,41]. The single line diagram of the power system under study is shown in Figure 3. For conventional power plants, automatic voltage regulators, turbine governors, and power system stabilizers are also included in the model. The existing library of the simulation software has been used for the representation of wind turbine machines and PVPs.

**Figure 3.** Single line diagram describing the existing transmission network in Egypt.


**Table 1.** Capacity of the generating stations.




**Table 3.** The Allowed values of voltage for the transmission networks.

#### **3. Simulation Results**

The technical potential of 27 sites covering all the territory of the country for installation of large-scale grid-connected PV power plants is assessed using software package RETScreen Expert in our previous work [42]. According to this study, the best sites for installation are located in the south of the country along the Red Sea and in the west to the Nile River. Four sites have been suggested for implementation of the new PVPs in KOMOMBO, KOSSIER, MINIA (B. Mazar) and 6.OCT\_PV. For integrating the proposed plants with the existing transmission system, four 220 kV double circuit transmission lines are added as follow: 90 km transmission line from KOMOBO to substation A.DAM with a thermal capacity of 850 MW, 70 km from KOSSIER to substation SAFAGA with a thermal capacity of 500 MW, 15 km from MINIA to substation SAMLUT with a thermal capacity of 1200 MW and 10 km from 6.OCT\_PV to substation 6.OCT with a thermal capacity of 1500 MW.

#### *3.1. Grid Capacity Assesment*

The Newton Raphson load flow method (PowerFactory software) was used to assess the electric network capacity to integrate PVPs in the case of the steady-state operation of the power system and determine the voltage profile and transmission line's power limits.

The capacity of the south Egypt region represented by the 850 MW PV power plant in KOMOMBO is restricted by the thermal limits of the 220 kV lines linking A.DAM to NOKRA and SELWA substations. Moreover, the capacity of the Canal region represented by the 400 MW PV power plant suggested in KOSSIER is restricted by the thermal limits of the 220 kV line between SAFAGA and HURGADA Substations. The capacity of the third site in B. Mazar (MINIA) of 1200 MW to a certain degree bounded by the overloading of the 500 kV line connecting SAMLUT500 and NORTH.GIZA substations. The capacity of the fourth location of 6.OCTOBER City is comparatively high due to the increase in demand for electric energy in that area, so 1500 MW PV power plant suggested in that location. The capacity of the new PV power plants can be increased to twice its suggested values with installing a third 220 kV line between SAFAGA and HURGHADA, 220 kV line between A.DAM and SELWA substations and another 500 kV line between SAMALUT500 and GIZA N. substations.

#### *3.2. Static Voltage Stability Analysis*

Voltage and frequency stability has recently become the two important parameters of electric power quality describing the power system performance. Equally important is to know how the elements of the energy system that can stimulate instability work. The voltage is one of the parameters, which has different values at each node in the power system. The voltage at the nodes of the system relies on the values of the impedance of the different elements, which are in the grid or out of the system such as control and protection devices. For maintaining the imposed voltage level at the node, which has a clear impact on the voltage at other nodes in the same zone, different types of voltage adjusting devices can be used. Electric power specialists concern the techniques used for adjusting the voltage level in the power system. The stability of the voltage within the system nodes is very important in the coordination and operation of the system. Problems of voltage instability led to the blackout of energy systems in countries such as Japan in July 1987, in August 2003 in the United States and Canada, in September 23rd, 2003 in southern Sweden and eastern Denmark, and a few days later in Italy and Central Europe as a result of the cascaded outage [43].

In the present study, the stability of voltage of the electric utility grid in Egypt is examined by elaborating the behavior of the power system as seen from four proposed locations; namely A.DAM, SAFAGA, SAMALUT, and 6.OCT. As they are considered the most prospected locations for integration of large photovoltaic generations.

The equivalent short circuit capacity (SCC) has been analyzed. Moreover, the basic definitions of short-circuit conditions are given in the IEC Standard 909 [44]. This standard is based on the calculation of symmetrical initial short circuit current (I"sc), for unloaded networks, i.e., in the absence of passive loads and any shunt capacitance. In order to calculate I"sc, the Thévenin's Theorem is applied to the unloaded network with a source voltage equal to Vn (Vn being the nominal voltage). IEC specifies two standard values for the factor c. The «maximum value» is to be used for apparatus rating purposes and it is fixed at 1.1 for HV systems. The «minimum value» is to be used for other purposes such as the control of motor starting conditions [44], which is typical of fast voltage fluctuations problems such as flicker, and it is fixed at 1 for HV systems. The (IEC standard) short-circuit power is then defined as:

$$S\_{\rm sc}^{\prime\prime} = \sqrt{3} \times V\_{\rm n} \times I\_{\rm sc}^{\prime} \tag{1}$$

The IEC approach perfectly suits either for equipment rating purposes or for non-critical voltage fluctuations problems.

Table 4 presents the equivalent short circuit capacity (SCC), short circuit current and X/R ratio at the four studied locations depending on the short circuit analysis performed used the built-in tools in DigSILENT. The voltage is expected to be more responsive to changes in generated power at sites with less SCC or with higher network resistance.


**Table 4.** The equivalent of the Grid as seen from the four locations under study.

Figure 4 presents the grid equivalent circuit as seen from the node of the proposed sites; A.DAM (Site-1), SAFAGA (Site-2), SAMALUT (Site-3) and 6.OCT (Site-4), where PV power plants are proposed to be installed.

**Figure 4.** *Cont.*

**Figure 4.** Single line diagram of the equivalent grid as seen from: (**a**) A.DAM (Site-1); (**b**) SAFAGA (Site-2); (**c**) SAMALUT (Site-3); (**d**) 6.OCT (Site-4).

Large-scale photovoltaic power plants would have a particular impact on the voltage stability of the power system integrated into it [15,21]. In this section, the voltage stability limit of PV power plants at different sites is assessed during steady-state normal operation. All the previously mentioned cases that have been simulated are examined to obtain the impact of integration of PVPs with high penetration levels on voltage stability of the Egyptian unified power system. During simulation and studying the behavior of the P-V curves the real power generated at the buses, to which the proposed plants are connected, was gradually increased until voltage collapse is reached.

The Q-V curve is a powerful tool for analyzing the limits of steady-state voltage stability and the network's reactive power margin by illustrating the relationship between the voltage at a certain bus and the reactive power injected to the same bus [45,46]. It demonstrates the distance in reactive power scale from the point of normal operation to the point, at which voltage collapse occurs. The system is called unstable, if the magnitude of the voltage at one bus in the system, at least, is decreased when increasing reactive power injected into the same point. This means that if the sensitivity of V-Q is positive for all buses the system is stable and when the sensitivity of V-Q is negative for one bus, at least, the system is unstable. The driving force for voltage instability is usually the loss loads in a certain section or tripping of transmission lines and other elements by their protective systems leading to cascading outages. Because of these outages, if the generator field current reaches its limit, some generators may lose synchronism [45].

Fundamental to any analysis of the electric power system is the know-how of per unit systems (p.u.). This system is widely used to represent voltages, currents, and impedances in a power system. The per unit systems allows the electrical engineers to solve a single-phase network where: all active power (P) and reactive power (Q) are three phase, voltage magnitudes are represented as a fraction of their original values "base value", all phase angles are presented with their original units.

Per unit (p.u.) system has many advantages over using the standard SI units such as:


For a given quantity (voltage, current, power, impedance, etc.) the per-unit value is the value related to a base quantity:

$$\text{p.u.} = \frac{\text{quantity expressed in SI system}}{\text{base value}}\tag{2}$$

Generally the base power "*S*Base" and the base voltage according to the line-to-line voltage "VBase" are chosen, then the value of the base current "*I*Base" and the base impedance "*Z*Base" are calculated:

$$I\_{Base} = \frac{|S\_{Base}|}{\sqrt{3}|V\_{Base}|}\tag{3}$$

$$Z\_{\text{Base}} = \frac{|V\_{\text{Base}}|^2}{|S\_{\text{Base}}|} \tag{4}$$

Figures 5–8 show the P-V and Q-V curves at the buses of the four proposed locations. The voltage profiles of the buses, to which PVPs are integrated and the buses of the nearby substations are also reproduced.

From Figures 5–8, it can be noticed that the voltage control has contributed in increasing the penetration level of the PVP at the points of integration by controlling the reactive power injected at these buses. The voltage control method supported the voltage profile at the terminals of the PVPs power stations and the surrounding substations as illustrated in the figures. Moreover, the voltage stability of a certain substation in the grid is directly affected by the equivalent impedance of the electric network as seen from that point. The higher is the equivalent impedance, the higher is the sensitivity of voltage and the lower is the level of real power that can be injected at that point.

**Figure 6.** P-V and Q-V Curves of Site-2 (KOSSIER) and surrounding substations.

**Figure 8.** P-V and Q-V Curves of Site-4 (PV OCT.) and surrounding substations.

#### *3.3. Dynamic Voltage Stability Analysis*

The dynamic voltage stability of the studied power system was examined by assessing the behavior of the voltage at the buses and the response of the proposed PVPs during the abnormal conditions of three phase to ground fault at the terminals of the proposed PVPs and loss of a generation of a large-scale conventional power plant. The behavior of the system under a three-phase short circuit has been studied and simulated at each of the four proposed sites. The variations in the voltage profile at the buses under fault and the surrounding substations, as well as the active and reactive components of current injected from the suggested PV systems, are established.

3.3.1. Three Phase Short Circuit on the Terminals of PV System\_1 (KOMOMBO)

Figure 9 shows the variation in the voltage profile during the application of a 3-phase to ground fault for 0.1 s at the terminals of the high voltage side of transformer substation of PV System\_1 (KOMOMBO). The upper graph displays terminal voltage variation at the terminals of the KOMOMBO site. The lower graph represents the variations in the terminal voltage at nearby substations A.DAM, SELWA, NOKRA, LUXOR, which are presented in the single line diagram of Figure 4a and the main power plants. Likewise, Figure 10 presents the variations of the active and reactive components of currents injected from KOMOMBO site and the H.DAM hydroelectric power plant and A.DAM. The results indicate that the system is stable with respect to the same voltage, reactive and active power after fault clearance.

**Figure 9.** Voltage variation in the case of a 3-phase fault at terminals of PV system at site\_1 and nearby bus-bars; Upper: Terminal voltage at PV terminals. Lower: Terminal voltage at nearby buses.

**Figure 10.** Variations in active and reactive components of current during a 3-phase fault at KOMOMBO (PV System\_1) and nearby substations.

#### 3.3.2. Three Phase Short Circuit on the Terminals of PV System\_2 (KOSSIER)

Figure 11 shows the variation in the voltage profile in case of the application of a 3-phase fault for 0.1 s at KOSSIER site. The upper graph presents the variation in terminal voltage at the terminals of the KOSSIER power plant. The lower graph shows the terminal voltage variations at surrounding substations mentioned in the single line diagram presented in Figure 4b. Likewise, Figure 12 represents the variations of the active and reactive components of currents injected at KOSSIER site and A.DAM hydroelectric substation. From the results, the system takes more time to reach the stability after fault clearing at the A.DAM hydroelectric substation, because this substation is near the fault location. However, the system each to the stability and has a good dynamic performance.

**Figure 12.** Variations in active and reactive components of current during a 3-phase fault at PV System \_2 (KOSSIER) and nearby substations.

#### 3.3.3. Three Phase Short Circuit on the Terminals of PV System\_3 (Minia, B. Mazar)

Figure 13 shows the variation in the voltage profile in case of the application of a 3-phase fault for 0.1 s at B. Mazar site in the city of Minia. The upper graph presents the variation in terminal voltage at the terminals of the MINIA power plant. The lower graph shows the terminal voltage variations at surrounding substations shown in the schematic diagram of Figure 4c. Likewise, Figure 14 represents the variations of the active and reactive components of currents injected at B. Mazar site and the thermal power plants of KURM1 and BINI SUIF. The results also indicate that the system is stable. Moreover, it has a good transient performance to rapid recovery of the terminal voltage and reactive and active power after fault clearance.

**Figure 13.** Voltage variation in the case of a 3-phase fault at terminals of PV system at site\_3 (B. Mazar) and nearby bus-bars; Upper: Terminal voltage at PV terminals. Lower: Terminal voltage at nearby buses.

**Figure 14.** Variations in active and reactive components of current during a 3-phase fault at PV System\_3 (B. Mazar, Minia) and nearby substations.

#### 3.3.4. Three Phase Short Circuit on the Terminals of PV System\_4 (6.OCT)

Figure 15 shows the variation in the voltage profile in case of the application of a 3-phase fault for 0.1 s at the fourth proposed site for installation of PVPs (6.OCT site). The upper graph presents the variation in terminal voltage at the terminals of the 6.OCT power plant. The lower graph shows the terminal voltage variations at surrounding substations presented in the single line diagram describing this part of the power system given in Figure 4d. Likewise, Figure 16 represents the variations of the active and reactive components of currents injected at 6.OCT site, OCT.GEN. and T.OCT.

**Figure 15.** Voltage variation in the case of a 3-phase fault at terminals of PV system at site\_4 and nearby bus-bars; Upper: Terminal voltage at PV terminals. Lower: Terminal voltage at nearby buses.

**Figure 16.** Variations in active and reactive components of current during a 3-phase fault at 6.OCT and nearby substations.

#### *3.4. Frequency Stability Response with the Interconnection of PV Power Plants*

In this section, the case of an outage of a certain generating station was simulated to examine frequency stability in the Egyptian power system. The following events have been considered:


Figures 17 and 18 show the variation of the electrical frequency after an incident tripping of a large generating power plant (NORTH GIZA), with a generating capacity of 2250 MW for the two predefined scenarios.

**Figure 17.** The response of electrical frequency of the power system following the outage of NORTH GIZA plant with 0% PV penetration.

**Figure 18.** The response of electrical frequency of the power system following the outage of NORTH GIZA plant with 3000 MW from proposed PV sites.

Figures 19 and 20 show the response of rotor angle of a number of the generators of the conventional generating stations operating on the system with No PV generation and with 3000MW renewable energy from the proposed sites of the PV power plants. From these figures, it can be noticed that all generators have reached steady-state condition after the outage of NORTH GIZA power plant.

**Figure 19.** Response of rotor angle of other conventional generating units in the power system with 0% PV penetration.

**Figure 20.** Response of rotor angle of other conventional generating units in the power system with 3000 MW from the PV power plants.

#### **4. Conclusions**

The performance of the national utility grid in Egypt is demonstrated using computational simulations using DigSILENT PowerFactory software package in case of high generation levels from photovoltaic power. The impact of high PV penetrations on the capacity of the Egyptian power system is performed using a Newton-Raphson load flow method. In accordance with the constraints on the capacity of transmission lines, the simulation results proved that the maximum allowable generations from the proposed stations have not to exceed 850 MW at KOMOMBO site, 400 MW at KOSSIER site and 1200 MW at B. Mazar, MINIA site to bypass line congestions. Moreover, the capacity of the PVP at 6.OCT site is comparatively high because of the high demand for electric energy at that industrial area and continuous network development. Transmission network amelioration to accept up to 3000 MW from PV plants by 2025 was introduced. Static and dynamic voltage stability of the network has been examined with respect to the integration of large-scale photovoltaic generation.

P-V curves, which have been obtained by increasing the level of PV penetrations at the selected sites, show that voltage control has contributed to raising the level of photovoltaic generation at different nodes of the system through the control of injected reactive power. Furthermore, the study of voltage's dynamic stability, after a three-phase short circuit occurrence, has manifested that the components of reactive power participated in supporting the value of the terminal voltage of the generating units. Analysis of frequency stability has been carried out after the outage of NORTH-GIZA steam power station. It has been observed that frequency stability in the Egyptian utility grid can be maintained for photovoltaic generations up to 3000 MW of the total demand for energy.

**Author Contributions:** H.M.S. developed the simulation model of the national utility grid of Egypt and analyzed the presented results in this paper under the guidance of A.A.Z.D. and O.N.K. H.M.S., A.A.Z.D. and Z.M.A. performed models simulations, analyzed the data and results and wrote the paper. A.A.Z.D., Z.M.A. and O.N.K. revised and edited the final text of the paper. Z.M.A. and O.A. contributed by drafting and revising. All authors together organized and refined the manuscript in the present form. All authors have approved the final version of the submitted paper.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Review* **Concentrating Solar Power Technologies**

#### **Maria Simona Răboacă 1,2,\*, Gheorghe Badea 2, Adrian Enache 1, Constantin Filote 3, Gabriel Răsoi 1, Mihai Rata 3, Alexandru Lavric <sup>3</sup> and Raluca-Andreea Felseghi 2,3,\***


Received: 3 February 2019; Accepted: 12 March 2019; Published: 18 March 2019

**Abstract:** Nowadays, the evolution of solar energy use has turned into a profound issue because of the implications of many points of view, such as technical, social, economic and environmental that impose major constraints for policy-makers in optimizing solar energy alternatives. The topographical constraints regarding the availability of inexhaustible solar energy is driving field development and highlights the need for increasingly more complex solar power systems. The solar energy is an inexhaustible source of CO2 emission-free energy at a global level. Solar thermal technologies may produce electric power when they are associated with thermal energy storage, and this may be used as a disposable source of limitless energy. Furthermore, it can also be used in industrial processes. Using these high-tech systems in a large area of practice emboldens progress at the performance level. This work compiles the latest literature in order to provide a timely review of the evolution and worldwide implementation of Concentrated Solar Power—CSP—mechanization. The objective of this analysis is to provide thematic documentation as a basis for approaching the concept of a polygeneration solar system and the implementation possibilities. It also aims to highlight the role of the CSP in the current and future world energy system.

**Keywords:** concentrated solar power (CSP), installed capacity; solar energy resources; solar thermal plants; thermal energy storage (TES)

#### **1. Introduction**

Prefacing the improvement of inexhaustible energy supply worldwide and non-polluting power sources [1,2], represents one of the major purposes of power generation at a global level [3–5]. Renewable energies (sunlight, sunscreen, water strength, biomass, wind power) and renewable raw materials are alternatives to fossil resources [3]. Solar activity represents one of the purest types of energy [6]. The huge amount of this type of energy underlies almost all natural processes on Earth [7]. It is, however, quite difficult to capture and store in a usable form (mainly heat or electricity) [8,9] that would facilitate its subsequent use [10].

The potential of solar resources, which far exceeds the potential of fossil fuels, is given by the following characteristics [1]:



Solar Thermal Energy (STE) is the most important type of solar power activity and represents one of the important technology resources, producing energy useful in various applications such as: building, electromobility and manufacturing. The integration with thermal energy storage (TES) comes with the possibility to make STE unique and dispatchable when mixed with other inexhaustible sources of energy. For a long period, expansion of the thermal solar activity industry has been associated with TES theory. It is important to provide high tech sources which facilitate the distribution of the demanded energy supply [11].

Unlike photovoltaic (PV) panel technologies, Concentrated Solar Power (CSP) has an inherent capacity to store heat energy for limited intervals of time for later conversion into electricity. When combined with thermal storage capacity, CSP plants are able to produce electricity even when clouds block the Sun or after sunset. [12]. Additionally, for instance, one megawatt of installed CSP avoids the emission of 688 tons of CO2 compared to a combined cycle system, and 1360 tons of CO2 compared to a coal/steam cycle power plant. One square mirror in the solar field produces 400 kWh of electricity per year, avoids 12 tons of CO2 emission, and contributes to 2.5 tons savings of fossil fuels during 25-year operation life time [13].

This work aimed to provide a state-of-the-art review of the development of CSP technologies over the last decade. First, the article provided a summary on the status of the EU's main objectives for renewable energy sources (RES) development, which intended to highlight the role of the CSP in the current and future energy system of Europe. The main CSP technologies are presented and the suitability map for installation of solar thermal power plants according to the direct normal irradiance (DNI) was illustrated. There is discussion regarding the worldwide stage of installation of capacities based on CSP technology, as well as CSP plants that are currently operating or under construction. The goal of this review was to provide a thematic documentation that can be a starting point for developing a research project within National Research and Development Institute for Cryogenic and Isotopic Technologies—ICSI Rm. Valcea, ICSI Energy Department. This work has already achieved results in some "smart city" examples in Europe [14]. It aimed to approach the concept of a polygeneration solar system, which involved the possibility of obtaining several forms of useful energy from solar resources: electricity, thermal energy (heat), mechanical work produced by steam, chemical energy in the form of hydrogen (fuel), cooling energy, light flux, etc.

Romania, one of the EU members, through energy policies, adopted strategies and research activities [15], has an intense orientation towards the world solar economy [16], which requires a revolution in energy technology, making the technical development of productive forces replicable internationally [1].

#### **2. Materials and Methods**

To compile the review based on a literature research of Concentrated Solar Power (CSP) technologies for sustainable power generation, existing relevant studies that were analyzed based on different types of CSP along with thermal energy storage (TES) technologies, and the worldwide state of implementation of these concepts have been identified.

A systematic literature search was carried out in Science Direct, MDPI, ResearchGate, Google Scholar and specialized technical platforms to identify relevant studies involving review analysis of different types of CSP and TES technologies and installed capacities, during the last 10 years. The concept of CSP technologies is not new, and a significant number of studies have already been conducted by researchers. The identified research works have been characterized according to the technologies reviewed, methodology adopted and the sustainability parameters discussed.

#### **3. Considerations Regarding the CSP Technologies**

#### *3.1. Main Policies and Objectives for Renewable Energy Sources (RES) Development*

The power of the sun and of the wind, as well as the power provided by biomass and biofuels, along with geothermal and hydro power energy are used as alternatives for fossil fuels to avoid the emissions that can trigger the greenhouse effect, and reducing the dependency volatility for fossil fuels, especially oil and gas [4,17,18].

The future post 2020 timeframe study, as well as the EU legislation on the promotion of renewable sources is under debate. The fundamental process of society's development is based on the availability of an inexhaustible power supply [3,19]. Contributing to most of the proposed energy requires, ensuring the transition to a sustainable energy system, the security of supply, reduction and even elimination of the greenhouse gas emissions, and the industrial development, that would lead to job growth and significantly lower energy costs [20]. The objectives of "20-20-20" strategy to be fulfilled by 2020 have set the following three key targets:


As the European energy system faces an increasingly pressing need for sustainable, affordable and competitive energy for all citizens, the European Commission adopted on 30 November 2016 the legislative package "Clean Energy for All Europeans", which seeks to implement strategies and measures to achieve the objectives of the Energy Union for the first ten-year period (2021–2030), in particular for the EU's 2030 energy and climate objectives, and refers to: energy security, energy market, energy, de-carbonization, research, innovation and competitiveness [15,19,20].

In a wider perspective, the EU established a set of long-term objectives in roadmaps by 2050. Regarding the building sector, the main three roadmaps are:


These roadmaps are a long-term aspiration that is not only desirable from a social and economic point of view, but also ecologically essential [15,19,20]. In many countries, the strong development of the heating sector from renewable sources [21] has been a key factor in achieving and surpassing the intermediate targets in these EU member states. This is true, for example, in Bulgaria, Finland and Sweden, where development was mainly driven by the use of low-cost fuel from biomass. The use of an inexhaustible supply in the field of transport [22] has lagged behind in most countries, except for Sweden, Finland, Austria, France and Germany.

However, most Member States are about to meet and even surpass their targets by 2020, based on the planning and assessment of current policies [3,19,20]. This makes it an entirely possible target not only for the EU members, but also for the entire EU (Figure 1) [15].

Solid biomass fuels were the main factor that produced heat from inexhaustible sources in 2013 [23], while during 2014, a report was published in regards to the production of heat and electricity based on the solid sustainable and gaseous biomass. This report was published by the European Union

and contained information on current planned EU actions that were supposed to maximize the benefits of the biomass usage while avoiding negative impacts on the environment [15].

**Figure 1.** Implementation chart of renewable energy sources (RES) in the EU member states and 2020 targets [15].

The power plants based on concentrated solar radiation (CSP) are considered to be an interesting alternative for generating electricity from renewable energy on a large scale worldwide [24,25]. Although their development has not been so rapid, some relevant projects were still in pending or cancelled in countries like Australia, and expansion of solar heat supply capacity is expected to be taken into consideration in the following years [26].

Electricity power, the fastest growing form of energy [27], is the power sector that contributes more than any other to the reduction of fossil fuels worldwide [28]. By 2040, an increase of around 40% (based on the current fleet) of the electricity requirement is expected. This increase by 7200 gigawatts may substitute the existing energy generators [5,29]. The strongest growth of renewables in many countries raises their share worldwide power generation, to one third by 2040 [15,30].

Due to global population industrialization and growth, the energy demand has increased dramatically. We can obtain renewable energy through natural resources such as geothermal, biomass, wind and solar heat power. This is the focus of most countries, especially due to the sustainability benefits by reduction of CO2 and greenhouse gas emissions.

Among the technologies based on the use of solar sources to produce energy, the technology using parabolic mirrors outstanding by high efficiency, compactness and the advantage of modularisation that allows them to be placed in isolated places, independent of access to conventional energy sources. This technology is part of the radiation focusing category, the mirrors having a role of capturing and concentrating incident solar rays in the focal area where the receiver is located, which is usually coupled to an electric generator. Thus, solar energy is transformed into thermal energy into the receiver, then to mechanical energy in the engine and finally to electricity in the generator [31]. Coupling of technologies through integration of thermal energy storage (TES) to concentrating solar power (CSP) brings uniqueness to this type of power converter among all other renewable energy generating alternatives [30,31].

#### *3.2. Direct Normal Irradiance (DNI)*

Direct Normal Irradiance (DNI) is defined as the solar irradiance collected by a normal plane, directly from the Sun, being of high importance, respectively is the basis of the functioning principle of CSP technology [32–34].

Figure 2 presents a map of the global distribution of direct normal irradiation, where four zones can be further distinguished [35–37] based on their suitability for the installation of solar thermal power plants (Figure 3) [37]. Thus, zone I—excellent—has great potential and records maximum DNI values of 3652 kWh/m2/y. It is followed by zone II—having a good potential for exploitation of DNI, which has average values of 2800 kWh/m2/y. Zone III allows to install thermodynamic solar power plants with DNI values of 1700 ÷ 2100 kWh/m2/y. And, finally, area IV—not suitable for these types of energy generation systems that have a DNI of 365 ÷ 1700 kWh/m2/y [37,38].

**Figure 2.** Distribution map of direct normal irradiation [36].

**Figure 3.** Suitability map for installation of solar thermal power plants [37].

For an efficient operation of thermal solar plants, DNIs need to record values above 1800 kWh/m2/y [39]. The most favorable CSP resource [37] areas are thus in North Africa, the south-western United States, northern Mexico, north-western India middle East, southern Africa, Chile, Australia, Peru and the western parts of China. Other relevant areas such as southern areas from Europe, Turkey and US, central area of Asia, Brazil and also Argentina and China, are included [40,41].

#### *3.3. The Concept of Concentrated Solar Power (CSP)*

One of the first studies on the possible use of sunlight, dating back to 1774, in the late 18th century, belongs to Antoine Lavoisier who created a large optical device containing a glass lens to focus and concentrate the sunlight on the surface of a burning material. Later, in 1878, a parabolic collector was

designed and built to test the impact of the Sun's rays on a steam boiler to heat the water from its interior to the boiling point, and to release steam under pressure. This boiler by means of a mechanical device, was ran and powered a printing press. [42,43] Figure 4a presents the first concept of a solar parabolic collector.

Sophisticated solar power (CSP) technologies are currently under development but are still not as accessible as conventional photovoltaic panels in providing confidence and reliability [44,45]. Figure 4b presents a modern solar parabolic concentrator concept [46].

**Figure 4.** Solar parabolic collector [43,46]. (**a**) First concept; (**b**) Modern concept.

Solar energy concentrating systems use parabolic mirrors that reflect the sunlight on a single point over the receiver's surface from where it is collected and transformed into thermal and electrical energy. Parabolic mirrors are designed to focus solar radiation on the receiver, which heats the gas to a relatively high temperature, which is then used to move a turbine or steam to power a Stirling engine running an electric generator, thus producing electricity [31,43].

Thermodynamic solar systems put into operation optical concentrators that exploited direct sunlight. The main specific of CSP technology(illustrated in Figure 5), when compared to other renewable energy conversion equipment is represented by a thermic stock system that generates electrical power during intervals of time with cloudy skies or the sun setting.

As compared to photovoltaic panels (PV), CSP uses DNI in order to provide heat and electricity without CO2 emissions where the DNI level is higher in comparison with others [43,47,48]. The CSP commercial technologies are the following [39,49–52]:


(c) *A Parabolic Dish (PD)* is made up of a parabolic dish-shaped concentrator that mirrors the Direct Normal Irradiance into a receiver located at the focal point of the dish. The main advantages of PD technologies include high energy efficiency (up to 30%) and modularity (5–50 kW), in addition to being particularly suited to distributed generation systems [39,49–52].

**Figure 5.** Main CSP technologies [53].

Based on the literature [54–62], the specific features of CSP technologies are presented in a comparative manner in Table 1.


**Table 1.** Comparison of CSP technologies [54–62].

PT systems occupy a large area, having low thermodynamic efficiency due to their low operating temperature. They have a relatively low installation cost and a large experimental feedback. Furthermore, LFR has thermodynamic efficiency due to the low operating temperature, but low installation cost. ST has high thermodynamic efficiency due to a high operating temperature; it occupies a large area, it has high installation cost and records high heat losses. Finally, PD occupies small area, features high thermodynamic efficiency due to high operating temperature, but it requires a high installation cost [54–62].

A percentage of 85% of all CSP research projects is carried out on the parabolic troughs technology; accordingly, the existing data regarding operating experience and cost information generally refers to PT energy systems.

Levelized Cost of Electricity (LCOE) estimated for CSP is still high as compared to the other renewable technologies, as shown in Table 2 [55,63–66].

Power generating technologies based on alternative energies are subject to constant research and development. Costs are expected to decline in the near future due to various pilot projects currently underway in this field being validated and implemented on a large scale (including energy storage solutions).


**Table 2.** LCOE estimates (€c/kWh) [55,63–68].

#### **4. Discussion Regarding the Stage Installation of Capacities based on CSP**

#### *4.1. General Considerations on RES*

Figure 6 shows a country ranking on total renewable energy production capacity as of the end of 2017 [68].


**Figure 6.** Country ranking on total renewable energy production capacity for 2017 [68].

From Figure 6, it can be seen, that the countries with the largest renewable energy production are China, the US and Brazil, followed by Germany and India. In terms of power generated by CSP capacity, Spain ranks first in the world ranking, being followed by United States that ranks second, and then South Africa, India, Morocco and China. In terms of solar water heating collector capacity, China ranks first in the world ranking, being followed by United States, Turkey, Germany and Brazil.

#### *4.2. Worldwide Capacity of CSP Technologies*

Figure 7 shows the concentrating solar thermal power global capacity by country and region over the period 2007–2017. From the statistical data presented in Figure 7, there can be observed a significant increase in renewable energy capacity from 0.4 gigawatts produced in 2007 to 4.9 gigawatts in 2017. Despite the fact that the global capacity increased by only over 2% in 2017 compared to 2016, the CSP industry was active, with a pipeline of about 2 GW of projects under construction around the world, especially in the Middle East and North Africa (MENA) region and in China [37].

Concentrated solar power technologies (CSP) have helped boost developing countries with a high level of direct normal irradiation (DNI) [35] and a specific strategic and/or economic alignment, benefitting from the advantages of these technologies [40]. CSP technologies benefit from better

support for energy policies, low oil and gas reserves with limited access to electricity grids, or stringent energy storage needs, thus achieving a strong industrialization and creating new jobs [33].

Ongoing research conducted mainly in Australia, Europe, and the United States, has kept concentrating on the development and improvement of Energy Storage Technology (TES) [69].

**Figure 7.** Concentrating solar thermal power global capacity by country and region over the period 2007–2017 [68].

Figure 8 shows the global storage capacity of solar thermal energy during the period 2007–2017. According to the statistical data presented in Figure 8, there is significant increase in the global storage capacity of thermal energy produced from concentrated solar radiation, from the supply of about 0.04 megawatts-hours in 2007 to 12.8 gigawatts-hours, in 2017.

**Figure 8.** CSP thermal energy storage global capacity and annual additions, 2007–2017 [70].

Figure 9 shows the STE worldwide capacity organized by main CSP technologies [71]. A significant percentage of installations in operation or under construction have linear gradient concentrating systems such as parabolic troughs, which operate both with and without storage. The trend was to increase the use of solar-tower technology. Forty one (41) percent of thermal energy storage systems and 41% of all STE plants under construction are in development under Fresnel reflectors, which operate without storage.

**Figure 9.** STE worldwide capacity categorized by technology and with/without storage [72,73].

Parabolic dishes are present in a low percentage of approximately 2% of the total STE plants in operation without storage, this type of technology being in the stage of research and implementation at the level of practical applications [72,73].

#### **5. CSP Sunflower 35 Experimental Equipment**

Under the research project "ROM-EST: Research Laboratories for Energy Storage", following the research study, a Sunflower 35 solar concentrator was purchased together with a Stirling energy unit to produce energy and heat from concentrated solar energy.

The performance of the Sunflower 35 module according to the manufacturer's specifications is as follows:



The Stirling engine did not require water consumption for power generation or for cooling cycles (closed circuit operation). The Stirling Technology Conversion Unit (PCU 35) of 35 kW was equipped with a 25% efficiency SBT V-183 thermal engine.

This equipment is sturdy and durable, adapts to and resists extreme operating conditions such as temperatures from −20 ◦C to +50 ◦C, 100% humidity, snow, sand, wind and dust.

The installation of the solar concentrator together with the power unit and the necessary accessories were initially underwent a preliminary study, which consisted in positioning simulation procedure, which was a preconception of the actual placement and installation mode in the location established. Taking into account these aspects, the installation and assembly positions of the main equipment will be anticipated in a preliminary manner.

The surface requirement for the solar concentrator is about 10 × 20 m2, with a favorable position, so as to ensure maximum solar radiation at the lowest shading factor. During the first stage of the installation and commissioning phase, the solar concentrator was installed together with the technical annex for the control and control of the processes.

At a later stage, the power unit with all necessary accessories will be mounted on the support arm of the concentrator, after prior testing of the equipment.

For the safety and proper functioning of the equipment, they must be mounted and installed in a safe location, where they must operate at a constant temperature and protected from bad weather.

For this purpose, a technical annex for the installation of technical equipment for process control and control is placed near the installation, being protected against dust and impurities, with a controlled thermal regime throughout the seasons.

The annex will be divided into two rooms with the following use:


Figure 10 shows the installation of the solar concentrator working position together with the technical annex for command and control.

**Figure 10.** Installed solar concentrator in its working position.

The parabolic metal frame of the solar concentrator was designed to be assembled to the ground by means of a special mounting system that allows quick and easy installation of the building.

The support pillar was positioned vertically in the reinforced concrete foundation on which the solar concentrate drive system was mounted, which is shown in Figure 11.

**Figure 11.** Solar concentrator support pylon.

After mounting the support pillar, the parabolic metal frame assembly was positioned by the arm and secured to the propulsion system.

Install the control system with all the quick connect connections that are installed on the control board interface of the propulsion system. The Stirling motor assembly of the test equipment is presented in Figure 12.

**Figure 12.** The Stirling motor assembly.

#### **6. Conclusions**

There is no doubt regarding the great global potential of solar energy which is a clean renewable energy form. It has the disadvantage of only supplying intermittent power for electricity generation. This inconvenience can be removed through CSP technology, which together with a suitable heat storage system can generate electricity even with cloudy skies or after sunset. Thermal Energy Storage allows the mitigation of short fluctuations and extension of electricity supply to more desirable periods, making Concentrated Solar Power dispatchable [74].

This work outlines an analysis of the latest scientific literature in order to achieve the state-of-the-art review of the development and worldwide implementation of CSP and TES technologies. In this respect, the following aspects were presented: CSP's position in worldwide policies and targets for Renewable Energy Sources (RES) development, global distribution DNI map alongside to the global map of suitability for installation of solar thermal plants, the concept and the main CSP technologies together with the main TES methods, and, in the end, there have been discussions about the worldwide stage installation of capacities based on CSP. The conclusions can be summarized as follows:


One R&D project allowed ICSI Râmnicu Vâlcea to purchase an integrated CSP Dish Stirling SUNFLOWER 35 type system for polygeneration of energy by concentrating solar irradiation, which has been installed in the current location, together with all the accessories necessary for good operation in optimal conditions. The Stirling engine power unit was installed within the CSP Dish Stirling SUNFLOWER 35 integrated system, but only after this equipment had been pre-tested and put into operation.

**Author Contributions:** Conceptualization, G.B. and C.F.; Methodology, R.-A.F., M.S.R.; Software, A.L.; Validation, G.B., R.-A.F. and M.S.R.; Formal Analysis, M.R.; Investigation, G.R. and A.E.; Resources, C.F.; Data Curation, C.F.; Writing—Original Draft Preparation, R.-A.F., G.R.; Writing—Review & Editing, M.S.R.; Visualization, A.L.; Supervision, G.B., C.F.; Project Administration, C.F.; Funding Acquisition, M.S.R.

**Funding:** This research received no external funding.

**Acknowledgments:** This work was supported by a grant of the Romanian Ministery of Research and Innovation, CCCDI-UEFISCDI, project number PN-III-P1-1.2-PCCDI-2017-0776/No. 36 PCCDI/15.03.2018, within PNCDI III.

**Conflicts of Interest:** "The authors declare no conflict of interest."

#### **Nomenclature**



#### **References**


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