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1. Introduction
Photovoltaic solar energy is the direct conversion of solar radiation into electricity through photovoltaic cells. These are manufactured with crystalline semiconductor materials which, due to the photovoltaic effect, generate electric current when exposed to solar radiation. Photovoltaic panels are composed of a grouping of cells [1]. This type of energy, like other renewable energies, is inexhaustible, clean, respects the environment, and establishes a basis for self-supply.
The study and optimization of the performance of this renewable source has the great potential to reduce greenhouse gases emissions, especially CO2, thus helping to fulfill the commitments assumed in the Kyoto Protocol, which aims to reduce greenhouse gas emissions and protect the planet from climate change.
The electric current generated from solar photovoltaic energy currently has a variety of applications, primarily in isolated systems [2] and systems connected to the grid, called grid-tied systems [3]. The focus of this study is grid-tied systems for a nominal 1 kW installation.
The PVsyst software application allows for the design, simulation, and data analysis of a photovoltaic system. It includes meteorological data from the most-used international databases, such as NASA, SSE RETScreen, and Meteonorm. This is essential for the calculation of an installation at our location, because in many cases the PVsyst program will not have data about the location where the photovoltaic generator is to be installed.
2. Grid-Tied PV System Sizing
Generally speaking, a photovoltaic system connected to a SFCR network can capture solar energy, convert it into electricity, and transform it into alternating current with the same characteristics as the AC grid [4]. To do this, optimum sizing is necessary for its elements to achieve synergy in their operation. Thus, the elements have to adapt to objectives of power, space, the climatic conditions of the area, and the objectives of produced energy (to either be completely consumed or delivered to the grid). In this context, a generator, consisting of a set of photovoltaic panels, is used, in addition to a grid inverter, which converts the DC power from the PV array to AC power to be transformed to allow correct grid coupling, as shown in Figure 1[5].
2.1 Components
The components of a photovoltaic system, shown in Figure 1, are detailed below. The main element is the photovoltaic array, which consists of the electrical interconnection of solar modules in such a way that the supplied voltage and current increase to match the desired value. The main sizing factor for the photovoltaic array is the power to be installed.
This depends on the area available, the load curve that energy can consume, and whether it is possible to deliver this energy to the public grid. Solar panel types are classified, according to manufacturing technology, into monocrystalline, polycrystalline, or thin film [6]. For this study, we selected polycrystalline modules due to their high efficiency and good performance under the influence of temperature.
A DC/AC power inverter converts the direct current produced by the photovoltaic generator into alternating current, fulfilling certain requirements of effective voltage, voltage wave frequency, and current. All of these parameters are synchronized with the local power grid [7]. Protection boxes ensure the safety of the grid-tied PV system and electrical safety [8].
Determination of the power of the PV system: We chose as reference a nominal power of 1-kW peak in order to carry out the analysis since this power value can quickly relate the calculation variables to the grid-tied systems.
Selection of the Photovoltaic module: We selected polycrystalline modules, due to their greater efficiency and offer of the best performance under the influence of temperature. According to the area available we decided to use four 250 Watts peak module units to form the photovoltaic array.
Inverter selection: According to the electrical characteristics determined by the photovoltaic array, there are requirements for an inverter of 1-kW nominal power, 69.8V, 14.36A in DC and 120V in AC, 60Hz single phase, and a network connection.
Installation location: The geographical location for the site of the grid-tied solar photovoltaic installation is in the metropolitan area of Bucaramanga, Colombia, whose geographical coordinates are:
Latitude: 7.035258 Longitude: -73.109584 Altitude: 907 amsl.
Database: The solar radiation data for the photovoltaic generator location were downloaded from NASA’s data bank, accessed by RETSCREEN software. In the table below we describe in detail the information obtained
Table 1 Irradiation, temperature, and wind speed; monthly average in Bucaramanga
| Global Irradiation (W/m 2 ) | Air Temp. (°C) | Wind Speed (m/s) | Ground Temp. (°C) | |
|---|---|---|---|---|
| January | 5.34 | 23.3 | 2 | 24.3 |
| February | 5.34 | 24.7 | 1.9 | 26.4 |
| March | 5.28 | 24.5 | 1.7 | 26.2 |
| April | 4.96 | 23.7 | 1.6 | 24.9 |
| May | 5.01 | 23.2 | 1.6 | 23.9 |
| June | 5.16 | 23.1 | 1.8 | 23.6 |
| July | 5.57 | 23.4 | 1.8 | 24 |
| August | 5.55 | 23.6 | 1.6 | 24.4 |
| September | 5.27 | 23 | 1.5 | 23.8 |
| October | 4.8 | 22.4 | 1.5 | 23.2 |
| November | 4.72 | 22.3 | 1.5 | 23 |
| December | 4.86 | 22.4 | 1.9 | 22.8 |
| Multi-year Average | 5.2 | 23.3 | 1.7 | 24.2 |
Energy study of grid-tied photovoltaic systems: Using the information provided by the NASA’s meteorological database, a quick and reliable calculation can be made, assuming the global irradiation data is on a horizontal surface Ga(0) and the radiation captured by the photovoltaic modules is on an tilted plane Ga(βopt).
First the annual global irradiation is calculated, in kWh/m2/year. We took the average daily value and multiplied it by 365 days, which gave us a result of 1898 kWh/m2.
The tilt of the optimal surface βopt. faces south and is related to the latitude ø with the expression:
βopt =3.7+0.69 *ø, = 8.55° (1)
Using the annual global irradiation on a horizontal plane, the incident at optimum orientation is calculated:
Ga(0)/ Ga(βopt) = 1-4.46*10-4. βopt - 1.19*10-4.( βopt)2
= 0.9875
Ga(βopt) = 18898/0.9875 = 1922.03 kWh/m2 (2)
With this result we obtain the effective annual incident irradiation in the plane of the array:
Gefa(β, ᾳ)/ Ga(βopt) = g1*( β - βopt)2 +g2(β - βopt) +g3 (3)
Values from the coefficients g1, g2, and g3 correspond to a medium level of impurity (5).
Gefa(β,ᾳ)/ Ga(βopt) = 1.218*10-4(8 - 8.55)2 + 2.892*10-4(8 -8.55) + 0.9314 = 0.931
Then: Gefa(β,ᾳ) = 0.931*1922.03 = 1789.4 kWh/m2 (4)
The energy produced by a grid-tied system EAC in an annual period can be roughly estimated with the following equation:
EAC = PG (Gefa(β,ᾳ)) * PR*(1-FS)
Where PG is the nominal power of the PV generator (kWp), PR is the Performance Ratio, and FS is the shading factor. The latter are two dimensionless parameters.
EAC = 1kWp * 1789.4 kWh/m2 * 0.74*(1-0.02) = 1297.7 kWh/year (5)
3. Simulation in PVsyst Software
PVsyst software has been selected for its internationally-recognized trustworthiness. Researchers around the world [9] [10] [11] have used it for the sizing and study of photovoltaic system performance. The simulation method used in the program is based on the realization of hourly energy balances throughout a year, by tracking the behavior of the system in order to calculate the appropriate combination for obtaining a system with the maximum amount of energy, in function of climactic variables such as global radiation, wind speed, and temperature, taking into account the installed capacity of the photovoltaic system.
A database associated with the software was used. It contains a variety of parameters and irradiation data collected in numerous parts of the world over the course of a year, as well as a vast gallery of photovoltaic components provided by the manufacturers in order to perform accurate simulations of the installation.
PVsyst allows designs at different levels.
The Pre-sized level makes a quick assessment of the dimensions of the system and its components. It likewise evaluates system production.
The Design level of the project makes a detailed simulation with hourly values, permitting the definition of the photovoltaic field and the correct selection of components.
At the Database level, PVsyst has radiation data from a large number of cities. We have the option to enter this information in from any external database, either manually or automatically. The radiation data for this specific location was imported from the NASA SSE RETScreen database. It only requires entering in the geographical coordinates and it automatically imports the meteorological information required for the photovoltaic project. In addition, the database has an extensive library of photovoltaic components with general characteristics and technical information supplied by the manufacturers.
The Tools level presents a wide range of digital aids for the climate database, components database, and data measured in order to facilitate the analysis and behavior of photovoltaic systems. Below, the main aspects considered when performing the PVsyst simulation are presented.
Figure 2 shows the effect of varying the tilt angles, azimuth, and a zone of 8 N latitude. It appeared that an tilt equal to the latitude and an azimuth of 0° generate the least amount of losses
Figure 3 shows the trajectory decomposed into angles and the movement of the sun. Based on the tilt and azimuth values, we can observe the solar incidence in the PV modules.
Figure 4 reflect how the irradiance primarily affects the current in a direct relationship. At a higher irradiance, the photovoltaic module provides a higher intensity. A different behavior appears with temperature variations. The short circuit current is independent of the temperature variation, while the open-circuit voltage and the maximum power decrease as temperature increases (-0.45% W/°C). Another aspect to be considered in the simulation is the efficiency curve of the grid-connected inverter, because depending on the operating point (placed by the power delivered by the field of PV panels), the inverter operates with differing efficiency. This variation in efficiency will have a significant influence on the energy performance of the system. Figure 5 shows the efficiency curve, according to power input, for the 1kW inverter. This curve was taken from the PVsyst library, and has been utilized in this simulation.
The results of the sizing of the 1kW installation, from PVsyst, are shown in Figure 6, and the performance results are shown in Figures 7, 8, and 9.
4. Analysis of results
The report provided by PVsyst details the factors affecting the performance of the PV generator. The temperature exceeded the normal operating temperature of the cell, recorded at 56°C. In these temperature conditions, photovoltaic modules lose efficiency because, as explained in Figure 4, according to the manufacturer's specifications, the power output decreases at a rate of 0.45%/°C for this module.
As for the production system, PVsyst gives us the following results:
The result from PVsyst foresees a produced energy of 1375 kWh/year (as seen in Figure 8) and a performance ratio (PR) of 72.7%. If compared with theoretical data by a simplified conventional method, which gave values of produced energy at 1297.7 kWh/year and a PR of 74%, the difference is minimal, in the order of 5.6%. This shows a great proximity between the data calculated by the theoretical model and the PVsyst validation. This slight difference can be attributed to the accuracy of the PVsyst model when calculating the actual operating points of the modules and inverters, and when including aspects such as wind speed and working temperature, aspects which are explained approximately in the theoretical model.
Figure 9 shows that the system is sized correctly, because there is a good correlation between the irradiance and the energy production, with minimal dispersion.
5. Conclusions
This study has shown the dependability of the PVsyst software in the sizing of grid-connected photovoltaic systems.
Comparing the production results calculated by conventional method and the results obtained by the PV system simulation demonstrates a difference of 5.6%, which is not significant, given the precision and detail that PVsyst provides compared to the theoretical model.
This analysis has validated the reliability of the PVsyst software in sizing and in evaluating the performance of this type of photovoltaic application.
The losses that most affect the performance of crystal technology solar panels are due to the working temperature of the photovoltaic cells. In this case, the low wind speed and the high environmental temperature increase the module temperature and results in a significant increase in system losses.
To conclude this assessment analysis of the grid-tied system, it is important to integrate the theoretical information performed through mathematical calculations with the simulation produced with specialized software (such as PVsyst) and compare it with real information, acquired via the monitoring of photovoltaic systems. This has to be done in order to analyze and detect possible causes that affect the overall performance of the photovoltaic system.
