Design Process

The design process is classified into five main parts:

(a) Establishing design criteria

The design criteria are based on several key factors as shown below in Figure 1.

Fig.1: Design criteria

A load survey should be conducted to investigate about the energy consumption followed by an energy audit using previous electricity bills to estimate the solar PV system size. The proposed site is located at geo-coordinates (Longitude, Latitude) with characteristics such as elevation, azimuth, slope that can be obtained from Solar GIS. The area of the site is estimated and the seasonal sunshine hours for the selected location should be calculated based on previous solar irradiance data. The roof’s load carrying capacity should be also determined. The environmental and weather variations are a direct function of the efficiency of any installed solar system. The main temporal elements are temperature, wind speed and irradiation levels variations on the site. The panels should be installed within the regulated norms to avoid poor mounting which can be unsafe due to fast winds.

(b) System configuration

A standalone PV power system (Off-grid) is one which is not connected to the grid. The three main types of off-grid PV systems and the general system configuration are as shown in Figure 2 and 3 respectively.

Fig.2: Types of off-grid systems

Fig.3: Standalone PV system configuration [1]

The main equipment are:

A grid-tied PV system is one in which the PV panels are connected to the load and the grid through an inverter and an input/output meter. Such systems do not necessarily need battery backup to ensure maximum power point tracking though there may exist grid-tied AC and DC coupled systems. Excess power not required by the load can be directly fed into the grid, which is considered to be an infinite power source.

Fig.4: Types of grid-tied systems

Fig.5: Grid-tied system configuration [1]

The main equipment are:

For Hybrid PV systems (PV as the main source), there is at least one more source such as wind, diesel, hydro and fuel cell.

(c) Component sizing and selection

This part includes the major components sizing and selection, metering and control, protection, switching and isolation, cable sizing and selection.

  1. The size of PV array is derived from the available area, the amount of solar radiation and the consumption forecasted for an optimized system. The panel is decided on the current and voltage requirements of the inverter, i.e., the short circuit current Isc of the panel should be less than the maximum DC current of the inverter and the string open circuit voltage Voc should also be less than the maximum voltage of the inverter. The PV size adjustment ensures that the variation in load demand in a year is well catered for.

  2. The PV arrays produce direct current (DC) at a voltage, which depends on the specific design and the solar radiation. The DC power then runs to an inverter, which converts it into standard AC voltage. Inverters commonly used in large scale applications are string inverters that offer easy installation and in some cases where efficiency is required at the expense of cost, micro-inverters are used. The inverter operates on the nominal power which is the output power. DC-to-AC ratio is normally used to prevent under- or over-sizing of inverters and is calculated as the nameplate capacity of panels to the nominal AC rating of the inverter. The ratio is normally maintained in the order of 0.8 to 1.2, based on the constraints and lifetime of the system.

  3. The Balance of System (BoS) equipment consists wiring cables, over current protection, switches, earthing, and safety signage among others. Some of the standards for the BoS equipment are mentioned in the Grid Code requirements of the scheme such as the Earthing shall be according to IEC 60364-5-55 which is a TT earthing system and cables should not have voltage drops must not be more than 5% and be of standards: MC3, MC4; BS7671.

  4. The size of the system needs to be determined primarily in order to select the appropriate components. It is denoted as Pp and calculated taking into account the number of units, annual peak sun hours and an arbitrary inverter efficiency value of 90% as follows:

(d) System optimization

This is carried on performance modeling softwares to optimize the system design. The software to be used for modeling the performance of the PV system is System Advisor Model (SAM) which is free software, developed by National Renewable Energy Laboratory, Washington. The software is able to analyze all solar technologies, as well as provide intensive financing and cost analysis. The results from the analysis can be presented in term of levelized cost of energy (LCOE), system energy output, peak and annual system efficiency, and hourly system production, in tables and graphs.

Prior to simulating the systems, a weather file (.EPW) is required. Moreover, in order to further improve the accuracy of the simulation, the irradiation values can be adjusted using the calibration factor which has to be determined. The optimization process is performed by varying the tilt angle from 0 to 45 in steps of 5 degrees and the energy produced is noted.

(e) Financial evaluation

  1. The capital cost, Ccap of the PV system is calculated and the costs that are not one –time are referred to as recurrent cost (calculated on an annual basis). The lifespan of the system is expected to be 20 years. The discount rate, d and inflation rate, i should be also deduced.

  2. The present worth, PWone of the system today with its inflation and discount rates is its worth in the future, with new inflation and discount rates. Since we have assumed that d > i, with time the purchasing power of money increases, the cost of the system will obviously decrease.

  3. To calculate the present worth of a recurrent cost, Frec, the equation below is used:

  4. The Life Cycle Cost (LCC) is calculated to find the cost of using the system during its lifetime. It includes all associated cost of the system that can be incurred over its existence. The LCC is performed before purchasing of any component for the system after analyzing different system designs and then select the best proposal.

  5. The capital cost (CAPEX) and operational & maintenance cost (OPEX) of the proposed system should be primarily estimated so as to determine LCC and subsequently the Annualized LCC (ALCC) and the Cost of Electricity (COE).

  6. The payback period (PP) is the amount of time that is obtained from the ratio of the initial investment in the PV system to the savings due to the system.


References:

[1] “Alternative Energy Tutorials.” [Online]. Available: https://www.alternative-energy-tutorials.com/solar-power/stand-alone-pv-system.html.