# Calculation of photovoltaic off-grid system

//Calculation of photovoltaic off-grid system

## Calculation of photovoltaic off-grid system

The scale and application of the photovoltaic system vary, such as the large scale of the system, small to a few watts of solar yard lights, and large to MW solar photovoltaic power stations. Its application forms are diverse, and can be widely used in many fields such as home, transportation, communication and space application. Although the photovoltaic system is of different sizes, its composition and working principle are basically the same. Solar power system consists of solar cell, solar controller and battery. If the output power is ac 220V or 110V, the inverter needs to be configured. The functions of each part are:

(1) solar panels: solar panels are the core of the solar power generation system and the most valuable part of the solar power generation system. The effect is to convert the sun’s radiant power into electricity, or to storage in a battery, or to drive the load.

(2) solar controller: the function of the solar controller is to control the working state of the whole system, and it has the effect of charging protection and over-discharge protection for the battery. In a large temperature difference, the qualified controller should also have the function of temperature compensation. Other additional functions such as light control switch and time control switch should be optional for the controller;

(3) battery: it is generally used for lead-acid batteries, small and micro systems, nickel-cadmium batteries, nickel-cadmium batteries or lithium batteries. The effect is to store the electricity generated by solar panels when light is available and release them when needed.

(4) inverter: in many cases, the ac power supply of 220VAC and 110VAC is required. Because the direct output of solar energy is generally 12VDC, 24VDC, 48VDC. To provide power to 220VAC electrical appliances, the direct current generated by the solar system needs to be converted into AC power, so the dc-ac inverter is required. In some cases, the dc-dc inverter is also used when the load of multiple voltages is required, such as converting the power of 24VDC to 5VDC (note that it is not a simple step-down).

The design of pv system includes two aspects: capacity design and hardware design.

Before for the design of photovoltaic system, need to understand and get some basic data to calculate and select the necessary: location of pv systems, including location, latitude, longitude and altitude; Meteorological data in the region, including the month by month of solar total radiation, direct radiation and scatter radiation, the maximum and minimum temperature, annual average temperature and the longest continuous rainy days, the maximum wind speed and hail, snow and other special weather situation, etc.

The design of the battery includes the design of the storage battery capacity and the series and parallel design of the battery. First, the basic method of calculating the battery capacity is given.

(1) basic formula

i. the first step is to multiply the amount of electricity required by the daily load times, and the initial battery capacity can be obtained by the number of self-supporting days determined according to the actual situation.

II. The second step is to divide the battery capacity of the first step by the allowable maximum discharge depth of the battery. Because the battery cannot be discharged completely in the self-supporting days, it is necessary to divide the maximum discharge depth to obtain the required storage capacity. The choice of maximum discharge depth needs to refer to the performance of the photovoltaic (pv) systems use battery parameters, can get details from battery supplier information on the depth of the largest battery discharge.Usually, if you use a deep – cycle battery, it is recommended 80%depth of discharge（DOD），If you are using a shallow cycle battery, use 50%DOD，The capacity BC of the battery is calculated as follows:

# BC=A QL NL TO/CCAh(1）

Where: A is the safety factor, between 1.1 and 1.4; QL is the average power consumption per day, which is the working current times the number of working hours per day; NL is the longest continuous rainy days; TO as the temperature correction coefficient, generally take 1 above 0 ℃, 10 ℃ above 1.1, the 1.2 – below 10 ℃, CC for battery discharge depth, general lead acid battery for 0.75, alkaline nickel cadmium battery for 0.85.

Below we introduce the method of determining battery series and parallel. Each battery has its nominal voltage. In order to achieve the nominal voltage of the load, we connect the accumulator to the load, and the number of accumulators in series is equal to the nominal voltage of the load divided by the nominal voltage of the battery.

The basic idea of battery component design is to meet the annual average daily load demand. Calculation of solar battery components is the basic method of using the energy needed to power a load on an average day (hours) divided by a solar battery components can generate energy in a day (hours), so you can calculate the system needs to be parallel quantity of solar battery, the use of these components in parallel system load required can generate electricity. The nominal voltage of system can be divided by the nominal voltage of solar battery components, can be the sun in the series of battery components require quantity of batteries, use these tandem solar cell components can generate the voltage needed to system load. The basic calculation formula is as follows:

the number of components in parallel, the load average (AH)/components, output (AH) series components number = system voltage (V)/components voltage (V) above is not revised formula. The following formula for reference solar cell phalanx design: Ns = UR/Uoc = (Uf + UD + Uc)/Uoc (2) : UR for solar cell phalanx minimum output voltage; Uoc is the best working voltage for solar cell components; Uf is the battery charging pressure; The UD is the diode pressure drop, generally taking 0.7V; UC is the pressure drop caused by other factors. Solar cell component parallel number Np.

(1) install the solar cell phalanx location of solar radiation Ht, converted to light intensity in the standard of the average daily radiation hours day radiation (see table 1) : H H H = Ht x 2.778/10000 (3) : 2.778/10000 (h. kJ/m2) for daily dose conversion into the standard intensity (1000 w/m2), the daily average radiation coefficient of hours. Solar power generation QpQp=Ioc * H * Kop * Cz (Ah) (4) : Ioc is the best working current for solar cell components; Kop is the slope correction coefficient (refer to table 1); Cz is the correction coefficient, mainly for the combination, attenuation, dust, charging efficiency and other losses, generally 0.8.(3) the shortest interval between two groups of the longest continuous rainy days Nw, the data for the uniqueness of this design, mainly considering in this period of time will lose battery power supplement, be the battery capacity of Bcb is: Bcb = A * QL * NL (Ah) (5) (4) solar modules in parallel method for calculating the number of Np is: Np = + x QL Nw (Bcb)/(Qp) Nw) (6) (6) the expression of meaning: the number of sets of solar array in parallel, in the shortest interval between the two groups of consecutive rainy days in power, not only for the use of the load, also need to make up the battery in the longest continuous loss of power in the rainy day.

(3) the solar cell phalanx of power calculation according to the number of series-parallel solar modules can be concluded that the power of the solar cell phalanx required P: P = Po * Ns * NpW (7) : in the Po for the power rating of the solar cell components. A really good designer should consider the following factors: 1. Where is the solar power system used? What are the solar radiation conditions in the area? 2. What is the load power of the system? 3. What is the output voltage of the system, dc or ac? How many hours does the system need to work every day? 5. How many days should the system be continuously supplied if there is no sunlight? 6. In the case of load, pure resistivity, capacitive or inductive, how large is the starting current? 7. The number of system requirements.

By |2018-03-08T21:28:56+08:00March 8th, 2018|Solar Panel|0 Comments 