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PV off-grid power system design and selection

Photovoltaic off-grid power systems are generally divided into: small DC off-grid power systems, small and medium-sized off-grid power systems, and large off-grid power systems. This system is now also in the category of micro-grid systems.

Small DC off-grid systems are primarily designed to address the most basic lighting needs in areas without electricity

Small and medium-sized off-grid systems mainly address the electricity needs of households, schools and small factories

Large-scale off-grid power generation systems are mainly used to meet the electricity needs of entire villages and islands

Photovoltaic off-grid power generation systems generally consist of photovoltaic arrays made up of solar modules, solar controllers, inverters, battery banks, loads, etc.

The photovoltaic array converts solar energy into electrical energy in the presence of light and supplies power to the load through the solar controller, inverter (or invertor) and at the same time charges the battery bank; in the absence of light, the battery supplies power to the AC load through the inverter.

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Modules

Photovoltaic modules are an important component of off-grid photovoltaic power systems, converting the sun's radiant energy into direct current energy. The irradiation characteristics and temperature characteristics are the two main elements that affect the performance of the module.

Reference formula: P0 = (P x t x Q)/(η1 x T)

* Where: P0 - peak power of the solar module, unit Wp; P - power of the load, unit W; t - -the number of hours of electricity used by the load per day, in H; η1 - is the efficiency of the system; T - the local average daily peak sunshine hours, in HQ- - the surplus factor of continuous cloudy period (generally 1.2 to 2)

Inverter

An inverter is a device that converts direct current (DC) into alternating current (AC) to meet the power needs of AC loads. According to the output waveform, inverters can be divided into square wave inverters, step wave inverters and sine wave inverters. Sine wave inverters are characterized by high efficiency, low harmonics, can be used for all types of loads and have a strong load carrying capacity for inductive or capacitive loads. According to the topology, they can be divided into high frequency inverters and industrial frequency inverters (the difference lies in whether they are equipped with an industrial frequency transformer or not)

Reference formula: Pn=(P*Q)/Cosθ

Where: Pn - capacity of the inverter, in VA; P - power of the load, in W; Cosθ - power factor of the inverter (generally 0.8); Q --margin factor required for the inverter (generally chosen from 1 to 5).

★ Note: a. Different loads (resistive, inductive, capacitive) have different start-up inrush currents and different margin coefficients. b. In high altitude areas, the inverter needs to be enlarged by a certain margin and used with reduced capacity.

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截屏2022-06-09 上午11.18.10

Controllers

The main function of the PV controller is to regulate and control the DC energy emitted by the PV modules and to intelligently manage the charging and discharging of the battery. Off-grid systems need to be configured with the appropriate specification of PV controller according to the DC voltage level of the system and the system power capacity. PV controllers are divided into PWM type and MPPT type, commonly available in different voltage levels of DC12V, 24V and 48V.

Reference formula: I=P0/V

Where: I - control current of PV controller, unit A; P0 - peak power of solar cell module, unit Wp; V - rated voltage of battery pack, unit V ★Note: In high altitude areas, the PV controller needs to be enlarged by a certain margin and used with reduced capacity.

Battery

The battery is the energy storage device of the power generation system. Its role is to store the electrical energy emitted by the photovoltaic module and to supply the load during power consumption.

Reference formula: C = P × t × T / (V × K × η2)  Lead-acid batteries

where: C - the battery capacity, unit Ah; P - the power of the load, unit W; t - the load The number of hours of electricity consumption per day, unit H; V - the rated voltage of the battery pack, unit V; K - the discharge coefficient of the battery, taking into account the efficiency of the battery, the depth of discharge, the ambient temperature, and the influencing factors, generally taking a value of 0.4 to 0.7; η2 --inverter efficiency; T - the number of consecutive cloudy days.

Reference formula: C=P×t×T /(K×η2)  Lithium-ion battery

where: C - capacity of the battery pack, unit kWh; P - power of the load, unit W; t - number of hours of power consumption of the load per day, unit H K - the discharge coefficient of the battery, taking into account the battery efficiency, depth of discharge, ambient temperature and influencing factors, generally taken as 0.8 to 0.9; η2 - the efficiency of the inverter; T -- the number of consecutive rainy days.

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Post time: Jul-29-2022