5000 Watt Off Grid Solar Power System Design

Today we will introduce a design of 5kW off-grid solar photovoltaic power system for small fish farmer including the configuration and some calculation methods step by step.

There are some basic data to be clarified for the preparation of solar power system design.

• First of all, the user’s voltage & phase should be identified, either to be the single-phase AC 110V, 120V, 220V, 230V, or 240V or the three-phase AC 380V, 440V, 480V, etc. It decides the output specification of the solar inverter.
• Second, the load type should be confirmed to be either inductive load or resistive load, because the load type can decide the inverter’s on-load power and output waveform.
• Third, the full-load operation time, namely average daily electricity consumption, should be determined. In case of the grid-connected photovoltaic power generation station, there are no energy-storage devices, then only a reasonable power of the photovoltaic module is needed. In case of the off-grid solar photovoltaic power system, the battery capacity should be calculated, including the system’s self-stored power when there are no photovoltaic power generation conditions during successive overcast and rainy days.

Design of 5000W off-grid solar power system

Now we take a design of off-grid solar power system for a small-scale fish farmer near a lake as case study. Due to long-distance power grid construction, not only the cost is high, but also the conductor power loss and voltage loss are huge. Meanwhile, the stability of power use cannot be ensured due to the typhoons, and the accidental power failure happens on a frequent basis, which can affect the power use for production and life. Therefore, the off-grid solar power inverter is adopted. The sunlight radiation intensity in the daytime is high, and the power generated by solar power system is directly provided to the solar inverter output to support the operation of electric devices. At the same time, the batteries are charged, and provides the power for the devices via the inverter at night.

1. Survey on electricity demand

Here is some basic data to be known before. The voltage here in daily life is AC 220V 50Hz, and the commonly used appliances or devices mainly include:

Ten sets of fishpond pump oxygen-generating machines (300W)
One set of Television + Satellite receiver (200W)
One electric cooker (750W)
One induction cooker (2,000W)
One small refrigerator (100W)
Lighting (100W)

These devices are not used simultaneously. The pump oxygen-generating machine operates in the daytime when there is sun radiation but rests at night. The power of other devices is around 3000W, and their daily electricity consumption is around 10 kilowatts. Since the lake surface illumination is adequate, the self-stored electricity on overcast and rainy days is not taken into consideration.

2. Solar inverter

According to the data provided by users above, at this off grid solar PV system design, the all-in-one solar inverter with MPPT charge controller is adopted. This 5000W solar inverter with MPPT charge controller has 48V 7kV power capacity, power factor ≥0.8 and conversion efficiency ≥85%. The actual on-load power can reach 5000W, which can satisfy the requirements of user’s devices output power.

3. Battery capacity

This off-grid solar power system adopts the commonly-used lead-acid battery as the energy storage, which has a large capacity and a high cost performance ratio. The reserved electric quantity of the storage battery is 10KWh. Since the DC input voltage of the solar inverter is 48V DC, the theoretical capacity of the storage battery can be calculated as below:

10,000VAh/48V=208Ah

According to relevant technological standards of the batteries, to set the battery discharge rate to be 0.5C2 is relatively economically reliable, which can ensure the battery’s circulated charge and discharge times, and effectively extend the battery’s service life. Thanks to the ample lighting on the lake, the photovoltaic directly adopts the inverter output in the daytime. Not through the repeated discharge procedures of the batteries, the electricity consumption of electric devices at night is small, and the discharge duration is short. Therefore, this design properly increases the battery discharge capacity to 0.6C2. Then, the actual batteries’ capacity can be given as below:

208Ah/0.6 = 347Ah.

Here, the battery capacity is set to be 400Ah, then the total capacity is 48V 400Ah. The lead-acid batteries is 12V 200Ah per piece. Four pieces are connected in series, while four connected in parallel. So eight batteries are needed in total.

4. Power of solar panel module

After the capacity of the batteries is calculated, the power of solar panel module is calculated. The lake is located in a position with a high solar radiation intensity, and the effective sunshine time is as long as 6 hours. The polycrystalline silicon photovoltaic modules are chosen, with the photoelectric conversion efficiency being as high as 16%.

The solar power generation can be given by the following equation:

System power generation = Power of solar panel module × Sunshine duration × Coefficient of colligation.

The coefficient of colligation refers to the loss coefficient caused by the temperature changes, line loss, and solar charge controller (or inverter) conversion efficiency. Its value is generally set to be 0.5 to 0.7, and in this case, the coefficient of colligation is set to be 0.6. Therefore, the photovoltaic module power can be given as below:

48V × 400Ah/ (6h × 0.6) = 5333W

The specification of the PV panel module is set to be 36V 275W, its dimensions to be 1900×980×45mm, and its area to be 2 square meters. Every two pieces (72V) are connected in series into one group. Then, there are ten groups connected in parallel. In total, 20 pieces of solar panel module are required, with the total power 72V 5500W. The solar array area of the photovoltaic panel module is 40 square meters.

5. Anti-thunder photovoltaic combiner box

The photovoltaic combiner box is used to reduce the ligature between the photovoltaic module array and the inverter. The user can connect photovoltaic modules of certain quantity and of the same specifications together to form the photovoltaic array. Then, connect several photovoltaic modules in series and connect them in parallel in the photovoltaic combiner box. After confluence in the photovoltaic combiner box, it can be output to the inverter through the DC breaker.

The lake belongs to the area with the frequent occurrence of thunders. The isolated sheds and the surrounding forests are vulnerable to lightning stroke. Therefore, the photovoltaic power station must pay attention to the lighting stroke of devices. To introduce the DC high-voltage anti-thunder module can effectively protect the use safety of the inverter, AC distribution cabinet and other devices. At the same time, the large-power anti-countercurrent diode within the combiner box can effectively avoid the back discharge of the batteries to the module and cause the module’s burning when there is no photovoltaic at night.

6. Photovoltaic supporter and cable

The solar panel supporter is an indispensable accessory to the photovoltaic power system, which can fix the photovoltaic module. The user can also set up the supporter onsite to reduce the costs, fasten the module and resist against rustiness.

The cable is a wire which connects the module, inverter and the DC distribution cabinet. Part of the cable is used in the open air. Considering long-term exposure to the sunlight and rain-wash, the cable, which is resistant towards high temperature, oxidization and UV, can be chosen to guarantee the system’s normal operation. The cable should be the rough copper core full line with a large cross section and a small resistance to reduce the voltage reduction caused by long distance, which, if not prevented, might affect the power generation efficiency.

To sum up, the case study above provides a comprehensive introduction of design of the off-grid solar photovoltaic power system. It is hoped that the above introduction can be helpful to you.