How to Design a Grid Tie Solar Power System for Your Home?

Today we will introduce a home solar power system design below 10kW, which is installed on the roof of the house and connected to the grid of 220V or 380V. The maximum installation capacity is 25% of the capacity of the upper-level transformer.

Choose the appropriate installation site

The household roof generally adopts the tile structure and cement structure, so it is necessary to go to the site for inspection before installation, because not every roof is suitable for the installation of solar panels.

First of all, it is required to determine whether the load capacity of the roof can meet the requirements. The loading requirements of the solar power system equipment for the roof are greater than 30kg/m². Generally, the cement structure houses built in the past 5 years can meet the requirements, while the houses of the brick structure built 10 years ago should be carefully investigated.

Secondly, it is required to check whether there are shadows in the surrounding area. Even a small amount of shadows will affect power generating capacity, such as water heaters, utility poles, tall trees and so on. In addition, if there are dust emissions near the road and houses, the modules should be polluted, thus affecting the power generating capacity.

Last but not least, it is required to check the roof orientation and inclining angle. When the solar panel faces the south and has the best inclining angle, the power generating capacity is the highest. If it faces the north, a great amount of power generating capacity will loss.

Choose the appropriate solar panel

The solar panels have three types of polysilicon, monocrystalline silicon and thin film. Each type has its own advantages and disadvantages. Under the same conditions, the efficiency of the PV system is only related to the nominal output of the solar panels, and has no direct relationship to the efficiency of the solar panels. The PV panel module has two kinds of 60 pcs and 72 pcs solar cells. The grid tie solar power system for home is generally small in scale and difficult to install. Therefore, it is recommended to use 60 pcs solar panels, which have small size, light weight and easy installation.

Choose the appropriate mounting bracket

According to the roof conditions, aluminum bracket, C-shaped steel bracket, stainless steel bracket and other brackets can be selected. In addition, after taking into account factors such as the strength of solar panel mounting bracket, cost of solar power system, and roof area utilization, the inclination angle of the solar panel should be minimized as much as possible to reduce the wind receiving surface, thereby increasing the strength of the bracket, reducing the cost of the bracket under the premise of not significantly reducing the system power generating capacity (the decrease no more than 1%).

Leakage is a problem that needs to be paid attention to during the installation of solar power system. The PV system will be safe if it has a good waterproof performance. The photovoltaic bracket is mounted on the roof to support the assembly and is connected to the roof. Its design adopts the top-top method, which will not perforate and damage the original waterproofing of the roof. The pressing block adopts the prefabricated structure, requiring no on-site casting and avoiding the hard destruction of the waterproof layer of the roof by the installation of the solar panel mounting bracket.

Solar panels series-parallel connection design

In this grid-connected solar power system, solar cells are connected in series to form a string branch. The series connection is used to raise the input voltage range from the DC voltage to the inverter. It should ensure that the solar cells do not exceed the inverter input voltage range under various solar irradiance and ambient temperature conditions.

The working voltage is around the rated working voltage of the inverter, which has the highest efficiency. The single-phase 220V inverter has the rated input voltage of 360V. The three-phase 380V inverter has the rated input voltage of 650V. As to the 3000W inverter, if it is equipped with 260W solar panel with 30.5V working voltage, the working voltage is totally 366V for 12 pieces and the power is 3.12kW, which is the best. The 10kW inverter is equipped with 260W solar panel, 40 pieces, 20 strings for each, the voltage is 610V, and the total power is 10.4kW, which is the best.

Cable selection

In the home solar power system, it is recommended to use the copper cable. Because the solar panel MC4 joint, the solar inverter output wiring terminal and the wiring terminal of the grid-tie switch are made of the copper core. If it adopts the aluminum wire, and the copper wire is directly connected to the aluminum wire, a kind of chemical battery will be formed, that is the electrochemical corrosion. It will cause the bad contact between copper and aluminum, thus increasing the contact resistance. In case of a current passing, the contact part has a temperature rise, which accelerates the contact corrosion and increases the contact resistance, thus causing vicious circle until being burnt. The output terminals of the solar inverter are also made of the copper. If the aluminum wire is used, the diameter should be larger. For example, for the 30kW inverter, it is designed to use the 10 m² copper wire, or 16 m² aluminum wire. If the cable area increases, and the water-proof wiring terminal has a limited area, it might be unable to tolerate. Besides, it is still required to select the BVR flexible copper wires of several strands instead of the BV hard copper wire, since the hard copper wire is easily to have bad contact with the wiring terminal, and the turning site still has the stress, it will easily cause bolt looseness and bad contact.

Common solar power system design scheme for home

As to the common household PV systems, the single-phase one is generally 3kW to 8kW, and the three-phase one is generally 4kW to 10kW. If permitted, it is recommended to use three-phase grid-tie solar power system, because under the same conditions, three-phase on-grid PV system has fewer investment and higher generating capacity than the single-phase grid-connected solar power system. For example, as to the 10kW grid tie solar power system, if it is connected to the single-phase grid, it will need 2 grid-tie inverters, 4 strings of solar panels for DC input, 8 DC cables and 2 switches for AC output. However, if it is connected to the three-phase power grid, it will only requires 1 on-grid inverter, 2 strings of solar panels for DC input, 4 DC cables and 1 switch for AC output. The solar system with three-phase grid connection has smaller current, lower loss and higher efficiency than the one with single-phase grid connection. The 3-phase on-grid solar system has less influence on the power grid, and will not stop because of promoting the grid-connection voltage.

For example, here is a brief design plan of 3000W grid tie solar power system for home.

The design requires about 30m² roof area, and adopts 12 pieces of 265W solar panels and the total power is 3.18KW. The PV system adopts 1 set of 3000W solar inverter, accesses to 220V power grid into the original indoor power distribution box, and then connects with the owner’s indoor low-voltage power distribution network through the 220V circuit before being sent to the mains supply.

The maximum DC voltage of the inverter (maximum array open circuit voltage) is 550V, the maximum power voltage tracking range is 70~550V, and the number of MPPT channels is 1 channel/1 grid. The rated operating voltage of each solar cell module is 30.8V, and the open-circuit voltage is 38.3V. Under the rated working conditions with the ambient temperature of 25±2℃, and the solar irradiance of 1000W/m2, the rated working voltage of the series branch of the 12 solar batteries is 369.6V and its open circuit voltage is 459.6V, which are both within the allowable input range of the inverter. Therefore, it can maintain normal operation.

In the variation of the working conditions, considering that the mean extreme environment temperature is -10℃, the working voltage of the solar cells at the maximum power point is 12 × 30.8 × (0.35% × 35 + 1) = 415V, meeting the input voltage requirement of the 550V maximum full loading MPPT point. When the mean extreme environment temperature is 42℃, the working voltage of the solar cells at the maximum power point is 12 × 31.2 × (-0.35% × 17 + 1) = 352.1V, meeting the input voltage requirement of the 70V minimum full loading MPPT point.

See the table below for the basic configuration of this home solar power system.