Guide to Detailed Design Methods for Solar Power Systems

Guide to Detailed Design Methods for Solar Power Systems

Up to this section, you have known the main parts and components used in solar power systems. The next step is to design a detailed solar power system, the more detailed it is, the easier it is to implement. Even with the simplest systems, you need to draw a diagram before construction.

Wiring and circuit diagrams will:

  • Make sure you don’t miss any components.
  • Supports the process of calculating wire sizes.
  • List all parts in the system and distribute work appropriately, especially when there is a group of people working together in the field.

Each system has its own unique wiring diagram, which varies depending on the types of components used. Please carefully read the product documentation accompanying each component or device, especially electrical information. At this stage, if you have not yet selected components or equipment, draw a general and detailed electrical diagram before proceeding with installation.

After drawing the wiring diagram, you need to remember the following points:

Ensuring safety: Conditions to be complied with

Like all other electrical systems, solar energy systems always have potential risks, but are often overlooked or not given adequate attention. You work with electricity and while individual components may be low voltage, the current can sometimes be large enough to cause problems.

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Furthermore, connecting multiple solar panels or batteries together can lead to high voltage. Therefore, it is important to properly evaluate safety during the detailed design phase and especially the installation phase. When designing a system, you should ask: “What is the worst that can happen?”

Solar power systems are relatively simple and the construction of all the components you will use is designed to keep risks as low as possible. However, there are still potential risks. If you pay attention to these risks, you can apply design measures to eliminate them.

Identify possible risks to the solar power system

With solar power, you will be working in several areas with potential hazards: DC power from panel arrays, high currents from batteries, AC power if using inverters, and high temperatures from solar panels. solar panels. Each of the above potential risk areas can lead to problems, both during installation and once combined.

These risks need to be assessed to ensure the design has the lowest level of risk.

Ground the electrical system

Except for very small solar power systems, such as powering alarms and garden lights, the solar power system must always be grounded. This means connecting a wire from the negative terminal to a ground rod buried in the ground. The grounding rod is a metal stake about 1 meter long, usually made of copper, sold at electrical stores.

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Grounding will prevent the build-up of static electricity and can prevent exposure to high voltages if the circuit fails. If you connect a panel array to your house, you must have a separate ground connection from that panel array. In other cases, this may just be optional, but is always a good idea.

You should ground from the panel array, if the array has a rated capacity greater than 200W. You must also ground the battery bank, because the current from the battery can reach very high currents. If there is both AC and DC power in the system, you must ground separately for AC power and separately for DC power.

Ground connection for electrical systems that cannot be grounded 

There may be situations where you have an electrical system set up but cannot be grounded. For example, a portable solar charger that can be carried around, or a yacht powered by a solar system. In general, these designs are very small, using only a few amperes of DC power.

If the panel array has a capacity of less than 100W, the system has a voltage of 12V and a current of not more than 10 A, it may not be necessary to connect a common ground for all components in the system. For large systems, ground wire is often used.

The ground wire is an electrical wire large enough to withstand high current, connected to the negative post on the battery, and each device that needs to be connected to ground is connected to this wire. The large wire that connects the batteries together is often used as the ground wire, and the smaller wire connects from the device that needs to be connected to ground to the ground wire.

Instead of the main ground wire, depending on what material the solar power system is installed on, you can use a metal frame to connect the general ground to the electrical system. The standard electrical system in a car, the car body, is made of steel, often used as a ground connection frame.

DC electricity

DC electricity in a solar power system is direct current generated by solar panels. This current is used to supply power to electrical devices in the system, or stored in batteries for use when there is no sunlight.

To ensure the safety and efficiency of the solar power system, it is necessary to pay attention to the following issues about DC power:

  • DC current is high voltage current. DC current from solar panels can reach hundreds of volts, so caution should be exercised when exposed to this current.
  • DC current has no phases. This means that DC current does not have alternating current, so it cannot be used directly for electrical devices that use AC power.
  • DC current cannot be transmitted long distances. Therefore, it is necessary to use an inverter to convert DC current into alternating current so that it can be used for electrical appliances in the home.

AC power

AC electrical safety is completely similar to household and industrial electrical safety. This is a high voltage system, 110V or 220V, and many countries state that the average person is not allowed to work on this electrical system unless you have the appropriate certification.

You should install two AC circuit breakers; one between the inverter and the power distribution board to completely isolate the solar power system when necessary, and one between the grid connection and the power distribution board to isolate the entire system from the grid, if your electrical system is connected to the distribution grid.

If you install a solar power system connected to the grid, you need to work with your branch or local power company to know the specific requirements and power purchase agreement agreements. The inverter must be a specialized type used in the system when the grid power is cut off. In case the system is connected to the grid, disconnect the power circuit from your solar power system if there is a grid failure, this ensures that electricity will not continue to be supplied to the grid, which can cause accidents for people working. work, repair, and restore the power grid.

High temperature

Solar panels are black and face the sun, on a sunny day they can get very hot; Although it may not be enough to cook eggs, it can burn the skin. Therefore, you should make sure the panel is placed in a place where curious children cannot touch it. If the solar panel is installed close to the ground, there needs to be a way to protect against curious people or accidentally touching the panel.

High temperatures become a serious problem if there is a problem in the panel array or the wiring between the panels. Damaged electrical wires or solar panels can cause a relatively large heat buildup, and can easily lead to a fire if not resolved promptly. An RCD, or GFI, will overcome this problem, allowing you to diagnose the problem before it becomes a serious problem. However, you should also install a manually operated DC circuit breaker to handle the problem. in case of emergency.

Safety issues are also presented in the system installation section; However, you need to remember, safety means no accidents. Consider safety issues when designing your system, and apply safety measures right into your solar power system. The additional cost of purchasing additional AC and DC circuit breakers, ground rods, RCD/GFI units, and the correct size wire, is completely reasonable and worth it.

Solar power system design methods

Every Panel in the array must face one direction. This ensures that each panel in the array must return to the photovoltaic cell and receive the same amount of light, which is a very important factor in generating electricity optimally.

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Sometimes you may have to install panels in different directions, for example on two different roof slopes. In this case, you need to make sure the two panels are separate, operate them as two separate arrays to supply power to an inverter or controller capable of operating with multiple panel arrays, or supply power to two inverter or two separate controllers.

Similarly, if you have solar panels of different sizes, you need to arrange them in separate arrays and wire them separately, use an inverter or controller capable of operating multiple panel arrays, or use separate controllers and inverters.

Panel array design – independent solar power system

If there is more than one panel and you want to operate the system with 12V voltage, you need to connect these panels in parallel to increase the power without increasing the voltage. If you want your solar power system to have a higher voltage, you can buy high voltage panels, or connect multiple panels together in series to increase the voltage to the desired value:

  • For a 24V system, you have the choice of using 24V solar panels, or two 12V panels connected in series.
  • For a 48V system, you can use one 48V panel, two 24V panels in series, or four 12V panels in series.

After achieving the desired voltage value, you can connect the panels in series and parallel, connect the panels in series to achieve the desired voltage, then connect the arrays of panels together in parallel. to achieve the required capacity, Figure 13.2.

Design of panel array – system connected to the grid using microinverter

With a solar power system connected to the grid using microinverters, the design is very simple. Each panel will be a complete solar power system, supplying power to its microinverter. Microinverters convert DC electricity into AC electricity and supply it to the main AC circuit.

Panel array design – system connected to the grid using an inverter 

If designing a grid-tied solar power system using an inverter, in general, all panels are connected in series together and supply high DC voltage to the inverter (Figure 13.4).

Due to the very high DC voltage present, additional safety measures are required. The solar panel array must be grounded, a DC circuit breaker installed between the panel array and the inverter, RCD or GFI to disconnect the panel array when a short circuit occurs.

In the diagram illustrated in Figure 13.5, there are 16 solar panels connected in series. If each panel has a rated voltage of 12V, this system will have a nominal voltage of 192V, a rated capacity of about 320V and an open circuit voltage of up to 416V.

Because the voltages are very high, it is necessary to install additional DC circuit breakers in the middle of the panel array to reduce the voltage in the array when the units are disconnected. This will make the system safer to maintain, reducing the risk of electric shock and fire. In this diagram there are two AC circuit breakers, one between the inverter and the distribution panel to completely isolate the solar power system from the house, and one to isolate the house from the grid.

The United States requires that all parts of the solar energy system connected to the distribution grid must have a capacity of no more than 600V. That means you cannot connect more than 20 12V panels or ten 24V panels in series to ensure the safety of the entire system.

In Europe, the open circuit voltage must be below 1000 V. In general, this means that no more than 30 12V panels or 15 24V panels should be connected in series. If the system is operating close to this limit, you have three options:

  • Install microinverter system.
  • Install a multi-branch system, use a common inverter for the branches, or use separate inverters for each branch
  • Connect the panels in a mixed series-parallel fashion.

Open-circuit voltage data are given in Table 10.1, information about multi-branch systems is given in Figures 10.4 and 10.5.

Acquy

The battery is wired in the same manner as the panel array. Depending on voltage and power requirements, you can connect multiple 12V batteries in parallel to get high power, or connect batteries in series to get high voltage, or connect them in series and parallel.

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When connecting batteries in parallel, it is important that one output wire must be from the positive terminal of the first battery and the second output wire must be from the negative terminal of the last battery in the battery bank, Figure 13.6, right. This ensures equal discharge and charging through the entire battery bank.

If both banks output from the first battery, Figure 13.6, left, connected to the controller and inverter, the first battery will discharge faster than the remaining batteries in the bank, and will receive the most charge from the network. panel. panel. This reduces the life of the first battery, and at the same time causes all batteries in the series to become unbalanced, leading to a marked reduction in life. The other batteries in the bank are not fully charged from the panel array, because the first battery is fully charged first, the controller will turn off the power, instead of continuing to charge the remaining batteries in the bank.

Remote controll

The controller has connections to the panel array, battery wiring, and DC loads. Although the heat generated in the controller is not as high as the inverter, it can still cause it to heat up. You should ensure that the controller is installed in a well-ventilated area that is easily accessible for inspection or maintenance.

Frequency converter

When used in stand-alone or standby power systems, the inverter is connected directly to the battery bank and not through a controller. You need to ensure that the inverter in the solar power system is located in a well-ventilated and easily accessible place for inspection or maintenance; At the same time, pay attention to the weight of the inverter.

Device

Devices must be connected to the inverter, if operating at mains voltage; or connected to the controller if operating with low DC voltage; It is not allowed to connect directly to the panel or battery array.

Characteristics of grid backup solar power system

Because grid backup (failure) systems do not connect the solar power system to the distribution grid, you are less limited in the components or equipment you can use.

However, you still must comply with basic legal regulations regarding the electrical system where you live. In some countries (e.g. UK), this means that your backup solar power system must be made by a qualified electrical specialist or technician, making the connections from the source. mains electricity into your house.

The design of a grid backup (incident) power system is similar to an independent solar power system, including solar panels, controllers, and batteries. The only difference is the part behind the battery bank.

The main advantage of a grid backup system is that it can operate in three ways: supplying power to the entire house, supplying power to pre-selected circuits in the house, or supplying power to just one circuit. . Information on circuit diagrams of grid backup (failure) power system configurations is given in Chapter 15.

Protect electrical circuits for solar power systems

All electrical systems must be protected to ensure safe shutdown when a short circuit occurs; Applicable to low voltage systems and high voltage systems. Low voltage systems can cause serious problems due to the high current from the 12V battery; If it exceeds 1000 A, even for a very short time, it can still cause electric shock with serious injury, even death.

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In case of a short circuit, the circuit will be very hot and the wire sheath will melt in just a few seconds, if not properly protected. This can easily lead to a fire, appropriate protective measures are needed to ensure the electrical system is not damaged by an accidental short circuit (ground fault).

Grounding

In all electrical systems, the negative terminal of the battery must be fully grounded. If a suitable means of grounding is not available, a standard ground rod or riser may be used.

DC circuit protection

For small systems under 100W, a fuse installed in the controller is sufficient to protect the basic circuit. In larger systems, the power supply to some devices does not go through the controller, the fuse is installed in the positive battery terminal.

When installing a fuse on the battery, you must ensure that all current output from the battery must pass through the fuse. In a DC system with many circuits, a fuse should be installed for each circuit. If you use 12V or 24V voltage, you can use the same household fuses and circuit breakers. For DC systems of the type used in high voltage circuits, you must use a dedicated DC fuse.

When connecting devices to a DC circuit, there is no need to use a separate ground wire for the device, because the battery negative terminal is already connected. Install a DC circuit breaker (circuit breaker) between the panel array and the inverter or controller. Install a second DC breaker between the battery bank and the controller or inverter.

Unless the inverter or controller has built-in ground fault protection, you should install an RCD or GFI device between the panel array and the controller or inverter.

Protects AC circuits

The AC circuit must be supplied through the electrical distribution board (distribution cabinet). This distribution cabinet must be grounded and have an integrated circuit breaker with cool run fault protection (RCD or GFI). After grounding the DC circuits, you must separately ground the AC circuits. You should also install an AC circuit breaker (circuit breaker) between the inverter and the power distribution panel.

For electrical systems connected to the grid, this is a mandatory requirement, but should also apply to other systems.

Cable size and selection

After having the electrical diagram, you need to determine the wire lengths in each part of the diagram, and determine each type of wire, according to the standard, needed for the electrical system.

Determine wire size

The method of determining the size has been presented in the previous section, and it is not redundant to repeat the wire, because the wire is an easily confused or misjudged part when installing an electrical system.

Low voltage systems suffer from large power losses through the wires. This is because the current is high when the voltage is low. Ohm’s law states that power loss through a conductor is proportional to the square of the current; the higher the current, the greater the resistance. To overcome this resistance, it is necessary to use wires with a large enough cross-section.

When using low voltage electrical wiring (from the panel array to the controller, and to all low voltage DC equipment), be sure to use the correct size electrical cable; If the wire size is too small, the voltage can drop to the point where the system fails. You can calculate the wire size according to the following formula:

  • (L x | X 0.04) ÷ (V ÷ 20) = wire cross-section
  • L: electric wire length, in m
  • I: current intensity, calculated in A
  • V: system voltage (12 volts or 24 volts) Wire cross-section in square mm.

To change gauges or sizes to American standard wire (AWG), see Table 11.1. The above wire cross-section is the lowest value that can be used; It is not allowed to use a smaller wire size, because the voltage drop will increase when the wire cross-section is reduced, thereby affecting the operation of the equipment.

 Protect power lines

When creating a wiring diagram, you need to make sure the power lines are protected, protected from destructive animals, protected from children, and avoided from accidental or intentional acts of vandalism. Mice biting the protective covering of electrical wires can lead to problems, even very serious. In general, electrical lines are often placed in appropriate conduit for protection.

Design the system with the shortest power lines 

If there are multiple devices located in different locations, you can design multiple power lines running in parallel to shorten the wire length, instead of one line running through all of those locations. By doing this you achieve two goals: reduce the length of each electrical cable and divide the load between several circuits, thus reducing the cross-section of the conductor, allowing you to save costs and installation processes. also becomes easier.

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If you wire your house, you can use an electrical distribution board connected to each circuit. For example, in a vacation home project, you should have separate circuits for upstairs lighting and downstairs lighting. By increasing the number of circuits, you can even use double household wires, with ground wires inside, for your home electrical network, instead of having to use specialized wires, which is not only simple but also economical. expense.

Full options for solar power systems

For solar power systems, non-standard design and poor installation will lead to poor performance but the cause is unknown. This usually happens in the first two to three years after the system is installed. The source of this problem is usually bird droppings or ultraviolet rays that degrade the wires leading from the solar panel to the inverter.

This is often due to not using connecting wires with covers that provide enough UV protection, are designed to withstand high temperatures and are resistant to the weak acidity of bird droppings. It is important to use the correct type of specialized electrical wire to connect the solar panels together and connect to the inverter or controller.

If necessary, you should look for electrical cords with specifications that meet UL 4703 or UL 854 (USE-2).

Electrical wire connecting the controller

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When calculating the cross section of the wire connecting the controller and the battery, you need to take into account the current entering the battery from the panel array and the current going out; In general, the peak current entering the battery is often much higher than the output current.

Electrical wires connect the batteries

You can purchase the battery wires along with the battery terminals (electrodes) from the battery supplier. Because the current between batteries can be very high, you should use the largest wire size to connect the batteries together (both in parallel and in series).

Solar power mounting frame

There are many types of frames available to install solar panels, from which you can choose the frame that suits your system.

But, sometimes they are not completely suitable for your project, both in terms of technique, taste, and cost. In that case, you can make your own, for example using angle steel, or have it made to your specifications.

Solar panels are not heavy, but you need to consider the impact of wind on the installation frame structure. If wind blows across the underside of the panel array, it will create a “lift” force and pull the array away from the mounting frame. However, the gap below the panel array serves to ventilate and cool the panels. This is especially important in hot climates, as panel performance declines markedly as temperatures increase.

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What is a solar power system? Should I install a solar power system or not?

Ensuring the panel mounting bracket is durable is important, as the panel array is angled at the optimal angle to capture sunlight. This often means that, even when placing panels on an existing roof, to achieve the highest possible performance the solar panels may have to be mounted at a slightly different angle to the roof slope.

Therefore, in principle, the solar panel mounting frame must be durable enough to work for at least 20 years, including in places with harsh environments, sun, heat, rain, snow…

For installing panel arrays on the roof

If installing panels on the roof, you must make sure the roof is durable enough to handle the weight of the panel along with other attached components. If you are not sure about this, you need to consult or review the roof structure or house construction documents.

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If you plan to install the panel array on piles or on a frame placed on the ground, you need to have a foundation design plan that is durable enough. The foundation consists of a reinforced concrete base on a compacted gravel or small stone base, a frame placed on this foundation with appropriate anchor bolts with a diameter of 25-30 mm.

For installing panel arrays on piles

If installing panel arrays on piles, you should follow the manufacturer’s advice. In general, piles must be placed into a concrete foundation no less than 1 m deep.

To install the panels on the rack frame, make sure to use high-strength bolts and self-locking nuts to avoid the possibility of self-loosening due to wind action.

If the panel array is installed in an easily accessible place, you should consider the possibility of using a bracket system that allows adjusting the tilt angle according to the months of the year, increasing the tilt angle in winter and decreasing it in summer; This will significantly increase the efficiency of the solar power system.

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For home solar power projects , the panels are installed in a garden shed with a sloping roof. The benefit of this option is that you can build the warehouse in a location that receives optimal sunlight. You can also position the batteries and controller close to the panel array, thus allowing for an “all-in-one” power station.

Good quality house structures, with repair and maintenance every 2-3 years , can have a lifespan of up to 25-30 years .

If you follow this route, you need to specialize in:

  • The angle of the roof must be correct so that the solar panels are optimally positioned
  • The roof must be strengthened to be able to bear the weight of the solar panel array
  • The floor of the warehouse, where the batteries are located, must be made of wood or laminate. Batteries do not work well on concrete floors, especially in winter
  • There must be good ventilation in the warehouse to expel hydrogen gas from the batteries out through the top of the warehouse roof.
  • The warehouse door must be large enough to easily install, inspect, and maintain the batteries.
  • You should consider insulating the warehouse floor, walls, and ceiling with polystyrene (Styrofoam) panels, to keep the batteries from getting too cold in the winter or too hot in the summer.

Solar power installation houses must have a hard concrete foundation. Gutter and rainwater drainage systems are also important, standing water on the floor can short circuit electrical wires or batteries.

Battery location (if any)

You need to choose a suitable location for the batteries. This position must meet the following conditions:

  • Watertight and weather resistant.
  • Do not expose to direct sunlight.
  • Good insulation, avoid temperatures that are too high or too low.
  • Good ventilation.
  • Avoid sources of sparks. Turk
  • Keep away from children and animals.

When charging, lead-acid batteries release a small amount of hydrogen gas. You must ensure that the place where the batteries are placed is large enough and well ventilated so that hydrogen gas does not accumulate.

Because lead-acid batteries have very high current, they must be placed in a safe place, away from children and animals (dogs, cats, mice, birds, etc.).

Do not place batteries directly on the concrete floor, dilute acid spilled from the battery will damage the floor.

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You need to ensure easy access to where the batteries are located for inspection and maintenance. Many deep cycle batteries require water changes several times a year and connections must be inspected regularly to prevent rust.

For the reasons mentioned above, the batteries are often placed on a durable frame, surrounded by thin wire mesh.

If you place the battery in a place that is too cold or too hot, you need to take measures to insulate the battery. Temperatures that are too high or too low will reduce battery performance. If the area where the battery is located has a temperature below 8°C or above 40°C, you need to insulate the battery.

You can use polystyrene (Styrofoam) sheets, placed under and around the battery for insulation, but you are not allowed to insulate the top of the battery, because the battery needs constant ventilation, and can short-circuit the battery if the object Insulation materials have electrical conductivity, even though they are very small.

Plan the installation

At this point, with a relatively complete shopping list, you need to determine the exact location of each part in the electrical system and the steps to perform the installation process.

Before placing an order or shopping, you need to go to the site and check everything one last time. Make sure the positions of panels, controllers, batteries… are all appropriate, and do not miss or confuse anything.

Once you are satisfied and sure of everything, you can submit the order or go shopping for equipment, wires, circuit breakers, inverters, etc.

Conclude

  • The detailed design ensures you do not miss any part of the design.
  • Consider carefully the safety issues in the design. At each stage, you should ask yourself, “What is the worst that could happen?” and design to avoid those problems.”
  • A circuit diagram or wiring diagram will help you visualize the operation of the electrical system.
  • You need to make sure that the wires are as short as possible. This can be done by designing several parallel lines, directly from the controller, through an electrical junction box, or through a distribution panel.
  • Dividing the power cables into parallel circuits, allows the current load on each circuit to be reduced, thereby reducing resistance and improving system performance.
  • If using an inverter to operate the system at mains voltage, only a legally certified electrical technician is allowed to make the electrical system connection, but your electrical diagram will help this person visualize the system. your electrical system easier.
  • You need to design the place where the battery is placed to ensure the best conditions for the battery to operate.