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Engineer talks. How solar panel systems work in 2023

Grid-tie systems Hybrid systems Off-grid systems Getting started Solar panels PV system design

Engineer talks. How solar panel systems work in 2023

04 Sep 2020
11 min
Intermediate

Vasilii SmirnovAuthor

Musk predicts that ordinary non-solar roofs will have become a thing of the past by 2032. So, you still have time to make money on the Sun by selling solar energy to the government or your neighbors. Here is how it works.

Basic principles of solar energy

Everything around you is energy

To understand how photovoltaic systems work, let's dive into the basics first. So, everything around us is energy. The food we eat is calories, the sound we hear is a vibration of air and the light we see is a stream of tiny particles called photons. Each photon has its energy, mass and impulse. One of the most powerful sources of photons is the Sun. The Earth intercepts 173 thousand terawatts of solar power. That's ten thousand times more power than the planet's population actually uses.

×10K

More power sun produces, than we use

The Sun radiates energy even when you can't see it: when it is cloudy or cold, in winter or even at night.

Solar panels turns light and sun radiation into an electricity

Solar panels are made up of smaller units called solar cells. The latter, in turn, are usually made of silicon, a semiconductor that is the second most abundant element on Earth. In a solar cell, crystalline silicon is sandwiched between conductive layers. Each silicon atom is connected to its neighbors by four strong bonds, which keep the electrons in place, so no current can flow. Here's the key: a silicon solar cell uses two different layers of silicon.

N-type silicon has extra electrons, and P-type silicon has extra spaces for electrons, called holes. Where the two types of silicon meet, electrons can wander across the P/N junction, leaving a positive charge on one side and creating negative charge on the other.

When one of these photons strikes the silicon cell with enough energy, it can knock an electron from its bond, leaving a hole. The negatively charged electron and location of the positively charged hole are now free to move around. However, they have only one way to go because of the electric field at the p/n junction. The electron is drawn to the N-side, while the hole is drawn to the P-side.

The mobile electrons are collected by thin metal fingers at the top of the cell. From there, they flow through an external circuit, doing electrical work, like powering a lightbulb, before returning through the conductive aluminum sheet on the back. Each silicon cell only puts out half a volt, but you can string them together in modules to get more power. Twelve photovoltaic cells are enough to charge a cellphone, while it takes many modules to power an entire house. Electrons are the only moving parts in a solar cell, and they all go back where they came from. There's nothing to get worn out or used up, so solar cells can last for decades.

The French physicist Alexander Becquerel first discovered the photoelectric effect in 1839, which explains why the solar cells generate direct current.

You can't use electricity directly from solar panel

In order for electricity to be used, a special box called an inverter converts direct current from a solar panel to alternating current that is used in the distribution grid.

Alternating Current (AC)
It is easily transmitted over long distances and delivered to the apartment / house via the distribution grid. It is used to power household appliances: coffee grinder, vacuum cleaner, stove, microwave, washing machine, light bulb...

Direct Current (DC)
It is converted from AC through power supplies of electrical appliances. It is used in all gadgets (smartphones, tablets, laptops) and the electronic components of devices that use alternating current.

Solar panel design

The most popular type of domestic PV systems is a crystal solar panel in a rigid aluminum frame. The frame makes it easy to mount solar panels, and also prevents them from overheating. Special tempered low-iron glass allows you to protect fragile solar cells from the environment and get the most out of the conversion of solar radiation into electricity.

Each solar panel mounted on a roof or a mounting rack consists of three components:

Solar cells
Solar cells can differ by the type of silicon used (monocrystalline or polycrystalline), by the type of production (crystalline or amorphous), and by the structure (organic or non-organic).
Junction box
The junction box is usually located on the back of the solar panel (though there are some exceptions) and serves as a connector between the cable and the solar cells. In order for the electric current to go only in one direction, there are special diodes in the contact box. Without them, electricity would be dissipated from the surface of solar panels at night, so they are called 'hot-carrier diodes' or 'Schottky diodes'.
Cables
There are two cables: positive and negative. Direct current flows from positive to negative, so it is important not to confuse them in order to avoid a short circuit. This is why male and female cable connectors were designed and marked to prevent incorrect connection. It is also important that the cable can withstand high temperatures and direct sunlight. Therefore, cables for solar panels usually have UV protection and double insulation.

How to choose a solar panel system type

Grid-tie solar system allows you to sell electricity

It consists of an array of solar panels and a grid-tie inverter. Solar panels generate direct current, which is transmitted to the grid with the help of the inverter. It converts DC to AC, suitable for household appliances, and sends it to the electrical grid.

The grid-tie inverter converts the current, the voltage of which is a little bit higher than the voltage of the public power grid (for example, 238-240V). That is why it is consumed first, so less electricity is taken from the distribution grid to power consumers. This is how savings are made.

If the power outage happens, the grid-tie inverter also stops working. This is due to security reasons, to prevent the electricity company worker from getting electrocuted.

The inverter is connected to the domestic power grid after the meter to account for the electricity delivered to the grid or consumed from the distribution grid.

If the country allows selling electricity to the grid, then, if there is some surplus power (when not all electricity is consumed), you can sell it via distribution microgrids to your neighbors or through municipal contracts to the public power grid. Thus, the household becomes a seller of electricity, i.e. a prosumer.

Grid-tie solar systems are suitable if

You want to save money and do not want to belabor.
You want to sell some of your energy to your neighbours or the state.
You need an uninterruptible power supply.

Not suitable if

You need an uninterruptible power supply.

Off-grid solar panel systems makes you independent

It consists of an array of solar panels, a battery charge/discharge controller, the battery itself, and a voltage converter (off-grid inverter).

Direct current from the solar panels is transmitted to the charge controller. The solar controller distributes the current at the right voltage and in the right amount, depending on the charge and state of the battery so that it does not deteriorate. An inverter is connected to the battery by means of conductors and converts direct current to alternating current, making it of the necessary frequency and voltage. All household consumers, in turn, are connected to it.

Controller prolongs the lifetime of a battery

In this case, you will not be able to give surplus electricity to the grid: when the battery is fully charged, the solar panels will stop transmitting current to it. On the other hand, the autonomous inverter works even when the public power grid is not functioning, since the it does not transmit electricity there.

Off-grid solar systems are suitable if

You need uninterrupted power supply.
The household does not have access to the public electricity grid.
The difference in cost of electricity during the day varies considerably.
You want to sell some of your energy to your neighbours or the state.
You need to periodically increase the capacity of the grid

Not suitable if

You want to sell some of your energy to your neighbors or the state

Hybrid solar panel systems can do both

The most expensive and complex type of solar power systems. It consists of an array of solar panels, a battery charge/discharge controller, the battery itself, and a hybrid inverter.

Direct current from solar panels first gets to the battery charge/discharge regulator, which charges the battery in a favorable mode or keeps it in a standby mode (a state of recharging). Surplus electricity is fed to a hybrid inverter, which sends it to the domestic power grid, which is a priority consumer. You can sell the non-consumed electricity as well.

If the public power grid is damaged, the hybrid inverter switches to the off-grid mode, so household appliances (house/apartment) can be powered by the battery.

Most modern hybrid inverters can be adjusted so that they 'inject' electricity from the battery into the domestic power grid at the right time. For example, when the electricity price is the highest.

Hybrid inverters can also be installed when it is necessary to increase the power of the domestic power grid. For example, if the maximum power allocated to a house is 10 kW, but there is a need to simultaneously turn on electrical appliances with a total power of 15 kW, the hybrid inverter will 'add' 5 kW of power from the battery.

The hybrid inverter automatically distributes the voltage, sending the excess to the city grid

Hybrid solar systems are suitable if