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How Does Solar Energy Work? The Energy Geeks Explain

September 22, 2020
By John Cole

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How Does Solar Energy Work? From Sun Power to Electricity

Converting solar energy into electricity begins with the sun. As sunlight hits the solar panels, the silicon cells in the panels generate an electric charge in the form of direct current. The electricity travels down through the wires where an inverter changes the direct current into alternating current – the form of electricity used in residential and commercial buildings. This electricity then goes into your home with any unused power feeding into the grid. The more panels you have, the more electricity you get!

This bird’s eye view is hardly the entire story. To understand this amazing technology called solar energy, we need to review some basics about both electricity and chemistry. Our simple 5-step process will best explain how we turn sun power into electricity. Let’s dive in!

Like with most things, it’s helpful to begin with a handful of definitions to make sure we’re all speaking the same language.

Definitions:

·  Direct current (DC) – a type of electricity generated by solar panels. This is the same type of electricity generated by batteries. It is a flow of constant current where the electrons travel at a consistent speed. This is also the electricity used by all electronics.

·  Alternating current (AC) – a type of electricity used in most homes and commercial buildings. Residential specifically uses 120v AC. The flow of current fluctuates up and down as the electrons move and inconsistent speeds. AC is incompatible with DC and cannot be used by electronics. However, AC allows more current to be run safely with a smaller gauge wire.

·  Inverter – A device that converts DC into AC.

·  Converter – A device that converts AC to DC.

·  Circuit – A flow of electricity that ends with the electricity going back to the source.

·  Photovoltaic (PV) cells/panels – The type of solar technology employed on most homes and commercial buildings. Each PV panel is made up of multiple PV cells.

·  Net meter – A smart electric meter that measures the net electricity usage. It goes up and down in accordance with the usage of the home and will indicate when you generate more electricity than what you use.

·  Electricity – Measures in watts, this is the energy caused by electrons flowing through a circuit.

·  Current (a) – The speed at which electrons travel through a wire. This is also measured in amps.

·  Voltage (v) – The amount of electrons that travel through a wire. Common voltages are 9v, 12v, 24v. 120v, and 220v.

·  Watts (W) – The measurement for electricity, or power. To find the wattage, you multiply volts with amps.

·  Kilowatt (kW) – 1000 watts.

·  Kilowatt hour (kWh) –This is the amount of kWs that is used in a one hour period. This is also the unit of energy that electric companies measure.

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Solar Technology: A Five Step Process

Transforming sunlight into electricity that can power your TV when it is plugged into a wall is a complicated process that took years to refine and is far from perfected. Understanding it completely requires just a touch of chemistry and a fundamental understanding of electricity, both of which we’ll look at later. Thankfully, the capturing of the sun’s energy has developed into a rather efficient process that is simple enough for most consumers to understand. Solar energy is best broken down into a 5-step process.

1. The sunlight activates the PV panels and produces DC.

2. DC flows across the individual cells and through the electrical wire.

3. Direct current is converted into alternating current

4. Alternating current powers your home

5. Excess electricity passes through a net meter and into the grid

Step 1: The sunlight activates the panels and produces direct current (DC)

Sunlight shines down on the PV panels. The energy of the sunlight, specifically in the form of photons, is absorbed by the panels. This means that the panel takes in the photon energy from the sunlight. It absorbs it. Now, PV panels are made of metallic compounds topped off with a glass cover. This makes a PV panel very reflective, this rejecting much of the energy produced by the sun. To compensate, an anti-reflective coating is placed upon the silicon cells underneath the glass to help it absorb as much of the sunlight as possible. To determine how much energy is being absorbed, solar manufacturers give panels an efficiency rating. The first largely manufactured solar panels used to have an approximate efficiency rating of 6%. This meant that the panel rejected 94% of the sun’s energy. Today’s panels hover around 35% efficiency. While we still have a long way to go, today’s panels are far above what they used to be.

But what actually happens to PV panels when the sun shines down on them? The energy from the sunlight knocks loose the electrons within the silicon atoms of the PV cells. An atom, of course, is made of three things: protons, neutrons, and electrons. The free movement of electrons generates an electrical current. Though we’ll dive more into the chemical process of this below, this electrical current is created in the form of DC. Depending on the amount of sunlight at the time, the panels might produce up 35-40v of energy. 

Step 2: Direct current flows across the individual cells and through the electrical wire

Once the panels have an electric charge, they are ready to send electricity to your home. DC generated by the individual cells moves along the conductive metal contacts on the surface of the panels. These are the metal strips that you see when you look at a panel. The metal strips act as a conduit from one cell to the next, to the next, and so on. The metal contacts then meet together at the wiring in the solar panel’s junction box. The wiring is made up of copper, just like any other electrical wiring. This copper cable then acts as a conduit to allow the free movement of electrons and they travel to other PV panels and into your home. The end of the chain will usually end up with a single set of wires that feed down into the inverter.

Multiple panels are wired together in either series or in parallel (depending on the size of the individual array). To explain this better, picture AA battery. One end of the chain has a ‘+’ while the other has a ‘-‘. Respectively, this is positive and negative. Batteries and solar panels can be connected together in identical ways. If you connect the positives to the other positives, this creates a parallel circuit. This means that the electricity from the source can flow through each circuit, or each battery, independent of the other. This increases the amount of current, or the amps, that travel through a wire. In a series circuit, each battery’s positive is connected to the next battery’s negative, and vice versa. This means that the electricity has to flow through each individual battery before going back to where it started. This increases the amount of voltage within the circuit.

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Step 3: Direct current is converted into alternating current

As stated before, DC isn’t usable in the home. The wiring within buildings is specifically designed for AC. The reason for this is simple, fire safety. In order to produce enough Watts for a home, AC is used because it allows for a higher voltage. And when you increase the voltage of a circuit, you decrease the current, or the speed of electrons. As the current comes down, the physical wiring runs cooler, literally. If you run too much current through a wire, it will heat up and cause a fire.

But now that your home has received DC, it must be converted to AC. Thankfully, this is an easy fix. An inverter is a common device that performs this conversion. They are used in many other settings, cars, RVs, boats, or any other system that uses DC, typically found in batteries. But a solar inverter specializes in turning the variable voltage from a solar panel into a usable 120v (the AC voltage for North America) with minimal losses. Note that if you happen to have a battery bank, then the setup will be a bit different. One possible setup is for the solar panels to hit a charge controller, then the batteries, then the inverter. Batteries use DC, not AC. Energy storage will be discussed more in depth below.

Step 4: Alternating current powers your home

The inverter sends AC directly to the breaker box. This is frequently done in conjunction with grid power as well. Note that this portion of the project requires the use of a licensed electrician. The breaker box then distributes the electricity to all of the outlets and lights in your home through the breakers as you would typically expect. While the source of your power might be different (sunlight and solar rather than grid power), you will not notice a difference in your home. The end result is the same – 120v. If you have energy storage, or batteries, as part of your system, then the inverter might hit a switchbox or a transfer switch before the breaker box. These devices can change the power source of the breaker box from grid to batteries.

Step 5: Excess electricity passes through a net meter and into the grid

If you are grid-tied (most residential installations are), then your system will tie into the meter. A meter is a device used by the electric company to see how much you use in a given month so they can invoice accordingly. Solar systems require a slightly different device. The specific meter for these installations is called a net meter. A net meter will not only measure the amount of electricity used, but will also indicate when a surplus amount of energy is created. For example, if your system is creating energy during the day while you are at the office, then your solar panels will provide electricity to the grid. In the evening, when there is no sunlight, the grid will supply electricity to your home. This net meter will account for this change and measure the usage appropriately. It goes up and down according to your usage and will indicate if you have generated a surplus. Some electric companies will pay you for any electricity you send back to the grid.

But How Does It Work – Chem 101

Now you might be wondering how solar panels create electricity in the first place. This is when we begin to dive into chemistry. A solar panel is composed of two thin slices of silicon called wafers. These wafers are sandwiched between a conductive metal base on the bottom and smaller metal contacts on top. The face of it is also covered with a layer of protective glass. But to get a solid grasp of this process, we need to get down to the atomic level.

How are Atoms Involved in Solar Panels?

Atoms are among the smallest particles known to man. They are made up of three things: protons, neutrons, and electrons. The protons and neutrons combine in order to create a nucleus. The electrons revolve around the nucleus, much like the moon revolves around the earth. The actual atomic element determines the number of protons and neutrons in its nucleus. The electrons, however, tend to vary. As these satellites circle around the nucleus at incredible speed, they sometimes get lost or borrowed, depending on the situation.

The number of electrons is very important. This is the basis for all chemical compounds. For an atom to be considered stable, it must have the correct number of electrons. Atoms can have three layers of electrons. The innermost layer wants to house two electrons while the outer two layers want to have eight electrons each. If an atom is considered unstable in this regard, then it will bond with another atom in order for both to achieve stability. Sometimes unstable atoms will give up an electron, or accept an extraneous electron, or even straight up share outer electrons with another atom.

Bonding, and we don't mean spending family time.

Bonding is the foundational chemical process that allows solar energy to be possible. Bonding is a lasting attraction in which two or more atoms combine to make chemical compounds. This is done by sharing, giving, or receiving electrons from one or more other atoms. As this is very important to understand the free-flowing electrons within a solar energy system, we will give you an example.

Sodium has 11 electrons. This means that the first two layers are filled with one electron just hanging out. Fluorine has 9 electrons, which means the second layer is short one. When these two bond, they combine and create the chemical compound sodium fluoride. This is a stable compound with both atoms having two full electron layers.

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The Chemistry Behind Solar Energy Systems

The magic of solar lies in the polarization of the two silicon cells, specifically the silicon atomic structure. Silicon atoms have four outer electrons. When bonded together, these create a stable crystal structure. During the manufacturing process, one layer of cells is infused with boron while the second is infused with phosphorus. A boron atom only has three outer electrons. When bonded with silicon atoms, the three-electron boron atoms are short one electron. This creates a hole in the structure. These holes are constantly being filled by nearby atoms. But when one hole is filled, another is created. Therefore, these holes shift around the solar silicon cell layers. This creates free-flowing electrons with a positive charge, or P-type silicon.

Phosphorus is the exact opposite. Phosphorus atoms have five outer electrons. When bonded with a four-electron silicon atom, this creates one extraneous electron that roams around looking for a home. Since all of the surrounding atoms have a stable structure, these extraneous atoms roam around the silicon cells. This combination creates free-flowing electrons with a negative charge, or N-type silicon.

The P-N Junction

Remember that a solar panel has a positive layer of solar cells (P-type) and a negative layer (N-type) of solar cells. When both the P-type and N-type silicon are layered together, the innermost atomic layers bond. This stable chemical bond creates an electric field between the two layers. This electric field blocks the rest of the free-flowing electrons from bonding together, thus resulting in charged polarization of silicon cells.

Sunlight’s effect upon charged silicon cells

The energy from the sun disrupts the bond at the P-N junction. Specifically, the photons from sunlight knock the electrons loose from their predetermined layers. A photon results from the nuclear reactions within the core of the sun. These photons are form of electromagnetic energy that have zero rest mass. Since they have no mass, they will not permanently alter the chemical compounds of the silicon atoms.

To create an electric current, these photons shine down on the panels and physically push the electrons from one layer to another. The P-N junction, or electric field, between the two silicon layers prevents the electrons from going back home by themselves. But these electrons know that they are in the wrong spot and want to go back home. When both layers are connected with a wire, this provides an avenue for the electrons to travel, thus making a circuit. The electrons then pass through the circuit to go back home. This process of moving charged electrons through a circuit is called current. It is this current that creates electricity. The solar panels are then all wired together, to your inverter, and to your home in order to provide power to all of the associated wiring.

Chemistry summary

That was a lot of information that can benefit from a summary. Solar energy is made possible by creating one positive layer of and one negative layer of silicon cells. The innermost atoms of both layers bond to form a P-N junction. This means that the extra electrons fill the holes on the other side to create a stable chemical compound. The rest of the free roaming electrons on one side of the panels are pushed beyond this electric bond by the sunlight’s energy. These electrons want to go back home but cannot push past the electric field of the P-N junction. When both sides of the panel are connected with a wire, this allows the electrons to flow back home, thus making an electric circuit, or electricity.

How Much Energy Does Solar Make?

That really depends on the size of your system and where you live. Solar systems are given a wattage rating – say a 7kW system. This means that in full-sun and under ideal conditions, your system will generate 7kW of power. If you have full sun for an hour, then the system generates 7kWh. The majority of the United States receives about 4 hours of full sun every day. Living in the southern areas of Texas or Arizona may bring up to 5 hours of full sun, while living in the northern parts of Washington might net you just over 3 hours of full sun.

It should also be noted that solar does generate some electricity during cloudy, and even rainy days. Not quite as much as full sun, of course. But sunlight still manages to permeate cloud cover and generates a modicum amount of DC to send to your inverter and power your home.

How Does My Solar Work with the Grid?

While most utility companies will receive electricity through a net meter, they do so at different rates. What you want is an electric company or a state policy that has a 1-for-1 net metering policy. This means that if you generate the same amount of energy that you consume in a given month, then you have a zero utility bill. If a utility provider does not have a 1-for-1 net metering policy, then it will not discount your bill as much. If the utility provider is only paying you a fraction of the retail cost of a kWh that you generate, then it might make more financial sense for you to look into battery storage and use your solar-generated electricity overnight.

Off-grid Variances: Powering Your RV or Mobile Home!

solar panels on an rv

For off-grid homes or RVs, the setup looks a little different. First, the net metering is only used if your system is tied to the grid. For independent systems, this simply isn’t needed. In fact, a truly off-grid system has no need for a meter except for personal energy monitoring.

Second, off-grid homes and RVs rely on energy storage for after-sun hours. The solar panels wire into a charge controller, which then charges the batteries. The batteries then feed to the inverter. The inverter will do the same thing as in a residential home and convert the DC into AC. The inverter then feeds the breaker box. If an RV wants to hook up to shore power and use the electricity from the park, then the system needs to have a transfer switch – automatic or manual – in between the inverter and the breaker box. This will switch the source of the power from battery to shore, and vice versa.

While turn-key solar providers will offer batteries specifically designed for large solar arrays, a smaller diy system can use any deep cycle battery. A deep cycle battery is a battery that is designed for a regular deep discharge. These batteries are rated in v (volts) as well as ah (amp hours). For example, a Trojan T-105 is 6v at 225ah. Using these two numbers, you can calculate the kWh available with one battery. Multiply the two numbers to get 1.35 kWh. This one battery might not provide much energy, but a battery bank with many of these wired together can accomplish quite a bit. For example, using Trojan T-105s, you can wire up a number of batteries in both series and parallel to achieve a battery bank that is 48v, 450ah – or 21.6 kWh! That is more than enough to power a tiny home. Just be sure that your charge controller can supply the proper voltage and amps to your battery bank.

What’s the Difference Between DC and AC?

Since the inverter is such an important part of the system, it’s helpful to understand what it actually does. DC, or direct current, is a form of electricity in which electrons move steadily in one direction. It is a constant motion of electrons at a steady speed. DC, however, tends to lose voltage as it travels. When using DC, thicker cable, or a lower gauge of cable, is required in order to maintain safety. When a wire is too thin, the movement of the electrons causes the wire to heat up, which can pose a fire risk. DC is the form of energy used in solar cells, car batteries, and household batteries. Its voltage is measured at lower voltage ratings, 9v, 12v, 24v, or even 35v for solar panels.

Alternating current, or AC, is the form of electricity where the electrons move both forward and backward. This is at a predictable rate, which is called a frequency. This alternating motion keeps the temperature lower. This allows a higher voltage to travel with a thinner gauge wire. This alternating current also holds its voltage better than DC. However, the alternating motion of electrons is incompatible with energy storage as well as all versions of electronics. This is why almost any device requires some power supply that converts the AC back to DC in order to function properly. In residential, AC typically measures at approximately 120v.

Still Confused About Solar Energy? Our Layman's Terms Definition.

Conclusion:

Solar power = light energy emitted by the sun. Simply put, your solar panels (made of PV cells) take the sun's energy and convert it into usable electricity for your home. Then, this energy is transmitted to your home to be used in appliances, electronics, or whichever device you're currently reading this on. OR if you have solar battery storage, it's then stored to be used at a later date.

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