Solar Power System. How does it work?
Needless to say that the Sun is the biggest source of renewable energy for the Earth. The fact is that even though the earth receives only a part of the energy generated by the Sun (i.e. Solar energy), that part of solar energy is also tremendously huge. The Earth receives solar energy in the form of light and heat. But in today’s world, the words ‘power’ and ‘energy’ are leaned more towards ‘electricity’. This article explains how electricity is harvested from the solar energy and how it is utilized.
Electrical energy can be harvested from solar power by means of either photovoltaics or concentrated solar power systems.
Photovoltaics (PV)
Photovoltaics directly convert solar energy into electricity. They work on the principle of the photovoltaic effect. When certain materials are exposed to light, they absorb photons and release free electrons. This phenomenon is called as the photoelectric effect. Photovoltaic effect is a method of producing direct current electricity based on the principle of the photoelectric effect.
Based on the principle of photovoltaic effect, solar cells or photovoltaic cells are made. They convert sunlight into direct current (DC) electricity. But, a single photovoltaic cell does not produce enough amount of electricity. Therefore, a number of photovoltaic cells are mounted on a supporting frame and are electrically connected to each other to form a photovoltaic module or solar panel. Commonly available solar panels range from several hundred watts (say 100 watts) up to few kilowatts (ever heard of a 5kW solar panel?). They are available in different sizes and different price ranges. Solar panels or modules are designed to supply electric power at a certain voltage (say 12v), but the current they produce is directly dependent on the incident light. As of now it is clear that photovoltaic modules produce DC electricity. But, for most of the times we require AC power and, hence, solar power system consists of an inverter too.
Photovoltaic solar power system
According to the requirement of power, multiple photovoltaic modules are electrically connected together to form a PV array and to achieve more power. There are different types of PV systems according to their implementation.
- PV direct systems: These systems supply the load only when the Sun is shining. There is no storage of power generated and, hence, batteries are absent. An inverter may or may not be used depending on the type of load.
- Off-grid systems: This type of system is commonly used at locations where power from the grid is not available or not reliable. An off-grid solar power system is not connected to any electric grid. It consists solar panel arrays, storage batteries and inverter circuits.
- Grid connected systems: These solar power systems are tied with grids so that the excess required power can be accessed from the grid. They may or may not be backed by batteries.
Introduction
In less than two hours, enough sunlight strikes the earth to satisfy the world economies’ annual energy demand. Despite this abundance of solar energy, the conversion of sunlight into usable energy forms only represents a tiny fraction of today’s global energy supply. Yet, the share of solar energy in global energy supply, especially in the electricity sector, is rising rapidly. Unprecedented deployment has taken place in the last decade, stimulated by efforts to improve energy access, security of supply and mitigate climate change. Between 2010 and 2017, the global installed capacity of solar generation increased more than tenfold from 34 GW to 437 GW (IRENA 2020). Steep learning curves and the economies of scale enabled technological improvements and, in consequence, have led to massive cost reductions.
Solar photovoltaics (PV), the conversion of light into electricity using semiconducting materials, were one of the most expensive electricity-generating technologies when first employed in astronautics in the late 1950s. By 2020, it has become an economically viable energy source for many applications. An alternative technical process to generate electricity from solar radiation is concentrated solar power (CSP). Yet, the latter, accounted for less than 3% of all solar power in global electricity generation in 2017 (IRENA 2020).
PV is the third most important renewable energy source in terms of global capacity after hydro and wind power. Globally, solar energy is mostly used in Asia, Europe and North America with the strongest rise in Asia, mostly driven by China and India (Fig. 9.1). According the World Energy Outlook of the International Energy Agency, solar PV may become the largest technology in terms of global installed capacity in the Stated Policies Scenario by 2035 (IEA 2019).
Technical Characteristics of Solar Energy
A brief introduction to the technical characteristics of solar energy provides the necessary background information to better understand its economics.
2.1 Solar PV
The main components of photovoltaic cells are semiconducting materials such as silicon and germanium. In these materials, sunlight releases charge carriers (electrons), which create an electrical field. As source of electricity generation, this field induces a direct electrical current. This process is known as the photovoltaic effect. Electricity generation exploiting this effect is not only possible from direct sunlight, but also from its diffuse components, implying that PV cells also generate electricity with cloudy skies.
Photovoltaic cells are integrated in solar arrays. Inverters (to invert DC current from solar panels into AC), transformers, electrical protection devices, wiring and monitoring equipment are summarized as balance of system (BOS). In some cases, BOS also includes sun-tracking systems, which increase the yield by positioning the panels towards the sun.
The three major types of solar PV technology are monocrystalline cells, polycrystalline cells and thin firm cells, of which the first two make up more than 95% of global module production (Fraunhofer ISE 2019).
Monocrystalline solar cells have the highest efficiency rates, typically 15–20% but the highest quality panels can reach up to 23% efficiency. As for all solar panels, the efficiency of monocrystalline panels depends on ambient temperature. On average, efficiency declines by about 10% when the ambient temperature rises by 25 °C (Quaschning 2019). Featuring high efficiencies, monocrystalline solar panels are space efficient, i.e. they require smaller ground areas to generate the same amount of electricity compared to other technologies. They also live the longest with most manufacturers putting a 25-year warranty on monocrystalline solar panels. Their main disadvantage is the high cost, because manufacturing requires the highest-grade silicon.
Polycrystalline silicon cells are cheaper because of a simpler production process and the amount of waste silicon is less compared to monocrystalline cells. The efficiency of these panels is typically lower (13–16%). They also have a slightly lower heat tolerance, which means that polycrystalline perform slightly worse in high temperatures than monocrystalline panels.
Thin film solar cells deposit one or several thin layers of photovoltaic material onto a substrate. Most thin-film modules have efficiencies of around 9–11%. Their mass production makes them cheaper than crystalline based solar cells. Thin film solar panels are mostly used in applications where panel sizes are not an issue. Another advantage is that they can be more easily integrated into facades and roofs.
When comparing efficiencies, it is important to differentiate between efficiencies of single cells, of panels and of the entire installation including converter and transformer. In the last 10 years, the efficiency of average commercial silicon modules increased from about 12% to 17% (Fraunhofer ISE 2019). Lab cell efficiencies of close to 50% when concentrating light rays and applying new materials demonstrate the potential for further efficiency increases at the production level (Geisz et al. 2020).
2.2 Concentrated Solar Power
Concentrated solar power (CSP) does not exploit the photovoltaic effect. Instead, mirrors are used to FOCUS solar rays to heat a fluid. Similar to conventional power plants, the thermal energy then drives a turbine to generate electricity. A downside of the CSP technology is that direct radiation is required for the process, because diffuse radiation cannot be focused. CSP plans are therefore mostly sited in countries with high direct radiation and a dry climate (see section on solar potential), for example, in northern Africa and the Middle East.
One major advantage of the CSP technology compared to solar PV is that heat can be stored at comparatively low cost. Equipped with molten salt vessels as thermal energy storage, most CSP plants have a steadier generation profile during the day and extend electricity generation long beyond sunset.
The four main construction types of CSP plants are solar towers, parabolic troughs, linear Fresnel reflectors and small-scale dish engines (Fig. 9.2). Parabolic trough and solar tower CSP plants are the most mature CSP technologies and lead new installations by far (REN21 2019).
CSP technologies can be grouped into point concentration systems (solar towers and dish engines), and linear concentration systems (parabolic troughs and linear Fresnel reflectors). Technologies based on point concentration systems achieve higher temperatures (up to 1200 °C) than linear concentration technologies (300–550 °C), and thus yield higher thermal efficiencies. However, focusing a large number of mirrors on a single point is highly complex and leads to high construction and maintenance costs. By contrast, linear concentration technologies require less land than point concentration systems.
Applications of Solar Energy
Photovoltaic systems have long been used in specialized applications as stand-alone installations (island systems). Grid-connected PV systems were first constructed in the 1990s. Nowadays, solar energy for electricity generation is applied on the wide range between small roof-top PV systems and large utility scale solar parks. In contrast to the modular solar PV, CSP is mostly deployed in large-scale power plants.
PV and CSP in large-scale solar parks, directly connected to the high voltage grid, are used to generate electricity on a commercial-scale. The largest solar power plants around the world are PV parks with installed peak capacities of up to 2 GW per site, the order of magnitude of a large nuclear power plant. The largest solar PV parks are located in India, China and the Middle East.
The modularity of solar PV (and dish engine CSP plants) also allows small-scale deployment. Roof-top PV systems have increased significantly, fostered by falling costs and governmental support policies. On a small-scale, roof-top PV serves self-consumption or supplies local mini-grids. In most countries, distributed residential systems already have generation costs below (the energy portion of) retail electricity prices, making the deployment of solar PV for self-consumption economically attractive (IEA 2020b). Behind-the-metre business models, increasingly comprising battery storage, allow to self-consume electricity generated by roof-top PV. In remote off-grid rural areas, particularly in developing countries with good solar resources, decentralized solar power feeding into local mini-grids may provide electricity access in places where a connection to the national grid is too expensive. In urban areas, roof-top PV could provide a back-up for an unreliable grid supply. In these applications, roof-top PV does not compete against large-scale power plants but against other small-scale generation units such as diesel generators. Often, solar is not only the most sustainable alternative but also economically viable. This increasing economic attractiveness of small-scale PV systems could lead to Rapid expansion of decentralized PV capacity.
Aside from power generation, CSP can also generate steam, which can be used in other sectors, for example, in enhanced oil recovery or steam-using industry processes. Thus, CSP technologies could be elements of sector coupling to enable further decarbonization of economies.
No fuel to burn
After installing solar panels, operational costs are pretty low compared to other forms of power generation. Fuel isn’t required, which means that solar power can create large amounts of electricity without the uncertainty and expense of securing a fuel supply.
Solar power and the environment
As a renewable CO2-free power source, the environmental impact of solar power is significantly smaller than other power generation methods. The impact is mainly related to the production and supply of the special materials and metals that are required to produce solar panels. The location and the water used to clean the solar panels also affect the environment. We are working hard to find alternative ways to clean our solar panels.
FAQ – Solar power
A power generation method that converts energy from the sun into electricity. It uses solar panels that are often arranged on a building or concentrated in solar farms to facilitate a reaction that converts the sun’s light radiation into electricity.
Photovoltaic cells in a solar panel turn sunlight into direct current electricity (DC). Then, an inverter converts the DC electricity into alternating current electricity (AC), and once this process has taken place, the electricity is used, fed into the grid or stored in a battery.
In the Nordic region, we have really good conditions for solar power. In the summer, the sun shines here a majority of the hours of the day. The slightly cooler climate in the Nordics is also very beneficial for solar energy, as warmer temperatures reduce the efficiency of the solar cells.
Several large-scale solar power parks are currently being developed in the Nordic region. In 2022, Sweden entered the top 10 in terms of the largest markets for solar energy within the EU, while Denmark has eighth place on the same list. According to calculations from Solar Power Europe, electricity production from solar power will reach one gigawatt in Finland by 2025.
The main advantage is that it is a renewable, clean source of electricity. Solar power is also scalable. This means that it can be deployed on an industrial scale, or it can be used to power a single household. When it’s used on a small scale, extra electricity can be stored in a battery or fed back into the electricity grid. Overall, the sun gives off far more energy than we’ll ever need. The only limitation is our ability to convert it to electricity in a cost-effective way.
Grid-Tie Solar Power Systems
Grid-tie solar is, by far, the most cost-effective way to go solar. Because batteries are the most expensive component of any solar system, but grid-tie solar owners can skip them completely!
So how do grid-tie solar power systems work?
First, let’s define what we mean by the “grid”. The grid is the utility company’s network of equipment that brings electricity from the power plant to your home or commercial building. If a building is getting electricity from the power company, it is connected to the grid.
Grid-tie solar systems send the energy they generate into the grid, where it is stored for later use. Under a net metering agreement, the system owner receives credit for anything they generate, and they can make use of that energy at any time.
It’s kind of like a bank account: sending energy into the grid is like making a deposit, and using electricity is like withdrawing against your account balance. If you overdraft i.e. use more energy than you produce in a given month, the utility bills you for the difference. No added fees, thankfully.
Advantages of Grid-Tie Solar Power Systems
Grid-tie solar is the best option if you want to offset your electricity bill and save money over the life of your system.
Most grid-tie systems pay for themselves within 5-10 years. With solar panels warrantied for 25 years, grid-tie solar is the only option that reliably turns a profit for the system owner over the life of the panels.
Another advantage is that grid-tie systems can be smaller — you don’t need to generate 100% of your power each month. The grid can supply additional power beyond your production, which is useful when bad weather hampers the output of your panels, for example.
Some people choose to size a grid-tie system for a partial offset of their bill, with plans to expand the system later once their budget allows for it. Design requirements are less demanding than in an off-grid environment, where you are fully responsible for your energy needs.
Disadvantages of Grid-Tie Solar Power Systems
The main disadvantage of grid-tie systems is that they are still vulnerable to power outages.
But wait, you might say, if I’m generating power from sunlight, why does it matter if the grid goes down?
Unfortunately, grid-tie systems are wired into the utility company’s infrastructure. In case of an outage, utility workers need to troubleshoot and fix the problem, and they can’t do that if connected solar systems are still energized and feeding power to the grid. For that reason, grid-tie solar systems are switched off during outages to allow utility workers to safely make repairs.
The solution? A hybrid system that connects to the grid, but draws on a battery bank in case of outages. We’ll cover those at the end of this article, but first.
Off-Grid Solar Power Systems
Off-grid solar is best for delivering power to remote locations where there is no access to a utility line.
Folks who live off the grid are solely responsible for generating their own electricity. This is usually accomplished by building an off-grid solar system that can cover a day’s worth of electricity usage, with a backup generator to supplement production during long stretches of bad weather.
Advantages of Off-Grid Solar Power Systems
The main draw of off-grid solar is the freedom to live wherever you want. It doesn’t matter if your property is 100 miles from civilization: if you have sunlight, you have a reliable way to generate power.
Although off-grid solar components are more expensive, there can be some hidden financial benefits to living off the grid that can offset those higher costs. Undeveloped plots of land located far off the grid will naturally cost less than a prime grid-tie location. In many cases, the lower land costs do more than enough to offset the higher cost of going solar off the grid.
Disadvantages of Off-Grid Solar Power Systems
Pretty simple, really: the need for a battery bank makes off-grid solar significantly more expensive.
However, it’s often wiser to invest in an off-grid solar system than it is to run a power line to a remote location. While an off-grid system may cost more than a grid-tie system, it is still more frugal than other remote power solutions, like running a new utility line or relying on a gas generator.
One way to keep costs down is to use propane appliances where possible to reduce your demand for electricity. Opting for a propane stove, clothes dryer, wall heater and on-demand water heater means you can get away with a smaller inverter and smaller battery bank.
It also helps to stagger electricity usage — for example, running laundry and the dishwasher at different times — to reduce your peak power consumption and relieve some of the costs of energy storage.
Backup Solar Power Systems
If you live on the grid, but you want protection from power outages, your best bet is a battery backup system.
Backup power systems connect to the grid, and function like a normal grid-tie system on a day-to-day basis. However, they also feature a backup battery bank that takes over in case of outages.
When grid power goes out, your inverter automatically disconnects from the grid and draws on energy stored in your battery bank, which will keep your appliances running when the grid goes down.
Battery backup systems have been gaining popularity recently, especially in light of news stories covering grid failures in Texas and wildfires interrupting service in California. They are also favored in climates that are vulnerable to fierce storms and natural disasters like hurricanes and tornadoes. The backup battery bank offers peace of mind to shield the owner from blackouts.
Lastly, battery backup is valuable if you have appliances which require uninterrupted power. If you are running a well pump, for example, service interruptions can be a massive headache. Adding backup power to your grid-tie system will keep these critical appliances running during a blackout.
Can I start with grid-tie solar and add battery backup later?
Yes, but it’s much easier to do if you plan for expansion in advance. Traditional grid-tie inverters like the SMA Sunny Boy aren’t equipped to handle a battery bank connection. Those can be paired with the SMA Sunny Island inverters to upgrade a hybrid battery based system, but you’ll spend more coupling a second inverter to your system.
Hence, iIf you think that you may want to add battery backup to your system down the line, we recommend a solution like the Sol Ark series, a string inverter which is engineered to handle all three applications: grid tie, off grid, and battery backup.
There’s also the Enphase Ensemble, a “grid-agnostic” micro-inverter system that is designed to seamlessly swap between grid power and backup power.
You’ll be able to start with Enphase IQ micro-inverters for grid-tie use, with the option to add the Encharge storage system later without any compatibility issues.
Watch this 5-minute video from Enphase to see how it works.
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