Solar panels transform solar light into electricity, but really the solar panels transform the sun into electricity.
PHOTOVOLTAIC SOLAR PANELS HOW THEY WORK
Below we will see the whole process of the operation of the solar panels
Transforming solar energy with photovoltaic cells
How a photovoltaic system works and how it produces electricity
How does a photovoltaic system work? And how does it produce electricity with the only energy from the sun? Nowadays, everyone knows what photovoltaic energy is, everyone knows that it is a technology that allows to produce clean energy by exploiting sunlight.
Everyone knows that it is a renewable source that reduces polluting emissions into the atmosphere. Many already know that, as a source of clean energy, will be the future (and perhaps already present) of a new energy model that will displace fossil sources in exhaustion. We know, in short, «what is» the photovoltaic energy, but not everyone knows how it works .
In this mini guide, we see how a photovoltaic system works and how it produces energy using the sun’s energy. Let’s see what the main components are and what are the factors that can compromise their performance.
We will see, then, how to size the correct system from the electricity bill, that is, its actual consumption and how to mitigate the losses due to shading and other inefficiencies to the maximum.
How photovoltaic systems work
The photovoltaic panels, composed by the union of several photovoltaic cells, convert the energy of photons into electricity.
The process that creates this «energy» is called the photovoltaic effect , it is the mechanism that, from the sunlight, induces the «stimulation» of the electrons present in the silicon of which each solar cell is composed.
Simplifying to the maximum: when a photon reaches the surface of the photovoltaic cell, its energy is transferred to the electrons of the silicon cell. These electrons are «excited» and begin to flow into the circuit producing electrical current.
A solar panel produces energy in direct current , in English: DC (direct current).
Then, it will be the task of the investor to convert it into Alternating Current to transport it and use it in our distribution networks. In fact, domestic and industrial buildings are designed for the transport and use of alternating current.
The components of a photovoltaic system
As many know, each photovoltaic system consists of at least two basic components:
- the photovoltaic modules , which consists of photovoltaic cells that convert sunlight into electricity,
- One or more inverters , devices that convert direct current into alternating current. Modern investors integrate electronic systems for intelligent energy management and conversion optimization. They can also integrate temporary electricity storage systems: AGM batteries, lithium batteries or other types.
Here is the «basic scheme» of a simple photovoltaic system, consisting of 18 panels connected in a single chain to a single DC / AC inverter.
In addition to these main components, there are also electrical panels, solar cables, support structures, control units, etc.
The factors that affect the photovoltaic efficiency
Not everyone knows that the conversion efficiency of each photovoltaic system is not 100%. That is to say: the panels, the solar cells, which are hit by the sun’s rays, do not transform all the received energy into electricity. They manage to convert only part of this: this is the conversion efficiency.
The best modules have a conversion efficiency of around 20-22%. This means that only a fifth of the solar energy that hits the panels is converted into electricity. Some «experimental» modules can achieve conversion efficiencies even above 30%, but for these the production costs are still too high.
In addition to this «physiological» factor, many others determine the actual performance of each plant. These are «losses» due to environmental factors and inefficiencies due to several electrical losses (cables, equipment, transport, …).
Typically, the factors that determine the performance of a photovoltaic system are:
- Temperature .
The efficiency of the photovoltaic modules varies according to the operating temperature: the higher the operating temperature, the less efficient the panels are. The overheating of the cell has a negative impact on the efficiency of the modules and on the performance of the entire system.
- Dirt .
The materials that can accumulate on the surface of the panels (soil, sand, pollution, bird droppings, leaves, resins, etc.) have a negative impact on the total reception of sunlight and hinder the performance of the photovoltaic system. In the long term, they could also compromise the economic performance foreseen by the investment plan. The efficiency losses due to this type of «inefficiency» can be very variable and depend to a large extent on the environmental conditions and the frequency of cleaning of the panel. In this case, cleaning is not only an «aesthetic» element , but «functional» .
- The shading .
They can be «passengers» (in certain time slots) and can derive from the surrounding presence of trees, other buildings or even chimneys on the roof. These are «calculable» inefficiencies. The other passenger shadows caused by clouds and the surrounding environment have a high rate of variability. However, there are technologies that can minimize the incidence of shading in the performance of the photovoltaic system. We see them later in this guide.
- Wiring and connectors .
Even the use of cables and connectors causes small yield losses. In this case, these are electrical leaks that only slightly affect the overall efficiency of the system.
- Mismatch .
We could translate the word mismatch as «mismatch» or better as «irregularities» . What does that mean? This means that not all panels of the same brand, of the same power and the same model, produce perfectly homogeneous. Between similar panels, subject to the same operating conditions, there are always minimal variations in performance. These are minimum «factory» variations that give the panels slightly different electrical characteristics. In addition, this «mismatch» can be one of the factors to take into account to estimate the losses of performance of a plant.
- Efficiency of the investor .
The process of converting direct current to alternating current by an inverter normally has an efficiency around 96-97%. The inverters usually have an optimum conversion efficiency when the «input» DC power is high, but always below the set nominal power.
- The antiquity .
The photovoltaic cells, which last from 20 to 25 years, do not produce homogeneously throughout their useful life: they have a decrease in the yield that is estimated at 0.5% per year . At the end of its useful life, a photovoltaic plant will have a yield of around 10-12 percent lower than at the beginning. This depends on a «physiological» degradation of the materials and components and must be considered from the beginning in the plant’s depreciation plan.
Here is a summary of the main and typical efficiency values of a photovoltaic system.
|temperature||-0.5% each degree Celsius |
(optimum temperature around 25 ° C)
|Efficiency of the investor||96.5%|
|Wiring and connections||98%|
|dirt||95% (high variability rate)|
|Aging modules||-0.5% per year|
|shading||Very variable depending on the context.|
How the efficiency of a photovoltaic system is calculated.
The above factors are combined in an index, to be precise: a coefficient , which serves to represent what in English is called the «System Derivative Factor», that is: the coefficient of reduction of the efficiency of a photovoltaic system. Some factors that affect are (more or less) «fixed» and calculable, others are extremely variable and depend on the place where the plant is installed.
Some simulators, including PVWatts Calculator from the Department of National Energy Laboratory of the United States, consider this index at a default value equal to ‘86% . However, this remains only an indicative value that can be extremely variable depending on the situations in which the photovoltaic system is installed.
As already mentioned, the efficiency index of a photovoltaic panel indicates how much solar energy that reaches the module is converted into electricity. To make this measurement, some «standard conditions» are usually considered : the so-called STC, Standard test conditions . These provide, in the laboratory, an operating temperature of 25 ° C and a solar radiation equal to 1,000 watts / m 2 .
Here is the formula used to estimate the efficiency of a photovoltaic system, or the conversion efficiency of solar energy into electricity:
Photovoltaic plant of general efficiency = (efficiency of the photovoltaic module) X ( factor of reduction of the capacity of the system )
Obviously, the estimates of the production of a plant are made with special software that can calculate and consider all the variables involved, including the «shading calculations».
How to dimension a photovoltaic system from the invoice
As usual, we start with «needs». What is the first tool we have to understand how much electricity we need? The electric bill .
Normally, the bill gives us some important indications on how to properly dimension a photovoltaic system: it shows us what our consumption is and shows us, above all, in what space of time they occur.
The other element that the bill shows us is … the cost: how much do we spend monthly to satisfy the energy needs of our home or our business?
With this important information, we can already do our calculations to understand if and how much photovoltaic energy can be useful for us, how much and how much can save us, considering that the plant produces a lot during the day «chasing» the day / night and summer cycles. / winter.
The electricity bill, therefore, reveals information about costs and electricity consumption. With this information it is possible to estimate the most suitable dimensions for our photovoltaic system that can compensate our consumption in an optimal way.
Daily energy requirement = monthly average monthly consumption
Assuming a monthly consumption of a small activity equal to 500 kWh and considering a month of 30 days, we have:
Daily energy requirement = 500 kWh / month30 = 16.7 kWh / day
We will need a plant that produces, on average, 16 kWh / day. The daily production varies a lot according to the season: in winter, the plant will produce less than this average. In summer you will have to produce more. The exchange mechanism in place will compensate (in part) for these seasonal fluctuations in production.
In addition to the daily requirements, we must take into account the value of solar radiation that varies according to the location of the photovoltaic installation. The equivalent Sun Hours for each specific area are used as a reference parameter.
To identify the «equivalent hours of sunlight» there are special tables, but in an indicative way we can define them as the hypothetical number of daily hours in which the radiation at 1,000 watts / m 2 would produce the energy produced on average by that area.
For example: «6 equivalent hours» means that in one area the energy received from the sun in one day is equivalent to the energy that would have received the same area in six hours with an irradiation equal to 1,000 watts / m 2 . It is a kind of «normalizer» to measure the production potential of a place and compare it with other places.
Graphically we can represent it in this way.
Assuming we are in an area equal to 5.2 hours equivalent day, this is the formula to identify the most appropriate dimensioning of the system that corresponds to our daily requirement.
Power of the photovoltaic plant = daily energy requirementHours of sun Daily equivalents = 16.7 kWh / day5.2 hours / day = 3.21 kWp
This would be the size of the photovoltaic system if our system had an efficiency of 100%. As said, it is not. For this we must «correct» this dimensioning considering an average inefficiency rate due to all the factors that we have already listed in this article: not only the conversion efficiency of solar energy, but also dirt, leaks due to wiring, connectors , Physiological degradation due to the age of the modules, etc.
A «standard» inefficiency rate that is usually taken into account is 0.8, but it can also be very different depending on the location and installation conditions.
Therefore, we must add this correction to the formula:
Size of the photovoltaic plant = Tasteoric energy inefficiency of the plant = 3.21 kW0.80 = 4.01 kWp
The optimum size of a photovoltaic system for a requirement of approximately 16 kWh / day in an area with approximately 5 hours of peak sunlight is, therefore, 4 kWp. Obviously, the example does not take into account the incidence of possible objections.
The effects of the shadow on the production of the photovoltaic system.
Since each photovoltaic system produces electricity depending on the sunlight it receives, the study of shadows is a fundamental issue to calibrate each solar installation well.
The effects of shading a tree, for example, that perhaps only affect a panel, can be worse than one might imagine because it affects the performance of all other modules in a chain. Contrary to what the intuition might suggest, in addition, the loss of efficiency of the plant is not proportional to the surface covered by shadows.
An experimental study conducted by Stanford University (one of the many available) showed that by shading only one of the 36 cells of a photovoltaic panel, the power output of the module can be less than 75% of the initial power.
How do shadows affect the flow of energy? Let’s think about water.
To understand in a simple way the incidence of shadows in photovoltaic production, we can imagine the photovoltaic system as a pipeline crossed by a current of running water. As the flow of water passing through a pipe is constant, then, for the same irradiation, the flow of electricity that passes through the photovoltaic modules is constant.
Shading a solar cell is equivalent to introducing an obstacle, an obstruction , to the free flow of water in the pipe: the whole plant will be affected by this obstruction. Similarly, when a solar cell, or part of a plant, is » covered » by a shadow, the entire flow of electrical current that passes through the photovoltaic chain is reduced (the chain is the set of several panels connected to it). series).
In this way, we will have a general decrease in the generation of electricity more than proportional to the shaded surface.
Graphically we could represent it like this:
The same mechanism of photovoltaic cells also occurs at the level of the panel and the photovoltaic chain: even if only part of the PV string is achieved by a shadow, even panels not shaded, which might operate at 100% of its potential, they actually work at lower levels, producing less than they could.
How to remedy the problem of shadows in photovoltaic modules?
The problem of yield losses due to shading on the panels can be mitigated. It can be reduced in 3 ways:
- using a different string configuration,
- using bypass diodes,
- using power electronics at the module level (see below for what it means).
Act on the configuration of photovoltaic ropes.
Several photovoltaic panels connected in series form a chain. Multiple chains can be connected in parallel. If, instead of connecting all the modules in series, we create more chains connected together in parallel, we can reduce the impact of shading on the production of the plant. This is a way to minimize the impact of shadows on the performance of the entire installation.
Thus, for example, if a system is installed on the flat roof of a building surrounded by a parapet, the modules that could be shaded by the parapet itself should be connected in a separate chain. In this way, the production of the plant can always be maintained at an optimum level, even in the hours when some modules will be in the shade.
The mechanism is simple and with a design everything becomes clearer:
The bypass diodes are small instruments within the photovoltaic panels that allow the electric current to «jump» ( «deflect») the shadow areas of the module. In this way, electricity flows regularly to the module, even if it is obstructed in some cells. This «elusion» occurs, however, at the cost of losing the energy generated by that group of cells.
In theory, to maximize the benefit of this mechanism, it would be ideal to have a «bypass diode» for each cell of the photovoltaic module, but the operation still makes it too expensive and uneconomical. To date, for classic 60-cell photovoltaic panels, there are 3 bypass diodes placed, as in the image below, every 20 cells.
Power electronics at the module level.
The «module-level power electronic components», which are MLPE in English, are nothing more than devices that allow you to increase the performance of photovoltaic modules not only when they work in optimal conditions, but also when they are in the shade. In addition to being useful in this regard, they can also monitor performance at the module level, which indicates potential performance problems or any anomaly. From a technical point of view, it is also said that they are capable of tracking and «pursuing» the point of maximum power of the photovoltaic module ( MPPT – Maximum Power Point Tracking) .
These are mainly two types of devices:
- the optimizers,
- the microinverters
The optimizers power (DC side) optimize the voltage and current of «output» of the modules to maintain the maximum power level of the module without compromising performance of the other panels. When a panel, for example, is hit by a shadow, the power produced decreases.
The optimizer can reduce the voltage of the module to minimize the decrease in power and, therefore, the impact on the other modules and the production of the plant itself.
The Microinverter , however, are nothing more than many small investors put at the service of each module. These convert the direct current produced by each panel into alternating current and replace the use of a single inverter that serves the entire chain (or the entire plant).
The use of microinverters allows each panel to work independently. Each panel will produce «tracking» of the maximum power point (MPPT) and will be completely independent of the incidence of any shading on the other modules. The malfunction of a module will not affect the performance of the other modules and the system itself.
On average, the performance improvement derived from the use of microinverters or optimizers is estimated at around 17 percent with respect to the efficiency of the systems installed with the classic configurations. The incidence of microinverters and optimizers is generally very similar and in most cases is equivalent.
Here is the diagram that shows the position of the microinverters and optimizers: