Photovoltaic Basics (Part 2): Integrating the Panels in a System
Getting electricity from the sun in the way that best suits your needs requires knowledge of the technologies and appropriate use of the elements of a system. In the first part of this series, we reviewed the types of photovoltaic (PV) panels available on the market, with all their different features and capabilities. Here we will see how to integrate them into the most common connection schemes with charge controllers, battery storage systems and inverters, both in off-line and grid-connected configurations.
Size/Performance Considerations for Photovoltaic Modules
The most readily available PV modules on the market (at least for the civilian sector) are still those made of monocrystalline and polycrystalline silicon. At present, crystalline silicon photovoltaic cell companies still use scrap from the electronics industry, but cost-effective availability is beginning to be scarce, partly because the demand for semiconductors for microelectronics has been growing for decades.
The best-selling panels are polycrystalline silicon panels, with a market share of around 55%. Monocrystalline silicon panels follow, at around 35%. The rest of the production consists of 10% amorphous silicon and thin-film modules, as well as other technologies that you are unlikely to find on the market in quantity, however.
Crystalline panels range in surface area from 0.5 m² to 1.5 m², with peaks of 2.5 m². It is common practice for manufacturers to avoid large modules, since the larger the surface area, the greater the performance loss the entire module suffers from the dimming or failure of a single cell. About peak power output, the most common ”cutoff” is around 200 Wp, but more and more modules with larger cutoffs are becoming popular.
The yield per unit area of crystalline modules is around 5 m²/kWp (for monocrystalline modules); therefore, about 5 m2 of area are needed to obtain 1,000 Wp. As for the thin-film technique, the yield is lower (typically 80 Wp/m², so to get 1 kW peak you need 12.5 m²). Therefore, panel sizes are usually larger, in part because they are designed to go over large areas.
Current production offers modules starting at about 50 Wp and going up to even 100 W peak. As for thin film, for example CIGS, it has a yield of about 125 Wp/m2. As far as cost is concerned, amorphous and thin-film technology promise to fall well below crystalline: amorphous silicon will soon come in below €2.4/Wp, while cadmium telluride/cadmium sulfide thin film is expected to cost just over € 0.8/Wp. Since the demand for photovoltaic panels grows, there is a big push on amorphous silicon thin film, which can derive more power for the same amount of semiconductor used.
Photovoltaic Systems
To exploit photovoltaic energy practically, except for mobile or isolated applications that require direct voltage, one must produce alternating current with similar characteristics to that of the power grid, to supply power to users designed for the power grid, whether civil or industrial; in the typical case one must derive 230 V AC of sinusoidal waveform and at the frequency of 50 Hz, at least for Europe.
In the simplest form, the system consists of an inverter that converts the DC voltage of one or more photovoltaic panels — connected in series to form strings — into AC; the inverter is chosen of the required power output, which must be supported by some margin of excess by the PV panel array. For example, if the users need a power source of 2 kW, it will be necessary to have panels delivering more than that power, and this is for three reasons:
- The efficiency of the inverter is always less than 100 %.
- The load may have absorption peaks above the rated power.
- The conditions of absolation (solar irradiance) and the average power available during the day must be considered.
Regarding the last reason, it should be kept in mind that the rated power for a photovoltaic panel is the peak power, corresponding to the maximum obtainable under the best absolation conditions and with incident sunlight within a certain angle; these parameters are provided by the manufacturer.
To give an example, a 400 Wp panel — with an angle of maximum efficiency of 90° ± 15° relative to its surface — will provide 400 W in full sunlight, and when the sun’s rays strike the cells within that angular range. Outside those conditions, the actual power available may be significantly lower.
Furthermore, there is the aspect of hourly power availability to consider, so how many kWh are needed: if a certain amount of power is needed with continuity over the hours of sunshine, given the variability of the conversion efficiency of the solar cells one has to think about storage, i.e., connecting the output of the photovoltaic panels to a charge controller, which in turn will supply batteries, capable of delivering electricity when the panels cannot cope.
Clearly the set of panels will have to be sized for a peak power not only higher than that which can be drawn from the load, but for an amount that will also compensate for the absorption by the batteries connected to the charge controller, especially when the batteries are almost flat.
For example, with a storage capacity of 2 kWh, assuming an average of 10 hours to charge the batteries, it would require a constant, 200 W extra power supplement to be delivered by the solar panels.
Let us now turn to the inverter: ideally it should have — for the same deliverable power and especially when a system exceeds a power of few hundred watts — the highest possible input voltage, since this reduces the current in the PV-side cables (DC) and with it the overall power loss, including that in the inverter.
An example of a combination of photovoltaic panels, charge controller and storage batteries, plus inverter with 230 V AC output is illustrated in Figure 1, which schematizes an independent system for generating electricity from the sun, both during the hours of sunrise and sunset, and in any case in the absence of sunshine.
In cases where alternating current is not needed, and it is enough to operate devices (but also simple light bulbs) running on DC, one can consider using the simpler system schematized in Figure 2, which shows a PV system with storage, charge controller and DC output.
output of the controller can be connected to the input of an inverter
Off-Grid and Grid-Connected Systems
When planning the installation of a photovoltaic system, the destination of the generated alternating current, i.e., whether it will be utilized locally or fed into the grid — with the aim of selling excess energy to the distribution grid operator — must also be considered when choosing the inverter.
The latter case demands a number of requirements and certifications on the plant, and a design that ensures a financial return. Among the requirements is the adoption of a grid-connected inverter, the characteristic of which is that the output connects to the power grid, typically on the bidirectional meter provided by the electric utility, to concur with the power demand or even transfer to the grid the excess energy produced by the solar panels.
The inverter of this type is very complex and expensive, however, because it must supply a sine wave voltage in phase with that of the grid, through a circuit capable of zero-crossing and starting the sine wave it synthesizes with a perfect synchronization. Furthermore, it must make a synthesis of the sine wave that ideally approximates the real one, and it is known that with step-reconstructed sine wave circuits, this is not always possible.
More interesting (not surprisingly, it has become increasingly popular) is the off-grid system, the output of which serves a number of loads placed on a section of the system not directly connected to the grid; in fact, it does not require certifications or contracts and allows, with special configurations, to work cooperating with the ordinary electrical system, but in a different way from grid-connected.
A very interesting solution consists of special so-called “hybrid” inverters that accept as input both a string of photovoltaic panels and the 230 V AC power grid; a contactor driven by the control electronics, allows switching the load to the grid or to the output of the inverter according to the power demand, i.e., the presence of photovoltaic voltage.
A more advanced version of this device is one where the power grid feeds the input of a UPS (uninterruptible power supply consisting of an AC/DC power supply that charges batteries, which power a DC/AC inverter) that has backup batteries, alternatively charged by the photovoltaic panels: as long as the power delivered by the latter is sufficient for the needs of the load, the inverter output to the UPS is powered by the photovoltaic, while when the latter is not enough the missing part of the power is taken from the power grid.
Such a system can power an entire house, taking care to feed the inverter with the incoming grid from the electric meter, then sectioning the downstream system, which will be connected to the output of the inverter. Such a solution will allow for the best combination of grid and PV resources, but also to exploit any storage, exclusively or in combination.
Also for PV exploitation, there are dedicated inverters that are completely isolated from the grid; however, these are reserved for grid-separated installations, and typically have a storage battery that makes up for the lack of solar power.
strings of 2 PV panels (24◦V) each and capable of delivering about 800◦W.
An example of a grid-connected system is the one proposed in Figure 3, and it is based on a 600 W (900 W peak) NEP micro-inverter. This “Plug&Play” type inverter is ideal for making mini photovoltaic systems for residential use, where you want to feed the energy produced by solar directly into the grid, thus going to reduce the energy taken from the grid. Meeting CEI 0-21 standard, combined with solar panels of adequate power and an upstream protection system, it is, for example, suitable for a balcony installation.
With a maximum peak power of 450 W per channel (two channels available), this microinverter can handle high levels of solar power. Furthermore, thanks to its built-in Wi-Fi connectivity, the microinverter can be monitored and controlled via a free app (NEPViewer), enabling easy and intuitive management of the PV system. Equipped with advanced protections such as overvoltage/undervoltage protection, over/underfrequency protection, and anti-islanding protection, the microinverter ensures safe and reliable operation in various environmental conditions.
This inverter operates only when the grid voltage supplied by your grid operator is present. It is possible to combine 12 V photovoltaic panels with this inverter by arranging two in series for each channel to obtain 24 V; for example, by using two 200 W panels for each input, it will be possible to obtain a total power of 800 W.
Hybrid Inverter System
The hybrid inverter is an effective solution for photovoltaic systems because of its ability to handle different energy sources. This device connects to solar panels and uses a built-in charge controller to keep storage batteries charged, while also providing a connection to the power grid.
Depending on requirements, the inverter automatically chooses whether to draw power from the panels, batteries or directly from the grid to power the devices at 230 V AC, without injecting excess electricity into the grid itself. An example is illustrated in Figure 4.
The hybrid inverter offers several operating modes, among which is the option of including an electronic switch. This allows the output of the inverter — which converts DC to AC — to be switched directly to the power grid through the use of a circuit breaker.
The choice of the operation mode depends on the complexity of the inverter and the specific purpose of the installation. In its most basic configuration, the hybrid inverter constantly monitors the AC output consumption. If it detects an energy demand that exceeds the capacity of the solar panels, the inverter automatically switches to the 230 V AC power grid, temporarily disconnecting the panels. In this way, the energy produced by the panels, when available, is used to charge the batteries via the charge controller, provided the batteries are installed and connected to the system.
More advanced versions can make a mix of the PV energy and the batteries, i.e., of the latter two and the domestic power grid; in the latter case, the superposition cannot take place on the high-voltage side but is done on the DC side; in practice, a UPS is adopted that consists of a power supply-battery charger with a 230 V AC mains input, the output of which goes to supply both the batteries (via a special charge controller) and the input of the DC/AC inverter.
With this solution, the inverter operation is independent of the DC power source and the AC output is independent (as well as decoupled) from the grid. The PV panels feed the input of the charger through the charge controller and also contribute to powering the DC/AC inverter.
A microprocessor circuit cyclically monitors how much power is being requested at the 230 V AC output and, while giving priority to power from PV panels and batteries, if it detects a draw beyond the possibilities of the latter (i.e., based on any scheduling) it draws to a greater or lesser extent from the grid to close the demand gap and maintain full operation of the electrical system.
Such a solution represents the best of the hybrid, since it does not suffer from the albeit small interval of absence of the high output voltage that occurs in hybrid inverters with mains switched through a contactor: a condition that in some applications is not acceptable and requires the adoption of a dedicated UPS for users such as, for example, Personal Computers.
Editor’s notes: Interested in ESP32 and DIY projects? This project originally appeared in Elettronica IN.
Questions or Comments?
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