1. Standard stand-alone PV system
A standard stand-alone PV system consists of solar panels, batteries, charge controllers and inverters.
Solar panels convert light energy into electricity, and batteries store electricity in an electrochemical form. The charge controller monitors the charge of the battery and it will avoid overcharging or overdischarging the battery; the role of the inverter is to convert the DC output of the battery to an AC output.
2. Solar Charge Controllers
The charge controller monitors and controls the state of charge of the battery. When the battery level is higher or lower than a certain level, the controller will disconnect the battery from the solar panel. The voltage of the lead-acid battery reflects the size of its power.
There are other types of more sophisticated controllers: Pulse Width Modulation (PWM) and Maximum Power Point Tracking (MPPT) controllers that monitor the battery’s maximum capacity charge.
A solar charge controller is necessary if a solar power system with batteries is to be used. The solar charge controller is used to control the power supply, which can be used to transfer electrical energy from the solar panel to the battery. If overcharged, the life of the battery will be reduced. The function of the simplest charge controller is to monitor the battery voltage, and when the voltage reaches a certain value, the circuit is disconnected to stop the charging process. Older controllers use mechanical relays to achieve this function.
Many charge controllers now use PWM technology. Therein, as the battery begins to reach a fully charged state, the power transferred to the battery gradually decreases. PWM is better for longer battery life because it puts less stress on the battery. It can also keep the battery in a fully charged state or in a permanent floating state. PWM chargers are more complex and more durable because they do not contain fragile mechanical connections.
Recent solar charge controllers employ Maximum Power Point Tracking (MPPT) technology. It’s an electronic tracking system that constantly compares the battery’s charge level with the output of the solar panels. By constantly adjusting the voltage and current of the battery, the power of the solar panel is stored into the battery so that the battery can discharge more efficiently.
Compared with the PWM controller, the MPPT charge controller is expensive, but the operating efficiency has been greatly improved. In addition to the type, the charge controller can also be divided according to the current (5A, 10A, 15A…50A…) or voltage (12V, 24V, 48V) it controls, and the voltage it controls is usually solar cell modules or solar cells. The voltage produced by the battery array. It can automatically switch between dual voltages, such as 12V/24V. It is also characterized by operating temperature and efficiency, which typically exceeds 95%. Its technical data sheet also shows its power consumption. Most 12V charge controller currents are generally controlled below 50A. Generally high-power photovoltaic systems are based on higher voltages (24V, 48V…).
In operation, the charge controller has a low voltage line, below this voltage line will no longer supply power to the load (the low voltage line of the 12V charge controller is generally between 10.5~11V); there is also a high voltage line, higher than This voltage value will disconnect the battery from the solar panel (the high voltage line of the 12V charge controller is generally between 13 and 14V).
Example of parameters of the charge controller
1) Rated voltage: 12V;
2) Maximum input current of solar panel: 20A; 3) Maximum operating voltage of solar panel: 23V;
4) Maximum input power of solar panels: 300W; 5) Efficiency of solar panels: 10%~30%;
6) Working efficiency: 95%~97%;
7) MPPT regulator;
8) Fully charged voltage line: 14V; 9) Internal voltage drop: ≤400mV; 10) Low voltage line: 10.5V;
11) Supply recovery voltage: 12.6V; 12) Temperature compensation: -3mV/battery;
13) Zero load loss: ≤45mA/12V;
14) Minimum wire diameter: 1~2A/mm2;
15) Dimensions: 188cm×118cm×55cm (7.4×4.65×2.17)
The inverter converts the DC voltage of the battery or directly converts the DC voltage output of the battery panel into an AC voltage output, generally AC110~125V or AC220~240V. There are many different inverters according to the shape of the converted AC signal: square wave inverter, modified sine wave inverter and pure sine wave inverter, of which pure sine wave inverter is the most complicated.
Waveforms only affect certain types of devices. These devices are susceptible to display problems when using square wave inverters or modified sine wave inverters: variable speed motor equipment, oxygen concentrators, fax machines, laser printers, high voltage cordless tool chargers, electric shavers and Garage door opener.
Generally, the performance of the modified sine wave inverter can meet the lighting needs.
Inverters can also be marked with the size of the low and high voltage lines. The inverter does not work when it is below or beyond this voltage range. Its general working voltage range is 10~15V.
(1) Peak or surge power and continuous power
As the shape of the output waveform changes, inverters are typically characterized by two powers delivered: peak (or surge) power and continuous power. Peak (or surge) power is the maximum power value that the inverter can achieve, but only for a short period of time specified by the manufacturer. Some household appliances, such as pumps, need to start at high voltage and then operate at low voltage. The surge power also has the function of overload protection.
Continuous power refers to the power when the device works for a long time. This is usually the power specified by the inverter.
(2) Square wave inverter
Square wave inverters are the earliest inverter type and are now obsolete. This is also the cheapest and least desirable type. Square wave inverters are not sufficient for most electronic equipment requirements.
(3) Modified sine wave inverter
Modified sine wave inverters are the most popular and common type on the inverter market. The power output from the modified sine wave inverter, while not exactly the same as the daily grid, is sufficient to efficiently run the following:
1) Portable mobile phone charger;
2) Portable computer;
4) Some fluorescent lamps;
5) Most DIY tools such as drills and jigsaw puzzles;
6) Small refrigerator;
7) Hair dryer and electric shaver.
However, some appliances that utilize the speed control of the motor may not be able to use the modified sine wave inverter normally.
(4) Pure sine wave inverter
Pure sine wave (true sine wave) inverters provide the most consistent waveform output. Of all power inverters, the output waveform of this type of inverter is the closest to a pure sine wave. In many cases, the output signal of a pure sine wave inverter is cleaner than that of the utility company. This will effectively run any type of AC equipment.
A pure sine wave inverter is the most expensive, but it also provides the most reliable and consistent waveform output. Some sensitive devices require sine waves, such as certain medical devices and variable speed or rechargeable tools (oxygen concentrators, fax machines, laser printers, variable speed motors, and garage door openers).
A battery is a device that stores electrical energy in chemical form. This is a DC device that is primarily described by voltage, capacity in ampere hours (Ah), and other technologies such as lead-acid, nickel-cadmium, lithium-polymer, etc. Most distributed batteries used in photovoltaic systems are deep cycle lead-acid batteries.
Most solar PV systems use 12V deep cycle batteries, 6V batteries are also available. It can be connected in series to obtain a larger voltage, or it can be connected in parallel to obtain a larger capacity without changing the voltage. Batteries used in photovoltaic systems are generally deep cycle batteries. The deep-cycle batteries are designed to deliver low-to-moderate currents for long periods of time, rather than short-term high-current batteries like those used in cars when they fire.
There are three main lead-acid battery technologies depending on the state of the electrolyte: submerged, gel and AGM (glass wool). The most common lead-acid batteries on the market are deep cycle immersion or wet batteries. Can be sealed or not. Sealed batteries do not require maintenance and are mainly recommended for ordinary users.
At present, other types of rechargeable solid-state batteries such as nickel-cadmium, nickel-hydrogen, lithium-lithium polymer batteries can be provided on the market according to application needs, but they are expensive. They come in different types of controllers and are generally used in smaller form factor or portable systems.
(1) Maintenance and storage
A battery is a device that has a self-discharge function. Its self-discharge rate can be measured according to its monthly discharge. Therefore, it needs to be connected to a float charger or charged periodically when stored. And should be stored and charged in dry, ventilated, and manufacturer-specified temperature conditions.
(2) Keter’s law: the capacity of the battery
The capacity of the battery is expressed in ampere-hour (Ah), which refers to the amount of current that the battery can transmit in 1h, or the number of hours that can continuously transmit 1A of current. Keter’s law gives the battery capacity as a function of current and time:
In the formula, C is the capacity of the battery to continuously transmit 1A; I is the actual current passing through the battery; t is the time; k is the Kert constant, dimensionless, generally between 1.2 and 2.
Ketor’s law states that when the battery is discharged at a small current below 1A, the discharge time is longer; on the contrary, the larger the discharge current, the shorter the time.
For example: for a battery with a capacity of 50Ah, if the discharge current is 50A, the discharge time is 1h; if the discharge current is 100A, the discharge time is less than 30min; if the discharge current is 25A, the discharge time will exceed 2h.
5. Choices of Standalone PV Systems
A photovoltaic system project first considers two basic parameters; first, the energy required depends on the load and usage time; second, the amount of solar exposure determines how much light energy the solar panel can use to convert it into electricity.
Assume a 20W stand-alone photovoltaic lighting system. Assuming that it is used for the lighting of the study, it is fixed for 10h every day, which means that 200wh of electricity is required every day.
Assume that our geographic location receives an average of 6h of light per day in sunny days and 4h of light in cloudy seasons. To meet our electricity needs all year round, the amount of light that can be converted into electricity needs to be calculated based on cloudy weather conditions. For 4h of light per day, the required power is 200wh, and the daily power required is 200wh/4h, that is, a 50w solar panel is required.
How big of a battery is needed to store the electricity converted by the solar panels? The voltage of the battery should be selected according to the voltage of the panel. For such a small system, the voltage is generally 12V. Therefore, a 12V battery can store 200wh of electrical energy, and the battery capacity is 200wh/12V, which is about 17Ah. Considering the cost-effectiveness relationship, lead-acid batteries are a good choice, and its use has some limitations. The maximum capacity stated by the manufacturer should not be exceeded when used. A better choice is to use 50% of the capacity. Therefore, the capacity of the battery is preferably 34Ah (17Ah/0.5 = 34Ah).
The charge controller should avoid low or high charge and discharge of the battery to ensure its life. For a 50W-12V solar panel, the correct and standard choice is a 12V-5A charge controller (50W/12V<5A). In fact, the charging voltage of a solar panel will exceed 14V under standard lighting conditions.
Whether an inverter is required to convert the DC output to AC output depends on the operating voltage of the lighting system.
Note: When using an inverter, take into account its losses, for example the efficiency of an inverter may only be 90%. Efficiency is usually specified by the manufacturer.
Through this simple example, we can consider the demand for light from another perspective by measuring the amount of light required on the tabletop in the study. So far we haven’t mentioned the technology about lighting systems.
it can be:
1) 20w incandescent lamp (usually too dim);
2) 20W fluorescent lamp;
3) 20W LED light.
(1) Comparison of three lighting systems
Now think about it from a different angle. The current situation is that our study needs a specific amount of light (illuminance), and we need to know which lighting system to choose is more reasonable. Depending on the technology we choose, a quantitative electrical input power is required to satisfy the lighting system.
Considering that 500lx illuminance is required on the desk surface in the study, try to find out the required power consumption according to the light source used. Light sources are expressed in lm units, but what we actually need is in l x units. The conversion between these two units is not yet clear for the chosen light source, as well as the geometry of the room. At the same time, the case of uniform or focused illumination needs to be considered, which further complicates the problem.
To simplify the problem, choose the most efficient lighting source, 20W LED, enough to reach 500lx on the desk under balanced lighting.
The electrical power required depends on the type and efficacy of the selected light source. Here, consider the following to obtain a lighting condition with a target illuminance of 500lx on a desk.
1) 20W LED light;
2) 22W fluorescent lamp;
3) 100w incandescent lamp.
This backward-thinking approach led us to systems based on lighting needs. The selected system is described below according to the chosen technology, using 50% of its capacity according to the deep cycle lead-acid battery:
1) A photovoltaic system that provides the required electricity for 20W-220V LED light for 10h per day:
①20W LED, needs 200Wh of electricity every day;
②The solar panel is illuminated for 4h: 200wh/4h = 50W;
③12V lead-acid battery (200 W h/12V=16.7Ah) 16.7 Ah/0.5 ~34 Ah;
④ 12V-5A charging controller (50W/12V=4.2);
⑤ 75W inverter.
2) The photovoltaic system that provides the required electricity for the 22W-220V fluorescent lamp for 10h of light per day:
①Daily required electrical energy: 22Wx 10h = 220Wh;
②The solar panel is illuminated for 4h: 220wh/4h =55W;
③12V lead-acid battery 18.3Ah/0.5~37Ah;
④ 12V-7A charging controller;
3) A photovoltaic system that provides the required electricity for 100W-220V incandescent lamps for 10h of light per day:
①Daily required electrical energy: 100W x 10h = 1000 W h;
②The solar panel is exposed to light for 4h: 1000wh/4h~250W;
③ 12Vt acid battery: 83.3Ah/0.5 ~167Ah;
④ 12V-25A charging controller;
To build a reliable and stable photovoltaic system, not only the average sunshine time on sunny days, but also cloudy days should be considered. For this reason, the photovoltaic system generally needs to be able to provide more than the daily power required to ensure 2-3 days of power supply under different circumstances.
Another solution is to combine photovoltaic systems with energy supply systems that do not require sunlight. The grid and generator can be used as a reliable power supply.
The selection of a photovoltaic system requires consideration of all aspects of loss in terms of wiring, from the solar panels to the charge controller, from the controller to the battery and inverter, and from the inverter to all loads. It also includes the loss of efficiency of the electronic charge controller or inverter and the loss of the battery not being able to deliver 100% of the stored charge. For these reasons, transmission lines should be kept as short as possible to increase the efficiency of photovoltaic systems. Other losses may come from a drop in the voltage of the solar panel caused by an increase in temperature, such as in the afternoon when the solar panel is not oriented perpendicular to the solar radiation. The cumulative loss of a well-installed solar panel can reach 20% of the energy converted by the solar panel. Optimizing the performance of a PV system requires attention to the following points:
1) Solar panels or arrays with the correct voltage should be oriented correctly;
2) The wiring should be as short as possible on the basis of correct estimation;
3) All equipment including batteries should be stored in a cool and dry place.
6. Grid-connected photovoltaic power generation system
The grid-connected photovoltaic power generation system means that the electricity generated by the solar panels is directly input into the grid through the inverter. In such a system, when more electricity is generated than is consumed, the meter will reverse. This is the concept of net metering. Electricity companies will return the charges to consumers in different ways depending on the contract between the company and the customer. Generally, grid-tied inverter systems do not store electrical energy. But certain grid-tied inverters allow backup storage outputs.