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Photovoltaic system energy storage battery principle and application
- May 04, 2018 -

The energy battery is an indispensable energy storage component for the solar photovoltaic power generation system. The main function of the energy storage battery is to store the photovoltaic power generation system power, and to supply power to the load when the amount of solar radiation is insufficient and at night and in an emergency state. The commonly used energy storage batteries include lead-acid batteries, alkaline batteries, lithium batteries, and super capacitors. They are used in different occasions or products. The most widely used batteries are lead-acid batteries, which have been developed since the 1850s. There are more than 160 years of history, and many types have been derived, such as flooded lead-acid batteries, valve-regulated sealed lead-acid batteries, colloidal batteries, and lead-carbon batteries. The fastest-growing lithium battery is currently a lithium iron phosphate battery and a lithium triple lithium battery (LiNiCoMn).


1 Lead-acid battery working principle, the basic structure


The lead-acid battery is a chemical energy storage device using lead and lead dioxide as the negative electrode and positive electrode active material of the battery, and dilute sulfuric acid as an electrolyte, and has high power conversion efficiency, long cycle life, high terminal voltage, high safety, and high cost performance. Features such as simple installation and maintenance are currently the preferred chemical power source for various types of energy storage, emergency power supply, and starting devices. The main components of lead-acid batteries include:

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1) Plates: The positive and negative plates are made of special alloy grids coated with active material. The plates store and release energy during charge and discharge to ensure the battery capacity and performance are reliable;


2) Separator: It is an isolation medium placed in the middle of the positive and negative electrodes of the battery to prevent direct and positive electrode contact and short-circuit. Different types of lead-acid battery separator material is different, the valve-regulated battery is mainly AGM, PE, PVC-based;


3) Electrolyte: The electrolyte of lead-acid battery is dilute sulfuric acid prepared with distilled water. The electrolyte plays a role in transferring ions between positive and negative electrodes during charge and discharge, so the electrolyte must be free of impurities;


4) Container (battery cover): The battery-covered container, electrolyte and plate are all in the container, mainly play a supporting role, and prevent the internal material from overflowing. The external material enters the internal structure and contaminates the battery.


2 The type of lead-acid battery


    The working principle of the lead-acid battery is through the electrochemical reaction, the mutual conversion between electric energy and chemical energy, the electrode is mainly made of lead and its oxide, and the electrolyte is a kind of storage battery of sulfuric acid solution. English: Lead-acid battery. In the discharge state, the main component of the positive electrode is lead dioxide, and the main component of the negative electrode is lead. In the charged state, the main components of the positive and negative electrodes are lead sulfate. There are many types, which are used in photovoltaic energy storage systems, and there are three more, flooded lead-acid (FLA) batteries, valve-regulated lead acid batteries (VRLA), Lead carbon battery and so on.


2.1 Flooded lead-acid batteries


    The sulfuric acid in the lead-acid battery electrolyte directly participates in the battery charge-discharge reaction process. In the conventional lead-acid battery, the remaining space in the battery cell to remove the polar plates, separators, and other solid assembly components is completely filled with sulfuric acid electrolyte, and the electrolyte is in surplus. Excessive state, it is called "rich liquid" battery, battery plate completely immersed in sulfuric acid electrolyte. The top of the flooded battery has a cover that can vent and can prevent the liquid from splashing out. Due to moisture evaporation and decomposition during use, it is necessary to periodically open the lid to add distilled water and adjust the electrolyte density, so it is customarily called It is an "open type" battery. The flooded lead-acid battery is characterized by its low price and long service life, and its disadvantage is that it needs regular maintenance.


2.2 Valve controlled sealed lead-acid battery


    Also called maintenance-free batteries, it is divided into AGM sealed lead batteries and GEL gel sealed lead batteries. The AGM type battery uses a pure aqueous solution of sulfuric acid as the electrolyte, most of which is present in the glass fiber membrane, while the interior of the plate absorbs a portion of the electrolyte. The AGM sealed lead battery has less electrolyte, the thickness of the plate is thicker, and the active material utilization rate is lower than that of the open type battery, so the discharge capacity of the battery is about 10% lower than that of the open type battery. Compared with today's colloidal sealed batteries, its discharge capacity is smaller. Compared with the battery of the same type with the same type of liquid-rich type, the price is higher, and has the following advantages: 1. The cycle charging capacity is 3 times higher than the lead-calcium storage battery, and has a longer service life. 2. Higher capacity stability throughout the life cycle. 3. The low temperature performance is more reliable. 4. Reduce the risk of accidents and reduce the risk of environmental pollution (since the acid is 100% sealed) 5. Maintenance is simple and reduces deep discharge.


    Colloidal sealed lead-acid batteries (ie GEL-type batteries), colloidal lead-acid batteries are improvements to ordinary lead-acid batteries for liquid electrolytes, using colloidal electrolytes instead of sulfuric acid electrolytes for safety, power storage, discharge performance and service life Other aspects have improved compared to ordinary batteries. Its electrolyte is composed of silica sol and sulfuric acid. The concentration of sulfuric acid solution is lower than that of AGM type battery. The amount of electrolyte is more than that of AGM type battery, which is equivalent to that of flooded type battery. This electrolyte exists in a colloidal state, filled in the separator and between the positive and negative electrodes. The sulfuric acid electrolyte is surrounded by the gel and does not flow out of the battery.


   Its advantages are as follows: GEL colloid battery is free of electro-hydraulic electrolyte gel, the probability of acid leakage is much less than the previous battery; its perfusion is more than dilute sulfuric acid, less water loss, so the colloidal battery will not be lost Failure caused by water; Colloid poured into the increase in the strength of the separator to protect the plate, make up for the defects of the separator to meet the acid shrinkage, so that the assembly pressure is not significantly reduced is one of the reasons for its prolonged battery life; colloidal fill The gap between the separator and the plate reduces the internal resistance of the battery, and the charge acceptance capability can be improved accordingly. Therefore, the over discharge, recovery ability and low temperature charge and discharge performance of the colloidal battery are superior to those of the AGM type battery.


Colloid battery excellent characteristics


  1 Can significantly extend the battery life. 2. The self-discharge performance of the lead-acid battery is good. Under the same conditions of sulfuric acid purity and water quality, the storage time of the battery can be extended by more than two times. 3, colloidal lead-acid batteries in the case of serious power shortage, anti-vulcanization performance is very obvious. 4. Colloidal lead-acid batteries have a strong recovery capability under severe discharge conditions. 5, colloidal lead-acid battery anti-overcharge capacity. 6. Late discharge performance of colloidal lead-acid batteries is good.


2.3 Lead carbon battery


   Lead-carbon battery is a capacitor type lead-acid battery. It is a technology evolved from traditional lead-acid battery. It adds activated carbon to the negative electrode of lead-acid battery, which can significantly improve the life of lead-acid battery. Lead-carbon battery is a new type of super battery that integrates both lead-acid battery and super-capacitor: it not only takes advantage of super-capacitor's instant large-capacity charging, but also brings advantage of the specific energy of lead-acid battery, and it has very Good charge and discharge performance. Moreover, due to the addition of carbon (graphene), the phenomenon of sulfidation of the negative electrode is prevented, a factor of failure of the battery in the past is improved, and the battery life is further prolonged. The cost of lead-acid batteries can be as low as 0.5 yuan/kWh. On the basis of large-scale production, lead-carbon batteries may even reduce the cost of electricity to below 0.4 yuan.


   Lead-carbon battery is the most advanced technology in the field of lead-acid batteries, and it is also the development focus of the international new energy energy storage industry, with a very broad application prospects. Energy storage battery technology is one of the key technologies that restrict the development of new energy energy storage industry. The energy storage fields of photovoltaic power station energy storage, wind power storage, and power network peak shaving require the battery to have characteristics such as large power density, long cycle life, and low price.


3. Lead-acid battery management


   Lead-acid batteries generally use three-stage charging mode:


  In the first stage of fast charge, constant current charging stage, the battery is rapidly charged with the maximum output current of the charger. The charging time depends on the battery capacity and the state of the battery at the beginning of charging. During the second stage of charge, the charging voltage of the charger is kept constant during the constant voltage charging phase, the charging power continues to increase, the battery voltage increases slowly, and the charging current decreases. The third stage float charging mode, the battery is basically full, and the charging current drops to a low level In the floating charge conversion current, the charging voltage is reduced to the float voltage.


● Charging current


The battery charging current is generally expressed as a multiple of the battery capacity C. For example, if the battery capacity is C=100 Ah and the charging current is 0.1 C, it is 0.1*100=10 A. The best charging current for lead-acid maintenance-free batteries is about 0.1C, and the charging current cannot be greater than 0.3C. Too large or small charging current will affect the battery life.


● charging voltage


For a single battery with a rated voltage of 2V, the float voltage is generally set to 2.2-2.3V. The average charging voltage is set to 2.3-2.5V. If the charging voltage is too high, the battery is easy to lose water and the heat is distorted. Otherwise, the battery will be insufficiently charged and the charging voltage may be abnormal. This may be caused by a charger configuration error or due to a charger malfunction.


● depth of discharge DOD depth of discharge


The percentage of the battery's discharged capacity in its rated capacity during the use of the battery is called the depth of discharge (DOD). There is a deep relationship between the depth of discharge and the battery life. The deeper the depth of discharge, the shorter the charge life. Therefore, deep discharge should be avoided during use. The discharge depth of the battery is 10% to 30% above and below the shallow cycle discharge; the depth of discharge is 40% to 70% above and below the medium cycle discharge; the depth of discharge is 80% to 90% above and below the deep cycle discharge.


In general, the deeper the daily discharge depth of a battery is, the shorter the battery life is, and the shallower the discharge depth is, the longer the battery life is. Shallow cycle discharge helps to prolong battery life. The shallow cycle operation of the battery has two distinct advantages: First, the battery generally has a long cycle life; second, the battery often retains more backup ampere-hour capacity, making the photovoltaic system's power supply guarantee rate higher. According to actual operating experience, a moderately moderate depth of discharge is 60% to 70%.


 ●Check battery


The battery has self-discharge phenomenon. If it is left unused for a long period of time, energy will be lost. Therefore, regular charge and discharge are required. Engineers and technicians can judge the battery's quality by measuring the open-circuit voltage of the battery. Take the 12V battery as an example. If the open circuit voltage is higher than 12.5V, it means that the battery energy is more than 80%. If the open circuit voltage is lower than 12V, it means that The battery energy storage is less than 30%, and the battery is already in the state of “fighting for food”. The maintenance-free battery adopts the absorption electrolyte system and will not generate any gas during normal use. At this time, the internal pressure of the battery will increase, and the pressure valve above the battery will be opened, which will cause the battery to bulge and deform. Even if it is leaked or even broken, these phenomena can be judged from the appearance. If the above situation is found, the battery should be replaced immediately.


● Battery installation


The battery should be installed in a clean, cool, ventilated, and dry place as much as possible, and protected from sunlight, heaters, or other sources of radiant heat. The battery should stand upright, not tilted. The connection between the terminals of each battery should be firm.


● Ambient temperature


Ambient temperature has a greater impact on the battery. If the ambient temperature is too high, the battery will overcharge and produce gas. If the ambient temperature is too low, the battery will be undercharged and the battery life will be affected. Therefore, the best ambient temperature is around 25°C.


● Regular maintenance


After the battery is used for a certain period of time, it should be checked regularly, such as whether its appearance is abnormal or whether the voltage of each battery is measured. If the battery is not powered down for a long period of time, the battery will always be in a charged state, which will deteriorate the battery's activity. Therefore, even if there is no power outage, periodic discharge tests are required to keep the battery active. The discharge test can generally be performed once every three months. The method is to carry the inverter with load, preferably over 50%, and the duration of the discharge depends on the battery capacity, which is generally several to several tens of minutes.


4 Lead-acid battery selection and design


 4.1 Battery capacity


It indicates the amount of electricity discharged by the battery under certain conditions (discharge rate, temperature, termination voltage, etc.), that is, the capacity of the battery, usually in ampere*hours. The battery voltage is 3V, 6V, 12V. The available battery capacity of the battery pack is not related to the series and parallel connection of the batteries. It is only related to the quantity. The available capacity = voltage*capacity*quantity*discharge depth, such as battery pack, total 4 12V/200AH, depth of discharge 0.7, then the available power = 12*200*4*0.7=6720VAH.


   (1) Effect of discharge rate on battery capacity


    The lead-acid battery capacity decreases with increasing discharge rate, that is, the greater the discharge current, the smaller the battery capacity. For example, a 10Ah battery can discharge with 5A for 2 hours, that is, 5×2=10; with 10A discharge, it can only discharge 47.4 minutes of electricity, for 0.79 hours, its capacity is only 10×0.79=7.9Ah. A given battery discharges at different rates and will have different capacities.


   (2) Influence of temperature on battery capacity


    The temperature has a great influence on the capacity of the lead-acid battery. Generally, the capacity decreases as the temperature decreases. When the temperature of the electrolyte decreases, the viscosity of the electrolyte increases, the ions receive greater resistance, the diffusion capacity decreases, and the electrolyte resistance increases. The resistance of the electrochemical reaction increases, and part of the lead sulfate cannot be converted normally, and the charge acceptance capacity decreases. As a result, the capacity of the battery decreases.


   (3) Effect of Termination Voltage on Battery Capacity


    When the battery is discharged to a certain voltage value, the generated voltage drops sharply, and the energy actually obtained is very small. If the battery is discharged for a long period of time, the damage to the battery is quite large. Therefore, the discharge must be terminated at a certain voltage value. This cut-off discharge voltage is called the discharge cut-off voltage. Setting the discharge cut-off voltage is of great significance for prolonging the service life of the battery.


        4.2 Calculating the ratio of storage battery of PV off-grid system


   (1) The voltage of the module and the voltage of the battery must be matched. The solar module of the PWM controller is connected to the battery via an electronic switch. There is no device such as an inductor in between. The voltage of the module is between 1.2 and 2.0 times the voltage of the battery. If it is a 24V battery, the component input voltage is between 30-50V, MPPT controller, there is a power switch tube and inductance circuit in the middle, the voltage of the component is between 1.2-3.5 times of the battery voltage, if it is a 24V battery The component input voltage is between 30-90V.


   (2) AGM battery charging current is generally about 0.1C10, fast charge does not exceed 0.15C10, such as a lead-acid battery 12V200AH, charging current is generally between 20A and 30A, the maximum can not exceed 40A, GEL gel battery charging current can Appropriately increased to 0.2 C10; battery discharge current is generally 0.2C10-0.5C10, different types of batteries, the discharge current difference, AGM battery maximum 1C10, GEL gel battery can be up to 2C10, lead carbon battery up to 5C10 .


  (3) In the PV off-grid system, the power consumption of the load is not fixed. When calculating the total power of the battery, it must be designed according to the user's requirements. For users with high power requirements, the available battery power is greater than the user's The highest value of electricity, for the average user, the battery available electricity is equal to the user's electricity average.


  (4) For the same battery pack, ensure that the battery is the same model. Try to make the battery in series, so that the battery charge and discharge equalization. The number of batteries connected in parallel is preferably no more than 3 sets. If it exceeds, consider adding BMS battery management system.


  (5) The battery cable design mainly considers the maximum current on the line. Divide the inverter power by the battery voltage to obtain the maximum discharge current, or the component power divided by the battery voltage to obtain the maximum charging current (less than The controller's maximum output current, such as a 3KW inverter, photovoltaic controller is 48V/50A, battery voltage is 48V, with 265W components 12 blocks, the battery pack maximum output current is 3000/48=62.5A, The total power of the module is 265*12=3180W, 3180/48=66.25A, which is the theoretical maximum charging current, but since the controller is 50A, the maximum charging current is actually 50A, so the cable should be designed according to 62.5A, if Cable length is less than 50 meters, optional 10 square, if the cable length is greater than 50 meters, or there is a tube, armored and other packaging, you must choose 16 square.


4.3 Interpretation of lead-acid battery parameters

1) Rated voltage, rated voltage of single battery, generally 2V, 6V, 12V;


2) Capacity, the battery's capacity, is represented by current A and time h;


3) Power, the rated output power of the battery, should be matched with the power of the load during design. If the power of the battery is less than the load power, it will cause the battery voltage to fluctuate, which may cause the battery management system to report falsely.


4) Maximum discharge current: different types of batteries, the discharge current is not the same, lead carbon battery up to 6C.


5 Lithium battery operating principle, the basic structure



    Lithium-ion batteries are rechargeable batteries that rely primarily on lithium ions to move between the positive and negative electrodes. During charge and discharge, Li+ reciprocally inserts and extracts between two electrodes: when charging a battery, Li+ is deintercalated from the positive electrode, and is inserted into the negative electrode through the electrolyte, and the negative electrode is in a lithium-rich state; the opposite occurs when discharging. A battery containing a lithium-containing material as an electrode is generally representative of modern high-performance batteries.


   Compared to other electrochemical currents, lithium ion batteries have many advantages, such as light weight, large energy storage capacity, large power, no pollution, long life, small self-discharge coefficient, and wide temperature adaptation range. Lithium-ion batteries can be used in power quality, reliability control, backup power, peak load shifting, energy management, renewable energy storage, and more.


       5.1 Types of Lithium Batteries


    Lithium batteries, which we usually refer to, are lithium-ion batteries, which are generally classified into energy storage lithium batteries and power lithium batteries according to applications. Energy-storage lithium battery is used for photovoltaic or UPS, the internal resistance is relatively large, the charging and discharging speed is slow, generally 0.5-1C, the power battery is generally used in electric vehicles, the internal resistance is small, the charging and discharging speed is fast, generally can reach 3- 5C, the price is about 1.5 times more expensive than the energy storage battery. According to the materials, there are mainly lithium iron phosphate batteries and lithium triple lithium batteries (LiNiCoMn). Comparison of two types of batteries:


Comparison project

Lithium iron phosphate

Lithium

1

Power density (mAh/g)

130

160-190

2

Discharge platform /V

3.2-3.3

3.6-3.7

3

Cycle performance 80%

>2000

>2000

4

High and low temperature performance

-20~+75

-30~+65

5

Material

Lithium ore, iron oxide phosphate reserves

Cobalt deficiency


5.2 lithium battery pack management


Lithium battery management system, BatteryManagement System, BMS is a device made up of microcomputer technology, detection technology, etc. Its function is not only to charge and discharge the battery, but also to accurately measure the remaining power of the battery, and the important function of the equalization charge of the single battery.


(1) Secure the battery: In the charge and discharge process of the battery, the terminal voltage and temperature, charge and discharge current and total battery pack voltage of each battery in the battery pack of the electric vehicle are collected in real time to prevent overcharge or overdischarge of the battery. .


(2) Accurately estimate the remaining battery power of the battery pack, and predict how much energy remains in the hybrid vehicle's energy storage battery or the state of charge of the energy storage battery. There is a certain relationship between the battery power and the voltage, but it is not a linear relationship. It is not possible to rely on the voltage to convert the remaining power. It needs to be calculated by the BMS.


(3) Equilibrium between individual cells: That is, the single cells are evenly charged, so that each cell in the battery group reaches a state of equilibrium.


5.3 Selection and Design of Lithium Batteries


Lithium battery system includes battery core and BMS battery management system, which is provided by the manufacturer. There are several points to note when designing:


1) The energy storage lithium battery pack needs to communicate with the inverter or the bidirectional energy storage converter PCS. To select the equipment that has the lithium battery function and the corresponding communication interface function.


2) Compared with lead-acid batteries, lithium-ion batteries have a smaller charge-discharge current.


3) If the system requires multiple battery packs, one battery pack needs a BMS system.