Problems are particularly acute in China's northern and northwestern regions. There is an urgent need for a stable and reliable power system suitable for use in the region. Nowadays, the commonly used separate power supply systems include generator sets, solar energy systems, wind power generation systems, and wind-solar hybrid systems.
The generator set uses a primary energy source as a power source to drive an internal combustion engine, an internal combustion engine drive generator, and a generator to make a magnetic force cutting motion to generate electrical energy. The primary energy resources are becoming scarce and the prices are getting higher and higher, and they seriously affect the natural environment.
Solar energy systems, wind power systems, and wind-solar hybrid systems use natural light and wind energy, which are inexhaustible in nature, as an energy source, and can be converted into usable electric power through certain installation devices. This energy source is natural and environmentally friendly. The concept of energy saving, environmental protection and love of the earth. The ability of such power supplies to provide continuous power supply requires energy storage equipment to provide protection. Currently, the main energy storage devices used are nickel-cadmium batteries and VRLA batteries. However, cadmium-nickel batteries were gradually replaced by VRLA batteries because of cost, lack of raw materials and other reasons. The following uses PV system as an example to introduce the application of VRLA battery in photovoltaic system.
PV system introduction
one. Photovoltaic system works under the condition of good lighting conditions, the solar cell module generates a certain electromotive force, through the series and parallel assembly of components to form a solar array, so that the matrix voltage reaches the system input voltage requirements. A part of the power supply system is used. A part of the battery is charged by a charge and discharge controller to charge the battery, and the electrical energy converted from the light energy is stored. When the lighting conditions do not meet the requirements, the battery pack provides the power required by the power system through the inverter.
two. The composition of the photovoltaic system
Photovoltaic system consists of solar cell arrays, battery packs, charge and discharge controllers, and inverters. The role of each part of its equipment is:
(1) Solar cell array: In the presence of light, the battery absorbs light energy, and the accumulation of different charges at both ends of the battery generates "photovoltaic voltage", which is the "photovoltaic effect". Under the photovoltaic effect, the two ends of a solar cell generate an electromotive force and convert light energy into electrical energy. This is an energy conversion device. Solar cells are generally silicon cells, which are divided into monocrystalline silicon solar cells, polycrystalline silicon solar cells, and amorphous silicon solar cells.
(2) Battery pack: Its role is to store the energy emitted by the solar array when it receives light, and it can supply power to the load at any time.
(3) Controller: Automatically controls the selection of power, and selects between mains, solar power, and battery to charge the battery.
(4) Inverter: It is a device that converts DC power into AC power.
Photovoltaic system requirements for battery performance
one. Battery operating conditions for photovoltaic power generation systems
In the photovoltaic power plant environment, when the light conditions are good (daylight hours), solar modules receive sunlight, output electrical energy, work on a part of DC and AC loads, and charge the battery on the other part; when the lighting conditions are not good (night or rainy days), The solar cell module cannot work, the battery pack supplies power, and the DC or AC load is supplied. The battery is in a circulating state. Therefore, in this use environment, the battery life is a cycle life.
The operating conditions of a battery used in a photovoltaic system differ from the operating conditions of a battery application in other applications. The main differences can be summarized as the following:
(1) The charging rate is very small. Due to problems such as cost and location space, the number of solar cell inputs will be greatly limited. In order to ensure the normal use of the power system, the charging power that is often provided to the battery becomes very limited, and the average charging current Usually 0.05C10 ~ 0.1C10, rarely reached 0.1C10A.
(2) The discharge rate is very small. Too much system design needs to take into account the maximum load capacity, the longest backup time, the configured battery capacity is larger, and the load is much smaller than the design load during actual use. The battery discharge rate is usually C20 ~ C240, or smaller.
(3) Due to restrictions of natural resources, batteries can be charged only when there is sunlight: that is, charging time is limited.
(4) The battery cannot be charged according to the given charging law.
two. Photovoltaic power generation system performance requirements for VRLA batteries
The batteries in photovoltaic power generation systems are frequently in the charge-discharge cycle. Due to the instability of sunshine, the disadvantages of over-charging and deep-discharging occur from time to time, and most of the photovoltaic power generation systems are used in the western regions. More than 2500M. Therefore, there are the following requirements for the storage battery in the photovoltaic power generation system:
(1) With deep cycle discharge performance, long charge and discharge cycle life;
(2) Strong resistance to overcharge;
(3) Strong capacity recovery capacity after overdischarge;
(4) Good charge acceptance;
(5) when the battery is used in a static environment, the electrolyte is not easy to layer;
(6) Has maintenance-free or less maintenance performance;
(7) should have good high and low temperature charge and discharge characteristics;
(8) It can adapt to the use environment of high altitude (above 2500M above sea level);
(9) The consistency of each battery in the battery pack is good.
three. The factors affecting the lifetime of the storage VRLA battery used in the photovoltaic power generation system (1) The positive active material is softened and detached. The VRLA battery is under cyclic use. The failure of the battery is mainly caused by the softening and falling off of the positive active material (PAM).
During the cycling of lead-acid batteries, the positive and negative active materials undergo a reversible dissolution and re-deposition process, changing the structure of the porous lead dioxide electrode. Especially for PbO2 electrodes, it may cause apparent volume increase, change the distribution of particles and pore size, and reduce the mechanical and conductive properties of the particles in porous PbO2 structure. As the cycle continues, this The situation will further deteriorate, resulting in softening and shedding of active substances in the area.
(2) Influence of discharge current on battery life In the photovoltaic system, the discharge current of the battery is very small. The formation of PbSO4 under low current conditions is much more difficult than the conversion of PbSO4 formed under high current conditions. This is because the PbSO4 crystal particles formed under a small current condition are coarser than the PbSO4 crystal particles formed under a large current condition, and the coarse PbSO4 crystal particles reduce the effective area of ​​PbSO4, thus accelerating plate polarization during recharge. This leads to difficulties in the conversion of PbSO4. As the cycle continues, this situation will be even more intensified. As a result, the plates will not be charged and eventually lead to the end of the battery life.
(3) Recovery of battery capacity after deep discharge In a photovoltaic system, the discharge rate of a battery is lower than that of a battery in other applications, and is usually between C20 and C240, or even lower. Deep discharge at low current means that the active material on the plate will be more fully utilized. In many photovoltaic systems, deep discharge does not normally occur unless the charging system fails or continues for long periods of bad weather. In this case, if the battery is not recharged in time, the vulcanization problem will be more serious and further lead to capacity loss.
(4) Effect of acid stratification on battery life The electrolyte stratification phenomenon is caused by the action of gravity on the charging and discharging process of the battery. That is, H2SO4 is generated on the surface of the positive and negative plates during charging, and its density is high due to gravity. The effect sinks. During discharge, both surfaces of the positive and negative plates consume H2SO4, so the density of the surface liquid layer is small, the low-density electrolyte rises between the plates, and the high-density electrolyte on the top of the polar group flows down from the side of the polar group, and electrolysis occurs. As a result of the liquid flow, the upper density is low and the lower density is high. The occurrence of stratification has an adverse effect on the service life and capacity of the battery, accelerating the corrosion of the grid and the shedding of the positive active material, resulting in sulfation of the negative electrode plate.
(5) The influence of the electro-hydraulic density on the life of the lead storage battery The concentration of the electrolyte is not only related to the capacity of the battery, but also related to the corrosion of the positive grid and the sulfation of the negative active material. An excessively high concentration of sulfuric acid accelerates the corrosion of the positive grid and the sulfation of the negative active material and leads to increased water loss.
(6) Effect of Grid Alloys VRLA batteries, due to long-term use, the positive grid will gradually erode and grow under the action of the electrolyte, and the longevity of the grid will reduce the binding of the active material to the grid, resulting in battery capacity. Gradually lost. The corrosion and growth of this positive electrode grid are mainly affected by the alloy composition of the grid, the electrolyte density, and the shape of the grid ribs.
During the battery charging process, a non-conductive layer is formed on the interface between the grid and the active material. These non-conductive layers or low-conductivity layers cause a high impedance at the grid and PAM interface, resulting in heat generation during charging and discharging and PAM near the grid. Swelling limits the battery's capacity (the so-called PCL effect).
(7) Influence of plate thickness The plate thickness should be a matter of battery design. In general, the cycle life of thicker plates is longer than that of thinner plates, and the utilization ratio of active materials is poor compared to some. But it is beneficial to the extension of the cycle life.
(8) Impact of assembly pressure Assembly pressure has a great influence on the battery life of VRLA. The elasticity of the AGM separator is poor. When assembling, the polar group is not pressurized or the pressure is too low. The separator and the plate cannot maintain good contact. The battery capacity is greatly reduced.
During the cycle, the expansion, loosening, and loss of the active material is one of the reasons for the early termination of the battery life, and the use of a higher assembly pressure can prevent the active material from expanding during the deep cycle. If the assembly pressure is too low, it will cause the separator to separate from the plate prematurely, causing electro-hydraulic transmission difficulties, and the internal resistance of the battery will increase rapidly, which will easily lead to the end of the battery life. Therefore, using a higher assembly pressure is a guarantee of a long cycle life of the battery.
(9) Influence of temperature The high temperature will accelerate the dehydration of the battery, thermal runaway, corrosion and deformation of the positive grid, low temperature will cause negative electrode failure, temperature fluctuations will accelerate dendrite short, etc., which will affect the battery life.
When discharged at a certain ambient temperature range, the use capacity increases with increasing temperature and decreases with decreasing temperature. In the range of ambient temperature 10 ~ 45 °C, the lead-acid battery capacity increases with the increase of temperature, such as valve-regulated sealed lead-acid battery discharge capacity at 40 °C, about 10% more than the amount of electricity discharged at 25 °C, but more than a certain In the temperature range, on the contrary, if the battery is discharged at an ambient temperature of 45 to 50°C, the battery capacity is significantly reduced. When the temperature is lower (<5°C), the battery capacity decreases with decreasing temperature. When the electrolyte temperature decreases, the viscosity increases, the ion movement is greatly resisted, and the diffusion capacity decreases. At low temperatures, the resistance of the electrolyte also increases. Electrochemical reaction resistance increases, resulting in decreased battery capacity. Second, the low temperature will also lead to a decrease in the utilization of the negative active material, affecting the battery capacity. For example, when the battery is discharged at an ambient temperature of -10°C, the capacity of the negative plate is only 35% of the rated capacity.
Under normal circumstances, if used at 25 °C, the battery life is 3 years, then when used under 30 °C, it will drop to 2.5 years; 40 °C down to 1.5 years. That is, with a temperature of 25°C, the service life is reduced by half every 10°C.
four. The design practice of the energy storage VRLA battery for photovoltaic system is based on the working conditions of the storage battery for the photovoltaic system and the special requirements for the performance of the battery for the photovoltaic system. In combination with the above factors that affect the battery life, a series of researches have been conducted on the basis of the original VRLA battery. Technical improvements have led to the design and development of VRLA batteries for photovoltaic systems. The specific improvement measures include the following aspects:
(1) Grid alloys: The use of lead-bismuth or lead-cadmium grid alloys that are suitable for and cyclical use can not only prevent the growth of corrosion during the use of the plate, but also eliminate the barrier layer at the interface between the grid and the active material and eliminate the barrier layer. Early capacity decay. Its charging efficiency and recovery after deep discharge are ideal. Since cadmium is a toxic element, it is now restricted to use. However, due to the lead-antimony alloy battery, the water loss is serious, and now it is generally required that the open-type storage battery needs regular replenishment and requires regular maintenance.
(2) Grid structure: A special grid structure is adopted to prevent the battery from being damaged due to grid growth, and the grid thickness is increased to extend the service life of the battery. Now commonly used tubular positive grid design, limited to solve the problem of poor contact between the active and grid.
(3) Lead Paste: In the positive and negative lead paste, add additives that can increase conductivity, such as graphite, acetylene black, etc., and improve the paste process and curing process to improve the battery's charge acceptance capacity, after discharge capacity Resilience and deep cycle life.
(4) Assembly pressure: The assembly pressure of the battery is increased to increase the cycle life of the battery. The use of high-strength, tight-fitting technology ensures that tight battery assembly pressures are achieved.
(5) Electrolyte: The specific gravity of the sulfuric acid electrolyte is reduced, and special electro-hydraulic additives are added, which can reduce the corrosion of the plate, reduce the generation of electro-hydraulic delamination, improve the battery's charge acceptance, and over-discharge performance.
(6) Control of impurities: Strict control of impurities (Sb, Fe, Ni, etc.) of various materials, especially the control of impurities in the alloy, reduces the self-discharge of the battery and eliminates the occurrence of negative electrode bus corrosion phenomena. .
(7) Proportioning of positive and negative active materials: For charging and discharging characteristics of energy storage VRLA batteries for photovoltaic systems, the ratio of positive and negative active materials is adjusted to improve the cycle life of the battery.
(8) Safety valve: The influence of the plateau climate above 2500 m above sea level was also considered for the safety valve, and the opening and closing valve pressure was adjusted in particular. A special safety valve was used.
(9) Battery structure: The total height of the battery is reduced. The use of a low profile construction can greatly reduce the adverse effects of battery life and capacity due to electro-hydraulic delamination. However, since the colloidal battery is not prone to electrolyte stratification, there is no such restriction.
(10) Consistency of individual battery cells: The consistency mentioned here refers not only to the open-circuit voltage of the battery, its initial capacity, but also the internal resistance of the battery, self-discharge, and charging efficiency. This requires adequate manufacturing. Accuracy, that is, from lead powder, cast piece, and paste, smear, solidification, chemical conversion, dry assembly, acid addition, and charging to the final four function tests must be controlled within a narrow tolerance range. Therefore, machine casting, Machine coating, assembly machine assembly and precise injection of acid are reliable guarantees to ensure battery consistency, minimizing human factors.
to sum up
Due to the low conversion efficiency and high cost of photovoltaic power generation systems, and the lack of corresponding laws and regulations to encourage development, the development of photovoltaic systems has been slow. However, the development of new energy sources is the trend of the times and it is bound to develop rapidly. The energy storage battery currently mainly includes nickel-cadmium batteries and lead-acid batteries, among which the nickel-cadmium batteries are gradually being eliminated. Lead-acid batteries include both liquid-rich and liquid-lean types, and will be widely used in photovoltaic power generation systems in recent years.
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