The capacity configuration of the solar LED lighting system mainly includes the calculation of the load power consumption, the power calculation of the solar cell array, the calculation of the optimal inclination angle of the installation, and the calculation of the battery capacity. In the solar LED lighting system, LED lamps, solar cell array power and battery capacity are the main components of the system cost. The calculation results can be used as a design reference during design, and sometimes it is necessary to make correct decisions on the use efficiency and appropriateness of the load. Judgment can get a cost-effective system design.
- Fundamentals of Design
The main function of the solar cell array design is to supplement the electric energy consumed every day, so that the battery can work normally for a long time. Due to changes in weather conditions, the power generation of the solar cell array is not constant, so it is necessary to make the solar cell array and the battery work in a matched manner under the weather conditions of the project.
The solar cell phalanx is designed to maximize the output in the worst season (usually winter) in the lightest season. The reasonable idea in the design of the solar cell array is to make the solar cell array meet the load needs in the worst season of illumination, that is, to ensure that the battery can be fully charged in the worst case of illumination. In this way, the battery can reach a full state throughout the year, which can prolong the service life of the battery and reduce maintenance costs. However, if in the worst season of the year, the illuminance is much lower than the average, and the size of the solar cell array is still considered according to the worst case, then the designed solar cell array will be far away at other times of the year. It is far more than the actual need, and the cost is high, so it does not have the requirements for promotion and use.
If the output of the solar array is designed to be equal to the average value of the load demand throughout the year, the solar array will provide all the energy required by the load, but the change in the amount of radiation will cause the battery to be in a state of power loss for many times. At the same time, the state of charge (SOC) of the battery will be greatly reduced under severe weather conditions for several days. (For more battery knowledge, please visit tycorun.com)
In a stand-alone photovoltaic system, there is no backup power supply to recharge the battery in poor weather, so the service life and performance of the battery will be greatly affected, and the operating cost of the entire system will also increase significantly. Do not consider fully charging the battery as fast as possible in the design of the solar cell array size. If so, it will lead to a large solar cell array, making the system cost too high; and in most of the time of the year, the power generation of the solar cell array will be far greater than the load usage, resulting in Unnecessary waste of the solar cell array; the main function of the battery is to supply power to the load when the solar radiation is lower than average; in the subsequent weather conditions when the solar radiation is higher than the average, the solar cell array will charge the battery .
- Battery capacity design
The design idea of the battery is to ensure that the load can still work normally when the sunlight is continuously lower than the average value. Assuming that the battery is discharged at night in a fully charged state, and the light radiation intensity on the next day is lower than the average value, then the electric energy generated by the solar cell array cannot completely fill the vacancy caused by the energy consumed by the load from the battery, and the battery will is not fully charged. If the light radiation intensity remains below average for the next few days, the battery will still be discharged to supply the load, but the battery state of charge will continue to decrease.
Such a discharge process can only last for a few days, after which the state of charge of the battery will reach a specified dangerous value, and continued discharge will damage the battery. In order to quantitatively evaluate the situation that the sunlight is continuously lower than the average value, an indispensable parameter needs to be introduced when designing the battery: the number of days of self-sufficiency, that is, the system can use the battery only when the battery is fully charged without any external energy. The number of days the load was working properly. This parameter directly determines the design battery capacity of the system.
Therefore, it is very important to determine the number of self-sufficiency days, and two related factors need to be considered: the degree of power requirements of the load working time and the meteorological conditions of the project site, that is, the maximum number of continuous rainy days. For the occasions where the load working time is not very strict on the power supply requirements, the self-sufficiency days can be determined as 2 to 4 days in the design, while the self-sufficiency days can be determined as 5 to 7 days in the occasions where the load working time is very strict. Usually, the maximum number of consecutive rainy days at the project site can be used as the number of self-sufficient days used in the system design, but the requirements of the load on the power supply should also be comprehensively considered. In addition, the situation of the project site should also be considered. If maintenance work is possible in a relatively remote area, the capacity of the design battery can be appropriately enlarged.
(1) Preliminary calculation
First of all, it is necessary to determine the power consumption of the load. For the LED system, the driving efficiency can be 0.8~0.9, and the total loss of the line is 10%. In this way, the total power consumption can be obtained according to the following formula (1):
In the formula, PLED is the power of the LED light source; η1 is the efficiency of the LED drive; and is the energy loss of the system circuit.
The total energy W consumed by the system every day is determined according to the working time of the system, and can be accumulated according to the difference of the power working in different time periods (2):
In the formula, Pi is the power consumption of a certain period of time; hi is the working time of a certain period of time. If the discharge is constant power, it does not need to be accumulated. When the power is changed, it can be accumulated according to the above formula.
The initial battery capacity can be obtained by multiplying the power consumption of the load by the working time and the number of self-sufficiency days determined according to the actual situation.
In the calculation, the maximum allowable depth of discharge of the battery should be determined according to the selected battery, because the capacity of the battery cannot be zero after the self-sufficiency days of discharge, so the required capacity of the battery should be the self-sufficiency days. The discharge capacity is divided by the maximum depth of discharge. The choice of the maximum depth of discharge needs to be determined with reference to the performance parameters of the battery selected in the solar LED lighting system. Detailed information about the maximum depth of discharge of the battery can be obtained from the battery supplier. Normally, if a deep cycle battery is used, 80% depth of discharge (DOD) is recommended; if a shallow cycle battery is used, 50% DOD is recommended. The basic formula for designing the battery capacity is as follows (3):
In the formula, C is the battery capacity; d is the self-sufficiency days; V is the voltage used by the system; DOD is the depth of discharge.
According to the calculated battery capacity, it is necessary to determine the method of battery series and parallel connection. Each battery has its nominal voltage. In order to achieve the nominal voltage of the load operation, the batteries are connected in series to supply power to the load, and the number of batteries connected in series is equal to the nominal voltage of the load divided by the nominal voltage of the batteries (4).
In the formula, ns is the number of batteries that need to be connected in series; VB is the nominal voltage of the selected battery. Then calculate the number of batteries to be connected in parallel (5).
In the formula, np is the number of batteries that need to be connected in series; CB is the nominal capacity of a single battery (10h discharge rate). Taking a solar LED street light as an example, assume that the power of the LED is 40W, the driving voltage of the LED is 12V, the full power discharge is 5h every day, and the half power discharge is 5h. Deep cycle battery with 80% depth of discharge. The calculation process is as follows.
First calculate the total power consumption, the drive efficiency is 85%, and the line loss is 10% (6)
Then accumulate the power consumption in different time periods (7)
Calculate the capacity of the battery (8)
Select 2V/40A h single battery, then the number of batteries to be connected in series (9):
Number of batteries to be connected in parallel (10):
In the design, you need to take an integer, you can take 6. Therefore, the required batteries are 6 rows in parallel, and 6 batteries in each row are connected in series, for a total of 36 batteries. In actual use, the high-power LED lighting system will be driven by 12V/24V, so the battery is mainly driven by 12V, and a battery is selected as much as possible to meet the power supply of the system, so as to reduce the unreliability caused by battery inconsistency to the system.
(2) Calculation correction
In practice, there are many performance parameters that have a great impact on the capacity and service life of the battery. In order to get the correct battery capacity design, the calculation results must be corrected.
When calculating the correction, the concept of the average discharge rate needs to be calculated first, and the calculation method is as follows (11):
In the formula, is the average discharge rate. For a photovoltaic system with only a single load, the working time of the load is the average daily working time of the actual load (unit is h). For photovoltaic systems with multiple different loads, the working time of the load can be calculated using the weighted average load working time. The calculation method of the weighted average load working time is as follows (12):
After the average discharge rate of the photovoltaic system is calculated, the battery capacity can be corrected according to the selected battery to find the battery capacity at the corresponding discharge rate. The capacity of the battery also changes significantly with temperature. When the temperature of the battery drops, the capacity of the battery will decrease. Usually, the capacity of lead-acid batteries is calibrated at 25°C. As the temperature decreases, the capacity at 0°C drops to about 90% of the rated capacity, and at -20°C to about 80% of the rated capacity, so the influence of the ambient temperature of the battery on its capacity must be considered.
If the temperature of the project site is very low, the calculated capacity may not meet the actual load consumption, which will lead to over-discharge of the battery, reduce the service life of the battery, and increase the maintenance cost. In this way, the battery capacity required in the design should be larger than the capacity calculated according to the battery parameters at the standard condition (25°C). In the case of 25 ℃, it can fully provide the required energy. According to the battery temperature-capacity correction curve, find the battery capacity correction coefficient corresponding to the temperature to correct the calculation result.
In addition, due to the influence of low temperature, a factor that must be considered in the design of battery capacity is to correct the maximum depth of discharge of the battery to prevent the battery from freezing and failing at low temperature, resulting in permanent damage to the battery. The electrolyte in the lead-acid battery may solidify at low temperature. With the discharge of the battery, the water continuously generated in the battery dilutes the electrolyte, causing the condensation point of the battery electrolyte to rise continuously until the temperature of pure water is 0°C. In cold climates, if the battery is discharged too much, the electrolyte may condense as the condensation point of the electrolyte rises, thereby damaging the battery. Even if a deep cycle industrial battery is used in the system, its maximum depth of discharge should not exceed 80%. Figure 13 shows the relationship between the maximum depth of discharge and the temperature of the general lead-acid battery, and the required adjustment factor can be obtained by reference during system design.
Considering all the above calculation correction factors, the final calculation formula of battery capacity can be obtained as follows. The basic formula for designing battery capacity is revised as follows (14):
In the formula, F is the capacity correction factor for different discharge rates of the battery; T is the temperature correction factor.
When designing a solar LED lighting system, since the power consumption of LED lamps is not very high, the maximum allowable depth of discharge can be appropriately reduced to increase the capacity of the battery to prolong the service life of the battery. Maximum allowable depth of discharge: Generally speaking, the maximum allowable depth of discharge for shallow cycle batteries is 50%, while the maximum allowable depth of discharge for deep cycle batteries is 80%. If it is in a severe cold area, it is necessary to take into account the low temperature antifreeze problem and make necessary corrections. For example, if a deep cycle battery is used, the maximum usable percentage of the battery capacity used is set to 60% instead of 80%, which can not only increase the service life of the battery, reduce the maintenance cost of the battery system, but also reduce the initial cost of the system. The cost won’t be a big hit. This can be handled flexibly according to the actual situation.
(3) Parallel design of battery packs
After calculating the required capacity of the battery, the next step is to decide how many single batteries to be connected in parallel to obtain the required battery capacity. In this way, there are various options. For example, if the calculated battery capacity is 500A·h, you can choose a single battery of 500A·h, or you can choose two 250A·h batteries in parallel, or you can choose five 100A batteries. · The batteries of h are connected in parallel. In theory, these options can meet the requirements, but in practical applications, the number of parallel connections should be minimized. That is to say, it is best to choose a large-capacity battery to reduce the number of parallel connections required. The purpose of this is to minimize the impact caused by the imbalance between the batteries, because some parallel batteries may be unbalanced with the parallel batteries during charging and discharging. The more groups connected in parallel, the greater the possibility of battery imbalance. Generally speaking, it is recommended that the number of parallel connections should not exceed 4 groups.
At present, many photovoltaic systems use two groups in parallel mode. In this way, if one group of batteries fails and cannot work normally, the group of batteries can be disconnected for maintenance, and another group of normal batteries is used. Although the current has dropped, the system can still maintain normal operation at the nominal voltage. . In short, the parallel design of battery packs needs to consider different actual situations, and make different choices according to different needs.
- Solar cell array design
The solar cell array mainly considers that the battery can be charged to meet the needs of the load consumption. The simple way to calculate the solar cell array is to divide the average daily energy (amp-hours) required by the load by a solar cell array that can generate in one day. In this way, the number of solar cell arrays that the system needs to be connected in parallel can be calculated, and the current required by the system load can be generated by using these components in parallel. In a solar LED system, the nominal voltage of the system is usually divided by the nominal voltage of the solar cell array to obtain the number of solar cell arrays that need to be connected in series. Using these solar cell arrays in series can generate The voltage required by the system load. The basic calculation formula is as follows (15):
In the formula, np is the number of battery components connected in parallel; ns is the number of series components; Ws is the average daily output energy of the solar battery components, in ampere hours; Vs is the charging voltage of the solar battery.
In actual work, the output of the solar cell array will be reduced by the influence of the external environment. Dirt, dust coverage and slow decay of module performance can reduce the output of the solar array. The usual practice is to reduce the output of the solar cell array by 10% during the calculation to solve the above-mentioned unpredictable and unquantifiable factors. In addition, considering the charging efficiency of the actual battery, it can usually be taken as 90%, which also needs to increase the output power of the solar cell array. During the charging and discharging process of the battery, the lead-acid battery will electrolyze water and generate gas to escape, which means that part of the current generated by the solar cell array will not be converted and stored but will be dissipated. The coulomb of the battery can be used. efficiency to evaluate this current loss. Different batteries have different coulombic efficiencies, and it can usually be considered that there is a loss of 5% to 10%. Therefore, in the reliability design, it is necessary to increase the power of the solar cell array by 10% to offset the dissipation loss of the battery. Therefore, when simply estimating the power of the solar cell array, considering these losses, it is necessary to appropriately increase the power of the solar cell array.
Considering the Coulombic efficiency of battery charging and discharging, and taking into account the reduction in the daily output of the solar cell array caused by environmental factors and the attenuation of the module itself, the conservative calculation value of the output of the solar cell array can be obtained (16):
In the formula, ηc is the coulombic efficiency of battery charging, usually 0.9~0.95; as is the output efficiency attenuation factor of the solar cell array, usually 0.85~0.95.
Due to the characteristics of solar energy, its irradiation energy varies greatly with the seasons, so the main starting point of the design is to meet the needs of any time, especially when the irradiation is the lowest. Changes in solar energy are difficult to predict, so the design can only rely on historical data for calculation. For the case of constant load throughout the year, the design calculation of the solar cell array is based on the month with the lowest irradiance. If the working condition of the load is changed, that is, the demand for electricity of the load in each month is different, then the best method to take in the design is to calculate according to different seasons or each month. The maximum number of solar cell arrays is required.
Usually in summer, spring and autumn, the power output of the solar cell phalanx is relatively large, and relatively small in winter, but the load demand may also be relatively large in summer, so in this case, only the annual average or a certain month is used. The design calculation is inaccurate, because the number of solar cell arrays required to meet the load requirements of each month is different, so the required solar cell arrays for that month must be calculated according to the load required for each month. The maximum value is the number of solar cell arrays required in a year. For example, it may be calculated that the number of solar cell arrays required in winter is 10, but in summer it may only require 5, but in order to ensure the normal operation of the system throughout the year, a larger number of solar cell arrays have to be installed, namely 10 components to meet the needs of the load throughout the year.
The output calculation of the solar cell array is also very important in the design and design. Since the output of the solar cell array is calibrated in a standard state, which is very different from the actual sunshine conditions and environmental conditions, its standard output cannot be Design basis. Usually, the rated output of the solar cell array and meteorological data can be used to estimate the output of the solar cell array under actual conditions. A simpler method is to use the peak time (unit: h) method to estimate the daily output of the solar cell array. The method is to convert the solar radiation energy received by the solar cell array into the peak time of standard solar radiation 1000W/m2, and then multiply the peak output current of the solar cell array to obtain the daily output energy of the solar cell array (amp hours) (17).
In the formula, Ht is the solar radiation energy received by the solar cell array on a certain day; the standard solar radiation H0 is 1000W/m2; N is the peak time, h; Imax is the peak output current. Because some simplifications are made in the peak time method, there is a certain deviation between the estimated results and the actual situation.
For example, the solar LED road light system in a project site has a load of 24V and 20A·h/day. If the solar radiation is the lowest month, the average daily solar radiation on the surface of the 30° solar cell array is 3000W·h/m2. Using a 12V75W solar cell array with a peak current of 4.4A, the calculation is as follows (18:):
Assuming that the coulombic efficiency of the battery is 0.95, the attenuation factor of the solar cell array is 0.9, and W20 (19)
Therefore, the system requires 2 large solar battery packs in series, and each battery pack adopts 2 parallel connection, so a total of 4 12V75W solar cell arrays are required.
- Design proofreading
The capacity of the solar cell array and the storage battery has been preliminarily designed and calculated before. In order to ensure that the design of the solar cell array and the storage battery can work in harmony, it is necessary to check the possible conditions of the system operation. It is mainly to check the depth of discharge and charging rate of the battery to avoid damage to the battery.
In the solar LED lighting system, the daily discharge depth of the battery is calibrated to ensure that the battery will not be over-discharged, which can effectively ensure the reliability of the system. It is usually necessary to calibrate a system with a small number of self-sustaining days. If the number of self-sufficiency days is large (more than 5 days), the actual daily depth of discharge is already very small, and no calibration calculation is required. If a photovoltaic system uses a shallow cycle battery of 100A·h and the daily load is 50A·h, then the average daily discharge depth reaches 50%, which has reached the limit of the battery discharge depth, which is easy to cause the battery to be over-discharged in the system. It is necessary to appropriately increase the battery capacity and reduce its depth of discharge.
In addition, it is necessary to check the maximum charging rate of the solar cell array to the battery pack. When the solar radiation is at the peak, the charging rate of the solar cell array to the battery should not be too large, otherwise it will damage the battery. The maximum charging rate of the battery can be obtained from the manufacturer, and the charging rate of the calibration solar cell array should be less than the maximum charging rate of the battery (20).
In the formula, Km is the maximum charging rate of the solar cell array to the battery. If two 12V75W solar cell arrays are used in parallel to charge a 12V80A h battery, the maximum charging rate is (21):
Compare the calculated value with the maximum charging rate of the selected type of battery provided by the battery manufacturer. If the calculated value is larger, the design is more reliable, indicating that the charging of the battery by the solar cell array will not damage the battery; If it is small, the design is unqualified, and it is necessary to redesign the battery to appropriately increase the capacity of the battery.
Read more: What are the advantages of VRLA batteries in terms of construction and use?