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98-100 Capacity design of independent photovoltaic power generation system

Author: Source: Datetime: 2017-01-14 10:01:58

4.3.1 Capacity design of independent photovoltaic power generation system


4.3.1.1 Calculate the load


Figure-4-46-load-characteristics-with-a-50KW-photovoltaic-power-plant

Load estimation is one of the key factors in the design and pricing of stand-alone PV systems. The names of all loads, power requirements, rated operating voltage, and power usage per day are usually listed. For AC and DC loads are also listed, the power factor in the AC power calculation should not be considered. The load classification is then grouped by the operating voltage, and the total power requirement for each group is calculated. The system operating voltage is then selected to calculate the average ampere-hours (A • h) required by the system at this voltage, ie, the sum of the average daily power consumption of all the loads. On the choice of system operating voltage, is often selected the maximum power load required voltage. In AC-based systems, the DC system voltage should be adapted to the input voltage of the selected inverter. Typically, the independent operation of solar photovoltaic power generation system, the AC load work in 220V, DC load work in 12V or 12V multiples, that is, 24V or 48V and so on. In theory, the load is indeed straightforward, but in fact the load requirements are often not sure. For example, the power required for a household appliance is known from the manufacturer's data, but the time is not known, and the daily, weekly and monthly usage time is likely to be too high and the cumulative effect will be photovoltaic The design capacity and the cost of the power generation system increase. In practice, some of the larger power loads can be scheduled to be used at different times. In the strict design, we must grasp the load characteristics of independent photovoltaic power generation system, that is, the load power at different time in 24h, especially for the centralized power supply system, to understand the law of electricity can be controlled in a timely manner. Figure 4-46 shows the load characteristics of a 50 kW PV plant. In the figure, FL, IL, TV, D, and H respectively denote fluorescent lamps, incandescent lamps, televisions, sterilizers, and heaters. You can also list the basic load of the local residents, as shown in Table 4-12.

Table 4-12 Loading power consumption statistics

No
Load name
AC/DC
Load power /W
Nomber of loads
Total power/W
Daily working hours /h
Power consumption/daily  KW/H
1
Fluorescent lamps






2
Incandescent lamp






3
TV






4
Sterilizer






5
Heater






Total







4.3.1.2 Calculation of the incident energy of solar panels
Design and installation of photovoltaic power generation system, of course, to grasp the local solar energy resources. The basic data required for the design calculation are as follows.
① the location of the site, including location, latitude, longitude, altitude and so on;
② Meteorological data of the installation site, including total solar radiation, direct radiation and scattering (or sunshine percentage), annual average temperature, longest continuous rainy day, maximum wind speed and hail, snow and other special climatic conditions.
These data are generally not predictable in the long term and can only be based on the average observed over the past 10 to 20 years. However, few independent PV systems are built in cities where solar radiation data are available, and solar radiation data in remote areas may not be similar to those in nearby cities. Therefore, it is necessary to grasp the factors of possible deviations when using only the meteorological data of neighboring cities or the data recorded by similar regional meteorological observatories. Solar energy resources need to estimate the impact will directly affect the performance of independent photovoltaic systems and cost.
The information from the meteorological department is generally only the level of solar radiation, to try to convert to the slope of the radiation. Here we give the calculation method.
The incident energy of the solar array, including direct radiation, scattered radiation and ground reflection of the three parts. Let the total solar radiation level of the whole day for the IH, IH0 it by the amount of direct radiation and horizontal scattering IHS composition. Then, the total solar radiation amount It of the solar panel inclination surface, which is set at the inclination angle? To the ground plane, is calculated by the following equation
It≅IHo • [cosθ+sinθcothocos(φ-∅)] + IH0 •(1+cosθ)/2+ρIH•(1-cosθ)/2 (4-1)
The first term on the right side of equation (4-1) is the direct component, the second term is the scattering component, and the third term is the ground reflection component. Ρ is the ground reflectivity, the reflectivity of different surface states can be found in Table 4-13. Engineering calculation, take the average of 0.2, with snow cover the ground when taken 0.7. Each of the angle relationships in the equation (4-1) is shown in FIG. 447.
Table 4-13 Ground reflectivity of different surface types

Surface condition Ground reflectivity /% Surface condition Ground reflectivity /%
Desert 24 〜28 Wet sand 9
Dry bare ground 10 〜20 Dry grass 15 〜25
Wet bare ' 8〜 Wet grassland 14 〜26
Dry black soil 14 New snow 81
Wet black soil 8 Can snow 46 〜70
Dry sand 18 Ice surface 69
Figure-4-47-shows-the-various-angles-of-incidence-about-the-sunshine

The amount of solar radiation from the horizontal plane to calculate the square solar cell tilted surface received solar radiation, the larger the workload. At present, RETScreen, a photovoltaic system design software developed jointly by the Canadian Environ- mental Energy Agency and NASA, is commonly used. Through this software, you can easily calculate the fixed angle of the square, the horizon coordinate azimuth axis tracking, the equatorial coordinate polar axis tracking and azimuth, tilt axis accurate tracking and other modes of operation under the square on the solar cell received Of the solar radiation energy.
Should the use of computer-aided design software, the square should be pre-tilt angle of the optimal design requirements of the total annual radiation as large as possible, and in the winter and summer radiation differences as small as possible, the two balance. This is particularly important in high latitudes. This is due to the large differences in solar amplitude between the winter and summer levels in high latitudes. Only consider the trade-offs, choose the best inclination, solar cell square on the surface of the difference between summer and winter radiation will be reduced, the battery capacity design can be reduced, the system cost reduction, design is more reasonable. In general, fixed angle solar cells on the front side of the radiation radiation level than the level of radiation by 5% to 15% higher direct component, the higher the degree of latitude, tilt the surface than the increase in the amount of radiation.Higher 5% ~ 15%, the greater the direct component, the higher the latitude, the slope surface than the horizontal increase in the amount of radiation.
In the design of small-scale independent photovoltaic power generation system, the azimuth angle should be set south of the northern hemisphere in China for the determination of the inclination and azimuth of the fixed square array. However, due to some restrictions can not be just south, as long as 20 ° in the South are within the square of the output power will not reduce the number. Non-south set, the power output roughly in accordance with a cosine function to reduce. As for the square angle, generally used in accordance with local latitude integer fixed set. If we consider that in order to be able to send more electricity in winter, square dip angle may be appropriate to increase some of the local latitude, generally within +5 ° ~ 15 °.


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