It is a consensus in the industry that solar energy resource data must be paid attention to in carrying out photovoltaic projects; However, many people ignore the “basic meteorological elements”.
What are the basic meteorological elements?
1. Temperature
Generally, it includes at least three items: multi-year extreme maximum temperature, multi-year extreme minimum temperature, and multi-year average temperature; A better weather station can also collect the extreme minimum temperature in the daytime and the extreme maximum temperature at night for many years.
2. Wind speed
It generally includes three items: multi-year average wind speed, multi-year maximum/maximum wind speed, and maximum wind direction.
3. Precipitation
It generally includes three items: multi-year maximum snow depth, multi-year average precipitation, and multi-year average evaporation.
4. Bad weather
It generally includes multi-year average thunderstorm days, multi-year average sandstorm days, multi-year average sand blowing days, multi-year average haze days, multi-year average fog days, multi-year hail days, gale days, etc.
5. Other
Standard frozen soil depth is generally included
Why should we pay attention to basic meteorological elements?
1. Temperature
In the design process of the solar power station, the temperature is used in at least three aspects.
1) Design of solar string
The extreme temperature must be considered when designing the series-parallel scheme of solar modules.
All PV modules have the parameter “open-circuit voltage temperature coefficient”, which is negative, that is, the voltage of PV modules will increase with the decrease of environment.
In the design, the more PV modules in series, the better, but the maximum open-circuit voltage after series cannot exceed the maximum system voltage of the module or the maximum allowable voltage of the inverter;
The working voltage shall be within the tracking range of the MPPT voltage of the inverter.
In some places with extremely low temperatures, such as – 30 ℃, when the open-circuit voltage temperature coefficient is – 0.32% / ℃, the open-circuit voltage of a single PV module will increase by 17.6%!
Therefore, there will be more PV modules in a single string in the South than in the north.
In practical work, we often see the design scheme of 22 265wp strings. There will certainly be no problem with this conventional scheme. But obviously, this scheme is an empirical scheme without detailed calculation.
In some places where the extremely low temperature is above – 10 ℃, there is no problem adopting the scheme of 24 pieces of 265wp in a string. Therefore, how many pieces of series are used in a string must be used: the open-circuit voltage value of the component, the best working voltage value, extreme high temperature and extremely low temperature, the maximum input voltage of the inverter, and MPPT voltage. The best scheme can be obtained by detailed calculation of these three groups of data.
It should be emphasized that the temperature of solar panels is generally higher than the ambient temperature. Therefore, when calculating the extremely high temperature, we should consider the extremely high temperature of solar panels.
In addition, some experts suggested that since the solar power station only operates in the daytime, the extremely high temperature and extremely low temperature in the daytime should be selected when selecting the extreme temperature, but it is difficult for most meteorological stations to collect the extremely low temperature in the daytime, and the extremely low temperature at night is generally used instead;
Other experts suggested that when matching with inverter open-circuit voltage and MPPT voltage, the temperature coefficient of voltage should be calculated by “open-circuit voltage temperature coefficient” and “optimal working voltage temperature coefficient” respectively. However, at present, few PV modules provide “optimal working voltage temperature coefficient”, so “open-circuit voltage temperature coefficient” is generally used.
2) Electrical equipment
All electrical equipment has an operating temperature range. During the review of the project, I found that the local extreme temperature is outside the operating temperature range of the inverter, so certain treatment measures should be taken during the design.
At the same time, the inverter will release a lot of heat when working. If the ventilation in the inverter room is not good, the temperature in the inverter room will be much higher than the ambient temperature, reduce the working efficiency of the inverter, and even higher than its operating temperature.
2. Wind speed
In the design, the reference values of wind pressure and snow pressure in a certain place will be given. However, I think the wind pressure value calculated according to the local wind speed and direction is of more reference significance, and the wind pressure value will directly affect the design of the support foundation.
3. Precipitation
On the one hand, the maximum snow depth must be higher than this value at the lower edge of the PV module in the design, otherwise, the PV module may be buried in the snow; On other hands, it can be used to calculate the snow pressure.
The value of local water resources can be seen from the local annual average precipitation and evaporation. For example, if the evaporation is much greater than the precipitation, it must be an arid area with valuable water resources. The depth of the water intake well must not be shallow, and the water may also have corrosive alkaline water. In the future, it is best to use a water-saving scheme to clean solar panels.
4. Bad weather
The average number of thunderstorm days over the years can be used as a reference in the design of lightning protection scheme;
The multi-year average number of sand storm days, the multi-year average number of sand blowing days, the multi-year average number of haze days, and other data can basically help you judge the cleaning frequency of solar power stations and the system efficiency loss caused by dust shielding.
5. Standard frozen soil depth
One of the parameters that must be considered in technical design is the depth of frozen soil. Generally, the depth of the foundation shall be below the frozen soil layer.
