1.-Droughts
In a context of increasingly frequent and persistent droughts, environmental awareness and sustainable development, the integration of technologies such as solar pumping with frequency converters is becoming an increasingly necessary option in water management. The search for new extraction points and the increasingly efficient distribution of water is leading to the installation of pumping points in remote areas where there is no access to the electricity grid or where a priority contribution of renewable energy is preferred.
Today we will explore the fundamental aspects related to solar module fields used in pumping with variable frequency drives, from the choice and design of the installation to practical tips and common mistakes to avoid.
The first thing we must be clear about is the type of solar pumping installation, as this will condition the design of our solar field.
2.-Types of installation
We will divide the installations into three types depending on whether the energy sources are connected to the power inverter: Hybrid, Switched or Isolated.
For good solar field design in hybridized systems, it is important that the solar field voltage is oversized to ensure that all available solar energy is always fed to the inverter.
3.-Hybridized and combined systems
Hybrid installations bring multiple energy sources to the drive, combining solar power with other sources such as conventional electric or wind power. This approach provides greater flexibility and reliability, as the system acquires the power it needs simultaneously. In the case of hybridizing a solar field with the grid (or a genset for example), the main advantage is that the inverter is not affected by sudden changes in environmental conditions or load requirements. In addition, the solar field will always provide as much energy as it can generate at any given moment, as the grid will supplement what is missing for the pump to run day and night.
-Network contribution
-Solar contribution
Another point to take into account in hybridized systems is the mandatory use of bias diodes on the DC side to ensure that current always flows from the solar panels to the DC bus of the frequency inverter and to prevent grid power entering the DC bus from damaging the solar field.
In a switched installation, either solar or grid power is used, but not both at the same time. This type of system is characterized by its ability to operate in automatic mode, alternating between sources according to environmental conditions. It is a necessary solution when it is necessary to ensure the disconnection of the conventional grid in those time periods where the cost of energy is expensive and a 100% solar contribution is ensured. The main disadvantage is that when working connected to the grid, the solar energy is decoupled and the production is not used.
In these cases, the solar field must be sized with a power output that ensures 100% load operation without any other external energy input. Normally the installed solar power is calculated between 1.5 to 2 times the pump power. The main factors to take into account are: the size and installation of the motor-pump assembly, the efficiency of the motor-variator assembly and the effective daily hours of water extraction required.
4.-Isolated installations
They are completely dependent on solar energy. They are not connected to the conventional electrical grid.
A stand-alone installation could be ideal in remote areas without access to the conventional power grid. Here the variable frequency drive must be able to modulate the energy with which it drives the pump to avoid shutdowns and maintain pumping for as long as possible. As with switch-mode systems, the solar field must be sized with a power output that ensures water extraction for as long as possible throughout the day.
In addition to the above, the choice of installation type for solar pumping will depend on factors such as location, flow requirements and water pumping hours, and associated costs. Each approach has its own advantages and challenges, but all represent important steps towards the sustainable use of solar energy in agriculture and water supply.
5.- Tips for choosing and designing the appropriate solar module field
The solar modules are connected to the DC bus of the inverter as shown in the figure. The number of modules in series required is determined by the minimum and maximum voltage of the inverter:
· The minimum voltage will be the voltage determined by the variable frequency drive to drive the pump and draw water at rated speed. In addition, the sum of the working voltage of the solar panels (Vmpp) connected in a series (called ‘string’), and not the open circuit voltage (Voc), will be taken into account for this calculation. A good analysis of the geographical area that determines the ambient temperature and available radiation is important, as heat negatively affects the voltage that the solar modules can effectively provide.
· The maximum voltage must always be below the maximum permissible DC bus voltage of the frequency inverter. The maximum voltage is calculated as the sum of the open circuit voltage (Voc) of all modules connected in a series (called ‘string’) at the minimum ambient temperature of the geographical area, since the voltage Voc increases with decreasing temperature.
The strings are connected in parallel to achieve the necessary power to drive the pump and draw water. The number of strings in parallel determines the total current to be supplied to the variable frequency drive. In this case, there is no limit to the number of strings that can be added in parallel. The minimum recommended is one that ensures a power of 1.5 to 2 times the power of the installed pump.
6.-Summary
The requirements for the solar field are:
· Power: must be adequate to that of the pump.
· Number of modules in series:
– Must be suitable for the rated DC bus voltage of the drive.
– May not exceed the maximum DC bus voltage of the drive.
· Number of parallel strings: As many strings are added as necessary to achieve the required power in the pumping installation, and may exceed the maximum rated current of the frequency converter.
At Vector we advise on the design and optimization of the solar field according to the type of installation and application. We are professionals in solar pumping solutions with more than 20 years of experience. Contact us and we will help you in the correct choice of the frequency inverter, accessories and protections for both AC and DC, technical support for the configuration of the equipment and the commissioning of your installation.
Please do not hesitate to contact us for further information and inquiries.
Tel. Offices: 935 748 206
Tel. SAT Automation: 937 618 313
Tel. SAT Sunvec: 937 617 771
Ricardo Molina,
Technical Director,
Vector Energy.