SOLAR PUMPS FOR IRRIGATION

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Solar pumps for irrigation are pumps that use photovoltaic panels to produce power which is used in running pumps for irrigation. Unreliable rainfall in Kenya has led to many farmers seeking alternative ways of ensuring reliable water supply to the farms. Many have opted to drill boreholes and to pump from rivers and streams to their farms. With the rising fuel prices in the world, the use of solar pumps seems like the best way to go. These types of pumps are especially useful in areas with no electricity, where the supply of electricity is not efficient, and in areas where the source of energy being used is not enough e.g., wind energy. These types of pumps have the added advantage of having low maintenance costs.

Types of solar pumps for irrigation

There are major types of solar pumps for irrigation available;

  1. Surface solar pumps.
  2. Submersible water pumps.
  3. DC solar pumps.
  4. AC solar pumps.

Surface solar pumps for irrigation

Surface solar pumps for irrigation are mostly used in shallow wells, ponds, or streams to pump water to farms. They are used when the depth of the well used is less than 15 m and the water can be pumped for a distance of 60 m or more. The impeller in surface solar pumps is mounted directly on the shaft and the motors used are permanent magnetic direct current motors (PMDC). These motors achieve high working efficiencies and are easy to maintain.

Submersible solar pumps for irrigation

These types of solar pumps for irrigation are used in deep wells where the depth is more than 15 m but less than 50 m. Submersible solar pumps can achieve high heads of up to 450 m depending on the size of the pump used. They come equipped with brushless direct current motors (BLDC)or alternating current induction motors. These motors are capable of achieving high torques and efficiencies.

DC solar pumps for irrigation

They are solar pumps that use direct current from the solar panels. The energy generated is capable of pumping water from wells having depths of up to 112 m

AC solar pumps for irrigation

These types of solar pumps make use of alternating current making an inverter necessary in the system. It is best to use solar pumps together with storage tanks. Solar’s are affected by the weather. Water can be stored in the tanks for irrigation use reducing the risk of bad weather.

Components of solar pumps for irrigation

Solar pumps for irrigation in Kenya consist of the following components;

panels, controller,  pump.

Solar panels

The function of solar panels in a solar pump for irrigation is to absorb sunlight and convert it to direct current. This process is achieved by silicon and conductors on the solar panels.

The type of solar panel and number of panels to use varies greatly depending on the following factors;

  1. Volume of water needed to be pumped – if the volume of water needed is a lot, then powerful or more panels or will be needed to generate the required power.
  2. Geographical location where the solar pump is to be used – when solar pumps are used in regions that receive minimal sunlight in the day, it is best to increase the power generated during the few sunlight hours. This can be achieved by using more panels or using more powerful solar panels. In areas like deserts or semi-deserts which receive a lot of sunlight then fewer solar panels can be used.
  3. Ratings on the solar panels – the ratings on the solar panels affect the number and types of solar panels to use. This is because the rating of the panels determines how much power can be generated by each panel.
  4. Head of water needed to be achieved by the solar pumps – for a solar pump to pump water great distances then more power will be needed. This fact determines the number of panels that will be needed and the type of panels.

What factors affect the power generated by solar panels?

The following factors affect the amount of power generated by the solar panels in solar pumps for irrigation.

  • Solar insolation – this refers to the amount of sunlight received within a specific region in a day. It is measured in kilowatt-hours per meter squared per day (KWh/m2/day ). The time of the year and the geographical location affect this factor.
  • Solar power – The panels can be connected either through a series connection or a parallel connection. Panels connected in series have a voltage equal to the sum of all the individual panel voltages while the current of the whole system is equal to the current of one panel. Panels connected in parallel have an overall current equal to the sum of each panel’s current and a voltage equal to the voltage of one panel.
  • Orientation of the solar panel – for maximum power generation, the solar panels should be facing the sun directly. The solar panels can be designed to be able to switch their angle according to the sun’s position maximizing on the available sunlight.
  • Sunshine hours – this is affected by the geographical location and the weather patterns within an area. There will be less power generated in areas that receive cloudy or rainy weather during the day compared to regions with hot and clear weather. In the northern and southern hemispheres where they experience the four seasons, maximum power will be generated during summer and little or no power will be generated during winter.

Controller

The main purpose of the controller is to regulate the current and voltage of the power produced by the solar panels. Since the light offered by the sun greatly fluctuates, the controller is used to take care of these fluctuations. The controller helps in starting the pump when the sunlight is low and also helps in preventing overheating of the motor during high sunlight quantities. The weather can be unpredictable at times and to cater for that, other sources of power can be connected to solar pumps for irrigation. These connections are made on the controller and examples of power sources that can be added are generators, batteries, and wind machines.

Solar pumps

Solar pumps for irrigation can either use direct current (DC) or alternating current (AC).  For solar pumps using direct current, an inverter is not needed. Those that use alternating current need an inverter to change the DC power generated to AC.  Solar pumps are designed to operate at a range of voltages between 24 v and 300 v. This range caters for even when the sunlight is low. The solar pump chosen should be able to overcome the total head to be pumped and the frictional losses in the system.

What is the working principle behind solar pumps for irrigation?

The photovoltaic panels in solar pumps for irrigation are able to convert the sun’s radiant energy to electricity. The electricity produced is direct current (DC) and there is need to be converted to alternating current (AC). Alternating current is the form of electricity that is used to run the pump and an inverter is used to convert the direct current to alternating current. The inverter is also capable of controlling the voltage and frequency of the of the power supply according to the sunlight intensity. The now converted alternating current is then used by the motor to run the pump.

Determination of the number of solar panels needed in solar pumps for irrigation

The number of panels needed to run solar pumps for irrigation depends on the power required by the pump. The power needed by the pump is usually indicated on the pump by the manufacturer. In order to find how much solar power is needed we use the following equation:

Solar energy needed by pump = load of the pump × hours of sunlight received per day × 1.5

The load of the pump refers to the amount of electricity needed to run the pump. Most countries receive an average of 5 hours of sunlight daily, hence we compute using 5 hours. The value 1.5 is used as a factor of the safety of the pump to ensure a constant power supply.

To find the total watts that the solar panels will need we compute using the following formula:

Solar panel wattage needed = total load of the pump/ average hours of direct sunlight

Solar panel output per day

Most solar panels come either having 60 cells or 72 cells. There are even some which come having 96 cells.  In the 60-cell panel, the cells are arranged in a 6 × 10 grid whereas in a 72-cell panel they are arranged in a 6 × 12 grid.  The 60-cell panel measures 1 m by 1.67 m whereas the 72-cell panel measures 1 m by 2 m. A 60-cells solar panel produces 270 – 300 watts per hour and a 72-cells solar panel produces 350 – 400 watts per hour. In order to compute the solar panel output per day we use the following relation:

Solar output per day = solar panel power produced per hour × average hours of sunlight in  day × 75%

The 75% represents the efficiency of the solar panel system.

Size of solar panel battery needed

A battery is required especially if water pumping is also being done during the night. The battery will also help when the weather conditions are not favorable i.e., rainy or cloudy weather. We use the following formula to calculate for the size of battery  needed.

Size  of battery needed = average power used per night + (average power used per night × 30% )

The 30% increase in the computation is to cater for cloudy or rainy days.

Size of inverter required

An inverter is an important device in solar power pumps for irrigation. It helps in changing the DC current from the panels to the AC current required by the pump. In order to find out the size of the inverter needed; the array must be found. The solar array refers to the amount of energy that a 1KW solar panel can produce. The solar array is computed using;

Solar array = Total solar energy produced per day/ Solar panel wattage

The size of the inverter is usually recommended to be of the same size as that of the solar array. That means the solar array found in the size of the inverter that is needed.

Advantages of solar pumps for irrigation

The advantages of using solar pumps for irrigation are:

  • The use of solar pumps is helping in reducing global warming and protecting our natural resources. Solar pumps use the sun’s energy reducing the use of fossil fuels. This has helped in protecting non-renewable resources and reducing harmful emissions.
  • Solar pumps for irrigation are durable and perform efficiently
  • The operating cost of solar pumps is low. Solar pumps don’t need fuel to work and this has helped in reducing costs.
  • Maintenance  done on solar pumps is very minimal
  • Portability of the solar pumps is very easy

 Challenges encountered when using solar pumps for irrigation?

The main challenge  encountered  using solar pumps in irrigation is power supply. The fluctuations in sunlight mean the power generated also is going to fluctuate. However, hybrid system maintains a steady flow of power when the charge is not enough. By using tanks to store the water pumped, it is also possible to counter the problem of sunlight fluctuation. This method of pumping water for irrigation is a little bit costly to initially set up. However, the long-term savings done when using this system compared to diesel powered pumps are a lot.