Design of Photovoltaic (PV) Pumping

Overview

In order to implement cost effective photovoltaic (PV) pumping system, it is necessary to follow some basic guidelines to design and size every system component. This page will explain about detailed calculation needed for sizing every system component.

The basic step to design photovoltaic (PV) pumping system is:

1. Determination of water supply need
2. Calculation of pumping head
3. Estimation of solar resource
4. Finding appropriate solar pump and inverter
5. Calculation of PV panel required

Determination of Water Supply Need

There are two distinct application for photovoltaic (PV) pumping system:

1. Drinking water supply (domestic water supply)
2. Irrigation water supply

Domestic Water Supply

For domestic water supply, the first data needed is to estimate the water requirement for one person/day.

A good estimate can be found in "Basic Water Requirements for Human Activities: Meeting Basic Needs" , that relate how much water required to sustain particular activities. Here is the estimate:

Purpose Recommended minimum (liter/person/day) Drinking water 5 Sanitary services 20 Bathing 15 Cooking and Kitchen 10 Total 50

For example: if there are a village with basic need of drinking water, bathing and sanitary services but no need for cooking; then it will require 5+20+15=40 liter/person/day. If the population is 300 persons, then the entire village will require 300 x 40 = 12 000 liter/day. Normally to compensate for water leakage on piping or distribution, 20% additional water will be required then the village will require 14 400 liter/day or 14.4 m3/day.

Calculation of Pumping Head

Pumping head is normally measured in meter (m), so sometimes there are misconception that pumping head is equal to pumping elevation. In actual, pumping head is divided into three components: elevation head, major losses head, and minor losses head.

Pumping Head (m) = Elevation Head (m) + Major Losses Head (m) + Minor Losses Head (m)

1. Elevation head is measured from the water source surface level to the point of outlet pipe level.
2. Major losses head is influenced by water flow rate, diameter of pipe, length of pipe, and type of pipe (PVC, HDPE etc)
3. Minor losses head is influenced by piping accessories including valve, elbow, inlet pipe etc

The detailed explanation can be found on "The Mathematics of Pumping Water, AECOM Design Build Civil, Mechanical Engineering".

Estimation of Solar Resource

Ideally, a ground based solar radiation measurement in location to be installed with solar water pumping system, is required in order to measure accurately the solar resource available for every part of the year. However, due to expensive equipment required, alternative data can be obtained from "Surface meteorology and Solar Energy" (NASA) for every part of the world freely or other solar resource data website available in the internet. 

Solar resources available per day is cited on unit of kWh/m2/day or PSH (peak sun hour) with 1 kWh/m2/day = 1 PSH. Common confusion of solar resource data understanding is usually with solar data cited on unit of W/m2 that describe power.

What is needed on solar pumping design is data cited on energy:

1. Solar Energy / Day ► cited on unit of kWh/m2/day or PSH     (commonly used for solar pumping design)
2. Instantaneous Solar Power ► cited on unit of W/m2               (only used for complex modelling)

Finding Appropriate Solar Pump and Inverter

The most important criteria on determining the optimum solar pump is by finding the pump that can satisfy both of daily water flow and pumping head requirement. Normally a solar pump can operate on wide range of water flow rate in response to variation of solar radiation. Generally there are three kinds of pump normally used as solar pump:

1. Centrifugal pump

2. Helical rotor pump

3. Piston (diapraghm) pump

Here is the table that explain the different characteristic of solar pump :

Type	Pumping Head (m)	Flow Rate (m3/day)	Mechanism
Centrifugal pump	0 to 80	6 to 20	Similar to conventional pump
Helical rotor pump	50 to 150	> 20	Robust, only one turning part
Piston (diapraghm) pump	0 to 150	2 to 5	Complex, lot of moving parts, require oil lubrication

For simplicity of design and maintenance, it is suggested that the inverter should come from one package (one supplier) with the solar pump. This will prevent operating mismatch and guarantee easier troubleshooting.

For pumping head, it is always imperative to keep the pump head specification higher than the required pumping head. A safety factor of 30% might be needed if the water level condition is not stable. If the pump cannot fulfill the pumping head requirement, no matter how bright the sunlight is, it will not provide any water at all.

For water flow required, some solar pump supplier usually provide a sizing chart that will help user to predict how much solar power required to provide for daily flow requirement. In order to use this sizing chart, user need data from previous step to determine how much solar resource available per day in terms of kWh/m2/day or PSH. This will in turn decide how much solar power you require for the solar pump. Unlike the pumping head requirement, if the pump water flow capacity is lower (due to specification or cloudy day), it still capable to provide water eventhough the sun is not as bright (it will only provide less quantity of water).

Calculation of PV Panel Required

The calculation of PV panel number required should be based on the specification of the solar pump inverter input. Normally they will suggest a range of DC Input Voltage and inverter power required. The number of PV panel required is related to the specification of individual panel. The common misconception is that you can estimate based on the number of panel (they had different capacity) or size (it might had different efficiency). 

Further Information

• Photovoltaic (PV) Pumping
• The Mathematics of Pumping Water, AECOM Design Build Civil, Mechanical Engineering
• Basic Water Requirements for Human Activities: Meeting Basic Needs