SOLAR SOLAR (PHOTOVOLTAIC) WATER PUMPING
Water pumping has a long history; so many methods have been developed to pump water. People have used a variety of power sources, namely human energy, animal power, hydro power, wind, solar and fuels such as diesel for small generators. The most common pumps used in remote communities are: • Hand pumps • Direct drive diesel driven borehole pumps • Electric submersible pumps with diesel generator • Solar submersible pumps The relative merits of the various pumping methods are laid out in Table 1 below. Advantages Disadvantages
Hand pumps Link
• • • • • • •
local manufacture is possible easy to maintain low capital cost no fuel costs more powerful than humans lower wages than human power dung may be used for cooking fuel unattended operation no fuel costs easy to maintain low cost long life high reliability unattended operation easy maintenance long life suited to local manufacture no fuel requirements unattended operation no fuel costs low maintenance easy installation long life (20 year) quick and easy to install low capital costs widely used can be portable
• • •
loss of human productivity often an inefficient use of boreholes low flow rates
Animal driven pumps
animals require feeding all year round often diverted to other activities at crucial irrigation periods
Hydraulic pumps (e.g. rams) Link
• • • • • • • • • • • • • • • • • • • •
require specific site conditions low output
Wind pumps Link
• • • • • •
water storage is required for low wind periods high system design and project planning needs not easy to install high capital costs water storage is required for cloudy periods repairs often require skilled technicians fuel supplies erratic and expensive high maintenance costs short life expectancy noise and fume pollution
Diesel and gasoline pumps
• • • •
Table 1: Comparison of pumping techniques.
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Solar (PV) water pumping
Solar pumps are used principally for three applications: • • • village water supply livestock watering irrigation
A solar pump for village water supply is shown schematically in Figure 1. The Village will have a constant water demand although there is need to store water for periods of low insolation (low solar radiation). In environments where rainy seasons occur some of this demand can be met by rainwater harvesting during the rainy season. Ideally in Sahelian Africa the storage would be 3-5 days of water demand. In practice some installed tanks do not have sufficient capacity and are smaller than a days demand leaving the
Figure 1: Village water supply.
tank empty at the end of the day. This is due to a mismatch between the sizing, pump capacity and the demand profile during the day. The main applications for solar water pumping are for livestock watering in the USA and Australia. In Africa the systems are used for village water systems and livestock watering. While applications of solar water pumping for irrigation are on the increase especially in India and China. A solar irrigation system (Figure 2) needs to take account of the fact that demand for irrigation water will vary throughout the year. Peak demand during the irrigation seasons is often more than twice the average demand. This means that solar pumps for irrigation can be underutilised for most of the year although there can be a reduction in strength of the sun during these times reducing supply side of the equation. Attention should be paid to the system of water distribution and application to the crops. The
Solar (PV) water pumping
system should minimise water losses, without imposing significant additional head on the pumping system and be of low cost.
Figure 2: Solar irrigation system.
The suitability of major irrigation systems for use with solar pumps is shown in Table 2.
Typical application efficiency 50-60% 70% 85% 40-50%
Suitability for use With solar pumps Yes No Yes No
Open Channels Sprinkler Trickle/drip Flood
0.5-1m 10-20m 1-2m 0.5m
Table 2: Suitability of major irrigation methods for use with solar pumps
The The technology
Photovoltaic pumps are made up of a number of components. There is a photovoltaic array which converts solar energy directly into electricity as DC. The pump will have an electric motor to drive it. The characteristics of these components need to be matched to get the best performance. The pump motor unit will have its own optimum speed and load depending on the type and size of the pump. Motor This can be DC or AC. If an AC motor is used then an inverter is also needed. AC motors are more widely available. Inverters have become cheap and efficient and solar pumping systems use special electronically controlled variable-frequency inverters which will optimise matching between the panel and the pump. A typical AC system would also need batteries which require maintenance and add to the cost as the system is less efficient and would need a larger array.
Solar (PV) water pumping
The most efficient type of DC motor is a permanent magnet motor. DC motors may have carbon brushes which replacing when they wear out, If a brushed dc motor is used then the equipment will need to be pulled up from the well (approximately every 2 years) to replace brushes. Brushless designs of DC motors exist where electrical circuits are used instead of commentators and brushes. These are becoming popular in solar pumping systems. Brushless dc motors would require electronic commutation. Solar Solar panels The basic principles of solar photovoltaic panels are explained in the Practical Action Technical Brief Solar Photovoltaic Energy. Some models use a GPS sensor to provide latitude, longitude and time data to enable the controller to calculate the position of the sun and position the solar array. The pump Pump options and the system configuration are described below
Submersible Submersible pumps
Often with electronic load controllers; the pump will be submerged while the load controller is above ground. The advantages of this configuration are that it is easy to install, often with layflat flexible pipework and the motor pumpset is submerged away from potential damage.
Multistage Multistage centrifugal pumps The centrifugal pump will start at low torque and can be matched with the solar array without electronic controllers. The pumps are not as efficient as positive displacement pumps using cheap electronic load controllers. Suitable for smaller heads. Older type set with AC motors operate at heads of 10-25 m.
Figure 3: Submerged multistage centrifugal motor pumpset.
helical Positive displacement helical pumps Helical pumps have the best efficiency and the smallest PV panel for the same specs of water delivery volume pressure and head. They have low rotational speed. The pump is made up of a metal helical rotor which rotates in a rubber casing. Suitable for bigger heads. A Mono solar pump will slow down when it is cloudy, but because it has no minimum speed (unlike a centrifugal pump) it will keep delivering water.
Solar (PV) water pumping
Submerged Submerged pump with surface mounted motor
The main advantage is the easy access to the motor for maintenance. The low efficiency from power losses in the shaft bearings and the high cost of installation have been its main disadvantages. In general this configuration is largely being replaced by the submersible motor and pumpset.
Floating Floating motor pump sets
The versatility of the floating unit set makes it ideal for irrigation pumping for canals and open wells. The pumpset is easily portable and there is a negligible chance of the pump running dry. Most of these types use a single stage submersed centrifugal pump. The most common type has a brushless dc motor. Often the solar array support incorporates a handle or 'wheel barrow' type trolley to enable transportation.
Figure 4: Submerged pump with surface mounted motor.
Figure 5: Floating motor pump.
Figure 6: Suction pumpsets.
Surface pumpsets Surface suction pumpsets
This type of pumpset is also suitable for low head applications. It is not recommended except where an operator will always be in attendance for maintenance and security of exposed systems. Although the use of primary chambers and non-return valves can prevent loss of prime, in practice self-start and priming problems are experienced. It is impossible to have suction heads of more than 8 metres. Less common types of solar powered pumps include solar PV powered reciprocating piston
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