2.7.1
Operating Conditions
The conditions under which a sustainable pump must operate are not expected to be constant and any environmental changes, such as increased or decreased rainfall in the wet and dry seasons, can impact upon a pumps performance. In the case of PV pumps, the reasons for this are twofold. Firstly, changes in the weather conditions can mean the difference in a relatively low or high borehole water level. Such a fluctuation in the pumping head can result in a decrease in performance for a standard centrifugal pump as well as any other type of pump. Ordinarily, it is not possible to compensate entirely for changes in environmental conditions such as rainfall, save implementing a pump with a good range of pumping heads. Secondly, for PV pumps, any changes in the intensity of solar irradiation in an area due to, for example, cloud cover or changes in daylight hours can impact upon the amount of power available to power the pump. Three ways of countering this effect are to employ a maximum power point tracking (MPPT) controller to the system, sufficient reserve water storage and/or battery storage. However as already mentioned in the argument for PVP’s, insolation levels are
35 often in-line with the power demand for highest water supply during periods of most demand. In the case of wind-electric and wind-PV hybrid systems, environmental changes in wind speed also have an impact on potential pumping capabilities of pumps.
2.7.2
Borehole Dynamics
In addition to changes in pumping head due to environmental effects, there also exists the issue of ‘dynamic drawdown’ [67]. During pumping, the water level in any borehole will drop as water is displaced as discharge flow. Water from the surrounding area must then balance the displaced water until equilibrium is achieved. Depending on several factors the degree to which the water level is ‘drawn down’ will vary. These include the discharge rate, soil permeability and area of the borehole. Naarvarte et al. provide a comprehensive summary of typical values of the static and dynamic levels of water in locations around Angola and Morocco, in addition to an informative guide on the phenomenon. These fluctuations in water level must be taken into account, especially in the case of centrifugal pumps, when specifying the pumping system for a particular location. Much as with any other site specific assessments, this adds to the installation cost and also adds another degree of complexity to the operation of a pumping system.
2.7.3
Installation Losses
When installing any pumping system, there is almost always a degree of human error involved in aligning and fitting all of the components as well as securing the pump in place in addition to component matching [12]. These errors can contribute to the pump operating outside of its BEP conditions due to an increased head from additional friction or losses through components. Further to this, and regardless of correct
36 installation or not, there lies a friction head within the system which, over time, will increase due to wear on the parts and internal corrosion of the pipes. Such losses can never be entirely avoided, but they can be lessened by introducing filters before the pump and ensuring all components are well protected. Poor component matching and improper installation have been previously shown to have a significant impact on a pumping systems operation, with entire systems failing [73]. This leads to the demand for either suitable technical expertise in selection and installation of equipment or the ability of a pumping system to self-calibrate and negate lack of expertise.
2.7.4
Power Utilization
Irrespective of the power source and degree to which power is freely available, a centrifugal pump is only able to utilize a finite amount of energy at any one time, dependant on current operating conditions. As previously established, the likelihood of a pump being able to continuously draw from its MPP is low, particularly in the use of direct coupled systems [12, 63]. Even in the case of optimization through an MPPT it would still be beneficial to utilize the optimum power available in the form of improved discharge flow, especially in pump/power systems where there is a surplus energy already available (this particularly relates to solar at high levels of insolation).
Additional generated energy may be stored in a battery system for later use, but ideally a simpler system would forgo the added complexity and expense of a battery for the storage of additional water in a reservoir. Thus, it would be more desirable for a pumping system to be able to utilize any and preferably all energy available to it at a given time, storing any surplus water for times of higher demand or lower pumping capacity.
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2.7.5
Appropriate Technology?
“Appropriate technology and village level operation are two design ideologies that, when considered together, can contribute towards the sustainability of
(solar powered) water pumps.” – T. D. Short 2003
In spite of being able to deliver a reliable supply of clean water to a developing community, there is still the challenge of deciding whether or not a particular type of technology is appropriate [68]. This begs the question: are centrifugal pumps, in the context of sustainable pumping, an appropriate technology? To consider an answer to this, the term appropriate technology must be defined. Dunn [74] summarises the term by considering the aims of an appropriate technology to:
Improve the quality of life of people.
Maximise the use of renewable resources.
Create work places where people live.
In addition to these aims, there are also a set of criteria that a technology must adhere to. As such, a technology must be able to:
Employ local skills.
Employ local material resources.
Operate within local financial constraints.
Be compatible with local culture and practices.
Satisfy local wishes and needs.
While automated pumps may fulfill the aims of an appropriate technology, it is clear to see that, for a pumping solution to be entirely suitable, the pump selected must be
38 easily maintained at a level within the capability of a developing community or independent rural location.