20-10-2016, 11:54 AM
Feasibility Assessment for the Replacement of Diesel Water Pumps with Solar Water
Pumps
1460111215-SolarPVwaterpumpingstudyFINALREPORTSinglesided.pdf (Size: 1.16 MB / Downloads: 9)
Executive Summary
A study was commissioned by the Ministry of Mines and Energy (MME) through the
Namibian Renewable Energy Programme (NAMREP) to examine the cost effectiveness of
solar water pumps compared to diesel water pumps.
Technology
The study provides an overview of water provision technologies for water supplies in rural
Namibia where no piped or open water is available and where the water needs are serviced
primarily through boreholes.
Current photovoltaic (PV) water pumping (PVP) technologies and diesel water pumping (DP)
technologies are discussed in terms of their performance range and their technical features.
The study considered the following PVPs:
The diesel pumps under review are in the power range of 2 to 12kW and the following engine
products were considered:
• Kia and Kirloskar of Indian manufacture; and
• Lister of South African manufacture.
The diesel pumping systems is based on a helical rotor, positive displacement pump (Mono
and Orbit elements).
Perceptions and experiences
Perceptions and experiences of PV and diesel water pumping systems were gathered
through interviews. Issues that were raised are:
• concerns about theft, in particular of PVP systems;
• the perceived inability of PVPs to supply variations in water demand;
• sophisticated technology of PVPs with serviceability mostly provided in Windhoek;
• performance concerns; and
• high cost.
The findings are:
• Theft is a valid concern and users need to consider counter measures where theft is
a risk;
• Some PVPs are able to operate in combination with a genset to pump additional
water at night or during inclement weather;
• The service infrastructure of PVPs is improving through the training and
establishment of solar technicians in the regions;
Recent developments have seen the introduction of durable, high efficiency, deep
well pumps which are improving the performance concerns of the past;
• The cost is discussed in the comparison with diesel pumps below.
In general the benefits of PVPs are well understood, in terms of their low maintenance
needs, automatic operation, the minimal attention that is required and their suitability for low
yield boreholes. PVPs provide environmental benefits by offering a clean solution with no
carbon emissions and risks of borehole contamination.
Cost comparison
A cost comparison for solar and diesel water pumps was conducted over a range of pumping
heads (10m to 200m) and a range of daily flow rates (3m3
/day to 50m3
/day). The life cycle
costs (LCC) were calculated over a 20 year period taking into account:
• the initial upfront cost;
• the operating costs (diesel fuel for the operation, inspections of pumping systems);
• maintenance costs; and
• replacement costs.
The graph shows the LCC for
PVPs and DPs which has been
averaged for different hydraulic
loads. The costing results of the
diesel pump are highly dependent
on the selected pumping schedule
(pumping interval and hours per
pumping session). In addition the
averaged PVP LCC has been
divided by the averaged DP LCC
for each of the hydraulic load
points. This yields the percentage
LCC of PVP as a function of the
LCC of the DP, measured against
the Y-axis on the right.
At low hydraulic load the PVP LCC is as low as 20% of the DP LCC. At higher hydraulic
loads this value reaches 55% which means that the PVP option still provides a solution at
half the life cycle cost of the DP option.
INTRODUCTION
Namibia as a semi-arid country with very little surface water has to obtain water from wells
and boreholes where the population reside in rural areas and are not in the vicinity of one of
the perennial rivers at Namibia’s borders or near a piped water scheme. It is quite common
to find hand-dug wells, in particular in parts of the north where the ground water levels are
shallow. However the larger part of water for the above areas is supplied from approximately
51,5002
boreholes.
1.1 BACKGROUND TO WATER PUMPING IN NAMIBIA
Historically pumping from boreholes in Namibian off-grid areas has been predominantly
achieved with wind pumps. Wind pumps have a long service life, are able to deliver water
from depths of 300 to 400m, require no non-renewable fuel, require basic skills but are workintensive
to maintain, and have a well developed service infrastructure. Wind pump systems
are however not simple to install and require larger water storage than for example a diesel
or solar pumps to provide for periods of low wind.
Diesel water pumps became more attractive during the second half of the twentieth century
with the development of the fuel supply infrastructure and the technology to allow diesel
driven engines to pump from boreholes. Diesel pumps have the advantage of pumping water
on demand (predictability), also in varying daily capacity, depending on the operating times
and over high heads (300m+). Diesel engines have a fairly low capital cost. On the down
side the diesel pumping system relies on fuel and is therefore “at the mercy” of fuel cost
variations and exchange rate fluctuations. Furthermore diesel engines require regular
maintenance, linked to the hours of operation and have a fairly short life expectancy (highly
dependent on the level of maintenance, the operating conditions and the quality of the
engine and the installation). Most diesel pumps require manual starting making remote
pumping installations more costly to operate.
It is quite common to find wind and diesel pumping combinations where either a diesel
engine can be used to drive the reciprocating pump or where a diesel generator can be used
to drive a submersible pump (fitted underneath the wind pump cylinder) to back up the water
supply during low wind period or wind pump maintenance.
Hand pumps are used for pumping from boreholes in particular in the communal areas.
These are rugged devices which require no non-renewable fuel, are easy to maintain and
have low capital cost. They are however limited in terms of the pumping volumes and depth
of installation (hydraulic load limit of less than 250m4
/day3
).
Solar photovoltaic (PV) water pumps were first introduced for water provision in off-grid areas
about 25 years ago. The technology has developed around many different designs and in
some PV water pumps (PVP) the reliability and maintenance requirements have improved
over the initial PVPs introduced to the market. PVPs are easy to install, require no nonrenewable
energy, operate autonomously and are generally “good” for the sustainability of
boreholes due to their low extraction volumes spread over eight to ten hours a day. The initial
capital cost of PVPs is high due to the cost of the photovoltaic modules. The maintenance
requirements of PVPs differ and range between annual and five year maintenance intervals.
PVP technology is sophisticated and maintenance on PVP’s does require skilled technicians.
The water storage for PVPs needs to incorporate storage for days of low irradiance
(inclement weather) but in general irradiation levels for Namibia are more predictable than for
example wind resources. The over-sizing of the water storage reservoir is therefore within
limits. A perceived limiting factor of PVPs is that they do not easily cater for fluctuating water
demands or increased water demand although solutions for this are being offered and will be
discussed in this study.
This study, which is assessing the viability of replacing diesel pumps with PV water pumps,
will focus on water pumping installations with borehole depths of less than 200m. Many
existing installations are fitted with wind pumps, electrical pumps (on grid) and hand pump
technologies. These technologies do not form part of the feasibility assessment.
1.2 SCOPE OF WORK
The objective of this study is to analyse the recent trends in the use and costs of PVPs and
to conduct a comparative cost benefit analysis between diesel and PVP, based on the life
cycle costing approach. With recent technical developments in the PVP sector and with
anticipated increase in diesel fuel prices as well as possible shortages, breakeven between
the two technologies may be shorter than expected.
In summary the scope of work includes:
1. Installation quantities of solar PV water pumps in commercial, communal and public
facilities and price developments of the capital cost of PVPs over the last five years.
2. Conduct a comparative cost benefit analysis between diesel and solar PV water
pumps taking into account the current diesel price (including variations of price within
the country) as well as anticipated fuel price escalation.
3. Identify the operating and performance conditions under which it is viable to replace
diesel pumps with solar PV pumps.
4. Identify the social factors, preferences and satisfaction levels that determine the
criteria for selecting a PV pumping or a diesel water pump.
5. Identify the barriers to PVP adoption in the commercial, communal and public sector
and make recommendations on how to address these barriers. Review and propose
financial incentives as well as government policies that can facilitate the adoption of
PVPs.
6. Evaluate the reduction in green house gas emissions for two PVP uptake scenarios.
The report presents the outputs for the above tasks in three sections titled:
• Water pumping in Namibia,
• Life cycle analysis, and
• Facilitation of PV Pumping.
The report summarises the findings in the conclusion and presents recommendations for
further activities.
1.3 PREVIOUS WORK
The first in-depth and comprehensive analysis of off-grid water pumping solutions was
conducted by Fahlenbock (1996), “Assessment of the Potential of PV Pumping Systems in
Namibia”. The work includes:
• Ground water resource assessment
• Analysis of existing borehole installations
• Institutional setup at Rural Water Supply
• Economic analysis of solar PV and diesel water pumps
• Technical, economic and social site selection criteria
• PVP suppliers in Namibia
• Assessment of PVP potential
Fahlenbock (1996) is the main document of reference in terms of market and economic
analysis from 1996 to 2006.
Hervie (2006) has written a thesis on “Solar water pumping versus diesel pumping in
Namibia”. The work conducts detailed case studies in two communities and calculates the
life cycle cost of solar and diesel pumps, yielding breakeven points between 6 and 11 years.
The thesis has been referred to in terms of capital costs and in terms of comparative figures
for breakeven points.
2 WATER PUMPING IN NAMIBIA
This section takes a closer look at the PV pumping and diesel pumping technologies traded
in Namibia and starts with an overview of the recent trends in installations and pricing of
these technologies. This is followed by an overview of the pumping configurations, their
features and their performance. The section ends with a summary on the perceptions and
motivations for using PVP or DP solutions.
2.1 OVERVIEW OF TECHNOLOGIES
The most common technologies of PVP and diesel pumps are described in this section in
terms of technical and performance aspects.
2.1.1 PV pumping technology
A PVP typically consists of the following main components:
1. Photovoltaic array: An array of photovoltaic modules connected in series and
possibly strings of modules connected in parallel.
2. Controller: An electronic device which matches the PV power to the motor and
regulates the operation, starting and stopping of the PVP. The controller is mostly
installed on the surface although some PVPs have the controller integrated in the
submersible motor-pump set:
a. DC controller: usually based on a DC to DC controller with fixed voltage setpoint
operation.
b. AC controller (inverter): converts DC electricity from the array to alternating
current electricity often with maximum power point tracking.
3. Electric motor: There are a number of motor types: DC brushed, DC brushless, or
three phase induction and three phase permanent magnet synchronous motors.
4. Pump: The most common pump types are the helical rotor pump (also referred to as
progressive cavity), the diaphragm pump, the piston pump and the centrifugal pump.
Some years ago there were PVP models on the market that operated with batteries and a
conventional inverter. However it was soon realised that the cost savings on the pump did
not make up for the overall substandard efficiency and the higher maintenance cost due to
battery replacements. Instead it became clear that it is more economic to rather store water
in a reservoir than electricity in a battery bank.
There are currently three pumping configurations commonly utilised in Namibia:
1) DC drive with positive displacement pumps. This consists of four pump
technologies:
a. Diaphragm pump driven by brushed DC motor: Submersible motor/pump:
Example: Shurflo, DivWatt, All Power Watermax.
b. Helical rotor pump driven by brushless DC motor: Submersible motor/pump:
Example: Total Energie TSP 1000.
c. Helical rotor pump driven by surface mounted brushed DC motor: Example:
Mono/Orbit pump with DC motor
d. Piston pump driven by surface mounted brushed DC motor: Example: Juwa
pump.
2) AC drive powering a submersible induction motor/centrifugal pump unit:
Example: Total Energie TSP 2000, 4000 & 6000 range; Grundfos SA 1500 and SA
400 which has been utilised extensively in Namibia but may be phased out in the
near future.
3) AC drive powering a three phase permanent magnet synchronous motor. This
category consists of:
a. Positive displacement helical rotor pump: Example: Grundfos SQ Flex,
Lorentz HR range.
b. Centrifugal pump: Example: Grundfos SQ Flex, Lorentz C range.
The above technologies have specific features which make them suitable for particular
applications:
Array voltage: Some of the pumping systems have high array voltages. This has the
advantage that the array may be further from the borehole without significant voltage drop
(dependant on cable size and current). Array positioning may be important where there is
potential for theft.
DC motors: DC motors reach efficiencies of up to 80% and are therefore significantly more
efficient than sub-kW three phase motors which have efficiencies in the region of 60% to
65%.
Brushless DC motors: This combines the high efficiency of DC motors with low
maintenance as opposed to brushed DC motors which require regular brush replacement
(approximately every one to two years – head and quality dependent).
Three phase permanent magnet motors: This similarly combines the high efficiency of
permanent magnet motors with low maintenance.
Positive displacement vs. centrifugal pump: Positive displacement pumps have a better
daily delivery than centrifugal pumps when driven by a solar PV system with its characteristic
variable power supply. This is due to the considerable drop in efficiency of the centrifugal
pump when operating away from its design speed. This is the case in the morning and the
afternoon of a centrifugal pump driven by a PV array, unless that array tracks the sun (which
is why centrifugal PVPs effectiveness improves more with a tracking array than a positive
displacement PVP). The efficiency curve of a positive displacement pump is flatter over a
range of speeds. However the efficiency of positive displacement pumps decreases with the
shallowness of the borehole (the constant fixed friction losses become a more significant part
of the power it takes to lift water). Therefore it is not surprising that both Grundfos and
Lorentz use centrifugal pumps for applications where the lift is less than 20 to 30m but switch
to positive displacement pumps for deeper wells.
Diaphragm pump: The diaphragm pump is used for pumping small volumes of water from
100/120m depth. The pump needs regular maintenance (diaphragm replacements, cleaning).
If the diaphragm breaks the motor chamber gets wet. The pump can run dry.
Juwa pump: The Juwa pump is manufactured in Namibia. It consists of a jack pump
(reciprocating piston pump) with a DC motor drive. The Juwa is suited for deep well
applications with low water requirements/low yield boreholes. The Juwa pump is based on a
rugged design and can operate in a hybrid pumping system with the addition of a diesel
engine. The Juwa pump is not able to compete on a pricing level with the submersible
options that are on the market (e.g. the Lorentz pump) and is therefore only manufactured for
special applications.