Solar PV water pumping study - FINAL REPORT ... - UNDP, Namibia

Feasibility Assessment for the Replacement of Diesel Pumps with Solar Pumps
FINAL REPORT: September 2006
The following losses are added to the hydraulic power to calculate the overall shaft power
required:
• Pump element efficiency: Variable as a function of the head
• Friction losses in the rising main: Variable as a function of the flowrate and the pipe
diameter
• Rising main losses due to a possibly non-linearity of taper threaded pipes (the quality
of taper threaded pipe has become poorer over recent years and there is a good
chance that the rising main is not perfectly straight – this “wobble” has efficiency, as
well as maintenance implications). This factor can be considerable but is virtually
impossible to predict or assess scientifically. It is taken as a fixed 5% value which is
considered conservative.
• Windage losses: Fixed at 10%. These are friction losses at the entry and exit of the
belts into the pulleys.
• Derating of the diesel engine for altitude and temperature: Fixed: 20% for 1,300 m
altitude (4% per 300m above 100m) and 35°C operatin g temperature (2% for every
5°C above 25°C).
The engine load factor is selected at 70%, providing the rated nominal power of the engine.
The specific fuel consumption per hour of the particular size of diesel engine is used.
The DRWS usually size their diesel engine system for a ten hour pumping day. This still
leaves some additional hours in case more water is required and also attempts to use
reasonable water abstraction rates for borehole sustainability (instead of pumping the
required daily water within two hours, should a borehole have that yield).
The costing model allows the pumping intervals and the pumping hours per pumping session
to be defined but limits the abstraction rate to 12m 3 /hour. This allows the modelling of diesel
pumping systems where the pump may be remotely located from the operator and is thus
only operated every second or third day.
3.1.4 Costing
The costing of a pumping system that has a life expectancy of a number of years is
comprised of the capital cost and the future costs, which include operating cost, maintenance
cost and replacement cost. In some life cycle costing calculations a residual value is
assumed which will provide a small return for the project again, for example the solar
modules or the lead of lead acid batteries. However, this is often something that is far in the
future and could be quite arbitrary. In this costing no residual values are considered for a
PVP or DP system.
Throughout the calculations a differentiation is made between a DRWS installation and a
private (commercial farm) installation. The differences are in the capital cost (lower prices for
the private system due to owner involvement), the operating costs (more costly for the
private system due to a diesel system which can be in the field, away from the home – not
applicable to DRWS system) and the maintenance and replacement costs (lower price for
the private installation due to more involvement of the owner).
There are three transport rates for different services involving transport. The transport rates
listed represent the reference case under current fuel prices (N$6.70/litre) and are adjusted
proportionally, based on AA rates with increase or decrease of the fuel price. The rates are
listed in Table 3.2.
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