DCOM Volume I Appendix F
APPENDIX F: SOURCE OF PUMPING POWER
The different types of power sources commonly used for water supply pumps include:
Diesel/gasoline generators and engines Natural gas/biogas generators
Solar Energy Wind Energy
The choice of pumping energy depends on several factors namely:
- Availability of and proximity to grid power,
- Capital costs of the alternatives,
- Operational costs of the alternatives.
In Tanzania when deciding on the pumping energy, grid power is considered as the basic source in the sense that when available it becomes the 1st choice. It is only when the grid source is too far from the pumping point that the other sources are considered. DRAFT
The three alternatives to grid power namely diesel/petrol/natural gas/biogas generators or engines have both positives and negatives. In the following subchapters each alternative shall be discussed.
National grid connection allows the water pumping station to remain connected to the larger, wide-area electrical grid. The electrical grid is responsible for generating, distributing and balancing electricity across a wide area. Since most of our national electric power supply is mainly from hydropower plants (64% of the total generated electricity in 2016) (Aly, Moner-Girona et al. 2019),and natural gas the price is stable and the costs of generation cheaper than the other sources except solar energy. Grid power is the most stable and trustable source of pumping power (Although other renewable sources can also be made stable and trustable). It is also the cheapest in terms of capital cost since electric motors are cheaper than other forms of prime movers such as generators which also require engines besides the electric motor of the pumps.
PV systems require higher investment costs due to the large area of PV required (It should however be noted that the costs of PV is getting lower). Similarly the running costs for electric motors is cheaper than that of engines except those utilizing own generated fuel such and gas or biogas fuel. However, in terms of running costs solar PV is the cheapest. Most of water supply sources requiring pumping are situated in rural areas where grid power may not be available. When deciding the alternatives the designers have to compare the costs of bringing grid power from the nearest point to the site against the capital costs of the other energy sources.
Diesel Generators/Diesel Engines
There are two approaches applied when considering engines for pumping. An engine may directly be coupled with the pump running on diesel or petrol. Most small irrigation pumps are of this type. The other way is a diesel run generator which provides electricity for running the pump. Here a normal electric driven pump is supplied with power from the generator instead of grid. Diesel run generators of different sizes (20 kW to over 3,000 kW) are available in the market to suit power requirements for various applications.
The designers of water supply system would need to estimate the power requirement for pumping and distribution of water in order to be able to order the right size of a generator. When properly sized generator sets are easy to install. The running costs are high due to the requirement of fuel (Diesel/petrol) and the maintenance of the engine.
There are also generators which run on natural gas or biogas in case this type of fuel is readily available. Natural gas is locally available in some areas of Tanzania and already there are plans to introduce natural gas run automobiles. Biogas on the other hand can be produced locally in a biodigester from industrial/bio wastes containing high loading of organic matter. These two gases provide cheaper source of fuel for generating power. Please note that when procuring the gas engines one has to be specific whether the generator shall use biogas or natural gas. Natural gas and biogas are slightly different in terms of composition.
Wind Driven Pump
The amount of water a wind-powered water pumping system can deliver depends on the speed and duration of the wind, the size and efficiency of the rotor, the efficiency of the pump being used, and how far the water has to be lifted. The power delivered by a windmill can be determined from the following equation:
Where: P is power in watts, D is the rotor diameter in metres, V is the wind speed in kilometres per hour, and e is the efficiency of the wind turbine. As can be seen from this expression, relatively large increases in power result from comparatively small increases in the size of the rotor and the available wind speed; doubling the size of the rotor will result in a four-fold increase in power, while doubling the wind speed will result in an eight-fold increase in power
Figure F.3: Windmill driven water pumping22
However, the efficiency of wind turbines decreases significantly in both low and high winds, so the result is that most commercially-available windmills operate best in a range of wind-speeds between about 15 km/hr and 50 km/hr.
For adequate usage of wind power, the wind speed should be higher than 2.5 to 3 m/s for at least 60% of the time. The wind mill should be placed above surrounding obstructions such as trees or buildings within 125 m; preferably, the windmill should be set out on a tower of 4.5 to 6 m high. Consider protection by provision of an automatic lubrication system or covering of the windmill driving mechanism. In order to directly pump water by a windmill, there is need to match the characteristics of the local wind regime, the windmill and the pump. This therefore means that, the manufacturers should always be consulted regarding the selection of the equipment. The discharge, Q that can be pumped by a windmill can be estimated by the formula below:
Q = (2.8D2V3e/H)
Where, Q = Discharge in litres per minute (l/min), D = wind rotor diameter in meters, V = wind velocity in meters persecond, H = pumping head inmeter, e = wind to water mechanical efficiency, value 0-1.
Windmills with rotor diameters between approximately 2 m to 6 m are usually available. The efficiency, e will rarely exceed 30%.About 5 years of data would give the designer reasonably representative averages following the fact that the monthly wind speed varies greatly between 10 % to 20 % from one year to the next. For successful operation of a wind pump, at least wind speed of 2 to 3 m/s is required. Effort should be made to acquire a wind map of the area to guide with the wind speed throughout the year. This information is available from the Meteorological Department however; it may require interpretation and organization to ensure that it is applicable to the area in question.
Solar-powered water pumps or photovoltaic pumps (PVP) are an effective alternative to conventional gas or electric pumps. Modern pumps are powered by solar energy effectively and used in different parts of the world. PVP systems offer numerous advantages over water supply system utilizing conventional power (adapted from Water Supply Manual, Uganda 2013):
PVP systems may be the only practical water supply solution in many regions where the logistics make it too expensive or even impossible to supply diesel generators with the required fuel; PVP systems are ideal for meeting water requirements for villages between 500 and 2,000 inhabitants and small scale irrigation purposes (up to 3 hectares); PVP systems run automatically, require little maintenance and few repairs; In areas where PVPs have entered into competition with diesel-driven pumps, their comparatively high initial cost is offset by the achieved savings on fuel and reduced maintenance expenditures; The use of solar energy eliminates emissions and fuels pills there by making photo voltaic pumps an environmentally sound and resource-conserving technology; There is no need for complicated wiring for the electricity and outside fuel is not needed; They can be designed not to require storage batteries, which are expensive and need a lot of maintenance; and The maintenance of a PVP system is restricted to regular cleaning of the solar modules. The three components of a solar-powered water pumping system are: Photovoltaic (PV) array (solar cells); Electric motor; and Water pump.
The PV array generates direct current (DC) electricity when exposed to sunlight. This electricity is fed into the electric motor and which in turn drives the water pump. Optional components of the system include the following:
Controllers (for regulating current and/or voltage); Inverters (for converting DC power from the PV array to alternating current (AC) power for certain types of motors); Electronic maximum power tracking devices, MPPT(to obtain a more efficient operation of the array and the motor); and Batteries (used for both voltage regulation and energy storage and also for generating the required starting currents needed to overcome high electric motor starting torques).
The selection of PV arrays and associated equipment should always be made in consultation with the relevant manufacturers. A solar generator provides electricity for driving a submersible electric pump, which in turn pumps water into an elevated water tank that bridges night-time periods and cloudy days. On a clear, sunny day, a medium- size PVP system with an installed power of 2 kW will pump approximately 35 m3 of water per day to a head of 30 meters. That amount of water is sufficient for communities with populations up to 1,400. Today‗s generation of PVP systems is highly reliable. For the most costly part, the PV generator, the manufacturers give a 20 year guarantee on the power output. A crucial prerequisite for the reliability and economic efficiency is that the system be sized appropriate to the local situation.
The power output of a PV array is directly proportional to the solar irradiation falling on to the array. The power flow through a typical solar-powered water pumping system is in Figure 3.23. Accordingly, the best efficiencies that are expected are as follows:
PV array -11% of the total solar power received by the PV array; and Motor-pump unit - 4.5% of the total power received by the PV array
The formula below can be used to estimate the daily solar power requirement of a water pumping system, in kWh.