SPWS concept and revolution
The benefits of solar-powered water systems (SPWSs) cannot be overempha- sized: simple, reliable, durable, modular, and low maintenance. In SPWSs, the solar energy is coupled directly to power an electric pump motor through a solar controller. The electric pump can be either a surface pump, submersible pump, DC pump, or AC pump. The controller can be either a DC control box or an inverter. Therefore a photovoltaic water pumping system is generally like any other pumping system, with the exception that the power source is solar energy.
Previously the capacity of SPWSs was limited. Fifteen years ago, the biggest solar pump on the market was probably a 4 kW DC pump with a daily hydraulic duty of between 1,500 and 2,000 m4 (approximately 10–200 m3/day at an inverse head of 10–200 m). The drastic reduction in solar PV prices in the last decade has triggered technological advancements of robust and reliable solar pumping equipment.
The development of variable-frequency inverters has extended the solar pump performance range tenfold since they work with standard electric motors. Literally any electric pump can be solarized and can also be powered using dual power sources (solar and AC power). These developments have led to a revolution in the use of solar for off-grid water pumping and the emergence of a vibrant private sector with good technical knowledge offering quality solar pumping products in most countries (see GLOSWI country reports from 2016 to 2020). SPWSs are technically non-restricting these days with large solar pumping systems feasible (see more on SPWS sizes in section 7.5).
One of the distinguishing factors of an SPWS is the feature of variable- frequency operation. Traditional water pumping using grid or diesel is typically configured to operate on constant pump speed, that is, the pump is designed to start and operate at a certain fixed minimum speed (usually 50 Hz or 60 Hz).
As seen in Chapter 2, available solar energy fluctuates throughout the day depending on the irradiation from the sun, thus limiting the operation of a fixed-speed pump. Solar pumping technology has therefore been engineered to overcome this hurdle. This means that solar pumping systems are designed to be able to start even at low frequencies and to adjust the operating frequency according to the available energy from the sun. Consequently, the flow delivered by the pump also fluctuates relative to the speed of the pump, allowing water to be delivered throughout the solar day, albeit at low quantities in the morning and evening when the sun’s intensity is low (Figure 3.1).
Different solar pumping configurations are possible based on the power source (solar stand-alone vs hybrid) and water source (submersible vs surface).
Solar stand-alone vs hybrid configuration
A stand-alone SPWS has solar as the only source of power. It consists of a PV array connected to a pump assembly via controller, as shown in Figure 3.2.
A hybrid SPWS will have solar as the primary source of power with an alternative source of power connected, such as AC power from either a diesel generator or grid supply, for pumping when solar energy is insuf- ficient to run the pump (Figure 3.3). It enables pumping where water demand cannot be met using solar and pumping is required beyond the solar day; or in cases where the water source is constrained and cannot provide enough water over a 4–9 hour solar pumping day; or where water demand is variable with the seasons and a supplementary power source can be switched on as needed to meet the increased demand (intermittent usage). More of this is discussed in section 5.3.8.
Whenever possible and where the solution can meet the demand duty, solar stand-alone should be prioritized over hybrid as it is the most cost-effective with the shortest payback period. Such a decision should always begin with availability of full water-source data, such as a test pumping report, as well as reliable water-requirements data.
Submersible vs surface configuration
Submersible configuration has a submersible pump installed completely submerged in the water source (mainly in wells and boreholes). Submersible pumps can also be installed horizontally inside water reservoirs at a minimum depth of 0.5 m, well anchored and fitted with a cooling sleeve (plastic or metallic cylindrical shroud put around the motor) to enable enough cooling to the motor.
Surface configuration has the pump mounted outside and near the water source (e.g. tank, river, dam, lake). They are mounted at ground level with the inlet connected to the water source through a suction pipe and the outlet to the delivery pipe (Figure 3.4).
Both submersible and surface pumps can also be installed floating in a surface water source such as a river, lake, or a dam. This is applicable in situations where it is not possible to install the surface pump close enough to the water source, which would otherwise result in a very high suction head that could cause cavitation problems (see section 3.3.1, ‘Surface pumps vs submersible pumps’). Where water levels fluctuate, to mitigate against flooding the surface pump in the rainy season and to manage the suction lift in the dry season, the pump is installed floating on the water source.
The optimal configuration (solar stand-alone or hybrid) is determined based on multiple criteria:
- Solar resource – locations where peak sun hours are insufficient to meet demand will require a hybrid configuration for prolonged pumping beyond the solar day.
Prevailing weather – some locations have seasons when the weather is overcast, necessitating a hybrid system which will allow intermittent diesel pumping during prolonged periods of cloud cover.
- Water demand – where the water demand exceeds that which solar stand-alone can provide, a hybrid system is necessary.
- Water source – a water source that has a limited flow will result in a small pump that will require prolonged pumping beyond daylight hours to meet demand. A surface water source will typically be equipped with a surface pumping system.
- Economic reasons – solar systems have a low cost of ownership and a short payback period, making it preferred over hybrid systems.
- Demographic factors – contexts where the population is unknown, uncertain, or expected to fluctuate unpredictably should be installed with a hybrid system to cushion against water supply fluctuations and shortages when the population increases.
- Social aspects – some communities may have low acceptance and may resist installation of solar, necessitating installation of a hybrid system until there is wide acceptance of the technology.