Design procedure for a solar water pumping system

ABSTRACT

In many arid countries rainfall is decreasing, making surface water scarce. This has increased the demand for groundwater, but the water table is also decreasing. Due to this, manual pumping has become more difficult. Diesel, petroleum, kerosene and windmills have traditionally been used to pump water from deeper levels, but solar photovoltaic pumps are becoming more common.

For several years many different types of solar powered water pumping systems have been field tested. In this paper, several steps are given to select a solar-PV water pumping system. The steps for selection of stand-alone water pumping system were: determining the type of PV module, type of controller unit, selecting pump type (diaphragm, piston, helical, or centrifugal), and analyzing the daily water demand requirement. Also to demonstrate how to determine PV array size, motor/pump rated power, and type of pump.

INTRODUCTION

A solar-electric powered water pumping system uses a photovoltaic (PV) array that powers an electrical motor which operates a pump. The water is pumped into an elevated storage tank. This converts the energy from the PV array into potential energy, eliminating the need for battery storage of the generated electricity.

Many solar-electric pumping systems are powered by photovoltaic arrays; however, solar pumps are best suited for small villages of 100 to 1,000 people and moderate agricultural uses. PV is preferable where there is ample solar resource, moderate demand and no access to the electric grid. Stand alone photovoltaic systems (as opposed to grid-connected systems) often rely on a set of back-up batteries for night time and outages. Solar energy is ideal for water pumping as the water requirement tends to peak during hot weather periods when solar radiation intensity is high, resulting in maximum array output. Similarly, the water requirement decreases during cooler weather when the sunlight is less intense.

In the rural areas of East Africa’s Arid and Semi-Arid Lands women and children may spend up to eight hours day collecting water. This reduces their time spent in school or earning an income (Ray 2007). Additionally, this results in adverse health effects due lack of access and transporting water daily (Ray 2007), and puts girls and women at risk for harassment (Short and Thompson 2003). In response to this, several non-governmental organizations (NGOs) and religious groups have installed boreholes. While these efforts have greatly increased the quality of life for some rural villagers, many of these deeper boreholes must be pumped using diesel or petroleum, which can be cost prohibitive.

Previous research conducted found that solar power in rural areas of the ASALs would be a viable option for water supply.

This paper will focus on the list of items above to help the reader in the selection of the best stand-alone water pumping system. Fig 1 shows a typical solar-PV water pumping system containing a PV array; disconnect switches, controller, and submersible motor with pump, and storage tank.

METHODOLOGY

To determine whether a solar water pumping system is the best one to use, the first step is to evaluate the solar energy resource at the location. Fig 1 shows the solar resource of Kenya.

Type of PV Modules

Currently there are two types of PV modules that are used for solar-PV water pumping: multi-crystalline and thin film (thin film modules used so far are amorphous-silicon and cadmium-telluride). The advantages of using multicrystalline modules for water pumping are:

1. Currently 85% of PV modules manufactured in world are multi-crystalline, so less worry on being able to find replacement modules or adding additional modules to array.
2. Module efficiency is higher than thin film (12 to14% versus 3 to 9%), so fewer modules are required for a specific power (takes up less space).
3. Lifetime over 30 years has been demonstrated and warranties up to 20 years are obtainable (thin film modules have only been around since 1988, and major problems with performance degradation with time for early models of thin film modules).

Controller

Controllers for PV water pumping systems can range from not using any controller to sophisticated smart controllers. For diaphragm pumps, the simple controllers can perform many tasks such as:

1. Limiting power to diaphragm pump motor in order to keep it from being damaged
2. Adjusting voltage and current to improve pumping performance at lower solar radiation levels
3. Providing manual disconnect switch between PV modules and pump motor
4. having a float switch to allow automatic disconnect of PV modules to pump motor when storage tank full.

Selection of Pump Type

There are four types of pumps which have been powered by solar-PV: diaphragm, piston, helical, and centrifugal. The first three pumps in the list above are referred to as positive displacement. Positive displacement pumps have the characteristic of being able to pump well at deeper pumping depths, but the flow rate is restricted. The characteristic of the centrifugal pump is opposite; it has higher flow rates, but not as well at pumping from deeper pumping depths unless the power rating is higher.

Solar sizing

The size of solar system depends on the amount of power that is required (in watts) the amount of time it operates (in hours) and the amount of energy available from the sun in a particular area. The user has control of the first two parameters, while the third depend on the location

In Kenya, expected increase in population is 3.0% per year, that is, r = 3.0% per year.

To determine the water demand, the average daily water requirement called per capita water consumption and the design population was used.

Hydraulic energy and the subsequent solar array power required were calculated using the following formulas. Using a water density of 1,000 kg/m³ with gravity at 9.8 m/s² and a conversion factor of one Joule to 2.78 x 10^-7kWh.

For Battery Sizing we consider the size of the battery required for certain number of days as backup capacity. Our load is the pump rating (power pumping hrs / day). To calculate the battery size required to provide for certain number of days as back-up in the event there was no sun.

 RESULTS AND DISCUSSION.

Data for Kendu Bay is not available from PV-Watts, so nearby towns or those with similar climates where used for the analysis. Kisii, Kakamega, and Kisumu were used to collect data. The locations of these towns are shown in Figure 2 Additionally; various angles representing typical roof pitches were compared to that of the latitude angle (the optimum angle) to determine if using a typical roof slope would make any difference in the effectiveness of the solar water pumping system.

Kisii town is 47 km south of Kendu Bay. The coordinates for Kisii are 0.67°S and 34.78°E with an elevation of 1,493 m. Based on the Kisii location, the ideal array tilt would be 0.67 degrees and face north at zero degrees.

Kakamega is about 100 km across Lake Victoria and northeast of Kendu Bay. Its coordinates are 0.28°N and 34.78°E with an elevation of 1,530 m. Based on this location, the ideal array tilt would be 0.28 degrees and face south at zero degrees.

Kisumu is approximately 50 km across Lake Victoria and northeast of Kendu Bay. Its coordinates are 0.10°S and 34.75°E with an elevation of 1,146 m. Based on this location, the ideal array tilt would be 0.10 degrees and face north at zero degrees.

A PV-Watts analysis was conducted to determine approximate solar insulation. Data was collected was for fixed tilt panels, followed by single and double axis tracking systems. Of the towns analyzed, Kisumu is the closest to Kendu Bay and is very similar in climate. Therefore, Table 4 shows solar radiation, AC energy and energy value data for the Kisumu region for an array at latitude as compared to one at various roof pitches.

Table 1.Data for fixed panels at various roof-pitches in Kisumu

The data points ranged from 4.79 to 6.54 kWh/m²/day, while peaking in February and September.

Throughout a typical year, the available solar radiation captured using fixed access panels ranges from 4.79 -6.54 kWh/m²/day for the selected towns. If single-axis tracking is to be used, then the solar radiation ranges from 5.89- 8.29 kWh/m²/day and if double-axis tracking is used then, the solar radiation ranges from 6.31 to 8.68 kWh/m²/day. The difference between the solar radiations captured using fixed or single-axis tracking for the town of Kisumu (the closest to Kendu Bay) ranges between 0.86 and 1.57 kWh/m²/day, and the difference between single-and double-axis tracking ranges between 0.09 to 0.57 kWh/m²/day.

The population data was obtained from population census results. The population for Kendu-Bay trading centre was 1,400 people. Kenya’s population growth rate is 3% per annum.

Therefore:

Current population) =1,400 persons.

Population growth rate (r) =3%

Design period = 20 years.

Therefore, design population, = 2,529 persons

Substituting a daily water requirement of 30 liters of per person per day (WHO guidelines aim for a per capita provision of 30 to 50 liters per day for domestic use only) in equation 3.2 above, we have

Q= 40 x 2,529 =101,160 liters per day.

= 101.160 /day.

Using the sunshine data from the area, the minimum hours of the sunshine are 5 hours, which is used in the design,  we have  20.23 /day/hour.

 RECOMMENDATION AND CONCLUSION.

In order to distribute water fairly to the rural community, it is recommended to first pump it to a storage facility and then distribute it by gravity. This way, enough pressures can be built up at the storage tank to facilitate water distribution by gravity. In addition, water will continuously flow in the tank, which helps to reduce growth of bacteria.

From the above results and analysis we have found out that a solar system can be put up in most areas of Kenya with the average solar irradiation in the country is adequate due to its geographical position.

Through this system, we can supply water to the larger part of the 65% rural Kenya lacking access to piped water. Using the engineering design principles of setting up a water pumping system with all factors involved under consideration, we can use these natural energy resources to supply water as well as, depending on output and demand.

The solar system is also eco friendly and seeks to preserve the environment by reducing pollution levels by a considerable amount when compared to diesel, for example, as an alternative.

The maintenance procedures are also not complex though proper education is required in order not to mismanage or inappropriately use of the system e.g. overcharging of the battery.

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