Category Photovoltaics is the process of converting

MOTORS

11.4.1 Introduction

Small motors (< 2 kW) of high efficiency that are well suited to photovoltaic water pumping are still relatively rare. Fig. 11.12 shows the typical fall off in performance as motor size decreases, which compounds the problem of reduced pump efficiency at smaller sizes (Bucher, 1988). Fig. 11.12 also clearly shows the superior efficiency of DC versus AC motors. Owing to the high cost of solar panels, it becomes justifiable in many applications to use more expensive DC motors to gain an efficiency advantage. However, the trend in photovoltaics is for steadily falling prices, with the potential for a further cost reduction over the next decades (Poponi, 2003)...

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Displacement or volumetric pumps

Displacement or volumetric pumps are the other class of pumps often used for water pumping applications, particularly for lower pump rates from deep wells or bores. Included within this class are piston pumps, diaphragm pumps, rotary-screw type pumps and progressive cavity pumps, such as the popular helical rotor (Fig. 11.7) (Revard, 1995).

Figure 11.7. Helical rotor type of displacement pump (after Revard, 1995).

Fig. 11.8 gives the performance curves of a typical positive displacement pump. The pumping rate with these pumps is directly related to the speed of operation, with a fairly constant torque required. The resulting flat torque-speed characteristic makes it almost impossible to drive these pumps directly from a photovoltaic source (Halcrow & Partners, 1981)...

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Centrifugal pumps

Centrifugal pumps have a rotating impeller that throws the water radially against a casing, as indicated in Fig. 11.5. The kinetic energy added to the water by the impeller is then converted to potential energy in the form of pressure or ‘head’ in the diffuser or volute section. As the water exits the pump through the diffuser, the cross­section increases, causing the velocity (kinetic energy) to reduce and, by conservation of energy, the potential energy to increase (United Nations Economic & Social Commission for Asia & the Pacific, 1991; Dufour & Nelson, 1993; Sahdev, 2004). They are normally used in low head/low pressure, high volume applications, particularly if direct connection to the solar panels is required...

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WATER PUMPS

There is an abundance of names and classifications for pumps but they may all be separated into two categories (Thomas, 1987; Krutzch & Cooper, 2001):

1. Dynamic—in which the water velocity is continuously incremented, then reduced at the output, leading to a pressure increase.

2. Displacement—including reciprocal and rotary types, in which energy is added periodically by forcing volume changes of an enclosure.

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SYSTEM CONFIGURATIONS

There is a range of possible components and configurations for photovoltaic water pumping systems, as shown in Fig. 11.2. Selection of the most suitable components and configurations for each specific application and site is critical to the economic viability and the long-term performance of the system (Sharma et al., 1995).

In the simplest photovoltaic water pumping systems, the solar panels are directly connected to a DC motor that drives the water pump. For such simplified systems, DC motors and centrifugal pumps are virtually mandatory, because of their ability to be matched to the output of the solar panels.

Volumetric pumps, often referred to as (positive) displacement pumps, have completely different torque-speed characteristics and are not well suited to being directly coupled to s...

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PHOTOVOLTAIC WATER PUMPING SYSTEM COMPONENTS

11.1 INTRODUCTION

Water pumping is essential world-wide for numerous purposes, including irrigation, stock watering, village water supplies and domestic use. An estimated 1.2 billion people do not have access to clean water (von Aichberger, 2003). However, the pumping of groundwater can be done sustainably only to the extent that the extraction rate equals the aquifer’s replenishment rate. Otherwise, a lowered water table leads to dry wells, and ecological and social damage (Pearce, 2004). Harvesting rainwater can be a better option in many cases. A wide variety of power sources are used for pumping, depending on local conditions. The advantages and disadvantages of many of these alternatives are listed in Appendix F.

Water pumping applications can vary widely, both in their requirements...

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Australia

Australia has had a number of renewable energy support programs since the mid 1980s, often state-based and usually specific to off-grid applications. A federal government scheme, the ‘Remote Renewable Power Generation Program’, is expected to continue until 2009. It is administered by the states under various subprograms and aims to replace diesel fuel used in off-grid applications via grants covering up to 50% of renewable energy component costs. Although not PV specific, almost all of the smaller installations incorporate PV and over 2 MWp of PV had been installed by 2003 (Watt, 2004).

A PV-specific program, the ‘PV Rooftop Program’, has operated since 2000 and is available for both grid and off-grid rooftop systems. It aims to develop BIPV technologies and installation techniques...

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China

China has a vast unmet demand for electricity (Hirshman, 2003), particularly for village mini-grid systems in the western provinces (Cabraal, 2004; Li, 2004). Some 800 village systems were installed in China in 2002-03, using 19 MWp of PV (Honghua et al., 2004). Systems ranged in size up to 150 kWp. In addition, plans are under way for the installation of 10 MWp of PV in smaller solar home systems by 2005, while PV is also widely used for mobile phone networks, microwave repeater stations, optical fibre links and train signalling (Ibid). The China Brightness Project aims to install over 100 MWp of PV in off-grid applications by 2010 (Yi, 2004).

For grid-connected PV in China, the ‘Renewable Energy Development & Utilisation Promotion Law’, expected to be implemented in 2005, is likely to ma...

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