Category Photovoltaics is the process of converting

EXAMPLE OF A DIRECTLY-COUPLED SYSTEM DESIGN

Specification

Design a directly-coupled pumping system (no batteries or power conditioning circuitry) for irrigation purposes near Melbourne (latitude 37.8°S). The only months of interest are December and January, during which period 2.5 million litres are required to be pumped. The relevant insolation data is given in Table H.2. The total pumping head is 8.8 m and this remains approximately constant during pumping and from season to season. There are no problems with the water source replenishment rate. Other months of the year are considered to be non-critical.

Table H.2. Average daily insolation data for a horizontal surface for the design months.

month______________________ S_____________ D____________ R

December 676 214 890

January___________________ 629___________ 210__________ 839

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PV MODULE CHARACTERISTICS

A typical module will have 36 cells connected in series, each cell typically having the parameters:

Voc = 600 mV (25°C) FF = 75%

Vmp = 475 mV (25°C) Vmp = 430 mV (45°C) Imp/Isc = 0.95

Each cell can be reasonably accurately represented by the equation

where V is the terminal voltage, I is the current, IL is the light-generated current, n is the ideality factor (taken to be 1.3), Rs is the series resistance, q is the charge on an electron (1.6 x 10-9 C), к is Boltzmann’s constant (1.38 x 10-3 J/K), T is absolute temperature (typically 318 K for field operation), and I0 is the dark saturation current, given by

= 2.17 x10~7 x IL (at 45°C)

where Voc is the open circuit voltage, which is typically 600 mV at 25°C for commercial solar cells, but falls to about 555 mV at 45°C.

For commerci...

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SYSTEM DESIGN FOR PV-POWERED WATER PUMPING

G. 1 INTRODUCTION

Aspects of insolation data manipulation are discussed in preparation for designing a stand-alone, directly-coupled PV powered water pumping system. This is followed by an analysis and discussion of typical PV module characteristics and output and the relevance for water pumping system design. An example system design is provided, including discussion of each part of the procedure.

H. 2 INSOLATION DATA MANIPULATION

Determine the maximum (solar noon) daily light intensity (I) incident on the array (at inclination angle P for each of the design months, for both sunny days (Is) and typical cloudy days (Ic). Refer to Chapter 1 for further details about insolation data and note that various computer programs are available to help with such calculations (Silvestre, 2003)...

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3 SANDIA NATIONAL LABORATORY APPROACH

The following example is based on the approach developed by Sandia National Laboratory, Albuquerque, USA (Chapman, 1987) and automatically incorporates over 23 years of insolation data.

An interesting outcome of the study involved in the formation of this model and approach is that accuracy of system design is not lost by basing the design only on data for the month with the lowest insolation levels over the year. This of course greatly simplifies the design approach. In addition, through the use of calculations similar to those in the approach described above, but over a wide range of possible design values and in conjunction with appropriately-treated average global insolation data, curves have been generated that facilitate:

1...

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STAND-ALONE PHOTOVOLTAIC SYSTEM DESIGN

F. 1 INTRODUCTION

The following material outlines two approaches used when designing photovoltaic stand-alone systems. Australian Standard AS4509.2, discussed in Chapter 7, is more recent than either and is normally the method to be used by accredited installers in Australia. The first method detailed here was used extensively by Telecom in Australia during its early days of photovoltaics application and represents a very conservative approach in which array size is optimised as a function of battery capacity. The second was developed by Sandia National Laboratories in the USA and is considerably more sophisticated, automatically incorporating many years of accumulated insolation data. Examples of both approaches are provided.

G. 2 STAND-ALONE SYSTEM DESIGN PROCEDURE

The following example ...

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DESIGN OF A DIRECTLY-COUPLED SYSTEM

A directly-coupled system is one where a low starting torque pump (such as a centrifugal pump) can be driven by a DC motor that receives its power directly from the solar panels. No batteries, inverters, or power conditioning circuitry are used, other than perhaps safety cut-out relays activated by level, flow or pressure sensing transducers. When the sun shines sufficiently brightly, the system operates and water is pumped either for storage or direct use.

An approach for designing directly-coupled PV-powered water pumping systems is provided in Appendix H. Important considerations are as follows:

1. The volume of water to be pumped and over what period...

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PV WATER PUMPING SYSTEM DESIGN

12.1 INTRODUCTION

Considerable emphasis in this book is placed on the design of photovoltaic water pumping systems, firstly because water pumping is a major application for photovoltaics and secondly because the design of each system is considerably more complicated than most applications, owing to the large range of water source types, consumer requirements and system configurations. Where batteries are required for storage, design procedures are relatively straightforward and follow the design principles outlined in earlier chapters for stand-alone systems. However, direct interfacing between photovoltaic panels and the water pump motor introduces significant mismatch problems as the light intensity varies...

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Array mounting

All support structures should be anodised aluminium, galvanised steel or stainless steel and need to be designed to withstand the maximum possible wind loading for the particular location (Ibid ). Lock washers or equivalent should be used on all bolts to remove risk of them coming loose during the subsequent 20 years. The structures should be located as close as possible to the water source to minimise wire lengths, and where necessary fencing may be used to protect from animals, theft, vandals etc. (Ibid.).

Tracking support structures can be useful to enable the solar panels to point more directly at the sun throughout most of the day. Motorised or passive tracking mechanisms in Madrid, for example, have been calculated to boost annual water flow by 40% or more...

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