Category STAND-ALONE SOLAR ELECTRIC SYSTEMS

Step 5: size the solar array

The size of the array selected for a pumping system largely depends on the type of motor and pump. Most solar-pump packages have standard methodologies that they use to size solar arrays for their pumps that are based on the hydraulic lift required and the design insolation value. A solar-pump supplier will be able to help you size the array for the pumping system you need (see also Chapter 12).

Consider other factors

• Borehole yield. Do not exceed the daily yield of the bore hole.

• Water condition and treatment needs. Have water quality checked by an expert.

• Site security.

Solar PV Pumping Case Study

This case study features a desert settlement with 300 people, 500 cattle and 2000 goats. The water source is a shallow well.

Step 1: consider all pumping alternatives and estimate t...

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Step 3: calculate the vertical pumping distance (head)

The depth of the well/borehole plus the height to which it will be pumped is called the ‘head’. The pump will have to push the water this full distance. A shallow well might have a head of 8-20m (25-65 feet), while a borehole might have a head of 60m (200 feet) or more. Solar electricity is expensive for use in deep borehole applications and is not normally considered where water is more than 150m (500 feet) below the surface.

Step 4: calculate the hydraulic lift and decide on type of pump

The amount of water needed (in cubic metres, m3) multiplied by the total head gives the hydraulic lift (m4). This number is used by engineers to select and size the pump and to estimate the amount of power needed to pump the water (i. e. the size of the array).

Figure 11...

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Case study 3: a hybrid clinic system

Off-grid solar PV systems often power clinics in remote parts of developing countries. The Kamabuye clinic, in Rwanda, serves several thousand inhabitants of an off-grid community. It is an example of a site where a wide variety of loads required a hybrid PV design.

A hybrid system is used because there are several loads that cannot be powered by a PV/inverter system (most notably the sterilizer) and because there are several months where cloudy weather limits PV array output.

Table 11.10 below provides information on appliances used in the centre.

Energy Requirement

Option Selected

AC Appliances

Lighting: Admin. block, maternity, ward, security

LED and compact fluorescent.

Cell-phone recharging

Staff phones recharged every day.

Centrifuge

Used twice a week.

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Selecting back-up generators

In many hybrid systems the diesel generator will already be on-site, but this is not always the case. An existing generator needs to be checked to ensure that it is suitable for the system, and if a system is being designed from scratch then the right back-up generator must be selected carefully and with expert advice. The generator must be sized as part of the overall system design.

Table 11.8 lists some of the key features to be considered in hybrid system generators.

Inverter-charger specification

Notes

AC output power (W) and AC output voltage

Enough power to power all AC appliances on circuit? Correct AC voltage? 120 or 240V AC?

AC input power (W) and voltage

Usually the same as the AC input power (W) and voltage but needs to be checked...

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Selecting inverter-chargers

Inverter-chargers are complex devices and before selecting one the designer should consult the manual to make sure its specifications are those required. It has two main functions: to provide AC power and to charge the battery. So, the AC output needs to be the correct wattage and AC voltage to power the loads, and its battery-charging function needs to provide sufficient current to charge the batteries (which can be adjustable) and be of the correct DC voltage. Inverter-chargers can also be powered by grid-power in back-up systems where the grid is unreliable.

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Generators as power sources for large loads

Sometimes provisions for occasional large loads are included in systems that cannot be powered by the inverter or PV system. For example, in off-grid hospitals, this might include X-ray machines or electric sterilizers. Other large loads not ordinarily powered by PV or inverters include workshop equipment such as welders, air-conditioners, borehole pumps or compressors. Connecting such loads directly to the generator, and running them from the generator when needed, can greatly increase the overall flexibility of the system. As an example, the clinic in Case Study 3 below runs the generator three times a week to power the sterilizer and laboratory HIV-testing equipment.

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Generators for equalizing batteries

As mentioned in Chapter 4, equalizing is important for large flooded-cell battery banks, especially during periods of low sunshine when PV arrays may not produce enough power to equalize battery banks (even if the charge controller has a setting for this purpose). Generators are useful tools to produce the minimum current required for this task (usually C10 to C20, see Chapter 4).

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Generators as back-up power sources

For large systems, powering 100 per cent of the load with solar is often expensive because the array and battery must be oversized to meet demand during the cloudiest months or during periods when demand for energy is heavy. To get around this, many designers plan so that solar meets 75 to 90 per cent of the total energy requirement. During cloudy periods (or when days are
short), a generator is used to meet energy shortfalls and to top up the battery when it falls to a low state of charge. This can reduce costs of PV arrays and batteries considerably and extend battery life...

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