Category STAND-ALONE SOLAR ELECTRIC SYSTEMS

. Hybrid system parts and operation

Figure 11.7 shows a schematic diagram of a hybrid system (without fuses, switches, etc.). Key parts of hybrid systems include the following:

• Generator. As explained below, the generator is used to provide additional power to the system.

• Inverter-chargers. In addition to converting low-voltage DC battery power to AC power for appliances, inverter-chargers convert AC power from the generator to DC power used for charging the batteries. It is important to select the right type of inverter-charger.

• Switching and monitoring equipment. With hybrid systems it is important to have additional switching equipment. This should be carefully designed for the needs of your system.

Back-up generators

Generators are used for three primary purposes:

1 As a back-up source for charging and toppin...

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Hybrid systems

Hybrid systems combine solar electric, generators and, occasionally, other renewable energy systems to increase energy availability. In hybrid PV systems, the solar array charges the batteries and supplies primary daily power requirements. A generator is used to charge batteries when there is not enough sunshine (or wind), to run battery equalization and to power specific large loads that the PV system cannot power. Hybrid systems are best used in remote locations where solar or wind resource is variable and where there is occasionally a need for large amounts of power. Most are PV-diesel hybrid systems.

In general, stand-alone PV systems are economically viable without gensets when energy requirements are less than about 5kWh per day and there is good constant sunshine...

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Olonana system loads

As can be seen from Figure 11.5, the Olonana system loads are equally divided between the lights, the refrigerator, the laptop and the band amplifier (the last of which is only used once or twice per month). On a heavy day, the total load demand is 1700-1800Wh (not including losses). Note, however, that the system is primarily used on weekends and holidays, so the average daily requirement is much lower. There are both AC and DC loads (most lights are DC).

Table 11.5 below summarizes energy systems at the cottage. Note that cooking, water-heating, water-pumping and drying do not use the PV system at all.

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Table 11.5 Olonana energy and electricity use

Energy requirement

Option Selected

Appliances Powered by PV system

Refrigeration Super-efficient electric fridge.

Ligh...

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Case study 2: off-grid residence (Olonana 500Wp house)

The author designed the Olonana cottage as a unique off-grid weekend resi­dence that can be used regularly for writing work, family outings and as a studio for practice sessions with musicians. Figures 11.4 and 11.5 provide information on the design process and show some of the system components.

Solar was selected for this system because there is no intention to connect the cottage to the nearest grid-line, located about 1.5km (1 mile) away. The system showcases how solar can work with an elegant design.

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Larger Residential Systems

Planning larger off-grid residential systems requires a focus on the needs of the people living in the house! Residential systems often work better than institutional or community systems because users have selected solar PV themselves and are fully committed to them. Planners should always work closely with system owners to ensure they understand the PV system they will be managing.

Firstly, planners should ask if the system is a full-time or a part-time leisure residence. Obviously, a full-time live-in residence will require a different design approach than one lived in during weekends and holidays only. This is especially true with regard to energy storage and cloudy season needs.

Secondly, off-grid households must review their appliance selections carefully...

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Small Institutional PV Systems

There is ample experience of solar PV being used in small off-grid institutions. For example, 200Wp can provide basic light in four classrooms for night-time study at a secondary or primary school, or for the examination rooms in a rural clinic. 1000Wp can light four to six buildings, as well as a few staff houses. Hundreds of schools and clinics use solar power for night-time lighting and small appliance power.

Institutional off-grid PV systems differ from small solar home systems in several ways. Firstly, they usually demand more energy (sometimes their energy demand is too large for a simple stand-alone PV system and requires hybrid solutions, as in the clinic described in Case Study 1 below)...

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Appliances that should not use Solar PV

As mentioned above, eliminate any appliances that use heat elements from your system. It is rarely viable to run heating appliances from solar electricity. Electric cookers, irons, kettles and water-heaters should never be used with PV systems.

• Cook with LPG gas, wood, charcoal, biogas, kerosene or with a solar cooker. In some large PV systems, microwave cookers (a few smaller DC versions are available) can be used but, because of the cost, this is best avoided.

• Use alternative water-heating systems such as wood, kerosene, LPG or solar thermal (see Chapter 1).

Table 11.2 describes alternatives for powering ‘standard’ appliances that cannot be cost-effectively powered by PV.

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Box 11.2 Solar refrigerators v. ‘normal’ refrigerators

Table 11...

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Choosing efficient appliances

It is far cheaper to purchase efficient appliances than to purchase extra modules to power inefficient appliances. Choosing efficient appliances can cut the cost of your PV system in half. Shop around for efficient units when making appliance purchases. (There are a number of catalogues and websites that offer efficient appliances: see Chapter 12 for more information.)

• Whenever possible use fluorescent or LED lighting (see Chapter 6).

• Use laptop computers instead of desktop PCs. They use about a quarter of the energy.

• Select efficient major household appliances such as refrigerators, washing – machines, fans and pumps which have been specially designed for PV and low-energy household applications.

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