different types of photovoltaic systems

2.2.1

Direct (Direct Coupled) DC System. The simplest photovoltaic system is made up of an array connected directly to a load. If the array includes more than one module, bypass diodes are used (Figure 2-4).

Applications requiring the most power during the sunniest part of the day are ideal for this type of system. Pumping water for irrigation or to a storage tank, running a fan for ventilation, or operating a pump to collect solar heat are examples of appropriate applications. Lighting and other loads which are rarely used during the daylight hours would probably never be supplied with power by a system of this type.

The load must run on DC (Direct Current) electricity. This normally means a DC motor is used to run a pump, fan, or other device. As the sunlight gets more intense, the motor runs faster. Thus, the more sunlight, the more water or air that is moved.

2.2.2

Power Point Tracking DC System. The performance of a direct DC system can be increased by adding a power point tracker (Figure 2-5).

The power point tracker constantly monitors the system performance and makes electrical adjustments to keep the system operating as close as possible to its maximum output. More information on power point tracking can be found in Section 2.5.5.

2.2.3

Self-Regulated DC System. If battery storage is added to the system, some means must be used to prevent overcharging the batteries. The simplest way to do this is to use self-regulating modules. These modules are designed to deliver a voltage that is too low to overcharge the battery. (Figure 2-6). Careful matching of component sizes and loads is critical.

Again, the loads are DC only. If adequate battery storage is provided, and the loads are used consistently, this can be a reliable system. Because electricity is stored, lighting and other devices can be used after dark or during cloudy weather.

2.2.4

Regulated DC System. Most systems do not have self-regulated modules. Furthermore, many systems require some way to prevent damaging the batteries from charge levels which are too high or too low.

A charge controller, sometimes called a charge regulator, is used to keep the batteries from being overcharged. An optional feature of many controllers is a load cutoff. This turns off some or all of the loads whenever the batteries' state of charge gets too low (Figure 2-7).

Although the additional component adds to the complexity of the system, draws additional power, and can reduce overall reliability, the charge controller extends the battery life.

This is probably the most common photovoltaic system. More information about charge controllers is in Section 2.5.2.

2.2.5

Direct AC System. In some cases, such as deep well water pumping, AC loads must be provided with power, but only during the day. If the DC output of a photovoltaic array is converted to AC with an inverter, it can supply the AC load directly (Figure 2-8)

This system is appropriate for many of the same situations as the direct DC system. The inverter must be protected from temperature extremes and inclement weather. The cost of an inverter is significant, and it reduces the overall system efficiency. However, if the application requires a device which cannot operate on or be converted to DC, this is the simplest way to do the job.

2.2.6

AC System with Storage. If the AC load must run during periods when the photovoltaic array cannot supply power, battery storage and a charge controller must be included (Figure 2-9).

The combined inefficiencies of the batteries and the inverter reduce overall system performance. Nevertheless, AC applications exist which will require the complexity and expense of this type of photovoltaic system.

2.2.7

Mixed AC/DC System. A good compromise is to supply every possible need with a DC device, and use AC only for those loads for which there is no alternative (Figure 2-10).

This compromise allows the most effective use of the energy available from the photovoltaic array, while satisfying the load requirements.