Terrace photovoltaic power system

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Illustration 1: 55 W PV panels on wooden frames.

Each of the 55 watt panels is mounted on a simple wooden frame, which are aimed such that the sun is perpendicular to the panel face at noon each day. The inclination angle is adjusted every now and then to account for the elevation angle of the sun in the sky as the seasons change.

The wiring to the panels consists of double-jacketed two-conductor 1.5mm2 stranded copper wire (actually the cable from an outdoor extension cord with the connector ends removed; it was actually cheaper than buying cut cable).

The panels are wired in a series-parallel configuration. As such, they are compatible with a 24V DC power system.

Illustration 2: 24 V Battery charge controller.

At this point, a simple 24V lead-acid battery charge controller is used to keep the batteries in a good state of charge. This controller charges the battery to 28.8V before backing off to a float charge of 26.4V. There is no provision for the occasional equalizing charge that should be done once every month or two. The equalizing charge is carried out using a bench power supply set to 29.5V for a couple of hours. The cells need to “boil” (generate gas) briefly to avoid acid stratification in the cells. Current should be limited to around 3-4A for my 100Ah cells to prevent damage. Equalizing charges should not persist for too long to prevent damage to the battery.

Illustration 3: Full energy storage and conversion system (inverter).

Here we see the series connected 12V flooded-cell lead acid batteries that make up the energy storage part of the system. Also visible is the 1500W DC-AC inverter. This permits direct connection of AC appliances to the system. I have successfully powered the following appliances with this inverter:

  1. Compact fluorescent lamps

  2. Large screen TV

  3. Stereo system

  4. Portable PCs

  5. Small electric kitchen appliances

  6. Small induction motors (fans, etc)

  7. Refrigerator

  8. Non-speed controlled power tools

Some things to note about the system behavior:

  1. Small induction motors seem to work well, but tend to generate a bit of noise as a result of the modified square-wave output of the inverter.

  2. Start-winding switching when the refrigerator starts up tends to cause some momentary voltage sag in the inverter output. This causes momentary flickering in lamps and causes TVs or computer systems to drop out.

  3. The current surge when connecting portable computer power supplies to the inverter has a similar effect.

  4. Certain speed-controlled power tools (the ones that use phase-angle switching to control speed) will not work properly.

  5. Some extra noise is generated in stereo sound system. This includes some hum in the transformer as well as a slight increase in hum in the speakers.

Some appliances, like speed controlled power tools and sound systems would clearly benefit from the use of a pure-sine-wave inverter. Moreover, as AC loads switch in and out, some way to avoid power drop-outs is needed. This is most likely remediated using current limiting on the computer power supplies to eliminate the surge on connection. For motor-starting, this is more difficult.

Illustration 4: Battery enclosure.

Everything is enclosed in a weather-tight box (but it is not air-tight, to allow ventilation for the batteries).

The system seems to recover full charge after 25-30% discharge on a bright summer day here in Belgium. This conclusion is based on the state of the charge controller and is estimated from the power consumption of the test loads (TV, computer, etc). This has not been tested rigorously yet, because a monitoring system still needs to be set up.

Last modified: 20090802 by Bill