Wednesday, March 05, 2008

PV System Automation Test

It was the best of times, it was the worst of times…

Those words from a Charles Dickens novel came to mind as I reviewed the data from my PV system automation test. The test was conducted during the last week in February, and sunlight on the solar panels was a rare occurrence here in Illinois. While it would have been great to see loads powered by the sun during the day, and batteries fully charged before sunset, that hasn’t happened much lately. The past few weeks have seen rain, snow, sleet, and mostly overcast skies. This was the worst of times for sunlight, resulting in the lowest PV system electricity production of the year.

Ironically, the bad weather makes this one of the best times to test the PV system. It is important to know how the system responds to these, the worst-case conditions. It will be equally important to see how the system responds to best-case conditions, a sunny summer day.

Without proper care, batteries can be ruined or weakened. The Charge Controller prevents over-charging, but does nothing to protect the batteries from over-discharging or chronic under-charging. The automation described here is an attempt to do just that. With the new automation, system loads are applied and removed depending on the battery state of charge (SOC). On a typical day, the sun shines on the panels and battery voltage rises as the batteries are charged. Once the battery voltage reaches a preset “high voltage threshold”, a relay closes. This relay connects the system to an electrical load, such as a refrigerator or freezer. The PV panels power the load, and can charge the batteries at the same time if the load is not too large. When the solar panels are unable to meet the demands of the load, at night for instance, the batteries must supply power to the load instead, causing battery voltage to decline. Once the battery voltage drops to match the “low voltage threshold” setting, the relay opens and the load is removed. Because the load is removed, the batteries do not discharge any further. To maximize battery longevity, the depth of discharge (DOD) should not exceed 20%. The automation described here amounts to little more than the low and high battery voltage threshold settings.

Using a data logger, battery voltage readings were taken every 30 minutes for several days. Battery voltage increases during the day, even on cloudy days, as a result of charge current from the solar panels. Battery voltage decreases at night, due to the loads. When charted, these voltage differences are seen as hills and valleys on the graph. For this test, a refrigerator and a freezer were used as the load. Since those devices have compressors that switch on and off at random times, the load was not constant throughout the duration of the test. When one or both of the compressors turn on, the heavy electrical load causes battery voltage to sag a little. This is followed by a voltage increase when one or both of the compressors turn off. The resulting voltage dips and peaks result in jagged lines on the graph.

Interpreting the data and making adjustments:

Data from the first set of tests suggests that battery damage could occur as a result of chronic undercharging. Although this was the result of a lack of sunshine, raising the high voltage threshold setting will help to ensure that the batteries are not chronically undercharged.

A high motor-starting current might result in a temporary voltage sag that can cause the relay to deenergize prematurely. Setting a high to low threshold delay will prevent this from occurring.

Although there was no evidence of false-triggering, a voltage spike could cause the relay to energize prematurely. This can be prevented by setting a low to high threshold delay.

It seems that the best strategy will be to apply different settings for summer and winter. Specifically, the high voltage threshold setting may be lowered in the summer, since more sunlight is expected. More sunlight will reduce the chance of chronic undercharging.

For now, the automation settings will be changed as indicated below:

Name and Purpose of Setting -- -- -- -- -- Present Setting -- New
H. Voltage Threshold - Energize relay --- 13.65 volts ----- 13.85 v
L. Voltage Threshold - Deenergize relay - 12.00 volts ----- 12.00 v
High to low threshold delay -- -- -- -- -- -- - Not set ------- 10 sec.
Low to high threshold delay -- -- -- -- -- -- - Not set ------- 2 sec.

This was the first of a series of tests that will be conducted throughout the year. I’ll continue to gather data and adjust the automation settings as necessary. Once I have sufficient data, I’ll list the setting changes I’ve deemed appropriate for the different seasons. Up to this point, the data I’ve gathered has been during a period of unusually bad weather.

It’s interesting to note that before automating the system, the average daily output was less than 0.5kwh. The system was capable of producing more, but I wasn't there to turn it on and off. Since I’ve automated the system I frequently get more than 1.2kwh from it. Unused energy from the sun is wasted energy, and I’m pleased with the results of this upgrade. I’ll post additional performance data as I get it. Check back later.

The settings described here are applied to the Morningstar Relay Driver, using MSView software. More information can be found on the Morningstar website:


No comments: