Friday, June 22, 2007

The Practical and Efficient Use of a Small PV System

Since the primary role of my small photovoltaic (PV) system has been to serve as an alternate source of electricity during grid power failures, a great deal of power was unused, and therefore wasted. To minimize the waste, I decided to put the system to use on a daily basis. After all, I reasoned, shouldn’t I be using this energy to reduce my electric bill?

My first task was to determine how much energy I could expect from the system. Since I have 340 Watts of PV capacity (4 – 85 Watt Panels), and I can expect about 4 hours of peak sunlight each day, the overall production will be 340 times 4, or 1360 Watt/Hours per day. Because I’ll be storing some of this energy in batteries, and because I’ll be converting DC to AC, system losses must be considered. Clouds will also limit production. With all of these things in mind, I’ll estimate that my daily capacity averages 900 Watt/Hours.

Looking over a list of appliances in my home, I decided to use a chest freezer as the load. The chest freezer is an ideal load for several reasons: It requires about 800 Watt/Hours per day and the energy requirements change very little from day to day. Because it uses a little less energy than the anticipated daily output of the PV system, the freezer could theoretically be powered indefinitely. I decided to carefully monitor system parameters for a few weeks, and log the results with comments. Here are some log excerpts:

May 6, 2007
Power up inverter and connect chest freezer.

May 13, 2007
Observed that in addition to providing power to the chest freezer, the PV array is able to fully charge the battery bank.

May 31, 2007
The past six days have been mostly rainy and overcast. Switch off inverter due to low battery voltage.

I predicted that the PV system would be able to produce more energy than needed to run the freezer, but six days of mostly cloudy weather depleted the battery bank.

June 2, 2007
Sunny weather has returned and batteries are fully charged. Switch on inverter and connect chest freezer.

June 7th, 2007
Severe thunderstorms are predicted. Switch off inverter in order to preserve batteries in the event they are needed in the event of a grid power failure.

No grid power failure occurred, but it was good to know that a fully charged battery bank was available just in case.

June 8th, 2007
Stormy weather has passed. Switch on inverter and reconnect freezer.

June 10th, 2007
Due to mostly overcast conditions, battery voltage has again declined. Switch off inverter and freezer load.

Overall the system performed pretty much as I expected it to. If not for an extended period of cloudy weather, it could have continued to provide enough energy to power the chest freezer for a very long time. I suspect that the short and cloudy days of winter will create the same situation.

Overall efficiency may seem to be optimized in this example, since the load is about equal to the capacity of the PV system, but a significant increase in efficiency is possible. The freezer could be put on a timer, set to allow the freezer to run only during daylight hours. If the compressor runs during the day, when energy comes directly from the solar panels, the losses associated with storing power in batteries and using power from the batteries are eliminated.

Based on the results of this test, I believe I can improve PV system performance by increasing the size of the battery bank. It is important to note that the battery bank must not be enlarged to a point where the existing PV array cannot fully recharge it. Chronic undercharging will shorten the life of the battery bank. It is also important to remember that in addition to charging the batteries, the PV array needs to provide power to the load.

The system continues to serve as an emergency source of electricity, and most days provides power to the freezer therefore reducing my electric bill. Additional opportunities to enhance system efficiency will surface as I enlarge the system. At this point it's good to know that I've reduced the waste.

Here's a picture of the freezer:

Visitors usually ask about the sign, giving me an opportunity to show-off my PV system.



Rodrigo said...

Oi, achei teu blog pelo google tá bem interessante gostei desse post. Quando der dá uma passada pelo meu blog, é sobre camisetas personalizadas, mostra passo a passo como criar uma camiseta personalizada bem maneira. Até mais.

d said...

Hi John,

Although my PV system is still tiny, I don't like to waste that expensively gathered energy, especially if I expand and overspec the system for summer to last more of the winter.

I'm intending to have an extra load make use of the extra power available when the sun is out and the batteries full, and switch off otherwise. In my case it's a computing project that can soak up an extra 30W of CPU power when available, but can safely go days without being run if need be...

All I have to work out how to do is get my battery-is-good-and-sun-is-out sensor (which I suspect can just be a 13V-threshold switch) into my laptop efficiently and safely via USB.

I think I'll also have a lower-threshold-driven changeover relay switch the laptop back to mains power, to always leave solar power for lighting.

The laptop is my 30W+ replacement for ~700W of always-on Internet servers...

That way I should be able to soak up all my sunlight usefully, but save some for a rainy/powercut day...