Monday, October 15, 2007

PV System Upgrade

While I’m looking forward to adding PV panels to my array, I’ll first have to replace my 20 amp charge controller with a bigger one. After careful consideration of available products, I’ve selected the Morningstar TriStar-60. I considered the Outback MX-60, but decided that the TriStar would be a better choice for my small system.

The Outback is a maximum power point tracking (MPPT) charge controller, which means that it can convert DC voltage to a higher or lower value. In some cases, this can result in more efficiency, but it probably wouldn’t benefit me much because my PV array is small. The TriStar is a pulse width modulation (PWM) charge controller, and it charges batteries in four stages. The strength of the charging signal is controlled by varying the pulse width. This has proven to be an efficient charging scheme for small systems.

The Outback charge controller can match any common PV array voltage to any common battery voltage. This feature allows the user to economize on wiring, and minimize losses that would otherwise occur. The TriStar can operate at 12, 24, or 48 volts, but cannot convert a high PV panel voltage to a lower battery voltage as the Outback can. While the Outback MX-60 is also a four-stage charge controller, its dc voltage conversion feature would not benefit me greatly, considering the limited wiring options I have with my small system.

The TriStar-60 uses less than 20ma (0.02amp) of current for its operation, much less than the Outback, an important consideration for a small system. More of the solar-generated power will be applied to the load. And the TriStar-60 comes with a 5-year warranty, while the Outback MX-60 controller is only warranted for two years.

Perhaps the most interesting feature of the TriStar-60 is its digital meter (option). I’ll be able to monitor system performance more easily, and I’ll return my digital multimeter to the toolbox where it belongs. In addition to instantaneous readings, the TriStar-60’s meter can display performance over time (data logging), and remote monitoring via the Internet is possible.

Before the year ends I’ll install at least one more solar panel. My ultimate goal is not only to power my home with PV-generated electricity, but to eliminate my use of fossil fuels entirely. I already burn corn to supplement natural gas heating, and I hope to purchase a plug-in car (PHEV) within the next two years. Using cfl lighting and replacing old appliances are other ways that I’m moving closer to my goal. I might just be the first person in my town to accomplish this. How cool is that!

John

8 comments:

Solar John said...

Update:

I installed the new TriStar-60 charge controller last night, and monitored system performance today. At this point I'm well pleased, and believe it was a good choice. The digital meter (option) was well worth the extra cost.

sj

Solar John said...

2nd Update:

I continued to monitor system performance today. I watched the transition from "bulk" to "absorption", but didn't get to the "float" stage. The size of the load was a factor, but pv panel output dropped to almost nothing at about 4pm due to shading from a tree limb. I spent the rest of the day trimming the tree. The problematic tree is tall and thin (thinner now), and in just the right spot to cause a problem, considering the suns position in the sky this time of year. This illustrates the importance of system monitoring.

sj

d said...

Hmm, I have a had to do a little tree trimming here and there too! Though autumn is helping by removing most of the problem for a few months...

Agree with you about the value of monitoring: I record a sample with my USB data input board every 2 minutes. My crude home-brew monitoring page:

http://www.earth.org.uk/off-grid-stats.html

which also means that I can tell what the weather is at home (roughly) from my mobile phone. Isn't technology abusable! B^>

If I expand my system I'll probably be making the same choice as you (MX60/TriStar) but I think the Outback is going to be most useful when all the panels are identical (manufacturer, age, etc) else there may not be a useful MPP to T...

Hadn't noticed the MX60's own power consumption: I'll check again if I am think of one. Thanks.

Rgds

Damon

Solar John said...

Thanks for the comment d.

I believe the Outback draws 20 watts when it's awake. I don't know how much power is consumed at night. Another troubling thing about the Outback is that it periodically checks for the presence of power from the panels. That's like picking up the telephone every few minutes to see if anyone is calling. An interrupt-driven system would be better.

sj

d said...

20W is high, though I guess I was thinking of the night-time draw.

On the whole I think polling is the right thing to do for slowly-changing inputs, rather than having to set an arbitrary voltage threshold for an interrupt and then debounce it. Indeed, all my hardware experience (eg from robotics) says to avoid interrupts unless really necessary!

Rgds

Damon

Niharika said...

Hi John,
Thank you for useful information. I am an Indian from a rural village, currently working in UK but goback to India in Feb08. I am very interested in Solar electricity. Our village has 6-8 hours power everyday. I would like to contact you over email. please let me know email id.

Regards,
santhosh

Solar John said...

niharika,

send me another message with your email address and I'll respond. I have to avoid spammers you know. I'll protect your email address by not publishing it.

sj

Anonymous said...

No fossil fuels, eh? You do realize that doing so will lead to further global warming? Which (oddly enough) will increase your PV efficiency because more solar radiation will reach earth due to fewer particles in the atmosphere. The best way to stop global warming is to burn as much carbon as well can to block on the sun (ONLY source of energy [read HEAT] on our planet besides nuclear)