[RE-wrenches] Higher LVR to force full re-charge in small OGPV systems

[Exeltech]

Eric .. Allan .. and all ...

Allan wrote:
> Re an MPPT controller: Nope, I wouldn't. First, this is equatorial west Africa.
> It's a tropical climate with hot modules most of the year. MPPT offers the
> greatest benefit when the voltage delta-T is greatest: cold modules and empty
> batteries.

*** While Allan is correct about deriving maximum MPPT benefit when the PV are cold, at 75W per panel, and 120A-H battery, the PV are undersized for all but very light-duty system usage.  Obtaining the maximum possible charge current will help lengthen battery life.  If two 12V 40W PV in series could be economically utilized (vs. one 75W panel) .. it's worth considering.


> Second, any controller with MPPT is more complex, and thus more expensive, than a simple PWM controller.

*** Hence my comment at the top of the suggestion list:  "...  if budget and circumstances permit...".  If not - go with a basic PWM unit and other cost-saving measures.  If they're going to hold your feet to the fire on the "seven year" battery performance requirement, you're going to have to limit the depth of discharge, and also institute other life-enhancing practices.



> Re temperature compensation:....

*** Temperature compensation in this application may be a toss-up between fully recharging the battery and obtaining maximum battery life.  The choice here could go either way.  If it's always warm (or hot), compensation may not be needed.  In fact, it may be beneficial to help maximize the recharge if the controller thinks the battery is cooler than it really is, which in turn would push the charge voltage set-point up a bit.  Not knowing the environment where the systems will be installed makes this decision an unknown .  Also .. and keeping in mind that there are elevated equatorial areas where the weather can be cool .. compensation may or may not be a consideration.


> To me the other features of the C12 - four adjustable settings, removable knobs and 15-minute reserve - were more important than MPPT.

*** There is much to be said for the KISS principle.


> Re an LED SOC display: agreed just as Dan stated. Is there one that can be programmed to lie a little? (The best of these I ever knew was the Photron Batterylite, gone since around 1998. I wish someone would resurrect that tiny gem.)

*** With several hundred units in the mix, I'd think a small design house may be interested.  As long as you're at it, how about adding a low-voltage disconnect option into the monitor design .. and perhaps even an audible alarm (with user reset).  Each feature adds $ and complexity.  If the users will routinely look at such an LED monitor and use power accordingly, neither of these features would be needed.


Eric .. there's no singular (or simple) answer to your question.  If the project is going to be awarded on a strictly low-bid basis .. and they're adamant about seven year battery life .. you're going to have difficulty meeting their budget AND achieving their system goals.


Happy New Year to all....


Dan




--------------------------------------------
On Tue, 12/31/13, Allan Sindelar <allan at positiveenergysolar.com> wrote:

 Subject: Re: [RE-wrenches] Higher LVR to force full re-charge in small OGPV systems
 To: "Exeltech" <exeltech at yahoo.com>, eric at solarnexusinternational.com, "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
 Date: Tuesday, December 31, 2013, 10:51 AM
 
Dan and Eric,

Now I'll respectfully argue with some of Dan's suggestions:

Re an MPPT controller: Nope, I wouldn't. First, this is equatorial west Africa. It's a tropical climate with hot modules most of the year. MPPT offers the greatest benefit when the voltage delta-T is greatest: cold modules and empty batteries. No chance for the former, and we're trying to get around the latter. If these systems were going to a cold climate I'd agree with you. Second, any controller with MPPT is more complex, and thus more expensive, than a simple PWM controller. Given the expressed competitive-bid budget limitations, MPPT is not a smart place to allocate costs. Dan is right about the weak C/rate. Spend the money (if you can) on a second or larger module.

Re cool batteries: agreed, although I don't know how to achieve this.

Re temperature compensation: as much as I push it strongly here, it may not matter in this case, and may be a hindrance. What's the annual ambient temperature swing? Again, if it's equatorial lowlands the temperature may stay around a constant 80º all year. If so, TC is irrelevant. And they occasionally fail, which can cause settings to mess up. Once again, decide if the added cost is worth it in this application.

Re an LED SOC display: agreed just as Dan stated. Is there one that can be programmed to lie a little? (The best of these I ever knew was the Photron Batterylite, gone since around 1998. I wish someone would resurrect that tiny gem.)

To me the other features of the C12 - four adjustable settings, removable knobs and 15-minute reserve - were more important than MPPT.
Allan

Allan Sindelar
Allan at positiveenergysolar.com
NABCEP Certified PV Installation Professional
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Founder, Positive Energy, Inc.

A Certified B CorporationTM
3209 Richards Lane
Santa Fe , New Mexico 87507
505 424-1112 office 780-2738 cell
www.positiveenergysolar.com

 
On 12/30/2013 7:32 PM, Exeltech wrote:
> Eric,
>
> Allan's recommendations on both points are excellent, and spot-on.
>
>
> I would add the following suggestions for your controller and system if budget and circumstances permit:
>
> 1) Use a MPPT model controller to enhance the recharge rate under less-than-ideal conditions.  The PV is already undersized for the battery you indicated (below).
>
> 2) If you get the job .. install the batteries in a location that will keep them as cool as possible.  Heat is just as detrimental to battery life as is failure to fully charge.
>
> 3) Temperature-compensated charging will enhance battery life.  However, the 75W PV vs. 120 A-H battery allows for only a C/20 recharge rate at best.  This is not conducive to full recharge on a regular basis unless the loads are very small and are lightly used.  If the budget allows for opportunity to double the PV wattage .. it would be wise to do so.
>
> 4) A simple "stop-light" style LED voltage display won't consume much power, but can go a long way toward providing a visual indicator for state-of-charge, especially if it's adjustable, and can be be biased toward a higher SOC (in essence, make it fib a little, and indicate a low SOC sooner than than is actually the case).  Worth considering as an option .. but easily omitted to reduce cost and/or simplify the system.
>
> I wish you success on winning the project.
>
>
> Regards,
>
>
> Dan
>
> --------------------------------------------
> On Mon, 12/30/13, Allan Sindelar <allan at positiveenergysolar.com> wrote:
>
>  Subject: Re: [RE-wrenches] Higher LVR to force full re-charge in small OGPV systems
>  To: eric at solarnexusinternational.com, "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
>  Date: Monday, December 30, 2013, 6:48 PM
>  
>  Eric,
> Yes, one of the classic problems with small systems and less educated users. Just a small contribution: it would seem to me that in this case the LVD setting is as key as the LVR. What if the LVD was set as high as possible - say, 12.0 V with sealed batteries, along with a 13.8 - 14.0 V LVR. The system would still be run until it shuts off, but the battery remains at a relatively high SOC. The next day the system will return to operation, but with a much higher range of SOC. The users will still experience shutdown, and will over time become familiar with when to expect it and how to live within its capacity, but the battery life will be protected.
>
> This approach seems counter-intuitive, but the more I think about it the more sense it makes. The system would also return to operation sooner following cloudy weather - in fact, it would provide a minimal amount during each of the cloudy evenings.
>
> Is the C12 still made? It would still seem ideal if so: bulk, float, LVD and LVR, all adjustable, with removable knobs and a 15-minute reserve button after initial LVD shutdown. Rock-solid reliable, too, in my experience.
>
> I'd be interested in what approach you select. This intrigues me.
> Allan
>
> Allan Sindelar
> Allan at positiveenergysolar.com
> NABCEP Certified PV Installation Professional
> NABCEP Certified Technical Sales Professional
> New Mexico EE98J Journeyman Electrician
> Founder, Positive Energy, Inc.
>
> A Certified B CorporationTM
> 3209 Richards Lane
> Santa Fe , New Mexico 87507
> 505 424-1112 office 780-2738 cell
> www.positiveenergysolar.com
>
>  
> On 12/30/2013 5:00 PM, Eric Youngren wrote:
>> Hi Wrenches,
>>
>> We are bidding on a project to provide several hundred small (75Wpv, 120AH battery) 12V DC off-grid solar home systems for rural villages in West Africa.  The project planners have requested that the systems be designed and built to provide 7 years of battery life.  That seems crazy optimistic to me but I'm trying to design the systems that will give the batteries a fighting chance of lasting that long.  
>>
>> In my experience with these types of installations, the system users have little or no understanding of how to properly manage their batteries and usually no metering or SOC indication to help them even if they knew what to look for.  So, the default control strategy becomes:  run the loads until the Low Voltage Disconnect (LVD) turns them off, then wait until the Sun returns and the voltage rises to the Low Voltage Reconnect (LVR) setpoint (around 12.5V is a common default) , upon which point the cycle repeats, with the result that the battery bounces between LVD and LVR, almost never reaches a full SOC, and the batteries are lucky to survive for maybe two years.  After that the system will provide a little power during the day while the Sun is shining but the batteries will quickly crash below the LVD after the sun goes down. 
>>
>> So, we want to offer a controller with a high LVR setting that will ensure the batteries reach a full recharge after each and every LVD incident.    I know the C-12 has an adjustable LVR setpoint and I see one Chinese brand (Manson) that can do it.   Does anyone have any other suggestions of small (<20A) PWM controllers that can be adjusted to not reconnect until 14V or so?     Price is going to be a factor in this project so they need to be low cost.  
>>
>> Any other suggestions?  What would you use for the optimal LVD and LVR in this situation?     These are rural household, DC only systems with primarily lighting and small device charging loads.   I know that this strategy will mean that they might not have any access to battery power for up to a few days if they hit the LVD during the rainy season and there is not enough sunshine to get the battery charged in a single day.  My thinking is that experience will provide good feedback that will encourage them to conserve and manage their batteries to avoid the LVD situation as much as possible.     That's the idea, anyway.  I'd appreciate any advice from you all.  Thanks!
>>
>> Wishing you all a happy and productive 2014!  
>>
>> Best energy,
>> Eric    
>>