MPPT or Dump load.

[Squirl]

I am researching my system design.  I had everything all set for a 9 panel 215 watt 24 volt array in a 3 panel series with a FM-80 controller.  It just seems like such a waste to shut off 2 kw of power. I would want to put any excess power to use in heat.  Thoughts?

[MountainDon]

I'm nor sure I follow the "MPPT or Dump" line of thought.

The FM60/80 have a 'diversion' setting under the Aux menu. Are you thinking of using the PV pwer not needed for battery charging by using the FM60/80 Aux setting to divert that "wild" DC power for something? Or thinking of using some other charge controller and method.

I also see possibilities of drawing DC power off the batteries for something requiring a constant voltage or via the inverter to use the excess as AC power?

There are problems with all above methods.
 

[firefox]

This is just a wild idea.
I was wondering if you couldn't use the power to lift a large weight like a 55gal drum
filled with water or something heavy. If you use some gear reduction so that you didn't have to lift it all that high and some good brakes you should be able to store a fair amount of power.
Then use the system to drive an automobile generator to extract the power. Use a lot of gear reduction.
Like I said, it is a wild idea, but I bet someone could make it work, and the parts shouldn't be that difficult to scrounge up from a junk yard.
Bruce

[Rob_O]

Just wire the dump output to the appropriate number of baseboard heaters. Install a disconnect in an easy to reach location so you can shut off the extra heat when it's not needed

[Squirl]

I didn't know the FM 80 had a diversion control.  I was hopping on using any excess solar for heating water or a baseboard.  Just trying to get closer to zero.  I had remembered the Xantrex C60 diagrams where a solar panel or wind turbine dumped power into the batteries and just dumps the excess to a diversion control.  I figured the power output can be pretty wild from a turbine also.

http://www.wholesalesolar.com/pdf.folder/controller%20pdf%20folder/XantrexCSeriesManual.pdf
P.47 of the manual shows a solar array, wind turbine, or hydro connected directly to the batteries and the C60 used as a diversion controller.

I've read a few books and have had a subscription to homepower.  I found no discussions of hooking the panels directly to the batteries or the downsides of doing that and just dumping excess power.   The inverter I'm looking at getting also has a battery equalizer.  I was hoping someone would have some insight into what I may be missing.

[MountainDon]

I've read a few books and have had a subscription to homepower.  I found no discussions of hooking the panels directly to the batteries...

If you gathered that thought from something I said, it was misinterpreted. I would never connect PV modules directly to batteries. The PV power always goes through some controller.   I'll be bacl later; have to see if I can find a bookmark.

[glenn kangiser]

The controller also controls the dumping of power after it regulates the power into the batteries.

When they are full it then diverts the power rather than disconnecting the solar panels.

A wind generator must always have excess power go to a dump load as it will overspeed without the load.

Many controllers are set up to either control panels and shut them down or to be used as a dump load controller.

[MountainDon]

The FM60/80 have an Aux setting that permits powering a relay. That relay can then carry diversion load. The relays are not cheap when you consider the high voltage it must carry.

9 modules; so 3 parallel strings of 3 modules in series?
What is your coldest likely outside temperature?

[MountainDon]

I can't find the bookmarks I wanted. A while back the Outback Power forum had a discussion on using the excess PV module power. Dave Sparks directed me to it when I was contemplating using the excess for heating. I ended up concluding it was more expense and bother for too little return, in my case. Perhaps do a search on their forum for "diversion load heating" or something. maybe Dave Sparks will drop by and slap my fingers for not being able to organize my bookmarks. 

OOOps. I made an error entering data in a chart. What was here was wrong.   

The Voc passes the Outback String Sizing tool, but fails the manual method of multiplying Voc x 1.25 x 1.25.   

[MountainDon]

I found some of what I was looking for...

http://www.outbackpower.com/forum/viewtopic.php?f=9&t=1192&hilit=water+heater
http://outbackpower.com/forum/viewtopic.php?f=9&t=3400&hilit=auxillary+mode
http://www.wind-sun.com/ForumVB/showthread.php?t=2718

[Squirl]

The coldest ever recorded temp at my location was -30 F or -34 C.  NEC states to use the actual panel ratings when available vs. the NEC charts.  The Voc for Standard Test Conditions is 35.6 with a temperature coefficient of .31%.  So with a 59 C difference (59x.31) 18.29% is the temperature adjustment per panel or 42.11.  With the 1.25 NEC safety margin it comes ot 52.6 volts per panel.  So that is a rounded 158 volts.  If I could use the nominal Voc of 32.9 instead of the STC it would pass at 145 volts.  I could always go with the midnight solar which is rated up to 196 volts and an output of 2700 watts at 24v. Between the 300 vdc breakers and the higher rated controller it would cost around $100.

I disagree that the solar panels cannot be connected directly to the batteries.  This is the configuration for diversion control in the Xantrex models, through a disconnect/circuit breaker of course.  The controller monitors the batteries voltage and dumps the excess power.  This is how I have seen every wiring diagram for diversion control.  Even though this is an approved charging method with wiring diagrams, I don't know why it is not used.  The default seems to run panels through the controller and waste the extra power.  This is a standard method for wind and hydro, but why not solar? 

p.46 of the manual
1. Connect your DC current source (PV, wind, hydro, etc.)
directly to the RE disconnect.
2. Connect another cable from the other side of the RE
disconnect to the battery positive terminal.
3. Run a negative wire from the DC current source (PV,
wind, hydro, etc.) to the battery negative terminal.

[MountainDon]

I was thinking after that post and I agree the actual measurements would be better, as long as an honest value is used. We've never quite reached the theoretical maximum for our system.

I am not well versed in wind generation but there are differences between it, hydro and solar.

You have solar so I'm thinking solar and maybe I am not properly understanding something in the diversion process as it would apply to PV modules and a controller like the C-35/40/60 series Xantrex,

Three modules such as the evergreen mentioned, connected in series would be putting out something like 85 to 90 volts on a great sunny day. I don't see how you could connect that directly to a 24 volt bank of batteries without some damage or at the very least boiling away the fluid much more rapidly. I suppose if the modules were closely matched to the batteries in voltage that could work. But then all the advantages of an MPPT controller are lost.

From I have observed with our three series connected panels and the FM60, there are many hours every day when the MPPT is working hard at producing the most it can for the batteries from both early and late day and cloudy day sunlight. It's able to make a little something from what otherwise would be nothing. To my mind I believe that using an MPPT controller has more advantages than using a PWM controller and a dump load for some ancillary purpose. But that may be just me.  It seems to me the battery will be replenished more rapidly with MPPT than with any other type and a diversion load/purpose. That's my primary goal; get the batteries full after the previous day and evenings use. We have clouds roll in many an afternoon and it's nice to get that recharge accomplished. It doesn't always happen but that's what a 3 day reserve is all about.

I believe it is more a case of the absolute need to divert power in wind or hydro generation systems, to prevent hardware damage, than it is a case of wanting to make real use of the excess power. PV modules can be safely shut down, disconnected even and left in full sunlight with no damage to the modules. Wind and hydro generators can not be handled like that.

Incidentally I have concluded that if air or water heating is desired, that is best served with a solar water or air collector. I'd have one on the cabin if it wasn't for the trees shading the south wall and roof.

[MountainDon]

Hmmm. Depending on how much excess there is, an efficient efficient use might be to have a highly efficient reversible heat pump. You can get more heat from a heat pump per watt than from resistance heating. It could use excess power in AC form to supply either heating or cooling. I'm not sure of there is an automatic controller that could be used to switch it on as excess becomes available (batteries full) and then off when there is no excess.   

[Squirl]

I agree about the heat pump.  Along with the wind turbine I am going to install with the right wiring and 600 sq. ft of space, I should be able to get to zero net energy.  An excess of about 4 kwh would equal 13,500 btu/h.  If I could get a 3:1 heat pump conversion, I could get 40,000 btu/h.  Pretty good for super insulated 600 sq. ft. house. The wiring of this would be key, and more complicated than a resistance dump load.

It is not like I am going to put the system together tomorrow, but I am getting together the components now, so that when the snow breaks in the spring, I have everything assembled, ready to setup.

I realize that solar, hydro, and wind put out power differently. Hydro and wind through off some wild power directly into the batteries and then dump the excess.  I would be happy running all the panels separately through a 12 panel combiner only if I could use the power. The cost difference would be $30 extra for the panel and the same cost for the circuit breakers because I would be going with 150 vdc instead of 300 vdc.

I understand the need, but what about the excess benefit.  Is the reason that people don't use the dump load from solar the excess cost of the equipment and wiring?  Is it from the benefit of the MPPT charge conversion?

[glenn kangiser]

I use the dump load from my wind generator and in fact it will dump when the voltage on the batteries is too high for any reason as it senses voltage and then dumps when too high.

I am on a 24 v system so took two 12v relays from a car the 1 1/4 inch or so square cube kind and wired then in series to give me a 24 volt relay with two sets of contacts.  I hooked the secondary for the 24v  relay to control a 120v relay that turns my 240 v pump on when the dump load is needed.  That way I can control much more power than just what the dump load of the wind generator could control.

I could also have shopped for a 24v dc realy but why do that when you can get off the shelf parts that will work.

[MountainDon]

Is the reason that people don't use the dump load from solar the excess cost of the equipment and wiring?  Is it from the benefit of the MPPT charge conversion?

In my opinion, Yes.

I am 'sold' on the advantages of MPPT controllers over any of the other. I've said before, I see the "magic" the MPPT performs when the sun is less than optimal for whatever the reason. The remote Mate display indicates how the high voltage, low/average amps is converted to just the right voltage for the state of charge and the maximum number of amps. The batteries get full as fast as possible.To me that is of primary importance. Therefore I vote for an MPPT controller.

When it comes to trying to use the excess power when using a charge controller like the FM60/80 the process and hardware needed did not seem to bring very great rewards. The recommended (by an engineer) quality DC rated SS relay was not cheap. There are cheaper ones available but they were not recommended. Resistance heating elements should be selected with an eye to the voltage, which changes enough in a three series system to create some issues. In the end I decided there was too small a return for the costs and bother. That was me, others may view it differently.

I should also point out something that now influences decisions I make. I don't like to build systems that might need an instruction manual for someone else to understand anymore. At my age I recognize more fully than I did when I was 30, 40 years old, that I will not be here all the time to provide maintenance. I don't want a system or sub-system that a hired electrician can not readily grasp. I'm thinking in particular about my dying and my surviving wife having to deal with any creative systems/features I may have developed.

I am reminded of a car I had when I first met my dear wife in '76; a modified Volvo 123GT. It could not be started without first running the pre-oiler pump to build up oil pressure in the turbo bearings. Of course that was while holding down the button that closed the relay to connect the battery to the cars' systems. You weren't supposed to shut it off until the turbo temperature cooled to below 400 F. Nowadays all that can be automated, but not mine, not back then; it just had a big pyrometer with a red paint line smack dab in front of your face. The high volume fuel pump had to be switched on separately; why? I really don't know other than I liked the pretty row of switches.     You wanted to drive it at night? There was a myriad array of switches and indicator lamps to choose between the regular headlights, the fog/turning lights, the driving lights and the pencil beam light. Some switches were inactive unless some other one were switched on. Try and explain all that to someone who eyes are glazing over. She did quite well, but I don't want to put even myself in such a complicated situation again.

So, making things "turn key simple" is a part of whatever I try to do. I don't always manage to live up to that.   

[firefox]

Boy, can I relate to that!
Bruce