Utilizing excess energy from solar panlel - not a dumpload question ;-)

**Txtinner** · 01-13-2013, 11:59 AM

Not a Dump load

Thanks so much for this thread, as I am designing a system to supply the energy load for a 40'X40' RV Solar Shop.
I am having issues with the exact load I will have, as the load will change often.
My plans are to have Radiant heating in the floors, but will only need this in the winter months. My thoughts are to
redirect this power to ceiling and exhaust fans for the summer months.
Just to let you know, this shop is in Texas, and we are challenged more with Summer heat than Winter cold.
I still need to put the numbers together for the load requirements for water pumps and ceiling fans, but I also plan to over-size.

At this time, I am having a challenge finding a reputable source for cells for custom panels.

Best wishes from Texas,

Danny
txtinner

**Sunking** · 01-13-2013, 12:04 PM

Originally posted by Roil

And please - this is not a tread about dumploads, and we don't need the replye's like, your load has do be big enought etc. Those of us who have this wish we just want to use the available energy to something, e.g add smal heater for air or water, or for those who live in the warmer part of the world - running a cooling fan

Well I am certain you have me and all other professionals very confused. Either that or you are very confused. You say you do not want to talk about dump loads, but that is exactly what you are defining and wanting to do.

**Txtinner** · 01-13-2013, 12:43 PM

Yes, somewhat confused, but getting better

I'm not sure why a "dump-load" is needed, instead of using fans or circulating pumps instead

**Sunking** · 01-13-2013, 12:53 PM

Dump Load Difinition

Dump Load: A device to which solar PV or Wind wind generators power flows to when the system batteries are fully charged so the otherwise unused power can be utilized rather than wasted. Generally it is a heating element because it does not require a minimum or set amount of power to operate under dynamic conditions.

They have been around as longer than solar and wind generators have existed.

**Roil** · 01-13-2013, 04:16 PM

Originally posted by Sunking

Well I am certain you have me and all other professionals very confused. Either that or you are very confused. You say you do not want to talk about dump loads, but that is exactly what you are defining and wanting to do.

Well, this might be a language issue, but every tread I have read about PV and dumploads has concluded, -you dont't want this, need it, etc

And since the basic design as I understand it of a dumload regulated system is based on direct connection of the source to the battery, and then a dumpload who are capable of putting a load on the system atleast the size of the source. I can't see how this can be established with the source beeing a PV regulated by a MPPT. In such a soluion there is now extra load to be utilized on the batteryside of the MPPT Consequently no energy to dump.

This is why I try to make difference between a normal dumpload solution and what I would like to do. Let me try to give some examples:

- MPPT does a bulk charge of the battery, no excess energy available
- MPPT goes into absorption, full current, no excess energy available
- MPPT in absorption, but 50% current, no the other 50% is available as excess energy
- MPPT in float, 95% of the energy is avilable as excess energy

However you do not need to comsume the excess energy, it's just an option?

If you put a load on the system when there is available energy the MPPT regulator should be in control of that load so the load can be reduced if the available energy is reduced. I cant understand how this can be achieved with current technologi - please enlighten me if I am wrong.

**inetdog** · 01-14-2013, 02:48 AM

Originally posted by Roil

If you put a load on the system when there is available energy the MPPT regulator should be in control of that load so the load can be reduced if the available energy is reduced. I cant understand how this can be achieved with current technologi - please enlighten me if I am wrong.

Some people in this and other forums make a distinction between a dump load, considered as a place to either use OR waste power to prevent overcharging of a battery bank or to keep a load on a wind or hydro turbine to keep it from running out of control when no power is actually needed. For wind, hydro, or shunt type battery charger, this load is not optional it is required.

When the specific goal is to take available power that would otherwise be wasted, as for example, the extra PV potential output when an MPPT controller does not need to use full output, it is more often called an opportunity load. It is completely optional in that if it is turned off or disconnected, nothing bad will happen to the system.

The most common opportunity load is water heating. This can be done directly with DC from the power source, but that requires finding heating elements with non-standard voltage and resistance specifications to make best use of the power. A few people have put forward the idea that as long as you have an inverter running off the batteries anyway, you can just as well power your opportunity load from the inverter whenever the battery bank is fully charged. The input current to the inverter will simply come directly from the panels rather than forcing a charging and discharging of the batteries.

The simplest way to control this is to use an output from the CC which indicates that it is in Float to drive a relay which energizes the heating element. That will not let you use surplus power during Absorb. That would be more complicated.

An idea which I have not seen explored would be to take off the excess power before the MPPT CC by sensing the current being drawn from the panels by the CC and pulling additional current into an opportunity load when the CC current is below the current Imp value. But that would require some way of knowing how much light is hitting the panels, or at least what the current Imp value is.
And too rapid a response could cause instability in the MPPT algorithm of the CC.

**Roil** · 01-14-2013, 05:36 PM

Now we are talking

Originally posted by inetdog

.........A few people have put forward the idea that as long as you have an inverter running off the batteries anyway, you can just as well power your opportunity load from the inverter whenever the battery bank is fully charged. The input current to the inverter will simply come directly from the panels rather than forcing a charging and discharging of the batteries.

Using the inverter is not a bad idea but typically in an off grid solution it's not turned on, that can be acomplished ofcourse.

Originally posted by inetdog

The simplest way to control this is to use an output from the CC which indicates that it is in Float to drive a relay which energizes the heating element. That will not let you use surplus power during Absorb. That would be more complicated.

But still there is a problem to be able to regulate the load to match the available PV power since this is not nessesary stable.

Could this be done using T-junction between battery and MPPT, putting a load on the system, but only as long as the current comes from the MPPT and never from the battery. Can a simple diode do this?

I must admit that I am about to conclude that the Solar Industry has missed this opportunity. As fare as I can understand the only proper way to achieve this is an advanced MPPT controller with to different output's
One charging the batteries, and another being able to supply the excess power to any load we want. The load will of course have to be able to accept whatever voltage and current avalable. I guess a heater element is fine, a fan might be more difficult.

Doesn't anyone know sommeone in the industri who can have a look at this?

**inetdog** · 01-14-2013, 05:45 PM

Originally posted by Roil

Now we are talking

Using the inverter is not a bad idea but typically in an off grid solution it's not turned on, that can be acomplished ofcourse.

But still there is a problem to be able to regulate the load to match the available PV power since this is not nessesary stable.

I must admit that I am about to conclude that the Solar Industry has missed this opportunity. As fare as I can understand the only proper way to achieve this is an advanced MPPT controller with to different output's
One charging the batteries, and another being able to supply the excess power to any load we want. The load will of course have to be able to accept whatever voltage and current avalable. I guess a heater element is fine, a fan might be more difficult.

Doesn't anyone know sommeone in the industri who can have a look at this?

If you do a google search for power management systems, you will eventually find a company that sells something to the UK market that regulates the opportunity load to exactly null out any power returned to the grid in a grid tie system. The only reason this makes sense it that the UK tarifs pay the user more for each KwH generated and used on the premises than for the same amount of power generated and returned to the grid. So the concept is around, just not apparently in the context of off-grid.
At some point the extra complexity and risk of failure is not justified by the energy saved. Also, it requires the CC and the inverter to be closely coordinated.

The problem with a resistive load, of course, is that it has a fixed rather than a variable resistance. So you have to either control the voltage applied to it or switch the load on and off in multiple segments. The best way to deliver a variable power to a resistive load is via a DC-to-DC converter or an inverter combined with an SCR type current regulator.

And, no a simple diode network cannot do this since the CC needs to be able to draw power back from the battery at all times and sense the battery voltage, while the inverter needs to see constant power or trip out from low battery voltage or overload on its output.

**Naptown** · 01-14-2013, 06:04 PM

Take a look at the Morningstar tristar charge controllers. Not the MPPT but the PWM type.
these can be used as a load controller, charge controller and diversion control.

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http://www.morningstarcorp.com/en/support/library/TriStar%20Datasheet%20English.pdf

This may do what you want it to do. In an unoccupied cabin however the possibilities for an opportunity load other than perhaps running a heater of some sort is limited.

**Roil** · 01-14-2013, 06:08 PM

Originally posted by Naptown

Take a look at the Morningstar tristar charge controllers. Not the MPPT but the PWM type.
these can be used as a load controller, charge controller and diversion control.

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http://www.morningstarcorp.com/en/support/library/TriStar%20Datasheet%20English.pdf

This may do what you want it to do. In an unoccupied cabin however the possibilities for an opportunity load other than perhaps running a heater of some sort is limited.

Yes but can it be done in the combination with a MPPT controller?

**Naptown** · 01-14-2013, 06:11 PM

Originally posted by Roil

Yes but can it be done in the combination with a MPPT controller?

Perhaps, I have not read the entire manual. I believe these can be used in conjunction with a Morningstar tristar MPPT but read the manual and check with Morningstar to be certain.

**inetdog** · 01-14-2013, 06:14 PM

Originally posted by Naptown

Perhaps, I have not read the entire manual. I believe these can be used in conjunction with a Morningstar tristar MPPT but read the manual and check with Morningstar to be certain.

When an MPPT controller is set up to work with a wind or water turbine, it is no longer in classic MPPT operation, and the dump load required by the turbine is usually provided separately for when the MPPT controller is not drawing enough. Maybe there is a config somewhere in the manual though.

**Roil** · 01-14-2013, 06:20 PM

Originally posted by inetdog

If you do a google search for power management systems, you will eventually find a company that sells something to the UK market that regulates the opportunity load to exactly null out any power returned to the grid in a grid tie system. The only reason this makes sense it that the UK tarifs pay the user more for each KwH generated and used on the premises than for the same amount of power generated and returned to the grid. So the concept is around, just not apparently in the context of off-grid.
At some point the extra complexity and risk of failure is not justified by the energy saved. Also, it requires the CC and the inverter to be closely coordinated.

And that coordination is the challenge of course, thats why I believe what it takes is a MPPT controller with a built in Regulator capable of diverting the computed excess energy to a DC load, it would be impossible ofcourse to run an inverter on this energy.

Originally posted by inetdog

The problem with a resistive load, of course, is that it has a fixed rather than a variable resistance. So you have to either control the voltage applied to it or switch the load on and off in multiple segments. The best way to deliver a variable power to a resistive load is via a DC-to-DC converter or an inverter combined with an SCR type current regulator.

This is what the Tristar PWM is capable of doing isn't it?

Originally posted by inetdog

And, no a simple diode network cannot do this since the CC needs to be able to draw power back from the battery at all times and sense the battery voltage, while the inverter needs to see constant power or trip out from low battery voltage or overload on its output.

Kind of knew this, but the Tristar does have a separate voltage sense kabel, and if the controller would have drawn it's needed power from that connection instead of the main connection, the diode idea could have worked? The load would still have to be DC and regulated.

My wish would still be that all of this was built into the MPPT

**inetdog** · 01-14-2013, 06:30 PM

Originally posted by Roil

My wish would still be that all of this was built into the MPPT

Some of the designers visit this forum, so you may get some feedback from them.

Utilizing excess energy from solar panlel - not a dumpload question ;-)

Utilizing excess energy from solar panlel - not a dumpload question ;-)

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