Using a Classic with a DC heating element

[Eric L]

I have a slightly unusual situation which has allowed me to have a working battery-based pv system (grid supported) but doesn't allow any easy way to implement solar thermal for hot water. Basically, the only clearing near our house suitable for solar is about 220 feet away from both the breaker panel and from the hot water heater. In between are a lot of trees which are staying put. It was possible to dig a trench by hand to bury wire in conduit, which I've done with the pv system, and it works well with two Classic 200s. But running pipes for a solar thermal system over that huge distance around all the tree roots, etc., is out of the question.

Consequently, I had always assumed that the best hope for ever getting solar hot water would be to someday expand my pv array enough to run a hybrid hot water heater from it, until I saw this thing:

http://www.presolarnet.com/products/liberty_box.htm

If you click on "PV Panels for use with the Liberty Box" you get some specs on the box itself. Then you see that it is input limited to 66 VDC from the panels. For my situation that's not enough voltage since it would require using very heavy wire.

So question: Could something roughly similar be rigged-up with a Classic (such as a 250 Lite, say), a small battery, some dedicated panels, and DC hot water heater element? I realize it would be a different system design from the Liberty Box which says it is a "digital impedance transformer that transfers the water heater element impedance according to the sun energy density. The more sun energy the lower the impedance the higher the heat transfer to the water tank".

My somewhat different idea is that there would be, say, a 1KW series string of panels running to the Classic at a fairly high voltage (190 VOC, say). That would keep the wire size reasonable. The Classic would be connected to a small battery bank at perhaps 24 volts (as small as possible to get everything to work) and a 24 volt DC hot water heater element. But, the idea would be to not have the heater element draw from the battery bank. The battery bank would basically be there to keep the Classic on and to regulate voltage. The heating element would be active only when there was sufficient input power from the pv (this is how I understand the Liberty Box to work as well). The batteries would at or near float voltage.

Could a Classic be configured to get something like this to work, perhaps using the diversion load function (which I've never used)? Or do I need to put down the crack pipe (figuratively speaking, of course)?

By the way, the idea behind the Liberty Box is that pv panels are getting cheap enough that this kind of hot water system could be cost effective.

[Photowhit]

Hi Eric,

I was curious about this as I have too much array for my battery and am looking for ways to utilize the energy. It appears in this PDF of their test results that this designed to increase the voltage! ...no it doesn't make sense to me either, they could have started at a higher voltage, but perhaps it has something to do with running a resistance load from a charge controller without a battery.

http://www.usa-eds.com/userfiles/file/Liberty_Box_Test_Results_final.pdf

The company , http://www.usa-eds.com/ ,that's creating this is from my home town, I think I even went to school with the electrical installer! I'm trying to find out more info and will pass it along if I have any discovery.

[Halfcrazy]

I have my Classics about 200ft from my water heater. What I did is pull a signal wire (Cat5 Cable) and I used the Classics Aux1 to run a solid state relay at the water heater to turn it on. So my inverters actually power my 3500 watt element.


Ryan

[boB]

Here is a post on the Otherpower forum that is relevant to this but is for
an AC water heater and uses an Aux 2  PWM function.


http://www.fieldlines.com/index.php/topic,147132.0

Remember that DC dump loads using Aux 2 PWM can be problematic because of
the inductance of resistors but water heaters may be less of a problem than
large wire wound resistors.

AC dump loads using AC SSRs is usually not a problem though.

boB

[Eric L]

Thanks for these great ideas. The idea of using a solid state relay off of the Classic's AUX function with an AC element looks so cheap and simple I'm going to try it.

Does anyone see a problem with using the same SSR as the guy on the Otherpower forum, but with a 120 volt AC 1500 watt element? I'd still try the AUX2 PWM settings he's using. Although my inverter is 240 volt, my solar distribution panel has only a single pole breaker left unused, so to go with a 240v element I'd have to disconnect another circuit, or get a bigger distribution panel. I'd also like the slightly lighter load on my inverter of just 1500 w.

My idea would be to power only the bottom element of the water heater from the inverter, and leave the top 240v element on the grid.

[mtdoc]

Hi Eric,

I was curious about this as I have too much array for my battery and am looking for ways to utilize the energy. It appears in this PDF of their test results that this designed to increase the voltage! ...no it doesn't make sense to me either, they could have started at a higher voltage, but perhaps it has something to do with running a resistance load from a charge controller without a battery.

http://www.usa-eds.com/userfiles/file/Liberty_Box_Test_Results_final.pdf

The company , http://www.usa-eds.com/ ,that's creating this is from my home town, I think I even went to school with the electrical installer! I'm trying to find out more info and will pass it along if I have any discovery.

It looks like they may have taken inspiration from This Paper

I see that their test results show that the PV power through the "Liberty Box" replaced about 4-5 kWh per day of AC use (AC for hot water went from 7-8 kWh to about 3 kWh per day.)

FWIW - I recently installed a Nyle Geyser Heat Pump on my 50 Gal hot water tank and am using about 5 kWh per day total to heat water for a family of 4.

[dgd]

Remember that DC dump loads using Aux 2 PWM can be problematic because of
the inductance of resistors but water heaters may be less of a problem than
large wire wound resistors.

AC dump loads using AC SSRs is usually not a problem though.

boB

Can be problematic how?
For a while I used aux 2 WASTE NOT HI to drive a Crydom D2D40 ssr that placed a 1kw 1.3 ohm ww resistor across the dc side of a rectifier from my ac turbine. PWM seemed to work nicely buzzing the ssr as it cycled the resistor into circuit... Never noticed any problems 
Still works now the resistor has been replaced by a 600watt 24vdc water heating element.
Dgd

[rumleyfips]

I need some overvoltage protection for my batteries when the wind turbine furling can't cope. The solar side is fine. I want to use an Aux funtion to divert to 3 clothes drier elements ( I have already durned up a 2k dump and two diversion relays. I bought a big contactor with a 110 AC coil but since figured that 110 seen between 2 legs is closer to 170v.

Which Aux would be best for a couple of Mitsubishi 600v, 75A IGBT's? There are too many choices but Aux1 diversion Hi seems OK.

My array, 5- 245 watt panels produces more than I can use lots of afternoons so I think I'll try opportunity with a water heater when I can find a good used one ( they're installing gas here now).

I have a no-name 2k Chinese 3 phase turbine, a no name Chinese 96 in 110 out inverter and 2 parallel strings of 8 Alphacell 105AH sealed gel cells.

John McManus 

[boB]

Remember that DC dump loads using Aux 2 PWM can be problematic because of
the inductance of resistors but water heaters may be less of a problem than
large wire wound resistors.

AC dump loads using AC SSRs is usually not a problem though.

boB

Can be problematic how?
For a while I used aux 2 WASTE NOT HI to drive a Crydom D2D40 ssr that placed a 1kw 1.3 ohm ww resistor across the dc side of a rectifier from my ac turbine. PWM seemed to work nicely buzzing the ssr as it cycled the resistor into circuit... Never noticed any problems 
Still works now the resistor has been replaced by a 600watt 24vdc water heating element.
Dgd

You were probably lucky.  If the Classic turns off (Resting) and the dump load is dumping lots of current, the voltage can skyrocket
because of the inductive kickback. When the Classic is running, it tends to hold its input voltage constant and you may
not notice a problem.  Also, the peak voltage across the SSR might go high enough to hurt the SSR if it is not strong enough.

Do you have an oscilloscope you can look across your SSR to see what it shoots up to ?
If your newer load is less inductance, then you might be OK too.

Our Clipper has to be able to withstand a few kilowatts of switching on and off and so we give it a bit more care than
if it was a smaller dump load.

boB

PS...   A snubber circuit also helps.

[jtdiesel65]

I'm looking into doing this too.

How would this work with two classics in Follow Me mode? Could you use the aux from each classic or would you have to only use the aux from one of them?  Sounds like you could use one from each.

[boB]

I'm looking into doing this too.

How would this work with two classics in Follow Me mode? Could you use the aux from each classic or would you have to only use the aux from one of them?  Sounds like you could use one from each.

One way to remember what is possible is to know that the Follow Me communications is fairly slow compared with the Aux 1 or 2 response time for
diversion.   So, each Classic responds independently for things like that that need to be very fast.

That does not mean though that there might not be some AUX functions that will be Followed from another Classic in a future
function.

boB

[dgd]

You were probably lucky.  If the Classic turns off (Resting) and the dump load is dumping lots of current, the voltage can skyrocket
because of the inductive kickback. When the Classic is running, it tends to hold its input voltage constant and you may
not notice a problem.  Also, the peak voltage across the SSR might go high enough to hurt the SSR if it is not strong enough.

PS...   A snubber circuit also helps.

I bought the Crydom d2d40 ssr 200volts 40amps to give some voltage headroom inserting it in the DC input to a classic 150. The turbine was 700watts at 24v and dump load 1000w 1.3ohm resistor.
The Crydom data sheet stated "Inductive loads must be diode suppressed" so following the wiring diagram I had inserted a spare Schottky power diode.
I kept it when change made to water heating element.
So that was my snubber..
Thanks for explaining

Dgd

[Psyfy]

I have a slightly unusual situation...

Forget the Liberty box. I know exactly how to solve your dilemma.

What how many watts of PV do you run, what is the avg daily Kwh (over the year) and what is the string voltage?

[dgd]

DC water heating update..
Its been three months now and my DC water heating system has been working well.
This uses a 120V 2Kw heating element  (from Ebay for under US$15+post) connected into a 180litre mains pressure water cyclinder.
I have my PVs connected in strings of 4, 5 strings in total. Each PV is 140W working V is 17.6 giving about 70v per string. When in FLOAT or ABORB and there is excess PV power I notice the input voltage rises to 75.
So connecting this to the heating element means I get about 780w max  (2000w/120v gives current 16.67A and element resistance of 120/16.67 or 7.2ohms. Connected to 75volts means current is 75/7.2 or 10.4amps so wattage is 10.4*75 = 781 watts). This is well within  the maximum heating wattage the element is designed for.
The Classic controls a Crydom D2D40 SSR (200volt 40amp DC ratings) with a snubber diode connected as outlined in the Crydom data sheet.
AUX2 setting WASTENOT high is used to switch the SSR, I think its in AC mode PWM.
This SSR is connected to the outputs of only two of the strings then after the SSR is a 75amp stud diode in the +ve line to stop current flow into the SSR from the other strings or Classic. I'm not convinced this diode is necessary but its there.

The element gets hot and almost every day I have hot water.
The SSR gets warm but not real hot. Its on a heatsink. I don't like the buzzing noises it makes. I suspect it may die sometime but it has went 3 months so far and still lives. Its over rated in this application so it will soon get swapped out for a D1D20 (100V 20 amps)
The stud diode stays cold always and it too is on a heatsink.

The C150 has worked normally and does not appear to have suffered and adverse effects.

Initially I tried the AC heating setup using an AC SSR, a Crydom D2450-10 connected to a 2Kw 230VAC element. It worked ok but my AC available power was limited to 3.3Kw (Trace SW3024E) and there were too many occasions when I felt uncomfortable driving the inverter towards its maximum output since its over 15 years old and never really pushed much over 2.5Kw output.
An option was to buy a cheap 2kw sine wave inverter and just use that for AC water heating, thats still a possibility..
The AC SSR run pretty hot and made some strange noises, I'm not sure it was happy with 100Hz or so PWM switching the AC current.

Anyways the DC setup is going nicely, the C150 has kept the smoke in and there is hot water aplenty 

dgd

[Psyfy]

DC water heating update..
Its been three months now and my DC water heating system has been working well.
This uses a 120V 2Kw heating element  (from Ebay for under US$15+post) connected into a 180litre mains pressure water cyclinder.

It's a good idea to raise the temperature of water instead of just dumping it into atmosphere. I do the same and presto, any excess BTU's boost my single SHWS panel that I have very nicely, a good match. The real beauty is that these 'Jug type' HWS elements are cheap and represent a purely resistive load with a pf of 1.0. So it doesn't matter if AC or DC is dumped into them as they just don't care.

I have my PVs connected in strings of 4, 5 strings in total. Each PV is 140W working V is 17.6 giving about 70v per string. When in FLOAT or ABORB and there is excess PV power I notice the input voltage rises to 75.
So connecting this to the heating element means I get about 780w max  (2000w/120v gives current 16.67A and element resistance of 120/16.67 or 7.2ohms. Connected to 75volts means current is 75/7.2 or 10.4amps so wattage is 10.4*75 = 781 watts). This is well within  the maximum heating wattage the element is designed for.
The Classic controls a Crydom D2D40 SSR (200volt 40amp DC ratings) with a snubber diode connected as outlined in the Crydom data sheet.
AUX2 setting WASTENOT high is used to switch the SSR, I think its in AC mode PWM.
This SSR is connected to the outputs of only two of the strings then after the SSR is a 75amp stud diode in the +ve line to stop current flow into the SSR from the other strings or Classic. I'm not convinced this diode is necessary but its there.

The DC SSR acts as a diode as internally it is essentially a MOSFET.
MOSFET's are essentially a diode with a variable forward voltage. A relay would be a different thing of course.
What is interesting is that a Stud mount power diode exhibits a Forward Drop Voltage of ~1.35vdc. (I unavoidably have to use a 300A/800V one in the +Ve of the UPS in order to effectively disable the onboard VRLA charger circuitry).
Recall from Ohms law that P=I2.R and that R=V/I. So, at a 2kw resistive load into the HWS heating element, diode losses would amount to ~38 watts.

A better option would perhaps be to place a Schottky fast recovery diode in reverse across the outputs of the SSR to protect the SSR from any inductive spikes at 'turn off' and remove the power stud diode.

The element gets hot and almost every day I have hot water.
The SSR gets warm but not real hot. Its on a heatsink. I don't like the buzzing noises it makes. I suspect it may die sometime but it has went 3 months so far and still lives. Its over rated in this application so it will soon get swapped out for a D1D20 (100V 20 amps)
The stud diode stays cold always and it too is on a heatsink.

Interesting!

A DC device such as an SSR shouldn't buzz, unless it is being fed PWM that is. In which case, you can smooth the PWM pulse train from the CC by placing an electrolytic cap of ~>1.3 the circuit DCV(max) across the SSR input rails, sized at about 470uF per amp(load). So, as you have about 30amps flowing at peak to the water heating element, you could put for example either a 100vdc/10,000uF or 100vdc/15,000uF cap in there. They run at about $15.00 on ebay.
Get a large can type with M6 or 1/4" (or even M8, M10) screw terminals. You'll find that most electros have metric terminal mounts though. There's lots of other easons too but I'll just leave it there for the moment.

That should solve the buzzing.

The C150 has worked normally and does not appear to have suffered and adverse effects.

Initially I tried the AC heating setup using an AC SSR, a Crydom D2450-10 connected to a 2Kw 230VAC element. It worked ok but my AC available power was limited to 3.3Kw (Trace SW3024E) and there were too many occasions when I felt uncomfortable driving the inverter towards its maximum output since its over 15 years old and never really pushed much over 2.5Kw output.
An option was to buy a cheap 2kw sine wave inverter and just use that for AC water heating, thats still a possibility..
The AC SSR run pretty hot and made some strange noises, I'm not sure it was happy with 100Hz or so PWM switching the AC current.

As mentioned, DC is absolutely fine for this application as it is resistive. Just watch the distance:I2R ratio loses associated with LVDC.  Keep cable runs as short as possible.

Anyways the DC setup is going nicely, the C150 has kept the smoke in and there is hot water aplenty 

dgd

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