Author Topic: More panels!  (Read 29763 times)

BruceM

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Re: More panels!
« Reply #30 on: March 15, 2018, 03:57:04 PM »
There seems to be fine line between open mind and hard vacuum in humans.  Even in people very gifted in one area, such as Newton, they are often delusional in others (he was into mysticism and trying to turn base metals to gold).

We have such limited mental capacity that we must specialize, and that does make us very susceptible to being misled in areas outside of our area of expertise.  Many are incapable of any expertise.  So no surprise that even well meaning leaders in government (often lawyers or businessmen) will propose/fund/mismanage all sorts of programs.  Add that  many sociopaths are attracted to the power of elected positions, and you end up where we are today. 


BruceM

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Re: More panels!
« Reply #31 on: April 11, 2018, 05:53:25 AM »
No matter how you do it, working on a steep pitch is hard work.  My first home here in the White Mountains had 600 SF of solar hot air panels and duct work built into the south face of a custom trussed roof at 53 degrees of pitch.   I used two large ladders against the face of the roof.  One was an extension ladder we made a "hook" end for that hung over the peak, with the two sections screwed together. From ground level you went up a ladder than transferred to the roof ladder.  That way we could have one guy on each side of the 4 foot wide fiberglass glazing which was a bit over 13 foot long.  Two guys would be a must for 250 watt panels, if one of them was me. 

It's hell on the feet and legs to stand on a ladder that isn't near vertical. 

BruceM

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Re: More panels!
« Reply #32 on: April 11, 2018, 06:07:20 PM »
I've never washed my ground mounted PV panels,  but I do wash my solar hot water panels occasionally in the winter when they are working hard to keep up. 

oldgoat

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Re: More panels!
« Reply #33 on: April 12, 2018, 02:23:03 PM »
I thought the same as glort and goot on the roof and cleaned the dust and crap off my panels. Could have saved myself the trouble the increase wouldn't have even paid for the detergent I used.

LowGear

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Re: More panels!
« Reply #34 on: April 15, 2018, 02:04:23 PM »
I find the rat nests under the panels to be of greater concern than the grit on top.  We have more of a grit here than dust.  Our panels are at about 20 degrees.  That's about our latitude hence equinox gets us pretty close to perpendicular to the Sun.
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mike90045

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Re: More panels!
« Reply #35 on: April 15, 2018, 03:26:45 PM »
Quote
Still trying to figure out how to switch High voltage DC current so I can direct power the water heater. 

Are you inverting any of the power, or using all DC ?

The 2nd easiest way is to switch an inverter on / off and allow it to provide AC to the heater , 
otherwise, it's SSR's, snubbers, and heatsinks

ajaffa1

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Re: More panels!
« Reply #36 on: April 16, 2018, 01:32:47 PM »
I`m not quite sure that I understand the problem, most PV/grid tied systems have a timer switch which feeds 240 volt electricity to the electric heater on your hot water cylinder. This timer is generally set to utilise the free solar power during the day with an override switch in case of failure or overuse. All of these systems have a thermostatic cut-out to prevent the cylinder reaching boiling point and exploding. If you really want unlimited shower/bathing facilities fit a larger, or secondary cylinder. If you want them to be totally solar powered use a liquid solar system with heating tubes on the roof and a small circulating pump.
Trying to feed 400 VDC through an immersion heater element sounds like a recipe for disaster, but maybe I have misunderstood what you are trying to do.

Bob

BruceM

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Re: More panels!
« Reply #37 on: April 16, 2018, 05:42:23 PM »
Warning!  Danger!

DO NOT use AC rated breakers or switches with DC over 24VDC.   I've tried that experiment with only a 500 watt load and fried a lot of heavy duty and commercial grade switches on the very first OFF.  Arcing and melt down is the result.  Putting a capacitor on the output won't solve the problem.  That can help with inductive kick back but that's not the only issue here.

I know you like to experiment and see things for yourself so make sure you use the 300V DC rated breakers to be able to reliably shut down.  Don't exceed that rating at the rated amperage.  DC arcing is dependent on both voltage and current; very light loads won't arc as much on opening the contacts.  As voltage and current go up, the arcing gets very strong.  In things like DC driven rail/subways, they actually use compressed air to blow out the arc.  In HV DC rated switches and relays, they use permanent magnets to bend the arc plus other methods to quickly move the contacts far and wide apart to extinguish the arc.

Unless you have found a Buck  converter (simplest of PWM) to regulate (lower) the output voltage, you must keep your string series voltage under load to match the rated heating element (RMS) voltage (or less).  Electro-mechanical relays for HV DC aren't commonly available.  If you don't want to so some component level soldering to add a opto-isolated gate driver IC to control a surplus IGBT module (cheap on Ebay) to do your own solid state relay, then you'll have to search for a solid state relay rated well above your operating voltage and current.  AC relays can't be used...they will arc and fry.

I use 120V nominal DC for my home, and the no load voltage of my PV array of 5 panels in series is 220 volts on so on cold days.  On cold days under load, the voltage is about 20V higher than needed for my 146 volt battery charging.  There is really that much swing in output voltage based on temperature.  Normally, your PWM/MPPT charge controller hides that variation from you, but for direct DC use, you have to think about things a lot more.

You must match the loaded PV string voltage to the load unless you can find a PWM/switching power supply to take a higher voltage and down regulate .   Most large switching power supplies with 230VAC rated input can be operated with 350VDC (Max- no load PV voltage) input.  It might be possible to find a large one or put two or more in series to get a useful DC output.
The issue will be the PV string no-load voltage exceeding the max input voltage...that will kill them fast.  Direct from PV well pump drives are designed to handle that big voltage range on input, so are another possibility.

I can make a schematic for the opto-gate driver (8 pin dip)/IGBT module method which would give you a SS relay that is bulletproof and can tolerate 500VDC.  But it is really the sort of thing for a electronics technician/hobbyist.

For direct DC drive, here's an example.  US water heater elements  are typically 4800 watts at 230VAC (same as 230VDC).  That gives us an element resistance of 11 ohms. 
At 200V it would be 3636 watts and 13.2 amps.
At 120V it would be 1309 watts and 10.9 amps

So as long as you keep the full load voltage of the PV string at or below the rated voltage of the heating element, you are right in that there is a wide range of useful heating being done on unregulated direct DC.  Brief moderate overdrive of the element won't fry it.  But remember to allow for the 20% boost in voltage of the PV on very cold winter days.  I would guess that you might be best off using one or two parallel strings of (9) 250 watt panels in series depending on how fast you wanted to heat the water. A single series string will be under 2000 watts actual.   Your no load voltage would be roughly 396 so a 600V IGBT module  is a good match. 

Please note that you cannot use the existing water heater  thermostat to switch the DC power directly, it must be done through your solid state relay.  The input to the opto isolator would be about 10 ma of 5-12V; that could be switched through the thermostat.

I use 120VDC for cooking and and other resistive heating appliances every day, and operate all my computer gear on it as well.   I use HV mosfets for switching loads in cooking appliances, using the built in thermostats to only control the mosfet(s).  For simple things without an active switching thermostat such as crock pots, no modification is  needed.  Voltage regulation of the DC isn't really needed since power co. AC was never well regulated either. 

I hope this helps.  I was not impressed with the prices of large switching power supplies, though I did look into that.  For myself, I'd go direct PV to IGBT module approach.  With some 4000 series CMOS logic and a second IGBT module, you could automatically switch the PV array low side back to an inverter when the water tank temperature was maxed out.  I can help with that as well, again, just one 16 pin dip IC will do it.














« Last Edit: April 16, 2018, 06:22:06 PM by BruceM »

BruceM

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Re: More panels!
« Reply #38 on: April 16, 2018, 05:52:28 PM »
PS, a DIY solid state relay ala opto gate driver and IGBT module is pretty simple.  A buck converter (PWM) is not something I would recommend you should try to tackle as a first electronics project. Frying the $50 IGBT module is the likely result, and an o-scope would be essential.  The same IGBT module could be used for either.

ajaffa1

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Re: More panels!
« Reply #39 on: April 16, 2018, 11:30:11 PM »
Hey Glort, Bruce is absolutely right about dc relays being dangerous, don`t do it!
My suggestion is to use a standard grid tie inverter. These only produce power when there is a 240 volt 50Hz supply connected to them, they produce nothing when there is a power cut. If you put your timer and the water heater thermostat between the grid and the invertor it will only produce electricity when there is a demand for hot water. Carefully matching the size of the invertor to the heater element should alleviate your issue with back feeding the grid.

BruceM

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Re: More panels!
« Reply #40 on: April 17, 2018, 01:44:06 AM »
Great solution. Bob, ingenious!

What are your Aussie water heaters like, Glort?  Do you just one element or two, with essentially two thermostats, as we do here in the US?  Ours have the upper element on for fast recovery when tank gets too cold, the lower element is switched on when the tank is just a little cold.  Elements are each 4800 watt and mutually exclusive-  never both on.  Both elements are controlled by one "thermostat" module. 





BruceM

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Re: More panels!
« Reply #41 on: April 17, 2018, 06:30:40 AM »
Bob's GTI approach of only applying the AC to the inverter through the water heater thermostat switch seems to meet your goal of not backfeeding on that meter/circuit, if you limited the PV array to less power than the water heater element could handle.
With a double pole, double throw relay the inverter outout could also be directed elsewhere when the water heater was hot, this by simply having the coil of the DPDT relay fed by the WH thermostat,  if that is to be the highest priority.

I can imagine how after the luxury of bargain priced but working GTI's and a wide range of PV voltage, the details of direct DC seem harder.   

The range of heater element wattages does make it easier.  Your dual element units are exactly the same as the commonl US models.  Having two elements for two different sources could be quite handy.



ajaffa1

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Re: More panels!
« Reply #42 on: April 17, 2018, 09:48:41 AM »
Hey Bruce, love your idea of DPDT relay to divert power elsewhere when the thermostat on the HW system opens. Perhaps Glort should buy some of those storage heaters and redirect current to them, this would give him unlimited solar hot water and heating at night with no fear of back feeding the grid. Adding an electric AGA to the system would be costly but also give him free cooking facilities (they are basically a glorified storage heater). Hope his wife doesn`t ever see this post or an Aga because she will want one and they are really expensive, there are probably cheaper alternatives available.

Bob

ajaffa1

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Re: More panels!
« Reply #43 on: April 17, 2018, 12:33:08 PM »
Hey glort if you are considering and Aga or similar unit, try to find one that runs on heating oil. They have a gravity fed wick burner that will run real good on WVO, hell of a lot easier and cheaper than an electric version.

Just a thought, if you have spare electricity after heating your hot water, what about a hot tub? I`ve been lucky enough to have sat in one in the Swiss Alps in minus 30 degree temperatures after a long day of skiing. Very good for reviving parts that other beers can`t reach.

Bob

BruceM

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Re: More panels!
« Reply #44 on: April 17, 2018, 05:11:44 PM »
Snubbers are simply a resistor and small capacitor with values selected to suppress the oscillations (aka ringing) from switching or the EMI emitted when diodes start and stop conduction.  There is a method to determine the values for optimum snubbing by trying different capacitor values while watching the ringing on the oscilloscope. A 50% reduction then lets the engineer calculate the optimal values.  This is only a modest reduction in the ringing or EMI, and if you use your trusty AM radio near any wire connected to your GTI, you will see that there is a whole lot still left.  The values used for this are small,  and excess capacitance just causes power loss in that the power transistors must overcome the capacitance when switching on. Values of 0.001 to 0.1 uF and R values of 1-100 ohms would be ballpark ranges.

For simple on-off controls that aren't pulsing rapidly as in a PWM controller (say at 50,000 times per second) they are often not used at all. 

When turning off an inductive load, there is also an inductive kick back which can cause some issues- if the low side (negative) is being switched, when the load is off, the voltage goes high.  It can be more than the switching transistor(s) are rated for.  Here a small capacitor can absorb some of that spike and keep the voltage within safe limits.  Selecting the transistors with voltage rating of near double the actual use will often provide margin to be able to ignore "snubbing".  The value for the cap is small- 0.01 to 1.0 uF.  AC or DC are handled differently, and there are several techniques commonly used for DC relay coils, a notorious inductive spike generator when switched off due to the very high inductance.  The techniques may include both a diode and snubber.  If the switching is infrequent, and the voltage rating of the transistors is high enough, no snubbing is needed. 

Slower switching, with limited slew rate also reduces this inductive spike problem and this is the technique I use for switching DC electric cooking elements along with picking transistors with a fair amount of head room- typically 250 volt rating for switching 120-140VDC.  I use no snubbing and limit the power transistor on/off times via high value resistor to gate...this can only be done when using MOSFETs rated for semi-linear operation.

AC relays, switches and breakers on DC typically fail on opening. DC rated relay contacts must be way tougher, they must open faster and further. Bouncing of contacts is normal so each open or close is actually a series of events...and even closing causes arcing.  DC also means that metal will always be transferred in one direction between contacts, which is very hard on the contact life.  Adding capacitance will  increase the current on closing of contacts but will reduce it on opening.  Arcing on opening will be helped by capactance since the difference in voltage seen at the contacts at opening will be smaller, and the current across the open contacts will be reduced somewhat.  So capacitance can improve things.  But I would not normally use AC relays, switches and breakers in and around the home on high voltage DC.  They WILL fail.

Even my 150VDC rated wall light switches fail over time.  Lights with regular daily use of say 10 times a day will start to fail (sound of arcing on switching) in about 5 years.  The old design rotary lamp switches also start to fail on DC in time, the ones most often used, again in about 3-5 years.  I have metal electrical boxes and all wiring in metal conduit, so there is no risk of fire.  The solid state switches (HV mosfet) in my cooking appliances have never failed...solid state switches are the way to go for DC.

DC mechanical switching is very different from AC; AC has spoiled us because the interruption of current flow every 8 or 10 milliseconds (for 60Hz or 50 Hz power) means arcs stop almost immediately, so all mechanical electrical contacts have a much easier and longer life. They are "value engineered" to just work well enough for AC.  The exception is rotary lamp switches which are still the same as the original DC version.  I also did find one double pole switch that has worked well for 120VDC at up to 3 amps ...but it is no longer in production, and I blew over $200 on testing others to find not one that would work.

"What I was wondering was if having a constant small load would help with the arcing?
My idea which I'm sure is way too simple to have legs, was to have something like a pair of 25W globes in series ( for voltage handling) across the switched terminals of the relay so effectively there was power to the heater element all the time.... Just very limited and inconsequential.
If the circuit was not completely dead, would that give the power a path to follow and reduce/ eliminate the arcing when the relay switched?"


The parallel small load would reduce switching current and voltage only very slightly. Not enough to allow use of AC relays and breakers.  Some AC breakers are said to work for 12/24V DC, but since the breakers sold by Midnight Solar are DC rated and approved (150 and 300V versions), it seems foolish to tempt fate.  Even with breakers, it is recommended that  for an off grid system, a DC rated fuse of high rated current be used between battery and breakers since a sudden dead short could cause instantaneous currents so high (before the breakers can open) that the breakers contacts will weld closed.

DC is much better (4x) than AC for shock safety, but greater caution needs to be taken regarding mechanical switching contacts.

"If I put a lamp between the AC side of the inverter and the supply, would that allow the  inverter to sync with the grid and stay connected?"

 Alas, no.  The inverter will immediately see an overvoltage when it tries to feed the circuit due to the resistance of the lamp.  Same as if you used a long run of tiny wire between the GTI and power panel.