| Making a solar water heater Beginning of solar idea Resource Final design |
| The
whole idea of lets just use standard elements and drive the water
heater with high voltage (120 to 240 VDC) power does have
merit. PV is cheap. Standard electric
water heaters
with standard elements are cheap. The trick is the power
control
to not burn the delicate AC rated relay/switches inside the ECO and
thermos. Here is an idea: I know it removes the UL listing because it mods the UL listed water heater but consider: Have a small 120 or 240 AC power source. Feed the water heater with it so that the ECO/Thermo receive the 120 or 240 VAC that they want but then use the 120 or 240VAC supplied at the bottom of the thermostats to drive a 120 or 240VAC coil relay(s) with *DC* rated contacts so its possible to switch the PV DC on and off and feed that to the water heater elements. For a typical water heater with two elements. Two relays will be required. One to switch the upper and one to switch the lower. End result: The ECO and thermos that come stock standard with the water heater are not themselves modified (just their wiring). They are never exposed to DC and therefore not driven by power they were not speced for. The DC is switched by a device that is UL rated to handle that power. There is no fancy shmancy controller that inverts the wave form that is power limited to 1250Watts. There is no loss of efficiency in converting the DC -> AC. Now I gotta go look up some specs. I have some big 40A power relays but not sure if they will handle 240VDC at 18.75amps (4500W/240V). -Dave |
| Attached
PDF, I don't understand the Table A: "Additional DC
rating with Magnetic Blowout" I don't know what that is or how it would increase the DC rating. ... but I like it! Need to do more research but these relays are fairly inexpensive. I run have two out of 4 of them (non-magnetic blowout) running in my controller box. Looks like the Magnetic Blowout type of AC operated only comes in 120VAC coil for this model series of relay but that is not a problem really. Table A: Limited to 20A at 110VDC, 15A at 144VDC, 8A at 220VDC So maybe solar limit is 2200Watts or find another model of relay that can handle more power. .... or unlike techluck which can not be paralled due to AC waveform phase issues, Have muliple strings of solar feeding multiple relay boxes. Another thought. It would be easy to wire for combo grid on top and PV on bottom (AKA Single Tank, Dual power type) with this idea. Just leave upper power element alone and wire the output from the ECO down to the lower thermostat and then use the output from the lower thermo to power the one relay needed for the bottom element. Techluck really needs two water heaters. This idea might be able to handle single tank setups. |
| Resource http://waterheatertimer.org/DC-relays-for-solar-panels.html |
| 3) Really good idea using
relays. Since the thermostats are mechanical switches... you could use any voltage that will activate the relay coil. 4) They make small cheap solid state relays rated for DC power... activate with low voltage https://amzn.to/3aYPhkc 5) Heres a variant on your idea. The key is finding correct relay if available Grab very low DC voltage from main PV lines... run the very low DC voltage through thermostats to run solid state relays http://waterheatertimer.org/images/Thermo-o-disc-upper-thermostat-Non-simultaneous-DC.jpg |
|
3) Really good idea using relays. Since the thermostats are mechanical switches... you could use any voltage that will activate the relay coil. You are very right! Could use any voltage or type really, just have enough current to drive the relays at the appropriate voltage. Could use 12VDC, 24VDC, 24VAC, 120VAC, 240VAC for typical choices. For my controller box I have a 120 to 12volt power supply in the box to drive the 12VDC relays with 120/240 volts on the NO/NC contacts. The key thing is to find relays that can tolerate the massive 20A or higher at 120 to 240 at DC current. The flexibility as to coil voltage opens the choice of relays. .... now to find one that isnt multiple hundreds of dollars each or the paralelling ide Still gotta lookup magnetic thing for the relays. Maybe it forces the contacts open/close quicker so they don't suffer as much from the arc flash thus preserving the contact material from being vaporized. |
| Let me know if you find a relay. |
| Resource http://waterheatertimer.org/DC-relays-for-solar-panels.html |
| They make 12 and 24VDC coil versions. This
one is only 25ampDC @ 250VDC but they make them up to 60 amp. Resource http://waterheatertimer.org/DC-relays-for-solar-panels.html I found them while looking for info on Magnetic Blowout. http://www.omron.com/media/press/2012/08/e0821.html Apparently the magnetic field of the added magnet (permanent or electro-magnetic) will legthen the arc of the flash and "blow it out" easier. Thus the rating on the DC contacts is increased. Or at least that's the theory as I read it. Learn something new everyday .... |
| Good find !! Very interesting research. Expecting you to develop a direct wire PV water heater kit ($399.99 + shipping)... DIY assembled using off-the-shelf parts. Look at these cheap 25 amp DC-DC relays, and give me your opinion Buy http://www.amazon.com/dp/B009AQN9CQ/?tag=waterheaterti-20 http://www.amazon.com/dp/B00FZPEBFA/?tag=waterheaterti-20 |
|
Expecting you to develop a direct wire PV water heater kit ($399.99 +
shipping)... DIY assembled using off-the-shelf parts. Big downside is that this whole idea does require mod to the water heater wiring (Bye Bye UL listing). Modding wiring is really quite easy to do on the Marathon (Ill send you pics of the conduit between the junction and element boxes). Other heaters I don't know where to begin. Either way, lawsuits & liability are probably not worth the 399$ - 150$ parts = 250$ profit. But it sounds good in theory! :) But if the thing can handle 4500 watts or whatever element wattage the performance and cost will blow away what the techluck can do. Look at these cheap 25 amp DC-DC relays, and give me your opinion Buy http://www.amazon.com/dp/B009AQN9CQ/?tag=waterheaterti-20 http://www.amazon.com/dp/B00FZPEBFA/?tag=waterheaterti-20 I like the lower cost of the SSR!!! The only Achilles heal for the SSR is apparently when they fail they typically fail shorted. An EMR will typically fail open. http://electronicdesign.com/components/electromechanical-relays-versus-solid-state-each-has-its-place The whole relay idea was too keep the ECO and thermos (but not the wiring to them) stock standard. Having a relay device that can fail without warning will remove 2 out of 3 of the fail safes built into the water heater relying on the last one (T&P) to save the day. If hot spewing water going all over can actually be counted as saving the day.... |
| Having
a relay device that can fail without warning will remove 2 out of 3 of
the fail safes built into the water heater relying on the last one
(T&P) to save the day. If hot spewing water going all
over
can actually be counted as saving the day.... |
| The following webpage was posted tonight. http://waterheatertimer.org/Convert-AC-water-heater-to-DC-water-heater.html I used quotes from your email if you permit ... plus I can add a link to your webpage or to anything you might be selling. |
| Looks
good! The quotes are fine. This whole thing is a
best left
in the open source experimentation realm because of the heater rewire
issue. It would be difficult to make a safe
consumer
install product let alone one that is UL listed. Diagrams: I suspected you made all your own diagrams. What software is that? One small change. The control side of the relay needs to be fed with low voltage DC (12 or 24 VDC coil or whatever range is good on the SSR). Maybe a 12 or 24VDC wall wart plugged into the wall. Or if you want it self contained a DC-DC step down converter. Wall wart power supply is cheaper. And the elements need something in the range of 120 to 240 VDC without going too much over. Too low of a voltage (like in the low 120 range) and the water heater element will only yeild 1/4 of its wattage. Too much over 240 and the element will burn out. Edge of cloud effect and cold PV panel temperatures increase voltage so extra cheap elements on hand will be needed for experimentation to find the sweet spot. For any given model of PV panel there will be a good number to run in series to get the optimal voltage. For instance I get about 30VDC at max power point on my 250W panels so 7 or 8 would be a good number to put in series. Now that I think about it ... It would be kinda freaky if this idea actually worked. Just don't know the gotchas. |
| Just
found this:. Its possible going slightly over the
voltage
of the element is ok as long as the wattage dissipated is not close the
to rating of the element. E.g. 8 of my 250 watt
panels is
2000watts. 10 of the panels is 2500 watts so it
will not
burn out the element. But I think its true as the guy says,
too
much mismatch between the volts from the panel and the heater element
specs puts the PV at the wrong place on the MPP (Maximum Power Point)
curve wasting some of its power. Resource http://www.solarpaneltalk.com/showthread.php?8230-PV-water-heater "Here is another way to look at it: If your element is designed for 240 volts AC, then applying 120 AC or DC volts to it is unconditionally not a problem (unless it is not in water, of course.) At 240 volts, it will pull about 4.17amps. Successfully applying 480 volts AC to it will destroy it. But if you have a panel whose Vmp is 480 volts and whose output power is 1000 watts, its Imp will be 2.28 amps, and its Isc will not be much higher, lets say 2.5 amps. Now if you connect that panel to the heating element, you will not be in any danger of burning out the element, because it will be able to pull at most 2.5 amps from the panel, and the panel voltage will be less than 240 volts at that time. It is a somewhat odd statement, but absolutely correct that if the panel maximum output power does not exceed the power rating of the heating element, then there is no combination of panel current and panel voltage that can damage that element. However, different combinations of panel specifications and heater specifications will determine whether most of that potential maximum power actually ends up in the heater or whether it is wasted because the panels are running far from their MPP. Just don't try it with batteries, generators, or power supplies because, unlike solar panels, they can be damaged by attempts to draw more than their rated power from them and they could damage the heating element while trying!!! |
| Let me summarize what I think is being said. Caveat that I am unfamiliar with DC power For self-contained unit, You suggest DC to DC step down instead of fuses? Will ordinary AC fuses work with DC? Do they make DC-rated fuses that will protect the line in event the relay shorts? For elements: The solar discussion forum is saying that AC elements will work with DC (as long as rating is not exceeded) ??? Are they also saying that AC thermostats will work without melting ??? Conclusion/ Confusion/ IF AC thermostats will work with DC... and AC thermostats will work with DC ... and IF fuses will work with DC ... why not just connect DC voltage to ordinary AC water heater and protect water heater with fuses?? I use $40 Photoshop Elements 7.0 for graphics work every day DC water heater image is a 70-piece, 70-layer, cut-and-paste mirage, just like assembling legos. |
| Let me summarize what I
think is being said. Caveat that I am unfamiliar with DC power No problem. PV solar is weird. I never understood the power curve thing for years. Then I had to figure it out when the MPPT tracking charge controllers came out a few years back. Anyways ... A solar array is a set of panels wired in series called a string. Each series panel increases the voltage of the array very much like stacking AA batteries together to get the required voltage for the device. e.g. Neg (0 VDC) ---->PV--->PV---->PV--->PV--->PV-->PV-->PV---> Positive (+240VDC Nominal) For self-contained unit, You suggest DC to DC step down instead of fuses Yes, lets focus on idea #1 self-contained DC only unit with no AC anywhere in it: e.g PV panels somehow feed the water heater. No AC, thus no ability to just plug in a wall wart to drive anything. |
| My
confusion with the diagram is that it shows the water heater elements
and the ECO/thermos/relay-inputs wired to the same
voltage. But it will not work since the
elements need
approximately 240 VDC and the relays that I have found so far only come
in 12VDC and 24VDC coil
voltages. I haven't
found a high current relay with 240VDC powered coil. "Fuses or DC-DC step-down transformer for prevented melted thermostats" Lets take this one at a time: Fuses: Fuses don't change voltage so if 240VDC is coming in from the panels then 240VDC will be connected to the input/control side of the relays. They aren't rated to take that high of a voltage so the SSR or the coil on the mechanical relay will burnout. DC-DC step-down converter. I don't know of any that take in 240VDC and step it down to 12 or 24VDC. Not claiming they don't exist. I just haven't seen any in the wild. So what to do? |
| I
think one good solution is to have a red wire labeled PV +240VDC coming
in from the panels and feeds the elements like you have in the
diagram. But then have a second wire of a different color (orange) labeled PV +30VDC also coming in from the panels (you would get this voltage by tapping into the lower middle part of the PV array, lets say the positive terminal on the lowest solar panel in series). Black wire from the PV panels labeled PV Neg like you have in the diagram. The orange wire goes like this: PV +30VDC -> Fuse --> DC/DC Converter---> top of the ECO Then another orange wire comes out of the thermostat and goes into input/control/coil side of the relay. Is the DC/DC converter required between the fuses and the ECO? If its an electro-mechanical relay: YES! They are picky about voltage. Putting 30+VDC into a 24VDC coil is going to make it fail faster. If its an SSR relay: Maybe. The orange wire voltage must always fall within the input range of the SSR. Be aware that PV voltages during cold winter morning can be sky high. Even above the VoC spec printed on the PV panel because they are so cold. Going way beyond the voltage limits of anything solid state tend to let out the magic blue smoke. A DC/DC converter can step down and regulate its output so that the relay always gets the proper voltage. Link for DC/DC conveters: http://www.trcelectronics.com/Meanwell/dc-dc-converter-sd15.shtml >>Will ordinary AC fuses work with DC? Some are dual rated. Depends on the fuse. I prefer breakers. There are some breakers that are DC rated. Squared QOU, midnight solar, outback. >>Do they make DC-rated fuses that will protect the line in event the relay shorts? Yes! But I like breakers. And now that you can get low amp DC breakers I love them way better than fuses... Resource http://www.solar-electric.com/mnepv.html (1 to 63amp options) DIN rail mountable! The solar discussion forum is saying that AC elements will work with DC (as long as rating is not exceeded) ??? Yes! The elements are clueless. Its only the ECO/Thermos that get obliterated by high current DC. Are they also saying that AC thermostats will work without melting ??? Yes! AC thermos will work with DC voltages ***BUT**** only if its low current. The kind of DC current needed to drive a 4500watt element directly will toast both ECO and Thermostats in short order. The thermos will likely burn up before the ECO since they switch on and off the most. The kind of DC current needed to drive a relay coil *should* be ok. I use $40 Photoshop Elements 7.0 for graphics work every day DC water heater image is a 70-piece, 70-layer, cut-and-paste mirage, just like assembling legos. Sweet! :) I love diagrams.... looking at them I mean. Glad you like making them. -Dave |
| Ok, thanks for explanation.. ...of course you are right about 240V ... the first drawing is completely wrong Will work on illustration showing low voltage drawn from 1 panel ... and will add more questions Here's another illustration taken from your general theory. Welcome critical review, naturally... Resource http://waterheatertimer.org/images/Thermo-o-disc-upper-thermostat-Non-simultaneous-DC-transformer.jpg Thanks for marathon photos showing conduit. |
| Will work on
illustration showing low voltage drawn from 1 panel ... and will add more questions Sounds good. Lets call that "PV Water Heater: Varient #1 Standalone PV DC only" Here's another illustration taken from your general theory. Welcome critical review, naturally... http://waterheatertimer.org/images/Thermo-o-disc-upper-thermostat-Non-simultaneous-DC-transformer.jpg Looks good. Awesome diagrams. Lets call that "PV Water Heater: Varient #2 AC power supply control" Whats the timer for? Save energy not running the relay during night? Im working on wiring for "PV Water Heater: Varient #3 Grid power upper element, PV powered lower element for single tank hybrid power application" |
| 1) Original drawing removed for obvious reasons. 2) Low voltage drawn from one panel: "PV Water Heater: Varient #1 Standalone PV DC only" Can you send a rough sketch how you would connect this? Including sketch (or description) showing the total number of panels wired in parallel that feed into water heater... with other necessary parts ... and suggested wire size. My drawing will mimic information from you 3) "PV Water Heater: Varient #2 AC power supply control" drawing is still posted http://waterheatertimer.org/images/Thermo-o-disc-upper-thermostat-Non-simultaneous-DC-transformer.jpg 4) Whats the timer for? People like options? Maybe folks are unaware of cheap 12-24 volt DC programmable timers? 5) Looking forward to seeing this project: .... wiring for "PV Water Heater: Varient #3 Grid power upper element, PV powered lower element for single tank hybrid power application" This is similar to techluck... except he converted to AC before feeding into water heater. 6) Marathon conduit certainly looks like off-the-shelf sch40 pvc |
| 2) Low voltage drawn
from one panel: "PV Water Heater: Varient #1 Standalone PV DC only" Can you send a rough sketch how you would connect this? Including sketch (or description) showing the total number of panels wired in parallel that feed into water heater... with other necessary parts ... and suggested wire size. My drawing will mimic information from you Sketch attached. I butchered your nice drawing with a paint program... LOL! Number of panels to use in series (Or even series parallel) will depend on : A) Voltage of element B) Wattage of element (Really A + B its a resistances (Ohms) thing) C) Model of PV panel D) Number of PV panels available. This is where a double E degree would come in handy instead of my CompSci. :) That link to that thread I sent you some guy wrote that Impedance should match solar array max power point according to the model specs for the PV panel used in the array. That concept needs testing. Personally I would try to hit the voltage expected by the water heater element but ... like I said PV panels are weird. They are NOT like batteries, grid power, generator power. They are kinda freaky. Suggested wire size (ampacity) and breaker sized is determined by NEC code according to expected PV output in amps. Derate for continuous operation 80% runtime > 3 hours. Yada Yada you know the drill. DC / DC converter required for highly recommended magnetic coil type relay. SSR fine for experimentation outside where its ok for water heater to explode with minimal damage and loss of life. Magnetic coil type relay required for production operation to keep the ECO and thermostat protection. 3) "PV Water Heater: Varient #2 AC power supply control" drawing is still posted Resource http://waterheatertimer.org/images/Thermo-o-disc-upper-thermostat-Non-simultaneous-DC-transformer.jpg 5) Looking forward to seeing this project: .... wiring for "PV Water Heater: Varient #3 Grid power upper element, PV powered lower element for single tank hybrid power application" This is similar to techluck... except he converted to AC before feeding into water heater. Working on it. Like I said it would be cool to get rid of the requirement for techluck (1000W=sucks) or a big ol inverter (4000W=expensive) when the PV power is just going to be disspated thermally. DC should be able to do it. But how to do it with what number of panels and doing it safely by not defeating the ECO are the big questions. |
| 1) Good illustration. Clear objective. 2) Oh, I see you can wire series and/or parallel Parallel more amps Series more volts 3) How many max volts and amps from each panel? 4) Are the wire connections on each side of panel, or one side, or back of panel? Are the connections screw terminals or pigtails? 5) Wire size is same for AC and DC 6) So the questions are? How many panels needed to draw low voltage. How many panels needed to draw full voltage. How to avoid too much or too little How to match element with expected voltage How to keep water heater safety features (I believe your $167 x 2 relays will solve that issue for certain) 7) Yes, I think were closing in on a solution |
| Gene, I'm not sure how to draw variant 3. I think the core of my idea was as follows: Feed 240VAC grid power into the top of the ECO just like a normal stock water heater.. If the upper is not satisfied then 240VAC grid will power the upper element as per stock water heater. The two elements are powered independently, Yes, the upper element can be consuming grid power at the same time PV power is heating the lower. The lower and the upper elements are protected by the ECO. NO second ECO required!!!. (BTW, this is not possible with the techluck product, without additional devices installed.) The lower element is CONTROLLED via relay + lower thermostat. The lower element is POWERED by DC PV solar array. Scheme requires a 240VAC to 12/24VDC power supply + one relay. The relay is POWERED by the 240VAC->12/24VDC power supply *IF* the 240VAC makes it past the ECO. Wiring: What I was thinking is that you take the wiring for a stock two element array water heater but instead of the lower element being powered when the upper is satisfied, its powered by a 240VAC to 12 or 24VDC power supply that feeds the input (coil) side of a 12 or 24VDC relay in series with the lower thermostat. The output (contact) side of the 12/24VDC relay is in series with the lower 4500W element and is powered by the DC PV solar array. Thus the lower element is: 1) Protected by the upper ECO 2) Switched on/off as determined by the lower thermostat 3) Powered by direct PV from the solar array 4) Runs independent of what is happening on the upper grid powered element. Take a stab at it and I will butcher it. I have my paint program ready! :) |
| Ok, will put together a drawing.... If I understand plan correctly: .... you want 240V on both thermostats. Upper thermostat is normal Lower thermostat controls relay... relay controls DC to lower element Upper element is powered by AC Lower element is powered by DC ECO will protect water heater by cutting power to water heater if overheated water is sensed Also consider: ECO trips when nearby wires are overheated, or short causes overheating nearby. Lower part of DC water heater might benefit from ECO.. ******************* Now help me with the particulars so drawing is more authentic. Do you use #10 wire on the PV panels? Are the solar panel connections on one side, or the back? Are connections made at terminals or pigtails? Send photo of wire connections. Are you wiring everything in series? Approximately how many solar panels? Approximately what dimension are the panels you are experimenting with? |
| Ok, will put together a
drawing.... If I understand plan correctly: .... you want 240V on both thermostats. No. Upper thermostat is 240VAC, Lower thermostat is 12 or 24VDC The 12 or 24VDC is created by the 240VAC->12/24VDC power supply. Upper thermostat is normal Yes, normal 240VAC Lower thermostat controls relay... relay controls DC to lower element Yes Upper element is powered by AC Yes Lower element is powered by DC Yes ECO will protect water heater by cutting power to water heater if overheated water is sensed |
| Yes,
It will cut both the 240VAC to the top element and because relay can't
activate it will cut the PV DC to the lower element. Also consider: ECO trips when nearby wires are overheated, or short causes overheating nearby. Lower part of DC water heater might benefit from ECO.. True, added safety feature, but not strictly needed right?. Also I can't replace non-ECO with ECO thermo on the lower part of a Marthon. Back plate not big enough. Take a look at the pics I sent. If a normal water heater experiences high resistance connection on a lug on the lower thermo will an ECO on the top thermostat trip? I guess an added ECO is an option, but will low voltage DC going through the ECO heat it enough to trip it? ******************* Now help me with the particulars so drawing is more authentic. Do you use #10 wire on the PV panels? Yes! AWG 10 will handle 30amps DC which is lots. Its my preferred wire size. The MC4 pigtails on the newer solar panels is typically #10 double insulated. My older ones have a junction box. If you want to get really fancy with multiple strings they can be combined into PV combiner box that has breakers or even fuses. Each string has its own breaker/fuse. http://www.altersystems.com/catalog/product_info.php?products_id=2265 Are the solar panel connections on one side, or the back? Most are on the back. Centered in the middle of the panel, maybe off towards one edge. Depends on the model. Are connections made at terminals or pigtails? Send photo of wire connections. I have 3 different models. Old ones (64Watt) have terminals. Medium old ones (175W) have MC3 pigtails. Newer ones (245W) are MC4 pigtails. I should round up some datasheets. We will need them to figure out the whole voltage, amps, impedance, resistance thing. Are you wiring everything in series? For solar hot water I would start with a single string. Once the optimum number of panels in string is determined (more panels = more volts) its easy I think to add more parallel strings with the same number of panels per string (more strings = more amps). Approximately how many solar panels? I don't know exactly, answering that question will make figuring out how to power the DC element(s) safely and drawing the diagrams look like a piece of cake. I'm guessing the magic number is 3 or 8 or some other number. The reasoning is gonna take a bit of time figure out cause im not a double E. I would draw the diagram with 4 or 6 panels in a single series string to give the general idea. Approximately what dimension are the panels you are experimenting with? I have Qty 15 of the old 64 watt ones I bought back in 1998 for my initial solar experments. They are about 5 feet by 3 I think. Old thin film. Not being used. Perfect for this experiment really. I would define success of this step as coming up with a formula to determine X. X = optimal number of panels in series. Given: Resistance of heater element in Ohms (figured from voltage and wattage of element) Vmp of single PV panel Imp of single PV panel And in the end to actually run the experiment with my volt meter and clamp on ampmeter. Attached some data sheets so you can get an idea of the differences between solar modules. The UniSolar 64 are what I have on hand to use. |
| ECO thermal heat protection is not necessary for
experimentation. Other details are noted: power supply, 10 gauge wires, pigtails on back, 4-6 panels etc. Will set up the drawing tomorrow. Thanks for solar resources ... you are a catalog of information. ############## I am exchanging emails with another fellow who is working on finding solution for solar water heater... like me, he is also very interested in your effort. Hopefully our collaboration page will yield webpage traffic and your ideas will spread. My images attract traffic to the website for sure ... and once we add your testing to the page... we will create something people will find useful for many years. If you are uncomfortable sharing your ideas with the world .. let me know. For example you can leave the final result un-revealed, or add copywright, etc. |
| Hi Gene! Before I get into horrible math and formula part of this problem (Im kidding its really not that horrible) I need to go over the MPP curve for solar panels or its not going to make much sense at all. Take a look at the datasheet I emailed you for the Sharp NT-180U1 module. On the second page there is a graph labeled IV CURVES. The DARK LINES on the graph show the MPP curve for this model of panel but what do they mean? Also of supreme importance is the Electrical Characteristics (just above the chart). The module properties of importance are Voc, Vpm, Isc, Ipm). and are usually based on a solar insolation of 1000W/M^2 Watts per Square Meter, Aka bright sunshine. at a module temperature of 20C/68F. Voc is Volts Open Circuit. Lay the 180W panel out in bright sunshine on a temperate day and a DVM in DC volts mode on the pigtails will read 44.8V The panel isn't connected to anything so there is 0 amps running through the wires. 44.8VDC @ 0A = 0 Watts produced when Resistance between the pigtails is infinity Ohms. Isc is Amps Short Circuit. Now do something really stupid and connect the pigtails together (Seriously do NOT do this in real life) but theoretically if you did a clamp on amp meter will read 5.60A for a short period of time while the panel is getting destroyed. The volt meter will read 0 volts so 0VDC @ 5.60A = 0 Watts produced when the Resistance between the pigtails is 0 Ohms (short circuit). So somewhere between 0 and infinity Ohms resistance this solar panel will produce approximately 180 watts of power. The question is what resistance is that? Looking at the specs the answer can be derived from the Vpm (Volts Power Max) 35.86V and Ipm (Amps Power Max) 5.02A specification. 35.86V @ 5.02A = 180.0172 Watts produced. Resistance = Volts divided by Amps (R = V/I) so 35.86V / 5.02A = 7.143 OHMSpm (Ohms Power Max) for a single 180W sharp panel. What happens if the perfect resistance is not used? The light colored line on the IV CURVES graph shows what happens. Not a big loss of yield if its close to OHMSpm but too far away the yield will suffer. So now its just a matter of figuring out: Panels/String Number of Strings Depending on: Element Voltage Element Wattage Panel Vpm Panel Ipm Easy! Gonna take a break before my next email with formula examples. Chainsawing up firewood while its nice out. -Dave |
| Im calling this part ... PV Array Series/Parallel Design for Direct DC connection to Water Heater: Originally I thought that all I needed to do is feed a 240VAC water heater element 240VDC or close to it and it will work perfectly. In the case of my 64Watt panels with a Vmp=16.5V using either 14 or 15 in series will come close to 240VDC. Of course my first take on this is wrong. A resistive element is totally clueless about volts and amps. It really doesn't care. Its specification on the store shelf might be defined in volts and watts but in reality it is just a resistor. Its real world spec is defined in Resistance (Ohms) and Power Dissipation ability (Watts). I see this effect already on my Solar Power controller depending on the sunshine im getting. Sometimes my water heater is fed 240VAC and burns 4500Watts (18.75A) and sometimes my water heater is fed 120VAC and burns 1125Watts (9.375A). So how to set things up so it all works right. Lets use a real example: I have Qty 15 of the Unisolar-64 laying around not doing anything. I have a 4500 watt element water heater. The heater element resistance is V*V/W or 240*240/4500 = 12.8 Ohms In order to maximize the power output from my solar array I need to match the impedance (In Ohms) of the panels Z=V/I with the resistance (In Ohms) of the water heater element. The UniSolar-64 specs from the data sheet Vmp=16.5V and Imp=3.88A Impedence Z=16.5V/3.88A = 4.25 Ohms. Totally does not match the resistance of the heater element. But if I take 3 of these panels and put them in a 3 panel string I get a string voltage of 16.5V * 3 = 49.5Vmp and keep my Imp of 3.88. My string impedence is now 49.5V/3.88A = 12.757 Ohms. I got lucky! Its almost matching the 12.8 of the element. Well now you say, 3 panels is only 192 watts and its going to take forever to heat up water to take a nice hot shower so lets just add more strings!. Cause that's what you do when you want to pump more juice into a charge controller but ... not so fast .. Adding more strings will not produce the expected results in this application. 2 strings of 3 panels each will have a Vmp of 49.5V and an Imp of (3.88 * 2) = 7.76A Each string will have almost matching impedance but the series/parallel array will be 49.5 / 7.76 = 6.378 Ohms. Way too low So what to do. Simple: Just put 3 more panels in each string for a total of 6 panels per string and 12 panels in the array. (16.5V * 6) / (3.88A * 2) = 99V / 7.76A =12.75 Ohms So now I have 768 Watts of power heating my water and 3 solar panels left over. Ok. So what. That just the same power output that can be handled by the crappy Techluck thing and I have lots of cash. I want to take unlimited hot showers on a sunny day. Free showers for the whole family ... and ... the dog. Just how many panels realistically could I use if I had unlimited funding, theoretically of course. Here are the options that all produce identical array impedance using the US-64 module: 1 string, 3 panels / string, 3 panels total, 192 Watts 2 strings, 6 panels / string, 12 panels total, 768 Watts 3 strings, 9 panels / string, 27 panels total, 1728 Watts 4 strings, 12 panels / string, 48 panels total, 3072 Watts 5 strings, 15 panels / string, 75 panels total, 4800 Watts. 4800 Watts! Wait a minute. That's a tad higher than the wattage rating of the element. Yup, Its a bit beyond spec but realistically an array typically does not pump out 100% of its rating except in perfect conditions. 95% of max is more typical so. I would be ok with that level of over powering ... Maybe. There is that darn edge-of-cloud-effect but that is very transient. Any questions so far? Im starting to think maybe this thing will actually work. -Dave |
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