 Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me. |
Forum Index : Solar : Solar electric hot water
Page 1 of 2   |
|||||
| Author | Message | ||||
| Darryl Newbie  Joined: 11/08/2013 Location: AustraliaPosts: 5 |
Hiya All I recently bought a 24V 500W water heater.I am using it to heat a home hot water system with Solar electricity the set up is as following. Solar panels 4 x 250W 24Volt poly. this feeds into a Tracer 24V 20A solar rectifier from here into 4 x 6V AGM batteries the heater is connected to the load circuit the battery is 20A fused and the heater circuit is 20A fused. I have 2 "Watt's Up" type meters one in the battery and one in the heater circuit. The batteries are new and fully charged and are 27.5 - 28.2 whilst on the charger. When i turn the heater on the Voltage drops to 24.3 and it draws 28A so i turn it off immediately i don,t want to burn out the heater. My electrical knowledge is limited so can you tell me what i am doing wrong and how to fix it. thanks |
||||
Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
28 Amps sounds OK. Watts is Volts times Amps, so in your case, the heater was drawing 680 watts. Allowing for tolerances of the Watts Up meters and the heater element, its probably within ratings. Also the amps will drop slightly as the water heats up, heater elements behave like that. Even if everything was spot on, it would be drawing just over 20 amps. You may need a 30 amp fuse on your heater circuit. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
||||
| davef Guru  Joined: 14/05/2006 Location: New ZealandPosts: 499 |
Darryl, I sent you a pm. Dave |
||||
Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Heating water with solar power via a regulator / battery storage seems to me a very expensive way of doing it. Why not heat the water directly from the solar panel power and do away with the other bits? If water heating is all you want from your solar set up that is the way I would do it, using the water heater thermostat to disconnect the solar power when the water tank is hot enough. OK, perhaps you want to do more with your solar power, such as heating the kettle occasionally or powering a few lights. Then your battery set up is the way to get solar electricity at other times when the sun goes to sleep. But, unless your battery bank is huge (1000+ Amp hours) it is better IMO to heat hot water directly when the sun shines. I doubt your battery bank is large enough to support a 500W heater, which may be on for a considerable time, and not drain the batteries down to low charge levels and drastically reducing their life cycles. Aim for only using 25% of battery capacity before recharging if you want them to last a few years. FWIW, I have 800 Amp hours of battery capacity here @ 24V and would not consider connecting a 500W water heater to them. I do use a 600W kettle but that's on only for a minute in the morning *if* there is good sunshine forecast for the rest of the day. Klaus |
||||
| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
Subsequently found out that he has burnt out one heater already. If the heater is is fully covered in water in a normal hot water tank situation and you are putting 24Volts on it, which is what it was designed for AND then it blows up that tends to suggest to me that the element is not really designed (properly) for this application. I would be talking with the manufacturer. 680Watts versus a 500Watt rating does not sound like the manufacturing process is under control. If the batteries are up at 28Volts then the thermostat switches on you will get more than 28Amps for a short period of time. I would be surprised if this would cause it to blow. > Running the heater straight off the panels Would not 1KW of panel capability in full sunlight put much more than 24Volts on a 500Watt heater element? |
||||
| Darryl Newbie Joined: 11/08/2013 Location: AustraliaPosts: 5 |
Hiya all me again It looks like i should fill you in on the whole picture. I am retired and this is a learning, hobby, thing I have 240V power to the house and do all the normal things with it eg.heating water when i need to, in summer this can be as little as once a week or even less so the success of this project is not vital. I have in the past set up two 12V pv systems for traveling, one on my motor cycle and one on my van, they both worked beautifully, hot meals and cuppas whenever i wanted so when the cash hungry politicians "privatised" our power to try and hang onto their power trip now our cheaper electricity has almost doubled in price (nothing to do with any carbon tax). I decided then why not do for the house what i have done for the bike and van just to keep the hot water warm for a longer time, not actually heat the water up totally but to just to top it up. So that is what i have done. Unfortunately the first attempt failed in that the element burnt out rather quickly because, I think that i trusted that this element, the same as below, would act like my little 500W bar heater i use in winter and just warm up to use the 500W that it was designed and manufactured to do. With this second attempt using another as described $120 emacandco, 24 VOLT WATER HEATER, IMMERSION, solar wind turbine 24V with a Regulated dedicated 24 Volt DC Supply from a storage battery / accumulator system, to feed the Immersion heater even with a regulated power supply of 20A at 24V ie 480W it actually uses 750W at 29.8A, there today's figures i left it on, the 20A fuse blew when it reached 29.8A. and i said a few naughty words. So i hope you can see that i am becoming frustrated with this "exact" science of electricity. My past work history is that of Clinical Nurse Specialist, Intensive Care and can you imagine the whaling and moaning from the populace if medicine was this accurate in its work. The PV panels are 4 x 250 polycrystalline, there are two separate 10A power leads with the active, neutral, and earth wires in each joined carrying the power the 30M to the heater it all went thru a tracer (same as Davef one) regulator, now also doing strange things (same as Davef one) the rest is as described in my first post Thanks for your patience,sorry about the life story, (I have enjoyed it). hope i have done something or maybe a lot of things wrong so that i can be corrected darryl |
||||
| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Well, I have 1.2 KW of solar panel power here and because they are on trackers they always get full sun. So, the highest voltage I have seen going into my MPPT controller was around 44V when the battery bank was equalising at ~29V. The equalising current was around 1-2A at this stage. On bulk charging, at 30A+, the panel output voltage sits around 30V. It would not be exactly the same with a 500W heater load but I doubt the panel voltage goes much higher than the quoted (panel specs) maximum power voltage (NOT max open CCT) of the panels. If the heater blows with that then its not up to the task, whether running of a solar charged battery bank or directly from the panels. I would be wary using an immersion water heater for this task anyway, these things may be great for camping when you are next to the pot it is immersed in and can watch when its boiling or the heater has moved a little out of the water. They last only seconds without water cooling. Klaus |
||||
yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
I would say that if you are pulling more than 20.8 amps in the regulated 24v supply then the element is not 500w 24volt. its either 800watt 24 volt or 500watt 12volt or there could be two elements that are supposed to be wired series and are instead wired parallel or one element is earthing through the tank or there is a short circuit. whatever the cause the heater element circuits resistance is way less than it should be. There is no fancy science here, the length of the nichrome wire in the element determines the resistance. 24v/20.8A = 1.15 ohms your element is more like 0.7 ohms wiring more resistance with something like under floor heating cable in the line will work but is pointless as you will be shedding heat (power) at the resistor and not in the tank which is kinda defeating the whole idea. I'm confused, no wait... maybe I'm not... |
||||
| Darryl Newbie Joined: 11/08/2013 Location: AustraliaPosts: 5 |
Thanks people especially yahoo2. this is the way my thinking, backed by my limited knowledge, was going. I need this kind of info so that i know when i question the supplier that he has sent me the wrong heater and i can ask for a replacement. I checked the resistance with my extech (not elcheapo) multimeter just now and it was 0.81OHM, with everything disconnected. So my readings and calculations that add up to the same as those you you have given me are correct. ie. 24.3V (reading whilst running)/29.8A (reading whilst running)=0.81OHMS(current reading). my faith in exact sciences are being restored. A warning that this supplier sends out with these heaters is that they should not be used as a dump load otherwise the warranty is voided and i think that connecting it straight up to the panels would qualify as a dump load scenario. If this is not right let me know please. Keep the info coming that result just seems to simple to be true. Darryl |
||||
| Darryl Newbie Joined: 11/08/2013 Location: AustraliaPosts: 5 |
hiya all last post on this item this is the result i got from my correspondence with emacandco Hi Darryl Very Sorry we are not experts on other manufactures equipment. We bought out a company that made these low Voltage immersion heaters all calculations on Wattage, length of heating element, type and thickness of heating element wire to obtain the desired Wattage at a preset Voltage where all worked out when we purchased the company. We use a simple chart to determine the element thickness and length to obtain the required Wattage, this is something we have been using for the last 8 years, we have sold over 2000 units which we manufacture and assemble here in Cheshire UK. Very Sorry I can't be of any assistance for you. Best regards Bob emacandco@aol.com EMAC & Co so i will pretend it is actually a 800W model and put in a 30A fuse and see what happens if it blows up i will shed a few tears then a few more naughty words and try something new, if it doesn't "what a great idea Gizmo why didn't i think of it". darryl |
||||
| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
So, the element would have about 30Volts on it if directly connected to the panel. If the 0.81Ohms is for real then the element would be trying to dissipate 1111Watts ... depends on whether or not the panel voltage would actually stay up at 30V. |
||||
| Georgen Guru Joined: 13/09/2011 Location: AustraliaPosts: 462 |
Would it be good idea to connect 2 elements in series? George |
||||
| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
It would definitely increase the chances of keeping the elements alive, but the power dissipated by them will be 1/4 of the power dissipated by one. Voltage being constant. In Darryl's application, operating two of them in series directly off the panel might be a good option. The panel voltage will go up with the lighter loading, so monitoring the panel voltage and working out the actual power dissipated would confirm suitability. Say the panel voltage went up to 35Volts with 2 * 0.81Ohm elements in series, you would get about 750 Watts or 335Watts in each element. |
||||
| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Just wondering if those solar panels could be configured for 12v? Some panels will let you set them up for 12 or 24v, and maybe a 12v system would work better in this case, just using the solar panels connected as 12 volts and fed directly into the heater, no controller. On a bright day they could reach over 18 volts, so no chance of damaging the heater element, but close enough to give 400 or so watts. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
||||
Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
I have not fully followed this thread as it seemed a total waste of energy and resources, with a pie eyed view to reinvent the wheel. 2 comments i have, first is why you would even want to take PV cells and use the power to heat water is beyond me when solar collectors will do a much better job, for a lower cost. Secondly why dont you simply buy a MPPT controller to feed the DC power to the heater element, or use a inverter to convert the solar DC to a working 240 VAC supply to the heater element. From what i had read thus far there is much dicking around with poor designs and little understanding to solve a problem that has more factors than an un skilled person can ever cope with. There is no magic solution of connecting wires one way or the other to solve the problem, its designing the problem out, and personally i think the OP has no knowledge of how to do this and no understanding of using energy to best suit a application. My advice is to use the PV to make power and use that in other areas of your house, and add a form of solar collectors to heat the water. In most cases hot water is run off "J" tarif (off peak) and that is less than half the price per kilowatt than normal (M tarif) supply charges. So why the hell do you want to take PV power and feed it into the lowest costing item per Kwh within your house dont add up to me. Lets say you pay $0.30 per Kwh for normal power and $0.12 for off peak (J tarif) (water heating) so why do you want to pump all your energy produced into a lesser costing consumption makes no sense to me. Perhaps you work with my sister in ICU as she too has backwards view of using energy, where its like taking the ICU budget and spending it in a general ward application expecting it to help ICU, and simply resulting in no greater return or benifit to ICU. Pete. Sometimes it just works |
||||
| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
My understanding is that this thread is about solving a technical problem rather than trying to save money. |
||||
| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
And there are advantages to using solar cells to heat water instead of solar water heating panels. Big one is the solar panels can be some distance from the water heater, and you can use the electric power to drive other loads as needed. Yes solar water heating panels are much more efficient than using solar electric, but every situation is different, and just because someone takes a different approach doesn't mean its the wrong approach. The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
||||
| Georgen Guru Joined: 13/09/2011 Location: AustraliaPosts: 462 |
Not to mention frost area (Just thought of it now after reading : "And there are advantages to using solar cells to heat water instead of solar water heating panels" ) George |
||||
| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
Georgen, Good point. I have to leave my solar controller powered all the time and one of the reasons is, in case of frost. 10Watts each hour to power the inverter and solar controller while I sleep :( |
||||
| Darryl Newbie Joined: 11/08/2013 Location: AustraliaPosts: 5 |
hiya all Thanks gizmo again you hit the nail on the head. For Downwind "the Guru"'s edification this for me is a hobby that i have just started. it is not a make myself rich scheme just a doing a small bit for the environment and learning whilst having fun. Gizmo has guessed correctly i don't live in the middle of the desert with 12 hours sunshine every day I live in a pine forest at the highest point in the Blue Mountains west of Sydney so at max 3 hours good sunlight and the panels are 30M from the house, Georgen is also right the max and average minimum temps this time of year are -3C to 9C, yesterday the max was 1C at 1400. My next scheme is to use a second hand solar hot water heater with an underground reservoir to stop the panel from freezing up. I hope Downwind isn't a plumbing expert as well as not even i can see this scheme working, but I'm going to give it a good try. Your poor sister. Darryl PS I shouldn't be so negative but unfortunately people like Downwind wind me up, so if this is a problem I,ll move on there are plenty of pimply faced teenager know it alls out there to discus things with |
||||
| Page 1 of 2 |
|||||
 Print this page |
