Solar panel DC power ==> column heater  - ridicule my idea...... - Page 1
Forum section: electronics / general

author: DTJ
author_rank: Super Contributor
timestamp: June 14, 2017, 11:07:00 am
content: Continuing on from a post I placed in the wrong sub-forum...Ok - I'm likely to have access to 8 x 190W panels in the near future, I can either offload them to the installer of my new system and get paid $400 trade-in or I can use them somehow.Has anyone used the DC from panels to provide heating during winter? I'm in Australia.I'm thinking along the lines of feeding DC from some of the panels direct to the element in a oil column style heater. I think if I keep the voltage under some value (170V??) there are no legal issues.Using DC at a higher voltage:Easier in terms of finding a suitable heating element (column heater)Dangerous as higher voltage DC cabling needed.Using lower voltage DC arrangement:~30Vdc with all the panels in // would be safer as a lower voltage would be present.Much higher current >40A perhapsHas anyone actually done this?It seems that using a plug in grid tie inverter in Australia is illegal and rigging these panels to a system generating mains voltage for isolated use may also be illegal if its connected to fixed wiring.The only other option I can think of is to see if the grid tie inverter I have can be hacked to run without being tied to the grid and run a mains heater off it directly.

author: Someone
author_rank: Super Contributor
timestamp: June 14, 2017, 11:47:19 am
content: Quote from: DTJ on June 14, 2017, 11:07:00 amRun as high a voltage as you are comfortable with, mains cable is usually rated for 400V+Quote from: DTJ on June 14, 2017, 11:07:00 amYes, its easy.Quote from: DTJ on June 14, 2017, 11:07:00 amYeah, its not always illegal but against your power contract. Either way avoid the hassles and don't back feed power to the grid unless its got all the signoffs.Quote from: DTJ on June 14, 2017, 11:07:00 amGood source of parts to build a DIY system from or hack to work as a standalone.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: June 14, 2017, 12:27:38 pm
content: AFAIK it's ok for non licenced people to string panels together up to 120Vdc using the 'nominal' voltage on the panels, you don't have to use the OC voltage.That should give you 3 or 4 per string. So you should able to parallel a few strings up if you have bypass diodes.This will keep you at extra low voltage where the AS3000 wiring regs don't apply.Probably want to make it all look like a professional install though. ie. DC isolator on roof. DC markings on conduit.Use the black DC cable if you can and standard connectors.

author: PTR_1275
author_rank: Frequent Contributor
timestamp: June 14, 2017, 12:33:06 pm
content: Be careful with switching the heater on and off (and thermostat if it's there) using DC. The switches and thermostat would be rated for AC, so the air gap of the contacts is smaller than the equivalent DC rated switch. AC crosses over 0 volts 100 times a second so the arc doesn't last long. DC doesn't cross 0v, so you can have a continuous arc.Turn the heater on (unplugged from the wall) and measure the resistance. This will give you a rough idea of what power at what voltage you can expect. If it's 2400w @240v (for example), the heater will be about 24 ohms. If you feed 50vdc into the heater, it will only dissipate 105w of heat. Increase the voltage and you will increase the heat output. Find the balance point with the panels you have, you might be better to Series a few panels and parallel that string with another string of series panels.

author: Codebird
author_rank: Regular Contributor
timestamp: June 14, 2017, 12:52:49 pm
content: This should work. A fully thermal system would perhaps be more efficient and lower cost, though also more difficult to install. Liquids are awkward things.I have a design for an MPPT optimiser designed for resistive loads like heaters, if that's any help.I also recall someone here months ago talking about their own design for an MPPT heater that worked in a rather novel way - connecting or bypassing heating elements in order to maintain optimal resistance to match the panels.

author: DTJ
author_rank: Super Contributor
timestamp: June 14, 2017, 01:00:32 pm
content: @Someone & HackedfridgemagnetThanks for the ideas & info guys.Are there standard connectors for DC applications like this? Maybe the 2 pole ones similar to a GPO but without a switch?@PTR75 Yeah I think if I do this I'd set the column heater up so it's simply a big power resistor with no thermostat.They are typically a couple of kW and all of the panels together are lucky to make just over 1kW.I don't think we'd be generating enough heat to have to regulate it, but it would be nice to lift the inside house temp by a few °C.@Codebird Thanks for the offer on the MPPT design but I'm just looking at a quick and simple installation for now. I've got plenaty of other things to keep me busy!

author: Seekonk
author_rank: Super Contributor
timestamp: June 14, 2017, 04:26:38 pm
content: A couple of things I don't understand. If you go on youtube and search PV water heating, there appears to be no one in the entire world that knows how to do it correctly without wasting a lot of potential PV energy. They are all connecting up directly to PV or going through an inverter in some discombobulated way. So now, you post on an electronics forum just about the worst way to do it without any regard to what might happen when the temperature gets to high. Were you seeking some kind of approval? There are a lot of ways you could do some easy pseudo MPPT control using anything from multiple heater elements, switching regulator chip to a small micro. Whatever you do, post it on youtube. That is where your idea belongs, not on a technical forum.

author: fourtytwo42
author_rank: Super Contributor
timestamp: June 14, 2017, 04:36:53 pm
content: It might help to have a picture of the panels specification label but I am guessing these are 72 cell panels ?In that case VMPP will be around 36V and IMP about 5 Amps.Given your extraordinary nany state laws in Australia and this limit of 120V the maximum number of panels you can series connect would be three giving Vmp of ~108V.You dont say what heater you are considering but if you use this calculatorhttp://www.rapidtables.com/calc/electric/watt-volt-amp-calculator.htmits fairly quick to find how much power you can expect from a given heater at MPP.Without an MPPT controller to match the suns output to the fixed heater load your performance will be quite poor, the problem is at lower insolations the heavy fixed load will drive the PV voltage down so the power will be way below MPP.I think you should take up Codebirds offer or just take the $400By the way as indicated switching medium DC voltages without correctly rated switches is a fire hazard, especially avoid built in switches or thermostats of household goods.

author: fourtytwo42
author_rank: Super Contributor
timestamp: June 14, 2017, 04:42:38 pm
content: Quote from: Seekonk on June 14, 2017, 04:26:38 pmI dont think the OP mentioned WATER heating! Also I dont think the OP was doing anything in a discombobulated way whatever that is. Why the obsession with youtube and what do you think is wrong with discussing the technical possibilities of what might be done in a forum for renewables ? Are you appointing yourself as a moderator on this site ?Personally I think we are all here to share our experiences

author: tronde
author_rank: Frequent Contributor
timestamp: June 14, 2017, 06:26:22 pm
content: Quote from: Seekonk on June 14, 2017, 04:26:38 pmI am not a native English speaker, so I might be wrong.With my understanding of the English language, the first post is full of questions with the intention of learning more. Your answer is mostly full of yourself...

author: jwm_
author_rank: Frequent Contributor
timestamp: June 14, 2017, 08:30:08 pm
content: You would want to run a heat pump rather than just do plain resistive heating for maximum efficiency. They can be 300% or so efficient because you not only get the waste heat (which equals what you would get running the electricity through a resistor) but also the heat pumped in from the outside.Another way to think about it is your DC supply has two resources to exploit, low entropy and high energy. Running it into a resistive heater makes 100% use of the energy but makes no use of the entropy sink. Running a heat pump makes use of the energy but also exploits the entropy to cause a thermodynamically unfavorable r thing to happen, heat going from the cold outside to the warm inside.

author: helius
author_rank: Super Contributor
timestamp: June 14, 2017, 09:02:46 pm
content: Quote from: Codebird on June 14, 2017, 12:52:49 pmhttps://www.eevblog.com/forum/oshw/digital-mppt-thermal-controller/