using solar to direct drive a resistive load - Page 1
Forum section: electronics / beginners

author: Jbates58
author_rank: Regular Contributor
timestamp: August 06, 2017, 11:25:19 am
content: hi alli am wanting to change my electric HWS from on grid AC (@~$.30/KW) and run it directy from solar, while i understand that it may not always be perfect, and what not, there are only 2 of us here, and its a well insulted 250L system, so should get at least 2ish days of good hot water from it if there is adverse weather, i am also prolly going to pump up the temp a bit and that should get me to 3 days.the element is a 240Vac, 3.6KW unit. comes in at 15ohms, using this, with the specs of the panels i am wanting to use, a 15ohm load on 7 of them in series, takes them to pretty much spot on their VMP/IMP range. so, that will give me ~1.75KW, and then i will connect another set into the system in parallel, taking it to 3500W, so just below what it will need, i have a feeling that this will drag it a little lower on the volatge side, which is fine, as the 7 of them is slightly out of VMP, so the extra 50W load per string should be spot on (or damn close to)but, my question is, with the AD vs DC side, will i actually need to use 240V DC? or as the AC is positive and negative half the time, the element only realy see's 120V positive, and then 120V negative. just not 100% clear on this.and to my understanding, as its a resistive load, and i have actually got enough panels to throw another 7 in to give me 7S3P, doing that will then cause it to be thrown way out of its range correct? as the load will be shared along the lot, causing the voltage to rise.for the control side, i am thinking of using an arduino and either some SSR's or using a large standard solenoid, and then putting a high vaule resistor across the 2 contact terminals as a pre-charge resistor. to my understanding, this will eliminate the arc when switching, as there will always be a voltage on the other side, and the current draw will minimal.i hope that makes sense, and i am atleast on the correct track of theory.Jason

author: SeanB
author_rank: Super Contributor
timestamp: August 06, 2017, 05:38:35 pm
content: Whatever you do add extra insulation to the water cylinder, at least double the existing thin insulation, and add as well insulation onto the hot and cold water lines, for at least the first meter of pipe on there. This reduces the static heat loss, and then you will find it takes less energy to reach setpoint, as the loss during the night is a lot lower.Power wise 240VDC is the same as 240VAC, as the conversion is based on the heating ability of the voltage, exactly like you want to do here, so just applying 240VDC will have the same heating effect. However you will still need to have the existing thermostat in the circuit, though it can be set at a higher temperature ( probably 80C) as it will be the safety thermostat, though they are not really DC rated, so you will need an extra non resettable 90C 10A thermal cutout as well in the supply wire in the cover, in contact with the cylinder inner but electrically insulated from it, to handle the case of failure of the thermostat and the control circuit, to prevent the cylinder heating up past 100C and becoming a large steam bomb.Control wise I would recommend not using a SSR, as they are generally only AC devices ( rely on the mains passing through zero volts to turn them off), and to use a contactor with a 16A DC rating. Very important to use a DC rated contactor, as it has to handle the arc on contact opening. Neat thing with solar panels is you can just wire them across the heater, and wire the contactor to short out the heater instead, the panels will not mind running into a short circuit, and using only the 1kW panels will still work, you do not really care if it takes all day to heat up the tank, even using 500W of panel will still likely get to set point during the day some time, even in winter with 8 hours of sun.

author: pantagruel
author_rank: Newbie
timestamp: February 01, 2021, 07:17:39 pm
content: I just made a video of how I use direct DC into my water heater .https://youtu.be/s7trVpHLts4Larry

author: langwadt
author_rank: Super Contributor
timestamp: February 01, 2021, 07:27:50 pm
content: Quote from: SeanB on August 06, 2017, 05:38:35 pmafaik the hot water should generally be no more than ~60'C to reduce the amount of limescale build up

author: fourtytwo42
author_rank: Super Contributor
timestamp: February 01, 2021, 07:48:55 pm
content: Dealing with switching a few hundred volts DC at a few Kwatts is a problem, putting a resistor across contacts is not gonna work! You really need something solid state.As Larry points out in his video VOC (Volts Open Circuit) is often high enough to blow standard off the shelf SSR's so you either need a high voltage one or like he has a high voltage IGBT.Your second problem is matching the load to the amount of sun your panels are getting, if you just slap a fixed load resistance across the panels you will only get full power transfer efficiency in a limited sweet spot but it's actually quite complicated to do anything better so it rather depends on your power electronics experience. Myself I use a buck/boost controller to do the matching, but others have tried many heaters switched in various ways (series parallel) and I guess you could do the same with the panels and a single heater, remembering that to switch high power DC you want something solid state to avoid fires!

author: wizard69
author_rank: Super Contributor
timestamp: February 02, 2021, 11:58:50 pm
content: The first thing that comes to mind is why use electricity at all? The sun can directly heat water through solar collectors design to do that. This would minimize losses.Now maybe you have an excess of solar power already, if the tis the case diverting the extra solar power to heat can make sense. So. a couple of points:The AC voltage rating is RMS which effectively means the equivalent heating capability of the same DC voltage. So 240 VDC will heat exactly like 240 VAC RMS.If you run some calculations through OHM law you will see that power into a circuit varies greatly with voltage. So a few volts here or there makes a huge difference in the power out of a circuit. However does this really matter? The output from your array will vary through out the day, even if it isn't delivering full power it is still heating your water. The real problem is understanding how the array reacts to various levels of energy input (sunshine) and how that is seen at the load.Since you are heating over the period of available sunshine, it might make sense to size a smaller array and use fewer heater elements. This of course depends upon your heater, if it is a single element heater then you are stuck with using one element. With a dual (or better) element water heater you could dedicate one element to being powered by the sun and a second element acting as back up from the line.A 250 liter tank really isn't much, one shower, some dish or clothes washing and your hot water will be gone. To me it is a bit optimistic to think you will be getting days of hot water.Safety is of extreme importance, you will need to use DC related components and have redundancy. At the very least have a safety transducer and a reliable DC switch to prevent over temps. This is not the same device as the control circuit to maintain temperature in the tank.Look for some of the solar power forums. I wouldn't be surprised if there isn't an already engineered solution for this problem. I'm not sure trying to DIY is a smart move in your case.Well more than a couple of points but your post below has me a bit concerned. I'm not sure you have enough background to tackle this safely. Don't jump until you have gotten a full grasp on what needs to be done and my post doesn't cover everything. In a nut shell you will be heating a pressure vessel and that is nothing you want to screw up.Quote from: Jbates58 on August 06, 2017, 11:25:19 am