DC water heater element?

[GLC]

 

[efficientPV]

It will be an experiment and nothing I tell you will matter untill you see how poorly it works.

 

[GLC]

The results will be based on what we currently use in hot water KW's daily vs using the solar elements as a pre-heater for the tank. may mean nothing and then it may mean a lot. It is a cheap experiment to do. I already have the solar panels just needed a 24 volt element to play with.

 

[efficientPV]

Can't tell the players without a scorecard. Be sure to get a wattmeter that totals and remembers the power sent to the element. What POZ doesn't tell you is what happens when sun intensity lessens to still very reasonable amounts. That 400W element is a little over 2 ohms. If panels are producing 2A, that will be a little over 4V and times 2A is 8W. What the panel can produce with that sun is 2A times 36V or 72W. Almost a 10 times increase. 5A times 2 = 10V or 50W vs 180W. 7A times 2 = 14V or 98W vs 252W. 10A times 2 = 20V or 200W vs 360W.

 

[GLC]

I understand, that is why we call this an experiment. Currently I have an cumulative watt meter installed in the wiring of the hot water heater.The score card will be the current average KW usage of the hot water heater now minus what the average KW usage will be with the DC element installed on the bottom of the hot water heater as a pre-heater. In other words replacing the lower element wit the DC element while keeping the AC 220 element in the top of the water heater. This will give me practical possible savings on that application. This may or may not effect daily KW usage but I think it will show a savings preheating the water. It is not so much as what the watt numbers are being sent to the DC element but what will be the possible reduction in the watts being used by the 220 element. Presently one of my 305 watt panels produce 35 volts at 7 amps to the solar charger in the peak period. It may or may not be an issue with the 24 volt element. Once again, this is an experiment.

 

[Craig]

It will be an experiment and nothing I tell you will matter untill you see how poorly it works.

Why cant you just let him try? Like I often say even if his experiment is a horrendous mistake learning is never a mistake. Glad you were not around Thomas Edison!

 

[GLC]

I am sure that Will when he first got into solar had hopes that certain experiments would always work out for the best. I am sure that some did and some did not. I consider myself a practical engineer. I will try this out practically, not on paper, and see how it works.

 

[GLC]

Well it came in today and i played with it a little. Hooked it up to two 305 watt in parallel pannels. Made open voltage of 40 volts and 15 amps. Put the 24 volt - 400 watt in a 5 gallon of water. Sun was about 2 hours before peak. Faucet water temp was 74 degrees. Checked water temp in an hour and it was 120 degrees. Forgot about it and checked again later and it was 130. It looked to top off at that temp. Dont really know how long it had been at 130 either. So all in all the 24 volt , 400 watt element worked pretty well. The thought is to possibly use it to preheat the water in the hot water heater as the lower element. I put a kw meter on the hotwater heater and will see the results in a week. After that i will replace the ac element with the dc element for a week and will check the difference. Still will have to tweak with different panel configs with volts and amps to see how far i can push this. This is straight panels no batteries or solar controller. The dc does have a thermostat on it.

 

[tazmann]

Sounds like it should help what voltage are you getting at the element ?

 

[GLC]

Did not check it, the element acts as a diode so i am sure it will be very low.

 

[Craig]

Well it came in today and i played with it a little. Hooked it up to two 305 watt in parallel pannels. Made open voltage of 40 volts and 15 amps. Put the 24 volt - 400 watt in a 5 gallon of water. Sun was about 2 hours before peak. Faucet water temp was 74 degrees. Checked water temp in an hour and it was 120 degrees. Forgot about it and checked again later and it was 130. It looked to top off at that temp. Dont really know how long it had been at 130 either. So all in all the 24 volt , 400 watt element worked pretty well. The thought is to possibly use it to preheat the water in the hot water heater as the lower element. I put a kw meter on the hotwater heater and will see the results in a week. After that i will replace the ac element with the dc element for a week and will check the difference. Still will have to tweak with different panel configs with volts and amps to see how far i can push this. This is straight panels no batteries or solar controller. The dc does have a thermostat on it.

Very cool. My plan is to place elements in my main water storage tank to keep water above freezing and help the instant water heater along by not having the water so cold when it starts heating. 5 gallons is obviously a lot less than 200 gallons but I dont need to make it hot just maintain 60 degrees or so.

 

[Ampster]

At the 10,000 foot level, I have concluded that moving heat is more efficient than creating heat. That is why I went to a heat pump water heater. As I often say it depends on where you are standing, I am on the grid so I am not reliant on just stored energy because I can use the grid as a battery.

In 1981 or 82 I put in a thermal solar system on my home because California was offering some kind of incentifve.. It worked well and dropped my gas bill by $20 or $30 a month, I sold that house a few years larter and it took years before I thought again about harnessing the power of the sun. By then (2008) I bought 3.0kW in solar panels for probably $15,000 and did my first solar panels and a few years later I bought a device that modified a standard electric water heater to an heat pump device that ran off 120 volts and recovered some of the extra energy that I was selling to the grid. Since then GE then Rheem have come out with HPWH that are self contained and work well. For me a HPWH is the mos efficient of using excess solar energy to heat water. It is more cost effective but I understand in an off grid situation it may mean more investment. I also understand that for space heating there is another element of load but it a temperate environment I have seen a HPWH used for radiant heating all powered by PV solar grid tied.

 

[sunshine]

KISS brand DC water heating.
12v 600w Dernord element from AliEx in poly housing as per photo.
4 x 190w 36Vmax in parallel (converted to 12v : 190w 18vmax plus with in series diode - not necessary if 24v element used but what I had)
Frame: Ex swingset - ex hammock frame - pv frame
15m 10mm twin cable.

Readings at 10am: PV end of cable 39.7amps 11.57volts
at element 9.13v ....362w output of element

 

[Ampster]

Yes, it can be done. Thanks for the photos.

 

[efficientPV]

Readings at 10am: PV end of cable 39.7amps 11.57 at element 9.13v ....362w output of element

Great post, I love to see real data. So, almost 100W was lost from the panels to the heater element. This is a teaching moment. Many will think just get bigger wire. Not true at all. If you had superconductor cables with zero resistance, the voltage at the panels and the heating element would be the same 9.13V. The resistance is fixed and the volts will just be amps times the resistance. This is true till you get to just about the power point voltage of the panels

 

[sunshine]

I used the voltage drop calculator from this site:

Electricity losses online calculator : AC and DC electrical wire voltage drop and energy losses

Quick online free voltage drop calculator and energy losses calculation, formula of electrical DC and AC power wire voltage drop for various cross section cables, power factor, lenght, line, three-phase, single phase. Formula to calculate voltage drop and energy losses.

photovoltaic-software.com

6mm wire was something like 23% loss, the 10mm an acceptable 10 % using the theoretical mpp voltage of 18v.

 

[efficientPV]

All these calculators are handy, but they don't take the place of knowing electrical fundamentals. Making the voltage drop in the wire go to zero in this case doesn't matter at all. Heater will see the same power.

I've never seen the resistance specified for these DC elements in any sales literature. I've assumed 12V or any multiple of would be 14.5V or at least 13.8V. This would be the nominal system voltage when operating in day. From your data, resistance calculates to be about . 0.23 ohms. Calculating 600W at 12.0V works out to be 0.24 ohms. Anyone calculating resistance of elements should use the exact voltage specified.

 

[sunshine]

The Vmax for the 190w panels is 18v. Most graphs I've seen show a significant fall going away from there yet at my reading of 11.57v there still seems adequate power??
0.24 ohm was what I based the original workings on from the v & p and to use 4 panels. Dernord elements are slightly dearer than others and I hoped this translates into better quality. They even have an image measuring the resistance as i've just noticed.
I've already removed the blocking diodes so its straight fom pv to element.
This is as basic as can be. I would have used a 24v 600w element if the panels were not already altered.

 

[efficientPV]

40A seems like a reasonable number for the array in good sun. So, the resistance of the heater should be almost double. A 24V element of slightly higher wattage would have been better. The currents are still so high and anything can dramatically increase the line loss.

Just what are you doing with this heater. It is outside with garden hose, pool heater?

That is a small heater and likely has a 120V 1500W or 2000W heating element. Resistance is about 7.2 ohms for 2K. 2K are cheap, I bought two on Amazon for $14 shipped, big box about $11 each. Switching the panels back to 24V and having a 2S2P arrangement would be ideal match and double the heating output. Bypass diodes would have to be placed back in the panels. Line losses would go to almost nothing

 

[sunshine]

Just what are you doing with this heater. It is outside with garden hose, pool heater?

The 600w element is in the poly housing just prior and below the standard hot water system as in the photo.
Still connected to mains with the thermostat set to min (60C)