Direct to water heating.

[efficientPV]

I hope you will share your project on this board. I had hoped there would be a community sharing their
ideas on this concept by now. I have received many pictures and private videos of these projects, but
none have wanted to open themselves to comments on this subject. There are dozens of designs that can
work and each has its attributes and limitations. This is not about just heating water with PV. It is about
getting FREE hot water from the panels you already have in place. Any successful PV system has excess power
which is just wasted. This topic should be in the Danger Zone as it is for advanced members.

Low density extra long folded over heating elements probably will make more noise than shorter high density
elements. On some elements the sheath is not electrically bonded to the flange. This possibly could cause
electrolysis. Electrically bonded sheaths would act as a shorted turn of a transformer and dampen any ringing.
I never found much of an issue with noise as mine has no set frequency in which vibrations can build up.

 

[curiouscarbon]

I think this concept is really cool (hehe) because it's treating photovoltaics with the same critical engineering eye as, say, an internal combustion engine under scrutiny. Just like @efficientPV said there's waste heat in these systems too.

As a side benefit, PV generally performs better when the photovoltaic cell material temperature is lower. Extracting heat from both the panel directly and using the electricity to resistively heat water could be quite efficient. Never done any empirical tests though.

Ok one more idea. An array of large (60x60mm) Peltier Elements (Thermoelectric Heating Cooling) could be used to generate the heat for the water. This could also simultaneously cool down a separate body of water and create both hot and cold water from extra power sitting around.

 

[Ampster]

I think this concept is really cool

I have probably already replied that from a high level system approach, a heat pump water heater is very efficient. The one I have can be programmed for different temperatures during different times of day. With some of the right tools I could probably get it to use excess solar to transfer heat to the tank. In my case, the economics of my NEM agreement favor selling the excess solar to PG & E.

 

[Hedges]

Ok one more idea. An array of large (60x60mm) Peltier Elements (Thermoelectric Heating Cooling) could be used to generate the heat for the water. This could also simultaneously cool down a separate body of water and create both hot and cold water from extra power sitting around.

Or that could be done with a compressor/refrigerant system. At all but the smallest scale, I think compressors will be more cost-effective than Peltier for producing heat. Even more so for producing cold.

A system that builds ice and heated water in a tank, then (or meanwhile) spilling excess heat/cold to the environment, could be useful. An A/C with its hot side in a 160 degree F water tank would be less effective than one with coils cooled to outside air temperature. So maybe its condenser could first go through the hot water tank, then further cool with a radiator to the air. A heat pump could first warm its cold side to freezer temperature, then further warm it to ambient air. Might require multiple valves to divert working fluid depending on temperature of each medium and needs for heat/cold.

Storing cold in an ice chest at night when rates are low (or in the morning when sun is shining for off-grid systems) then using ice as cold side during high demand and hot times of day could boost efficiency. Same for making heat for later use. But cost to implement always has to be weighed.

I have probably already replied that from a high level system approach, a heat pump water heater is very efficient. The one I have can be programmed for different temperatures during different times of day. With some of the right tools I could probably get it to use excess solar to transfer heat to the tank. In my case, the economics of my NEM agreement favor selling the excess solar to PG & E.

So you can run the heat pump when rates or low, or during power failures when you have surplus PV. Also taking into account temperatures of air the heat pump uses as its heat source.
For me, the funny thing is use of an efficient heat-pump water heater to extract ambient energy from an electrically heated house.

 

[Ampster]

For me, the funny thing is use of an efficient heat-pump water heater to extract ambient energy from an electrically heated house.

Yes that is ironic. Hopefully the house is heated with a heat pump. As we know, it is more efficient to move heat than create it with resistance heat. My HPWH is in the garage. I cut a vent in the ceiling to pull warm air from the attic which is warm even in the winter.
Of course, you and I live in a temperate climate so there is plenty of heat to be transferred.

 

[curiouscarbon]

100% agree that heat pump will be more efficient than peltier in effectively every case

def preferable to not draw from a temperature gradient that required system power to create in the first place (eg fridge condenser coil in ac cooled room)

ideally any heat considered waste would be ejected into the great outdoors

in my perfect world the air conditioner would reject its waste heat into the water heater or something and then only vent to atmosphere after the water is hot.

of course this would necessitate re-engineering due to higher delta t across elements

i’m still drawing stuff and thinking about it

i’m weird and like solid state ? and brazing is not in my skill set (yet!)

ok last edit:

haha i lied one more edit:

electric resistive heating + heat pump water heater

 

[efficientPV]

Everyone seems to approach this with mono thought. Many heat pumps can't elevate temperatures high enough to get you thru the night. As a dump load, HPWH just can't take that much energy <1KW. Even at lower efficiency resistive heating can be a far better solution. I get excess power that may only last 3 minutes, you just can't be starting and stopping a heat pump. Hardware cost/return of PV water heating are similar to HPWH. I supplement a HPWH with PV resistive heating. If heating water looks simple, you haven't studied it enough.

 

[Ampster]

Everyone seems to approach this with mono thought. Many heat pumps can't elevate temperatures high enough to get you thru the night.

Yes, if efficiency is mono thought I am guilty. My solution to getting through the night is to run the HPWH at 135 degrees and use a tempering valve to expand the effective volume of 120 degree water. Worse case if I wake up in the middle of the night and get an urge for a long hot shower, I could always get on my phone and switch my HPWH to high recovery mode.
No worries, I am not trying to change your mind or denigrate your idea. I just thought some other readers might be interested in alternatives.

 

[efficientPV]

Everyone likes to think their particular situation is the only one in the world. Going higher temperatures lowers efficiency and non CO2 systems have a hard time achieving the higher temperatures. Running higher temperatures at times of the day when excess energy is not available does cost. There is a lot of smoke and mirror associated with HPWH. The following data from an engineering study is an example. Note the changing COP with temperature and humidity.

– COP performance data
provided by Rheem for the
HP50 model considered in
the analysis.
– The correlation equation is
obtained to allow
automatic calculation of
COP based on hourly
ambient conditions from
the eQuest model output.
Dry-bulb
Temperature
Relative
Humidity %
24 HR Energy
Factor (COP)
65 30 1.69
65 40 1.79
65 50 1.88
65 60 1.97
70 30 1.85
70 40 1.95
70 50 2.05
70 60 2.16
75 30 2.03
75 40 2.15
75 50 2.28
75 60 2.38

I have a HPWH at home because it males sense. If youhave made an enormous outlay for a solar system you can do anything you want and HPWH can make sense. At camp I didn't buy any extra panels, just $20 on a heater controller and used existing tanks. For me to use a HPWH at camp would cost me an additional $3,500. Efficiency is mono thought.

 

[Ampster]

Everyone likes to think their particular situation is the only one in the world.

My first solar system was a hot water system which I DIY'd in 1979 to chase a California State tax credit. My apologies if my comments come off as if a HPWH is the only one in the world.

Going higher temperatures lowers efficiency and non CO2 systems have a hard time achieving the higher temperatures. Running higher temperatures at times of the day when excess energy is not available does cost.

Yes those are tradeoffs. My needs are small enough that i no longer need to do that unless my sister comes to visit. I will sacrifice a little efficiency to reduce the chatter about "not enough cold water." It all depends on where you are standing. I also drive two EVs so most of my excess solar is diverted to those.

 

[curiouscarbon]

i totally did not mean to derail anything by bringing up heat pumps. absolutely do not think they are perfect for all situations and all people! those technology connections videos are real great educational material for less experienced visitors (that was my thought in sharing them). probably just goes over a lot of stuff the people in this thread already know

despite a thermoelectric peltier element being less efficient than a compressor based heat pump, the peltier can very easily switch on and off to transiently shed extra power. the TEC is a resistor after all, so being cycled does not harm it*, whereas a compressor has minimum on times and fairly high current requirement to operate usefully. TEC can efficiently operate at <100W easy and for microseconds at a time safely.

i'm trying to design various thermal management systems, and a more integrated approach can have some upsides for my use cases.

just thinking out loud, if i do build a DIY water heater, probably would test/evaluate peltier element to get the water up to a moderate temp and get free cooling, then use resistive heating to get it to the peak temp.

also add like 4-6" of polyisocyanurate on all sides of the thing

hope this is not too OT, cheers


*peltier elements can degrade from certain frequencies of heat cycling over longer times, so keeping it coupled to the water would help ameliorate that design risk factor.

 

[K W]

I hope you will share your project on this board. I had hoped there would be a community sharing their
ideas on this concept by now. I have received many pictures and private videos of these projects, but
none have wanted to open themselves to comments on this subject. There are dozens of designs that can
work and each has its attributes and limitations. This is not about just heating water with PV. It is about
getting FREE hot water from the panels you already have in place. Any successful PV system has excess power
which is just wasted. This topic should be in the Danger Zone as it is for advanced members.

Low density extra long folded over heating elements probably will make more noise than shorter high density
elements. On some elements the sheath is not electrically bonded to the flange. This possibly could cause
electrolysis. Electrically bonded sheaths would act as a shorted turn of a transformer and dampen any ringing.
I never found much of an issue with noise as mine has no set frequency in which vibrations can build up.

Here's the PWM circuit I built. It works, but the MOSFETs get hot when driving a 1.5KW load. I was using it to shunt my panels to reduce the voltage to 140v on cold days so the charge controller wouldn't over volt and turn off. I'll keep working on it so I can drive a KW or two without roasting the FETs.

 

[curiouscarbon]

some concerns about electrolysis with water and voltage potentials was mentioned



for my version, probably will consider something like heating mineral oil and exchange the heat to water with a heat exchanger like above. i'm figuring it might be worth the trouble of two pumps due to avoiding the need for water to be around a voltage potential difference.

 

[efficientPV]

engineer775 is now selling the Cyboiverter for $2,000 that uses dedicated panels to heat water with resistance.
I'm sure it will find some users even though the cost/payback seems really poor. Not just the price, but it uses
dedicated panels which become useless when the tank isup to temperature. And I thought the techluck ws pricy.
According to him they are no longer available or maybe he just got tired of replacing bad ones. Dedicated panels
for heating is a poor solution. Far better to use existing panels in a diversion process. Not all resistive PV
solutions are equal.

Here is a screen shot I took a week ago showing the spikes of sunshine emerging from clouds around noon and how
my heater was able to adjust to by changing apparent resistance the panels see. This dta is from a Atorch D3010
"100A" bluetooth power meter. Don't trade in your old phone or tablet! While it has many shortcomings, it is only
$15 and gives a great insight to the secret life of panels. I've been quite surprised at many of the things I've
seen. Although it can be irritating at times I've come to like this little gadget. There is also a AC version which
uses the same software.

 

[efficientPV]

KW I get everyone confused, but I thought you said there was no FET heating. There is just no advantage to going that fast. offhand the compensation capacitor is also too large. I think with some scope traces any problem would be obvious. I think the noises you heard when operating at low frequency was the TL494 going into oscillation.

 

[z80]

This thread has become so complicated with expensive hitech solutions to what is a simple problem.

Yes...you CAN connect your series/parallel combo of used solar panels that you got for 5 bucks each to heat hot water directly in your el-cheapo resistive element hot water unit.

Yes... you will get a humungous arcing spark across the thermostat contacts when the water comes up to temperature and disconnects the load.

Frightening videos indeed abound everywhere.

But!

if you can get your hands on a small start capacitor off a washing machine motor etc...
Then to keep the design boys happy fit a 100k resistor across the capacitor terminals to discharge the capacitor....
then fit the capacitor/resistor combo cross the 240v terminals of the hot water unit.
(Don't try this when using AC...hopefully you know the difference?)

Voila...no more sparking...arcing..or blow upping...just hot water...total cost of arcing snubber mod...$1

You can watch this guy play with it or skip to the 6 minute mark for some action...

 

[efficientPV]

It is getting complicated. This should be in another topic.

KW, I wouldn't be making circuit boards for your friends yet. The schematic has a serious design issue and should be immediately
deleted. Schematics on the internet soon have a life of their own and then you have no control of them. Take some time to consider
all potential conditions and why your design looks different than other TL494 designs..

 

[falgsc-al]

I know I am late to the game here but I wanted to share since I actually do this.

A little background: I have 3kw of panels, 80a charge controller, 3500/7k 120v inverter, and 5 used 5.63kw BYD packs in my off grid setup.I have paralleled an existing 50 gallon hot water heater's elements bypassing the series thermostat on the top element and allowing both to run simultaneously on 120v. This drops the unit to a total of approximately 2kw, it heats the water more slowly (about 45min-1:30 turn time from summer to winter) but it absolutely works. Have been using the set up for several months. I also painted the hot water tank black and put it in my greenhouse so it can be a solar collector on its own.

cheers and good luck

 

[z80]

I know I am late to the game here but I wanted to share since I actually do this.

A little background: I have 3kw of panels, 80a charge controller, 3500/7k 120v inverter, and 5 used 5.63kw BYD packs in my off grid setup.I have paralleled an existing 50 gallon hot water heater's elements bypassing the series thermostat on the top element and allowing both to run simultaneously on 120v. This drops the unit to a total of approximately 2kw, it heats the water more slowly (about 45min-1:30 turn time from summer to winter) but it absolutely works. Have been using the set up for several months. I also painted the hot water tank black and put it in my greenhouse so it can be a solar collector on its own.

cheers and good luck

how do you switch the heating element off and on?

 

[falgsc-al]

how do you switch the heating element off and on?

I let the factory thermostats do the work. Usually the top element thermostat interrupts the power to the lower element thermostat when it's on, so I bypassed that by hooking both the top and bottom thermostats directly to 120v. If I'm feeling skiddish about my SOC at the time I just turn off the breaker to it.

edit: should've chose my words more carefully and said thermostats instead of elements. The elements are using their factory installed thermostat.