unique solar application

[C Keith]

First off I am totally new to solar. I want to run something past you people who have done solar for a long time.

i want to run an electric water heating element for as many hours a day as the sun allows. Possibly a 1500w 120v element , but we are open to other options. I am preheating hotwater for my domestic hot water. I don't really require storage of any power at this time. I possibly have a lead on 2kw worth of solar cells (to be assembled and installed by me).

Any thoughts on this crazy arrangement?

I haven't yet really thought out what to do in case I get enough hot water and have to go to a dump load .

(I realize a solar water heater is a more practical way to do this. I'm an electrician - not a plumber .... LOL )

[russ]

It will cost you far, far more and be more complex to use solar PV for hot water.

Plumbing is simple compared to your trade. You really have to know three things - 1) hot is on the left, 2) cold on the right and 3) s**t don't run uphill.

I really suggest you look at solar thermal for hot water - far more efficient. At a bare minimum it is twice as efficient (in a cold climate) and in many locations much more.

Russ

[petertearai]

"Watt power crisis" in newzealand were selling such a thing .They may be able to advise on the economics. With the cost of pv coming down and the simplicity of it, I think not such a bad idea.
Regards Peter

[arkieoscar]

I haven't yet really thought out what to do in case I get enough hot water and have to go to a dump load .

(I realize a solar water heater is a more practical way to do this. I'm an electrician - not a plumber .... LOL )

No need for a dump load. The PV doesn't care if it's "open" or "shorted". You just need a thermostat that will open the DC circuit without arcing too much.

[icarus]

Use direct solar gain to heat hot water instead of using PV.

Using Pv to convert to electricity, and then to convert that electricity to heat is very inefficient. PV converts ~ 15% of available power, while solar hot water converts ~75%.

Direct solar hot water is way cheaper and way more efficient.

Tony.

PS For example, a simple homemade solar hot water collector, made from scrap parts, and not very efficient (the last one I made) cost ~ $500 including the pump and controller, and produces ~ 75 gallons of 100F rise water on a good day.

[Cariboocoot]

Just going to note here that if you are off-grid and using the PV for primary electric power the situation becomes somewhat different as it makes sense to use electric hot water as a "dump load" once the batteries are full and you still have sunlight to harvest.

On the whole, electricity is a poor way of heating anything.

[BB.]

Also, from your first post, it sounds like you may be assembling your own solar panels from raw solar cells?

If so, do you have a good way to assemble reliable solar panels? In general, most home made panels probably only last a few months before they need repairs (water intrusion, solder tabs lifting, etc.)... Also many folks build them with wood backing and/or plastic sheeting which makes for a substantial fire hazard too.

-Bill

[raydias]

I am looking at something similar and have seen DC water heater elements listed out there that may make it more efficient. I have an older electric water heater that was replaced recently (nothing wrong other than it was old and inefficient as well as the elements died) and was considering adding the DC elements, connecting the diversion load from a charge controller (Xantrex Charge Controller for DC Charging Sources - 40 Amp, Model# C40) to the water heater.

Missouri wind and Solar has the following in 12, 24 and 48 volt versions:



Starting at: $49.00
8 Inch 24 volt model -
Can be used direct with solar panels and hydroelectric generators and as a wind turbine dump diversion load. Can also be used as a solar panel dump load. Make sure to use our #6 or #4 welding cable for long runs from your wind turbine, hydroelectric, or battery setup.
Ratings:
• (Ni-Chrome coils / Ultra Low Wattage)
• 12 volts @ 30 watts
• 24 volts @ 100 watts
• 48 volts @ 225 watts
• Thread-in model (1" NPT bottom tap)

Livestock water tank deicer (low amp draw means you won't drain your battery as fast). For keeping dog, cat, and other domestic animals' water from freezing.
Standard American water heater element replacement for on or off grid applications
Water preheating
Campers, Motor homes
Emergency hot water
Portable commercial, industrial machines and equipment
Runs off your car battery for portable hot water anywhere
Direct wind and solar applications
CONCRETE MIXING TRUCKS
Use for diy solar batch water heaters
For heating motor oil, vegetable oil, or most any other oils or liquids
and many, many more uses where heat or diversion loads are needed.

[Cariboocoot]

I am looking at something similar and have seen DC water heater elements listed out there that may make it more efficient.

Nope. The only difference between an "AC" resistive heating element and a "DC" resistive heating element is the Ohms; i.e. the intended operating Voltage/Amperage to produce the Watts. You should also be aware that the thermostatic controls made for AC will not handle the same power levels on DC; the contacts will weld shut.

Electricity just isn't an efficient way to heat things and there's no way to change the laws of physics on this. The economics of it aren't very good either.

[BB.]

From what I have seen, DC heating elements are just standard AC water heater elements spec'ed at the lower 12-48 volts dc system voltage. Don't pay anything extra for the "dc" versions.

From your numbers, that maybe a 5-10kw 240vac water heater element. The numbers do not quite make sense--although if this is measured power, ni-chrome wire has less resistance at lower temperatures--so this may be a 4.8 kw 240vac element.

Bill