unique solar application

C Keith · March 2011

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 · March 2011

Re: unique solar application

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 · March 2011

Re: unique solar application

"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 · March 2011

Re: unique solar application

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 · March 2011

Re: unique solar application

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 · March 2011

Re: unique solar application

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. · March 2011

Re: unique solar application

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 · March 2011

Re: unique solar application

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 · March 2011

Re: unique solar application

raydias wrote: »

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. · March 2011

Re: unique solar application

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

raydias · March 2011

Re: unique solar application

Cariboocoot - i understand that electricity isn't and efficient way to heat things and at this point for my situation i am look at economical option. I have the used electric water heater, solar panels, charge controller with diversion load capabilities and a small battery setup. the DC elements would cost about $120 to replace the two in the used water heater.

So my thinking is that instead of either tying the solar to the grid or just wasting the solar panel power when the batteries are charged my not use the diversion load option to preheat water durring the day and using DC elements would remove the inefficiency of adding an inverter for DC to AC.

Just ideas i am exploring.

thanks

BB. · March 2011

Re: unique solar application

For $10-$20 each, you can get the ac versions from a hardware store.

-Bill

mike95490 · March 2011

Re: unique solar application

BEWARE using AC thermostats in the water heater, to switch the DC current. Of course, the thermostats are designed for 600W AC, and will switch much less DC amps, but the contacts may arc and burm on DC

raydias · March 2011

Re: unique solar application

The same site has a DC thermostat that should do the trick.

this is basically what I was thinking of doing.

Cariboocoot · March 2011

Re: unique solar application

You know what the basic flaw is in that design? It gets coldest at night, when the panels produce nothing. I wouldn't rely on it to keep water from freezing. Warm it up a bit before going into the regular water heater would work. Not as efficient as direct solar thermal, but easier to adapt to existing apparatus.

BB. · March 2011

Re: unique solar application

Remember too that Solar panels are, more or less, constant current devices.

And the heaters are resistors (somewhat variable resistance) and the panels will have vary poor efficiency unless they are matched to the elements at near full sun... At less than full sun, there will be much lower efficiency.

-Bill

raydias · March 2011

Re: unique solar application

Cariboocoot wrote: »

You know what the basic flaw is in that design? It gets coldest at night, when the panels produce nothing. I wouldn't rely on it to keep water from freezing. Warm it up a bit before going into the regular water heater would work. Not as efficient as direct solar thermal, but easier to adapt to existing apparatus.

The reason I am considering this setup is the fact that it would keep the water indoors so freezing isn't a problem.

Bill - understood - even if at less than full sun i only get 10 degree uptick in water temp it's 10 degrees less that the DHW system has to heat from the grid source.

C Keith · March 2011

Re: unique solar application

Thank you all for your comments. Very helpful.

As silly as it sounds , I am not so interested in the economics of doing this. As I said I am an electrician and in desperate need to learn some green energy tricks. Rather than sell a customer a flawed product - I want to get it right. I can do some plumbing (if I have to .... I guess...) in the name of economy. I believe strongly in direct solar hot water and it is the next adventure I go on.

regarding the home made solar panels . I am making every possibly effort to seal the panels from moisture. We have a good southern exposure , and the design of them is still being pushed around in my head.

I was intrigued by that drawing of solar panels hooked directly to an element does anyone have additional comments on that. The fact that switching DC is a problem is very true , but I have solutions for that. Useing the thermostat to switch an oversized DC rated contactor. I doubt any of those elements could actually preheat 40 gallons of cold water significantly over the short period of time that I have "peak" sun. I envisioned 1500 watts full bore for 5-6 hours to make any hot water.

I might plumb in my "preheat tank" and start some research on how many watts it would take to actually get the job done. someone mentioned having a used electric water heater - which is exactly what I have for the job.

C Keith · March 2011

Re: unique solar application

russ wrote: »

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

Your only a second year plumber with that information . .. . ..
in third year and fourth year you will learn

4) Keep your fingers out of your mouth

5) every [avoid language please] is a potential customer.

C Keith · March 2011

Re: unique solar application

Also if anyone has comments on assembling , configuring solar cells please comment.

as I said earlier - I have no prior training on this and probably about to open my mouth and stick in both feet but here it goes.

I have at my disposal 500 6"x6" cells . 3.97 watts per cell . nominal voltage of .5-.6 grand total of 2 kw.

potentially each panel could be 125 cells for an ocV of 62vdc -75vdc . about 500 watts. ( darn close to 32 sq ft - maybe a bit big for a sheet of plywood .... er plastic..... er glass . . . they'll be mounted to something. )

My reasoning for this was for the purposes of charging a 48v battery bank , if no viable hot water solution is found. At that point i have to go to the drawing board and figure out loading and battery sizing.

additionally since I am not trying to be offgrid - a suitable house load needs to start consumeing some sun power. I saw an auto transfer switch recently that could switch over a 60 amp panel from grid to alternate source of energy.

Cariboocoot · March 2011

Re: unique solar application

There's quite a lot of discussion on this forum regarding DIY solar panels. Can't say it's very positive, though. All right for a learning experience, but what you learn is you can't build one as good as a commercial unit. Maybe not even as cheap, considering the per Watt pricing on panels today. You'll also learn how hard it is to solder the connections even with a temp controlled iron, and how difficult it is to keep moisture out. Hopefully you won't learn the hard way that you can burn your house down by using plywood as a backing.

russ · March 2011

Re: unique solar application

DIY solar PV is for the real DIY type - the guys that love DIY and do it all the time with the proper tools on hand.

People that don't even know where to start have practically zero chance of success.

You are worried about plumbing (which is easy in comparison) and want to jump into DIY PV?

It will get expensive and you will end up with a pile of junk most likely.

If you do start, try doing so with a 300*300 mm (12''*12'') panel and work your way up.

mike95490 · March 2011

Re: unique solar application

Here's a pretty detailed thread about fab'ing a panel. Practice your soldering on junk cells first (99% of cells on flea bay are likely 2nds, 1sts go into production panels)

http://fieldlines.com/board/index.php?PHPSESSID=2217ba36ada3e9efa590eb08bf4dac94&topic=145005.0

raydias · March 2011

Re: unique solar application

you could always build something like the following:
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm

C Keith · March 2011

Re: unique solar application

While it is humbling to realize you are wrong - I am a humble guy and can admit that I am chaseing a pipe dream. Moving forward from here won't be so easy. I really fail to see the difficulty in creating a functioning sealed panel . I knew it would be time consuming , but had no idea it is considered to be impossible.

a year ago I built a homemade solar air heater with good results , and 6 months before that I thought that was a crazy stupid idea also.

I checked both of the mentioned web sites both are very good.

Seven · March 2011

Re: unique solar application

Not impossible, just futile. Home made panels rarely make 50% of what you expect. There is a thread here about a guy who bought panels that were home made(he didn't know it) and the panels actually caught on fire, on his roof!

You are going to spend a lot of time and not get much from it. If the cells you get even work.

XRinger · March 2011

Re: unique solar application

I've been trying to (DIY) heat water with PV for a while now.
So far, I've found that 400 or 500 watts just isn't enough..

There has been some work done in this area, by the gov, starting in the late 90s.
http://www.bfrl.nist.gov/863/pvsolar/pvwater/index.htm

It works, and it's not real complicated. It's just expensive.

Here's part of my simple setup.
http://ecorenovator.org/forum/solar-power/1284-cs6p-200-canadian-solar-pv-xantrex-c40-project-4.html#post12348

I just got two more panels, so we can try out 800w (2730 BTUh).
Maybe I can save a little heating oil this summer. 8)

Plan B:
If the heat loss of the boiler is too great, I might resort to buying a small
hotwater heater and install some real heating elements.

Plan C:
Make AC power and use an Air Source heat pump to make hot water..

~~~
Back in the 80s, we had three Novan hotwater panels on the roof.
After about 10 years, the water started to eat the copper..
Then, our 84 gallon tank started leaking. I made repairs, but after
about 15 years of use, the heat exchanger went bad (leaks).
We needed new shingles on the roof, so the solar stuff was removed.

IMHO, PV hot water is going to last longer and have less problems in the long run.

One side effect, my wife NEVER wants panels on the roof again..
So, all my PV is located in the backyard (a poor site)..

Cheers,
Rich

Cariboocoot · March 2011

Re: unique solar application

Consider this:

A typical electric tank-type water heater has two 3500-4500 Watt elements which are activated alternately (only one on t a time). It usually take 1-2 hours to heat a tank full of water (depending on incoming water temp, how hot it's heated too, radiant factors, et cetera).

So how much heating are you going to do with 1/10th that amount of Watts? Got half a day to wait for your shower?

XRinger · March 2011

Re: unique solar application

Cariboocoot wrote: »

Consider this:

A typical electric tank-type water heater has two 3500-4500 Watt elements which are activated alternately (only one on t a time). It usually take 1-2 hours to heat a tank full of water (depending on incoming water temp, how hot it's heated too, radiant factors, et cetera).

So how much heating are you going to do with 1/10th that amount of Watts? Got half a day to wait for your shower?

Most folks know the sun is very unreliable source of energy in New England.

If it was sunny for 5 hours (assuming real sunny), then that 800w (2730 BTUh)
could add up to 13,650 BTUs total during the day.
That seems like it should help supplement the main heat source (oil burner).

The oil burner is on a timer and runs 1/2 hour (1/2 gallon) each morning.
Depending on hot water use for that day, we might need to fire it up again
for showers in the evening. If solar was good that day, maybe not..

We need to keep the oil burner running in the winter, since it's our back-up
heating system. If the temperature gets down under 10 deg F, we burn oil.
Otherwise, our Sanyo mini-split heats the house.

I would really like to shut down the oil burner during the summer.
So, I'm dreaming about an air-source heat pump water heater, powered off PV.. 8)

Bottom line, if 800w of PV can save us from burning a gallon of oil every week week or so, I'm cool with the expense.

solarvic · March 2011

Re: unique solar application

C Keith wrote: »

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 )

So if you really want to cut down your electric use, just get an electric hybred waterheater. I have been using a GE GEOSpring for 5 months now and have used 205 KWH.. Electric in my area is about 12 cents a KWH. Figures out to around $5.00 per month worth of electric. I had around $1400.00 in cost of wheater self installing it. Got a $450.00 rebate on my taxes and a $300.00 rebate from the power co.
Net cost was $700.00. The efficiency factor is 2.36 times over a regular electric waterheater . Not figuring the exact cost I think it will take about 2 years to recoup the extra cost I have in the cost of the hybred water heater over the cost of a regulat electric waterheater. I am a 1 person household and with a larger family you should save even more. :DSolarvic:D

solarvic · March 2011

Re: unique solar application

C. kieth
One of my neighbors has done something like you want to do, only he bought factory made solar panels. He looked at my GT system but didn,t want to invest in all the equipment need at this time. He has 4 270 watt blem panels that he bought at a good price connected to 1 dc heating element in a 40 gal tank. That tank goes to an 80 gal electric waterheater. Says the setup raises the water temp to about 100 to 110 degree F. Ha uses no thermostat. What he has is a combiner box with 2 strings of 2 panels in the combiner box with 1 set of wires connecting directly to heating element in the preheater water heater. He thinks he saves about $20.00 per month. I can,t substantuate it , but that is what he reported to me. He has his solar panels on other side of road from his house and had to bury his cable under the road. When he run his electric cable to his house he run extra set of wires to accomidate a gridtie system later when he can afford the other components he will need. :Dsolarvic:D

unique solar application

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