Directly using PV panel (without batteries/inverter)

[TyrionLannister]

Hey guys,
i want to know if can can directly hook up some load (like LED) to PV panels directly, (Of course voltage/power ratings are almost equal).
ie- no intermediate device connected .. wires from PV panels connected across LED strip (12V).
Will it work ? if so what about stability?


honestly speaking i'm quite new to practical solar applications. but detailed electronic explanations are welcome.
Also please do suggest some books/sources to deeply understand PV panels output,

[SunEagle]

Hey guys,
i want to know if can can directly hook up some load (like LED) to PV panels directly, (Of course voltage/power ratings are almost equal).
ie- no intermediate device connected .. wires from PV panels connected across LED strip (12V).
Will it work ? if so what about stability?


honestly speaking i'm quite new to practical solar applications. but detailed electronic explanations are welcome.
Also please do suggest some books/sources to deeply understand PV panels output,

You may already understand the issue. Due to the way a pv panel generates power it is due to the "lack" of stability that makes it unwise to just wire one directly to a dc load.

Not only are you fighting the issue of a voltage the panel generates being higher compared to what load 12v DC load is expecting, but the panel amp output will change as the sunlight hitting the panel changes.

To combat the voltage difference you need to install a device to regulate the voltage to the load. But you still do not have a guarantee the panel will produce enough power to keep the load working.

While batteries are expensive IMO they are a better investment to provide a stable voltage and amp source to safely run a DC load. The only place I might wire a small wattage panel directly to a load would be to use the panel as a trickle charger for a battery.

There is a "Solar for Dummies" book that can be downloaded for free. That book might provide some more details on how a pv panel works and what are the best path to use them.

[gfry2267]

I received my 200w solar kit today. Just for experimenting. I can't afford batteries now. I might try connecting the panel/s directly to a dc water heater element, set in a metal 5 gal bucket full of water. Just have to find the correct heating element for my system. Doing this just for residual heat. Getting cold here and I don't have the energy to build a big can collector system. My little one pumps out good heat at 120f. Not enough. Do you have any recommendations on a cheap battery? Maybe a 12v 150ah. I've researched and can't afford it right now. there was an old ups from work i was going to play with but that is gone now. When it stops raining here I have to trim 3 silver maples that block a lot of sun.

[SunEagle]

I received my 200w solar kit today. Just for experimenting. I can't afford batteries now. I might try connecting the panel/s directly to a dc water heater element, set in a metal 5 gal bucket full of water. Just have to find the correct heating element for my system. Doing this just for residual heat. Getting cold here and I don't have the energy to build a big can collector system. My little one pumps out good heat at 120f. Not enough. Do you have any recommendations on a cheap battery? Maybe a 12v 150ah. I've researched and can't afford it right now. there was an old ups from work i was going to play with but that is gone now. When it stops raining here I have to trim 3 silver maples that block a lot of sun.

I am not sure what you are trying to accomplish with solar. If you can't afford batteries and the amount of sunlight is limited due to trees and rain why would you spend anything on solar panels?

Also using solar to power a heating element is pretty much a waste of time and money.

So what are your plans for solar?

[lkruper]

I received my 200w solar kit today. Just for experimenting. I can't afford batteries now. I might try connecting the panel/s directly to a dc water heater element, set in a metal 5 gal bucket full of water. Just have to find the correct heating element for my system. Doing this just for residual heat. Getting cold here and I don't have the energy to build a big can collector system. My little one pumps out good heat at 120f. Not enough. Do you have any recommendations on a cheap battery? Maybe a 12v 150ah. I've researched and can't afford it right now. there was an old ups from work i was going to play with but that is gone now. When it stops raining here I have to trim 3 silver maples that block a lot of sun.

UPS batteries are not designed for cycling, but if free perhaps worth it for experimentation. I recently tried to survey a lot of batteries for cents / kWh and Costco 6v batteries came out at about 11 cents / kWH but you would need two of them at $84 each + 15 core to get to 12 v and that would be 207AH. The Costco Marine battery would be less expensive for $79 plus $15 core but that would be only 70 AH. That is, if, your Costco has the same Interstate flooded lead acids that mine do. Not as good as Trojan, but much less expensive for learning on.

[gfry2267]

Thanks for your input. I am by no means rich, but I like to ocd on experimenting with solar when things get slower here in the winter. I understand the efficiency of solar vs water/passive systems, but I won this pv system on ebay so now it is time to play with it. I figure I can generate on average 1kwh per day, so without battery cost I could recoup my cost in maybe 6 years. Right now solar for me is having some fun and learning a little. Maybe recreating the mouse trap, but I am trying to think outside the box. I hooked up the panels and got 15v and .175 amps. Cloudy rainy day here. They are in my living room. Would work better outside. Now I will try to see what what old pc fans I can run. Any thoughts on ptc heating elements?

[lkruper]

Thanks for your input. I am by no means rich, but I like to ocd on experimenting with solar when things get slower here in the winter. I understand the efficiency of solar vs water/passive systems, but I won this pv system on ebay so now it is time to play with it. I figure I can generate on average 1kwh per day, so without battery cost I could recoup my cost in maybe 6 years. Right now solar for me is having some fun and learning a little. Maybe recreating the mouse trap, but I am trying to think outside the box. I hooked up the panels and got 15v and .175 amps. Cloudy rainy day here. They are in my living room. Would work better outside. Now I will try to see what what old pc fans I can run. Any thoughts on ptc heating elements?

No, sorry. But let us know how your experiments go!

[Sunking]

I received my 200w solar kit today. Just for experimenting. I can't afford batteries now. I might try connecting the panel/s directly to a dc water heater element, set in a metal 5 gal bucket full of water. Just have to find the correct heating element for my system. Doing this just for residual heat. Getting cold here and I don't have the energy to build a big can collector system. My little one pumps out good heat at 120f. Not enough. Do you have any recommendations on a cheap battery? Maybe a 12v 150ah. I've researched and can't afford it right now. there was an old ups from work i was going to play with but that is gone now. When it stops raining here I have to trim 3 silver maples that block a lot of sun.

You are pretty much SOL and Dead in the Water.

[createthis]

I received my 200w solar kit today. Just for experimenting. I can't afford batteries now. I might try connecting the panel/s directly to a dc water heater element, set in a metal 5 gal bucket full of water. Just have to find the correct heating element for my system. Doing this just for residual heat. Getting cold here and I don't have the energy to build a big can collector system. My little one pumps out good heat at 120f. Not enough. Do you have any recommendations on a cheap battery? Maybe a 12v 150ah. I've researched and can't afford it right now. there was an old ups from work i was going to play with but that is gone now. When it stops raining here I have to trim 3 silver maples that block a lot of sun.

Interesting idea. I read a year or two ago that it's cheaper to install solar PV to run an electric water heater than to install solar thermal heat collectors. I didn't do the math, but I could see that being a thing.

I don't think 200W will give you a ton of heat, but with a battery you might be able to coax it into powering a 12V electric blanket: http://www.amazon.com/Roadpro-12-Vol.../dp/B0009Y2CFS

Batteries can be pretty cheap if you think outside the box. Consider buying a cheap PWM charge controller or two (if you can afford 200w PV panels, you can afford cheap Chinese PWM charge controllers) and go searching for old 12v car batteries. You might be able to revive some of them by charging/watering them. Wear face protection around lead acid batteries (acid). I'm not an expert at reviving old batteries, but I've heard of people doing it. Some people just have a knack for taking what most people consider trash and making it do something useful (unfortunately, I'm not one of them).

[J.P.M.]

Interesting idea. I read a year or two ago that it's cheaper to install solar PV to run an electric water heater than to install solar thermal heat collectors. I didn't do the math, but I could see that being a thing.

I don't think 200W will give you a ton of heat, but with a battery you might be able to coax it into powering a 12V electric blanket: http://www.amazon.com/Roadpro-12-Vol.../dp/B0009Y2CFS

Batteries can be pretty cheap if you think outside the box. Consider buying a cheap PWM charge controller or two (if you can afford 200w PV panels, you can afford cheap Chinese PWM charge controllers) and go searching for old 12v car batteries. You might be able to revive some of them by charging/watering them. Wear face protection around lead acid batteries (acid). I'm not an expert at reviving old batteries, but I've heard of people doing it. Some people just have a knack for taking what most people consider trash and making it do something useful (unfortunately, I'm not one of them).

Considering the case of solar thermal vs. grid tie PV, depending on how the generated electricity is utilized for the water heating and the local climate, PV may be more cost effective. For off grid, solar thermal probably still has the edge.

In a moderate to warm climate, a PV/ heat pump water heater scheme can probably be more cost effective than solar thermal. However, if the PV generated electricity is used to supply resistance heat, maybe not. Big reason: You'll need about 2X the PV surface area to do the same job as a solar thermal collector when firing a resistence heater. Mitigating that cost somewhat is the lowering costs on PV while solar thermal is more material and mfg. intensive. Solar thermal is also pretty mature so prices due to mfg. improvements are harder to come by.

Colder climates have different considerations for both solar thermal and PV. For most moderate climates, PV/heat pump can be more cost effective than solar thermal and another big factor can be that the PV/heat pump is probably easier to maintain.

All in all, where available, CH4 as a fuel for domestic water heating is probably the least expensive way of all available ways to heat domestic H2O in the U.S. at this time.

[lkruper]

Considering the case of solar thermal vs. grid tie PV, depending on how the generated electricity is utilized for the water heating and the local climate, PV may be more cost effective. For off grid, solar thermal probably still has the edge.

In a moderate to warm climate, a PV/ heat pump water heater scheme can probably be more cost effective than solar thermal. However, if the PV generated electricity is used to supply resistance heat, maybe not. Big reason: You'll need about 2X the PV surface area to do the same job as a solar thermal collector when firing a resistence heater. Mitigating that cost somewhat is the lowering costs on PV while solar thermal is more material and mfg. intensive. Solar thermal is also pretty mature so prices due to mfg. improvements are harder to come by.

Colder climates have different considerations for both solar thermal and PV. For most moderate climates, PV/heat pump can be more cost effective than solar thermal and another big factor can be that the PV/heat pump is probably easier to maintain.

All in all, where available, CH4 as a fuel for domestic water heating is probably the least expensive way of all available ways to heat domestic H2O in the U.S. at this time.

I just read someone post in another thread that one needs a battery when powering a DC resistance heater for hot water because of the resistance of the heater. Do you agree and also know why that is?

[inetdog]

I just read someone post in another thread that one needs a battery when powering a DC resistance heater for hot water because of the resistance of the heater. Do you agree and also know why that is?

Not quite true.

The problem with a fixed resistance heating element is that to make the most of partial power from the panels you would have to be able to increase the resistance of the element under automatic control since the PV output is close to constant current.
At half power from the panels, with a fixed resistance optimized for full power only 1/2 of the available panel power will be used. (i.e. 1/4 of max power).
With a variable resistance the full available power (1/2 of panel max) can be used.

If you put a battery in place, you drive the element with a near constant voltage and the MPPT CC can feed power into the batteries at solar max to be pulled out to the element when panel output is lower.
You still need a mechanism to shut the heating element off when panel power goes too low and the battery is getting pulled down below your threshold (~20-25% DOD?).

[lkruper]

Not quite true.

The problem with a fixed resistance heating element is that to make the most of partial power from the panels you would have to be able to increase the resistance of the element under automatic control since the PV output is close to constant current.
At half power from the panels, with a fixed resistance optimized for full power only 1/2 of the available panel power will be used. (i.e. 1/4 of max power).
With a variable resistance the full available power (1/2 of panel max) can be used.

If you put a battery in place, you drive the element with a near constant voltage and the MPPT CC can feed power into the batteries at solar max to be pulled out to the element when panel output is lower.
You still need a mechanism to shut the heating element off when panel power goes too low and the battery is getting pulled down below your threshold (~20-25% DOD?).

If a resistance water heater, say 200w at 12v, is optimized so that it puts out 200w at a particular amperage, what happens to the current/amps when the panel output is much lower? Without knowing any better (which I do not!) I would have assumed that all the power from the panels would go through the heater and turn into heat. Does the resistance from the heater somehow stop the current from flowing through it so that some is wasted?

[LETitROLL]

If a resistance water heater, say 200w at 12v, is optimized so that it puts out 200w at a particular amperage, what happens to the current/amps when the panel output is much lower? Without knowing any better (which I do not!) I would have assumed that all the power from the panels would go through the heater and turn into heat. Does the resistance from the heater somehow stop the current from flowing through it so that some is wasted?

Ohms law gets in the way, P=I(squared) x R, so with fixed resistance you can only have optimum performance with one specific current flow, and of course solar panels current output is all over the place.
Some popular elements I have looked at are around 1 ohm so a average 200w panel 7 amps x 7=49 x 1 is a whopping 49watts at max current, then early or late in the day when panel can only put out 3 amps (3x3=9x1)= 9 watts, so it is not linear, 3 is almost 1/2 of 7 yet you have dropped from 49w to 9w, when a solar panel is hooked direct to a fixed resistance load all of the power does flow through the load, it is just not usually a lot of power because of the constant mismatch of the current vs the resistance. In the example above your maximum capability of 200w would need about a 4 ohm element (7x7=49x4ohm)=196w, but power falls off the cliff fast when conditions are not perfect for max current. 3x3x4= only 36w from a pretty decent 3 amp ouput.

[lkruper]

Ohms law gets in the way, P=I(squared) x R, so with fixed resistance you can only have optimum performance with one specific current flow, and of course solar panels current output is all over the place.
Some popular elements I have looked at are around 1 ohm so a average 200w panel 7 amps x 7=49 x 1 is a whopping 49watts at max current, then early or late in the day when panel can only put out 3 amps (3x3=9x1)= 9 watts, so it is not linear, 3 is almost 1/2 of 7 yet you have dropped from 49w to 9w, when a solar panel is hooked direct to a fixed resistance load all of the power does flow through the load, it is just not usually a lot of power because of the constant mismatch of the current vs the resistance. In the example above your maximum capability of 200w would need about a 4 ohm element (7x7=49x4ohm)=196w, but power falls off the cliff fast when conditions are not perfect for max current. 3x3x4= only 36w from a pretty decent 3 amp ouput.

Thanks that helps immensely. It was not intuitive to me that heat dissipated through resistance was not linear. So if one plots Amps (I) against (Resistance) to show (Power) there is a sweet spot.