Dirrect Current For Resistence Heating?
rjw
| Posted in Energy, Heating & Insulation on
The Haiti mission group I do some work with has received a great donation from First Solar of a lot of high capacity solar panels.
One of the likely uses is to heat an oven in which they bake high nutrition muffins for the school children at the Brad Reddick School (http://www.missionsinternationalofamerica.com/BRS.html)
Since the power from the panel’s is DC, we need to find out how we can directly supply power to resistance coils. (Currently, they spend 9,000 a year for charcoal!)
Any advice?
“Ask not what the world needs. Ask what makes you come alive… then go do it. Because what the world needs is people who have come alive.”
Howard Thurman
Replies
P=I*V=I**2R=V**2/R
Resistive heating is exactly the same with direct current as with AC.
Just need to have the appropriate resistance for whatever the supply voltage is and the right number for the wattage needed.
First you need to figure the energy needed. And then the look at the energy that the panels can supply.
And you need to look a the time that the baking takes and how much temp fluctuation that it can withstand and what the thermal mass of the oven is. I am assuming that this will be used without batteries so that when a cloud passes over the heat stops.
And I don't have any idea of how well these store and how much capacity that they have to make them ahead of time and stock pile them. Might needs to keep the charcoal as backup, even online dual fuel.
William the Geezer, the sequel to Billy the Kid - Shoe
distance from the collectors to the unit would be a factor as well.Seems high tech where a reflector oven might do the same job cheaply.But maybe they feed too many people. 9K a year for charcoal is a lot, and that kills off a lot of trees too.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Yes, there is too many unknowns to be give much of a specific response..
William the Geezer, the sequel to Billy the Kid - Shoe
RJW,
Sounds like a fun and worthwhile project.
Since the power from the panel's is DC, we need to find out how we can directly supply power to resistance coils.
1. Resistance coils can operate with DC voltage - so that's good for you.
2. But bad for you, the PV panels can't be connected directly to the coils in the oven(s), for a couple of reasons.
A. The power output of the panels is related to the amount of sunlight hitting the panels - so the temperature of the oven(s) would vary depending on the weather, time of day, etc. This would be bad for muffin quality.
B. The power produced by a solar cell (or panel) at a given level of insolation varies as the amount of current varies. You need a controller to move the output voltage and current to the optimal power producing point. This is where the cell produces peak power. Now if you're not concerned with efficiency, then maybe this can be done manually - but there are safety considerations.
Perhaps your donation also includes some or all of the equipment you need in addition to the actual panels. Get some help from the donor.
Regards,
Jim x 3
thx...
"Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."
Howard Thurmanhttp://rjw-progressive.blogspot.com/
I just got this info:"These cells put out 60% in clouds, and we can use Batteries as a buffer and would do that.
now the question is, where to find DC heater coils"we're still waiting for detailed tech specs from First solar.And trying to figure out how to get the container into Haiti without it's being hijacked while moving through the port!
"Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."
Howard Thurmanhttp://rjw-progressive.blogspot.com/
"A. The power output of the panels is related to the amount of sunlight hitting the panels - so the temperature of the oven(s) would vary depending on the weather, time of day, etc. This would be bad for muffin quality."For something like this all you need is a thermostat. For example if the solar cells can supply enough power at 60% then size the heating elements for that. And it will run 100%. When it you get more power the duty cycle just drops.Bob - if you do google you there are thousands of suppliers heating elements for all kinds of stuff. And there are some premade elements that might work. Things like heating coils used for electric cars and I would suspect that there are some the marine industry and also off grid people, but that is probably rare.And there are suppliers of resistive coiled wire, just like was used in grandma's space heater and toaster. Just need to size the size of the wire and the lenght. And the under of units. Might be able to get the frame and insulators out of some old heat pump air handlers and restring them with the new wire..
William the Geezer, the sequel to Billy the Kid - Shoe
Bill,
I'm sure you and Junk could do it.
all you need is a thermostat - make sure your thermostat can handle the inrush current of the heating elements at the open circuit PV voltage. And that it can switch DC.
there are some premade elements that might work - this is a good idea, then the elements can be replaced 1 for 1 - the system doesn't need to be re-engineered for new elements with different electrical characteristics. Junk's idea for using regular oven heating elements is good.
But see my other post about operational considerations.
Regards,
Jim x 3
One of the beautiful aspects of the NiCr alloy used in stove and oven elements is that the temp coefficient of resistivity is < 10% that of tungsten, some NiCr alloys are zero, hence no worry about inrush.
edit BTW, the person we had throwing switches (just once a week, but it was a tracking array) was a U of HI student, 1 hr training <G>
If the voltage is under 150 Vdc minimal safety worries. Probably would be good though to make it ultra safe would be a few $2 FETs as controllers with 10 V switches for the gates.
Edited 9/7/2009 2:16 pm ET by junkhound
From what little I've read, you need a breaker panel as well.
You are running power into a structure. The controls for PV can get pricey. "There are three kinds of men: The one that learns by reading, the few who learn by observation and the rest of them have to pee on the electric fence for themselves."Will Rogers
A solar panel we built for a demo on Haleakala simply used resistance heating as the load, we were mainly concerned with measureing the cell and array efficiency at altitude. (ref) All we heated was the mountain air, about the same as a home oven.
To get the solar array producing the max power (like Jim said) we simply switched different resistance elements into and out of the circuit.
Your setup sounds like you are 'starting from scratch' for the design, including the overn heating elements.
If this were here and doing it at home on the super cheap, and depending on the size of the ovens, I'd simply go to thrift stores and get a number of old stove elements and wire them is series and parallel combinations to roughly match whatever the solar array configuration is. Would have a panel with a simple $10 power meter and about 10 plain 'ol wall type switches per oven, for every approx 200 sq ft of solar array.
I'm assuming on a clear day the array produces constant power for 10-15 minutes - so the cook would need to look at the meter every 10-15 minutes and throw switches one way or another so the meter reads the highest with the oven on. Setup dependent on if the array is a tracking array or simple tilted array (have assume fixed tilted, a tracking array would probably need more maintanence than available?)
Oven Temperature control by normal on-off thermostat, or the cook just throws switches to keep the temp constant, but that would take some interpretation and training.
If you can send me any specifics on the array (size, efficiency, orientation, volt-amp curves for various insolation levels, etc) I'd be glad to provide design guidelines and a schematic. A good designer should be able to come up with a manually operated system concept in a day or so, and which could be built in a few weeks for less than a few hundred $$. An Automatic design effort and system cost would be open-ended, depending on degree of automation - up to millions.
ref: http://gltrs.grc.nasa.gov/reports/2004/TM-2004-213361.pdf
Thanks for the offer.I've forwarded the info to the "bossman."
"Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."
Howard Thurmanhttp://rjw-progressive.blogspot.com/
Junk,
From reading your post (and your many previous posts in other threads) I'm confident you could design, build, and operate this manual system to bake the muffins. You are an impressive guy (in more ways than one).
But the cook in Haiti doesn't have your background (I'm assuming). Some significant training would be needed to operate this manual system safely and effectively. It can't be done by an untrained person just throwing the switches.
The system needs to be designed with an eye to the expected operational methods.
Regards,
Jim x 3
PS. You may want to rethink the use regular wall switches given the likely open circuit PV voltage and the low temp (low) resistance of the cold oven heating elements. Or maybe a startup switch needs to be added to this Rube Goldberg.
An inverter. Or the other one, whichever converts DC to AC (converter? Heck, one of the two).
No you don't.
Appliances, breakers, disconnect switches, fuses .... all have limits when it comes to DC. Either the rating is greatly reduced, or they're simply not approved for use with DC.
For example, the magnetic element in a breaker simply won't work on DC.
Don't cut corners, do it right. Panels -> disconnect -> inverter -> panel -> breaker -> appliance.
"do it right. Panels -> disconnect -> inverter -> panel -> breaker -> appliance."hmm...pretty techy - and expensive...."One of the likely uses is to heat an oven..."so, you are planning to take solar radiation, convert it to electricity, and convert that to infrared radiation in an oven via a 5 step process?I'd like you to consider an alternative - http://userwww.sfsu.edu/~urbs530/Solar/Oven/oven_link.htmhttp://www.globalcrisis.info/solarcooking.html#CChttp://www.solarovens.org/http://breadbaking.about.com/od/solarovenbaking/Solar_Oven_Bread_Baking_Instructions.htmuse the panels to run the computers - "there's enough for everyone"
I was thinking solar oven myself. I read a while back about a simple solar oven some college kids had developed for cooking use in Africa - it was a simple sheet metal affair that could be broken down flat for storage or transportation, it seemed to work pretty well too.
Pretty techie? There's a word you're leaving out: SAFE. Ordinary equipment isn't rated for DC. There may be times to hillbilly-rig things, but this isn't one of them. Sending the latest in technology off to third world countries, to br operated by ignorant personnel is not the place to be taking short-cuts.
Sending the latest in technology off to third world countries
Totally agree, no advanced technology, just on-off switches! That keeps it super simple, 70V dc (per the spec sheet earlier posted by Sam T) is plenty safe in a dry climate with just minimal safeguards.
Is there ANY codes in Haiti to 'worry' about. IMO 85% of NEC is to protect insurance companies, 5% common sense, the other 10% self serving supplier 'make them buy this' - not to get off the subject, but aka 'arc fault in every bedroom' ,etc.
I have no qualms about spitting on both hands and grabbing 70 Vdc, about the same body current (which is what safety is all about, not voltage) as 40Vac. Yes, I have purposely grabbed even 120 Vdc (dry) just to show folks it is not something to be paranoid about.
Would have no qualms about being able to teach my 6 YO grandson how to operate a simple switch system, should be easier for an adult?
Heck, my 10 YO grandson and 13 YO grand-daughter could build the whole thing with donated solar arrays. Think of the education the kids in Haiti would get beling let help with hooking up a solar array to help cook their muffins. May be a few future EE's in the group, once they get exposed to what is and what can be... which is what the probable purpose of the solar doanation is vs. just cooking muffins.
For 70W, I'm guessing about 2' by 4 ' panels, 70 Vdc approx at peak power per the list posted by Sam. Biggest simple system cost is a bunch of 18 AWG wire (smaller and it breaks to easy) run to a central control panel.
A typical surplus calrod small stovetop heating element just happens to be about 70 ohms. 30 of these under the oven and thermally insulated from the Carribean air. Pulls just the right current to operate the First Solar panels at peak power. Serendipity! Add a few surplus smaller elements for load peaking for light haze, cosine angle/time of day, etc.
Say Solar provides just 30 panels, connect to each with an 18 AWG pair (or a 4AWG Al common to minimize cost) running to a 'control' panel with only about 8 switches and a thermostat. Betcha some of the kids in Haiti have more complicated donated old gameboys and thus would intuitively know how to throw switches to keep a meter at the right number. Probably intense competition amonst the kids about who got to control the array while the muffins cook.
An example of 'kids' adaptation to technology of any level. 2 YO kids run Windows easily with a mouse. The folks who designed the dual hand controls with double thumb switches to fire stinger missiles from Humvees thought it would take (this was early '90s) 2 months to teach Army recruits how to efficiently target and operate the system. First recruits to try it took 2 HOURS to be more proficient thant the designers!
junkhound, in terms of electrocution, voltage makes no difference. Current, or the amperage is what determines a hazard.
Yep, that is what I said, not so?
grabbing 70 Vdc, about the same body current (which is what safety is all about, not voltage) as 40Vac
Current only counts when it's inside you.
It's the outside voltage that'll put it there.
I been complaining about that bit of safety propaganda since it came out in the 70's.
Had a senior radioman in the Navy who was deathly afraid of changing a fuse on a 60 amp circuit.
Only 16VDC driving that 60 amps.
I walked up, took my shoes off, moved the damme rubber mat out of the way, licked my fingers and placed them across the blown fuse.
Then I pulled the fuse and replaced it.
Shoulda seen his face (|:0)SamTA Pragmatic Classical Liberal, aka Libertarian.
I'm always right! Except when I'm not.
NO FAIR SAM, you knew whut you was doing!
>>or they're simply not approved for use with DC.
We're talking Haiti.
I don't think the AHJ in Savanette has a french copy of the NEC <G>
"Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."
Howard Thurmanhttp://rjw-progressive.blogspot.com/
Is thiss one of those locations where it is ccommon to 'borrow' power by limbing the pole and jury rigging a tag line onto the power companies supply?LOL, I saw some photos from our mission to Haiti twenty years ago that must have had twenty danglers for every 150' between poles in that barrio
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
First Solar panel specs:
Model Numbers
(+/- 5%)
You're gonna have to get the DC voltage to about the RMS of 240VAC, so maybe, sets of three panels in series.
Then, to get to 3kw to run a big oven, you'll need about 15 sets (of three) in parallel.
If the bakers are using a massive earth type oven, you can probably cut the total power in half, but it would take longer to get the mass up to temp.
An advantage of a massive earth oven is that it is very forgiving of short term (15mins) input variations.
An ordinary oven thermostat of the correct wattage will regulate the temperature.
See also:
System Designs
First Solar works closely with our customers to optimize the complete system design in order to obtain the highest system efficiencies and economics.
They may donate the design too.
SamT
A Pragmatic Classical Liberal, aka Libertarian.
I'm always right!
Except when I'm not.
Is this the oven?!?!?
View Image
You might need three or four standard 240VAC 2KW-3KW oven coils for that thing.
SamT
A Pragmatic Classical Liberal, aka Libertarian.
I'm always right!
Except when I'm not.
usually the bank of converters is connected to an inverter that is planned and sized to send AC online. DC requires larger wire to carry the same current due to heat loss.
I've heard of an alternative where each solar panel has it's own small inverter and they can be linked together as the system grows in size. For a larger system, this get too expensive by comparison, but for a small system, the individual inverters are supposed to be the way to start small and be able to grow.
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