Dump all excess PV to air heater?

Thank you just the same, I choose to beleive the engineers at MidNite who say in unison not to put anything brtween a MPPT controller and the array that it is controlling..., not panel optimizers, not anything......but then i do not have any price point equiptment, Its all the best of the best...MidNite,Outback,Trace,Magnum Energy, Morningstar, Exeltech, Dynamote Brutus.......the very first high/low frequency inverter.

Read the wiki at wikipedia: maximum power point controllers.....

Personally I plan on experimenting with Peltier Heat Pump for divert load to simultaneously heat and cool water in separated insulated containers. Should work the same with whatever load be it a heater or whatnot.

I intend on using battery pack voltage sense to determine when the arduino will turn the load on or off.

Given knowledge of the LFP charge curve, simply divert load when pack exceeds 3.375 volts per cell. Adjust that threshold to optimize.

I’m too chicken to muck with the higher voltage PV side so I’m sticking to battery diversion due to lower and more consistent voltage.

Peltier heat pumps are horribly inefficient, modern dc permanent magnet refrigeration is far, far, far more efficient than Peltier heat pumps

Tecnodave said:

Peltier heat pumps are horribly inefficient, modern dc permanent magnet refrigeration is far, far, far more efficient than Peltier heat pumps

Click to expand...

Peltier run at *high voltage* is horribly inefficient.

Peltier run at 1-2V per 127 junction module is efficient comparably so to refrigerant based compressor units. CoP of 2-4 can be achieved which is comparable with compressor.

Edit: trying to provide actionable info..

Peltier are sold in grades based on their resistance. The rating is for how many amperes a single module can handle. For the 40x40mm modules the lowest efficiency rating is 3 ampere and the highest efficiency is 15 ampere.

15 ampere modules have ~1 ohm ac resistance
6 ampere modules have ~2 ohm
it just gets worse the lower ampere rating you go.

this means twice as much Joule heating if one sources TEC1-12706 instead of TEC1-12715 modules. Joule heating is inefficient compared to pumping heat.

not all peltier are the same. they can operate efficiently. happy to give more advice.

peltier element have a Max Voltage rating. Often 15.4V. do not run the module at this voltage! run at 10-20% of Vmax. this is how to access Efficient Peltier Life.

The trick is that the modules can be stacked to sum the pumped heat. When run at 10-20% of Vmax the temperature difference across a single element is too low alone.

With ambient air 25C I’ve cooled water down to 7C and heated up to 70C using a stack of 20 units of TEC1-12715 modules arranged five side by side and four layer between two aluminum water blocks. It uses 80-100 W and it drives the peltier direct from the LFP battery. solid state heat pump

3S LFP battery. 9-11V. Using an H-bridge to drive all twenty peltier modules as one string. Electrically like a solar string it’s 5S4P wiring.

9/5 = 1.9 V per peltier module
11/5 = 2.2 V per peltier module.



This is a coefficient of performance vs voltage graph for a TEC1-12715. Check the blue curve. It represents 10C delta temperature which is good for four layers. 4 layer x 10C = 40C max delta. Can get down to freezing if it’s ~30C ambient..

Anyways.. here’s a pic of the test setup:


the center display shows hot vs cold (primary vs secondary) side temp. 34C hot side 10C on cold side with 78W usage.

I was able to get that same side from as low as 7C all the way up to 70C. All while using <100W I think it’s fairly efficient. Haven’t evaluated CoP quantitatively yet. I expect CoP 1.5-3 depending on specific operating temperature.

What kind of compressor....? Not a Danfoss permanent magnet motor, My Grape Solar 5 cu.ft. danfoss powered referigerator uses 42 watts with a 15-20% duty cycle, my coleman cooler uses 48 watts non stop 4.0 amps at 12 volts (it does not have a thermostat).....and it will not freeze using 5 times the power of the Danfoss compressor.

I have never seen a Peltier cooler that is as efficient as a Grape Solar or Sun Danzer, can you name a product that uses Peltier technology that uses less power than a SunDanzer..........commercial product with a listing available today.....I would like to see that published information

I can’t name a product; all the commercial peltier fridges I’ve seen are total crap and not worth anyone’s time. they all appear to use a single layer, a hopeless endeavor. thin insulation. futile.

that’s why I’m building it myself ?

i’m building a fridge using 2” of vacuum insulated panels on all sides.. ~R40.

this same TECnology will work perfectly for my hot water needs if scaled up.

another issue with commercial peltier fridges that make them silly is that the peltier directly interfaces with the inside volume!! heat flows rapidly back into the box!! futile.

i plan on using two water circulation loops. the inside cold area will be cooled by pumping cold water through silicone tubes and into an aluminum water block and maybe copper heat spreader..

by modulating the pump you can modulate the insulation. this is fundamentally superior principle of operation compared to direct conduction peltier fridge as you will find in the “travel coolers” aka battery killing food spoilers

curiouscarbon said:

I can’t name a product; all the commercial peltier fridges I’ve seen are total crap and not worth anyone’s time. they all appear to use a single layer, a hopeless endeavor. thin insulation. futile.

that’s why I’m building it myself ?

i’m building a fridge using 2” of vacuum insulated panels on all sides.. ~R40.

this same TECnology will work perfectly for my hot water needs if scaled up.

another issue with commercial peltier fridges that make them silly is that the peltier directly interfaces with the inside volume!! heat flows rapidly back into the box!! futile.

i plan on using two water circulation loops. the inside cold area will be cooled by pumping cold water through silicone tubes and into an aluminum water block and maybe copper heat spreader..

by modulating the pump you can modulate the insulation. this is fundamentally superior principle of operation compared to direct conduction peltier fridge as you will find in the “travel coolers” aka battery killing food spoilers

Click to expand...

Yep, you said it....all the commercial peltier coolers are krap...........I do not need to waste my time with that, I have been using Danfoss powered referigerator for 12 years now.......

curiouscarbon said:

not all peltier are the same. they can operate efficiently. happy to give more advice.

Click to expand...

If I order from my favorite Chinese store-

10pcs/lot TEC1 12715​

and my voltage range is 12.6-14.8v or 0-21v how would you arrange these?
Application- decide during transit time.

Please dont get me wrong, i am a technologist, retired from the industry, I am tinkering with the latest.

sunshine said:

If I order from my favorite Chinese store-

10pcs/lot TEC1 12715​

and my voltage range is 12.6-14.8v or 0-21v how would you arrange these?
Application- decide during transit time.

Click to expand...

please help me understand these two voltage ranges.

based on bench power supply experimenting i decided 1.9 volt to 2.2 volt per TEC1-12715 in series is the band i would focus on for maximum efficiency without having to stack too many layers. even 1.4 volt applied resulted in a noticeable amount of heat being pumped.

napkin math for 12.6 volt to 14.8 volt:


thus, i suggest you evaluate 7x TEC1-12715 in series for this voltage range. that will result in 1.8-2.1 volt per TEC1-12715 and thus highest efficiency.

0 volt to 21 volt:

0 volts would result in no heat being pumped, so i would avoid that and focus on the 21 high end.

napkin math:

thus i would suggest you evaluate 9x TEC1-12715 for this voltage range. that will give you strong yet efficient heat pumping at 21 volt input to 9 series peltier, at 2.33 volt per TEC1-12715. Performance will go down as the voltage goes down.

hope this helps, good luck with any experiments you do.

https://diysolarforum.com/threads/solving-the-climate-control-crises.23322/post-291839

https://diysolarforum.com/threads/i...on-setup-using-solar-energy.22301/post-263312

two other posts with additional info.

here’s my experimental data spreadsheet from testing 3x TEC1-12715 in series stacked on each other with thermal pad between each. used a cpu cooler with heat pipes.
https://diysolarforum.com/threads/will-im-sorry-but.525/post-288747

curious carbon,

I did hong haul sailing for a while, that is where i saw the best of the peltier coolers, installed through hull, if you do develop this to a commercial product there is a ready market. After 15-20 days you would do anything for some fresh food......

Tecnodave said:

installed through hull

Click to expand...

wow cool!!! dissipating heat directly through the hull sounds so cool! either through the hull or by drawing in sea water through a heat exchanger. i would worry about biology clogging a pumped heat exchanger. thank you for mentioning this interesting application!

and it worked very well, though it was expensive, too long to remember a number but at 20 days we had fresh food, cooler built into yacht with maybe 5-6 inches insulation, thru hull radiator , the yacht had a small wind generator for power, i just was along as crew, not my boat, i suffered with a ice chest on my small sailboat. ....ugh.

Tecnodave said:

and it worked very well, though it was expensive, too long to remember a number but at 20 days we had fresh food, cooler built into yacht with maybe 5-6 inches insulation, thru hull radiator , the yacht had a small wind generator for power, i just was along as crew, not my boat, i suffered with a ice chest on my small sailboat. ....ugh.

Click to expand...

thank you for sharing your experience, very interesting to learn about challenges encountered during extended sailing. shame they didn’t have you set up with one too. sounds like they at least shared a bit ?

rhino said:

I have a Victron system and was wondering if there is any example of how I can make sure the solar charge controllers are always running at 100% and any excess the battery does not need gets diverted to some sort of electric heater to heat air. I used to have a system that used relay when the batteries were full but that would cause some power to be pulled from the batteries and it was always either fully on or off. If I have a 2kW heater for example, it would turn the full 2kW heater, wait for the voltage to drop on battery, then turn off when the battery voltage was below a setpoint. so you'd get wild fluctuations... I want the battery to stay 100% full and dump in this case everything else to the heater. Does someone know how you'd set up such a system?

Click to expand...

I am planning on using peltier to simultaneously heat and cool separate loops of water efficiently.

Peltier are actually strictly more efficient than resistive heaters. Fun fact!

This is because they act as both a typical resistor that does 100% efficient Joule Heating but also Joule Pumping!

All this assumes you have a latent source of heat around such as ambient air or ground conduction or nearby water body etc…

Managing the complexity of many peltier modules operating in cooperation is totally a big downside of this as compared to resistive heating. Way easier to use resistors and rely only on Joule heating.

That said, I’ve been doing experiments with 20x peltier in one brick sandwiched between two aluminum water blocks. One water block goes to an aluminum radiator. One water block goes to another water block that I hold in my hand. Each side has a dedicated dc motor pump and the radiator has a 12v pc fan. MOSFET to toggle each component with arduino. It uses 80-100W with 20 peltier modules 40x40mm

edit: It’s fun to cool down the hand side and feel the heat showing up at the radiator. Then flip the H-bridge and heat up my hand. 42C kinda feels nice and warm. the weirdest part is that the radiator side becomes colder than ambient for the heating on the other side! totally blows my mind but i guess physics is real! ?

curiouscarbon said:

please help me understand these two voltage ranges.

Click to expand...

Thanks for the reply.
These are just the voltages either side of the pwm CCs.
Your info has clarified a few things.
Probably use a few on the fish tanks. Heating or cooling depending on the season
Another possible use is sticking these on the back of a normal fridge to assist...are there any heat sink adhesives?

Been using some VHM-009 over/under battery voltage modules. Very reliable when switching another relay.

​

sunshine said:

heat sink adhesives?

Click to expand...

This is what I used between the peltier/copper/aluminum interfaces: https://www.amazon.com/Smalloo-100x100x1mm-Conductive-Resistance-Temperature/dp/B0922C83Z7

it is merely sticky think vinyl statically clinging to glass maybe a bit stronger?

thermal conductivity 8 W/(m•K) is decent.. peltier face is 30. aluminum is ~200. copper is near 400.

it’s weak adhesive though and won’t resist much pulling force and probably would eventually give with even a tiny constant force. i have wrapped the entire inedible sandwich with tape to keep the thermal pad firmly squeezed against the interfaces.

thermal epoxy exists and if you search newegg or other places it should be abundant. that sort of thing in a tube can hold much stronger and longer.

if you want to be a total crazy person you can use thin indium sheets which has 80 W/(m•K) thermal conductivity which is great for thermal interface material which usually ranges like 2-8 for PC thermal paste from my reading. it has no adhesive properties though. very easy to disassemble though. food for thought

really like those applications!

lol now i’m curious what a peltier assisted minifridge with overbuilt insulation would be like *think think*

the midea fridge i have has the condenser embedded into the left and right outer walls. using thermal pad to mate a few 240x80mm aluminum water blocks to cover the area that gets hot(condenser) and then even just pump that through to an external radiator might allow for overbuilt insulation.

anyways trying to focus on heating, but refrigeration with heat pumps has the flip side of pumped heat to be utilized

here’s the data from my recent test:




ambient ~25C

btw i’m using an ac resistance meter to spot check modules and will be using it to match modules going forward. most individual TEC1-12715 modules exhibit 0.8-0.9 ohms ac resistance as measured by yr1035 meter.

it’s so weird, i can feel the cycling heat very gently from the AC resistance measurement waveform. just mentioning this so people can check if they got a fake module

you can be fairly certain that your TEC1-12715 is fake if the ac resistance meter indicates in excess of 1.5 ohms.

I dump excess power into a grid tie inverter from midday onwards during the sunny seasons using a voltage sensing relay and a 24hr timer.

The issue that many don't think about with peltier they are a window to the world when they aren't powered. Heat loss is an issue when they are opportunity loads.

pollenface said:

I dump excess power into a grid tie inverter from midday onwards during the sunny seasons using a voltage sensing relay and a 24hr timer.

Click to expand...

Statistically these systems work well. I favor timed systems that operate a device for a short period of time and then check voltage again instead of waiting for a drain down voltage. Again looking at panel voltage is probably a better indicator charge state.

efficientPV said:

Statistically these systems work well. I favor timed systems that operate a device for a short period of time and then check voltage again instead of waiting for a drain down voltage. Again looking at panel voltage is probably a better indicator charge state.

Click to expand...

You're completely right. I'd do better if I had a system to disconnect the panels and repurpose them but I'm not that advanced yet.

That said I do use a 10w panel as a "sun sensor" to trigger automotive relays for other projects.

curiouscarbon said:

here’s the data from my recent test:

ambient ~25C

btw i’m using an ac resistance meter to spot check modules and will be using it to match modules going forward. most individual TEC1-12715 modules exhibit 0.8-0.9 ohms ac resistance as measured by yr1035 meter.

it’s so weird, i can feel the cycling heat very gently from the AC resistance measurement waveform. just mentioning this so people can check if they got a fake module

Click to expand...

Have you measured temperature change and water volume though a heat exchanger? Or rate of temperature change for a volume of water?
That would let you determine Joules or BTU/hour for a given power input and temperature delta between hot/cold sides.

curiouscarbon said:

wow cool!!! dissipating heat directly through the hull sounds so cool! either through the hull or by drawing in sea water through a heat exchanger. i would worry about biology clogging a pumped heat exchanger. thank you for mentioning this interesting application!

Click to expand...

Do a search for 'keel coolers'. They have been used decades.

Hedges said:

Have you measured temperature change and water volume though a heat exchanger? Or rate of temperature change for a volume of water?
That would let you determine Joules or BTU/hour for a given power input and temperature delta between hot/cold sides.

Click to expand...

good idea ? i’ll try running the pump continuously and measure the weight of the water it pumps over one minute or more and note the grams and seconds elapsed. this should give a decent approximation of the grams per second flow rate.

measure mass of aluminum water block empty vs full of water to evaluate volume in block.. somewhat annoying to measure total volume in loop…hmmm i could use known flow rate from above and then time how long a lone air bubble takes to transit that side of the loop.. hmm…. thanks

attached is the csv log of this test run that lasted about a half an hour real time. had to change to txt to upload, but it's csv.

1. cool down the secondary side to ~8C
2. deactivated, backflow mode
3. heated secondary side ~54C
4. deactivated, backflow mode
5. cooled secondary side to ~8C again
6. force both side temperature to converge to ~25C ambient at the end

easy to see how fast the heat backflow is when deactivated. These are measured at the Water Block. The process volume is separate from the Water Block Heat Exchanger.


there are three thermometers mounted right now. one touching the back side of the big water block “primary side” and two on the “secondary side” one back of water block and one right on the copper surface that interfaces with the water block. the arduino writes all three temperature to serial usb every 200ms for computer log

the thermometer touching the plate has quickest response of all three of course. i used thermal pad to affix each thermometer to maximize heat flow into them. don’t want to measure wire temperature. the two on back of water block have delayed response speed relative to the plate one.

efficientPV said:

The issue that many don't think about with peltier they are a window to the world when they aren't powered. Heat loss is an issue when they are opportunity loads.

Click to expand...

effectively not an issue if you have them insulated from the body you’re cooling or heating and use a water pump to transport the heat. you’re right that heat back flows across the peltier module when it’s turned off. the graph above reinforces that you’re right. camper coolers allow this back flow heat to reach the food, that’s a design flaw as you say. silicone is not very conductive.

the tiny water pumps i’m talking use ~5W each at 12V so it costs 10W to gain ability to modulate the back flow of heat. never let the peltier directly touch the process volume unless you’re okay with instant back flow of heat when deactivated. check out my graph and you’ll be able to even observe how quickly the back flow occurs

heat will very slowly back flow along the water and the silicone tubing, but water has thermal conductivity of 0.6W/(m•K) and high specific heat capacity and the inner diameter of the tube is ~7mm so the back flow will be at least one order of magnitude less than a 40x40mm peltier plate which has 30W/(m•K) thermal conductivity for the alumina surface but only ~2W/(m•K) for the bismuth telluride inside

(pi*(7/2)^2)*0.6 vs (40*40)*30 ? is what we are talking about



thus, the coleman peltier cooler type should be about 2000x worse at managing heat backflow than the solution i’m talking about with pumps

Hedges said:

Have you measured temperature change and water volume though a heat exchanger? Or rate of temperature change for a volume of water?
That would let you determine Joules or BTU/hour for a given power input and temperature delta between hot/cold sides.

Click to expand...

Math...

The secondary side has four main components other than water.

1. aluminum heat block touching peltier stack
2. aluminum heat block touching process volume
3. silicone tubing
4. water pump

Rough numbers..

1. 176 grams. 200x40x12mm. aluminum 2.7g/cm^3. solid block would be 259.2 grams. implies 30.8 cm^3 volume inside this block.
2. 75 grams. 80x40x12mm. aluminum 2.7g/cm^3. solid block would be 103.7 grams. implies 10.6 cm^3 volume inside this block.
3. ~40 cm length. inner diameter 7mm. 0.38 cm^2 area inside. implies ~15 cm^3 volume inside tubing.
4. just going to guess and say 3 cm^3.

Ok.. so I'm going to estimate 30.8+10.6+15+3 = 59.4 cubic centimeters of volume in this loop.

Water is ~1 gram per cubic centimeter. 4,184 Joules to increase 1 kilogram of H2O temperature by 1 degree celsius.

4.184 Joules to increase 1 gram of water by 1 degree celsius. ~60 grams of water.

So if I haven't gone totally off the rails with my calculations and estimates... approximately 250 Joules per degree celsius changed in the water loop.

This is purely an estimate of the water volume, and ignores the specific heat capacity of the aluminum blocks.

I wrote a script to take the previous 25 log entries and tabulate the average temperature change over that period. That worked out to about 21 seconds of past data being taken into account. Attached is that printout of the derived data showing delta temperature of the secondary aluminum water block over time.

During the period of heating the secondary side, a steady 0.25-0.35 degrees celsius per second was averaged. Since this thread is talking about air heating and water can service that..

That means... 75 Joules per second... 75 W thermal transfer? 255 BTU/hr? Doesn't really seem like every element is running at good efficiency.

edit: mass of aluminum in loop is ~250g and at 0.9 joule / gram Kelvin it should account for an additional 225 Joules per degree change. ? any thermodynamics knowledge advice here would be greatly appreciated.

edit2: there are copper heat spreaders inside the stack.. 250x63x1mm -> ~15.75 cubic cm.. it's 140g... 0.38 J/g K for copper.. there are 5 of these copper plates... that implies an additional 266 Joules per degree Celsius change... probably better to delete those, not clear if they actually provide a benefit.. i included them in the hopes of ensuring the adjacent peltier modules at each layer would stay close in temperature.

My end goal is to individually power each layer, and individually monitor the Seeback Voltage of each string/layer. This would allow for realtime feedback of the delta T per layer... still working on that.. As I move on to 7-10 layer designs, individual power control and monitoring of Seeback Voltage will be essential to efficient operation under a variety of conditions. This test was with 4 layers because i got lazy and got tired of applying thermal pads on the way to 7 layers prototype. It's so tedious to apply the thermal paste that I want to design 3D printed brackets with edge space carved out so I can just add a bunch of thermal paste and put it in a vice and have the excess go into the seam.

I ignored the thermal mass of the water blocks, so I am probably very off the mark by now. But I tried.

this makes me want to add more layers and lower the voltage. in my tests, 1.0V per module resulted in twice as many degrees change per watt as 2.0V per module.......

at this point just writing a script to model the individual surface area, mass, specific heat capacity, instantaneous temperature, of each material in the stack seems like my only real chance of being rigorous.

Anyways, feedback welcome.