air conditioner or thermal cooler?

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pfromero

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Jan 30, 2017
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60
Hi all,

Thermolectric coolers (Peltier cooler) seem perfect for our battery boxes, but they use a lot of DC electricity. I will be facing 90+ degree fahrenheit temperatures this summer with my outdoor battery box/inverter box. Does anyone use these? A/C units are off the shelf, easily returned under warranty (vs. Asian sourced products), and can be locally serviced if they break, but they are bulky and require an AC outlet installed near the battery box. As there are many, I think, putting their battery boxes together, I thought it would be worth some discussion.

Thanks,

Paul
 

Korishan

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Jan 7, 2017
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1) peltiers use a lot of current. I would not recommend them. plus they are expensive
2) some a/c units can be adapted to use DC power, or you can get a DC powered one for an RV

However, what I would recommend, and what I plan on doing for cooling my house, is use geo-thermal cooling. Not sure how much acreage you have, but if you able to, you could dig a pit about 4 foot deep (for cooling the battery shed, this would be fine) and make it about 12x12, and then use a flex tubing you can get from Homer/Lowes and a DC powered water pump (which are relatively cheap), and pump the water through a radiator of some sort; I'm using the coils from an old central unit and put adapters on the coils to hook up to a water source.

Temperature at 4' deep is about 80F, depending on where you live. At 7' deep, the temperate is closer to 72F. The nice thing is, once it's built, it's virtually maintenance free. Just two motors that have to be replaced/repaired if something goes bad (water pump, blower fan).

The peltiers would have an adverse affect on the batteries. Drawing current causes heat to increase, the increase in heat causes the peltier to need more current to move the heat. It's semi-endless and could lead to a runaway event. Secondly, how would you remove the heat from the building? The peltier only moves the heat from one side to the other. You'd still need to move the heat away from the batteries efficiently/effectively.
 

pfromero

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Jan 30, 2017
Messages
60
Korishan,

Luckily I read your post before I bought an air conditioner! I have had some training in ground thermal heat pump systems, and I wish I had one for my house. The main problem is that my soil is way too Rocky to ask a poor backhoe operator to dig hundreds if not thousands of linear feet of six foot trench to put the pipe in.

However I've been interested in this Energy System since 1991 when I went to a seminar about it. So yes... I'm going to dig probably two trenches x 20 feet each for some coiled low pressure pipe from Home Depot. This will help with the winter too! Thanks so much!

Paul
 

Korishan

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Glad to be of help and help save your wallet ;)

What you could do in the mean time if you wanted cooling now, is take some coils of tubing and put in a kiddie pool and put the pool is shaded location. Just keep the pool filled and it'll help decrease the temps in the shed (if hooked up like I mentioned earlier about the fan, water pump, radiator). Depending on how hot the shed gets, any little bit can help.

You could also look into a similar set up that an RV uses for the fridge unit. Most of them use an ammonia based system. The sun provides plenty of heat if you concentrate it a little bit. Not sure of your skills in those areas, but it's worth looking into as well. For me, those things are a little out of my comfort zone. I'm still learning to weld, solder, sweat, and braze properly :p
 

Elmo

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Feb 19, 2017
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189
I just built a Peltier cooled camera chiller for my Canon DSLR, it uses an 80mm x 40mm water block and two thermoelectric devices.. For long exposure night sky photography getting the noise level down is critical and cooling is the best way to accomplish that.

How much heat you can pump with a Peltier is strongly dependent on the thermal differential, 10 degrees C is one thing, 35 C like I'm pumping is another.

The first thing I would do is a rough calculation of the heat to be removed from your pack under a worst case scenario.

For instance if you have 1000 cells each of which holds six watt hours and say five percent of that energy will be released to heat during charging or discharging then you will have 1000 x .05 x 6 = 300 watt hours of energy in the form of heat to get rid of from a 6 kWh pack upon either charge or discharge. I have no idea of the actual numbers but my ebike packs don't get all that hot even when I have had them fairly stifled in containers.

It also occurs to me that if you have excess solar capacity at a time when your pack is both fully charged and hot then Peltier cooling would be more or less "free" at that point no matter how inefficient it may be.

On edit: One other thing, the amount of heat released by a cell is dependent to some extent on its internal resistance, lower Ri will result in less heat generated during both charging and discharging all else being equal. So if heat is a major concern for your installation then you should cull your harvested cells of those that exhibit a high internal resistance or generate excess heat in testing.
 

Korishan

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It also occurs to me that if you have excess solar capacity at a time when your pack is both fully charged and hot then Peltier cooling would be more or less "free" at that point no matter how inefficient it may be.


I agree here. Otherwise, that energy captured is just wasted. Might as well put it to good use.

Plus, when your shed would be getting hot, would be when most likely the packs are charged. So, I guess from that perspective, it would be acceptable. I'm now thinking of all the extrawiring you'd have to run. And if you add in water cooling in addition, lots of other extras :p
 

pfromero

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Jan 30, 2017
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Hi all,

Elmo, yes Ive built a DSLR cooler, too (smiley face here like Korishan's, above) ...small world eh? I really like the way you've really thought things through with the heat loss. I agree though that a Peltier cooler with a fan would save space, and the electricity would not be a drain when its hot and sunny.

I will have at least 1260 cells x .05 x 6 = 378 watt hours of energy. But at the very least, the heat from the sun beating down on a dark plastic deck box would be much greater. If you think 378 watt hours = about 6 75 watt light bulbs which get too hot to touch and then compare that to how hot the plastic of the deck box would be in the sun, and you spread the area of those light bulbs over the roof and 2 sides of my deck box, in my estimation, the heat from the suns is about 12 x the heat energy of the batteries.

a geothermal cooling/heating loop could be passed through the battery packs as part of its normal route...no need for heat exchangers...placing the cooling capacity right where it needs to be and leaving the rest of the box alone. An air conditioner or peltier would have to cool the whole deck box.

But it is alot of work to put a ground based cooler in versus an air conditioner or Peltier. Since you have knowledge of these devices, would you have a manufacturer in mind that might work for my deck box?

Thanks,

Paul
 

station240

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Oct 9, 2016
Messages
177
You can get 12V DC powered refrigeration systems, they are called car/camping fridges. No idea how much power one would draw just cooling a battery to 25C or something.

Someone gave me one, sadly the electronics proved unrepairable due to completely submerged in muddy water.
Compressor still works, just need to attach an ebike controller to make the motor spin properly again.
 

TheBatteries

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Oct 8, 2016
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I'll be curious to see how this plays out. I too will be facing these kinds of temperatures. It's not usual for it to hit 100F during the summer in my area, 90F+ is pretty typical during August. My plan was just to install some high speed fans to keep air circulating. I assume the batteries existing in 90F isn't a problem considering they use them in cars, bikes, etc. I just don't want the shed to heat up to 150F like what usually happens with no air circulation. I would prefer not to use refrigeration of any kind considering the amount of electricity that will be wasted.
 

Elmo

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Feb 19, 2017
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189
Korishan said:
It also occurs to me that if you have excess solar capacity at a time when your pack is both fully charged and hot then Peltier cooling would be more or less "free" at that point no matter how inefficient it may be.


I agree here. Otherwise, that energy captured is just wasted. Might as well put it to good use.

Plus, when your shed would be getting hot, would be when most likely the packs are charged. So, I guess from that perspective, it would be acceptable. I'm now thinking of all the extrawiring you'd have to run. And if you add in water cooling in addition, lots of other extras :p


The TECs I'm using are 40mm^2 and draw about 2.5A @ 8.5V each for maximum cooling effect in my particular use more current gets warmer as does less, that's not to say they would operate at the same parameters for what you want to do though.

Water cooling is optimal but that's not to say that a big heat sink with a fan on it can't work. Some of the computer coolers with the heat pipes leading to the fins are excellent. I'm only pumping a few watts of heat but I'm trying to maintain a 35C air temperature difference which means my cold side is closer to -50C below ambient so keeping my hot side as cold as possible is vital, also vibration with fans is an issue on a camera as is the starting and stopping torque of a fan when you do that for temperature control, water cooling eliminates both problems.

The real problem is that active cooling will eventually fail somehow or at least you should certainly plan for it to because if you don't your 18650 goose will most likely get cooked.
 

Elmo

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Feb 19, 2017
Messages
189
pfromero said:
Hi all,

Elmo, yes Ive built a DSLR cooler, too (smiley face here like Korishan's, above) ...small world eh? I really like the way you've really thought things through with the heat loss. I agree though that a Peltier cooler with a fan would save space, and the electricity would not be a drain when its hot and sunny.

I will have at least 1260 cells x .05 x 6 = 378 watt hours of energy. But at the very least, the heat from the sun beating down on a dark plastic deck box would be much greater. If you think 378 watt hours = about 6 75 watt light bulbs which get too hot to touch and then compare that to how hot the plastic of the deck box would be in the sun, and you spread the area of those light bulbs over the roof and 2 sides of my deck box, in my estimation, the heat from the suns is about 12 x the heat energy of the batteries.

a geothermal cooling/heating loop could be passed through the battery packs as part of its normal route...no need for heat exchangers...placing the cooling capacity right where it needs to be and leaving the rest of the box alone. An air conditioner or peltier would have to cool the whole deck box.

But it is alot of work to put a ground based cooler in versus an air conditioner or Peltier. Since you have knowledge of these devices, would you have a manufacturer in mind that might work for my deck box?

Thanks,

Paul

Paul,

Reading the EXIF on my 500D I can get the sensor down to 0C when the ambient is 30C, the sensor tends to run about 15C above the ambient when taking multiple long exposures so -15C ambient inside the cooler. Small world yes, and getting smaller.

Back on topic, what you are trying to do sounds like a recipe for overheating your cells. Insulate your battery from the rest of the enclosure and cool it with a smaller ground source loop.. That sort of heat isn't going to help the longevity of your electronics either so put them in the insulated box also. I'm not a big fan of active cooling because sooner or later it will fail, if your water flow gets blocked for whatever reason, pump quits, bacteria build up in the system, leak drains the water and so on you'll have to have a fail-safe or damage/lose all your cells. Coming from ebikes BMS can mean Battery Murder System or Suspect, I don't consider them a universal panacea to keep from damaging batteries. Yeah, Tesla can engineer a decent BMS and active cooling, I know I'm not remotely in their league so it's going to be KISS for me.

Since resistive heating in the cells/fuses is equal to current squared having more parallel cells for a given current can drop the heat load substantially. Twice the cells gives one fourth of the resistive heating per cell so half the heat generation from that source.

Yours is a tough problem, I'm not sure there are any good answers, even an insulated box will eventually get hot if you leave it in a hot place all day.

There is one thing to consider though and that is how long you have to dissipate however many watt hours of heat might be generated in the packs, to dissipate 400 watt hours over 8 hours is only 50 ongoing watts of cooling required. If you are draining a laptop cell based pack faster than about two hours you are pushing it into territory the cells were not really designed for in the first place.
 
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