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Prototype Thermoelectric Heat Pump

Post in 'The Green Room' started by Thomas Anderson, Sep 11, 2012.

  1. Thomas Anderson

    Thomas Anderson Member

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    You guys may or may not be interested in this. I'm currently tweaking a prototype thermoelectric heat pump I designed and built from basic HVAC components and a thermoelectric chip (TEC).

    In the world of off-grid living, we can only afford to run a costly (power-wise) window air conditioner or conventional dehumidifier on a very sunny or windy day, and even when trying hard to keep the window opening sealed, it's often self-defeating. I needed something that could run almost continuously if necessary and maintain the integrity of our tight building envelope. The primary goal is dehumidification with cooling secondary. For efficiency and reliability, DC is highly preferred. And for self-reliance, simplicity and minimal moving parts is highly preferred. A thermoelectric chip fits the bill perfectly. It is a solid-state device kind of like an LED which uses the Peltier Effect to cool one side and heat the other. But there are no commercial thermoelectric air conditioners, so I had to build my own.

    This is the prototype. It contains extra sensors, controls, and visibility for testing purposes. My production model will be opaque from the outside, will be controlled by a remote thermostat, will contain a larger TEC, and will be better insulated. Now that the proof-of-concept is complete, I'll be fitting my house for the necessary wires and ducts to install several production models in different rooms.

    By way of comparison, the smallest window air conditioner you can find in Walmart/Sears/Home Depot/etc puts out about 5000 BTU (nominally, anyway, but much of it may be wasted), draws about 500 W (EER of 10), runs on AC current (which ups the total wattage due to inverter losses), uses a compressor and refrigerant, leaks air around the window opening, and is a security hazard.

    My thermoelectric air conditioner puts out perhaps 100-200 BTU (the demo/prototype model here wastes some through the plexiglass window), draws 82 W (although the demo/prototype model here is wasting some for the flashy lights and sensors), runs on DC current (efficient from the house batteries), uses solid-state components with minimal moving parts (only DC computer fans), exhausts air efficiently though vents, preserves security, and requires almost no maintenance. That's a pretty poor EER (around 2), but the advantages outweigh the disadvantages.

    Now, 82W of cooling won't do diddly squat for most homes -- it would conventionally cool about 4 sqft. But for a very tight and well-insulated one, this is sufficient to make a bedroom or office comfortable. I'm installing my prototype unit in my utility room to keep the moisture down and to keep the heating system from over-heating the computer servers.

    I'll also be using my experience in creating these units to design my walk-in refrigerator's cooling system. It will have R-50+ insulation and require minimal cooling.

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  2. woodgeek

    woodgeek Minister of Fire

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    I like tinkering as much as the next guy....but you killed me at the EER of 2. Why not run a cheapo window ac on a 10% duty cycle to get 500 BTU/h cooling at an average of 50W consumption?? Or better yet, a 2 ton minisplit with an EER of 20, and run it 30 minutes a day??

    The reason there are no thermoelectric ACs is b/c of poor eff of peltier devices. Why they would be better in an off grid home eludes me.
  3. Ehouse

    Ehouse Minister of Fire

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    Bravo! I'm a fan of simplified/combined function/directech solutions. I foresee the day when a comfortable home can be inexpensivly built with no energy requirements other than what can be gleaned from the ambiant surroundings. Don't need no stinkin' solar panels!

    Ehouse
  4. Jags

    Jags Moderate Moderator Staff Member

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    A sleeping human produces an average of 81 watts. That leaves 1 watt for cooling my bedroom.

    Gotta love the tinkering side, though. Good work on putting it together.
  5. Thomas Anderson

    Thomas Anderson Member

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    I thought I covered that above, but maybe it wasn't clear. When you're off grid, you can't run several 500+ watt units, because you may not be paying attention and all of a sudden your batteries are almost drained. Also any AC device will lose 10%+ off the top from conversion. Also, AC motors (compressor) face power factor losses. Motors, pumps, refrigerant tubes, etc are also more prone to failure than a solid-state device. Also, more powerful units will cycle more, causing inefficiencies and temperature swings. A small solid-state device like this is simple, reliable, has a predictable and manageable power draw, and will provide a more constant conditioning of the air. I would love if it were also more efficient, but in this case it's not the most important thing. That said, for my next one, I'll use the slightly larger and slightly more efficient TEC.
  6. Thomas Anderson

    Thomas Anderson Member

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    That's an interesting stat. What it means then is that your body heat won't warm up the room while this is running. It will also be dehumidifying at the same time, also making it more comfortable. But I'm definitely going to up-size the TEC in the next one or maybe even use two of the smaller ones. This way it can fight both your body heat plus whatever gain there is through the windows mostly and the walls to a much lesser extent. I just have to watch the total energy draw.
  7. Thomas Anderson

    Thomas Anderson Member

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    Also, the material cost of this unit is under $200. I looked into DC compressor refrigeration components and it was like $800 minimum.
  8. woodgeek

    woodgeek Minister of Fire

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    Sorry. I'm dense. 10x higher efficiency has to trump 'you might forget and leave it on' and 10% conversion loss to AC. Ok, I put my AC unit on a timer so I don't forget, and I am still getting 9x as many cooling BTUs as you are per watt.hour input. And I refuse to worry about power factor.

    Also, I would expect your muffin fans and TEC unit to have at best similar operating lifetimes to a cheapo AC unit.
  9. Thomas Anderson

    Thomas Anderson Member

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    Well I'm not going to repeat the dozen things of import again, but efficiency, while desired, is not at the top of the list. The fan lifespan is 10,000-15,000 hours. The TEC much longer, probably 100,000+ hours. Compressors might last 5,000 hours if you're lucky. Less for a cheap one. Condenser coils will also probably spring leaks in under that time frame. Moreover, a DC fan only costs about $10 new, plus I've got a huge supply of old ones from scrapped computers. And it's a simple procedure to unscrew one with failed bearings and replace it. Not so simple or inexpensive to replace compressors and coils.
  10. Grisu

    Grisu Minister of Fire

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    I am fascinated by your project and regardless of its efficiency if you find it worthwhile that is all that counts. Nevertheless, I find it kind of funny that you many times try to support your arguments with some funny facts that don't hold up when someone takes a closer look. An AC compressor that lasts at best 5000 hours would need to be replaced easily every 3 to 4 years. (70 days x 24 hr use = 1700 hours x 3 years = 5100 hours) Living in VT I am not a big expert in AC units but find that rather unlikely. Why not sometimes admit that you do not know everything? It is also entirely your business what you do in your spare time; no need to get defensive about it.
  11. Thomas Anderson

    Thomas Anderson Member

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    I find your response curious. You make a strange calculation based on admittedly not being an expert and then accuse me of not admitting that i do not know everything? Really, that is an odd response. Anyway, if you were indeed running your compressor 24/7 for 70 days straight, then you'd probably replace it in 3-4 years or less. I'm thinking more along the lines of 5 hrs/day for 70 days gives you 14 years. That would be more likely and corresponds to my research. Perhaps if my argument doesn't sit right with you, you should ask for clarification instead of accusing me of not knowing what I'm talking about. I don't know everything, but I know what I know.
  12. jharkin

    jharkin Minister of Fire

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    I gotta say I see Grisu's point on this one. I and probably everyone else here on this board has personal experience with window AC unit that are 20 or 30 years old or more and still work just fine with zero maintenance, so I really don't see how the lifetimes can be that bad.

    I also would be really amazed if you could build a room that as less than 100W of heat gain no matter how well insulated and how good your windows are... Jags rightly pointed out that one persons body heat will overwhelm this thing. Then add a couple people, a TV or computer and suddenly you are dealing with a lot more heat than your 82w peltier can manage.

    No matter how you spin it you cant get around the fact that even the best peltiers have an efficiency of 10% or less, compared to the 40-60% achievable with refrigerant systems. I really don't think its physically possible you are getting better results even factoring in the DC to AC losses. As others have said you are probably better off finding a DC air conditioner and running it on a timer... I don't know first hand but Id bet there are DC based aircons for RV applications etc.
  13. Thomas Anderson

    Thomas Anderson Member

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    Again, I've researched this, and most air conditioner units last an average of 14-16 years, and they don't run 24/7 always on, but cycle on and off during the day and probably don't run much at night. If you run it 24/7, it'll fail much sooner. Also, compressors tend to have even shorter lifespans when powered by inverters. Mine's a true sine wave inverter, so that might not apply, but one of my goals is high reliability and lifespan with minimal maintenance, so it makes sense to take that out as a variable. Another goal is to try to eliminate the inverter for as many loads as possible so that it's not a single point of failure that can bring down my whole system.

    There are DC air conditioners, and I've heavily researched that before deciding to build my TEC system. They are expensive and don't solve a lot of the issues I mentioned above. I even bought a large portable DC refrigerator/freezer on eBay for the purposes of scavenging the parts, but ultimately decided that it would be plan B if my TEC system didn't work out.

    Heat gain is really not the biggest problem in my house. It's the summer humidity that's uncomfortable. As stated in the initial post, dehumidification is the number one goal. Cooling is secondary. My walls are composed, from the outside-in of 3-4 inches of stone veneer (or 1/2 inch of stucco on a few), 1/2 inch foam board, taped vapor barrier, 1/2 inch plywood, 5.5 inches of R-7.5/inch closed-cell spray foam, aluminum radiant barrier, 1/2 inch foam board furring strips, and 5/8 inch sheetrock. My windows are all low-e, high efficiency, with insulated radiant barrier curtains. And my home incorporates large amounts of thermal mass to maintain steady diurnal temperatures. That said, cooling is desired now and then, but not a lot of BTUs are required unless someone leaves doors wide open.

    But removal of moisture is the foremost purpose of my doing this. I preceded this experiment with just using low-powered dehumidifiers. Besides the aforementioned drawbacks of AC-powered compressors, I found that these added too much heat. So that had me thinking of ducting the heat out of the house. And as long as I was going to go through the effort of installing ducting, I would also try to improve on my other goals related earlier.

    I purposely used a lower-powered TEC in my prototype to evaluate how well it performs. I'll likely use a slightly higher powered one (100-120 W) in my production units. But I'm still evaluating the prototype performance. Maybe I will ultimately go back to a DC-powered compressor system, but right now I'm fairly hopeful about the TEC system and I'd appreciate if people dial back the negativity on here.
  14. begreen

    begreen Mooderator Staff Member

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    I hear a healthy amount of skepticism but folks are not being negative. Just challenging the assumptions and seeking data as they should.

    To improve the efficiency of the unit I would lose the windows and insulate it more thoroughly. Be sure to have good condensate drainage too.
  15. woodgeek

    woodgeek Minister of Fire

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    If my cheapo AC has 10x the BTU capacity of your peltier, then its ~10,000 hour lifetime (my minimum compressor lifetime guess) only runs a 10% cycle to match your units output, it then lasts 100,000 hrs of demand time, matching your TEC lifetime running 100% cycle?? And still uses 77% less energy to do it?? (I.e. EER of 10 vs 2 + 10% inverter loss)

    And since when does energy consumption not matter in an off-grid situation??

    I've always wanted a low draw DC timer that I can put in front of an inverter for stuff like this....and could never find one on the market....can you build one of those using off the shelf components??
  16. maple1

    maple1 Minister of Fire

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    My walls are composed, from the outside-in of 3-4 inches of stone veneer (or 1/2 inch of stucco on a few), 1/2 inch foam board, taped vapor barrier, 1/2 inch plywood, 5.5 inches of R-7.5/inch closed-cell spray foam, aluminum radiant barrier, 1/2 inch foam board furring strips, and 5/8 inch sheetrock.

    Isn't vapor barrier supposed to be on the warm side of insulation? Or maybe you mean air barrier (Tyvek)?
  17. Thomas Anderson

    Thomas Anderson Member

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    Yes, that's the plan for the actual production units. This is just a prototype to evaluate it's function and effectiveness, so I wanted to be able to peer in. They'll definitely be closed up more tightly with joints foamed. And no superfluous lighting or thermostats either. Drainage is good -- I made a little drip pan out of a 4" PVC cap and piped it out.

    It's not quite that simple, but I understand your point. You're overestimating your compressor expected lifespan and there are still drawbacks you're refusing to recognize. I already have window AC units. I've gone down that route already because it was easy. But they're not as effective or efficient as you would like to believe. The efficiency marked on the box is in an ideal lab setting, not the real world. In the real world, the window AC unit sits in the sun, is exposed to wind-driven rain, has leaks around the edges of the window opening, cycles on and off a lot, has power factor losses, is noisy, and it's a security hazard. If I were happy with window AC units, I wouldn't have started down this route. Also, several of the rooms in which I want air conditioning don't have windows suitable for one of those or don't have windows at all, so I need to be able to target them as individual zones without trying to condition the entire house. So this requires a ducted system.

    Energy efficiency certainly matters, but it's not the top priority in all cases, because smarter zoning, insulation, ducting, and usage can end up being more energy efficient and because having smaller, more predictable loads that can be managed better means not inadvertently over-discharging the electrical system. I'm not thrilled with the TEC efficiency, but its advantages outweigh its disadvantages for my purposes.

    There are DC timers available online. It wouldn't make sense to put a timer in front of your main inverter in an off-grid setup and a cheaper point-of-use inverter would probably send a dirty wave form to your compressor, shortening its life.

    You're right, that was a brain fart on my part. I meant moisture barrier or house wrap. The closed cell spray foam is my vapor barrier.
  18. woodgeek

    woodgeek Minister of Fire

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    Seems a better app would be a ASHP kettle for boiling water. Retrofit an insulated electric kettle with the cold half of your apparatus, and get a COP or 2+ to boil water on your counter??
  19. Thomas Anderson

    Thomas Anderson Member

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    Following my philosophy of redundancy and experimentation, I'm still working with the TEC-based experimental units, but I've also decided (given some good points made in this thread) to install an inexpensive 23 SEER 120V AC mini-split for my two offices. At around $800, it cost twice as much as my TEC-based units, albeit with a much higher BTU rating.

    It's got many of those downsides I've mentioned, however these are fairly commodity now with good reliability, and I'll try to control the other factors as best as possible. E.g. I'll keep the thermostat set high except when the offices are occupied to prevent inadvertent drawing down of the batteries. Also, the high efficiency means it will only draw about 5A when operating at max and only operate for a short period or at a reduced level (a built-in inverter powers down the motor under low load). The high efficiency should also offset conversion and power factor losses. The main issues remaining are that it is dependent on AC availability, so the main house inverter is a single point of failure. I'm trying to eliminate inverter-dependent loads. Also, it is unknown how the compressor will respond to the inverter-provided power. It's supposed to be a clean, pure sine wave, but I've had a refrigerator fail once already. Admittedly, it was an old fridge though, and may have simply been its time. So this will be an experiment to see how well and how long the mini-split performs.

    At the same time, I'll also be installing my TEC air conditioners, and based on their performance and that of the mini-split, these will be either the back-ups to the mini-split, or the other way around. It's certainly more expensive to have both, but my whole house is basically like this, with redundant systems. It's both research and fail-safe.

    I also found a 100% DC mini-split on alibaba.com, and I may try that if the AC one bites the dust early. It was way more expensive though and comes directly from a Chinese manufacturer with no U.S.-based support. It would eliminate my inverter concerns and even be more efficient (28 SEER), but at about 10x the cost of my TEC units, it's hard to justify it unless the TECs had to run constantly and still couldn't keep up. We'll have to see how they actually perform though. If neither the AC mini-split nor the DC TECs work out, the DC mini-split may be on the shopping list.

    Also, for my walk-in freezer/refrigerator, I'm now thinking of trying to use a DC compressor-based truck freezer unit as the primary, with TEC back-ups. I'm leery of compressor reliability, but they are indeed much more efficient. I'll continue my work with the TECs though to see if I can't squeeze better performance out of them. I definitely want a DC compressor for my freezer/refrigerator though rather than AC, to ensure better reliability and that the inverter is not a single point of failure that could ruin thousands of dollars worth of food. I may even put in a redundant compressor unit just in case, unless I can get the TECs to perform adequately.

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