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Old 02-08-20, 06:46 AM   #1
gadget
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Default DIY multi stage Heat Pump from multiple compressors

I like the idea of the variable output heat pumps but since my budget is in the DIY range, I'm thinking a home made 2 or 3 stage. Also, I like the idea of being able to use different power sources for each stage. Example would be a solar setup that is not tied to the grid/house. It could be used for some of the stages.

- first stage may be just an small fractional HP compressor comes on at say 98F
- Second stage another fractional HP compressor at say 95F
- Then a 1 HP compressor at 93F

They would all use the same water source that could also come on in higher flow rates as the stages ramp up. They would all be heating the same buffer tank.

So basically a multi stage DIY HP that is accomplished by using multiple recycled compressors with separate refrigerant systems but common heat exchangers sharing the same source water and heating tank.

Any thoughts or challenges? I may want to have the ability to change my buffer tank temp to change radiant heating rates but maybe I can just cycle the radiant flow on and off?

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Old 02-08-20, 08:16 PM   #2
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There are a few ways to approach this one.

First choice: Mad Max bartertown rube Goldberg contraption. Everything is independent of everything else and is simple as it can be. Cheaper but not so efficient or reliable.

Second choice: big single speed compressor and duty cycle control. Larger heat exchangers equate to longer cycles and higher efficiency. Price depends on what you can scrounge up.

Third choice: variable speed digital control and equipment. Efficiency, price, and reliability all depend on homework done up front and thorough testing and trials. Not usually cheap but more set it and forget it.
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Old 02-08-20, 08:19 PM   #3
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It would be simpler to use a VFD, possibly in combination with a switchable displacement compressor for more dynamic range.

I'm actually looking at something like that for a thermal storage system, using a 2-4 ton switchable displacement compressor in combination with a VFD to try to match compressor power draw to surplus PV power available. That's pretty far out, however, as I will work on other parts of the solar power setup first.
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Old 02-08-20, 10:57 PM   #4
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Quote:
Originally Posted by NiHaoMike View Post
It would be simpler to use a VFD, possibly in combination with a switchable displacement compressor for more dynamic range.

I'm actually looking at something like that for a thermal storage system, using a 2-4 ton switchable displacement compressor in combination with a VFD to try to match compressor power draw to surplus PV power available. That's pretty far out, however, as I will work on other parts of the solar power setup first.
VFDs are getting cheaper. Still out of my price range though. What type of thermal storage are you working on?

I was working on a setup where a solar load diverter would power a compressor with off the shelf parts. There are some DC compressors that could work. I was thinking one of the 3 phase brushless units and a load diverter solar charge controller. I know of one charge controller thats PWM output. Not sure how that would work with a brushless motor controller.

I have a load diversion solar controller but I think I'm going too try diverting power to some LED modules for adding extra grow light in my greenhouse for now.
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Old 02-08-20, 11:04 PM   #5
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Originally Posted by jeff5may View Post
There are a few ways to approach this one.

First choice: Mad Max bartertown rube Goldberg contraption. Everything is independent of everything else and is simple as it can be. Cheaper but not so efficient or reliable.
This will be the route I will probably go since I already have the compressors. I would think efficiency should be good assuming my refrigerant systems are built correctly(I'm still learning in this area) Typically, cycling should be your biggest hit to efficiency due to pull down time. Also, it should be more reliable since a stage can go off line and the others keep working making heat.

I'm not sure how long the compressor would last in the first stage since it would be running constant for days and days.
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Old 02-09-20, 10:32 PM   #6
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Quote:
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VFDs are getting cheaper. Still out of my price range though. What type of thermal storage are you working on?
A Prius inverter is about $100 or just above and will get you two 3 phase inverters to work with. Just replace the control board with some STM32s to turn it into a VFD.
https://openinverter.org/wiki/Toyota_Prius_Gen3_Board
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I'm not sure how long the compressor would last in the first stage since it would be running constant for days and days.
Why would you run it at night (discharging the batteries) when you can use thermal storage instead, which is orders of magnitude cheaper?
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Old 02-10-20, 01:36 AM   #7
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Why would you run it at night (discharging the batteries) when you can use thermal storage instead, which is orders of magnitude cheaper?
Good point!

I don't have it up and running yet but I bought a lister 8/1 clone last summer that could charge/power the batteries at night. They run off of any fuel oil. Im hoping to run it on used veggie oil, or...keep it just for back up to save on engine wear and go with a thermal store. I already have 3 indirect water heaters I have found cheap on local adds. I have an 104 Gallon Marathon plastic, 80 gallon IBC stainless, and a 50 gallon stainless(forget the brand).

I have not done the math yet on BTU needs/storage so they may be to small for over night. Do you know what size tank an average house would need for over night storage? Are we talking thousands of gallons?

I like the idea of over night storage. Thanks
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Old 02-10-20, 02:12 AM   #8
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Good point!

I don't have it up and running yet but I bought a lister 8/1 clone last summer that could charge/power the batteries at night. They run off of any fuel oil. Im hoping to run it on used veggie oil, or...keep it just for back up to save on engine wear and go with a thermal store. I already have 3 indirect water heaters I have found cheap on local adds. I have an 104 Gallon Marathon plastic, 80 gallon IBC stainless, and a 50 gallon stainless(forget the brand).

I have not done the math yet on BTU needs/storage so they may be to small for over night. Do you know what size tank an average house would need for over night storage? Are we talking thousands of gallons?

I like the idea of over night storage. Thanks
I couldn't sleep so I stayed up and did some research. 1 of my tanks could probably store about 50K BTUs (depending on temp). My 70's 2 story 2,000 sq ft home in a 6500 deg day area could loose that 50K BTU's in an hour.

One site said 500-1500 gallons storage is typical for over night or 10hrs of heat for average homes.

I don't have that kind of space so it looks like I can't do thermal storage for over night. That would of been a nice option. It explains the poor performance of my 200 sq ft greenhouse heat battery I tried earlier this winter. It was about 385 gallons of water but only at 100F
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Old 02-11-20, 12:58 PM   #9
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Ok so have you done an energy audit in the house? Reducing your heating needs is a much bigger better strategy to pursue than devoting your time and effort towards effective feeding the heat gobbling. Leak sealer and insulation is an investment that starts paying back immediately and continues essentially forever.

That being said, if you are trying to go modular with your idea presented in the other thread, and want to go mad Max on the cheap, I would propose a few ideas that work good for me:

Ditch the cap tubes and use txv valves as metering devices. Surpluscityliquidators dot com is an awesome source.

Learn to size and wind helical "worm coil" condenser assemblies and use flare fittings at first. Flare nuts and fittings are common and inexpensive.

Look through a few build threads and start swaging and sweating copper tubing. Start out with safety silver solder and flux and a propane blow torch.
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Old 03-04-20, 10:08 AM   #10
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I ran some calculations on my battery banks. I have a large 12v and a large 24v. With my 12v bank, I could run 2,000 watts with a 5 amp draw per battery. My 24v would be about 2-3amps per battery at that rate. So I may be able to run the heat pumps more off solar then I was planning at night. I have a good source for free batteries so I don't mind wearing them out.

I also have a slow RPM stationary diesel generator that I was planning on running at night. It will be co-gen and runs off any oil source. Its an 8/1 lister clone if your familiar with those. It will have a 12v alt and a 24v alt for keeping the batteries topped off. I could put a belt drive compressor and heat pump directly. I will also be getting tons of waste heat from the motor. The motor will also be running a large 240v split phase genhead (stamford clone) the can run the heat pumps via AC power. Its an 8kw head but I can get a little over 4kw out of it on this motor if noting else is running.

Ideally I would only use the generator for backup so as to not wear it out though they are designed to be ran and rebuilt and ran...etc

I have about 8.3kw of solar panels that are not yet in use. That I have been collecting for a while now. Not all are going up on the roof. I may keep about 3-4kw that I put out on my back lawn for winter time use running a heat pump.

I also am putting up some solar thermal collectors on the side of my garage. I'm hoping to get about 200-250 sq ft of area.

The one area I am lacking in is a large thermal storage. Just don't have the space. Guess its time to move and get some land.

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