Retrofitting a thermoelectric refrigerator with a conventional compressor system

A couple years ago, I built a nice wooden table that housed a small refrigerator. The idea is that it keeps drinks handy in the living room for parties and guests. It's also cool, and fits with my idea of what "functional furniture" should be.

I chose a refrigerator meant for storing wine because it had a nice glass door, was pretty small, was very quiet, and could operate facing upwards because the thermoelectric device doesn't care about its orientation. I knew that thermoelectric refrigerators generally suck at actually refrigerating, and this one was not exception.
On really hot days, the fridge would get up to 5o-60*F, which is cool, but not nearly as nice as drinking 40*F beverages. The other problem is that it draws 70W, essentially constantly. This comes out to 613 KWh/year. Most small conventional refrigerators use around 300 KWh/year. Also, I like to tinker and wanted to mess around with a refrigerant system.



I bought a small 1.8 cu ft Haier fridge off craigslist for $15. Geez, can't beat that! My first task was to pull out all of the system components from the insulated box. This required draining the system of refrigerant through a small hole that I drilled in the compressor fill tube. Before doing this, I checked the label, and it indicated the system used 1.6oz of R134a. I can buy R134a at the auto parts store, so I will be able to refill the system.


I de-soldered the tiny capillary line from the accumulator / dryer and de-soldered the suction line on the compressor. I used an oxy-acetylene torch to heat the joint, then I just pulled the tubes apart when the solder became molten.

Here is the evaporator liberated from the fridge.



Luckily, with one flap unfolded, the evaporator fit perfectly in the thermoelectric cooler (TEC) fridge.


I mounted the compressor underneath the table (also getting very lucky that there was enough clearance). I cleaned the copper tubes carefully, then re-soldered them with silver solder and some paste flux. I had to extend the suction tube by a few inches, and so I just got some tube that fit around the existing line and soldered it to the outside.


I soldered the capillary tube back into the dryer, and put a valve on the compressor fill tube. The system was sealed at this point. Now, I connected my vacuum pump (Welch 1400) with a micron vacuum gauge and a tee that connected to a can of R134a. I positioned the can on a sensitive scale so that I could meter out 1.6 oz of refrigerant.

I pulled a 375 micron vacuum in about 15 minutes or so. I even ran the compressor while the system was under vacuum. It raised the pressure just a couple hundred microns, then it settled back down quickly, so I felt the system was dry and sealed.


After I metered out 1.6oz, which only took a few seconds with the valve just cracked open, I tightly closed the valve that I added to the fill tube, and was very pleased to see feel the evaporator getting very cold.

Tomorrow, I will re-insulate the TEC fridge and hopefully give it a final test.

DIY aquarium chiller

See update here:
http://benkrasnow.blogspot.com/2010/04/titanium-heat-exchanger-for-diy.html

My nano reef aquarium is usually 2-3*F hotter than the ambient room temperature (after the heater setpoint has been reached). This is a problem, since the temperature in my living room is often higher than 82*F in the summer. This puts the tank water at an uncomfortably high temperature (84+), and I think that the corals suffer from the temperature swings as well as the overall high values.

So, how to lower the tank temperature? For a 5 gallon tank like mine, a peltier heat pump like the Coolworks Ice Probe would seemingly be a good choice. I tried building just such a device a few years ago, and it was a big failure. I learned that peltier heat pumps cannot be controlled by raw pulse width modulation (PWM) signals, and they don't do well in thermostatic (on/off) systems either. One reason is that the semiconductors inside the Peltier device do not like the thermal shock of the constant on/off switching. Also, Peltier heat pumps are already horribly inefficient, and using PWM or on/off control makes things even worse. During the "off" cycle of either the PWM pulse or the on/off cycle, the heat will flow backward though the device -- the same heat that the device just pumped during the "on" part of the cycle. Think of bailing out a sinking boat with a bucket that has a huge hole in the bottom. The best way to control the peltier modules is to generate high-frequency PWM, then smooth it out with an inductor/capacitor filter. There is still the problem of the peltier junction's inefficiency, and the hot-side heatsink must be massive with a massive fan to make the system viable. Anyway, I haven't heard anything great about the Ice Probe, nor any other Peltier cooling systems designed for any application that requires a good amount of cooling. I have a thermoelectric refrigerator that is just marginally good enough for its purpose.

So, today's design for a new aquarium chiller will NOT use Peltier junctions, as much as I love the idea. I bought a $99 water cooler that uses a conventional compressor and r-134a refrigerant.
I filled the cooler with tap water, and mounted a Rio pump with an outgoing hose and return line.


The two hoses connect to a stainless steel coil. I've had this thing laying around my shop for a long time. It came out of junked, expensive lab equipment. It is non-magnetic, which indicates 3-series stainless steel. I'm guessing it's 316, which is highly corrosion resistant. Of course, the aquarium purists would insist on titanium, but I don't have any, nor do I think it's really necessary. I'd love to hear from anyone who saw a stainless steel chiller coil corrode, or definitively caused tank poisoning.

I melted a couple slots in my hang-on cheapo protein skimmer (it's not a Skilter, but very similar). I would have used a dremel, but I didn't feel like taking the filter off the tank, and I also wanted to avoid getting plastic shavings in the water. The stainless coil sits down into the slots and is just held by gravity.

The cooler is plugged in all the time. It keeps its insulated water chamber around 47*F. The Rio pump in the cooler is turned on and off by the PID temperature controller that I mentioned in a previous blog post. The controller can be configured to use a longer cycle time (eg 30 seconds) since it is controlling a pump, and it would not make sense to turn a pump on and off once per second as it would be for a heater.

The cooler is rated 86 watts. If this is what the compressor draws while normally running (I didn't check it). I would estimate the cooler can pump about 170 watts of heat (about 580 btu/hr). The coefficient of performance is around 2 for small compressor systems. For comparison, a large peltier device can move around 70W under ideal conditions, at a very specific current/voltage. The coefficient of performance for Peltier devices usually tops out around 1, and is often about 0.5 for realistic situations. So, a peltier pump drawing 86 watts, would only pump about 43 to 86 watts of heat.

I just installed this chiller today, so I'll monitor it on hot days and make another post about its performance.