Did a little rearranging and optimizing in hopes of characterizing just a little better before I make the plunge into the chip control.
Hurricane Arthur, while being a "disappointment" for our area (in terms of actually getting much of anything from the storm itself), turned out to carry a hidden lesson about the overall design of the fridge.
In preparation for the storm's arrival, I went through and neatened some things up, added a battery backup for the blower/recirc fan, and manually pulled it down as hard as I could as any power failure would have been useful in terms of an excuse to see what this thing was really made of.
In the process, because I've been using thermisters and had only a rough calibration to go on, I missed the mark on the water mass in the fridge chamber. I was only off by a single degree F, but that was enough to cause a rather significant malfunction.
While I was aware that I was likely supercooling the mass, I was having trouble preventing it. As the water sits, the crystal seeding points dissipate and eventually disappear. I only briefly looked for a way to keep a persistent seed source as I needed to find out exactly where this point was, but nothing simple was coming up. My end solution involved periodically shaking the bottle up vigorously to get some air bubbles trapped in it so the ice would seed if it was going to form, right at 32F, and no supercooling would occur. The night of the hurricane, apparently, it all came together and I discovered ice the next morning.
The result was, because of the separate nature of the control for the blower, the air temp inside was beginning to warm up just enough to cause the blower to turn on, bringing cold air from the core up into the chamber and further perpetuating the ice in the sensor mass. Because of the ice, the compressor never started, and it was simply "draining" the core instead.
The good news in all of this is that I was able to pinpoint the exact freezing point and set the lower threshold to avoid it in the future. Had I been using digital sensors to begin with, this never would have been a problem, as I could have just set it slightly above from the beginning, but you go with what you've got sometimes, right?
While there was no actual power failure, there might as well have been. It ran for just shy of 12 hours without compressor, and the core held it's ground. A good sign ultimately. It would have easily run much longer than that, but I stopped everything to thaw out the frozen fridge mass bottle.
I decided that this point would also be a good time to take the center bottle out of the core and put the sensor on it so I could gauge the core's ice temp. I removed the bottle, and put it in the cab of the van, in the sun, to thaw it as quickly as possible. It took about 3 hours or so to completely get rid of the ice, during which time I worked on other aspects of the system.
The core bottle with the sensor attached and insulated from the surrounding air.
The thaw was necessary to stop the formation of condensation which would have interfered with the adhesion of the tape. After the main thaw, I was running out of sun and still needed to raise the temp somewhat to get it up to room temp. Nothing a couple of minutes in the microwave didn't handle with ease.
The bottle placed back in the center of the core, ready to resume it's purpose.
Up until this point, I had only been measuring air temps in the core, and suspected that the temperature plateau observed due to heat of fusion when fresh water freezes was skewed, and it turns out I was right. Monitoring the bottle directly shows that the temperature falls relatively quickly as it approaches the freezing point, but doesn't then simply level off as the ice forms. Instead, the freezing point slowly, but progressively drops for the remainder of the brine due to fractional distillation and increasing saline concentrations. The result is a slight slope during freezing/thawing, which turns out to be a useful characteristic that can be used something like a "fuel gauge" for remaining cooling store. I was skeptical that this wouldn't be possible when first venturing into this because of the well established behavior of fresh water as it changes states. Now I know, and ultimately, even if I were using the capacitive ice sensor in the design, this would supercede it. Live and learn.
Another note about thawing out just a single bottle...
I have been pondering from early on how I would go about defrosting this beast when it gets so clogged up that air can't efficiently flow through it and the cooling power diminishes to the point of hopeless. I can say as of now that the "one bottle at a time" approach will be a highly unlikely method.
The thawed bottle reminds me of what happens when there's a dead cell in a battery... It provides a constant strain on the remaining bottles, and throws the performance off, and a lot of energy can be wasted trying to pull the one bottle down. The heat doesn't propagate quickly from bottle to bottle in there, and so there's a lot left simply to time to sort out. I'm still pulling down on this one bottle, and it's still nowhere near the rest of the core yet, and I've dumped about half (so far) of the total energy required to pull the entire set down from room temp.
One possible solution I'm considering is yet another fan to circulate air down there, but that comes with it's own set of problems. Aside from adding to the parasitic power load, I've noticed that the bearings on these small brushless fans do not like the extreme cold and tend to chatter readily. I've been able to mitigate this somewhat so far with speed limits on the other two fans, but a fan in the core would be exposed to some really extreme temps, particularly during hard pulldowns. At times, the core air can go below -10F. I've heard the blower bearings chatter under the right circumstances at a paltry +10F, so the lifespan and reliability of a fan in such conditions may be rather limited at best.
One thing is certain, eventually, I will have to thaw it out, either partially or completely, and it's an energy intensive process either way, not to mention must be prepared for ahead of time (consume all perishables etc) and produces a lot of runoff that must be dealt with in the process.
I also moved the core air sensor to just inside the downtube for the blower intake. So far this looks like a good location. Whether or not this sensor remains here when it's all over and done with is yet to be determined, but so far is helping draw the big picture by providing confirmation of things like heat load and air flow etc. Time will tell.
A shot of the back side of the freezer, not included in the original update because of pic restrictions. It's just as well, because I just recently added the door switch that kills the blower/recirc fan. You can see the wires going up into the right hand side hinge. Currently, they simply interrupt power going to the two buck converters that power the motors, but will eventually just tell the PICAXE that the door is open and software will deal with controlling power to the fans. The compressor cooling fan is down on the lower left.
Detail of the door switch. There's only one spring loaded opening assist mechanism, and it's on the other hinge, leaving this one open with plenty of room to put a leaf type microswitch in.
This was the only "permanent" modification made to the freezer, two holes drilled in the side of the hinge arm to hold the switch in place. Even the PFC cap is just plugged in.
Last but not least, the final version of the top gasket. Up until this point, and not visible in any of the other pics, I was using just a full piece of foam with a gap on the left side to make it easier to get out and to give the wiring a place to go that wasn't restrained so I could move them around if I needed to.
With this in place, I can now stand 2L bottles of soda upright inside and close the door properly. The lack of extra insulation in the access port also lends nicely to a little extra heat leakage that helps keep the differential between the two chambers under control (when I'm not foolishly trying to manually push things to the limits, that is
).
The two temp sensors along the bottom of the fridge chamber are still there and the wiring still runs along the surface, which interferes with the physical placement of objects to be cooled. In the final design, I'd like to move these, but I'm not sure of the best location for them. I've experimented with moving the control sensors, recirc fan, etc around and the results aren't necessarily "better" in one location or another, just different. But controlling the blower speed has the greatest effect when it is running without power, and so I'm having trouble deciding if there would be a better location for them, and if so, where? If not, should I just trench the wiring in the bottom of the chamber?
Any input on this concept (or the defrost process as well) would be greatly appreciated. I'm weighing the pros and cons of everything on this, so I'm open to whatever ideas anyone might have.
Till next time...
Steve