Michael Pletcher

Matthew Nistico wrote:
@ Walter Jeffries - Walter, I find your concept of a lunar panel fascinating.  Very creative.  But I am confused about something: what about your system do you feel is an advantage over a classic passive solar cooling ventilation scheme?  Have you done a study on the thermodynamics that suggests it is worth the extra cost and trouble?  I'd really like to know.  Like you, I am fascinated by the simple elegance of passive systems.

To elaborate my question... You are proposing a fluid loop powered by thermosiphon effect that transfers heat collected from the living space (via a radiator-type fixture, I presume) to a panel, from where the heat is radiated to the outside.  So that requires two heat exchange fixtures (one inside, one on the roof), plus fluid, plus piping.  Whereas at night (or whenever it is cooler outside than inside), I instead open my clerestory windows and my lower room windows and let the hot air exhaust from my house, drawing a steady flow of cooler air in at the ground level. This also works on a thermosiphon effect, but my heat transfer fluid is just air, and my heat exchange fixtures are just open windows (one high, one low).  Yours is a closed loop system, whereas mine continually exhausts the cooling fluid.  But then air is free.  Clearly my system is simpler, lower cost, and more foolproof since there is no apparatus to require maintenance.  But that doesn't necessarily mean it is more effective.  Also, if one didn't build a clerestory into one's home, an exhaust fan would be needed instead, which then introduces an apparatus to break down as well as electricity to burn.

Interested in your opinion.  Thanks!

I tend to agree, there are so many other options for expelling heat, or avoiding it in the first place.  Night sky radiative cooling is a neat technology, and if properly applied, it has the potential of dissipating more heat than other passive systems, or even active ventilation schemes.  I've seen systems applied in the tropics that cooled water on the roof at night that could absorb heat in the ceiling during the day, after insulation was re-applied in the morning.  The version I would like to play with would use a simple two-phase thermosiphon loop containing a volatile refrigerant that naturally circulates between a heat source and the radiative panel.  The size of such a system to cool a home down is impractical in my view, as you would need a rather large surface area, as well as pretty clear and dry skies, and few terrestrial sources of heat like trees.  

White roofs, radiant barriers, natural ventilation at night, and shade should be more effective and cheaper to install.  There's no substitute for good basic design, unless you want to brute force cooling with heat pumps.

This is the problem with modern refrigerator/freezer combos.  They are supposed to maintain two different temperature cabinets with one compressor and one evaporator coil.  Some of the new fancy junk actually has multiple coils and switching valves to cool any compartment that calls for cooling.  The simple fact is, these two functions should be handled separately, and anyone interested in a sustainable lifestyle (short of re-evaluating refrigeration altogether), should seek separate refrigerators and freezers.  

Peter is right about the fridge design.  Most of the modern ones I've seen have an evaporator coil in the freezer, with a fan to circulate the air.  The freezer thermostat actually adjusts the temperature the compressor kicks on at.  The refrigerator knob adjusts a dampener that allows air to circulate into the refrigerator cabinet.  I could have this backward, but I think the freezer is the main controlling factor.

Variations in ambient temperature can seriously influence the balance between the heat budget on the refrigerator and the heat budget on the freezer.  If the compressor doesn't run often, less air will actively circulate between the two compartments.

If it were me, I would find the end of the thermostat sensing tube, and locate it so that the cabinet of interest is maintained at the desired temperature.  If you just need a refrigerator, lower power consumption by cutting some holes between the two compartments to allow better air circulation, higher freezer cabinet temperatures (less compressor work), and essentially no need for a defrost heater.  I would even unplug the heater and see how it goes.

Or just install an aftermarket thermostat, and play around with sensor position and dampener settings.

And yeah, these are engineered to work in a certain ambient operating range.  Not only can the temperatures in the cabinets get a little goofy, but the vapor compression system can run very poorly.  Usually they can still keep up, albeit with less than desirable balance between the two compartments.

Good luck!

Non-electric forms of refrigeration are actually a great interest of mine, as well as refrigeration generally.  I really can't say for sure if you're idea will work, but I could take a stab at it!  

First thing to understand, is that an RV absorption refrigerator (Diffusion Absorption) calls for heat whenever the refrigerators thermostat calls for it.  A propane flame is ignited which heats the generator (boiler) portion, and also heats the bubble pump (if it is separately plumbed).  The size of the burner was specifically engineered for the refrigerator, such that the generator reaches an appropriate temperature.  Not so hot or the water in the generator will boil excessively and waste heat in the rectifier and condenser, and the bubble pump may cease to run correctly, but not so cool or flow rates internally will slow, and the bubble pump again cease due to inactivity.  Getting near this temperature would make your project more of a success.  There's definitely wiggle room, so don't worry!

One way to determine this, might be to enable the electric heating element portion it probably has, in in the case of small hotel refrigerators, the only heat source it has.  I would monitor both the power consumption (it will be high!), and the generator temperature.  This might be best done by strategically placing thermocouples on the generator.  You might have to remove the gas flame flue to do this, but make sure you reinstall for accurate readings.  I'm guessing they run at somewhere between 100 to 150 Celsius.  A bit of research online will also help here.

I think temperatures like this should be possible with an evacuated tube, especially in a parabolic trough.  Something else to consider, is the need to keep these refrigerators well leveled.  A slight angle can cause serious problems and early failure.  You would have to find a way to circulate the heat between the heat collector and the generator.  I have a suggestion.

Unless you plan on rotating the parabolic trough around the stationary evacuate tube, you can forget about adjusting for the best angle.  This thing is arranged for best overall sun exposure and fixed in position.  The evacuated tube is presumably angled to face south (if you're in the northern hemisphere), and the generator is elevated above, perhaps behind a wall to protect the fridge from direct sun exposure.  Heat would be carried from the tube by a simple two-phase thermosiphon, probably a copper pipe with water sealed inside under a vacuum.  Water carries heat extremely well, so it is probably the best refrigerant for the application.  It will be in a positive pressure when heated up, but the pressure shouldn't be getting too terribly high (take precautions and understand this nonetheless).  The issue now is to get the heat to the generator effectively.  A simple straight pipe is not best here.  I would instead, remove the flue and carefully wrap copper tubing as many times and as tightly to the generator as possible.  You want good thermal contact.  This will be difficult, so wrapping the coil with a thin piece of sheet metal and tightening it around the coil with tubing clamps or wire might be better.  Some kind of thermal paste or putty mashed into the coils could be a plus.  The top of the coil is soldered or brazed to the top end of the copper pipe in the evacuated tube, and the bottom of the coil runs to the bottom of the tube (or just enters the top of the pipe and extends to near the bottom).  Wrap the whole assembly in high temp fiberglass insulation.

So when the pipe heats up the few ounces of water (you'd have to figure out an appropriate amount), it vaporizes and travels to the top of the coil where it dissipates the heat and drives the refrigerator.  The vapor condenses and falls back to the bottom of the pipe to pick up more heat.  I'm guessing one evacuated tube won't be nearly enough, so you could plumb several together in parallel.  One of the limiting factors with this design is getting enough surface area in the generator coil (the condenser) to effectively heat it.  Nonetheless, I think it could work!  Maybe make an inside and outside condenser coil.

Sorry for the long post; it was fun to write though!  I have many more suggestions if you want to go down this road.  If this device worked, the collectors could even be placed on the roof, with lines running down to the kitchen below by using a two-phase geyser pump.  The fridge will over-cool on very sunny days, so provisions would need to be made for that.

Good luck.  Build it!  

Walter Jeffries wrote:

Tyler Ludens wrote:

Walter Jeffries wrote:
Lunar cooling panels

Do you have any links or other information about this?  

Unfortunately no, I don't have a link because I haven't written the article about it yet. I invented them. The idea is simple though so let me give you the short version: A lunar panel is the opposite of a solar panel. With a solar panel you capture the light of the sun and turn it into heat energy which you then pump into a storage vessel. Rewind. With a lunar panel you take the heat from a vessel and run the fluid up through the panel pointed at the darkest part of the night sky (not actually the moon) and the heat radiates away. The now cooled fluid is denser so it falls back down into the vessel (your house, concrete slab, water tank, etc) and picks up its next load of heat. Once it is warmed it rises back up through the up side of the tubing to the lunar panel where it once again releases it's load of heat. Presto: Lunar Cooling Panels. I like passive systems that use thermosiphons so it has no motor burning energy but you could build it with a motorized pump if you were upside down (e.g., you were sending the heat down into a pond or the ground.)

Hey Walter, have you done much work with your Lunar Cooling Panels?  Are you referring to two-phase thermosiphons?  I have been interested in toying around with a TPTS night sky radiative cooling device for a few years now.  There are many other important projects to get underway first, like a passive refrigeration system to make ice in the winter, similar to the Scott Nielsen version.  

Love to see a DIY radiative panel example.  It would be neat to continue ice production in air temperatures above 0 Celsius, assuming clear skies!  

Interesting and funny read.  So many puns!

First post!

Direct drive, or "Open-drive" compressors are abundant, but not like they used to be, and generally not on the small side.  I also think a car AC compressor would be the best option for the size of the heat load.  Specifically, a small capacity York compressor, still manufactured under a different name, could be rigged up with an appropriate belt to a gas engine.  In reality, this would be a lot of work to get it working smoothly and efficiently, unless you have a lot of experience with HVACR design.  There aren't too many examples of DIY open-drive systems, because it's noisy, and impractical.  I for one love the idea of a repairable compressor, but something like this could have a lot of maintenance issues such as leaky shaft seals.  

Ammonia absorption air conditioning systems exist, but you'd better have a good source of waste heat to run them, and a deep wallet.

Off-grid air conditioning is a tough nut to crack, especially if you're seeking any semblance of sustainability.  My suggestion would be to reduce your need for AC as much as possible.  Insulate.  Exterior window shades.  Sun blocking trees.  Night time ventilation to bring in cooler air, solar chimney for passive ventilation, and maybe dig in some Earth tubes for cooler ground air.

AC is great, but it is a luxury of the cheap energy age.  Time to learn to live without.