Does anyone know of existing systems similar to what I describe below?  It just popped into my head one day...

Problem 1: Multiple appliances in a household use phase change principles (i.e., refrigerants, condensors, evaporators) for thermal management.  In my case, I have HVAC, refrigerator, freezer, Vinotemp, keg fridge, and computer CPU coolers.  The net efficiency of these small, cheapo, self-contained units cannot be anywhere close to the efficiency of a single, well-built shared system. 

Problem 2: Except for the HVAC, all these devices have their condensers located within the same air space that the evaporator takes its air from, and that air has to be thermally managed by the HVAC.  In other words, the fridge transfers heat to room air, and that heat has to be transferred outside by the HVAC.  If it takes 50W to move X Joules of heat from one compartment to another, the effective cost of cooling the inside of the fridge is 100W (not accounting for different device efficiencies) due to the HVAC having to move the heat a second time.

Problem 3: Each device has a compressor motor that must kick on, run, then shut down--over and over again--drawing electricity inefficiently due to the surges required for motor startup.

Idea:  Deploy a large, open-drive compressor unit that runs continuously at low speed, accumulators/receivers to store compressed refrigerant during low-demand periods and release during high demand, and a condenser outside the conditioned air space.  The condenser could be coupled to a dehumidification system or a cooling tower to collect water at a higher elevation, which then powers a turbine through gravity flow... but I digress.  This single system runs refrigerant throughout the house to high-side and low-side wall ports, just like any other utility.  Any device employing phase-change cooling is simply plugged into the dual wall ports.  The device would require no compressor, condensor, or electrical supply (except maybe for an internal light bulb).  Thermal regulation inside the device's compartment would be controlled by solenoid valves at the wall ports, operated by a software management system with thermistor data from inside the device.

Are there any errors in my thought process?  I'm not an HVAC guy, just a technical nerd.

I've seen some experimentation with domestic refrigerator waste heat recovery for hot water heating supplementation. To get beyond that to your grand scheme is a bridge too far.

Much of the year, the HVAC is not running full out, but the fridge and others run on a continuous cycle. The degree of turndown to have a full house HVAC running at load small enough to support just a refrigerator would likely result in significant inefficiencies at part load.

Most of the uses require different refrigerant pressures. And over pressurizing wastes significant energy. Removing 50W of waste heat from a refrigerator (or any appliance) only requires about 17W (and that's only when the AC is on - in the Winter, the heat is beneficial). Motor startup is insignificant in terms of energy use.

It is a shame that the typical AC system can't produce hot water.

Posted By mtrentw on 29 Jul 2016 09:36 AM
I've seen some experimentation with domestic refrigerator waste heat recovery for hot water heating supplementation. To get beyond that to your grand scheme is a bridge too far.

Much of the year, the HVAC is not running full out, but the fridge and others run on a continuous cycle. The degree of turndown to have a full house HVAC running at load small enough to support just a refrigerator would likely result in significant inefficiencies at part load.

I should mention that I am in FL, where cooling and dehumidification are a year-round operation.  Heating almost never gets used.  The world of efficient building design is thoroughly biased toward solving the problems of wintry climates.

It is a shame that the typical AC system can't produce hot water

Why couldn't my proposed system also heat water?  The physics of phase-change allows for both absorption and release of heat.  Sure, it's easier to heat water directly by piping it through a solar array (which I plan to do), but phase-change materials are already being used to augment solar water heating.  Think abstractly when considering my idea; don't envision a Trane unit connected to a Whirlpool fridge, etc.  Think compressors, condensors, evaporators, distribution piping, and insulated boxes.  This is not conversion of COTS devices; it's the design of a system from the most basic components.  I am treating phase-change material as a basic utility distributed through the building, like water, electricity, ethernet, coax, fiber, etc.

I'll need to study up on the efficiencies and losses of various materials as they change phase, their latent heat values, and what pressure differentials are required to store and extract energy from them.  It may turn out that a multi-stage system using different materials may be necessary to overcome efficiency barriers.

There's also a practical problem of purging/bleeding when connecting and disconnecting devices from the system.  Perhaps there are some self-purging components out there that could be installed in-line.

And I'll defer the question of safety until the math shows that it has a potential benefit.

Also consider that your refrigerator runs more efficiently when surrounded by 75F air than 95F air. So the two stage cooling process isn't as bad as expected.

In Florida, I'd consider a heat pump water heater. And a low energy CPU that doesn't need anything more than a fan.

Overall, I expect that the ROI is better on more mainstream approaches - air sealing, insulation, efficient AC, attic duct leaks, solar panels.

Posted By jonr on 29 Jul 2016 02:51 PM
Also consider that your refrigerator runs more efficiently when surrounded by 75F air than 95F air. So the two stage cooling process isn't as bad as expected.

The refrigerator is surrounded by 75°F air in both scenarios.  It's the heat given off by the refrigerator's condenser, located inside the 75°F compartment, that the AC has to deal with.  My idea relocates all condensers outside the 75°F compartment, reducing the workload on the AC.

And a low energy CPU that doesn't need anything more than a fan.

What about two 42U racks filled with servers (approx. 50 CPUs) rendering at 100% CPU load 24/7/365?  Because that's what I have in my house.

Overall, I expect that the ROI is better on more mainstream approaches - air sealing, insulation, efficient AC, attic duct leaks, solar panels

Maybe for an existing structure, but not when you are designing one from the ground up to be NetZero/LEED Platinum/etc.  I also derive academic value from my complicated ideas, which is somewhat immeasurable.

My idea relocates all condensers outside the 75°F compartment, reducing the workload on the AC.

And assuming that this relocated condenser is now outdoors and the AC is on, then it's at a higher temperature (perhaps 95F). This increases the workload on the compressor supplying refrigerant to the refrigerator. The AC bill goes down and the refrigerator bill goes up.

Posted By jonr on 29 Jul 2016 08:45 PM

My idea relocates all condensers outside the 75°F compartment, reducing the workload on the AC.

And assuming that this relocated condenser is now outdoors and the AC is on, then it's at a higher temperature (perhaps 95F). This increases the workload on the compressor supplying refrigerant to the refrigerator. The AC bill goes down and the refrigerator bill goes up.

We're misconnecting here, I think.  There is only one^* condenser: that of the centrally plumbed phase-change system.  All devices that use PCM ("refrigerant"), are connected to the same system.  I also never said anything about putting the condenser "outdoors", only that it would be outside the compartment that the AC has to regulate.  That could be a cooling tower, integrated underground with a geothermal system, or inside a sealed compartment within the house as part of a water heating or dehumidification system.  Is your stomach inside your body?  From a 3D geometrical perspective, perhaps; but physiologically, no.  Same general concept here.

*There actually could be more than one, if I implement multiple uses for the heat released by the condenser, some of which I already listed.  But there is only one "system", to be sure.  A "fridge" in this context would really be a custom-built, insulated box that I run an evaporator through.  Once you aren't tied to an appliance-sized 5lb refrigeration system, why bother with the limited form factors available in a typical fridge?

I guess it could work, in theory. At least appliances like refrigerators and freezers wouldn't be releasing the compressor heat into the conditioned envelope. It would probably be more effective in a true tropical climate, which FL is not. Even in Miami and the Keys, there is plenty of non-A/C weather. We lived near Tampa, and didn't A/C at all for about 5 months of the year except during an occasional mid-winter heat wave. During the non-A/C season, the system would be way too big for the relatively tiny usage of appliances. I've never considered the possibility of 'storing' cooling capacity; it would be interesting to study that. Has it ever been done for some other usage? If so, a coastal home could use wave action and tides to compress the refrigerant. You could also use a Sterling cycle engine to use all of that FL heat for cooling purposes. Solar, of course, would be another source. You'd have to compare whether it's more efficient to store energy in the form of batteries, or store cooling, in the form of compressed refrigerant.

All of this is theoretical, of course. By building a really efficient house in the first place, our power bill is so low that we would never recoup the investment. Ideas like these have more practical appeal for people living off-grid, where every watt counts, and you are willing to spend more not to save money, but to have enough power to live comfortably.

Yes, work on that. When the sun shines, collect PV solar and freeze water.

When the AC isnt needed in mild weather, my windows are open. No problem with the refrigerator releasing heat into conditioned space. When the AC is on, having the refrigerator completely in cooled space isn't as much of a loss as people typically think.

Posted By jdebree on 30 Jul 2016 07:25 AM
You could also use a Sterling cycle engine to use all of that FL heat for cooling purposes.

All of this is theoretical, of course. By building a really efficient house in the first place, our power bill is so low that we would never recoup the investment. Ideas like these have more practical appeal for people living off-grid, where every watt counts, and you are willing to spend more not to save money, but to have enough power to live comfortably.

Funny you should mention Stirling engines; I was just thinking about them last night.  It is a given that solar (passive and PV), wind, and geothermal will all be part of my physical plant.  Possibly a fuel cell as well.  The house will be in a rural area, away from the coast, and probably off the grid.  The grid thing is tricky because I may colocate a business there, which will require 3-phase power, so that's a separate issue.  But the academic endeavor for the house is to be 100% energy and water self-sufficient.  The water thing is why I see some potential collateral uses for the phase change system to scavenge atmospheric and subterranean water (while controlling moisture in the underground part of the house).

Saving on energy $, while certainly important, is not the driving force here.  The key objectives are (in roughly descending order or importance):
--Net zero energy & water balance/production (i.e., off grid)
--Ecological integration ("greenness"), which is really just part of the mass balance equation above
--Gee Whiz fun factor of the intellectual and engineering challenge

Other than the power/water consumed during construction, there should be no utility bill to measure, so cost is really a factor of amortizing the construction and maintenance costs over my lifespan.  I plan to live there until I die (40+ years, maybe?), so I'll be satisfied as long as the break-even point comes while I'm senile enough to celebrate it.  Furthermore, the grants and tax incentives available (not including any potential equipment/materiel donations by manufacturers in exchange for my proof-of-concept research) will really enhance my TCO/ROI curve.