The KillCap Refrigerator


As it stands right now, the machine I would most like to develop is a pedal powered refrigerator and possibly freezer.  The compressor and drive mechanism will consist of a metal framework supporting a heavy flywheel perhaps 40 centimeters across and 25 kilograms.  This is mounted in the center of the frame and is driven like the drive wheel on a bicycle by a set of gears and pedals mounted to the framework.  A human operator sits in a seat on top and pedals away.  The flywheel will be able to freewheel and will serve as an energy storage between the operator and the open drive compressor which is chain driven by a set ratio and mounted in the same plane as the flywheel.

In order to provide the refrigeration needs for any 24 hour period, a human operator should not be expected to to pedal for more than an hour; less if possible.  To provide the refrigerating effect, a thermal mass can be employed.  Water ice is an obvious choice because of it’s high enthalpy of fusion and phase change temperature near the desired refrigerator cabinet temperature.  To avoid the well known problems of water ice insulating effect around a direct expansion evaporator coil, I think it’s better to cool a water brine tank with phase change material vessels (water bottles) immersed in it.  The evaporator could passively cool the water brine/PCM or if active brine circulation is advantageous then mechanical energy from the operator could be used.  An efficient use of that energy might be a standing wave held in the tank with some sort of mechanical governor to guide it.

It should be noted that my initial interest is to evaluate how much ice a human being can produce in a reasonable amount of time.  I have not given great thought recently to the design of a refrigerator cabinet itself and how the phase change material should be utilized.  Some cabinet materials being considered are: stainless steel, copper, plastic, glass, wood and cork insulation.  Cork was at one time used extensively for refrigeration before being replaced by petroleum alternatives.  Areas I would like to explore are door(s) improvements, windows, multiple compartments, cooling by radiation rather than convection and what I call right now ‘wet cold’.  If convective cooling were used then perhaps water ice could cool the air directly and so avoid the drying out effects that plague modern refrigerators.  No doubt some old texts on ice box construction could be of use in this matter.  Another way to remove heat from the cabinet to melt the PCM, would be the use of heat pipes with thermostatically controlled valves to regulate temperature.  A complication I wish to avoid.

For the evaporator, I’m abandoning the dry type in favor of a flooded evaporator with low side float.  There are several advantages to this including good thermal contact between the wet refrigerant and evaporator wall, less chance of slugging liquid to the compressor (if the float is working correctly), and the float itself.  A simple mechanical float maintains refrigerant level within the evaporator and may avoid the problematic ‘hunting’ found in thermal expansion valves or the complicated controls of an electronic expansion valve.  Whatever pressure is maintained in the suction line will determine the evaporator temperature so long as the float valve adds a steady ‘trickle’ of makeup refrigerant thereby ensuring it is never starving.  As I currently understand it, the evaporator can remove the maximum amount of heat given the constraints of delta T, surface area, heat capacities and so on.  The regulating factor in the operating pressure will not be an expansion valve which is most common, but instead the speed of the compressor.  Lower evaporator temperatures are acquired by an increase in compressor speed.  This will increase the heat removal, but also the compression ration, thus lowering overall efficiency.  This machine, like most other vapor compression systems, is best run at a low speed over the course of the day rather than cyclicly allowing for smaller delta Ts, higher suction pressure, lower discharge pressure and thus smaller compression ratios, higher system efficiency and coefficient of performance.  Since the machine is made to run less than 1/24 of the day, the surface area of the evaporator must be maximized in order to keep the delta T to a minimum.

As far as the condenser goes, the waste heat pumped from the phase change material representing the heat load removed from the refrigerator cabinet the previous day could be used for preheating the domestic hot water supply.  It could also be emitted into the local environment if heating were needed.  It should be noted at this point that if the refrigerator were to be used in a climate where seasonal temperatures dipped below freezing for even just a few weeks out of the year, that a passive heat pipe radiator could be installed on the outside of the home capable of keeping the PCM frozen.  If the condenser heat is to be radiated into the local environment, even a rather warm one, then the delta T may have to be impractically high in order to cool the whole day’s load.  On the other hand, if the condenser heat could be stored while the machine is running only to be released throughout the day, then the discharge temperature and thus the high side pressure can be much lower assuming sufficient surface area.  I am of course talking about another application for PCMs.  In this case I am thinking of something with a fusion temperature 15 to 30 degrees Fahrenheit warmer than ambient.  Eutectic salts and Paraffin Wax comes to mind.

The driver of this contraption would have mechanical pressure gauge readouts with indicators of appropriate operating ranges to get the desired effect.  The machine is entirely mechanical however, an electronic microcontroller could be used to monitor temperatures throughout the system and provide the operator with interesting and useful information about the re-freeze progress, system performance, log system history, cabinet use and habits as well as provide bio-feedback of the person’s heart rate and calories burned.

The entire machine could of course be run off a small DC electric motor rather than pedal power, but there is much to be gained by rethinking the construction of a refrigerating machine by framing it around the physical limitations of the human body.  It would be also interesting to experience the amount of work it takes for domestic refrigeration.  With a DC motor this machine might adapt well to a solar power system or other off grid battery system with limited joules to spare.  Better yet is it’s application to people who have no access to electricity or hydrocarbon fuels needed for absorption refrigerators.  The last point goes hand in hand with making this an appropriate technology that is long lasting and reliable, built from accessible materials.  The exception to this is at least the compressor and evaporator/float valve.  To make this technology available to people, a detailed illustrative book could be made describing the theory, materials needed, construction and operation of the machine.  A kit could be sold with the book and any of the hard to find components at or near wholesale value.  Direct profit may only inhibit it’s adoption and an alternative means of benefit would need to be explored.  This needs to be non-proprietary, un-patentable and owned by the people.  I don’t even know if the thing would work, but I’m sure interested in finding out.

By the way, if you are wondering what ‘KillCap’ is :

-M. C. Pletcher

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Posted in Refrigeration, Refrigerator
2 comments on “The KillCap Refrigerator
  1. pauljs75 says:

    Interesting idea. It’s feasible to some extent at least. But I think for something that would make the best public demonstration would have a goal involving a minimal amount of thermal mass. An evaporator coil that could get a typical bottled or canned beverage icy cold for a few minutes of reasonable effort would be best. (And possibly practical? At least depending on location.)

    • mcpletcher says:

      All very valid points. Many of the features I talk about are things I would like in my own refrigerator and I suspect there are other people that would appreciate them as well. Many people? I don’t know. Like anything with a lot of thermal mass, it would require a great deal of effort to get it down to temperature initially and so would benefit from some other motive force. As a demonstrative piece, something with low mass and excellent insulation would be ideal.

      In terms of a permanent installation, I think the human powered option might be possible, but would have be be performed in a PCM maintenance function and not to actively cool the contents of the cabinet. It might be a pipe dream, but it’s a fun to ponder over,

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