The Passive Refrigerator:

The Vision:

To design a low cost cooling system using basic materials and physical principles to obtain a passive and sustainable solution to maintain the quality of products

Diseñar un sistema de enfriamiento de bajo costo utilizando materiales básicos y principios fisicos para obtener una solución pasiva y sustenible de mantener la calidad de productos

Group Members:

Brian Croshal: Recent graduate from Cal Poly with Mechanical Engineering.
He likes doing cool stuff with cool people. Thus making Gilbar and the Refrigerator group a perfect match.

Gilbar Morales:


We started the design process by reading through the write-up of the previous Refrigeration Group (UNIV392) to understand their process and learn from their results. Their page can be found here

Their initial design decision was to use a design matrix to determine the most appropriate style of refrigerator to further develop. The heat chimney design was chosen as the best candidate, and various experiments were run in order to test and eventually prove the concept.

After reviewing the matrix and discussing among ourselves, we agreed to also design in the heat chimney style. At a glance, there are a few differences that we experienced in the materials and the environment of San Pablo versus their environment of San Luis Obispo.

  • The ground in San Pablo was measured to be 12°C below 0.5 meters below the ground. (versus 15°C in SLO)
  • Ambient temperatures in San Pablo (During July and August) would rise only as high as 20°C in the early afternoon (versus 30°C in SLO)
  • The location of the experimental refrigerator was highly affected by the logistics of placing it in the Guateca House.

The initial intention was to install our refrigerator at the Café Coyote so that Danny could make use of it to keep vegetables and potentially chill beverages. When we began discussing size considerations and the exact setup of the different chimneys, we decided instead to design an experimental setup at the Guateca House to avoid hassle of commuting up the hill for both construction and experimentation at the Café Coyote.

The Design

The design and construction of the refrigerator can be broken up into four unique parts: the cold (intake) chimney and pipe, the insulated transfer tube, the insulated box, and the hot (exhaust) chimney and pipe.

Our first major decision and deviation from the previous group’s design was to place the cold intake tube in the ground at a downward angle (~35° to the horizontal). The concept that we are testing is to achieve buoyancy driven flow not through the warm air rising up and out of the exhaust pipe, but through warmer ambient air entering the intake tube, cooling and becoming more dense underground, and then downhill into the relatively warmer box. This new design eliminated the need (though not the possibility of future application) of a hot exhaust chimney.

Design Schematic


The Box

The first and most straightforward part of construction was the insulated refrigerator box. In defining the system as experimental and not intended for long term usage in the house, the sizing of the box was relatively arbitrary. As such, we opted to use the waste pieces of wood from trimming the ends of the floorboards. To provide adequate insulation, we purchased multiple (but used one) sheets of one inch thick extruded polystyrene foam which we then cut to size and lined the inside of the box with. The resultant usable space within the box is nearly 1 ft^3. Large (3″) holes were cut into the top and side of the box respectively and a hinged top was applied to the box which, with the help of the foam layer, closed snugly.

Isometric Fridge Shot
Side Top View

The Trench and Intake Tube

It was determined in the UNIV392 group’s experiments (and confirmed in San Pablo) that the under 1 foot of soil, there is no daily fluctuation in soil temperature. Nonetheless, we decided to dig a trench and place the intake chimney at a depth between 1″ (downhill side) and 3″ (uphill side). This was for multiple reasons:

  • To protect the pipe from being dented or otherwise squished by foot traffic up or down the hill or by the occasional leap from the second floor of the house onto the hill.
  • To increase the total thermal mass available for heat transfer and minimize the possible effect of sustained flow heating up the surrounding soil and rendering the refrigerator useless.

The intake tube is made up of three separate straight lengths: the insulated length that spanned the gap from the hill to the house, the galvanized steel length that traveled completely underground up the hill, and the last length of galvanized steel that pointed at~70° to the horizontal, ending with 1′ outside of the soil and capped with a generic chimney attachment. Each joint is connected with healthy amount of duct tape, with the exception of the connection with the box which is snug fit and sealed with silicon sealant. It was concluded that the duct tape would be appropriate because it would withstand the soil and moisture for the short term nature of the experiment and allowed for the arbitrary angles that we required to fit the tubes in the trench and line up with the box in the house. The length that spans the gap to then enter the house is appropriately wrapped with old clothes (courtesy of Mace) to act as insulation to maintain the cold temperature of the air inside the tube even when the tube leaves the ground.

The chimney attachment (top right neon green in the picture below) is the generic wide conical top used with all galvanized chimney rigs. We decided to customize ours by connecting it in such a way to only allow a small gap to prevent water, dirt, or insects but still allow for air to flow into the tube. In order to minimize any heating of the part of the chimney that protrudes from the ground, we packed soil all around the base (soil displaced from digging of the french drain) and wrapped the remaining protrusion in old t-shirts.

View of Complete System
View of Insulated Tube Entering Refrigerator


Due to various factors, we only ran one experiment with our setup. Using the portable HOBO logger we collected data over multiple days to see if the the inside of the box maintains a lower temperature than the ambient air.

We covered the hole on the top of the box (for the exhaust tube) with a layer of duct tape, then peeled back a quarter of it to allow for that much air flow. Our theoretical expectations were that the temperature in the box would remain fairly close to 12°C
C (the temperature of the ground). This would be achieved because when the ambient air rose higher than 12°C, the warmer air would be induced into the intake chimney, cooled to nearly 12°C, maintaining that temperature in the box. When the ambient dropped to below 12°C, the intake flow would cease but the cooler temperature gradient between the box and the ambient would maintain the box temperature.


TABLE 1. Temperature and %RH results from HOBO logger

Black Line (Left Axis) gives Temperature (°C)

Blue Line (Right Axis) gives Rel. Humidity (%)

The vertical red line marks an example (and recurring) time of 9AM where the box has a temperature of 12°C. This is what we would expect or hope for knowing that our ground temperatures measure 12°C. Unfortunately, as shown, both the temperature and the humidity seem to consistently rise and fall with the natural rhythm of the day. This would suggest that the insulation is insufficient to maintain constant low temperatures within the box.

It must be noted that the exhaust port consisted of a half covered hole in the top of the box, potentially hindering the effectiveness of the insulation. This could be addressed by adding a stop valve in the exhaust port to minimize back-flow into the box.

TABLE 2. Interior and Ambient Temperatures measured over two days

The interior temperatures were taken from the HOBO logger placed inside the box over the course of two days. The exterior temperatures were taken from the ambient temperature readings of the Stove Water Heater Group measured with the Octo-logger (the 8-probe unit).

The two important lessons to be learned from this graph:

  1. The interior of the box lags behind the ambient temperature. This implies that the insulation is having some effect in restricting heat flow across the box.
  2. The box did remain at a temperature lower than ambient during the day and a temperature a bit higher than the ambient at night.

With data taken over such little time (2 days vs 2 months) this graph mainly demonstrates the form and the process of taking temperatures inside the system and comparing them to the ambient temperatures throughout the year in order to infer certain aspects of the system.

Suggested Further Experiments

There are four main variables that could be adjusted in experimentation:

  1. The size of the exhaust hole
  2. Both the size and the insulation of the insulated tube
  3. The effect of the temperature and the rain variations on a daily basis
  4. The effect of the weather variations on a seasonal basis.

1. The size of the exhaust hole is the bottle neck in the current system. The larger the hole, the less resistance to flow exists within the entire system. This variable can be adjusted over a couple weeks (with all other variables held constant) and would quantify the effect that the exhaust had over the flow rate. This test would be most accurate in the dry season because the effect of the rain can be removed as an inconstant variable.

2. To replace the current 4″ old T-shirt wrapped insulated tube would alter the effectiveness of that length of pipe greatly. A smaller tube (using a nozzle to adapt to the box, would allow less heat to enter the most exposed (relatively) section of the system. One could use 2″ thick plastic tubing and cover it with a variety of possible materials (cork, pine needles, fiberglass, etc.). These tests could be run with thermocouples placed at multiple locations inside the tube to note the changes in temperature as the air flows from ambient to inside the box.

3 and 4. Experimentation aimed at understanding the effect of the climate requires consistent tracking of temperature and humidity over a long range of time. One could analyze this data over an entire year noting the maximums and the minimums to draw conclusions of the effect that the ambient climate would have on the operation of the box.

Any further research into this project is that it is relatively simple but that it must be consistent and sustained in order to yield yearlong data to best understand the long term performance of the refrigerator.