Intro

It’s easy enough to ventilate a building. Just add a fan that extracts the air – and consequently draws in fresh air from outside. This is good, because it improves air quality and helps to eliminate excess humidity.  The down side though, is that if it’s cold – or hot – outside, then you are wasting precious energy – literally pumping it straight out of the building!

What you need is a heat recovery system that (in a cold climate) scavenges the heat out of the outgoing air, using it to heat the incoming air. There are plenty of commercial systems that will do this for you, but they are large and costly and if you only want to ventilate a small building, then they may be prohibitively expensive.

I decided to build this system to ventilate a small cabin constructed from a 20′ shipping container.

Requirements:

The system shall be:

• Very quiet
• Efficient at recovering heat from the ventilated air
• Able to swap all the air in the building within 24 hours
• Cheap to run
• Achievable to build without specialist materials or tools
• Able to heat incoming air to within two degrees of internal temperature
• Able to eliminate any condensation that might build up in it

The system should be:

• Able to swap all the air in the building within six hours
• Able to heat incoming air to within half a degree of internal temperature

Design:

The shipping container will be lined with conventional timber wall frames with silver sarking and a thin foam insulation layer on the outside (closest to the container walls), fibreglass insulation bats in the wall frame cavities, and tongue and groove chipboard flooring as the wall lining (for durability – if the container has to be moved in the future plaster board my well crack if the container flexes while loading or lifting). I have made one of the cavities between wall frame studs wider, and left out the nogging and insulation to allow the heat recovery unit to be installed in the wall.

The unit itself will have two flat cavities separated by a thin layer of copper foil. Warm internal air will be forced through one. Cold external air will be forced through the other cavity traveling in the opposite direction (commonly called a counter flow heat exchanger). In this way the two air bodies pass each other, but are kept separated by a highly conductive membrane with very little mass. This should allow the two air bodies to exchange heat efficiently.

The critical factors in a design of this nature are:

• Surface area for heat exchange
• Radiation absorbtion and conductivity of the dividing membrane
• Air speed

If there isn’t enough surface area then not all of the air has a chance to come into contact with the membrane or radiate its energy and be cooled/heated. If the air speed is too high, then it won’t be in contact with the membrane long enough to be cooled/heated. Either way the system won’t be as efficient as possible. Commercial systems deal with this by using complex shapes like honeycombs or multiple layers in their heat exchangers, all in an attempt to increase the surface area that the air will be exposed to.

My system will be a compromise, only having two, thin cavities, but it will have the advantage of a very low mass, high conductivity membrane which should transfer heat very quickly.

For more information on heat exchangers, see the Wikipedia article:

https://en.wikipedia.org/wiki/Heat_exchanger

Inlet and outlet position

For best effect, the system should draw the hottest, most moist air from the room. To achieve that the inlet vent should be close to the ceiling and approximately half way between the front and the back of the room. Conversely the warmed external air should be fed back into the room at floor level, which will provide a degree of air mixing and help to distribute heat more evenly between the floor and ceiling.

Condensation:

One potential problem with this system is that warm moist air may be cooled sufficiently to cause the water vapor in it to condense inside the heat exchanger. This may lead to damage to the heat exchanger, so I will need to make sure that any condensation that forms can escape rather than simply fill up the bottom of the unit!

Control:

To ensure that the system runs at maximum efficiency irrespective of the temperature inside and outside I am going to control the speed of the fans using a single board computer, some digital thermometers, and a DC motor controller. All of this will be driven by a PID algorithm that will vary the fan speeds to attempt to get the best possible heat exchange. When it is very cold outside, the fans will run slower to give more time for heat exchange. When the external temperature is higher the fans can run faster as the required heat transfer will happen more quickly.

Materials & components:

• Heat exchange core
  • Frame
    • Timber
    • Plywood front and back panels
    • PVA glue
    • Wood screws
    • Staple gun
  • Membrane/conductor copper foil
  • Wall lining of reflective aluminium foil
• Fans and control system
  • Hardware
    • 12 volt DC power supply (for fans)
    • 5 volt DC power supply (for Raspberry Pi)
    • Fans: 4 x 12 volt 80mm computer case fans
    • Raspberry Pi 2 B
    • Motor control hat – Adafruit DC motor control hat
    • Temperature sensors – 3 x Adafruit I2C enbled temperature sensor
    • Assorted cabling
    • Heat shrink insulation
  • Software
    • Raspbian Linux
    • PID algorithm
    • Python control code

Materials & component discussion:

Frame:

The frame will be constructed from timber – for convenience and for insulation. 12mm thick hard wood for the battens and thin ply wood for the front and back.

Membrane/conductor:

I will use copper foil .001 inches thick as the membrane (that’s about 50% thicker than household aluminium foil). Being very thin and an excellent conductor means that it should allow the two bodies of air to readily equalise temperature.  Heavier copper sheet would be easier to work with, but is likely to make the system less efficient overall.  I am planning to do a small science experiment to investigate this.  You can see more here.

Fans:

I am using four 80mm, 12 volt DC, computer case fans.  Two each for the inflow and outflow sides of the unit, so four fans in total. Each fan can move up to 0.59 cubic meters of air per minute. Having two fans will help to overcome the internal resistance of the air channels, so I may get a little more than that volume, depending on the internal resistance.

You can see the fans here:

http://www.jaycar.com.au/80mm-silent-hydrodynamic-bearing-case-fan/p/YX2570

Temperature sensors:

I am using Adafruit MCP9808 high accuracy I2C temperature sensor breakout boards to monitor the external temperature, the internal temperature, and the temperature of the air as it exits the heat exchanger into the room.

Strictly speaking, I only need the last two pieces of date to make the system work, but knowing the outside temperature (and therefore how many degrees difference the heat exchanger must recuperate) may let me better optimise the controller.

These sensors have an accuracy of approximately +/- 0.25 degrees Celsius, they use I2C bus to communicate, and can be powered with 3 or 5 volts.

You can see the temperature sensors on the Adafruit website here:

https://www.adafruit.com/products/1782

Design drawings and 3D models:

 

 

Tools:

Under development

Instructions:

Under development

Notes:

Under development

Finished product:

Under development