Thursday, January 30, 2014

Electric Shoe Dryer

This is not actually a Renewable Energy project but more of a weekend project necessitated following a rainy week. In Winter it's always a nightmare to dry up wet shoes and putting them in front of a very hot source (such as a heater or fire) will risk of ruining the shoes. A more gentle approach is necessary, number one option being to place them outside in the sun, however when this is not possible, this solution will dry up the shoes gently without damaging them.

Dryer Description

The built is pretty simple and straightforward. I used a sheet of aluminium (30cm length x 45cm width) and bolted underneath 4 power resistors. The heat generated by the power resistors will be transferred to the sheet which will be eventually radiated to the shoes to dry them up.
Two very important points which must be taken into account for a successful built are;
1) The sheet thickness. A thicker sheet will provide better heat conduction inside the metal.
2) The resistors placing.
Both will help to reduce 'hot spots' and provide a better heat distribution across the whole surface. 'Hot spots' occur when an area gets hotter than the rest. This normally occurs just over the heat source (power resistors) and is not desirable (at least in this application).
I then fixed 3 wooden legs to the aluminium sheet to elevate the power resistors from the floor. Finally I covered the aluminium sheet with a thick cloth to help reduce further 'hot spots'.
*Heat transfer paste will help in transferring the heat from the resistors to the sheet and is recommended that it is used.


An underneath view of the finished assembled shoe dryer.

An Upper view showing the finished assembled shoe dryer.
Here I started fixing the thick cloth to the upper side. I have used Pattex glue and held everything in place using G clamps.


An underneath view of the finished show dryer including the thick cloth.

The finished shoe dryer.


Circuit Description

The circuit is very simple. It just consists of 4 power resistors (R1, R2, R3, R4) rated 2.2 ohms 100W wired in series. The values are not critical and in fact I used whatever I had in stock. I would have preferred if the resistance was slightly higher to keep the current/power further down. The power rating of the resistors could also have been less since I'll be just using a small fraction of their power dissipation. The advantage of the higher power rating is that the components will not operate next to their maximum rating and should last longer.
The resistors are connected to an external 24v power source. I connected it to my off-grid 24v setup. I have not shown in the diagram however a fuse is indispensable for these types of circuit. A 5 amp in-line fuse should do the job nicely.
Diode D1 in series with resistor R5 provide an indication when the Dryer is on.

Using ohms law V=IR
The voltage is 25.4v (12.7v x 2)
The total resistance is 8.8 (2.2 x 4) ohms
Therefore the current is 25.4v / 8.8 = 2.88 amps

Using the Power formula P=IV
The voltage is 25.4v (12.7v x 2)
The current is 2.88 amps
Therefore the power dissipated is about 73W

In actual fact, the power dissipated would be slightly less simple because the battery voltage would be lower due to other loads. Also, power is wasted in the wiring. In my case, I'm not plugging in the resistors right next to the batteries but I'm using the 24v off-grid wiring which I have going round the house. The length of wire is introducing extra resistance.

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