Blower stove pulsing to extend battery life

Introduction

The ultralight blower stove has excessive power for most cooking needs because it has been geared to burning damp Gippsland sticks. I discuss various practical ways to reduce the fans air output by using choking, lower voltages and fan pulsing. This is in order to have; an option for more gentle cooking and or have a greatly extended cooking time per battery.

The extended battery life should be most important for extended hike situations where recharging facilities are absent (unless you have a good solar charger and sunshine) or new batteries cannot be purchased. It also should be noted that many of my Blower Stoves can provide cooking for a second pot as shown below. With low airflow, the heat flowing to the second cooking position will be reduced for active cooking, but it will still be hot enough to keep cooked food simmering or at a pleasant temperature that is ready for eating.

When fan pulsing is used, it introduces the possibility of damaging flame blowback through the blower fan. Consequently, ways of preventing blowback are also examined.

Blower fan choking

Closing the air intake shutter on the blower fan can reduce the airflow and reduce the motor current by a small amount (est. 10%), so while this is good for starting the burner and simmering meals, it, unfortunately, does not save much battery energy. It does, however, contribute to blocking of flame blowback. “That is, if you remember to shut it!”

Blower fan voltage reduction

The blower fan is designed to run on a regulated 5V DC power supply and draws about 0.3A. However, the fan will run effectively at lower voltages with commensurately lower current and air flows at voltages between 2. 5 and 5V. The fan sounds a bit rough at 2.5V. There are various ways of reducing the voltage to the blower fan motor to reduce the airflow:
1. Put a resistor in the fan motor circuit.
2. Put a voltage step down regulator in the circuit.
3. Use a bank of Four AA batteries (~5V) that can be tapped in the middle to provide lower voltage when required.

Method 1 does not save battery energy as the resistor simply ‘wastes’ the energy as heat.
Method 2 could save energy, but the extra quiescent and active current to run the step-down device is similar to the motor current saved so this is not helpful.
Method 3 can save significant battery energy and can for example approximately double the cooking time from a set of four AA batteries  Power Supplies. Unfortunately, in my experience, these batteries do not match the performance and versatility of my preferred Li-Ion rechargeable 18650 batteries Power Supplies. Also, the centre tapping of the battery bank also requires considerable management to ensure correct polarity and that all batteries are discharged evenly. “Even with my experience with using the blower stove, in the ‘hurly-burly’ of backpacking camp cooking that is often in darkness, these issues are not easy to manage.”

Blower fan pulsing

Pulsing the voltage to the fan should provide variable stove power and save battery energy. I have found that an ‘on period’ of as short as 15s coupled with an ‘off period’ of 30 seconds or more is adequate for keeping the fire burning and maintaining good cooking temperatures with dry fuel sticks. Longer ‘on periods’ would be required for burn maintenance when using damp fuel and little or no ‘off period’ for wet or frozen fuel sticks. Several approaches to voltage pulsing are discussed below:

Unplugging the power supply. One of the simplest battery saving techniques that I have used is to simply turn off the fan by removing one of the power supply connections when there is a break in ‘active cooking’ or gentle simmering and meal warming is all that is required.

I have used this ‘crude switching’ this for several years by periodically disconnecting the USB plug from the power supply by partially withdrawing the USB plug. The stove will ‘hold fire’ for 30 min or more when I have a break in cooking. However, this is fiddly, tedious and not easy to do repeatedly while cooking a meal. Furthermore, with unfavourable winds, the blower unit must be removed from the stove to prevent smoke, hot gases or even flames being blown back through to the plastic fan body. This blowback is an issue with all the fan pulsing options discussed.

Thermal feedback voltage pulsing. In an effort to eliminate the tedium of manual switching I have put a thermal switch in the motor circuit that opens and closes according to the temperature sensed in the stove. I have tried this method but there was too much delay between the change of temperature in the stove and the switching of the thermal switch (too much hysteresis).

Electronic voltage pulsing. Another possible method is to have a pulse generating circuit that drives a relay to switch the motor on and off for various periods. However, as with the voltage regulator above, the quiescent and active current will largely nullify any battery energy saving.

Manual pulsed voltage using a switch. While writing this post I thought of another simple option “See, there is the value of writing”. I was about to test a simple on/off push button (as used on bed lamps of my childhood). Importantly, this option would not use any precious energy from the battery (or me) for logic control or switching. This switch would enable the cook to select a moderate or strong burn with the push of a switch.

Three-week field trial of a switched blower stove.

I tested the switched USB blower on a 3 week through hike. I am pleased to report that the switch worked well and provided a 3-4 fold extension of battery life on this long hike.  I took 6*18650 rechargeable lithium-ion batteries as a precaution. Between ~weekly resupply points (where battery charging was possible). For two people, we cooked extensive dinners, desserts, soups and hot drinks and provided bulk water sterilization (when required for Brumby pooh contaminated water) mainly with the use of only one battery. In one longer supply period, we needed to use a second battery for one evening.

The frugal use of the battery power was made possible by having the fan turned off for much of the cooking and simmering time whilst still providing good cooking temperatures. Running the stove this way also meant that it made a bit more smoke, but this was quite acceptable and I think it may have kept mosquitos away.

I should also add that we had long days of walking (~20+ K/day) so we started our daily breakfast preparation at ~6:00 am and took a small gas stove for a quick breakfast coffee and porridge cooking, and also used this when outdoor cooking was occasionally not possible.

It is worth noting that the success of the trial was helped by the dryness of the aged fuel sticks that were available for most of the trip. I am unaccustomed to such dry wood but, speculate that the moist and fresh Gippsland sticks, that the stove has been developed for, may need a little more ‘fan on time’ to sustain effective cooking, but there still should be an immense improvement battery life. Also, the energy density of the solid damp fuel and very volatile fuel gases contained in them may compensate somewhat.

The bad news. You will see if you read my full stove development article, that “challenges breed solutions and the new solutions breed their own new challenges”. The battery life improvement using the pulsed operation of a blower fan was a delight but smoke/flame blowback became the new challenge. When the fan is stopped an unfavourable wind gust can send smoke and hot flames backwards through the hot stove, along the air tube and into the plastic blower fan. This will not be good for fan survival.

During the above trip I simply removed the whole blower unit from the stove when the fan was stopped if there was significant wind. This was tedious and fiddly, particularly at night time when we did most of our cooking.  Another partial solution was to face the stove exhaust opening away from the wind. However, “as all campfire experts will know, the wind blows the smoke in your direction wherever you choose to sit around the fire.” So it is not a reliable solution.

Prevention of blowback while blower fan pulsing

For a start, I made a more conical ‘sugar scoop’ air entry port for the stove to simply make it much easier to dock the long air tube into the stove. I also added locking tabs to the port so that it would remain in the burner. This makes a big improvement in ease of preventing blowback damage (and I will probably include this in all my blower stoves), but it’s not quite convenient enough and forgetting to remove the blower assembly is still likely. How many times during busy cooking did I hear “Tim there is smoke coming out of the fan!”? Luckily, no harm was done and I had a spare fan in my pack.

While on the hike a friend (who is a talented techno-boffin) suggested that I should integrate an air tube check valve with the switch so that smoke blowback would automatically be prevented. Whilst this was an excellent idea my efforts to make an air tube butterfly valve proved to be complex and expensive and even more so when it came to interlocking it with the blower switch. Also when I field-tested this ‘beautiful valve creation’, under windy conditions, it still allowed a little blowback.

While making the complex butterfly valve the simple idea of closing the air intake shutter came to me. My traditional slide shutter did a reasonable job of restricting the smoke blowback, but it still leaked a little too much with strong wind. Subsequently, I have made an improved shutter that seals better with the aid of a 14g lead weight and it is smooth and much easier to use than my original shutter. Disappointingly, field testing of the improved shutter showed that it still leaked a little too much with strong wind.

In what I thought was my last desperate anti-blowback strategy, I combine the improved shutter with a conical ‘sugar scoop’  air entry port, as described above, but this time I set the long air tube in the port so that there was a large cutaway vent on the top of the outside of the cone.

Surprisingly, with the cutaway, the blower air still effectively enters the stove burner and impacts on the charcoal in the stove as is intended LINK. Previously, I had assumed that the entry joints needed to be as tight as possible to prevent leakage, but the above test revealed that a tight joint was not important. This means that; the bottom of the air entry cone simply cradles the end of the long air tube without any seal or grip, keeps the air stream flowing in the correct place and in the right direction. The good news is that the simple combination of vented entry cone and improved shutter appears to prevent damaging blowback. It is early days with my testing, but I think either measure alone will reduce the risk of rapidly damaging the fan with blowback. It will give the cook a little time to remove the blower assembly without harm if there is a sudden wind gust.

The bottom line is that the blower assembly is easy to remove and replace with this new improved cutaway conical port.  At night time the reflected glow from the inside of the port also provides a nice guide for docking the blower unit.

Automatic blowback prevention

I was not entirely satisfied with the above blowback protection. I still could see a tiny amount of smoke leaking back through the shutter during strong wind gusts and failure to close the shutter would be a natural human mistake.

I was sleeping very well (“To my friends, I tell them that this is my only skill where I claim to be an expert”) the thought came to me;

“Don’t give up on automatically coupling blowback protection with the switching of the blower fan as suggested by my walking friend. I thought in a flash; speed up the jet of air by reducing the nozzle diameter (I have suggested this elsewhere for a better stove design full stove development article) and provide it with a flat upward sloping face. Then put an ultralight non-return valve flap on the face so that the air jet can lift it up and it will close automatically when the jet stops.

I even remembered that in my kit of goodies I had some stainless steel sheet or foil that is only 0.025mm thick (what technical magic) and that with a little tricky cutting and welding this would make a robust feather-weight valve flap (0.07g).

I also had another trick up my sleeve in that I knew how to make a very lightweight refractory air nozzle withasloping face. I used sodium silicate and aluminium cooking foil or other lightweight refractory minerals to make it. DIY Refractories.

“For me, often such creative dreams dissolve somewhat when they implemented in the real world. In this rare case I made a prototype and it ‘works like dream’ first time!”

Valve protection. The valve flap only weighs 0.007g so it is very delicate to be used in a backpacking camping context. Consequently, I have modified the air port yet again so that it is integrated with the nozzle and valve. This means that the delicate valve flap is protected from mechanical damage during use and backpacking. The air port is exposed to very high temperatures and abundant oxygen in the fan-forced air. This means that it will erode with use and may need replacement, so I have designed it to be separable from the blowback valve so that replacement is easy and inexpensive. “Even though I have made replacement provisions for the air port, I have not yet destroyed one in this type of blower stove with many hours of testing and field use”.

Conclusion.

Battery energy saving. Manual pulsed voltage operation of the blower stove should now be practical and automatically safe for the fan housing even if the cook gets distracted. There was no visible smoke leakage from the fan housing while the fan was switched off in the many test that I did.  I did many tests as I had a strong but fickle wind and I wanted to get the timing right for the photo so that there was a good blowback flame (much more impressive than blowback smoke). I think my beautiful improved shutter has had a glorious short life and has been made redundant.

The last word/s

The blower stove is particularly suited to this type of pulsed burning. Without the fan running the fire burns down to a slow-burning charcoal bed that becomes covered with ash in the primary burner. The remnant fuel sticks eventually will be out of reach of the combustion zone, but they will be warmed and dried by the strong radiant heat from the glowing charcoal and will be primed ready for easy combustion when required information on charcoal burning. With most fuel sticks this idle state can last for 30 min or a lot more with large dense quality sticks. When the fuel sticks are pushed into the burner and the fan is started the stove will fire into action after a brief ‘smoky phase’. My tiny waxie fire starters or a dry gum leaf or two can make this restarting even quicker and avoid the smoky phase.

Also, I would be interested to hear from anyone who has managed to make a small and ultralight thermoelectric generator that could use heat from the stove to make electricity to charge a spare battery on extended walks.

Addendum.

Sticky valve flap. On a recent three-day ski trip I used the stove with an automatic blowback protection valve for cooking all meals for two people. Initially, the valve functioned well, using damp and cold wood which made a considerable amount of smoke when the fan was stopped. “I was getting ready to crow about the success of this little stove improvement.”

However, the first sign of trouble was that the valve failed to close or open on a couple of occasions. Because of this problem, I bypassed the valve for the rest of the trip. When I got home I made a close examination of the valve and found that the tiny valve (0.007g) was coated with sticky tar that gummed up the hinge and probably caused the occasional failure. Nevertheless, it just demonstrated the value of real field testing under adverse conditions. So for me, it’s back to the drawing board for a more robust anti-blowback solution that I think is buried in the above story.

Running the blower stove in a fireplace. For some time I have wondered about the practicality of running the blower stove for efficient and convenient cooking in a hut with a fireplace. During the above-mentioned ski trip I tested this idea out and found that it worked very well and was much better than cooking directly in the fire. It meant that we could have the comfort of the warmth from the fire as well as the convenience of stable easy cooking on the blower stove. There was a small amount of smoke leakage back into the hut because the stove was set forward near the mouth of the fireplace in preparation for lighting the main fire. If the main fire was not going to be lit then the stove could have been placed further back to eliminate smoke leakage. However, we soon lit the main fire using burning sticks from the blower stove and the heat from the fire sucked all the smoke in and up the chimney. All in all, we conclude that it was a very practical and comfortable arrangement for cooking and keeping warm on a very cold night.

Tim

ggs