Solar fire blower- using the sun to make damp wood burn well

This post describes my testing of a 150g backpacking solar panel that can run a solar fire blower stove and charge a USB power pack.

Background to solar fire blower

Most wood combustion is improved by forced air draft from a device such as bellows or a fire blower particularly if there is not a strong natural draft. This is even more important when backpacking camping when a wood stove must use damp and crappy sticks for fuel.

My blower stoves and USB fire blower are good examples of utilizing forced air to turbo-charge such combustion. They are designed for lazy cooking with long sticks in remote areas while walking skiing camping and normally depend on batteries to drive the tiny fan.

Techno boffins like myself and The inimitable Steve of Theultralighthiker often dream about using Peltier or thermoelectric generators (TEGs) to use stove heat to make backpacking electricity. My small efforts in this regard revealed that such a device for backpacking would currently be expensive, complex, heavy, bulky, clunky and produce just enough electricity to run its own fan and provide very little extra electrical power for practical rapid battery charging.

I can’t help thinking that the source of heat used to run the candle-style TEGs mentioned by Steve (heated by a candle, gas cylinder or alcohol) would weigh more than an equivalent USB power pack that could be used as a simple substitute.

The BioLite camp stove puts this dream together in a neat but heavy stove package (2.06lb). I know experienced walkers who have one and they never take it on serious backpacking trips. It is neither a good remote area stove nor a good charger.

I would also add that its burner is designed to burn a load of very small fuel pieces from the top downwards. It does burn hot and clean and wonderfully well. However, if cooking or charging for a long time, the stove must eventually be refuelled from the top (or cooled and started again).

I have not used the Biolite, but have made DIY wood gas stoves and have a Stickman fan-driven wood gas stove. They are designed to also burn ‘top-down’ and they suffer the same fuelling problems after the first tiny fuel batch is burnt.

In contrast, there are exemplar domestic biomass gasifier stoves that have large fuel chambers that overcome these shortcomings. However, they would not be welcome in my backpack.

Having a small solar source of supplementary power for the stoves has always seemed like a practical option. This would be especially the case for long trips (away from trail-heads with USB recharging power) with multiple nights out and other backpacked items may benefit from remote charging. I think it should be infinitely better and more versatile and practical than a backpacking TEG, as evaluated below.

Can a cheap small backpacking solar panel run a blower stove and charge batteries?

Small solar USB power pack. I have used a variety of ultralight power supplies and one was a power pack with a small solar panel on it. The solar charging was slow and so it was really only good for infrequent use when it could be solar charged for a long time (e.g. on a car dashboard) or mains charged instead. It did not have enough power to run a blower stove directly on solar only.

Larger backpacking solar panel. I recently collected from the post office a small flexible backpacking solar panel after a typically long shipment that we have all grown to accept (ETFE SunPower Folding 10W Solar Cells Charger 5V 2.1A USB Output Devices……….). The bare weight is 150g without the cute but clunky complimentary carabiners that will find a much-appreciated but more functional use in my kit elsewhere.

From my own experience of battery specs etc and reading reviews of solar panels, I took the advertised specifications with a pinch of NaCl. To be fair, I often acknowledge the value for money that some of these cheap devices represent when they even include free shipping.

However, the panel had quite an area and even if its output was just 20% of the stated wattage, it still may do the job. So I was keen to see if I had wasted my money and time.

The direct solar blower stove running

I thought that charging a battery with the solar panel would take too long to determine anything conclusive. It was about the middle of an early Autumn day with high cloud cover over the sun for much of the time. Consequently, for a quick test, to see if I had been ‘sold a pup’, I plugged in an inline USB power monitor to the output socket on the back of the solar panel and then plugged the fan cable of my blower stove into it.

As soon as I removed my silly shadow from the panel the fan started to run. I crudely turned the panel to face the then-invisible sun. I could immediately tell there was a gentle whirring sound coming from the fan and I knew that it could ark up my tiny blower stove to make a celebratory cuppa with it. “It would be my first solar fire blower.”

The way the power monitor plugged into the cable made its display awkward to read in the bright outdoor light when pointing the panel at the sun (working on that). The readings were approximately 5.2V and the current fluctuated around 0.25A with various amounts of cloud cover (as shown in the video).

The expected full power current of my fan is much the same at about 0.26A at 5.2V when using my normal 5V regulated USB power pack. “I have found that even half voltage the fan will still work the stove, even if I need to flick start the blades, so working in poor sunlight is still possible.”

Here is a little video of the solar blower happily driving the hot cooking stove.

Charging a USB battery pack

I was delighted to see that the panel could charge a USB power pack under the same overcast conditions (as indicated by the light on the battery pack). I think the power monitor was showing ~5.2V and ~0.4A. However, it was difficult to read the display and I was somewhat mistrusting of the current, given that it was considerably higher current than the fan current under similar conditions.

The next day the sun was less obscured. There was a thin Sirrus cloud layer and light smoke from fuel reduction burn-offs. I had rigged a small paper shroud around the power monitor screen to make the viewing better. The power reading for the fan was ~5.2V and ~0.2A. Similarly, while connected to the USB power pack the readings were 4.56V and up to 0.82A.

“This is about 39% of the stated charging amps (2.1A). So it looks as though my bullshit discounted estimate of 20% was overly pessimistic. So I am delighted with its performance.”

Is a backpacking solar panel a practical backpacking tool?

Direct blower stove operation

No doubt, it will be fun to use my blower stove during sunlit times to make hot drinks, soups cooked food in cold weather if the lunch break and weather permitting. However, cooking for breakfast before early morning pack-ups or for dinner after a long day of trekking will seldom be practical because of the weak solar radiation at these times. Consequently, battery recharging during the day, on the top of my pack, would seem to be the most practical use of the panel, in the context of backpacking.

Nevertheless, it would be great to use when power fails at home and to take on fishing trips, bush cook-outs and picnics which are usually daytime events that only go ahead with OK weather. USB battery power can always be used as a backup. I think that hybrid solar-powered cooking with a few fallen sticks has a lovely bugout appeal.

Battery charging

My preliminary results indicate that the solar panel can output 0.82A at 4.56V and this is 3.74 volt Amps or watts in bright sunshine. “Yes, it is a long way short (37%) of the advertised 10watts, but on the upside, it is better than my pessimistic 20% guess.”

If we assume a conversion efficiency of 70% (in and out), this would mean that for each hour of good solar charging, the batteries could output 2.61 Watt-hour of energy (or 1.7h of full power blower stove cooking time). If 3h of solar charging were available, this could add about 8Wh to the battery (~5.2h of cooking time). “More than enough cooking time for the most gluttonous backpacker.”

Aso in another post (Blower stove pulsing), I also describe how the cooking time per Watt-hour can be greatly extended by strategically turning the fan on and off while still maintaining cooking power. This trickery is at its best with dry fuel sticks, but damp fuel will still make charcoal that is great for simmering meals without the fan working.

These volt*amps or watts, volt amp hours or watt-hours are doing my head in. So it is time for a green nuclear ode:

Green stored energy is our past and future you see,
Whether stored in battery, oil, dams, old plastic or dead branch from a tree,
All from nuclear fusion of hydrogen in sun be the source,
Forget not sunlights photosynthesis in trees of course,
Then any time, with clear conscience, we can enjoy a nice hot USB cup of tea .

Non-backpacking use of a solar blower

If you follow my pyromaniac posts, you will know that I use my little USB fire blower for many things such as starting bonfires with wet wood, burning out stumps and running forges and furnaces. Consequently, I was curious to see if the directly solar-powered blower would work with my ceramic furnace (in its low power charcoal making mode).

The tiny solar fire blower was able to pump enough air to keep the furnace ticking over very nicely to make charcoal and preheat ceramics before their final firing in the charcoal with the full power of a mains-powered vacuum cleaner. “No doubt, it will work well when burning out stumps and the power pack can be kept for overnight use.”

Conclusion about a solar fire blower

I am delighted to report that, with sunshine, the solar panel can directly run a blower stove. That would be great for lazy lunches. The direct running will not be effective for that main game of early breakfasts or latish dinners when out on the backpacking trail. However, if the panel is used to charge a battery during the day (on top of a backpack and at lunch breaks and fishing excursions etc), it should be able to store ample energy for blower stove cooking and other requirements.

I was tempted to call it a solar cooker because both the fallen sticks and the electricity are made by the sun, but then it would leave out the solar battery charging function.

Tim

Addendum1. Optimized orientation

I found that optimization of the orientation of the panel toward the sun was not easy. However, I found that a thin plastic tube that was mounted square to the panel could be used to set the best orientation when it cast no shadow. The rod is held firmly in place with a small screw through one of the four mounting eyelets. It works very well, but when I brought my experimental reflective light trap into play, it made the shadow much harder to see. More on that later….