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Portable off-grid power

What is the best way to generate power portably? Say you wanted to go wild camping for more days than you can carry USB power banks, what is the best way to charge radios, lights, phones, etc? I'm in the UK and I'm not up to winter camping, so let's call this the typical use.

Requirements

At least 5W of USB power (1A at 5V) and under 1.6kg.

Why USB? It's the common standard and you can buy rechargeable radios, lights, etc. It also means you can recharge a standard powerbank and then charge from there.

Why 5W? Many items will recharge at 1A, so anything less and you'll spend more time recharging than using.

Why under 1.6kg? Well a web search said “A loaded backpacking pack should not weigh more than about 20 percent of your body weight.” and say the average adult weight is 80kg, then that's 16kg for the pack and no way should power be more than 10% of the pack weight.

Efficiency means different things to different people. Here we want the most power for the least weight, so that's the greatest average Watts per kilogram (where the average is taken over all times of day and all expected weather).

Choices

Let's consider all the options. If I've missed something out then do email me. I used Amazon for links because they have the most honest reviews and the most permanent links, but ebay and banggood are cheaper.

Batteries

Batteries, specifically USB power banks, don't really count as they don't produce power off grid, they only store it for off-grid use. If power is needed for only a small number of days then buy batteries and don't bother with anything else.

Beware of inflated capacity claims when buying USB power banks, especially on eBay.

I put fuel cells into the same category as batteries, they have higher energy density but are far less practical.

Completely unsustainable, but a 4xD cell holder (£1.24) and a 5V LDO regulator (£1.15) makes a very cheap and super lightweight backup.

Hand Crank

Let's start with the failures:

My current candidate is Manual Crank Generator Hand Crank Generator (£21.99). You can get 5W continuous out of these (I've measured it), but be aware that the crank is 16cm long, so it's smaller than you might expect. The core generator and crank can be had for half the price above and weights only 360g. But who wants to crank and walk, or spend evenings cranking for power?

NEW: Dynabow: It's easy to make a lightweight bow out of aluminium tent poles. A single turn of the cord around the axis of a conventional bicycle dynamo provides suitable gearing. One hand holds the dyanamo, the other one end of the bow. Moving both hands together and then far apart with arms outstretched gives over 1m of movement and the dynabow generates power in both directions.

Walking

There is much on the internet about using walking to power mobile devices, but it's not very well thought out. Typically piezoelectricity is touted, but the transducers are almost incompressible so little work can be done and little energy generated.

Using rough numbers, the horizontal and vertical oscillation from walking is at about 1Hz. A person with rucksack way weight 100kg. The useful horizontal movement is about 1cm, vertical is about 1m for feet and 0.5m for hands. g is about 10 N/kg.

Let's say we put all of the vertical movement to work, so 100kg moves through 1cm in 1s, That's 10W, which is about right, and it would be equivalent of always climbing a 1% incline, which is also acceptable. For this we need two shoes which would accommodate the 1cm movement - right now I don't know how to do that.

Now let's think about taking energy from a mass that can travel vertically. The movement at back/shoulder level is only a few centimetres, so lets say that there is only 1cm of movement for work. How big should the mass be? Well, it's the same range as above, so it needs to be about 100kg, which is clearly unreasonable. The only way to get a high mass is to use the mass of the backpack, let's say that is 20kg max, but I'd find having my 20kg backpack jiggle about independently of me quite disconcerting. Someone may build a generator into a backpack frame, but it's not going to be me.

Moving on to horizontal motion, let's say you can accelerate a mass to 1m/s every second, what mass do you need for that to be 5J every second? 1/2mv^2 = 5, so m=10kg. No thanks, I'm not walking with 10kg strapped to each leg.

Fire

Heat will boil liquids and create a useful pressure to power an engine. Liquids are heavy so the only practical liquid would seem to be water. The only people playing with steam engines seem to be model enthusiasts, a working example is here. The closest I can find is that I can find in mass production is the FD4-Model-Dynamo which claims to work with a model engine and generate 1A at 12V. There are a few engine and generator assemblies on eBay.

I don't fancy leaving a boiler in a camp fire whilst I sleep. so that limits the generation time to a few hours per day.

Thermoelectic is also possible, some even claim that small amounts of power can come from wellies. I can't find commercial thermopiles beyond milliwatt sensors, so I assume they are just not practical for power generation.

Solar

Solar is the the most marketed solution with many options available. They all assume that the sun shines at 1000W/m2, which is just silly. I don't yet know what you can get in the UK at 1pm on a sunny day, but I'm sure it's a lot less. And who wants to be sitting around for the best part of the day? Solar would only seem to be viable if you are prepared to leave it charging and come back to collect it at the end of the day. That said, there is a clear winner, the class that includes BigBlue 3 USB Ports 28W Solar Charger at 590g. I bought one, but they sent a 21W model and on a very sunny day in winter it produced no useful power at all, so I sent it back.

It should be possible to increase the power to weight of solar by adding reflective foil to make a funnel. Given a narrow enough angle pointing towards the sun and perfect reflectivity the effective area is the area of the opening. Mylar aka Space Blanket is very light and reflective, but work would need to keep the funnel open (as well as occasional angle adjustment).

Wind

Wind seems to be the least developed option, but it also has the most potential. 8mm aluminium tent poles weigh 100g/m, the above generator is 360g, so there's scope at about 1.5kg.

Most commercial wind generation is designed to be permanent, have a lot of storage and operate best at the average and high wind speeds (power as V2 biasses towards high speeds). That's not what we want here, we want some power at low wind speeds and not to break at high wind speeds. Moreover, varying the pitch of the blades by anything other than bending/stretching is too complex.

This publication gives the basics of wind turbine designs and is well worth reading.

Vertical Axis

The main advantage of vertical axis wind turbines is that they don't have to be oriented to point into the wind. The main disadvantage is that they are just not as efficient as vertical axis wind turbines.

There are lots of kickstarter campaigns s uc as http://windpax.com (failed). Also on Kickstarter is Trinity (underpowered, broken home page), RMRDTECH (underpowered) and AE3D (too heavy but 300W!).

There are many DIY efforts, e.g. flapturbine.com. Vertical axis is very popular with DIYers, probably because they look really interesting. We care about power per kilogram so vertical axis isn't necessarily too inefficient.

NEW: A Flappy turbine: When the wind blows on the power side it forces the flaps to vertical, however on the other side they are free to rotate to horizontal. This is more of a “windwheel”, i.e. a waterwheel powered by wind, than a turbine, but it could have good power to weight.

Horizontal Axis

Small (100W to 400W) generators are common for marine and camping use, but these come in at about 10kg and so are too heavy. Infinite Air is light at 626g, but not powerful enough.

Others:

Decision: Kite based horizontal axis wind turbine

I went with a horizontal axis wind turbine because:

  • I'd get board hand cranking - anyway it's always an option if there is no wind
  • I don't believe that solar works in the UK - not enough time the sun is shining and everything is set up
  • Steam was out of my reach
  • I think that horizontal axis imposes less stress on the system, so it can be lighter and bigger

It would seem possible to mount the 20W generator with horizontal axis and have four trailing “blades”. As we want light weight and low wind speed, I envisage the blades being more like kites, that is light weight fabric without complex aerofoil shaping.

Prototype

What size?

It's important to be as large as possible within the weight constraint, as there will be days with only a gentle wind, and it's the number of wind generating days that counts, not the average power generation.

At taller than 2m to the generator it gets hard to rig and adjust. Let's start with 2m to the horizontal axis, that's about 3.3m tall - that's big!

Components

These are the items I've bought, I may not need all of them and the links are to eBay and so will expire. Prices include delivery (normally free).

Part Weight Length Item Cost
upright 340g 2m 4x1M Outdoor Backpacking Steel Tent Poles £12.32
generator 290g DC6V 12V 24V Miniature Hand Crank Wind Hydraulic Generator £9.29
spars 530g 3 x 2.61m ShineTrip Aluminum Rod Tent Pole 2 Pack £11.59
shaft 30g 1m 10mm x 1m Aluminimum tube £4.25 B&Q
splits 30g 3x 4x tent pole repair splint £1.56
sails ~360g 4.5m * 1.5m To be purchased, example 1.41mx1.41mx3x60g/m2 = 360g £14.99
guy ropes ~60g To be purchased
pegs To be purchased
total 1640g approximate - total materials ~ £65

I ordered aluminium alloy for the upright, they sent steel so it's heavier and I didn't pay for it. I may well replace one day.

Overall it looks as though it'll come in just over the 1.6kg budget. Whether it'll generate useful power is the bigger question.

portable_off-grid_power.txt · Last modified: 2019/05/15 08:58 by admin