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portable_off-grid_power

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.

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.

Steam

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.

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've got one on order.

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 and have a lot of storage, so it operates best at the average wind speed. That's not what we want here, we want some power at low wind speeds and not to break at high wind speeds.

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.

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.

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 quickly. Prices include delivery (normally free).

When/If I build it then I'll post results here.

portable_off-grid_power.txt · Last modified: 2019/03/05 10:39 by admin