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Small scale off-grid power supplies

I am a 3rd year engineer and I am working with a team to create a portable energy device to charge laptops for school children in remote off-grid nepali students. The laptops were donated by NGOs but the villagers have no way of charging them. We are reaching out to charities and other experts who work in similar situations so we can tailor our device to some of their needs as well. Our project is still in the brainstorming phase so any input will help make the final product more useful and will be greatly appreciated. What are the energy needs? What is the energy used for? What are the current sources of power used or current solutions? Do you think small scale power generation can be useful?

For example, small battery charging devices that are powered by an ox while the ox is ploughing the land, or battery charging shoes using piezoelectric, or using the pit fires that villagers typically make to charge small batteries throughout the day. Do you think these kind of devices are helpful for charities in bringing power to off grid communities? During this brainstorming phase, we are considering microhydro power systems, rain energy, wind, solar, or even somehow using wood fire or coals.

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The starting point is to assess how much energy in watt-hours (Wh) is needed. Laptop batteries are typically 11 – 15V voltage with a capacity of 2,500 mAh (= 2.5 Ah) to 7,000 mAh.

Voltage (volts, V) x Current (amps, A) = Power (watts, W).

Power (watts, W) x Time (hours, h) = Energy (watt-hours, Wh).

Capacity (Ah, = 1,000 mAh) x battery voltage (V) = Energy storage (Wh).

Laptop batteries are generally between 40 and 80 Wh. It’s useful to convert from the typical laptop battery specification of milliamp-hours because the voltage that you generate may not be the same as that used by the laptop. Defining the energy required is the best way to compare different sources.

The next question is how often you need the battery recharging? A fully charged laptop will typically run for 3 to 6 hours depending on the model, activities, power settings, and age of the battery. Is this enough for one day, or several? This will tell you the minimum amount of energy you need to generate per day.

How will you charge the laptop? Laptop power supplies convert from 230 Vac (or 110 Vac) to 15 to 20 Vdc. If you have a 230 Vac source – an engine or hydro generator – you can use the standard laptop power supply. Whilst it might sound feasible to connect a DC (direct current) source, such as a solar PV panel, direct into the laptop charging socket, you will probably damage the charge regulator in the laptop. A practical solution is to use an intermediary 12V battery and either a small inverter to convert to 230 Vac or a 12 Vdc car adaptor. Inverters are easier to find (or replace) locally but may result in other loads being connected.

How to generate the energy? Unconventional sources like piezoelectric (shoes, pavements) or thermoelectric (stove or fire-charger) sound like an interesting research project. Small stove chargers are available but generally output to a USB socket which at 5.2 Vdc is not enough for a laptop. Although technically less exciting, the best approach is to use an easily available, well proven source. Typical examples would be solar PV, engine generator, wind turbine, hydro generator or, maybe, pedal generator.

For the small amount of energy required, an engine generator is overkill, and requires regular and costly fossil fuel to operate. Nepal has for many years been a world leader in the use of micro hydro for village power supplies. There may well be one available locally, in which case the laptops could be charged directly, or possibly via a portable battery (with inverter) that can be taken for charging to the hydro system once a week. Hydro systems are quite expensive to install, so would not be cost effective just to run a few laptops alone. As a village electrification supply it’s a different story.

Wind turbines are well proven and can be made locally – see Hugh Piggott’s designs at scoraigwind.co.uk which have been adapted ... (more)

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