How Soil Lamps Function

Important Insights

When it comes to traditional electric lighting, the power source is usually quite simple: it comes from the grid. When you switch on a light in your room, electrons flow from the wall socket into the lamp’s conductive parts, completing a circuit and lighting the bulb (for more information, see How Light Bulbs Work).

Alternative power sources are gaining popularity, and lighting is no exception. For instance, the Demakersvan streetlamp uses a sailcloth turbine to generate electricity in windy conditions. The Woods Solar Powered EZ-Tent, on the other hand, harnesses energy from solar panels mounted on the roof to power LED strings inside the tent once night falls. Philips has created the Light Blossom streetlamp, which combines both solar power and wind energy to provide illumination—solar during sunny periods, and wind during calm days. Of course, we can't overlook human-generated power: the Dynamo kinetic flashlight requires the user to pump a lever to produce light.

While we all know about wind, solar, and kinetic energy, a device presented at last year's Milan Design Week introduced an energy source that's rarely discussed: dirt.

This article explores how a soil lamp operates and its various uses. It's an energy-generating method that's been known for quite some time, dating back to its first demonstration in 1841. Today, there are two primary ways to harness electricity from soil: one where the soil acts as a conduit for electron flow, and another where the soil itself generates the electrons.

Let's begin by looking at the Soil Lamp showcased in Milan. This device incorporates dirt in a manner similar to how energy is generated in a typical battery.

Earth-battery Lamp

The Dutch designer Marieke Staps is behind the creation of the Soil Lamp. While the design itself is modern, it draws inspiration from an ancient concept known as an 'Earth battery.'

Back in 1841, inventor Alexander Bain demonstrated that dirt could generate electricity. He buried two metal pieces—copper and zinc—about 3.2 feet (1 meter) apart and connected them with a wire circuit. This setup produced around 1 volt of electricity, enough to power the clock attached to the circuit [source: EE].

This arrangement resembles the classic Daniell-cell battery, dating from the 1830s. The Daniell cell consists of copper (the cathode) suspended in copper-sulfate solution and zinc (the anode) suspended in zinc-sulfate solution. These liquids, known as electrolytes, contain ions that enable the exchange of electrons between the zinc and copper, thereby creating and channeling electrical current. The Earth battery, as well as potato or lemon batteries, function similarly to the Daniell cell, though less efficiently. Instead of using copper and zinc sulfates, the Earth battery relies on dirt as the electrolyte.

When copper and zinc electrodes are placed in wet dirt, a chemical reaction occurs because zinc loses electrons more readily than copper. Since dirt contains ions, wetting it turns it into an electrolyte 'solution.' This allows the electrodes to exchange electrons, just like a conventional battery.

If the electrodes were to touch directly, they would generate a lot of heat during the reaction. However, with the separation created by the soil, free electrons must travel across the wire connecting the two metals in order to move between them. By attaching an LED to this completed circuit, you can create your own Soil Lamp.

The process isn't indefinite—over time, the soil will degrade as its electrolyte properties are used up. However, replacing the soil can restart the cycle and get it going again.

Staps' Soil Lamp is a conceptual design, not yet available commercially (though you could probably make your own version by swapping 'potato' with 'container of mud' in a potato-lamp experiment).

A more recent variation of the Earth battery utilizes soil in a more active role in generating electricity. In microbial fuel cells, it’s the contents of the dirt that are crucial to the process.

Microbial-battery Lamp

If you have a compost heap in your yard, you're likely aware that soil is a very active substance. It's teeming with life, as microbes in the dirt work constantly to break down organic matter into valuable products. In compost, this process produces fertilizer. But some microbes do even more powerful work: they generate electron flow.

Bacterial species like Shewanella oneidensis, Rhodoferax ferrireducens, and Geobacter sulfurreducens—found naturally in soil—not only generate electrons when breaking down food (like our waste) but can also transfer these electrons from one place to another.

Lebone Solutions, a startup, has found a way to tap into this microbial electricity, using it to provide lighting and charge cell phones in rural African communities.

Microbial batteries, or microbial fuel cells, have been explored in research labs for some time, but their power output has been too low to make them practical for most uses. They couldn't power a clothes dryer, for example. However, Lebone Solutions has discovered a practical application for them: it takes very little power to light a bulb or charge a phone.

The device is straightforward to build, mainly consisting of a graphite cloth (serving as the anode) placed at the bottom of a container, then covered with soil. A length of chicken wire acts as the cathode, and a conductive wire connects the anode and cathode to form a circuit. An LED is attached to this circuit.

As microbes consume waste in the soil, they generate electrons. These electrons naturally move toward a more positive charge, traveling through the bacterial network. They flow from the graphite-cloth anode, pass through the conductive wire, and reach the chicken-wire cathode. The flow of this current lights up an LED in the circuit.

Lebone estimates that a fuel cell covering 10.7 square feet (1 square meter) could produce 1 watt, enough to charge a mobile phone. A larger setup of 53.8 square feet (5 square meters) could power a lamp or fan [source: Grifantini].

While a microbial fuel cell may not be an ideal power source in the developed world, it holds significant potential in rural Africa, where access to grid power is scarce. In such areas, this technology could provide an alternative to the long journey needed to charge a phone. Lebone is currently introducing the fuel cell in several African villages.