What are the key advances in mini-grid generation technologies?

Innovations in manufacturing will continue to decrease the cost of conventional silicon solar PV technologies. Over the past decade, solar PV has become much more affordable. Between 2007 and 2016, the cost of solar modules decreased from $4 per watt to $0.55–$0.65 per watt. Technology innovations such as diamond-wire sawing for PV wafers, advanced metallization solutions, increased automation and larger cell sizes are decreasing manufacturing costs.

New semiconductor materials are improving efficiency and lowering the cost of solar PV cells. Super-efficient semiconductor materials like perovskite and gallium-arsenide, as well as cells made with tiny but powerful solar-absorbing “quantum dots,” are under development or in limited production.

Perovskites are especially promising. Perovskites are made from inexpensive materials such as lead and ammonia. They are easy to build at lower temperatures, unlike silicon solar cells, which require a very high temperature (1,425 degrees C). Scientists have discovered ways to “paint” perovskite material onto thin, flexible material such as plastic.

Wind speeds increase significantly with altitude, so a generator must be high off the ground to harness wind energy. Conventional wind technologies use tall towers to position wind turbines. Towers for wind generation require a significant amount of steel.

Airborne wind turbines, a promising new technology, use conductive tethers or wireless transmission instead of steel towers. Turbines capture energy from high winds; a generator converts this mechanical energy to electricity. Then, electricity is transferred to a transformer on the ground, either through a conductive tether or by using lasers or microwave radiation. Airborne turbines don’t yet produce enough energy to power large systems, but they are perfect for smaller mini-grids and are a compelling way of delivering more power than a simple tower-based wind turbine. They also are well suited to windy climates that lack good conditions for solar power or water to power mini-hydro plants.

As of 2017, no commercial airborne wind turbines were yet in operation. Several companies, however, are developing and testing airborne wind turbines. Promising preliminary results have attracted investments from Google and other large investors.

Adding renewable energy generation to diesel-powered mini-grids is an effective way to decrease costs, reduce emissions and provide high-quality energy services. Diesel hybrids have drawbacks, however. Conventional diesel generators can’t handle sudden increases or decreases in energy input from intermittent sources of renewable energy.

When solar or wind resources generate more than 70 percent of a hybrid mini-grid's needs, conventional diesel generators can’t maintain a stable frequency. When a burst of sunlight strikes the PV cell or a gust of wind spins the wind turbine, for example, the injection of energy causes a conventional diesel generator to spin too fast, generating electricity at too high a frequency. Similarly, if a cloud suddenly blocks the sunlight or wind ceases, a conventional diesel generator might not be able to ramp up its own power in time to maintain a stable frequency.

Unstable frequency can damage end-use equipment. Many electrical loads (especially equipment with motors or transformers) require electricity at a specific frequency. In Africa, Asia and Europe, utilities aim to keep the frequency constant at 50 Hz, while North America’s system uses 60 Hz. If the frequency becomes too high or too low, the electricity can harm equipment powered by the system.

In conventional diesel mini-grids, a feedback mechanism increases or decreases fuel input as needed to keep frequency constant while loads vary. Adding small amounts of intermittent renewable energy doesn’t impact the system. But as the amount of renewable energy increases relative to the amount of diesel-generated electricity, the feedback mechanism begins to fail.

Diesel generators in hybrid systems can deteriorate more quickly without special maintenance. When diesel generators produce low levels of power for a long time, as when solar or wind energy is providing most of the power, unburnt carbon residues build up in the generator’s pistons. Running engines at low loads also decreases the temperature of exhaust. At lower temperatures, acidic water can condense in the exhaust system and cause corrosion. To protect against damage, manufacturer Caterpillar recommends that during periods of light operation, diesel engines should operate at 30 percent or more of their generation capacity for about 30 minutes every four hours.

New diesel generator technologies can accommodate automatic and continuous variation from 0 to 100 percent generation from intermittent renewable sources. Innovations include:

• Rapid start-up and shut-off. New diesel generators can shut off and start up quickly as needed. These improved generators can start up and reach required speed in 1–3 seconds.
• Dump load operation. Dump load operations absorb excess power when intermittent generation exceeds consumption.

Fuel cells are emerging as a promising source of both primary and backup generation for mini-grids. Fuel cells generally convert energy more efficiently than technologies that use direct combustion, like engines. Fuel cells convert stored chemical energy into electrical energy. Fuel cell energy conversion efficiency is 40–60 percent, while gasoline engines are typically 20 percent efficient and diesel engines are 35–49 percent efficient.

Pilot projects have demonstrated the effectiveness of fuel cells in mini-grid applications. As of 2017, stand-alone and hybrid fuel cell systems manufactured by Sirius Integrator are supplying off-grid electricity in applications in the United States and Canada. In 2014, fuel-cell manufacturer Ballard Power Systems and Anglo American Platinum launched a pilot fuel cell mini-grid project at the Naledi Trust Community in Kroonstad, South Africa. The installation provides 230 V of alternating current (AC) power to 34 households, which use the energy for cell phone charging, TVs, radios, lighting, refrigeration and cooking.