Two things are often scarce in the aftermath of a natural disaster: electricity and fuel. A team of engineering students at Florida International University has designed and built a hybrid emergency generator that uses no fossil fuels. Instead, it harnesses the power of sunlight and wind, coupled with a battery bank for storage, to provide electricity in disaster-stricken areas. How does it compare to a fossil-fueled generator?
Image credit: Florida International University
Dimensions
Weighing in at 215 pounds (98 kg), the unit can be transported by small pickup truck or SUV. The wind turbine is 59 inches (1.5 m) in diameter; its tower is 149 inches (3.8 m) tall, extendable to a maximum height of 203 inches (5.2 m). The PV panel is 47 inches (1.2 m) wide, and the battery bank is 32 inches (0.8 m) wide.
PV
A single 100 watt monocrystalline solar panel provides most of the power for this unit. Monocrystalline is slightly more expensive than the more common polycrystalline solar panels, but monocrystalline panels are more efficient. On a sunny day in a temperate zone, the panel can generate about 500 watt-hours of energy per day - even more in tropical climates. With the price of solar panels being so low, I’m surprised that they didn’t use two or more panels in this system. (More on that later.)
Wind
The wind turbine sweeps an area of 2732 square inches (1.76 square meters). It feeds a Freedom Permanent Magnet Generator (PMG) whose maximum output is 1600 watts. That sounds impressive until we look at the power curve for that generator. In order to generate 1600 watts, a wind speed of 22 m/s (50 mph) would be required. If the unit operates under those conditions, I hope that baby is anchored down with guy wires!
In reality, at a hub height of only 173 inches (4.4 m), the typical wind speed is very low. In fact, most wind maps give the wind velocity at 33 feet (10 m) - more than twice the height of this tower. For the sake of argument, let’s say the average wind speed is 3.6 m/s (8 mph). The maximum power in the wind, given that velocity and the area swept by the rotor, is about 50 watts. Most small wind turbines are about 30% efficient (at best), so in this scenario the turbine will generate about 15 watts of power. If that’s a steady wind, then the turbine would produce 360 watt-hours per day. (For a detailed analysis of small wind turbines, check out my article about rooftop wind turbines.)
Storage
A pair of 12V, 115 Ah Everstart deep cycle marine batteries provides up to 2760 Wh of storage - adequate for emergency lighting and charging small electronics. While it could power a hot plate for a short time, I think a small camp stove with a propane cylinder would be a better source of heat for cooking.
Based on my power calculations, it would take about three days to fully charge the batteries using renewable power. Of course the unit would arrive with fully charged batteries, so the PV and wind would need to top off the batteries rather than charge them from zero.
Inverter
An inexpensive Cen-Tech modified sine wave inverter turns the battery’s DC output into AC power. It can deliver 1500 watts continuously, and up to 3000 watts in short bursts. Unfortunately these small inverters are only about 87% efficient.
Output
According to the press release, this hybrid generator is capable of producing 1.5 kWh of energy per day. From my calculations, which are pretty generous as far as wind speed is concerned, it’s likely to produce about half that amount.
Solar, Wind, or Both?
The reason that designers put solar and wind together is because they’re complementary - sunny days tend to be calm, while cloudy days are often windy. Also, the turbine can generate power at night, assuming the wind is blowing. At the utility scale that works well because large wind turbines are more efficient and reach higher velocity winds. But small turbines on short towers produce so little energy that they’re usually not worth the expense and complexity. Some designs cannot be scaled down effectively; this is one of them.
As you can see from the numbers, it would be more cost-effective to use extra solar panels and a larger storage system than to include a small wind turbine. For the cost of the permanent magnet generator (not even including the blades and the tower), another 100 W solar panel could have been added. Even on a cloudy day, I suspect that the PV panels would still generate more energy than the wind turbine.
Why Not a Gasoline Generator?
In my garage I have an emergency generator that weighs 140 pounds and cost $450 when I bought it in 2011. It can deliver 3500 watts continuously, as long as I have a supply of gasoline. And that’s the key: as long as I have fuel. For a short-term emergency, a gasoline generator is the most reliable and cost-effective source of power.
When Superstorm Sandy hit the east coast of the United States, power was out for weeks and fuel supplies were severely limited. A few residents who had solar panels were letting their neighbors charge cell phones and other portable devices. That’s where a portable generator that uses renewable energy can make a difference. But I’d skip the wind turbine and add more solar panels, or include a generator option like the military-grade (and much more expensive) X3 Energy systems.
It’s an Engineering Student Project
One goal of this project was to show that one could produce an affordable emergency generator that uses only renewable energy. In this case, the students successfully built a functional prototype for about $1700. But the larger goal of a student design project is to learn real world engineering skills. If I were designing a commercial version of this, I’d go with all solar and no wind turbine. But for a class project, it’s important that the students learn the mechanical concepts associated with a wind turbine, because those same concepts apply to other mechanical systems. When they field test the unit under realistic conditions, I think they’ll learn that some designs can be scaled up or down and still work effectively, while others - like small wind complementing solar - don’t work out so well.
Overall it’s a commendable senior design project that created a useful product and draws attention to the need for generators that don’t rely on fossil fuels.
Product images (except where noted) courtesy of their respective manufacturers