As we wind down toward the end of another year, it's time to take a look back at some of the innovations that helped make 2013 a big year for green car technology. Highly anticipated cars like the Porsche 918 Spyder and Cadillac ELR made their debuts, while green concept cars provided a peek at what the future holds. Our favorite eco-friendly concepts from the past 12 months rethink power technologies, aerodynamics and architecture to push fuel economies into the triple digits.

Peugeot Hybrid Air

One of the earliest green concepts of 2013, the Peugeot Hybrid Air was also one of the most interesting. Putting a different spin on hybrid technology, the powetrain uses a long, tubular compressed air tank in place of batteries for energy storage. That air tank powers a hydraulic motor-pump, which assists a gas engine in three modes: zero-emissions, air-powered driving, hybrid, and engine-only. The motor-pump recovers air energy during braking and deceleration, refilling the air tank. The French automaker estimates that the powertrain is capable of 2.9 l/100 km (81 mpg) and CO2 emissions of around 69 g/km.

In addition to detailing the layout of the powertrain in January, Peugeot Citroën revealed the Hybrid Air-powered Citroën Cactus concept at the Frankfurt Motor Show in September. It plans to bring the technology out of its research facilities and into B-segment production cars by 2016.

VW CrossBlue Coupe

The CrossBlue Coupe made several appearances throughout the year, leading folks to speculate that Volkswagen is serious about ushering it to the market. The plug-in hybrid concept seamlessly combines fuel economy, SUV utility and zippy performance.

Volkswagen's latest specs give the CrossBlue Coupe – which really abuses the term "coupe" – 13 miles (21 km) of all-electric driving from two motors. When those motors team with the turbo V6 engine, they put out 415 horses, which inspire a 5.8-second 0-60 mph (96.5 km/h) time. VW estimates that the powertrain is capable of around 70 mpg-e and up to 570 miles (917 km/h) of range.

The CrossBlue is based on Volkswagen's new Modular Transverse Matrix and could be outfitted with various six- and four-cylinder engines, including diesel and compressed natural gas options. We won't be surprised if we hear a lot more about the concept in 2014.

The Elio

The Elio from Louisiana start-up Elio Motors doesn't use any type of alternative power. Instead, it uses a unique design and a small, fuel-sipping three-cylinder engine to get up to 84 mpg (2.8 l/100 km). That combination also powers the narrow three-wheeler to a respectable top speed of more than 100 mph (161 km/h) and a range of up to 672 total miles (1,081 km).

As a three-wheeled vehicle, the Elio faces some regulatory hurdles before it's available for regular use. Elio has had success in getting some US states to recognize it as a separate class of vehicle, but others apply motorcycle regulations to three-wheeled vehicles. Elio is working with the intent of launching the car in the fourth quarter of 2014 and is still quoting a price under US$7,000.

VIA Motors XTRUX

Maybe it's just that we've come to expect hybrids to come in compact and subcompact packages, but it was nice to see something completely different at the 2013 North American International Auto Show. That something was the VIA Motors XTRUX, a large GM pickup truck with a range-extending 4WD powertrain.

Instead of replacing the pickup truck's V8 engine, Utah's VIA Motors puts it to work as a gas-fueled range extender. Driving the wheels are two 402-hp electric motors. The XTRUX has the power and capability of a big, burley truck, the instant torque of an electric, and the range of a gas truck. VIA says that it can get up to 40 miles (64 km) of all-electric driving and average 100 mpg-e.

The XTRUX's fate remains unclear, but VIA launched other range-extending pickup trucks and vans last month. Those models use a single 402-hp motor along with a V8 engine purposed as a range-extending generator.

Yaris Hybrid R

Toyota's Yaris Hybrid R concept is a stretch of the term "green," but it is an interesting look at how hybrid technology can turn a car as mundane as the Yaris into a quick, aggressive track car. The Toyota Yaris Hybrid R debuted at the Frankfurt Motor Show equipped with trickled-down racing technology. Instead of the usual lithium battery pack, the hybrid hot hatch has a super capacitor to power its two rear-mounted 60-hp electric motors. The motors assist the 300-hp turbo four-cylinder engine in delivering quick, lively AWD acceleration, and the driver controls their output with a dual-mode track/road system. The hybrid system can also alter torque to the rear wheels, providing enhanced cornering and control.

Buick Riviera

During the course of the year, GM gave Cadillac Volt-like plug-in hybrid technology in Detroit and showed what a future Buick plug-in might look like in Shanghai. While the Riviera concept, and its glacial-blue lighting and white-leather cockpit, was clearly built as a showpiece only, the technological ideas it presents may just underpin future models. Those ideas include a wireless plug-in hybrid system with a chassis-integrated recharge panel, integrated 4G connectivity and car-to-car communications.

Outside of those basic details, Buick was rather vague about the Riviera's equipment, focusing more on its design language. The concept car might influence future styling and technology, but we don't anticipate it showing up as a Buick model.

Daihatsu FC Deco Deck

Fuel cells climbed out of the shadows a little during the year, with several major manufacturers preparing for the next generation of fuel cell technology. Daihatsu showed one of the most interesting fuel cell concepts in the form of the FC Deco Deck from Tokyo. Daihatsu's liquid fuel cell eliminates the platinum used in other fuel cells, saving money and resources. It's compact enough to slide under the floor, adding space and flexibility.

The FC Deco Deck uses big visuals to advertise its big technology: a cubed cabin with wraparound-glass is planted atop a rectangular trailer chassis.

Toyota i-Road

Part tandem, three-wheeled car, part enclosed trike, the Toyota i-Road is all innovation. Toyota debuted the vehicle at the Geneva Motor Show, solving the naming dilemma with the simple term "Personal Mobility Vehicle." The design offers a narrow footprint and maneuverability comparable to a two-wheeler, with a safer, more comfortable, more weatherproof driving experience. It's powered by dual 2.7-hp front-wheel electric motors.

We came out of Geneva thinking the i-Road was a crazy concept that might never be seen again, but Toyota said in September that it plans to build a limited number of i-Roads for use in its Harmonious Mobility Network trials in Toyota City, Japan beginning early next year.

Mitsubishi Concept GR-HEV

From its sleek, modernized pickup car form to its diesel-hybrid powertrain, the Mitsubishi Concept GR-HEV is a future look that we want to drive right now. The Geneva-debuted concept is powered by a 2.5-liter diesel engine and electric motors, which work with a full-time 4WD system. Mitsubishi claims that the concept puffs out just 149 g/km while cruising from city to worksite.

Mitsubishi didn't say whether or not Geneva was a stop on the way toward GR-HEV production, but the concept was one of several hybrid and electric concepts that Mitsubishi showed in Geneva and Tokyo, so we anticipate more electric power in its future line-up.

Nissan BladeGlider

One of the most intriguing concepts of 2013 from every angle, the Nissan BladeGlider reteaches everything we thought we learned about car design. Nissan's design team wiped the slate clean and drew influence from aircraft and racers like the ZEOD RC to design a new type of street car. That car is a rolling triangle with a 1-m (3.3-ft) front track and widened rear track, a platform that delivers superior aerodynamic performance and increased maneuverability. The car's unique shape and CFRP construction allow the RWD, in-wheel electric powertrain to deliver an efficient but exhilarating ride.

During its Tokyo debut, the BladeGlider's future seemed optimistic. Nissan used language like "when BladeGlider matures into a production car" and "exploratory prototype of an upcoming production vehicle." Later in the month, however, the design became the subject of a lawsuit filed by Nissan's DeltaWing partner Delta Wing Project 56, throwing its future in doubt.