Hydraulic versus Electric
The Lightning Hybrids ERS is a hydraulic hybrid. Although electric hybrids are well proven and are well-suited to passenger vehicles, electric hybrid technology is less suitable for heavier vehicles. In fact, the heavier the vehicle, the more hydraulic hybrids out-perform electric hybrids in a number of key areas:
Second generation batteries that can charge faster, go farther, and last longer are currently prohibitively expensive, ranging from $10,000 to over $40,000. Many battery systems are rated at 500 cycles—or about 18 months of daily charging before replacement is necessary. Equivalent hydraulic pump/motors and accumulators cost $5,000 – and they last much longer before needing to be replaced.
Charge time not only impacts overall efficiency and utilization, but also how efficient the brake regeneration system is. Toyota currently estimates their electric brake regeneration system at 30 percent efficiency, while hydraulic brake energy regeneration efficiency is over 80 percent, since hydraulics are not limited by battery charge limits during the regeneration cycle. The following diagram shows the difference in regeneration efficiencies (click to enlarge):
Once the batteries have reached the end of their useful life, there’s the problem of recycling them in an environmentally responsible manner. In fact, recycling these advanced lithium ion batteries is usually not cost effective, and dumping them poses a hazard to the environment. Hydraulic components are mostly aluminum and steel, which are easily recycled or repurposed.
Weight has a dramatic impact on overall transportation efficiency, and even lithium ion battery technologies add significant weight, ranging from 350 to well over 1,000 pounds. Equivalent storage in carbon fiber composite tanks weighs 200 pounds. Even the largest ERS hybrid system, designed for trucks over 35,000 lbs GVW, weighs around 1,200 lbs with fluid. The weight of batteries that would be needed for such a heavy vehicle would be much higher, compromising payload capacity and vehicle safety.
Maintenance talent availability
Electric hybrid vehicles are highly complex and the DC electrical power can be dangerous if not handled by specifically trained mechanics. There are not a large number of specifically trained mechanics in the US today, and the training requirements are significant. In contrast, most fleet mechanics already have training and a strong comfort level with hydraulic technology.
No fire hazard
The batteries in electric hybrid vehicles can cause vehicle fires. On the other hand, hydraulic fluid has a very high flashpoint and nitrogen is inert making the hydraulic hybrid safe and reliable.