Hydraulic hybrids: Boosting fuel economy without batteries

[Elderberry]

Carbon fiber helps to lightweight hydraulic accumulators, increasing efficiency in hybrid vehicles and myriad other applications.

UPS has 50 hydraulic hybrid delivery trucks in the Chicago metro area. These vehicles, which use carbon fiber composite wrapped accumulators, offer a 15-55% reduction in fuel usage and CO2 emissions and cut NOx emissions in half.
SOURCE: Lightning Hybrids and Steelhead Composites.

Composites have generally benefitted from the growth in hybrid vehicles, because lightweight materials are used to help offset heavy batteries and extend driving range on a single charge. According to Plastic Omnium’s (Île-de-France, France) Dec 2016 Investor Day presentation, all vehicles will be some type of hybrid by 2050.

A vehicle is a hybrid if it uses two or more power sources to achieve propulsion. Typically, the primary power comes from fossil fuel — gasoline or diesel — with supplemental power coming from batteries and/or energy recaptured during braking (regenerative braking) using a generator and electric motor. This reduces the power demand on the engine, which in turn lowers fuel consumption and emissions.

However, a hybrid vehicle does not necessarily have to involve batteries or electrification. According to the US Environmental Protection Agency (EPA, Washington, DC) and Lightning Hybrids (Loveland, CO, US), hydraulic hybrids are more efficient and easier to implement, especially for heavier vehicles.

Hydraulic hybrids use many of the same principles as electric hybrid vehicles, but instead of batteries, they use a lightweight hydraulic system to capture and reuse braking energy for powering the vehicle at slow speeds. The EPA has been experimenting with hydraulic hybrids since the late 1980s. Today, companies such as UPS, DHL, Kiessling Transit, Denali National Park, Hyatt Hotels, Miami Dade County, and many others are running hydraulic-hybrid systems from Lightning Hybrids and Parker Hannifin (Cleveland, OH, US).

Hydraulic hybrid systems are efficient, compact and affordable, reusing braking energy to provide power for acceleration, which reduces vehicle fuel use and emissions.
SOURCE: Lightning Hybrids (left) and EPA (right).

How Hydraulic Hybrids Work

Hydraulic fluid is stored in a low-pressure reservoir (aluminum tank with bladder). A pump moves the fluid from the reservoir to a high-pressure accumulator (composite tank with bladder). The accumulator holds not only the fluid brought over by the pump, but also pressurized nitrogen gas in the bladder.

During regenerative braking, kinetic energy from the vehicle’s motion powers the pump via the drive shaft, moving fluid from the reservoir to the accumulator. This slows the vehicle down, which means the hybrid system acts as the primary braking system. During this phase, pressure builds in the accumulator due to the nitrogen gas being compressed, and this acts as a mechanical battery, storing energy to be released when the vehicle needs to accelerate again. As the vehicle accelerates, the accumulator sends its energy, in the form of pressurized hydraulic fluid, to the pump, which now acts as a motor, turning the vehicle’s driveshaft and thereby reducing the load on the engine. This is how fuel use and emissions are reduced. (Note, in an electric hybrid vehicle the regenerative braking energy is transferred first to an electric motor which then turns the driveshaft.) As the vehicle accelerates, the hydraulic fluid returns to the reservoir, ready to charge the accumulator again during the next braking event.

Composites World
Blog: 1/26/2017 Ginger Gardiner

http://www.compositesworld.com/blog/post/hydraulic-hybrids-boosting-fuel-economy-without-batteries

[endicom]

I've been waiting for this to take off for many years but I believe the named manufacturer is the only one still pursuing it.

[thackney]

I've been waiting for this to take off for many years but I believe the named manufacturer is the only one still pursuing it.

Typically an energy storage system that uses compression has an efficiency problem due to heating during compression.  I'm more familiar with systems using a gas such as air or others, but I'm not sure if fluid hydraulics have the same significant issue.

[r9etb]

Typically an energy storage system that uses compression has an efficiency problem due to heating during compression.  I'm more familiar with systems using a gas such as air or others, but I'm not sure if fluid hydraulics have the same significant issue.

I'd think cooling of the nitrogen over time would be the more pressing issue, as that represents a pressure loss, and therefore an efficiency loss. 

However, considering the likely use -- storing and releasing energy during frequent stops and starts -- the losses wouldn't be that great, since the amount of time is measured in minutes at most.  So some insulation on the tank would tend to mitigate the loss.

[thackney]

I'd think cooling of the nitrogen over time would be the more pressing issue, as that represents a pressure loss, and therefore an efficiency loss. 

However, considering the likely use -- storing and releasing energy during frequent stops and starts -- the losses wouldn't be that great, since the amount of time is measured in minutes at most.  So some insulation on the tank would tend to mitigate the loss.

Any heating is energy lost for a pressure system.

[Oceander]

Most likely these systems are really only beneficial for vehicles that do a lot of stop and go in generally slower traffic, such as in big cities like NYC.  They'd probably also be good in cabs in big cities. 

[r9etb]

Any heating is energy lost for a pressure system.

Not sure about that.  If it's an adiabatic process (i.e., insulated tank), thermodynamics says that compressing the gas will heat it up, and it cools off again when you expand the gas. 

It's heating losses that matter most.  If you let the gas cool, then you lose pressure and that us where your losses will occur.  Insulating the tank reduces the heating losses.

Of course, you're going to lose heat across the bladder into the hydraulic fluid, but if the bladder's a reasonably good insulator and the cycle time is short, maybe those losses aren't too large.

[r9etb]

Most likely these systems are really only beneficial for vehicles that do a lot of stop and go in generally slower traffic, such as in big cities like NYC.  They'd probably also be good in cabs in big cities.

Yes.  If you're on a long highway drive you don't do much starting and stopping.  But for something like a UPS truck, I imagine the fuel savings would be considerable.