I know the "umbilical" cord would be a pain, but I can't help wondering if excavators in urban settings could be electric (using ultracaps for peak power requirements).

Neil,

Beyond being a pain, I would suspect that an "umbilical" might post a safety hazard in an active construction site. With all sorts of cranes, excavators, vehicles, structures and manpower milling around, a severed umbilical could easily create a shock hazard, as well as bringing work to a sudden halt at an inconvenient moment.

Moreover, the power demands of such heavy-duty equipment may overtax the local electric grid connections, and there is little reason to beef up an infrastructure for an admittedly temporary usage.

At the same time, diesel construction equipment used in a dense urban setting can create noticeable local emissions impacts -- Boston during the Big Dig comes to mind. A good solution to this problem, though, would probably involve hybrids, and other low sulfur / clean diesel technologies.

I did the math a while ago, I may have been wrong, but pound for pound, hydraulic fluid has the same energy density at 5000 psi as lead acid batteries.

They already have a hydraulic system. Why not add a few accumulators for high pressure, and make it a hydraulic hybrid. When the engine is not in use, they can turn off the engine, and use the stored hydraulic energy.

If they put some work into it, they can even recover hydraulic energy as the excavator lowers a weight.

Nick & Michael,
I was wondering the same thing, whether it would be simpler or cheaper to use hydraulic set up instead?

And then it dawn on me that:
1) hydraulic hybrid requires storage of energy via compressed nitrogen gas tank at 1000-5000psi, meaning that there is not a steady pressure required to operate the hydraulic actuators directly from the nitrogen pressure storage tank. This means that you will need a separate hydraulic pump and hydraulic motor, both having variable displacement mechanism to account for the high variation in storage pressure. You cannot use the single hydraulic pump to both charge the tank and power the actuators at the same time. This implies no major cost nor complexity reduction from using generator-motor electric route. The displacement of the pump & motor must be constantly adjusted, and that increases complexity, whereas the generator simply charges the battery at the same voltage, and the motor simply runs from the battery at the same voltage each time.

2) "hydraulic fluid at 5000psi has equal energy pound for pound with lead acid battery..."

While this may be true, have you calculated the weight of the high-pressure nitrogen tank strong enough to hold 5000 psi of pressure? It probably will weigh >200 lbs if made out of carbon fiber, but may cost $1500 or more, or will weigh much higher if made out of steel...And the battery type used will be either NiMH or Li-ion which will be much lighter than lead-acid at much smaller volume requirement.

I'm sure that Komatsu engineers must have considered these and many more factors before they settled for this type of parallel hybrid drive.

If the picture is of the size machine they are planning on building it would be a 150 size or smaller. The reason for electric hybridization is in many cases an excavator the power usage is under 10 hp in standby or light usage.
Current hydraulic technology is not very efficient at lower power levels.
As to accumulators, revisit the EPA hydraulic hybrid info. They have better energy densities than batteries of ultra capacitors for high power short duration demands.

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