Sunday, August 21, 2011

Agricultural Robots: Power Source, Batteries, Why Not?

There are no doubt new battery technologies, being worked on in the lab these days, that will out-perform the Chevy Volt's Li-ion battery.  But for now, the Volt's battery can be considered a good representative of what is "state-of-the-art" for a "light-weight" large capacity battery, manufacturable in production quantities. 

Chevy Volt Battery, without cover
It's a "T" shape, roughly 5-ft long and 3-ft wide.  It has a weight of 435-lb (197-kg), and volume of approximately 15.0 cubic-ft.  Its capacity is rated at 16-kWh, but for reasons of longevity, its usable capacity has to be de-rated down to about 12-kWh.

The Chevy Volt's listed curb weight is 3750-lb, (1750-kg).  The weight of the Volt's battery is not a factor in the car's overall performance because the battery represents only 12% of the car's total weight.  This means that the marginal increase in rolling friction this extra weight brings is only 12%. 

At 35-mph, the aerodynamic drag for a typical car is comparable to its rolling friction [1].  So the extra drag on the Volt, caused by the extra battery weight, will amount to less than 6% of its total energy consumption when driving down the road at freeway speeds.   This is the reason that the Volt can get away with being battery powered. 

Now look at the situation for our robot harvester/tender.  Its speed of travel, for most cases, will be on the order of 2-20 ft/min.  Aerodynamic drag will never be a factor.  The overwhelming amount of the energy it uses for locomotion will be expended moving its weight around.  In this case, the weight of the robot will be the dominant factor in determining its power needs.    

The lighter the robot, the quicker it can move, and the faster it can perform its harvester/tender duties.  Adding a 400 lb battery to a robot that is going to weight less than 300-400 lb to start with, will compromise its performance fatally.

One could put a smaller battery in our robot harvester/tender, but then it could only run for an hour or so before needing a recharge.  Now imagine trying to run a cluster of ten to twenty robots in a field at one time, each of them needing to be taken out of service every hour or so for recharging.  Our field supervisor would need to bring in extra help to service the 'bots that needed charging, and also bring in extra 'bots to fill in for the 'bots being cycled out of service for recharging.  This scenario becomes a major violation of the WID Rule.       

So what is the long term outlook for battery powered industrial robots?  One only need look at the specific energies for the different power sources to get an answer. 

--One gallon of gas, burned efficiently in an ICE, has a specific energy of about 10,000 Wh/kg.
--The Chevy Volt’s Li-ion battery has a rated specific energy of about 80 Wh/kg [2].

By weight, a gallon of gas holds over 100 times as much energy as the same weight of a Li-ion battery.  So even if there were some major breakthrough in battery technology that yielded a factor of 10 times the storage capacity over the current Li-ion batteries, it would still fall short of the energy storage capacity of a liquid-fuel based power source by another order of magnitude.

For a liquid-fuel, energy is stored in the chemical bonds of its molecules; therefore every molecule is an energy storage unit.  In contrast, a battery stores energy in the chemical potential of two ions physically held separate.  Thus, a battery will always require the presence of some inert matrix to hold the two ions apart.  A battery also requires the presence of cathode and anode terminals to provide a pathway for the battery's stored energy to reach the outside world.  In other words, a battery will always contain a lot of extra structure that contributes nothing to its energy storage capacity.  For this reason, on a pound-per-pound basis, a battery will never be able to compete with a liquid-fuel.

To avoid violating the WID Rule, once a robot is in the field and working, it needs to be able to run without attention for a full 8-10 hr shift.  This requires a power source based on an energy storage capacity that will forever and always be out of the reach of batteries. 

Sadly for the pro-battery folks, batteries may be a workable solution for robot toys, but they will never be a viable power source for industrial robots that will be required to work in the field for extended periods of time.

Next Post:  New options for liquid-fuel based power sources

[1] When I can, I'll get the calculations behind this estimate posted over at my web site.
[2] For a comparison, a standard 12V lead-acid car battery has a specific energy of about 35 Wh/kg.

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