ATR3: Engine

I am not inclined to work with combustion engines, given their vibration, heat, noise, and maintenance requirements.  Moreover, combustion engines have a significant zero offset in their fuel consumption vs energy output function; ATR3′s engine consumes 1/3 gallon (1.25 liters) of gasoline per hour at idle.  Unfortunately, while electric motors can readily convert electrical energy to a rotary form, I am not aware of an alternative energy storage medium that can accomplish all three of the following:

• produce either rotary or electrical energy at a rate of 10kW;
• approach the weight per deliverable energy (1-4kg/kWh, depending on ratio of tank capacity to engine size) of gasoline;
• approach the price per delivered energy ($0.73/kWh) of gasoline.

The above weight and price numbers are based on 4-stroke gasoline 120VAC generators.  I am open to alternatives within a few hundred percent of the weight and cost of gasoline.  For instance, $1.50/kWh and 15kg/kWh would hamper rover mobility, but would be worth implementing to eliminate the shortcomings of a combustion engine power source.

Sealed lead acid  (SLA) batteries actually store energy at a lower cost than gasoline: initially $130/kWh with a 600-kW lifespan and a typical $0.15/kWh utility rate for a lifetime cost (considering charge efficiency of 80%) of $0.40/kWh.  However, SLAs are significantly heavier than gasoline (20kg/kWh) and bulky.  ATR3 could carry a maximum of 5-7kWh worth of such SLAs, which would limit the rover to around two hours of lightly loaded operation between recharges. As most batteries require a recharge time of at least an hour, this implies either a significant downtime after each discharge or a physically rotating stock of batteries where one bank can be recharged while the other is in use.

Lithium polymer batteries approach the weight/energy of gasoline (5kg/kWh), but with much higher lifetime costs ($1.84/kWh) and complex charging requirements.  Hydrogen fuel cells have a reasonable weight/energy, but are astronomically expensive.  I have not been able to determine exact pricing for cells in the kilowatt output range, as they must be custom ordered.

Below is a chart of the Price Weight Power Ratio (PWPR) for various energy storage media.  The PWPR is the product of weight and price for a given medium as a ratio to the bar set by 4-stroke gasoline: 4kg and $0.73/kWh.  A medium with twice the weight but half the cost of gasoline would also have a PWPR of 1.0; twice the weight and twice the cost would be 4.0.

PWPR by Medium

The graphed numbers are for a 2-stroke 1000W 120VAC gasoline generator [ETQ TG1200, $140], 4-stroke 3000W 120VAC gasoline generator [i.e. Gentron 3500W, $385], 1kWh deep cycle SLA [Sears Die Hard, $130], and the cheapest ($/kWh) LiPo batteries I could find [various hobby stores].  Inverter/regulator pricing and efficiency are not taken into account for the batteries.  2-stroke oil cost is included at 1:50 ratio. The numbers for propane and compressed air are not included, as they dwarf the other bars.  Simplified spreadsheet here.

[UPDATE: The ETQ TG1200 generator is actually more efficient than I projected here. See fuel consumption measurements.]

[UPDATE: Power Sources 2 examines 2-stroke gas generator versus SLA in further detail.]