For the past few years I have wanted to make some sort of recreational vehicle that runs on multiple-source renewable energy. For example, a kayak with a trolling motor that gets its electricity from pedal power and solar power.

I am no electrical engineer, but I imagine that one would need some configuration that converts the generated input from the pedals and solar panels to a correct voltage that can be accepted by a common battery that in turn runs the electric trolling motor.

Recently, however, I have read about fuel cells and I am wondering if it wouldn't be a more simple design to power the trolling motor with fuel cells and then use the solar and pedal energy to make the hydrogen from water. It seems to me that this would avoid the need to convert the electricity coming from pedals and solar cells to any common factor.


What is the most efficient way (in terms of cost, complexity and safety) to collect and store energy from multiple inputs for a recreational vehicle?

Please don't be limited to the above example electrical inputs. For example, when the kayak is stationary, a raft with a solar collector and stirling engine could be deployed which takes advantage of the difference in temperature of the cold water beneath and the heated water above to provide a third source of electrical input.

Deploying a propeller like the one at this link could also be used as a sail/turbine.

Open Source

Once designed, this entire project will be open source. Please let me know if you would like to be a part of that.


4 Answers 4


More later maybe. Quick thoughts:

Various electrical inputs via MPPT (maximum power point tracking) converter to LiFePO4 (Lithium Ferro Phosphate) battery is usually likely to be the most efficient and cost effective way of storing electrical energy. See end for MPPT comment.

Brushless DC motor drive is a top contender for drive efficiency with regeneration of "braking energy" back into the battery where approriate.

Stirling engines are great but are unlikely to be practical from cost or mass density or volume density points of view in real world solutions.

Energy conversion from low grade heat sources such as temperature differentials is extremely inefficient due to Carnot efficiency limit of (delta temperature)/(Maximum temperature) with actual efficincies being a fractin of that. Very low % :-(. Such may be OK for static applications where getting free energy is much more important than weight or cost or size.)

Fuel cells have their place but Hydrogen is hard to deal with well and the technology to use it compactly is still evolving. It's very low mass density and high diffusion rates and other factors make it an unlikely solution in compact portable storage and powering applications. Methanol cells can have higher energy densities but are not yet good as storage solutions.

LiFePO4 batteries can store energy at > 90% efficiency have very good but not superb density compared to th very best battery technologies and have good life-cycle costs (but higher initial costs than eg lead acid.) Lead acid can be extremely good on conversion efficiency with care and has lower initial costs but higher long term than LiFePO4. Various other LiIon storage systems are not as good as LiFePO4 energy wise but have higher energy storage densities.

"Just paddling" has its place but can be over-rated :-).

@Rocketmagnet's suggestion of a sail is even better than he suggests. A practical sail for a Kayak can be of modest size and can be highly practical and provide a very good motive source on lakes and in the sea. You may need Exalted-Grand-Master status to use it above a class 2 rapid - but that may apply to trolling motors as well ;-).

Rocketmagnet's suggestion of using flowing water as an energy source when stopped is also a good one (and is related to my braking energy and regeneration comment). Total potential energy in falling water is mgh = mass x gravity constant x head ~= 10 Watts per kilogram.meter/second or about 12 Watts per gallon.foot/second. Extractable energy is probably around 10% of this in a portable propellor situation.
Of more likely interest is energy due to velocity = 0.5 x m x V^2.
Below -
density = kg/m^3
V = metres/second. 1 m/s ~= 2.2 mph,
Area = prop area in metre^2

Mass/second = Area of prop x velocity x density so
Power at 100% conversion =~ 500 x V^3 x A Watts. V in metres/second. 60% of this max unducted.
Near 100% of this in superb design ducted.
Say 20% or so in real world. So
Power ~= 100 x A x V^3 Watts.

Interest only: The above formula also works for wind turbines with a factor of about 1000 x less power per area for air due to the density difference.

Carrying a small wind turbine with fold out blades for use when stopped can make sense. Use when in motion in a kayak again needs Grand Master status.


Wikipedia on MPPT

They say:

  • Maximum power point tracking (MPPT) is a technique that grid tie inverters, solar battery chargers and similar devices use to get the maximum possible power from one or more solar panels.[1] Solar cells have a complex relationship between solar irradiation, temperature and total resistance that produces a non-linear output efficiency known as the I-V curve. It is the purpose of the MPPT system to sample the output of the cells and apply the proper resistance (load) to obtain maximum power for any given environmental conditions.[2]

MPPT 2 page introduction useful.

Similar + product info similar


MPPT is useful with many energy sources.
An excellent way of thinking of it is as being an electronic gearbox that takes current and voltage at inout and outputs a different voltage and corresponding current such that

  • Vout x Iout = Vin x Iin x K

where K is the efficincy of concersion.
For a given set of operating conditions MPPT adjusts the effective load resistance and thus the voltage and current such that maximum power is being obtained and adjusts output voltage or current to suit the target output device.
An example would be an industry standard nominally 12V crystalline silicon PV panel (= photovoltaic = solar panel) charging a 12V lead acid battery. A stanradd panel has 36 cells and an output voltage in full sun of 18V or more. At peak power point (the MPP that MPPT tracks) the voltage will be ABOUT 15V but this varies wit cell efficincy, insolation (sunshine level), age of PV panel, cleanliness of glass, atmospheric conditions and more. The bttery may be optimally charged at a voltage of anything from about 10V (rather dad battery) through 14V+ (certain specialist modes). The MPPT controller matches these different voltage and current levels. If the battery was best charged at 12V and the PV panel MPP was at 15V then if the two ar just joined together the efficiency of PV energy use is 12V/15V = 80%. The 20% extra is lost. This does not mean that the battery necessarily uses the optimum energy as well as it should but the problem then changes from a PV panel loading one to a battery chemistry one.


Long cycle life, good temperature range, superb current charging efficiency, very good to excellent energy charging efficiency, relatively robust. relatively flat voltage range, excellent high temperature performance acceptable to good low temperature performance, lowest whole of life cycle-cost of any battery.

This applies t

  • \$\begingroup\$ Thanks again Russell. So your thought is that all of this kinetic energy is converted to electricity and stored in LiFePO4 batteries--what else would be needed in that process between the source(s) and battery array? Any kind of transformers, inverters, etc? \$\endgroup\$ Apr 22, 2012 at 7:32
  • \$\begingroup\$ As noted " ... Various electrical inputs via MPPT (maximum power point tracking) converter ..." . I'll add to answer at end. \$\endgroup\$
    – Russell McMahon
    Apr 22, 2012 at 8:34

I would have thought that your best sources of energy would be 1. rushing water, 2. wind, 3. the sun. Any thing else such as temperature differences are going to power an LED and that's about it.

Flowing water contains a huge amount of energy which is yours for the taking whenever your kayak is stopped. E.G. if you're holding onto a rock in some rapids, or if you have tied up the boat for the night.

My suggestion would be a LiFePO4 powered brushless motor connected to a propeller which can be raised out of the water to reduce drag when you're paddling. Whenever you stop, dip it in the water, and let it charge up the battery. Whenever you're tired, cruise along on the motor.

Use the whole kayak as a propeller. I don't know a lot of hydrodynamics, but I'm sure I've heard of kayaks being dragged under the water and spun around. I don't know if you could orchestrate such an event easily, but imagine how much energy is available there!

Slap a flexible solar panel on the kayak too for an extra top up.

Attach a computer controlled kite to the boat for huge speed, but only if you're doing this on a large open stretch of water.

Boat Kite

  • \$\begingroup\$ Thanks Rocketmagnet! In my head I had pictured the kayak being used on large lakes or saltwater--but your point of capturing the energy of rushing water is well made. I guess I just wouldn't have thought you would need extra propulsion if you were already going swiftly downriver. However, your concept could still make good use of tide changes. \$\endgroup\$ Apr 22, 2012 at 7:35
  • \$\begingroup\$ Also, do you have a link or any extra info about the source of this image? \$\endgroup\$ Apr 22, 2012 at 7:36
  • \$\begingroup\$ @MatthewPatrickCashatt - The image was from Kite Sailing from California to Tahiti. I just found it by using Google Images to search for boat kite. \$\endgroup\$ Apr 22, 2012 at 9:08

Well collecting from the water movement will slow you down - lose energy in the weight of batteries, generator,lose some energy in the storing process and then also lose more energy in the using process from heat.. so your only making it harder on yourself.

Instead of pedaling with all that lead and iron... just paddle. less work.

Storing suns energy is slowing you down from solar panel and battery weight and also losing energy in the storing and using process. Better without that also.

I believe anything you do will be wasting more human energy instead of making it easier on you. Unless you use some kind of fuel like gasoline (which is nothing new).

One thing I know that would be good for this kind of thing that i thought of (if we're talking any vehicle/not boats) would be to use generators for brakes on your car to charge a capacitor that would aid in accelerating back to cruise speed.

So normally you slow down and all your momentum is dissipated as heat in your brake pads... this would use the generation process to slow you down while storing the energy it collects in a capacitor so when you hit the gas the capacitor discharges in to an electric motor to aid in acceleration, saving a ton of gas.

  • \$\begingroup\$ It's not about adding features to save energy, it's about examining where your wasting energy and figuring out a way to use it instead. \$\endgroup\$
    – Emery King
    Apr 21, 2012 at 20:51

It may be a waste of energy to find a way of storing energy for a kayak. There are many more factors to consider than efficiency which the suggested solutions above fail in at least 1 practical sense.

1) cost

2) size

3) weight

4) safety

5) maintenance

6) technical expertise to manage

7) development cost

8) water damage

9) salt damage

10) thermal issues

11) theft or insurance liability

12) other risks

I might consider an electric trolling motor but think money is better spent in making it longer to increase paddling efficiency.

Bye the bye Hydrogen generators use more energy than they create if using conventional methods.

  • \$\begingroup\$ you may want to remove the comment about the prior answers failing to address all your points as people can see for themselves that it's not rue and may downvote accordingly. || He's specifically asking for a powered solution || Hydrogen generators (as opposed to "crackers") ALWAYS use more energy in than out - it's just that some energy sources are "free-er" than others. \$\endgroup\$
    – Russell McMahon
    Apr 22, 2012 at 0:49
  • \$\begingroup\$ I appreciate the comment (this wasn't an answer), but feel the need to point out--layman though I am--that there would be no innovation in this world if we all thought this way. I will never understand folks who hone the skill of finding the reasons they can't do something rather than finding the one overlooked reason that makes it possible. \$\endgroup\$ Apr 22, 2012 at 7:26

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