Really busy recently taking on some extra work to help finance the desperately needed replacement battery bank. I was thinking about other ways to store our power and had an idea on how we might be able to setup a mechanical system to store energy by lifting a large weight. Might this complement the batteries substantially? So, I decided to run the numbers on the physics involved to see.
Stored Energy in Lifted Block
This system would convert electrical energy into stored potential energy by lifting a large weight off the ground. Whenever we had excess power from windy stretches the extra power could be diverted to this system. The implementation would involve a small/medium size DC electric motor that would drive a reduction gear box, that in turn would rotate a spool of cable. The cable would be attached to a pulley on a sturdy large steel frame that could support a large weight or block fastened to it.
When excess power is available, or we just want to “charge up” all our energy systems, we could apply the DC voltage to run the motor. With the gearbox reduction and increase in torque, the cable would tighten and lift our block slowly. Power is removed by a mechanical reed switch or something when the block reaches the top of our system.
To retrieve this extra stored power, we simply unlock the hanging block and it slowly descends spinning the gearbox and DC motor which is now producing a current that we can charge deep cycle batteries with, or run the inverter for AC power.
Test the concept
At this point, I’ll ignore many of the real world details like speed/voltage regulation when we extract power. I just want to see if this could be useful to compliment my battery purchase. I won’t consider heat loss and other mechanical inefficiencies yet, I just want to know if there is enough energy stored to start with.
A block weighing X kilograms lifted to a high of h meters will represent the stored energy in joules, which is also watt-seconds. 9.8 is the acceleration constant of gravity on earth.factor is the gravitational constant for earth.
Energy in joules = (weight kg)(9.8)(h meters)
Joules/3600 = watt-hours
Ok, lets test this out for a big system that would store a lot of power. For the math, we’ll select a big concrete block that weighs 10,000 kg, or roughly 11 Tons. We’ll lift it really high, say 10M, or about 33 feet. The lossless raw energy stored in this system would be:
10,000kg * 9.8 * 10m / 3600 s = 272 watt-hours, or 22 amp-hours at 12V
Ker Flop!! I guess this giant storage Idea would not yield any results worth pursuing any further! However, when you think about it this way, it’s really amazing how much power can be packed in a deep cycle lead acid battery.
For example this result is less than the 30AH we use from a marine deep cycle battery to run a little trolling motor! Can you imagine? Hooking up something the size of a car battery, and lifting 11 tons 33 feet in the air and only using half the charge??
wait, it can be done with water
Storing your power in the form of potential energy is nothing new, this will work with water if you property has the right layout and terrain. Lets take this system, remove the gearbox, the cable, and the weight. Now we put an impeller on the motor shaft, or drive a pump. Now lets say we have a storage pond that is 33 feet above the pump. Remember water is heavy, about 8 pounds per gallon.
Lets take a small pond, say its 50 x 50 feet and 5 feet deep. ( I know, no one has a perfect box pond, but this helps the math.)
1 cubic foot = 7.48 gallons, and 1 gallon water = 8.35 pounds
pond volume = 50 x 50x 5 = 12,500 cu ft.
pond water weight = 2304* 8.32 = 104,000 pounds, or 47,270 kg
47,270kg * 9.8 * 10m / 3600s = 1287 watt-hours, or 107 amp-hours at 12V
Wow, is amazing what you find running the numbers! You really have to store a good deal of water for this to work. This is how hydro projects are created. Still probably not a practical off grid power storage technique, but if you have year round running water on your property, always consider micro-hydro power.
Did you know that if your pond was 50 x 50 feet and 50 feet deep that you could store 28,243 AH at 12V on this system?
What other ideas do you have for storing power? Do you know of practical systems that are more efficient that current battery technology? Please tell us about your experience in the comments below!!