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Portable Solar Power System for Fence Installers

I while back, I was watching a crew of two people install a fence along a street near my house.  The main power tools they were using were a Skill type circular saw and a couple cordless drills. 

They had a large flat bed truck with a small gasoline generator chugging along to supply the 120vac needed for the saw and to charge the batteries for their cordless drills.  They started their day early in the morning and worked until the late afternoon.  I figured they put in a solid 8 hours of work each day.


I wondered if instead of a gasoline powered generator, could they use a solar panel, a storage battery and an inverter to run their equipment18  It would certainly be quieter and would not belch out any exhaust.  Would this idea work18  Letís run some quick math and find out.
Letís first figure out how much energy the two installers need for their operation.
The rotary saw was used by only one of the crew members.  The saw ran for just 4 seconds for each operation and was used only about once every 2 minutes.

The saw most likely demanded about a kilowatt of power.  The duty cycle of the saw usage was 4 seconds every 120 seconds or about 3.3%.  That means over the course of 8 hours, the saw was in operation for about 16 minutes or 0.27 hours.  Multiply that by 1000 watts and you get 270 watt-hours of energy for a day of work for the saw.

The electric drills were used more often and two of them were in constant operation. The drills were mostly used to secure the fence pickets.   It seemed that the installers were changing out their battery packs every 30 minutes. It looked like their drills used standard 18v 5 lithium ion cell battery packs with a 1 Amp-hour capacity.  Two battery packs were being charged in 30 minutes so the burden on the 120vac power source to charge the two packs would be 4.2v x 5 cells x 2 Amps x 2 packs or 84 watts.  Multiply that by 8 hours and you get 672 watt-hours.

If we take the 270 watt-hours for the saw and 672 watt-hours for the drills, we get a total of 942 watt-hours.  Letís round it off to 1000 watt-hours.
A good size solar panel for this kind of application would be a 250 watt device as shown below.  The panel contains 60 cells and is designed to charge a 24v battery.  The panel would be mounted on a movable platform on the bed of the truck, so it could be quickly pointed due south at a 45 degree angle.  This position should work until the crew moved down the street and changed direction.
The average sunny day has about 6 hours of useable sunlight, so the energy needed by the crew for 8 hours of work, would have to be generated during this shorter time.

250w Solar Panel

250 watts times 6 hours is 1500 watt-hours. That is how much energy would be available for the workers.  That exceeds the 1000 watt-hours needed by the crew with a good 50% margin.  To maintain operation even during overcast periods, some kind of energy storage battery would be needed. The rule of thumb for battery systems is to make the battery energy storage twice as big as needed.  This insures a longer battery life.  So, if 1500 watts is generated by the solar panel, then the battery should be rated at 3000 watt-hours.  If you divide that figure by 24 you get a needed battery rating of 120 Amp-hours.  Perhaps two 12v 150 Amp-hour batteries as shown below would do the job.
The output of the solar panel would be routed through a battery charge controller, then to the pair of 12v lead acid batteries.  The controller insures that the battery is not overcharged.  Finally, a 24v DC to 120vac power inverter would be used to supply power for the saw and for the two 18v lithium battery charge stations, powered by the 120vac.
12v 150 Amp-hr Sealed Lead Acid Battery  
Battery Charge Controller 3KW Inverter
The complete power system is shown below.

In conclusion, it certainly appears that a single 250 watt solar panel could provide all the energy needed for the two fence installers.  They could use their power tools without burning a single drop of gasoline. Perhaps other similar solar power systems could be designed for other portable power needs.  As gasoline prices continue to increase, these renewable energy systems will become more appealing.


August 2011

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