Friday, January 20, 2006
Solar mower?
A potential way to mitigate the impact of a rechargeable, electric mower is to use solar photovoltaic panels to recharge it. In some ways, solar seems a natural choice. Grass growth may be roughly proportional to sunshine. In the winter, only a small trickle charge is required to maintain battery life. (See a previous post about a rechargeable mower).
Some companies have periodically marketed either complete mowers or add on systems for solar charging, but I have not found a currently available one, so I will analyze a hypothetical home built one.
Power from grid:
2 mowings/month * 400 W *0.5 hour/mowing * 1 kw/1000W * 31 sq.yd.-month/kw-hr = 12.4 sq.yd
Trickle charge of battery:
5 W * 24 hour * 31 day/month * 1 kw/1000W * 31 sq.yd.-month/kw-hr = 115 sq.yd
Total: 127 sq.yd
Cost: $0.29/month (@$0.08/kw-hr)
Power from panels:
To recharge once a week in summer, about 10 W nominal power is the minimum required, assuming 5 “equivalent sun” hours per day. This allows no margin for non-optimal location or other factors. (20 W would be much more comfortable). Small panels are relatively expensive on a $/W basis. About $120 was the cheapest 10 W panel I have found available on a consistent basis (there are occasionally sale or used panels cheaper).
A charge controller is required. I have not found a small, affordable charge controller that can take 12 VDC nominal input and charge a 24 VDC battery pack. One could add wiring and switching to charge the two, 12 VDC batteries in parallel, and then use them in series, but this requires a multi-pole, fairly high current rated DC switch. This is a technical challenge that I haven’t quite worked out. Alternatively, one could have two, 5 W, 12 VDC panels in series to supply 24 VDC, but those panels are even more expensive than one, 10 W panel.
Footprint: 10 W * 5 hr/day * 31 day/month * 1kW/1000W * 0.3 sq.yd-month/kW-hr = 0.47 sq.yd.
(footprint value for “PV power)
Ecological benefit: 127 -0.47 = 126 sq.yd (a clear winner for “footprint”)
Alternative analysis: 4 lb of “electronics” with a 20 year life
4 lb/240month * 1325 sq.yd-month/lb = 22 sq.yd. (still much less than 127 sq.yd.)
Cost: $120 (panel) + $30 (charger) = $150 (not all costs are captured)
Economic Payback: $150/($0.29/month) = 517 month = 43 years
This is an example of the economic costs not being well aligned with the ecological benefit. A clear benefit for ecological footprint (by either analysis method) has a long economic payback (at least 43 years).
copyright 2006 by Milliwatt
Some companies have periodically marketed either complete mowers or add on systems for solar charging, but I have not found a currently available one, so I will analyze a hypothetical home built one.
Power from grid:
2 mowings/month * 400 W *0.5 hour/mowing * 1 kw/1000W * 31 sq.yd.-month/kw-hr = 12.4 sq.yd
Trickle charge of battery:
5 W * 24 hour * 31 day/month * 1 kw/1000W * 31 sq.yd.-month/kw-hr = 115 sq.yd
Total: 127 sq.yd
Cost: $0.29/month (@$0.08/kw-hr)
Power from panels:
To recharge once a week in summer, about 10 W nominal power is the minimum required, assuming 5 “equivalent sun” hours per day. This allows no margin for non-optimal location or other factors. (20 W would be much more comfortable). Small panels are relatively expensive on a $/W basis. About $120 was the cheapest 10 W panel I have found available on a consistent basis (there are occasionally sale or used panels cheaper).
A charge controller is required. I have not found a small, affordable charge controller that can take 12 VDC nominal input and charge a 24 VDC battery pack. One could add wiring and switching to charge the two, 12 VDC batteries in parallel, and then use them in series, but this requires a multi-pole, fairly high current rated DC switch. This is a technical challenge that I haven’t quite worked out. Alternatively, one could have two, 5 W, 12 VDC panels in series to supply 24 VDC, but those panels are even more expensive than one, 10 W panel.
Footprint: 10 W * 5 hr/day * 31 day/month * 1kW/1000W * 0.3 sq.yd-month/kW-hr = 0.47 sq.yd.
(footprint value for “PV power)
Ecological benefit: 127 -0.47 = 126 sq.yd (a clear winner for “footprint”)
Alternative analysis: 4 lb of “electronics” with a 20 year life
4 lb/240month * 1325 sq.yd-month/lb = 22 sq.yd. (still much less than 127 sq.yd.)
Cost: $120 (panel) + $30 (charger) = $150 (not all costs are captured)
Economic Payback: $150/($0.29/month) = 517 month = 43 years
This is an example of the economic costs not being well aligned with the ecological benefit. A clear benefit for ecological footprint (by either analysis method) has a long economic payback (at least 43 years).
copyright 2006 by Milliwatt
# posted by Theepdinker @ 6:38 PM 
