Milliwatt: Cheap choices about energy

My 12V rechargeable string trimmer died (burnt out motor windings), but it still had a functioning battery and charger controller. So, those got recycled into a solar light for our horse arena. The battery and charge controller are housed in the "bird house". A 5W panel from the prior Solar mower project was reused. I ordered a 12W, 12VDC light and timer switch. The light certainly doesn't light up the whole arena, but gives just enough light to take the tack off of the horse after a ride at dusk, and to hook up the harrow to the tractor.

The timer lasted about 6 months. It still works as a switch, but now the timer doesn't work.

Well, I took a brief hiatus from blogging.

A brief accounting:

Solar lawn mower:
Still have the mower, but we have moved to a much larger yard, so it only gets used for trimming odd areas, such as under the trampoline (you can fold the handle and crawl underneath). The charge controller failed after 3 years of use. The solar panels were re-deployed to a fence charger and a lighting system (more on those to come).

Rain barrel:
The rain barrel was hard to use, as it got a lot of sediment from the roof, and had to be flushed out semi-annually. Also, it didn't help a lot when we needed it the most: our "Mediterranean" summers with little rain. It could sit empty for weeks, waiting for rain to fill it. And rain collection is supposedly illegal [!] in our state. It has been re-deployed as a horse training obstacle.

Solar light:
The lantern worked for about four years. An internal trace on the lanterns charge control board failed, and it was beyond my soldering skills to fix. The battery was shot, so the lantern was replaced. The same panel is hooked into the new lantern in a new location, and it serves well for a few minutes use each night.

Lessons Learned:
- solar panels, even cheap ones, are generally durable (at least for 5 years)
- Cheap charge controllers are not so durable, and may ruin batteries (due to no charging) when they fail. I will buy better quality ones from now on.

I finally decided to recharge my electric lawn mower with solar power (despite the long payback, as discussed in a previous post).

I bought two, 12 V, 5 W, amorphous cell panels, and a 12 V/24 V charge controller (the mower is 24 V). This will provide plenty of trickle charge to keep the batteries from degrading over winter, and enough power to recharge within a week in the summer (even with the mower and panels parked in a marginally sunny spot). The panels and charge controller have a quick disconnect so that all that stuff doesn’t need to be hauled around while mowing (and potentially damaged). It also freed up about 6 sq.ft. in the garage (should I count that in the ecological benefit?).

An “ecological footprint” is an estimate of how much land would be required to sustain some use. You may find calculators at sites such as http://www.redefiningprogress.org/ . I use the footprint values from “Radical Simplicity” by Jim Merkel. He notes that there is a balance between simplicity of use and accuracy to the method, so any calculations are of course estimates.

By one calculation, the Earth provides 4.7 acres of productive land per person. The typical American may use 20 acres (!). By some estimates, world wide consumption equals three or four entire earths, which is obviously not sustainable.

The energy estimates are of particular interest to me. The “footprint” of consuming, for example, a gallon of gas per month accounts for the energy needed extract and deliver the gas, as well as enough forest and ocean to absorb the carbon dioxide The footprint of consuming one gallon of gasoline a month is 500 square yards, or about one-one hundredth of an acre. The footprint of operating a car is greater than that of just burning the gasoline, as the metal and other materials in the car had an impact from mining, manufacturing, etc.

This method offers a tool other than economics to make choices about how we use energy. The “ecological payback” is often different than “economic payback”.

For small choices, it is more convenient to express the footprint in sqare yards (abbreviated sq.yd.) than in acres. There are 4840 sq.yd. in one acre.

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

One of the things that I enjoyed, that may not have a positive benefit for the environment, is my electric mower. The first mower I had was a gas one. Fortunately, I hit a big rock and bent the crank shaft irreparably. About that time our county had a rebate program that gave you credit for an old gas mower to get a non-gas powered one. I got a manual push mower. Aah, silent mowing with just a little “thwip, thwip, thwilp” sound of the reel on the cutter. The best part was being able to mow barefoot on warm summer days. But it was didn’t do well in my lumpy, branch-strewn yard, or if the grass got too tall. I got tired of either raking the yard before each mowing, or getting an unexpected Heimlich maneuver every time the mower got jammed with a branch or Douglas Fir cone. This one was eventually given to an interested carpooler.

So, the next time the county offered a rebate, I got a rechargeable electric mower. While it is a bit heavy, it doesn’t stop dead every time it runs into a pitiful branch. It is quiet, and has no fumes (my son has asthma, so removing every possible irritant is important to me). No direct pollution. There is some pollution involved in generating the electricity, but not near as much as a small, relatively inefficient, gas engine produces.

One just has to remember to recharge it. Lead-acid batteries do not recover well if left for long in a discharged state. This is how the first set of batteries met an early demise. A friend mowed our lawn while we were on vacation, and did not hook it up to the charger. Two weeks later we returned, and I finally plugged it in. It did recharge, but it no longer mowed the entire lawn on one charge. Over the next few months it degraded further, and stopped working at all at about three years age. It was easy to install a replacement, but it made me think about the environmental cost.

The battery shop took the old battery for recycling. The Battery Council International says that “more than 93% of the lead” from a battery is recycled (http://www.batterycouncil.org/news-041902.html). The EPA estimates that "Nearly 90 percent of all lead-acid batteries are recycled" (http://www.epa.gov/epaoswer/non-hw/muncpl/battery.htm). That sounds pretty good. But what happens to the other 7% to 10% of the lead?

Ecological footprint analysis:

(footprint values from “Radical Simplicity” by Jim Merkel)

Push mower: 20 lb (mostly metal), 20 year life
20 lb / 240 month * 397 sq.yd.-month/lb = 33 sq.yd

Gas Mower: 30 lb, 10 year life
30 lb / 120 month * 397 sq.yd.-month/lb = 99 sq.yd
2 pint gasoline/month (average over year) = 0.25 gal/month
0.25 gal/month * 500 sq.yd.-month/gallon = 125 sq.yd.
(I am not sure that this includes all of the pollution effects of a small, inefficient engine)
Total: 224 sq.yd

Electric, rechargeable mower: 30 lb, 10 year life frame
30 lb / 120 month * 397 sq.yd.-month/lb = 99 sq.yd
Battery: 30 lb, 3 year life
30 lb / 36 month * 335 sq.yd.-month/lb (recycle metal) = 279 sq.yd
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: 506 sq.yd
(I am not sure how accurate my estimate of the footprint of the battery is, but by this estimate it is larger than the entire footprint of the gas mower!)

The most environmentally sound solution would probably be to not have grass to mow. We are slowly headed in that direction. Our garden takes up the small, prime sunny spot. The back yard is already partly wild, and laziness and neglect may soon send it the rest of the way.

copyright 2006 by Milliwatt

I bought a rain barrel for the front yard. It was from a seasonally existing garden store that mostly featured rain barrels- dozens of them. My 60 gallon barrel had "cut carrots" written on the side with grease pen. The store added two spigots, one low and one high on the barrel. I cut a hole in the screw lid that the downspout fits into.

Our city allows rain barrels without a building permit, so long as they are on the ground, height to width ratio less than 2, and under 4000 gallons (!). (Now that is saving for a non-rainy day!)

I got it too late in the season to do much useful watering with. So far, I have used it mostly for rinsing out the compost bucket. The kids will have fun with it this summer, sporadically over-watering the garden.

How long will it take to save enough domestic water to off-set the ecological footprint of a plastic rain barrel?

Ecological footprint analysis:

(footprint values from “Radical Simplicity” by Jim Merkel)

20 pound, plastic barrel, used previously as food packaging

Estimated benefit: 400 gallons/year, or 33 gallons per month (about 10 barrels full of useful garden watering per year, water that would have come from a domestic water supply otherwise)

Water cost: $0.0031/gallon (water in + sewer out [which one has to pay whether it went down the sewer or not])

Economic benefit: $0.105 per month

Footprint benefit: $0.105 * 157 sq.yd/$ = 16.4 sq.yd
(alleviates the need for that much resource to provide domestic water)

Economic cost: $50

Economic pay back: $50/($0.105/month) = 476 months, about 40 years (!)

Footprint “payback”: 20 lb plastic * 331 sq.yd/lb / 16.4 sq.yd = 404 months, about 33 and a half years (roughly equal to the economic payback, so the economics are roughly in line with the footprint)

The footprint “payback” could be less if the barrel is credited as used (about half the footprint, so payback in half the time)

Intangibles: Eyesore? Status symbol (acquired for the “wrong” reasons)? Educational/fun for kids? A measure of independence from the grid? Emergency backup water supply?

Like many of the things that I have considered to reduce impact, this has a long payback time (however, not quite to the seventh generation), which some may consider marginal, or negative. The only measure sure to have a smaller footprint is to use less.

copyright 2006 by Milliwatt

I have the lamest solar power system that I know of.

I have a 6W nominal solar panel hooked up to a 12V rechargeable latern. This provides me with about 6 minutes of light twice a week in perpituity while I do the compost. It is also a backup light source for if the power fails. The sealed, lead-acid battery in the lantern is continuously trickle charged during the day (even in summer, when I do not need the light), so the battery may last a long time.

Is this "sustainable"? Is this a simple, low impact choice?

I bought the panel and the lantern with a gift certificate that a relative gave me from an online retailer, so it didn't use any cash (except for an automotive 12V outlet I needed for a neat installation). I was pleasantly surprised to see these types of things at the retailer. With another certificate, I bought a watt-hour meter to audit how much power various appliances use. Perhaps I will post more on that later.


copyright 2006 by Milliwatt