Tuesday, May 31, 2011

solar water

Despite the on-and-off sun and unseasonably cool temperatures (highs are forecast to be in the 60s all this week), the solar powered irrigation system meets our needs. It's core elements are a 3,000-gallon tank on a thirty-foot tower, a Danish-made Grundfos pump in the ground, and 4 130-watt Kyocera photovoltaic panels on the barn. The system came together last August after about 9 months of planning, design and fabrication (and digging of trenches!).

It works well. The Grundfos pump works on available power and thus pumps from slow to fast depending on the quality of the sunlight. On a typical day it fills the tank in 6 hours. At 30' elevation, the tank provides about 15 pounds of pressure to the drip irrigation system, just exactly what is needed. With all of my irrigation valves turned on, the tank empties in about two hours. With all valves on and the pump on, it empties in about 4 hours. With about half the irrigation system turned on and the pump on, the system is more or less in stasis, with the tank filling as fast as it empties. It has, in other words, plenty of capacity for my current need plus a bit more.

The water source is groundwater. Because we're located very close to the tidal zone of the Sacramento-San Joaquin Delta, the groundwater level is high - about sea level. Our elevation here is 25-30 feet above sea level, so the combined lift from groundwater elevation to the top of the tank is only 55-60 feet; a higher lift would mean lower output. Groundwater is being used unsustainably thorughout the world, with much more being withdrawn annually that is replenished naturally. The San Joaquin Valley is a particular problem spot, with an estimated 60 million acre-feet withdrawn in excess of replenishment since 1960, or about 10 percent of the total holding capacity of the groundwater basin. The result has been land subsidence in some places and higher pumping costs as the water needs to be pulled from lower in the ground. Groundwater withdrawal globally has even had a measured small impact in increasing ocean elevations, since water used in municipal and agricultural applications ultimately ends up in the sea. (Thermal expansion of ocean water and the melting of polar ice - both of which are real and accelerating impacts of global warming - are much more significant contributors to sea level rise.)

Groundwater is a scarce resource and we do our best to use it wisely by irrigating only when needed, by extensively mulching, and by concentrating as much of our production as possible in the winter months, when natural precipitation can meet plant needs.

The solar panels also charge batteries located in the workshop. They in turn run a 12 volt Sundanzer freezer and some local lighting. The system is semi-automatic, in that I need to manually direct power to the pump, but it shuts off automatically when the tank is full.

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