Your Reading List

Home-built solar basics

I recently had the privilege of talking to two groups of producers in Saskatchewan regarding construction of solar water systems. We run a wide variety of systems in our operation, and they are basically built using the principle of being the most cost-effective solution for our problem. Included in these solutions are gas pumps, nose pumps, a straight siphon (this summer’s project) and a variety of solar water solutions.

We started seriously pumping water in 2002 when we were blessed with three-eighths inch of rain over the course of six months. The benefits were so blatantly obvious that we have continued to develop our watering systems. Among the benefits are increased calf gains, lower health costs, better conception rates, enhanced pasture management/animal control, extended dugout life, and vastly improved riparian areas. A combination of curiosity and Scottish heritage kept us from trying commercially produced systems. They are generally very good and while they can come with a sizable price tag you are also buying the expertise of the company and the ability to drag and drop an entire system onto your operation.

Sizing the system

For anyone interested in building or buying a watering system, some basic math skills are essential. The size is determined by the number of cows you need to water. Generally speaking a pair requires 15 gallons per day. I use 20 gallons in my estimates. With this extra slack I avoid the need to figure out the efficiency of panels and batteries and the impact of latitude on available daylight. So, with my system, if I want to water 100 cows, I need a setup that pumps 2,000 gallons per day.

Sizing the pump

If you have a pump that will pump five gallons per minute it will have to run for 400 minutes (2,000 / 5) or six hours and 40 minutes each day to provide enough water for the cow herd. A pump with a capacity of 15 gallons per minute needs to run for 135 minutes or two hours and 15 minutes. Pumps need to be able to accommodate the lift and volume required and most will have that rating readily available.

Sizing the battery

Power for the pump is stored in one of two ways, in a battery bank or a water tank. A water tank that gravity flows stores the power as elevated water. An on-demand type of system stores power in batteries. Battery storage capacity is rated in “Amp Hours.” Using the pump in our previous example that operates at five gallons per minute and draws three amps, we can figure that we need 20 amp hours (62/3 hours x 3 amps) of reserve power to run the system for one day. A perfectly efficient 100-amp hour battery fully charged would run the system with no additional power for five days. It is extremely important to use deep-cycle RV/marine-type batteries as they discharge and recharge more fully without damage than a typical car battery. This is also the type of battery we use on electric fencers.

I like to have quite a large-reserve battery capacity, but I would strongly advocate using water storage. The advantage of water storage is that in the event of a failure (hailstones destroy panels) it is possible to pump the water storage full using alternate means such as a gas pump.

Sizing the panels or wind generator

Lifting 20 gallons of water takes quite a bit of power. Watts are the electrical equivalent of horsepower and generally speaking “Watts move water.” A watt is equivalent to 1 amp x 1 volt. This is useful as it helps us to figure out the sizing of the power generator (either solar or wind). Again we will use our previous example of a pump that needs 20 amp hours per day. Let’s look at an 80-Watt/12-volt panel. Working 100 per cent full out, the panel would generate 6.7 amps (80 / 12). That means in one hour the panel could restore 6.7 amp hours to the battery bank. We need 20, so the panel would have to work for three hours (20/6.7). We also need to account for clouds, rainy days, latitude and day length.

About the author

Sean Mcgrath's recent articles

Comments

explore

Stories from our other publications