Designing a Resilient Energy/Water System for Our Farm 6

We get 20 gallons per minute of water from our 228-foot well, powered by electricity from Central Alabama Electric Cooperative. But we’re becoming convinced that the US electrical grid is overtaxed, under-maintained, and increasingly brittle. What happens when we can no longer depend on our grid power to be there?

We have a standby propane generator. Although we slept through it both times, the standby generator has kicked on twice in the past year during brief power outages. It’s ideal for a “quick” interruption, say an hour or two, or overnight. But what happens if the power interruption continues for three weeks? Or more likely, what happens when the hours of good service are outnumbered by the down hours,

as it is now in Iraq? Will we run a propane generator all day and all night? If you’re a typical citizen of the US, that sounds ludicrously far-fetched. OF COURSE we can depend on having electrical service! This is America! What’s this doomer been smoking? But you pay me (in a manner of speaking) to connect the dots, so I do. I look at the proportion of electricity in Alabama that depends on burning coal for its production (about 65%), and I read that we are much closer to peak coal than most of us realize. Even if we had an unlimited supply of coal, dare we burn it when we know the effect that the burning of coal has on catastrophic climate change? (Parenthetically, the answer, distressingly, appears to be yes). And even if we had an unlimited energy source available at every power plant, maintaining the grid takes trucks, and chain saws, and big mean mulchers to chew up trees, and miles and miles of wire, all creatures of cheap oil. And even the most optimistic among us must accept that our days of cheap oil are waning. What becomes of the reliability of the grid when utility companies can no longer justify inspections, or routine maintenance? Each of us is free to prepare in his or her own way for this uncertain future. Even as Amanda and I work to reach out to our community and our neighborhood and build resilient relationships, another part of our preparation is to envision living comfortably without a reliable electrical grid. Every system upgrade needs to start with a thorough understanding of the system already in place. Here’s what comprises our energy/water flow now:
1. Connected via the grid to Central Alabama Electrical Coop. Service is above ground until it gets to our property and underground on our property.
2. Standby propane generator with sufficient power to run all systems when the grid power is not available. Transfer switch activates it automatically when the grid goes down, and turns it back off again when the grid power returns.
3. Pumping water now for household use and for drip irrigation from our 228-foot well using a 2 hp electric pump (pump #1).
4. Pole barn is equipped with robust gutters that flow through downspouts to the beginning of a rainwater collection system. At present, the rainwater collects in a 6-inch PVC pipe and runs freely down the hill below the pole barn. We hope to attach gutters to the similar-sized lodge so that its rainwater joins with that of the pole barn, which would roughly double the rainwater harvested from each rain event.
5. We are fortunate to have a 1-acre pond that has never run dry or even lost water depth, even in the worst drought on record in 2007. We are just now beginning to design our “dream” energy/water system. Here’s my current thinking. Let me acknowledge that right now all of this is just thinking; we haven’t yet ordered any of these components.

  1. Add a collection tank (the lower tank) down the hill from the pole barn so that it catches the rainwater from the pole barn (and probably later the lodge) after a first flush diversion. The lower tank would be about 10,000 gallons and would be equipped with a “push” pump (pump #2).
  2. Add a storage tank (the upper tank) on what we call the North Knoll, which is the highest point in our Core Campus. My guess is that the upper tank would also be about 10,000 gallons and would also be equipped with a “push” pump (pump #3).
  3. Add a pump (pump #4) and filter in the pond to use for drip irrigation of Veg Hill, the blueberries and greenhouse, and the orchard.
  4. Add photovoltaic panels to the south-facing roof of the pole barn sufficient to drive any two of pumps #2, #3, and #4 and to provide additional power as needed for household use. At 10,000 gallons, the lower tank would accommodate a little more than three inches of rain before it fills up (an inch of rain on our 5,000 sq. ft. pole barn roof produces 3,000 gallons of water), or a little more than 1 1/2 inches if we harvest rainwater from the yet-to-be-constructed lodge. If the lower tank fills and there’s no PV power, we would either turn on the pump in the lower tank with grid power or begin letting the tank overflow. In all likelihood, we would begin pumping with grid power if it’s available, but a hurricane-style thunderstorm would eventually overflow the lower tank, because the pump in the lower tank would use little power and would pump slowly, on the order of 5-6 gallons per minute. The reason we’re content for the pump to work so slowly is that under normal circumstances, rainfall events at Longleaf Breeze are infrequent, and they’re rarely more than two inches of rain in an event. And whenever we get that much rain, there’s generally some daylight intervening during which we could pump some of the water up to the upper tank. And of course once the upper tank is full, there will be no more pumping, and if both the lower tank and the upper tank are full, the water will simply overflow. On days after a heavy rain, we may be pumping from the lower tank to the upper tank from sunrise to sunset. Rarely will we need to hurry. We will draw household water from the upper tank. When power is available, the “push” pump in the upper tank will deliver household water pressure (50 psi+/-). When power is not available, the water will still flow, not by pump pressure but by gravity. The altitude difference between the upper tank and the lodge is 30 feet, and the altitude difference between the upper tank and the pole barn (where Amanda and I live) is 55 feet. I’m no engineer, but I have to think that would give us sufficient pressure even at the lodge, and certainly at the barn, to fill a coffee pot, to take a shower, or to brush our teeth. The flow would be slower than most of us are accustomed to having now, but we would have running water. And yes, it would also provide enough pressure to refill a toilet tank, but by the time the grid is this unreliable I expect to have converted everyone else in our household to the delightful

composting toilet I love so much. I am estimating that each person living in the core campus would use about 40 gal. of water per day. I know this is considerably less than the 80 gal. per day that the average American uses today, but if we have reached a point where the grid is unreliable and we are monitoring our water level daily, I expect we will all be frugal with our water use. Amanda and I have decided to size the core campus to accommodate 10 people including us. So at 10,000 gallons, the upper tank would provide 10,000 / 10 / 40 = a 25 day supply of water. If we have 10,000 gallons in the lower tank, that’s a 50 day water supply. If the supply from both tanks ever dipped below a one-week supply (for 10 people, that would be about 10 x 7 x 40 = 2800 gallons), we would pump from the well (using whatever power we have available – PV if available, then grid power if available, then the standby propane generator if necessary) until the upper tank had at least a seven day supply. We always want to maintain enough water in the upper tank for normal household use! Drip irrigation would come from the pond, also powered whenever possible by PV. If we need to irrigate and the PV is spoken for with pumping household water, we’ll use grid power, or we’ll wait until the load on the PV is lower. Drip irrigation is critical for our growing success, but pushing the watering back by a day or two will rarely cause a problem. I understand that NRCS has funds available to help farmers convert their irrigation from groundwater to surface water. I hope and expect that we would qualify for some financial help with this portion of the project. Once the conversion is complete, I expect we would never again irrigate with groundwater. I haven’t mentioned it yet, but now it’s time to use the “B” word – batteries. Batteries seem to be the weakest link of every off-grid electrical system, so we’re working to minimize their use at Longleaf Breeze. But we don’t stop living at sundown, so I expect we will need a modest battery array to give us enough power when the grid is down to run a couple of ceiling fans, a few lights, and maybe a television and a computer without the need to fire up the propane generator. We will make charging them a priority each day using PV power and if necessary grid power. If their charge level becomes critically low, however, we’ll fire up the generator long enough to recharge them. I would love to have all this governed by a master controller that would monitor tank levels and decide when we need to pump from the lower tank to the upper tank, when we need to recharge the upper tank, etc. Maybe one day. At present I lack both the confidence and the patience to try to connect all the sensors in this way. At first, anyway, we’ll just twiddle the knobs by hand based on human monitoring. One last thought: many of you would quickly realize this system would be inappropriate for you, because you can pump water directly using windmills. And if you can, you would be right. Windmills that pump water (as opposed to windmills that produce electricity) are relatively affordable, simple, and almost maintenance-free. We would have loved to design a system using windmills for pumping. We couldn’t make it work, for two reasons: first, and most important, a windmill needs to be above any obstruction within a 400 foot radius, and it needs to be at least 20 feet taller than any obstruction within a 200 foot radius. We fail on both counts in three of the four places where we would need to position a pump. Second, although a windmill is affordable, four windmills quickly get to be cost-prohibitive. That’s why I’ve taken pains to number the pumps in the system we’re contemplating.

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6 thoughts on “Designing a Resilient Energy/Water System for Our Farm

  • Lee W

    Hi Lee,

    For your storage tank, you can figure gravity will give you approximately 1psi of pressure for every 2.31 feet of height the tank is above the outlet. I’d recommend sizing the supply line as large as is economically feasible to minimize your pressure drop during a flow condition. For instance, a 10,000 gallon tank 55′ above the pole barn would give you roughly 24psi during a no-flow condition. If you’re supplying the pole barn through a 1″ line 100′ long (not sure how far it is between the tank and the barn), you’re going to have about a 5psi pressure drop when you’re flowing water, giving you around 20psi at your fixtures. For comparison, a 3/4″ line the same length gives you about a 8psi pressure drop.

    The way it was explained to me by my engineer friend, the more full the tank is, the better your water pressure will be due to the height of the water column, depending on the design of the tank. A tall, skinny tank will provide more pressure when it’s full than a corresponding short, fat tank, but you’ll experience a more pronounced drop in pressure as the tank empties (since the top of the water level will fall faster).

    The solution here is to elevate the tank to achieve a higher water column, but at the cost of having to provide more pumping power to fill from your lower collection tank to your higher storage tank. In this case, you may end up having to increase the size of the pump, which would cost more up front, but since you’re hopefully getting free solar energy, you won’t have to factor in energy costs to run it.

    I’m interested in the concept, if you’re interested in running some “real” numbers, please let me know and I’d be glad to help with the calculations.

  • Lee

    Thanks Lee,

    I had no idea you had this expertise! I would LOVE to visit with you and show you more what we have in mind. Distance from the upper tank to the lodge is roughly 435 feet. Distance from the upper tank to the barn is roughly 715 feet.

    There’s a slight dip on the way to the lodge. I’m wondering whether it might make sense to use larger pipe (like 3″) for the longer downhill portion and smaller pipe (like 1 1/2″) for the shorter uphill portion.

    And now you have me wondering what 20psi feels like when you turn on a faucet. Would it provide enough pressure, say, for a somewhat satisfactory shower? Or would we need more like 50 psi for that?

    I understand that elevating the tank would increase the pressure, but I’m hoping to avoid that option, simply because it adds so dramatically to the cost. What I’m hoping to do instead is to place the tank directly on a concrete pad. Within reason, I’m willing to live with lower pressure at the outlet to avoid the cost of building a tower.

    I cannot tell you how exciting it is to learn that my friend Lee knows this stuff!

  • Lee W

    Based on my very brief research, it looks like most shower heads are designed to operate at pressures of 30psi and above – that’s where the “low flow” part kicks in.

    Here’s a pretty typical shower head: Click on the text that says “graph only” (I couldn’t link directly to the graph for some reason). Notice how the gpm curve flattens at 2.5 gpm for all pressures above 30 psi. At a supply pressure higher than 30 psi, the little cartridge inside the fixture starts working to cap the flow at 2.5 gpm. Below 30 psi, the supply lines simply can’t supply enough water to reach 2.5 gpm of flow and activate the choke mechanism. Going further down this rabbit-hole, it would stand to reason that at pressures lower than 30 psi (for this particular fixture), there is no value in installing a low-flow cartridge in the shower head!

    That said, it’s a personal call as to whether or not the lower flow is going to provide enough cleaning power to take care of your needs. Based on your sustainability goals, the last the we want to do is have you taking longer showers and ultimately using more water in order to get clean. As will all sustainability decisions, there are certainly tradeoffs involved, and there is an art to making those decisions.

    As for the pipe sizing, I’ll have to get my pressure drop curves out and take a look at it. Off the top of my head, I think 2″ is the practical limit for line size, as that’s the largest size PEX tubing that is manufactured. Not sure about your feelings on PEX vs. copper, but once we get the line size it will be fairly simple to run a cost comparison. Keep in mind, materials prices increase exponentially as pipe size increases.

  • Lee

    That fits in with what I was expecting. I’m going to look for a way to “dial down” the pressure on our pump to that 20 psi range to see what it feels like to take a shower at that pressure. My guess is that when we arrive at that point, a hot water shower of any kind, “droopy” or not, will feel terrific, but we’ll know more if we’re able to experience something like the actual flow.

    I need to remember that our expectation is that we will have “normal” household pressure (50 lb. +/-) during most of the time the shower is operating (because I still believe we’re likely to have electricity available at some point during the day, and I suspect people will be likely to time their shower for the time when the flow will be robust), so there will continue to be utility to having low-flow shower heads.

  • Lee

    I just learned today (thanks, James!) that there is a simple relationship between “head” the difference in altitude between water source and water outlet) and pressure. It’s .43 PSI per foot. So the 55 vertical feet between the upper tank and the barn translates to 55 X .43 or 23.65 PSI at the barn. At the lodge, it’s 30 X .43 or a measly 12.9 PSI. That tells me that in the barn, Amanda and I would probably be okay. Not great, but okay. In the lodge, the pressure would be too low for a shower, but high enough to fill a toilet slowly, and high enough to wash your hands, brush your teeth, or slowly fill a coffeepot. And the higher I can boost that upper tank, the higher the pressure I can deliver. So if I could put the tank up on a 6-foot clay-gravel mound, that would help. If I could put it atop a 15-foot wooden tower, that would be even better (if I am confident the tower is strong enough to support the weight even in a high wind).

    The more I think about this last idea, the wooden or steel tower, the less excited I am about it. We’re talking about a huge tank, so the structure needed for support to withstand the strongest storm expected would be almost breathtaking. Now I’m thinking more about building a gravel mound of 4-8 feet and sitting the cistern atop it. Six feet of additional elevation gives us 2.58 more PSI, and eight feet gives us 3.44 PSI. Nothing to sneeze at.