Short and fast versus long and slow pumping?

peterthepainterpeterthepainter Registered Users Posts: 3
We have inherited a system which pumps water from a deep well using a powerful pump (sorry no details available at present). We have a huge tank to store the pumped water ready for use. The idea, according to previous owner, was to switch the pump on 3-4 times a day for a few minutes, whereupon the available water in the well would run dry and we have to wait for it to slowly replenish itself. The powerful pump, it would seem, is putting excessive strain on our limited solar electrics too.
This seems all wrong to me. Does it not make more sense to pump very slowly (using minimal power) for a long period? perhaps constantly during the day? Hopefully the well would never dry using this method.
Any thoughts on why one system is better than the other appreciated. Thanks.

Comments

  • mike95490mike95490 Solar Expert Posts: 8,030 ✭✭✭✭
    This would be a job for Grundfos SQL Flex pumps
     They run off solar panels, and will fill the tank in daylight, but not in cloudy or dark weather.     it's not good for a well or the pump, to suck it dry and then wait for recharge, a slower withdraw rate has always seemed better, but I don't sell big well pumps either !

    SQL Flex pumps are expensive, with their control box and some PV panels, but you don't need a big honking inverter and battery bank for them.


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  • jonrjonr Solar Expert Posts: 1,135 ✭✭✭✭
    Would be far less expensive to use a 24VDC submersible well pump.  Or possibly a lower power version of your current pump.
  • MichaelKMichaelK Registered Users Posts: 79 ✭✭
    A neighbor up the road from us has exactly the same problem.  What's happening is referred to as the well's "refresh rate".  That is, the rate in gallons per minute that water enters the bore hole.  So, it you have a well bore with a refresh rate of say 2.5 gallons per minute, and a pump that removes water at say 5.0 gallons per minute, the well will run dry.  At least for a while

    The big problem with this is a lot of submersable pumps are damaged running dry.  They are typically wired with a dry "shut off" switch that's positioned directly above the well, so when the water level drops down to the pump it will shut off. That is of course till the switch malfunctions and does not switch off in time to keep the pump from being destroyed.

    Does the well driller have a logo/placard on your water tank?  The original driller (or maybe your county records office) will have data concerning the bore's refresh rate.  That info will help you rationally decide what size pump is "supposed" be to there.  My guess would be somebody down the road replaced a worn out pump with a bigger model, not taking into account the low refresh rate.
    15 Renogy 300w panels,  Midnight 200 CC, 8 Trojan L16 batteries, Schneider XW6848 NA inverter, AC-Delco 6000w gen.
  • peakbaggerpeakbagger Solar Expert Posts: 341 ✭✭✭

    It makes a big difference on the type of pump down in the hole. A centrifugal pump needs to run at minimum speed in order to develop enough pressure to get the water up and out of the well and over to the tank. These pumps are normally fixed speed and they are sized for high flow. Pumps are frequently sized to supply a flow in excess of the well recharge rate. The reason for this is that frequently the well has to be drilled down deep enough to get to a seam in n the bedrock that is actively flowing, once this seam is hit, the water will fill the hole to a much higher level then the depth where water was encountered. As an example my well was "dry" until around 300 feet of depth, once it sat for awhile the water level rose to about 40 feet deep. The hole was drilled to total of 330 feet and the pump was set around 320 feet. The well recharge rate is low around 1.5 GPM but the pump is sized for 6 GPM. The sizing was done taking into account that the pump would not run full time and that there was a 8" well casing full of water up to 40 feet of head to cover the times when the pump does run. The other thing to consider is a centrifugal pump doesn't put out one set flow rate. We need to go to pump 101 class and review that the required pump pressure is related to the difference between the level of water in the hole versus the level of the tank inlet plus any frictional losses in the piping.  The pump trades off pressure to make flow. As the hole full of water gets drawn down, the elevation difference between the water in the hole and tank increases so the required pressure the pump needs to generate goes up so more power is used for pressure and less for flow so the flow goes down. Note pump 101 adds pressure loss for friction, the faster the water is moving in the pipe, the more pressure required. Ideally you want you want to run a low flow to reduce friction loss and that also benefits you in that the lower flow means the elevation in the well doesn't drop as much as the flow going out is approaching the recharge rate. Ideally you would want to adjust the flow so that the flow out to the tank is always matching or less than the recharge rate. This can be done but you need a variable speed drive pump. Variable speed pumps are getting popular for new installations, they allow the installer to not require a pressure tank in the basement.

    The alternative approach is switch to a positive displacement pump. It moves a certain volume of water every revolution of the motor. Run the motor faster and it moves more water. Pressure really doesn't factor in as much on the pressure delivered until the friction loss becomes noticeable. Pressure does factor in for sizing the pump motor, the higher the pressure the more power required. Typically these pumps are supplied by a variable speed drive. They are much more efficient than a centrifugal pump, but compared to a fixed speed centrifugal pump they cost more. I think its bit of a wash once you compare a centrifugal pump with a variable speed drive to a positive displacement pump.

    In your case a positive displacement pump is the way to go. You want it to run slow and preferably stay below the recharge rate of the well as well as reduce friction loss. Ideally you fill the tank during the day when you have surplus solar generation and then run off the tank overnight. Controlling it could be fancy to optimize efficiency or simple. The fancy way is put in PID loop controlling the pump speed to maintain a high level in the tank with level transmitter on the tank. This would be enabled by a contact from your PV system when its in float mode. If the PV isn't in float then the system would control to a lower setpoint on the tank. The simpler approach is replace the level transmitter with four level switches two high up on the tank and two down low. If the PV is in float it controls to the high level and if not it controls to the low level. I expect you would put a timer in the high level control circuit so that momentary losses of PV would not switch it off but extended periods would shift to low level control.

    Good pump design requires good understanding of the system and desired operation. A typical pump service tech is not going to take the time to look into the details and is going to put in something that is good enough that they have on the truck or back at the shop. They are used to grid connected installations where power is unlimited to their solution is probably not the best for someone that is off grid and power limited. The good thing is you have the large tank.


       

  • cow_ranchercow_rancher Solar Expert Posts: 109 ✭✭✭
    There are several ways to correctly use a low flow well, manually turning the pump on and off is not one of them.

    One way is to use a windmill pump.

    The best way is that there are several devices that will detect when the pump runs out of water, they measure the current, and when the current flow goes down, which it will because the pump is not doing as much work as it is rated for then the  current will drop below a pre-settable level they turn off for another pre-determined amount of time to allow the well to recharge.  One of those devices is a Pump-Tec by Franklin.

    Another way is to use a valve to throttle back the output of the pump so it never over pumps the well, and will not always stop the over pumping, because you can drain the aquifer if it is severely limited in volume. 

    When the pump runs out of water it will air lock and must be turned off and restarted.

    Pumps, Wells and Tanks questions can be answered at http://www.pumpsandtanks.com/ .   Admin please delete the link if not allowed here.

    Rancher


  • southforksouthfork Registered Users Posts: 28 ✭✭
    I use the Pump-Tec by Franklin. with a 10 GPM pump 220V 1HP in a  2 - 8 GPM well depending on what time of the year with 2500 gal storage works great. Grid tied but I only use for watering during the day so solar takes care of my power needs. My second well is 18-20 GPM and uses same basic system with 2500 gal storage for domestic use.
     24 solar world 285 watt panels with 24 Enphase 250 s ground mount grid tied .                                                                
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