Standing Column vs. Closed Loop System

Discussion in 'Geothermal Loops' started by CT Geo, Apr 6, 2009.

  1. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Typographical error?

    I can see a 1250 foot horizontal loop equaling a 200' vertical loop, not 125' horizontal.

    Or I am missing something
     
  2. Nexsen

    Nexsen New Member

    Standing column well versus closed loop power use

    In this discussion there seems to be an assumption that it will cost more to operate a standing column well than a closed loop system. I think the assumption comes form a normal well where you lift water from below and discharge it at a much higher level. The work against that "head" is wasted in addition to the pipe friction whereas in a circulating loop you do no work against a head but only have the pipe friction losses.

    A properly designed standing column well is much like a closed loop system in that you do no work against a "head" if you return water to the well. It doesn't matter how deep your pump is. What matters is the difference in the level of the water leaving and returning to the well and that should be zero in a SCW. If you allow "bleed" where some water is discharged during peak loading which will make a SCW even more efficient then you "do work" against the head created by the difference in the level where the discharge occurs and the SCW level. I hope that makes sense.

    The only reason to allow bleed is to lower (or raise) the water temperature during peak loads to gain efficiency or capacity but usually bleeding more than 20% will cost more in pumping losses than you gain. But bleeding can be justified by high peak loading that doesn't occur often.

    For example our church may have an average 30 ton load during the week but for a few hours on Sunday morning it may jump to 50 tons. An SCW sized for the 30 ton average load that's allowed to bleed for a few hours on Sunday to handle the peak may be very cost effective. It would "effectively" be a pump and dump for a few hours a week.

    The water quality issues discussed are a big issue and need to be thoroughly investigated and continuously monitored for changes but operational costs are very similar unless you bleed a SCW which is really operating as a partial pump and dump while bleeding.

    Since a SCW returns the same water to the same well changes in water quality are less likely over time but if you allow bleed that does draw water form the aquifer and increases the chance of a water quality change. Of course adjacent users can deplete water causing things to change in the aquifer so monitoring over time is necessary but my point is that an SCW is less likely to suffer water quality changes over time than a pump and dump well.

    But the really significant fact is initial cost. A 1500 foot SCW can handle a 30 ton load and that would require 6000 feet of vertical drilling for 12,000 feet of HDPE tubing fully grouted. Our church would be looking at around $84,000 for the vertical loop exchanger versus maybe $15,000 to $20,000 for the SCW so we are looking at SCW.

    The other plus I mentioned is the ability to bleed and boost capacity to perhaps 50 tons for a few hours a week. A closed loop can be allowed to rise (or drop) in temperature during paks at a loss of efficiency but it is more limited. That is we cold tolerate 90 degrees water rather than 70 degree when cooling at a loss of efficiency for a few peak hours a week but we can't accept 110 degree water so the % over capacity we could allow is more limited. The SCW can just bring in more colder (or warmer) water from the aquifer and dump the water for those few hours.

    Back to the power consumption issue: There is no reason that a properly designed SCW should have higher pumping costs than a closed loop system with equal flow. In fact, to maintain high enough flow to guarantee turbulent flow in the vertical loops you might actually have higher pipe friction in the closed loop system. That's one problem with slowing down the flow. You save on pumping cost as the power drops as the cube of speed reduction (1/2 speed equals 1/8 power) but the reduced flow may cause laminar flow in the tubes and you lose the heat transfer coefficient. This is a bigger problem when you have several heat pumps using one loop field but somewhat less of a problem with a SCW since the annular ring is usually turbulent over a wide range of flow rates due to the roughness of the rock bore hole.
     
  3. Nexsen

    Nexsen New Member

    That must refer to another post as I did not mention any numbers comparing horizontal and vertical loops but I think it deals with the same issue I was addressing.

    The pumps for horizontal or vertical closed loops (as well as standing column wells when not in bleed mode) do "no work" against a head (except when initially filing the loop). Therefore the pumping power should be the same for the same length and same diameter tubing at the same flow rate. The pipe friction and flow rate and length would determine the pump energy consumption.

    In vertical loops we tend to think we are lifting water against a head but keep in mind on the downside (return) gravity is giving us back the energy we used lifting the water. Once the loop is closed the only work is against friction not gravity. We can lower resistance by increasing pipe size but if we lower it too much and go from turbulent to laminar flow the heat transfer coefficient falls and loop capacity diminishes. We want to guarantee turbulent flow for maximum heat transfer in all these loops whether horizontal or vertical or in the annular ring around the porter shroud in a standing column well.
     

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