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Open loop versus Closed Loop Design

  
  

closed loopWhen it comes to installing geothermal systems vertical closed loop has many advantages over open loop systems.

Open loop systems:

Require water from one or multiple aquifers be pumped to each unit and then directed back to a return well(s).  Depth could be as shalow as 50 feet and are generally average approx 200ft.  These systems are very effective and are in use through out the country on many succesful installations.

 

 

The advantages to open loop design are limited as compared to closed loop:

  • Generally less expensive to install. 
  • Less area required for installation

The downfall to these systems are as  follows:

  • Cannot guarantee the furture performance of the aquifer.  There is a certan level of risk associated with the systems a far as quality of water, quantity of water available to maintain adequate flow rates.   
  • Must have access to the well for pump and treatment of aquifer. 
  • Frequency of well maintenance cannot be easily predicted.  Therefore maintenance costs cannot be easily projected and can vary. 
  • Damage to the landscape and pavements as a result of trucks accessing the wells.
  • Due to impurities in the aquifer water (hardenss, softness, copper magneisum etc) the heat exchanger form the time the system is brought on line loses heat transfer efficiency. 
  • Any savings associated with open loop design as far as unit COP's will be offset by higher electrical usage from the well pump and increased electrical usage of the machine due to higher refrigerant operating pressures. 

 Closed Loop Systems:

 closed loopRequire  a series of 5" borings drilled into the grod generally 200 to 400 feet deep. High density poylethelyne piping is then installed into the boring. A high efficiency bentonite groute is then pumped into the boring to maintian the shape of the boring and promote even heat exchange. The HDPE piping is run from the geoexchange or source loop system into the building or home. The HDPE piping is charged with a glycol (antifreeze) soluiton  The geo exchange loops move the fluid utilizing a pressurized or non pressurized pumping system. 

The down fall to these systems are as follows:

  • More expensive to install than open loop.

The advantages to installing a closed loop system are many:

  • Loop perfromance can be easily calculated with computer software.
  • Loop design can be predicted at least twenty five years out.
  • There is no need to access the loop field after it is installed.
  • Warranty on loop field generally 25 to 30years.
  • Due to cooler loop design tempratures unit operating pressures are lower yielding lower amperage draw and electrical use.
  • Cost to run closed loop pumps much lower than open loop pumping.
  • Because the loop field is a closed loop the efficiency of the heat exchangers are not compromised and can be predited for the life cycle of the equipment which is generally 25 to 30 years.   

Hybrid Systems:  

What is a Hybrid System:

In our region the heating load is generally greater than the cooling load.  For this reason we must design the geo exchange loop for the worst case which would be the heating BTUs required for the installaiton.  The peak heating BTU's only are required on a design day heating day which is generally less than 2% o fthe heating season. 

If the geo exchange loop can be designed to handle the heating and cooling requirements and made smaller by not sizing to handle the peak load we can reduce the amount of borings required.  This can be accomplished by installing or utilizing an existing boiler to inject the heating BTUs into the geo exchange loop during these design day conditions which again are less than 2% of a normal heating season.  The cost savings in most cases would adjust the price of a closed loop installaiton to be equal or less than an open loop design. 

By installing a Hybrid closed loop system the only real poisitive to installing an open loop over a closed loop design would be eliminated.  There would be no need to have a system installed that the operation and design output of the geo exchange system be left to chance.   

 

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