WorldWide Drilling Resource

25 WorldWide Drilling Resource ® MAY 2013 Environmental Monitoring by Thomas Kwader, Ph.D., P.G. Ground Source Heat Pumps - Well Design Considerations The use of open- and closed-loop geothermal heating and cooling systems is rapidly gaining popularity. There are all types of systems in use. However, in this article, I would like to discuss sys- tems using a well(s) to supply water to a heat exchanger and a well(s) to rein- ject the water back to either the same aquifer or a different one. Lately, I have been involved “after the fact” in many poorly designed sys- tems where the system is not performing adequately. The most common problem is the temperature of the pumping well rising due to the supply and return wells being too close to each other for the volume of water circulated. The cause of this problem is basically that ground- water pumped out creates a low pres- sure area or “cone of depression” and groundwater returned through a well creates a mound of “high pressure”. The area of high pressure will try to move the area of low pressure, or pumping well, if the wells are too close together. The distance depends on many fac- tors, so there is no simple formula to calculate recommended distances. Rock and sediment formations in the earth have a large capacity to absorb heat from the returned water. The greater the distance between the supply and return well, the better. Large commercial sys- tems may require hundreds to over a thousand feet depending on the thick- ness of the water-bearing zones (aqui- fers); permeability of the water-bearing zones; or the “layering” of clays or other low-permeability beds present in the borehole of the supply and return wells. An ideal situation includes pumping and returning water from two distinct hydro- logic zones separated by a relatively aerially extensive clay layer that does not readily allow the exchange of water between the two zones. Another important factor is to deter- mine the natural direction of groundwater flow. Always try to place the supply well up gradient and the return well down gradient of each other. This configura- tion will help the supply well capture the water naturally coming into the area and the return well will dispose of the water naturally leaving the area. The money spent on additional pip- ing to separate the supply and return wells is well worth it in energy cost sav- ings. Tom Tom Kwader may be contacted via e-mail at admin@ worldwidedrillingresource.com The July issue deadlines are: Space Reservation: May 24 th . Ad Copy - Display & Classified: May 31 st .