WorldWide Drilling Resource

41 WorldWide Drilling Resource ® APRIL 2015 Backup by Peter Tavino Litchfield Geothermal This is not about what the drill helper says to the rig driver, who is backing the truck onto the borehole stake. Instead, it's about backing up a geothermal heating system with auxiliary or emergency heat, if the system cannot keep up with the heating capacity requested. Even for sys- tems designed to supply 100% of heating needs, backup is incorporated for cold wind chill days and prolonged days of cold, when ground heat borehole climate design capacities are overloaded. (As engineer Scotty sort of said in Star Trek : "She's burstin’ apart at the seams Cap'n!") Loops don't burst with antifreeze mixtures, but monitoring data show Entering Water Temperatures drop off as the winter goes on, and heat pumps will lock out to protect the coaxial heat exchangers, if threatened with freezing loop water that can damage the thin corrugated copper inner pipe. In my opinion, a building owner could probably add 50% more borehole to en- sure 99% instead of probably 95%of heat- ing typical northern geothermal systems provide. But it would not be cost effective. In Connecticut, rebates were only for sys- tems supplying >95% of design load. In Massachusetts’ current program, rebated geo heat pumps must provide between 80% and 120% of design load. There are many terms for backup heat, but there is no backup cooling un- less it is a separate window unit. Auxiliary or Supplemental or Emergency Heat dis- cussion receives minimal attention in the IGSHPA (International Ground Source Heat Pump Association) manual. Most commonly, it is a duct or electric strip heater (operating at COP [Coefficient of Performance] of only 1.0 compared to COP = 4.0 for the geothermal heat pump) that acts as a third stage, if a two-stage compressor cannot satisfy the thermostat request. Activation is through the 24-volt wiring from the upstairs thermostat to the heat pump. Air source heat pump con- densers work the same way. Within the heat pump, the electric strip engages, usually while the compressor still runs. But multizone geothermal control from thermostats sometimes requires using the control panel instead. Turning the geother- mal compressor off to rest the ground loop and allow it to absorb more subsurface heat is another option. More complicated, is backing up to an existing fuel oil or propane furnace or boil- er remaining in a retrofit. The heating/cool- ing installer decides whether it should be activated through the control panel de- pending on outside air temperature, low loop Entering Water Temperature, or on excessive run time. And this third stage should be automatic, and not for the home- owner to turn a switch, other than the ther- mostat when the house is cold. The same applies for homeowners returning from winter vacation to their house which has been set to 50ºF (10ºC), and they want it at 70ºF (21ºC) immediately. For water-to- water geothermal heat pumps, a 40-watt crank case heater, for example, must also be properly wire controlled. So where does this leave the well drilling contractor and designer figuring the best borehole depth(s)? We do not undersize boreholes by 50% to win the " ; # 511-,(2 0) 20'5&43 %9 4+(3( ,/( $/5)$&452(23 ; 08 029'0/ # ! : : $8 $,- 3$-(3 *,6(/3,/4(2/$4,0/$- &0. "(% ,4( 777 *,6(/3,/4(2/$4,0/$- &0. Tavino cont’d on page 44.