Assessing the Thermal Impacts of an Open-Loop Ground Source Heat Pump. Vicky L Freedman Scott R Waichler Rob D Mackley Jake A Horner

Transcription

1 Assessing the Thermal Impacts of an Open-Loop Ground Source Heat Pump Vicky L Freedman Scott R Waichler Rob D Mackley Jake A Horner

2 Open Loop GSHP Ground source heating and cooling system has been installed at the BSF/CSF facility on the Battelle Campus at PNNL Located in close proximity to the Hanford Site and Columbia River Heat rejection will occur yearround 2

3 Potential Thermal Impacts Create thermal plume to Columbia River Cause thermal drift at extraction wells which lowers the system thermal efficiency Balance between thermal efficiencies and thermal environmental impacts may need to be obtained 3

4 Objectives Determine potential thermal impacts to the Columbia River Examine shoreline temperatures before mixing occurs in the hyporheic zone Compare groundwater thermal discharges to river flows Determine potential thermal impacts at extraction wells Examine thermal trends under different heat rejection scenarios Identify operational scenarios that minimize thermal drift at extraction wells and thermal impacts to the Columbia River Can operate system by rejecting heat to larger volumes of water Steeper cones of depression/impression Lower injection temperatures Can operate system by rejecting heat to smaller volumes of water Shallower cones of depression/impression Higher injection temperatures 4

5 Numerical Modeling Approach Steady-state flow Field tests used to obtain site specific geology and hydraulic conductivity estimates (e.g., constant rate pumping test, slug tests) Range of hydraulic conductivity estimates for Ringold ( m/d) Multiphase flow simulator STOMP (White and Oostrom 2006, 2000) Saturated and unsaturated flow under non-isothermal conditions Water Air-Energy Two different heat rejection scenarios Average 3 X Average = Peak operational scenarios Assumed pumping occurred 24/7 year-round over period of 20 years 5

6 Conceptual Model Unconfined aquifer system is located in the Hanford and Ringold formations Groundwater generally flows east to northeast from the Yakima River to the Columbia River 6

7 Hanford Formation Unconsolidated sands and gravels Typically the most transmissive, upper zone of the unconfined aquifer Limited saturated thickness at BSF/CSF Field hydraulic conductivity estimates near shoreline: ,000 m/d Not as transmissive at BSF/CSF Site 7

8 Ringold Formation Contains sands, gravels, and muds that are typically more consolidated and less permeable than those in the Hanford Contains confining layers Water table is predominantly in the Ringold Formation at BSF/CSF Site Field hydraulic conductivity estimates: m/d 8

9 Potentiometric Surface Water table map, Spring 2007 Regional groundwater flow northeasterly at BSF/CSF site 9

10 River Stage and Groundwater Levels BSF/ CSF River stage controlled by dams upstream Diurnal and seasonal variations Groundwater levels impacted by river stage Dirurnal variations in groundwater levels occur in near-shore wells (100 m from shoreline) 10

11 Simulation Domain North BCs: Aqueous Flux, Fixed Gas, Outflow Energy West BCs: Aqueous Flux Fixed Gas Fixed Energy East BCs: Aqueous Hydrostatic Fixed Gas Fixed Energy Top BCs: Aqueous Neumann (recharge) Fixed Gas Fixed Energy Bottom BCs: No Flow for 3 phases South BCs: Aqueous Flux, Fixed Gas, Fixed Energy 11 Domain: 5050m x 7000m x 44m Variable grid spacing: 50m < Δx, Δy < 150m; Δz = 1.0m 49 x 58 x 44 = 125,048 nodes; 21,150 inactive cells Bottom boundary represents low permeability Ringold Lower Mud; vertical domain extends 3 m above the water table

12 Wells 4 Extraction Wells Upgradient Boundary (1 4) Total pumping rate divided by capacity Well 1-40% Wells 2 & 4 25% Well 3 10% 4 Injection Wells Downgradient boundary (5 6) Total pumping rate evenly divided at each of the wells

13 Observed vs. Simulated Head Ringold K sat 200 ft/day Ringold K sat 500 ft/day Well # Obs in 2006 All Observations (a) (m) 2006 Mean Observed (m) Simulated (m) Mean Absolute Error (m) Mean Error (m) Simulated (m) Mean Absolute Error (m) Mean Error (m) 699-S34-E S34-E S32-E13A S32-E S32-E10D S32-E10B S30-E15A S32-E13B S31-E S31-E10E S31-E10C S31-E10A (a) Mean of observations for all years. 12 wells, avg error 60 m/d: m 150 m/d: m 13

14 Heat Rejection Scenarios Ambient Temperature = 16.4 o C Avg heat rejection (1.58 MBTUs/month) 782 gpm/18.1 o C 196 gpm/23.1 o C Peak heat rejection (4.32 MBTUs/month) 2143 gpm/18.1 o C 782 gpm/23.1 o C Examined two estimates of hydraulic conductivity Examined thermal plumes via temperature plots Extraction wells Shoreline for region impacted by the thermal plume No mixing considered in the hyporheic zone No mixing considered with the Columbia River 14

15 Areal Extent of Thermal Plume Ringold Ksat = 60 m/d Ringold Ksat = 150 m/d 15 Areal extent of thermal plumes similar for both Ksat estimates 60 m/d: more lateral spreading 150 m/d: narrower plume; higher temperatures extend closer to shoreline

16 Impacts at Extraction Wells (Avg) Rate = 196 gpm Temp = 23.1 o C Rate = 782 gpm Temp = 18.1 o C Temperature drift does not occur under average pumping conditions 16

17 Impacts at Extraction Wells (Peak) Rate = 782 gpm Temp = 23.1 o C Rate = 2143 gpm Temp = 18.1 o C More significant impact of temperature drift at higher pumping rates As high as +0.7 o C for higher pumping rate, lower injection temperatures Less than +0.2 o C under lower pumping rates, higher injection temperatures 17

18 Potentiometric Surfaces at Wells Rate = 2143 gpm Temp = 18.1 o C Rate = 782 gpm Temp = 23.1 o C 18 Drawdown cones much steeper for higher pumping rates which draws more of injected water Minimal mounding occurs due to injection into Hanford Ringold Ksat = 60 m/d Hanford Ksat = 2000 m/d

19 Impacts at Shoreline (Avg) Rate = 196 gpm Temp = 23.1 o C Rate = 782 gpm Temp = 18.1 o C Both pumping rates yielded similar shoreline temperature predictions (> o C) 19

20 Impacts at Shoreline (Peak) Rate = 782 gpm Temp = 23.1 o C Rate = 2143 gpm Temp = 18.1 o C More significant impact of shoreline temperatures under lower pumping rates As high as +2.9 o C for lower pumping rate, higher injection temperature ~ +1.0 o C under higher pumping rates, lower injection temperature 20

21 Impact of Hydraulic Conductivity Extraction Wells (Peak) Ksat = 60 m/d Ksat = 150 m/d Ksat estimates yield similar temperature trends at extraction wells Wells 1-4: +0.1 o C o C 21

22 Impact of Hydraulic Conductivity Shoreline (Peak) Ksat = 60 m/d Ksat = 150 m/d 22 Ksat estimates yield similar temperature trends at shoreline Average: +1.6 o C Max: +2.7 o C (60 m/d) ; +3.0 o C (150 m/d)

23 Assessing Impact at the Columbia River Total flux represents model flux discharging to the river 35 > 16.5 o C represents flux at temperatures greater than ambient > 17.0 o C represents flux at temperatures greater than 17.0 o C > 17.5 o C represents flux at temperatures greater than 17.5 o C Daily average flows Columbia River are 40, ,000 cfs Groundwater flux to river is % of flux to Columbia River during low flow Mixing in hyporheic zone will attenuate heated groundwater fluxes Mixing in river will further attenuate temperatures Groundwater Discharge to River (cfs) Total Flux > 16.5 C > 17.0 C > 17.4 C Injection Temperatures 23 C 18 C 23

24 Thermal Impacts at BSF/CSF Average operational conditions demonstrate no real thermal impacts at extraction wells and the shoreline Peak operational conditions Lower pumping rates/higher injection temperatures can result in higher peak temperatures at shoreline Small area impacted by highest temperatures Small flux relative to flow in the Columbia River Mixing in hyporheic zone will also likely attenuate fluxes Higher pumping rates/lower injection temperatures results in more thermal drift at the extraction wells Due to steepness of drawdown cone Results in decrease of GSHP thermal efficiency Likely operational scenario lies between average and peak conditions System recently became operational (July 2010) Currently collecting data to determine operational conditions Potential for cooling needs to increase due to placement of supercomputer 24

25 Summary Downgradient thermal plumes can be significant for surface water bodies and other GSHPs Important to assess thermal impacts as well as hydraulic Thermal drift at extraction wells is more likely when rejecting heat to larger volumes of water Dependent upon Ksat, distance between extraction and injection wells, regional flow Potential need to strike balance between system efficiencies and environmental impacts Operational scenarios that promote thermal efficiency may have a greater potential to negatively impact nearby surface water 25