Adapting Geothermal Flow to Heat Pump s Requirement When efficiency and flow meet

Transcription

1 WELCOME! Adapting Geothermal Flow to Heat Pump s Requirement When efficiency and flow meet By PATRICK LAMBERT, P. Eng, M.M.T., CGD CEO of Geo-Energie inc. Simon Fraser University GeoExchange TM CONFERENCE May 2 nd, 2013

2 Conference description RESUME: Getting the most efficiency from a geothermal system is a goal that most designer will be aiming at. Efficiency is often evaluated by the ratio of the energy input to the energy output of a system. ln any ground source heat pump system, the pumping energy requirement is a fact that needs to be factored in the equation. Unfortunately, many installed systems are not meeting the energy performance anticipated during the design phase. Frequently flow issues are the heart of the problem. These issues are general/y caused by the incapacity of the installed system to optimally meet, simultaneously, the heat pump's flow and geothermal field flow requirements. This presentation will tackle this problematic in a theoretical and a practical way. Flow requirements vs COP, viscosity, Reynolds, pressure drop and hydrodynamics are topics that will be discussed about.

3 HERE WE GO!

4 The Basics: GSHP Flow requirement Pump and geothermal flow direction Refrigerant gas flow Canadian GeoExchange Coalition 2010

5 The Basics Heat Transfer fluid circulation is provided by the operation of a circulating pump. Pump size must be compatible with: - Desired Flow rate - Total Dynamic Head - Fluid type - Operating Temperatures - Fluid Viscosity Canadian GeoExchange Coalition 2010

6 The Basics Big or Small, the GHX pumps should always be sized properly Photo Geo-Air Industries 2008

7 Heat Pump vs Flow 1 Heat Pump: 3 Charted Flows 1 0 : Open Loop Recommended Flow (1.5 GPM/Ton) Take a look at the associated: - Pressure drop (PSI) - Heating Capacity (HC) - Power input (KW) - Geothermal Heat Input (HE) - COP 1.5 ton Heat Pump Capacity Data Canadian GeoExchange Coalition 2010

8 Heat Pump vs Flow 2 0 : Closed Loop Min. Flow (2.25 GPM/Ton) Take a look at the associated: - Pressure drop (PSI) - Heating Capacity (HC) - Power input (KW) - Geothermal Heat Input (HE) - COP 1.5 ton Heat Pump Capacity Data

9 Heat Pump vs Flow 3 0 : Closed Loop Max. Flow (3.0 GPM/Ton) Take a look at the associated: - Pressure drop (PSI) - Heating Capacity (HC) - Power input (KW) - Geothermal Heat Input (HE) - COP 1.5 ton Heat Pump Capacity Data

10 Heat Pump vs Flow Flow rate through heat pump influences many performance Characteristics! 1.5 ton Heat Pump Capacity Data Canadian GeoExchange Coalition 2010

11 Heat Pump vs Flow Heat Pump gain: Btu Power Input: 500 Watts Less!

12 Canadian GeoExchange Coalition 2010 Heat Pump vs Flow Example: Flow rate impact on energy consumption Lets consider a 27.5 Ton Heat pump, water-water with the following characteristics: Operation time (Annual): hours Average Heating temp: 105 F Average Loop Temp: 42 F Considering 2 different flow rates: 45 GPM COP = 4.75 ; kw = GPM COP = 5.33 ; kw = 14.34

13 PUMPING EQUATIONS: Pump Law (Pumping Power): Pump Law (metric for all liquid): Relation of Flow vs Pressure Loss: PumpHP P shaft * g * h* Q h gpm* 2.31psi* spec. gr 3960* o h Q * Q1 2 Gpm, Q, l/s: Flow rate Psi, h, kpa : Press Drop : density (lbm/ft 2,kg/m 3 ) : pump efficiency (%) g : Gravitational acceleration

14 Canadian GeoExchange Coalition 2010 Heat Pump vs Flow Example continued: Flow rate impact on energy consumption Pressure drop through 45 GPM: 4.3 psi Pressure drop through 65 GPM: 8.6 psi Pressure drop through 45 GPM: 7.2 psi Pressure drop through 65 GPM: 15.1 psi TOTAL PRESSURE 45 GPM: GPM: 23.7 psi

15 Heat Pump vs Flow Given: TOTAL PRESSURE 45 GPM: GPM: 23.7 psi PumpHP Applying the pump law: Assuming 60% efficiency Assuming 100% water (spec. gravity = 1.0) We obtain the following pumping power 45 GPM: 0.50 HP ( GPM: 1.50 HP (1 117 Watts) gpm* 2.31psi* spec. gr 3960* Assuming the pump run the same # of Hours per year as the GSHP, We obtain the following annual pumping energy 45 GPM: GPM: kwh/yr

16 Heat Pump vs Flow Annual Energy Balance sheet: GSHP Energy production GPM Btu/hr GPM Btu/hr kwh/yr GSHP Energy consumption 3000 hr/yr Pumping Energy Consumption 3000 hr/yr COMBINED COP (kw produced / kw total consumed Pump + H. Pump) kw kwh/yr kw kwh/yr / COP = kw kwh/yr kw 3 351kWh/yr / COP = 4.94

17 Heat Pump vs Flow QUESTIONS: - What is the BEST flow rate to use? - Is it OK to circulate more than 3.0 gpm/ton? - Am I investing too much in pump energy? - How does my pump selection impacts the bore field? Answer: For one project corresponds a set of good answers. There may be multiple sets of good answers (and bad ones) too!

18 Geothermal Field (GHX) vs Flow QUESTIONS: - What is the MINIMAL flow rate to use? - Is laminar flow recommended? - How does viscosity impact my GHX performance? - Does the 3 gpm per ton/bore applies?

19 Geothermal Field (GHX) vs Flow For the system on the left, the installed capacity is 54 tons (18, 3-ton heat pumps). The load diversity is such that a maximum of 40 tons is used at any given time. Flow rate in the GHX should be based on?: 54 Tons? 40 Tons? Maintaining turbulence in the geothermal field? (Re>2500)

20 Calculating Flow Rate and Pressure Loss In geothermal systems it is important to select the correct flow rate and to evaluate the resulting pressure loss Pressure drop / pump head Proper Flow rate These two values will determine circulating pump size and the resulting energy consumption

21 Calculating Flow Rate and Pressure Loss Pressure drop Kavanaugh and Rafferty* make recommendations regarding pressure drop in geo-exchange systems. These recommendations are shown on the tables at left. Can this be applied to northern climates as well? What about the antifreeze considerations? Canadian GeoExchange Coalition 2010 Tables & * Kavanaugh, S.P., Rafferty, K., Ground-Source Heat Pumps Design of Geothermal Systems for Commercial and Institutional buildings, ASHRAE,

22 Vertical Boreholes Configuration Closed Loop Drawing courtesy of Water Furnace

23 Horizontal Trench Configuration Closed Loop

24 Closed Loop Example A 34 Ton heat pump (79 kw of Heating capacity) Nominal Flow rate: 50 GPM Fluid type: Propylene Glycol; 25% / volume Site Geology: Silts and clays for more than 500 Limitation: Water and fractures at 280 Max Borehole depth: 265 Min EWT: 30 0 F Sizing results: 24 Boreholes of 254 Single loop, 1 DR 11

25 Closed Loop Example 50 GPM (Heat Pump) divided by 24 BH = 2.08 GPM If we put 2 boreholes in series (adjusting the length): GPM Min Flow rate for Re > 3300 : 7.31 GPM / group - HP flow rate is insufficient. What should we do?

26 Closed Loop Example A de-coupled pump arrangement can help solving flow difference requirements, while contributing in minimizing the pumping energy and pipe sizes. One pump ensure circulation in the Heat Pump, while the other, more powerful, manages the GHX.

27 Turbulent vs Laminar Flow Three (3) types of flow behavioral: Laminar if Re <2500 Transient if 2500 <Re <4000 Turbulent if Re > 4000 It is recommended to provide a flow that will provide a Reynolds number superior to 3300 on any portion of the ground heat exchanger. Turbulent flow increases the pumping energy but also increases the rate of heat transfer between the fluid and the pipe wall.

28 Turbulent vs Laminar Flow Reynolds Number: Re a dimensionless number Influenced by flow rate (flow speed in pipe) (V; m/s, ft/s) Influenced by Fluid Density ( ; kg/m³, lbs/ft³) Influenced by fluid viscosity (m; Ns/m², Pa.s, lbm/s.ft) Considers a smooth pipe inner surface (D; m,ft) Re VD m 68.4 F (20.2 C) = 1.0 cps (centipoise)

29 Turbulent vs Laminar Flow Density variation table for Propylene glycol solution Re VD m A 25% / vol solution of propylene glycol, from 90 0 F ( C) to 30 0 F ( C) varies in density from lbs/ft 3 - TO lbs/ft 3 A 1.36% increase Table courtesy of Dow Chemical

30 Turbulent vs Laminar Flow Thermal expansion of various pipe materials Note: all values are to be multiplied by a factor of 10-6 Re VD m Carbon Steel 6.5 in/in* F 11.7m/m* C Copper 9.3 in/in* F 16.8m/m C PVC 28.0 in/in* F 50.4m/m* C HDPE 67.0 in/in* F 120.0m/m* C Example: A 1 1/4 nom. HDPE pipes expands, from 30 0 C to 0 0 C by mm (minus 0,36%)

31 Turbulent vs Laminar Flow Viscosity variation table for Propylene glycol solution A 25% / volume solution of propylene glycol, from 90 0 F ( C) to 30 0 F ( C) varies in VISCOSITY from cps - TO cps A 336,89 % increase! Re VD m Table courtesy of Dow Chemical

32 Turbulent vs Laminar Flow Given the previously presented variation of Density, Viscosity and Pipe ID, of a 25% / volume solution of propylene glycol Considering a 1 ¼ Nom. Pipe and a 6,0 GPM, running in 1000 pipe - At 90 0 F: Re = :::: Turbulent - At 30 0 F: Re = :::: Laminar. Re VD m A 332 % variation Meaning that, at 30 0 F, flow rate needs to be 332 % higher to maintain the 90 0 F level of turbulence. Table courtesy of Dow Chemical

33 Dynamic Head in Pipes Darcy-Weisbach Equation 2 L v H L f * * D 2g f Re Given the previous example (25% PG, 1 ¼ Pipe DR11, 6 GPM, 1000 ), We obtain the following pressure drop: At 90 0 F: 8.11 ft of water ( 3.51 psi ) At 30 0 F: ft of water ( 4.74 psi ) increase in pumping energy: 301.1% a 35% increase in pumping energy no enough to ensure anticipated thermal transfer. (At Re = 3 300; Flow = 8.02 GPM and PD = 18.2 ft)

34 Impact of Loop Temperature on Pump Example: Previous : 34 Ton heat pump (79 kw of Heating capacity) Design Loop Temp: 30 0 F (4 0 F delta T) Fluid type: Propylene Glycol; 25% / volume Loop qty & length: 24 x Re flow rate : 7.31 GPM / Loop Pressure drop per BH: 3.53 psi Total pumping power: Watts

35 Impact of Loop Temperature on Pump Example: We modified the previous example with the following design parameters: 34 Ton heat pump (79 kw of Heating capacity) Design Loop Temp: 27 0 F (30 0 F) (4 0 F delta T) The impacts are: Fluid type: Propylene Glycol; 30% (25%)/ volume Loop qty & length: 24 x 226 (254 ) - 12% 3300 Re flow rate : (7.31) GPM / Loop Pressure drop per BH: 7.9 (3.53) psi + 224% Total pumping power: 2045 (556.8) Watts + 367%

36 CONCLUSION Do not forget the following considerations: Chose antifreeze type wisely Limit antifreeze concentration and viscosity behavior Being design aggressive with loop temperature is a double edge knife Limit Flow, while maintaining Re > 2500 at ALL times Liquid Calculate dynamic pressure drop at peak heating Aim at less than 100 Watts of pumping energy / ton

37 THANK YOU! Geo-Energie Inc Gay-Lussac, Boucherville, QC T: Patrick Lambert, P. Eng, M.M.T, CGD Geo-Energie, 2013