RECENT DEVELOPMENTS IN GROUND SOURCE HEAT PUMP RESEARCH. Jeffrey D. Spitler Oklahoma State University

Transcription

1 RECENT DEVELOPMENTS IN GROUND SOURCE HEAT PUMP RESEARCH Jeffrey D. Spitler Oklahoma State University

2 Outline A brief history Recent research developments Foundation Heat Exchangers (Residential) Simple simulation tool (Both) GSHP vs. VRF - ASHRAE HQ Building (Commercial) Renewable?

3 William Thomson (Lord Kelvin) 1852: proposed a heat pump for heating buildings or, in tropical climates, cooling them. 1853: provided mathematical proof of same, interacting with the works of Joule, Carnot, Mayer, Rankine

4 Heinrich Zoelly A Mexican-Swiss turbine engineer, born in Mexico in Better known for development of steam turbine as an alternative to steam engines for locomotives. Issued Swiss patent in 1912 for a ground source heat pump.

5 1940s Some pre-world War II installations were done in the U.S.; numbers increased after the war.

6

7 A start a fizzle 1940s: A dozen research/monitoring projects reported in the literature After the early 1950s reports of ground source heat pump systems essentially vanished from the U.S. literature. Why? Apparently, problems with drying around horizontal ground-loop heat exchangers, leakage, and undersizing.

8 Take Two 1970s In 1974, Oklahoma State University began a research program in response to a request from some local businesses. First GSHP by modifying an air-source unit first residential installations in Oklahoma Late 1970s several US projects on solarassisted ground source heat pump systems

9 Early OSU Work The major problem is the complexity of the controls From Bose, et al. (1979)

10 Challenges addressed 70s-90s Leakage: Fused plastic (HDPE) pipe Undersizing: Design tools Ground thermal properties: Thermal response test Research-to-practice: Commercialization, IGSHPA, technology transfer

11 Mid-1990s - Present Transition from residential to commercial. Primary challenge remaining: economic. Addressed by: Hybrid systems Improved ground heat exchangers Identification of niche applications, e.g.: Schools Light retail Dissemination of best design practices Avoid over-pumping Avoid excess controls Improved design and simulation tools

12 Recent research developments a sample Foundation Heat Exchangers (Residential) Simple simulation tool (Residential and Commercial) GSHP vs. VRF - ASHRAE HQ Building (Commercial)

13 Foundation Heat Exchangers

14 Ground source heat pump (GSHP) systems First cost the most significant barrier. For typical US house, extra cost for drilling boreholes is $3000-$6000 (USD)

15 An alternative: Foundation Heat Exchangers (FHX) Experimentally-proven technology! For well-insulated houses For houses with excavated basements (or drainage) Significant cost reduction possible.

16 FHX

17 Experimental Houses Two low energy houses have been constructed with FHX at Oak Ridge, Tennessee, USA. Data collected over a one year period has been use to validate a number of design tools and simulation models

18 After earlier experimental success - questions Proven in a temperate climate where else might they work? Proven for highly-insulated houses how good does the insulation need to be? How can we design such a system? How big of a problem is short-circuiting? How can we calculate energy consumption in EnergyPlus in a reasonable amount of time? Most of these questions can be at least partially answered with an experimentally-validated simulation.

19 Simulation Which phenomena need be modeled? Conduction heat transfer Surface convection & radiation Evapotranspiration Freezing/thawing Moisture transport Snow Methodology? Speed? Accuracy?

20 Simulation Approaches Numerical Models 2d & 3d FVM using boundary-fitted coordinates 2d coarse grid finite volume method (FVM) 3d dual coordinate system FVM Response Factor Model Dynamic Thermal Networks Analytical Model

21 Dual-coordinates FVM Combines nonuniform coarse grid with radial grid surrounding each pipe. Final solution implemented in EnergyPlus 4000 rectangular cells; 360 radial cells Similar approach developed by Piechowski

22 Heat pump Entering fluid temperature (C) Experimental Validation Experimental result DCS-FV E+ model HVACSIM+ model 2D/3D E+ Model Days

23 FHX for well-insulated house Marginal may require additional horizontal ground heat exchanger

24 Simple simulation tool for vertical GHE

25 GHX Simulation Approaches Analytical Numerical Response factors (g-functions) Short time-step g-functions DST model But 25

26 GSHP System Simulation To be useful, needs to be part of a modeling tool, e.g.: equest EnergyPlus HVACSIM+ TRNSYS Modelica 26

27 Problem What to do when heat pump or system is non-standard? equest EnergyPlus Wait or approximate HVACSIM+ TRNSYS Modelica Use existing components or write new Fortran code. Write new Modelica code (Good Luck!) 27

28 Our Solution Problem complicated by simultaneity Use successive substitution with full-duration, separate, simulations of GHX Heat pump(s) and supplementary devices 28

29 Our Solution Hourly, multi-year simulation GHX simulation: standalone, pre-compiled exe. (derived from HVACSIM+) Heat pump / auxiliary components: Excel/VBA G-functions from database or Javed and Claesson (2011) Post-processing: Excel/VBA Converges rapidly : 4 or 5 iterations

30

31 ASHRAE Headquarters Building Study

32 ASHRAE Building 2008: Major renovation Three state-of-the-art systems: 2 nd floor: Ground source heat pump (GSHP) system 1 st floor: Variable refrigerant flow (VRF) system a multiple-split air source heat pump system Dedicated Outdoor Air System (DOAS) to provide fresh air 1600 data points are measured! Includes: Total power of each system Lighting power consumption Plug loads Objective: Compare performance of GSHP and VRF systems

33 Analysis of loads on both systems Measured: Lighting Plug Loads DOAS Estimated: Envelope (Walls, windows, roof) Occupants

34 Monthly Net Loads, kwh/sq ft Net monthly building loads 1,2 1,0 GSHP VRV 0,8 0,6 0,4 0,2 0,0-0,2

35 22 20 Average Power, W/m VRF Outside Air Temp, C GSHP

36 36 VRF system power contributions of cooling/heating heating cooling Average VRF Power Use, W/m Outside Air Temp, C

37 37 GSHP system power contributions of cooling/heating heating cooling unallocated Average GSHP Power Use, W/m Outside Air Temp, C

38 Temperature, C 38 Ground Loop Supply Temp. 40 Ambient Dry Bulb Temp. Ground Loop Supply Fluid Temp /1/11 12/31/11 7/1/12 12/31/12 7/2/13

39 39 Conclusions System performance 2 nd floor (served by GSHP system) has (per unit floor area): Higher cooling demand than 1 st floor, but Lower heating demand Total cooling energy requirements >> total heating energy requirements The GSHP system used less energy per unit floor area than the VRF system while maintaining similar room temperatures Up to 40% less energy in summer Up to 70% less energy in winter and shoulder seasons

40 40 Conclusions Reasons for the Difference Ground loop supply temperature was more favorable than the ambient air temperature for heat pump operation The control strategy of the VRF resulted in more simultaneously heating and cooling than the GSHP, especially in shoulder season VRF system over-controlled leading to heating mode operation even in summer. Defrosting operation of VRF in winter

41 Renewable Energy Perspectives

42 Perspectives What is the goal of renewable energy? Not (in my opinion) an end in itself Rather: to maximize human comfort and productivity while minimizing consumption of non-renewable energy, also minimizing adverse environmental effects, and do it cost-effectively.

43 Comparison Comparing GSHP system to an advanced air source heat pump system (VRF), GSHP system: Reduces electrical energy required for cooling (~40-70%) Reduces electrical energy required for heating (~65%) Delivers (mostly) renewable heat to the building Gives the same human comfort and productivity Has ~25% lower initial cost, including boreholes.

44 Conclusions Like any other cooling system, GSHP systems require electricity. They use less energy, so for any mix of power sources, they use less non-renewable energy. They can use even less non-renewable energy as the power source mix becomes more renewable.