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Geothermal _Ground Source_ Heat Pumps

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					GEOTHERMAL (GROUND-
 SOURCE) HEAT PUMPS
     Introduction

         Dr. John W. Lund, PE
National Renewable Energy Laboratory
           Golden, CO 80401
          john.lund@nrel.gov
      World Utilization of GHP
   Largest geothermal direct-use growth
   Almost 7x growth since 2000
   Mainly USA, Canada and Europe – 43
    countries
   Capacity: 35,200 MWt – 9,800 GWh/yr
   Typical unit: 12 kW (3.4 tons)
    – Range: 5.5 kW to 150 kW
   Approx. 3.0 million units installed worldwide
    – 1.0 million in the U.S.
Examples of water-
source heat pumps
¾ to 10 tons (2.5 to 35
kW) most common
              GHP Example - 1
   Galt House East Hotel, Louisville, KY
    – largest installation in the USA
   Heat and air conditioning provided for
    –   600 hotel rooms
    –   100 apartments
    –   960,000 ft² of office space
    –   Total 1,740,000 ft²
   Using 2,800 gpm from 4 wells @ 57 F
   Provides 15.8 MW cooling, 19.6 MW heating
   Energy approx. 53% of similar non-GHP
    building
   Saving approx. $25,000 per month
     BALL STATE UNIVERSITY, MUNCIE, IN
>40 buildings
<4,000 holes
400 ft. deep
Saving $2 mill./yr
Saving 80,000
tons of carbon/yr
Estimated 5,000
tons of
cooling/heating
Under
                     Cost: $70 million - $40 mill. from
construction
                     state + $5 mill. Stimulus Funds
           Fundamentals - 1
   HP = machine causes heat to flow
    “uphill”
    – From lower to higher temperature
 Work done – “pump” used to describe
 Refrigeration unit – reversible
 Heat absorbed = “source”
 Heat delivered = “sink”
 Difference in temperature = “lift”
 The greater the lift – greater power input
            Fundamentals - 2
   Geothermal (ground-source) heat pumps
    – Uses ground or groundwaterresource between 40
      and 90 F - available throughout the U.S.
    – Either removes heat from a low temperature
      resource to a higher temperature reservoir
      (heating)
    – Or removes heat from a high temperature
      resource to a lower temperature reservoir
      (cooling)
   Geothermal uses a constant temperature
    resource
    – The ground or groundwater (below about 30 ft.)
           Fundamentals -3
   Air-source heat pumps – dependent on
    outside air temperature, which is:
    – Lowest when heating demand is highest,
      and
    – Highest when cooling demand is highest
    – Thus, supplemental energy (electric)
      source needed
Annual Soil Temperature Variations, Stillwater, Oklahoma
                                     MONTH
          J        F   M     A     M J   J    A   S     O      N       D
    92                                                                       33oC
               GROUND SURFACE
    82                X = 2 ft (0.6 m)
                    X = 5 ft (1.5 m)
                   X = 12 ft (3.6 m)
    72


    62                                                                       16.5oC


    52                                       AS


    42        tO
                                                      MOIST SOIL
                                                    " = 0.6 FT 2/DAY
    32                                                                       0oC
      0       40        80       120 160 200 240      280     320      360
                                  DAY OF THE YEAR
        Advantages of GHP
    (as compared to air-source)
   They consume less energy to operate
   They tap a constant temperature resource
   They do not require supplemental energy
    during extreme outside air temperature
   Often utilize less refrigerant
   Simpler in design and maintenance
   Does not require a unit outside exposed to
    the weather
   Longer equipment life
      Disadvantages of GHP
    (as compared to air-source)
   Higher initial cost due to excavation for piping
    or drilling of a well
   Lack of trained and experienced designers
    and installers
   Lack of understanding by government
    regulators
   Shallow horizontal heat exchangers are
    affected by surface (air and sun) temperature
    variations – thus, requiring 30 to 50% more
    pipe in the ground
                Definitions - 1
   General terms:
    – Ground-Source Heat Pumps (GSHP)
       » Used by engineers and technical types, and
       » The International Ground Source Heat Pump Association
    – Geothermal Heat Pumps (GHP)
       » Used by individual in marketing and government
       » Often confused with direct-use geothermal
    – Geoexchange
       » Used by Geothermal Heat Pump Consortium
    – Geothermal Systems – in many countries –
      confusing
    – Below ground = “geothermal”
                Definitions – 2
   Ground-coupled or earth-coupled (closed
    loop) – tubing network directly buried in the
    ground - generally a thermally-fused plastic
    pipe (HDPE) with water or antifreeze (20%
    propylene glycol) solution circulated through
    the tubing - >50 year life
    –   Horizontal
    –   Vertical
    –   Spiral coil in vertical hole
    –   “Slinky” in a horizontal trench
    –   Encased in a foundation pile
    –   Direct expansion (no heat exchanger)
   One or more loops in a single hole or pile
                Definitions - 3
   Groundwater or water-source (open loop)
    systems use well or lake water. Water quality
    may be a problem due to calcium carbonate
    (hardness) and/or iron bacteria causing
    scaling or fouling of the heat exchangers
    –   Well water
    –   Lake water
    –   Mine water
    –   Tunnel water
    –   Standing column
                         Ground Coupled Heat Pumps (GCHP)
                         a.k.a. closed loop heat pumps




       vertical                      horizontal
                                                                              slinky



                         Groundwater Heat Pumps (GWHP)
                         a.k.a. open loop heat pumps
                                                                                 Disposal to lake,
                                                                                 pond, river,
                                                                                 creek, etc.

                                                                                                     Hot water
                                                                                                     tank
                                                                                                                 Heat pump

                                                                                                                         Low-temperature
                  two well                                      single well                                              underfloor heating



                         Surface Water Heat Pumps (SWHP)
                         a.k.a. lake or pond loop heat pumps                                                       Borehole
                                                                                                                   heat exchanger




indirect                                                   direct



                        pond
                                                                    pond
Trenching
Horizontal loops
Spiral slinky
Examples of Installations
                   Equipment
   Most common single package – water-to-air
    – Refrigerant-to-water heat exchanger
    – Refrigerant piping and control valves
    – Compressor
    – Air coils (heats in winter; cools and dehumidifies
      in summer
    – Fan
    – Controls
    – Desuperheater?
                Equipment - 2
   Refrigerant
    – All use R-22, an HCFC – transition refrigerant
       » Ozone depletion value (ODP) = 0.05 (5% of the most
         damaging R-11 and R-12) – phase out by 2010
    – By 2003 or 2004 – new refrigerants:
       » R-410A or R-407C
   Desuperheater can be added for hot water
    – Waste heat in summer – “free”
    – Diverts some heat in winter
    – Provides heat only when running – thus, provides
      only a portion of the heat
    – 25% savings in colder climates, 50% in warmer
      climates – US$ 100 to $200 per year.
Heating Mode
Cooling Mode
          Performance Ratings
   Coefficient of performance in the heating
    mode:
                    COP = Qh/Qe
    = heating capacity (kW)/electric power input (compressor) (kW)


   Energy efficiency ratio in the cooling mode:
                   EER = Qc/Qe
    = cooling capacity (Btu)/electrical power input (compressor) (kW)


   That is: for a COP of 4, for every unit of
    electrical input, the system puts out 4 units of
    heat energy
                O s W
               C Pv E T
5.5

5.0
                                                          Efficiency depends
4.5
                                                          on entering water
COP



4.0
                                                          temperature
3.5

3.0
      -5   0    5     10   15    20     25    30
                n r g a r e p r tue c
               E tein Wte T mea r , ( )




                                                    E v W
                                                   E R sE T
                                 7.0
                                 6.5
                                 6.0
Examples of Ground-              5.5
Coupled Units for
                                 EER


                                 5.0
                                 4.5
various entering water           4.0

temperatures (EWT)               3
                                 3
                                  .5
                                  .0
                                       10    15    20     25   30    35     40   45
                                                    n r g a r e p r tue c
                                                   E tein Wte T mea r , ( )
    Estimate of Annual Energy
         Consumption 1
 Denver (Stapleton): 924 F-days cooling (>65 F),
  6,282 F-days heating <65 C)
System      Cooling       Heating DHW        Total
             (kWh)        (kWh) (kWh)       (kWh)
Electric     3,910        17,640 4,120 25,670
ASHP         1,960         8,820 4,120 14,900
GHP          1,220         4,650 2,510        8,380

For a 2000 ft² home, newly constructed with desuper-
  heater: est. annual savings @ 8¢/kWh = $1,385
  (electric vs GHP), and $522 (ASHP vs GHP)
     Estimate of Annual Energy
           Consumption
 Steamboat Springs, CO: 300 F-days cooling (>65 F),
  7,300 F-days heating <65 C), the annual loads are:
System      Cooling     Heating DHW Total
             (kWh)       (kWh)     (kWh) (kWh)
Nat. Gas       520       25,400 5,150 31,070
Electric       500       20,300 4,120 24,920
ASHP           400       10,150 4,120 14,670
GHP            340         5,400 3,150       8,890

For a 2000 ft² home, newly constructed with desuper-
  heater: est. annual savings @ 8¢/kWh = $1,280
  (electric vs GHP), and $462 (ASHP vs GHP). For
  natural gas at $12/million Btu, the annual savings would
  be $910 compared to GHP.
    Estimate of Annual Energy
          Consumption
 Steamboat Springs, CO: 750 FLH cooling (>65 F), 750
  FLH heating heating <65 F) – annual loads
System       Cooling        Heating DHW           Total
               (kWh)        (kWh)     (kWh)      (kWh)
Nat. Gas       47,100      137,500 10,000 194,600
Electric      45,000       110,000 8,000 163,000
ASHP          30,000        55,000 8,000         93,000
GHP           23,550         27,500 4,000        55,050
For a 10,000 ft² commecial building with desuper-heater:
  est. annual savings @ 8¢/kWh = $8,640 (electric vs
  GHP), and $3,040 (ASHP vs GHP). For natural gas at
  $12/million Btu the annual savings would be = $5,700
  compared to GHP = 8 year payback for new
  construction for 35 tons and 16 years for retrofit.
      Cost of Installation (USA)
   Costs more than conventional system;
    however, savings are greater
   Costs depend upon:
    – The availability and experience of contractors
    – Cost of drilling or trenching (or wells)
   Total installed cost (10.5 kW – 3 tons) for a
    2000 ft² home with duct work and controls
    – ASHP and Gas with AC ~ US$ 4 to 5,000 ($1300–
      1,700/ton)
    – GW GHP ~ US$ 7,000 (+ well cost) ($2,450/ton)
    – Ground-coupled GHP ~ US$ 10 to 15,000
      ($3,300 – 5,000/ton)
   Substantial variation possible by 2x to 3x
    Savings in Energy Costs in
     USA Compared to GHP
        ($ per Btu or kWh)
 4x for electric resistance
 3x for propane
 2x for ASHP
 2x for fuel oil
 2x for natural gas
      Commercial Installations
          (large-scale)
   Cooling demand usually the largest
   Using central or multiple control units
   Groundwater systems the oldest and most
    widely used approach – with two wells, often
    reversing source from winter to summer
   Multiple ground-coupled system becoming
    popular – vertical bore holes 100 to 400 ft.
    deep in large fields under parking lots
                Conclusions
   Geothermal (ground-source) heat pumps are
    not a new technology – Lord Kelvin
    developed the concept of heat pump in 1852.
   GHP – popularity started in the 1960s and
    70s
   Growing at least 15%,yr; now 20%/yr
   Are economically competitive in areas of high
    alternate fuel cost
   Best suited for large building loads, such as
    schools, commercial buildings, etc.
   Best suited for new construction, as retrofits
    are expensive
             Contacts (USA)
   Geo-Heat Center, Oregon Institute of
    Technology, Klamath Falls, OR, USA
    http://geoheat.oit.edu
   American Society of Heating, Refrigeration
    and Air-Conditioning Engineers, Atlanta, GA,
    USA: www.ashrae.org
   International Ground Source Heat Pump
    Association, Oklahoma State University,
    Stillwater, OK, USA: www.igshpa.okstate.edu
   Geothermal Heat Pumps Consortium,
    Washington, D.C.: www.geoexchange.org.
                  References
   An Information Survival Kit for the Prospective
    Geothermal Heat Pump Owner by Kevin Rafferty,
    HeatSprings, 2008.
   Ground-Source Heat Pumps – Design of Geothermal
    Systems for Commercial and Institutional Buildings
    by Stephen P. Kavanaugh and Kevin Rafferty,
    ASHRAE, 1997.
   Outside the Loop – a Newsletter for Geothermal Heat
    Pumps Designers and Installers, Univ. of Alabama –
    also available on the GHC website.
   International Course on Geothermal Heat Pumps,
    Proceedings, International Geothermal Days
    “Germany 2001” IGA and Internat. Summer School

				
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posted:9/3/2011
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