Geothermal EnergyGeothermal Heat Pumps
San Jose State University
FX RongèreApril 2008
District Heating
Direct use of geothermal hot water
Heat Conduction in the Ground
One-dimensional Conduction equation is:
Along the year, the surface temperature is a sinusoidal function:
The analytical solution is:
2
2 ,.
,..
x
txTk
dt
txdTC
TtTtT .sin.),0( 0
Txk
Ctx
k
CTtxT
.
.2
...sin..
.2
..exp., 0
Ground Temperature Ground temperature remains constant around the
year for a depth greater than 10m.
Ground Temperature Profiles
0
5
10
15
20
25
-20 -10 0 10 20
Temperature (C)
Depth
(m
) Spring
Summer
Fall
Winter
Ground characteristics:T0= 20oC ω = 2.10-7 s-1 ρ = 2,300 kg.m-3 C = 900 J.kg-1.K-1 k = 1.5 W.m-1.K-1
It depends on annual average temperature
Ground Temperature at 10m
Average Annual Temperature
Heat-Pump/Chiller
A Heat-Pump as a Chiller transfers heat from the cold source to the hot source using compression to change the evaporation/condensation temperature
0 50 100 150 200 250 300100
101
102102
h [kJ/kg]
P [
ba
r]
40 C
30 C
10 C
-10 C
R134a
lnP-h Diagram
Heat-Pump cycle
compressorW
EvaporatorQ
CondensorQ
Valve
Geothermal Heat Pump
Two types of exchangers
Vertical loop More expensive Reliable Smaller foot-print
Horizontal loop Less expensive May be perforated Large foot-print
Geothermal Heat Pump
One can also use the aquifer or a pond
Open water loop Excellent transfer Water quality Regulatory issues
Surface water loop Very good transfer Size Temperature limitations
Renewable Energy for Heat In Winter
Heating energy provided to the building. For each increment of
4 kWh required for heating (COP 4.5):
3.5 kWh (11.9 Btu) “free” taken from the earth
Renewable Energy used for the total comfort need of the building: 77%
1 kWh of Utility electricity
consumed
3.5 kWh “renewable”+ 1.0 kWh “utility” 4.5 kWh required
3.5 / 4.5 = 77%From renewable source
Renewable Energy for Cooling In Summer
Cooling energy provided to the building. For each increment of 4 kWh required for cooling (COP=4):
Renewable Energy used for the total comfort need of the building: 75%
5 kWh (17 Btu) are rejected and “renewed” in the ground
1 kWh of electricityRequired for cooling
5.0 kWh “renewed”-1.0 kWh “utility” 4.0 kWh required
4.0 / 5.0 = 75%From renewable source
Pros and cons
Advantages Energy efficiency Low life-cycle cost Simplicity Low maintenance Water heating No outdoor
equipment Packaged equipment Lower peak demand No freezing in
heating mode
Disadvantages First (capital) cost Limited qualified
designers Messy construction Geographically
limited contractors Supply/demand =>
higher vendor markups
Freezing When the air is cooled at the evaporator of
the heat pump, the air moisture condensates and freezes
-10 0 10 20 30 400.000
0.010
0.020
0.030
0.040
0.050
T [C]
Hu
mid
ity
Ra
tio
Pressure = 1.0 [bar]
0.2
0.4
0.6
0.8
0 C
10 C
20 C
30 C
AirH2O
Outside air is 5oC and 40% humidity
When cooling on the evaporator water condensates and freezes
Components
Residential traditional configuration (USA)
Reversible heat pump
Blower enhanced freon-air exchanger
Components
Hydronic System
Floor heating or cooling
Hydronic system advantages
0 50 100 150 200 250 300100
101
102102
h [kJ/kg]
P [
ba
r]
40 C
30 C
10 C
-10 C
R134a
Traditional HP on air
COP = 3
-15C
55C
Hydronic system advantages
0 50 100 150 200 250 300100
101
102102
h [kJ/kg]
P [
ba
r]
40 C
30 C
10 C
-10 C
R134a
Geothermal HP on air
COP = 4.5
4C
55C
Hydronic system advantages
0 50 100 150 200 250 300100
101
102102
h [kJ/kg]
P [
ba
r]
40 C
30 C
10 C
-10 C
R134a
Geothermal Hydronic HP
COP = 9
4C
Hydronic Advantages
Better Comfort
Heating and Cooling needs
Heating and Cooling load of a house can be calculated using energy software
Sizing and Construction
Pond Closed loop
Typically 15 tons/acre (depth15-20 ft) or as high as 85 tons/acre for well stratified deep lakes
Sizing and Construction
Horizontal Ground loop
4 – 6 ft burial depth
Sizing and Construction
Horizontal Ground Loop
Sizing and Construction Not recommended, use a software package to
include interaction and long term dynamic behavior
0
50
100
150
200
250
300
350
400
450
44-47 48-51 52-55 56-59 60-63 64-67 68-70
Avg. Ground Temperature (F)
Len
gth
of
Tre
nch
or
Bo
re p
er T
on
Slinky 6-Pipe 4-Pipe 2-Pipe Vert. U-Tube
Ground Characteristics
Conductivity
Ground Characteristics
Source: McQuay Geothermal Heat Pumps Design Manual
Unit Conversion:1 Btu/(h.ftF) = 1.8 W/(m.K)1 Btu/(lb.F) = 4,420 J/(kg.K)1 lb/ft3 = 16 kg/m3
Sizing and Construction
Vertical Loop
200 Ft.
20 Ft.
25 Ft.
4 Ft.
Typically, a 300 ft bore hole provides 3 Tons
But interactions and long term behavior are complex; use of software recommended
30
35
40
45
50
55
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Time (yrs)
He
at
Pu
mp
Ent
eri
ng
Flu
id T
em
pe
ratu
re (o
F)
Critical Design Temperature
Sizing and Construction
Better case when balanced heating and cooling loads
Software Packages
Available software packagesSoftware Product Vendor
CLGS Intl. Ground-Source Heat Pump Assoc., Stillwater, OK, USA
ECA Elite Software, Inc., Bryan, TX, USA Earth Energy Designer (EED) University of Lund, Sweden GEOCALC Ferris State University, Big Rapids, MI, USA GeoDesigner ClimateMaster, Oklahoma City, OK, USA GchpCalc Energy Information Services, Tuscaloosa, AL, USA GL-Source Kansas Electric Utility, Topeka, KS, USA GLHEPRO Intl. Ground-Source Heat Pump Assoc., Stillwater, OK,
USA Ground Loop Design GBT, Inc., Maple Plain, MN, USA GS2000 Buildings Group, Natural Resources Canada Lund Programs University of Lund, Sweden RIGHT-LOOP Wrightsoft, Lexington, MA, USA WFEA Water Furnace Intl., Fort Wayne, IN, USA
Drilling is the main cost
Drilling cost: $5 to $10 per footCost of a GHP installation
Exchanger15%
Manifold35%
Drilling50%
Piping and Grouting
The grout aims to keep optimal conduction from the pipe to the ground
Grout conductivity is very important. It varies from 0.8 W.m-1.K-1 to 2 W.m-1.K-1
Markets
Geothermal Heat Pump market in the USA
LEGENDE :> 50> 20> 10> 0
IGSHPA
Nb of IGSHPA certified installers per state
References and Companies to follow
Water Furnace www.waterfurnace.com Earth Energy Systems
www.eartenergysystems.com McQuay www.mcquay.com Geoexchange www.geoexchange.com Climate Master www.climatemaster.com
Winery in Livermore equipped with geothermal heat pump and hydronic system. Source: Earth Energy Systems
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