Transcript geothermal HP

Geothermal Heat Pumps
North Seattle Community College HVAC Program
Instructor – Mark T. Weber, M.Ed.
Objectives
• After studying this unit, you should be
able to:
– Describe an open- and closed-loop
geothermal heat pump system
– Explain how water quality affects an openloop geothermal heat pump
– Describe different ground-loop configurations
for closed loop geothermal heat pump
systems
Objectives (cont’d.)
– Explain the advantages and disadvantages
of series- and parallel-flow configurations
in geothermal heat pump systems
– Explain the different system fluids and heat
exchanger materials
– Describe different geothermal well types
and water sources for heat pumps
Objectives (cont’d.)
– Explain some of the most common service
problems with geothermal heat pump
systems
– List and explain the governing formulas
that calculate the amount of heat rejected
or absorbed by the water-side of a
geothermal heat pump
– Describe a direct geothermal heat pump
system
Reverse-Cycle Refrigeration
• Geothermal heat pumps use the earth,
or water in the earth, as heat sources
and heat sinks
– Heat pumps use the energy stored in the
earth’s crust for heating
• Air-conditioning loads transferred to the earth
– Can be used for space heating and cooling
• Uses four major system components, four-way
reversing valve
Geothermal Heat Pump
Classifications
• Open-loop
– Water source heat pumps
– Water is used as the heat transfer medium
– The water is then expelled back to the
earth
– Typically use a well, lake, or pond
Geothermal Heat Pump
Classifications (cont’d.)
• Closed-loop
– Earth-coupled system
– The same heat transfer fluid is reused
– The fluid is circulated in buried plastic
pipes
– Used primarily where there is not enough
water to support an open-loop system
Open-Loop Systems
• Heat is transferred between a water
source and the air from the space
– Water is then expelled back to the earth
– Heating mode
• Heat is absorbed from the water source and
transferred to the air in the space
– Cooling mode
• Heat is absorbed from the space and
transferred to the water source
Figure 44–3 An open-loop, water-source heat pump with boiler and cooling tower to
maintain the loop temperature
Water Quality
• Water flow must be able to handle
required capacities
• The temperature of the water must be
within desired range
• Water temperature determines heat
transfer capability
• The water must be clean
Water Quality (cont’d.)
• Water and refrigerant piping is
configured in a counter-flow design
• The heat exchanger is a coaxial tube-intube type
• Heat exchangers are usually made of
copper alloys to extend the service life
Closed-Loop Systems
• Utilize ground loops or water loops
– Many yards of buried plastic pipe
– Loops are completely sealed
– Water or antifreeze solution is circulated
through the loops
– A low-wattage centrifugal pump is used to
circulate the liquid
Closed-Loop Systems
(cont’d.)
Figure 44–7 A ground loop showing a series-vertical
configuration in the heating mode
Closed-Loop Systems
(cont’d.)
Figure 44–8 A ground loop showing a parallel-vertical
configuration in the cooling mode
Closed-Loop Systems
(cont’d.)
• The circulating fluid exchanges its heat
with the refrigerant loop
– Heat exchange takes place within the heat
pump’s cabinet
– The heat exchanger will not get fouled
• The air loop is used to distribute
conditioned air
Closed-Loop Systems
(cont’d.)
• Domestic water can be heated by
compressor discharge gas
– Requires a separate heat exchanger
– Domestic water is circulated by a pump
– Uses a counterflow tube-in-tube heat
exchanger
– The hot gas is desuperheated while the
water is heated
Ground-Loop Configurations
and Flows
• Vertical systems – used when there is a
shortage of land
• Horizontal systems – used when land is
available without hard rock
• Slinky loop
– Designed to reduce trench length
– Can be installed in lakes or ponds
Ground-Loop Configurations
and Flows (cont’d.)
• Series flow
– Only one path for the fluid to flow
– Trapped air can be removed easily
– Have a high rate of heat transfer per foot of
pipe
– Larger diameter plastic pipe is needed
– Installation costs are higher
– Larger pressure drops
Ground-Loop Configurations
and Flows (cont’d.)
• Parallel flow
– Use smaller diameter plastic pipe
– Installation costs are lower
– Air is difficult to remove from the system
– Requires less antifreeze than series
systems
– Water flow balancing is difficult
Figure 44–17
Different flow
paths in ground
loops
System Materials and Heat
Exchange Fluids
• Buried pipe usually made of
polyethylene or polybutylene
• If there is not threat of freezing, pure
water can be used in the ground loops
• Antifreeze solutions
– Salts
– Glycols
– Alcohols
System Materials and Heat
Exchange Fluids (cont’d.)
• System components must be chosen
carefully when they are to be used with
salts, glycols, or alcohols
• New pre-mixed geothermal loop fluids
have great antifreeze, anticorrosive, and
heat transfer properties
• R-410A is the leading alternative to
replace R-22 in new equipment
Geothermal Wells and Water
Sources
• Drilled wells
– Equipped with submersible well water
pumps
– Water is pumped to the individual units and
is then discharged
– Discharge water can be directed to lakes
or streams
– Most wells are grouted to prevent water
contamination and rusting
Geothermal Wells and Water
Sources (cont’d.)
• Return wells
– Return the discharged water back to the
ground
– Supply and return wells should be located
far enough apart to prevent the supply and
return water from mixing
– Supply and return wells should be at least
100ft apart
Figure 44–21 A return well system
Geothermal Wells and Water
Sources (cont’d.)
• Slow closing solenoids in the return line
– Prevent water hammering
– Keeps heat exchanger and pressure tank
pressure equal
– Helps keep minerals dissolved in the water
Geothermal Wells and Water
Sources (cont’d.)
• Dedicated geothermal wells
– Closed-loop system
– Uses only one well
– Supply water comes from top of the well
– Return water is introduced at the bottom of
the well
– Used when there is not enough water for
other standard well systems
Geothermal Wells and Water
Sources (cont’d.)
• Dry wells
– Used for the discharge water in an open-loop
system
– Basically large reservoirs filled with gravel
and sand
– Water is filtered as it seeps through the
gravel
– Water then returns to the underground
aquifer
Geothermal Wells and Water
Sources (cont’d.)
• Pressure tanks
– Used on well systems and open-loop
geothermal heat pumps
– Pressurized tank for water storage
– Prevents the well pump from short cycling
– The well pump fills the pressure tank to a
predetermined pressure
Geothermal Wells and Water
Sources (cont’d.)
• Pressure tanks (cont’d.)
– When the tank pressure drops to a
predetermined pressure, the pump comes
on again to fill the tank
– The tank should be sized so that the pump
comes on about once every 10 minutes
Geothermal Wells and Water
Sources (cont’d.)
Figure 44–24
The operation of
a well system’s
pressure tank
Water-to-Water Heat Pumps
• Utilize two coaxial heat exchangers
• Configured as either open-loop or
closed-loop system
• Common to see a buffer tank installed
on the condenser water side to prevent
high head pressure and to function as
the water supply tank for the radiant
heating system
Water-to-Water Heat Pumps
(cont’d.)
Figure 44–26 Heat exchanger
configuration on a water-towater heat pump system
Figure 44–27 Buffer tank
location on a water-to-water
heat pump system
Troubleshooting
• Similar to the methods that are used for
air-to-air heat pumps
• Ground loop pressure and temperature
readings are needed
• A temperature probe measures the
temperature difference between the
inlet and outlet of the water’s heat
exchanger
Troubleshooting (cont’d.)
• Pressure gauge
– Used to determine the pressure drop
across the heat exchanger
– Helps to determine the flow rate through
the heat exchanger
• Troubleshooting the refrigerant and
electrical circuits of geothermal heat
pumps is similar to other refrigeration
systems
Troubleshooting (cont’d.)
• Ground loop (water loop) provides
means for a water-to-refrigerant heat
exchanger
• Amount of heat transferred = gpm x
temp differential x 500
• Temperature differential: temperature
difference between the water entering
and leaving the heat exchanger
Troubleshooting (cont’d.)
• A conversion chart from pressure drop
to gpm may be needed
• Low antifreeze flow rate be caused by:
– Defective circulating pump
– Air restriction in the piping
– Contaminated or kinked pipe in closed-loop
– Low water supply pressure in open-loop
Troubleshooting (cont’d.)
• Symptoms
– Reduced antifreeze flow (heating)
• Low suction pressure, large temperature differential
– Reduced antifreeze flow (cooling)
• High head pressure, large temperature differential
– Mineral deposits in heat exchanger (open-loop)
• Lower-than-usual temperature differential
• High head pressure in cooling mode
• Low suction pressure in heating mode
Direct Geothermal Heat Pump
Systems
• Direct geothermal systems
– Refrigerant lines buried in the ground
• Refrigerant loop acts as the evaporator in the
heating mode
– In the cooling mode, conventional aircooled condenser is used
• No coaxial heat exchangers or centrifugal
pumps are used
– Can be used as first-stage heating
Direct Geothermal Heat Pump
Systems (cont’d.)
• Installation and refrigerant-loop piping
– Installation costs are lower than a stand
alone geothermal system
– Existing condensing unit acts as the pump
and heat generator
• Copper loops are buried 3 to 4 feet; no buried
joints underground
• Loops are connected by brazing to a header
– Existing refrigerant lines are tapped for
connections
Direct Geothermal Heat Pump
Systems (cont’d.)
• The earth loop (the refrigerant loop),
may be three different configurations:
Diagonal, Vertical, Horizontal
• Connected to a refrigerant distributor or
manifold, which divides the refrigerant
flow equally to each loop
• Manifold is connected to the heat
pump’s compressor unit
Direct Geothermal Heat Pump
Systems (cont’d.)
• Heating mode
– Heat is transferred from the warmer earth
into the refrigerant loop
• Cooling mode
– Refrigerant temperature entering the
refrigerant (earth) loop is higher than that
of the earth and will now be transferred to
the earth
Refrigerant Management
System
• System consists of two components:
– Liquid Flow Control, Active Charge Control
• Three main objectives:
– Improve system efficiency, reliability, and
serviceability
– Continuously return lubricating oil back to the
compressor without returning liquid refrigerant
– Stabilize liquid and vapor refrigerant flow in
long refrigerant
Refrigerant Management
System (cont’d.)
• Liquid Flow Control
– Regulates the rate of liquid refrigerant
flowing from the condenser to the
evaporator by responding directly to the
amount of vapor bubbles arriving at the
control from the condenser’s outlet
– End result is a larger condenser with lower
condensing pressures, lower compression
ratios, and higher system efficiencies
Refrigerant Management
System (cont’d.)
• Active Charge Control (ACC)
– A thermally insulated reservoir replaces the
standard accumulator
– Purpose is to constantly deliver refrigerant
vapor and oil to the compressor in the
optimum conditions and quantities
– Determines when the system is properly
charged without using gauges, wet and dry
bulb readings, or charging charts
Summary
• Energy is transferred daily to and from
the earth by solar radiation, rainfall, and
wind
• Geothermal heat pumps use the earth,
or water in the earth, for their heat
source and heat sink
Summary (cont’d.)
• Because the earth’s underground
temperature in the summer is cooler
than the outside air, heat loads from
summer air conditioning can be rejected
underground more efficiently
• Geothermal heat pumps are very similar
to air source heat pumps in that they
both use reverse-cycle refrigeration
Summary (cont’d.)
• Geothermal heat pumps are classified
as either open- or closed-loop systems
• Water quality is one of the most
important considerations in the design
of an open-loop geothermal heat pump
system
• Open-loop systems usually use well
water as their heat source and heat sink
Summary (cont’d.)
• Heat exchanger fouling can be a
problem if water quality is poor
• Water sources for open-loop systems
may be an existing well or a new well
• Pressure tanks are used in conjunction
with wells in open-loop systems
• Closed-loop heat pump systems
recirculate the same antifreeze fluid
Summary (cont’d.)
• Closed-loop or earth-coupled systems
are used where there is insufficient water
quality or quantity
• Loops can have series or parallel fluid
flows
• The buried piping or underground heat
exchanger is usually either polyethylene
or polybutylene pipe
Summary (cont’d.)
• The antifreeze solutions inside the buried
piping are used to prevent freezing of the
heat pump heat exchanger and for heat
transfer purposes
• Water-to-water heat pump systems often
use a buffer tank to store the heated
water until it is needed by the heating
circuits
Summary (cont’d.)
• Waterless heat pump systems utilize
buried refrigerant lines instead of buried
water lines
• Waterless heat pump systems transfer
heat into and out of the refrigerant by
using the ground as the heat source in
the winter and as the heat sink in the
summer