Title: MEBS 6008
1MEBS 6008 Heat Pumps
2What Is a Heat Pump?
- A heat pump is a self-contained, packaged
cooling-and-heating unit with a reversible
refrigeration cycle. - A heat pump is basically a device that transfers
heat from one substance to another substance. - It has these same basic refrigeration components
compressor, condenser, evaporator, and expansion
device. - The difference is that it can also reverse the
refrigeration cycle to perform heating, as well
as cooling, by reversing the functions of the two
heat exchangers. - The operation of the refrigeration cycle changes
depending on whether the unit is in cooling or
heating mode. - Heat pump is generally reserved for equipment
that heats for beneficial purposes, rather than
that which removes heat for cooling only.
3What Is a Heat Pump?
- Dual-mode heat pumps alternately provide heating
or cooling. - Heat reclaim heat pumps provide heating only, or
simultaneous heating and cooling. - An applied heat pump requires competent field
engineering for the specific application, in
contrast to the use of a manufacturer-designed
unitary product. - Built-up heat pumps (field- or custom-assembled
from components) and industrial process heat
pumps are two types.
4Heat Pump Cycles
- Most modern heat pumps use a vapor compression
(modified Rankine) cycle or an absorption cycle. -
- Although most heat pump compressors are powered
by electric motors, limited use is also made of
engine and turbine drives. -
- Applied heat pump systems are most commonly used
for heating and cooling buildings, but they are
gaining popularity for efficient domestic and
service water heating, pool heating, and
industrial process heating. -
5Introduction of heat source and heat pump system
- Heat sources include the ground, well water,
surface water, gray water, solar energy, the air,
and internal building heat. - Frequently, heating and cooling are supplied
simultaneously to separate zones. - Decentralized systems with water loop heat pumps
are common, using multiple water-source heat
pumps connected to a common circulating water
loop. - They can also include ground coupling, heat
rejecters (cooling towers and dry coolers),
supplementary heaters (boilers and steam heat
exchangers), loop reclaim heat pumps, solar
collection devices, and thermal storage.
6Review of a Typical Vapour Compression Cycle
- Refrigerant enters the evaporator in the form of
a cool, low-pressure mixture of liquid and vapor
(I). - Heat is transferred to the refrigerant from the
relatively warm air or water to be cooled,
causing the liquid refrigerant to boil. - The resulting vapor (II) is then pumped from the
evaporator by the compressor, which increases the
pressure and temperature of the refrigerant
vapor. - The resulting hot, high-pressure refrigerant
vapor (III) enters the condenser where heat is
transferred to ambient air or water, which is at
a lower temperature. - Inside the condenser, the refrigerant condenses
into a liquid.
7Review of a Typical Vapour Compression Cycle
- This liquid refrigerant (IV) then flows from the
condenser to the expansion device. - The expansion device creates a pressure drop that
reduces the pressure of the refrigerant to that
of the evaporator. - At this low pressure, a small portion of the
refrigerant boils (or flashes), cooling the
remaining liquid refrigerant to the desired
evaporator temperature. - The cool mixture of liquid and vapor refrigerant
(I) travels to the evaporator to repeat the cycle.
8Heat Pump Cycle
A heat pump cycle comprises the same processes
and sequencing order as a refrigeration cycle
except that the refrigeration effect q14 or qrf,
and the heat pump effect q23 ,both in J/kg, are
the useful effects.
where h4 h1 enthalpy of refrigerant
entering and leaving evaporator, respectively, J
/kg Win work input, J/kg The coefficient
of performance of the heating effect in a heat
pump system COPhp is
9Basic types of heat pump cycles
- Closed vapor compression cycle
- This is the most common type used in both HVAC
and industrial processes. - It employs a conventional, separate refrigeration
cycle that may be single-stage, compound,
multistage, or cascade.
10Basic types of heat pump cycles
Mechanical vapor recompression cycle with heat
exchanger
- Process vapor is compressed to a temperature and
pressure sufficient for reuse directly in a
process. - Energy consumption is minimal, because
temperatures are optimum for the process. - Typical applications for this cycle include
evaporators (concentrators) and distillation
columns.
11Basic types of heat pump cycles
Open vapor recompression cycle
- A typical application for this cycle is in an
industrial plant with a series of steam pressure
levels and an excess of steam at a
lower-than-desired pressure. - The heat is pumped to a higher pressure by
compressing the lower pressure steam.
12Basic types of heat pump cycles
- Heat-driven Rankine cycle
- This cycle is useful where large quantities of
heat are wasted and where energy costs are high. - The heat pump portion of the cycle may be either
open or closed, but the Rankine cycle is usually
closed.
13HEAT SOURCES AND SINKS
Air
- Outdoor air is a universal heat-source and
heat-sink medium for heat pumps and is widely
used in residential and light commercial systems. -
- Extended-surface, forced-convection heat transfer
coils transfer heat between the air and the
refrigerant. -
- Typically, the surface area of outdoor coils is
50 to 100 larger than that of indoor coils. -
- The volume of outdoor air handled is also greater
than the volume of indoor air handled by about
the same percentage. -
- During heating, the temperature of the
evaporating refrigerant is generally 6 to 11 K
less than the outdoor air temperature.
14HEAT SOURCES AND SINKS
Air
- When selecting or designing an air-source heat
pump, the outdoor air temperature in the given
locality and frost formation in particular must
be considered. - As the outdoor temperature decreases, the heating
capacity of an air-source heat pump decreases. -
- This makes equipment selection for a given
outdoor heating design temperature more critical
for an air source heat pump than for a fuel-fired
system. -
- The equipment must be sized for as low a balance
point as is practical for heating without having
excessive and unnecessary cooling capacity during
the summer.
15HEAT SOURCES AND SINKS
Air
- When the surface temperature of an outdoor air
coil is 0C or less, with a corresponding outside
air dry-bulb temperature 2 to 5.5 K higher, frost
may form on the coil surface. - If allowed to accumulate, the frost inhibits heat
transfer therefore, the outdoor coil must be
defrosted periodically. -
- The number of defrosting operations is influenced
by the climate, air-coil design, and the hours of
operation. -
- It was found that little defrosting is required
when outdoor air conditions are below -10C and
60 rh (confirmed by psychrometric analysis). -
16HEAT SOURCES AND SINKS
Air
- Under very humid conditions, when small suspended
water droplets are present in the air, the rate
of frost deposit may be about three times as
great as predicted from psychrometric analysis. -
- The heat pump may require defrosting after only
20 min of operation. - The loss of available heating capacity due to
frosting should be taken into account when sizing
an air source heat pump. -
- Early designs of air source heat pumps had
relatively wide fin spacing of 5 to 6 mm, based
on the theory that this would minimize the
frequency of defrosting. -
- With effective hot-gas defrosting a much closer
fin spacing is permitted that reduce size and
bulk of the system. -
- In current practice, fin spacing of 1.3 to 2.5 mm
are widely used.
17HEAT SOURCES AND SINKS
Water
- City water is seldom used because of cost and
municipal restrictions. - Groundwater (well water) is particularly
attractive as a heat source because of its
relatively high and nearly constant temperature. -
- The water temperature is a function of source
depth and climate (Any information on water
temperature of HKs situation ?). -
- Frequently, sufficient water is available from
wells for which the water can be re-injected into
the aquifer. -
- The use is non consumptive and, with proper
design, only the water temperature changes.
18HEAT SOURCES AND SINKS
Water
- The water quality should be analyzed, and the
possibility of scale formation and corrosion
should be considered. -
- In some instances, it may be necessary to
separate the well fluid from the equipment with
an additional heat exchanger. -
- Special consideration must also be given to
filtering and settling ponds for specific fluids.
-
- Other considerations are the costs of drilling,
piping, pumping, and a means for disposal of used
water. -
- Information on well water availability,
temperature, and chemical and physical analysis
is available from U.S. Geological Survey offices
in many major cities (Again, Hong Kongs
situation?)
19HEAT SOURCES AND SINKS
Water
- Heat exchangers may also be submerged in open
ponds, lakes, or streams. -
- When surface or stream water is used as a source,
the temperature drop across the evaporator in
winter may need to be limited to prevent
freeze-up. -
- In industrial applications, waste process water
(e.g., spent warm water in laundries, plant
effluent, and warm condenser water) may be a heat
source for heat pump operation. -
- Sewage, which often has temperatures higher than
that of surface or groundwater, may be an
acceptable heat source. -
- Secondary effluent (treated sewage) is usually
preferred, but untreated sewage may used
successfully with proper heat exchanger design.
20HEAT SOURCES AND SINKS
Ground
- The ground is used extensively as a heat source
and sink, with heat transfer through buried
coils. -
- Soil composition, which varies widely from wet
clay to sandy soil, has a predominant effect on
thermal properties and expected overall
performance. The heat transfer process in soil
depends on transient heat flow. -
- Thermal diffusivity is a dominant factor and is
difficult to determine without local soil data. -
- Thermal diffusivity is the ratio of thermal
conductivity to the product of density and
specific heat. -
- The soil moisture content influences its thermal
conductivity.
21HEAT SOURCES AND SINKS
Solar Energy
- Solar energy may be used either as the primary
heat source or in combination with other sources. -
- Air, surface water, shallow groundwater, and
shallow ground-source systems all use solar
energy indirectly. -
- Using solar energy directly as a heat source for
heat pumps can provide heat at a higher
temperature than the indirect sources, resulting
in an increase in the heating coefficient of
performance. -
- Compared to solar heating without a heat pump,
the collector efficiency and capacity are
increased because a lower collector temperature
is required.
22HEAT SOURCES AND SINKS
Solar Energy
There are two basic types of solar-source heat
pumps systems direct and indirect.
Direct
- The direct system places refrigerant evaporator
tubes in a solar collector, usually a flat-plate
type. A collector without glass cover plates can
also extract heat from the outdoor air. - The same surface may then serve as a condenser
using outdoor air as a heat sink for cooling.
23HEAT SOURCES AND SINKS
Solar Energy
- Indirect system
- An indirect system circulates either water or air
through the solar collector. - When air is used, the collector may be controlled
in such a way that - The collector can serve as an outdoor air
preheater, - The outdoor air loop can be closed so that all
source heat is derived from the sun, or - The collector can be disconnected from the
outdoor air serving as the source or sink.
24AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
- In an air-source heat pump system, outdoor air
acts as a heat source from which heat is
extracted during heating, and as a heat sink to
which heat is rejected during cooling. -
- Since air is readily available everywhere,
air-source heat pumps are the most widely used
heat pumps in residential and many commercial
buildings. -
- The cooling capacity of most air-source heat
pumps is between 1 and 30 tons (3.5 and 105 kW). -
- Air-source heat pumps can be classified as
individual room heat pumps and packaged heat
pumps. -
- Individual room heat pumps serve only one room
without ductwork. -
- Packaged heat pumps can be subdivided into
rooftop heat pumps and split heat pumps.
25AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Split System Heat Pump
Roof top package unit
26AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Most air-source heat pumps consist of
- Coils through which air is conditioned,
- Outdoor Single or multiple compressors,
- Indoor coils where heat is extracted from or
rejected to the outdoor air, - Expansion valve
- Reversing valves that change the heating
operation to a cooling operation and vice versa, - An accumulator to store liquid refrigerant, and
other accessories.
27AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Indoor Coil
- In an air-source heat pump, the indoor coil is
not necessarily located inside the building. - The indoor coil in a rooftop packaged heat pump
is mounted on the rooftop. - But, an indoor coil always heats and cools the
indoor supply air. - During cooling operation, the indoor coil acts as
an evaporator. - It provides the refrigeration effect to cool the
mixture of outdoor and re-circulating air when
the heat pump is operating in the re-circulating
mode. - During heating operation, the indoor coil acts as
a condenser. - The heat rejected from the condenser raises the
temperature of the conditioned supply air. - For heat pumps using halocarbon refrigerants, the
indoor coil is usually made from copper tubing
and corrugated aluminum fins.
28AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Outdoor Coil
- The outdoor coil acts as a condenser during
cooling and as an evaporator to extract heat from
the outdoor atmosphere during heating. - When an outdoor coil is used as a condenser, a
series-connected subcooling coil often subcools
the refrigerant for better system performance. - An outdoor coil always deals with outdoor air,
whether it acts as a condenser or an evaporator. - Like the indoor coil, an outdoor coil is usually
made of copper tubing and aluminum fins for
halocarbon refrigerants. - Plate or spine fins are often used instead of
corrugated fins to avoid clogging by dust and
foreign matter.
29AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Reversing Valve
- Reversing valves are used to guide the direction
of refrigerant flow when cooling operation is
changed over to heating operation or vice versa. -
- The rearrangement of the connections between four
ways of flowcompressor suction, compressor
discharge, evaporator outlet, and condenser
inletcauses the functions of the indoor and
outdoor coils to reverse. It is also called a
four-way reversing valve. -
- The efficiency losses altogether with leakage,
heat transfer, and the pressure drop across the
reversing valve cause a decrease of 4 to 7
percent in heat pump performance. -
- Other accessories include filter dryer, sight
glass, strainer, liquid level indicator, solenoid
valves, and manual shutoff valves.
Compressor.
- Reciprocating and scroll compressors are widely
used in heat pumps.
30AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Expansion Device
- A variety of expansion devices may be used in
heat pumps. - The most common types are thermal expansion
valves (TXV), electronic expansion valves, and
capillary tubes. - All of these devices reduce the pressure and
temperature of the refrigerant within the cycle. - Expansion valves, such as the TXV, have the added
capability of metering the quantity of
refrigerant flowing through the cycle in order to
match the load to enhance the efficiency of the
cycle. - TXVs used in heat pumps may be bi-directional
(that is, refrigerant flows in one direction
when in cooling mode and in the opposite
direction when in heating mode). - Another way is to design the refrigerant piping
inside the heat pump to ensure that refrigerant
flow through the valve is in the same direction
in either mode.
31AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Cooling Mode
- When the discharge air temperature sensor detects
an increase in the air temperature above a
predetermined limit at the exit of the indoor
coil, cooling is required in the air-source heat
pump. -
- The indoor coil now acts as an evaporator and
extracts heat from the conditioned air flowing
through the indoor coil. -
- After evaporation, vapor refrigerant from the
indoor coil passes through the sliding connector
of the slide and flows to the suction line. - Hot gas discharged from the compressor is led to
the outdoor coil, which now acts as a condenser. -
- An economizer cycle can be used when an outdoor
air sensor detects the outdoor temperature
dropping below a specific limit during cooling
mode.
32AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Heating Mode
- When the discharge air sensor detects a drop in
air temperature below a predetermined limit at
the exit of the indoor coil, heating is required.
- The outdoor coil now acts as an evaporator.
- When the discharge air temperature sensor detects
a drop in air temperature further below preset
limits, the electric heater (that is
supplementary heater) would be energized in steps
to maintain the required discharge air
temperature.
33AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Heating Mode
- Supplementary heating is energized only when the
space heating load cannot be offset by the
heating effect of the heat pump. - ASHRAE/IESNA Standard 90.1-1999 stipulates heat
pumps equipped with internal electrical
resistance heaters shall have controls to prevent
supplemental heater operation when the heating
load can be met by the heat pump alone during
heating or setback recovery.
34AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Cycling Loss and Degradation Factor
- For split packaged air-source heat pumps, indoor
coils are located inside the building and outdoor
coils are mounted outdoors. - When an on/off control is used for the
compressor, during the off period, refrigerant
tends to migrate from the warmer outdoor coil to
the cooler indoor coil in summer and from the
warmer indoor coil to the cooler outdoor coil
during winter. - When the compressor starts again, the transient
state performance shows that a 2- to 5-min
operating period of reduced capacity is required
before the heat pump can operate at full
capacity. - Such a loss due to cycling of the compressor is
called cycling loss.
35Water-to-Air Heat Pumps
These heat pumps rely on water as the heat source
and sink, and use air to transmit heat to, or
from, the conditioned space. They include the
following 1) Groundwater heat pumps 2)
Surface water heat pumps
36Water-to-Air Heat Pumps
Groundwater heat pumps
- They use groundwater from wells as a heat source
and/or sink. - These systems can either circulate the source
water directly to the heat pump or use an
intermediate fluid in a closed loop, similar to
the ground-coupled heat pump.
Surface water heat pumps
- They use surface water from either a lake, pond,
or stream as a heat source or sink. - Similar to the ground-coupled and groundwater
heat pumps, these systems can either circulate
the source water directly to the heat pump or use
an intermediate fluid in a closed loop.
37Water-to-Air Heat Pumps
Internal-source heat pumps
- They use the high internal cooling load generated
in modern buildings either directly or with
storage. - These include water loop heat pumps.
Solar-assisted heat pumps
- They rely on low-temperature solar heat as the
heat source. - Solar heat pumps may resemble water-to air, or
other types, depending on the form of solar heat
collector and the type of heating and cooling
distribution system. -
38Water-to-Air Heat Pumps
Wastewater-source heat pumps
- They use sanitary waste heat or laundry waste
heat as a heat source. - The waste fluid can be introduced directly into
the heat pump evaporator after waste filtration,
or it can be taken from a storage tank, depending
on the application. - An intermediate loop may also be used for heat
transfer between the evaporator and the waste
heat source.
39Water-to-Water Heat Pumps
- These heat pumps use water as the heat source and
sink for cooling and heating. - Heating-cooling changeover can be done in the
refrigerant circuit, but it is often more
convenient to perform the switching in the water
circuits. - Direct admittance of the water source to the
evaporator is one approach. - Alternatively, applying the water source
indirectly through a heat exchanger (or
double-wall evaporator) to avoid contaminating
the closed chilled water system, which is
normally treated may be necessary.
40Ground-Coupled Heat Pumps.
- These use the ground as a heat source and sink.
- A heat pump may have a refrigerant-to-water heat
exchanger or may be of the direct-expansion (DX)
type. - In systems with refrigerant-to-water heat
exchangers, a water or antifreeze solution is
pumped through horizontal, vertical, or coiled
pipes embedded in the ground.
41Ground-Coupled Heat Pumps
Direct expansion ground-coupled heat pumps use
refrigerant in direct expansion, or flooded
evaporator circuits for the ground pipe coils.
42Ground-Coupled Heat Pumps
- Soil type,moisture content, composition, density,
and uniformity close to the surrounding field
areas affect the success of this method of heat
exchange. - With some piping materials, the material of
construction for the pipe and the corrosiveness
of the local soil and underground water may
affect the heat transfer and service life. - In a variation of this cycle, all or part of the
heat from the evaporator plus the heat of
compression are transferred to a water-cooled
condenser. - This condenser heat is then available for uses
such as heating air or domestic hot water.
43Refrigerant-to-water heat exchanger
- It may be a tube-in-tube, tube-in-shell, or
brazed-plate design. - The example shown here is a tube-in-tube, or
coaxial, heat exchanger. - It is constructed as a small tube running inside
another larger tube. - The water flows through the inner tube and
refrigerant flows through the outer tube. - In the cooling mode, the refrigerant-to-water
heat exchanger acts as the condenser. - The water flowing through the inner tube absorbs
heat from the refrigerant flowing through the
outer tube. - In the heating mode, it acts as the evaporator
and the refrigerant absorbs heat from the water.
44Benefits of using water-source heat pump
- In the heat recovery mode gt saves energy by
reducing the operating time of the cooling tower
and boiler. - Allowing different space temperature in many
spaces with dissimilar cooling and heating
requirements (each independently controlled space
is served by its own heat pump and own
thermostat). - The same piece of equipment is used to provide
both cooling and heating to the space. Even
though a separate cooling tower and boiler may be
included in the system, only one set of water
pipes is required. This can reduce the system
installation cost. - A water-source heat pump system typically
requires less mechanical floor space than
centralized systems. This increases the rentable
space and revenue in tenant-occupied buildings. - If one heat pump fails and must be replaced, it
does not affect the operation of the rest of the
system.
45Key issues associated with water source heat-pump
system.
- Outdoor air for ventilation may bring a few
challenges. Most commercial buildings have a
separate, ducted ventilation system. - Next, because a heat pump is located in, or very
close to, the occupied space and contains both a
compressor and a fan, the resulting noise level
in the space must be considered during system
design. - Proper maintenance of the heat pumps requires
that they be located in accessible areas. Units
that make access as easy as possible increases
the chance that the equipment will be properly
maintained.
46Water-source heat pumps Configurations
Configurations available to suit various building
types.
Horizontal units
- Horizontal units are designed for installation in
ceiling plenums, especially for spaces where
floor space is at a premium. - Typical applications include offices and schools.
47Water-source heat pumps
Vertical units
- Vertical units are designed to be installed in
separate spaces such as closets or maintenance
rooms. - Common applications for small vertical units
include schools, apartments, condominiums, and
retirement homes. - Larger vertical units are generally used in
spaces that are more open, such as cafeterias and
gymnasiums, or used as a dedicated ventilation
system to condition the outdoor air brought into
the building.
48Water-source heat pumps
Console units
- They are designed for installation under windows,
in perimeter spaces or in entryways, where ducted
systems cannot be used and floor space is not a
constraint. - Typical applications include offices, apartment
buildings, motels, and dormitories. - Because of their rugged design, they are
typically used in schools.
49Water-source heat pumps
Vertical-stack units
- They are designed for corner installation in
multistory buildings such as hotels, apartments,
condominiums, and retirement centers, where a
minimum amount of floor space is available. - They are designed to be stacked above each other
to minimize piping and electrical installation
costs.
50Heat adder rejecter
Water-source heat pumps
Use of water to water heat pump
Ground loop
51Water-source heat pumps
Operating strategy
Warm weather
- Water-source heat pumps can run in either heating
or cooling mode. - During warm weather, when all the heat pumps are
operating in cooling mode, heat removed from the
air is transferred to the water loop. - This causes the temperature of the water in the
loop to rise, making it necessary to remove heat
from the water. - A cooling tower or evaporative water cooler
rejects this heat to the outdoor air, maintaining
a leaving-water temperature of approximately
32ºC.
52Water-source heat pumps
Operating strategy
Cold weather
- During cold weather, when most of the heat pumps
are operating in heating mode, heat is removed
from the water loop and transferred to the air. - This causes the temperature of the water in the
loop to drop, making it necessary to add heat to
the water loop. - A boiler or water heater adds heat to the water
loop, maintaining a leaving-water temperature of
approximately 16ºC.
53Water-source heat pumps
Operating strategy
Mild Weather
- During mild weather, such as spring and fall, the
heat pumps serving the sunny side and interior of
the building operate in cooling mode and reject
heat into the water loop. - The heat pumps serving the shady side of the
building operate in heating mode and absorb heat
from the water loop. - Heat rejected by the units operating in cooling
mode can be used to offset the heat absorbed by
the units in heating mode. - If the water temperature stays between 16ºC and
32ºC, neither the boiler nor the cooling tower
need to operate. - Under this situation, a water-source heat pump
system provides a form of heat recovery and an
opportunity to save energy. - In case heat generated by lights, people, and
office equipment may require year-round cooling
in the interior spaces, this heat recovery
further reduces boiler operation during the
winter months.
54Ground-Source Heat Pump Systems
- A ground-source heat pump uses the earth as the
heat rejecter and heat adder. - These systems take advantage of the earths
relatively constant temperature, and use the
ground or surface water as the heat rejecter and
heat adder. - Ground-source heat pump systems dont actually
get rid of heatthey store it in the ground for
use at a different time. - During the summer, the heat pumps absorb heat
from the building and store it in the ground. - When the building requires heating, this stored
heat can be recaptured from the ground. - In a perfectly balanced system, the amount of
heat stored over a given period of time would
equal the amount of heat retrieved.
55Ground-Source Heat Pump Systems
- In a properly designed ground-source heat pump
system, neither cooling tower nor boiler may be
necessary that saves initial cost and floor
space. - Ground-source heat pump systems offer the
potential for operating-cost savings when
compared to the traditional cooling-tower-and-boil
er system. - However, a significant amount is on the
installation cost of the ground heat exchanger. - Installation requires excavation, trenching, or
boring, and in some areas there are very few
qualified contractors for installing the ground
heat exchanger.
56Ground-Source Heat Pump Systems
There are several types of ground-source
systems Ground-coupled system
- This system uses a closed system of special,
high-density polyethylene pipes that are buried
in the ground at a depth that takes advantage of
the earths natural heat sink capabilities. - When the building cooling load causes the
temperature of the water loop to rise, heat is
transferred from the water, flowing through the
buried pipes, to the cooler earth. - Conversely, when the temperature of the water
loop begins to fall, the water flowing through
the buried pipes absorbs heat from the earth. - In a properly designed, ground-coupled system,
operating and maintenance costs are low because a
cooling tower and boiler are not required in the
system.
57Ground-Source Heat Pump Systems
Pipe pattern
- The pipes that make up the ground heat exchanger
can be oriented in a vertical or horizontal
pattern. - The choice depends on available land, soil
conditions, and excavation costs.
Vertical loops
- Vertical loops are the most common in commercial
applications due to the limited land generally
available. - Vertical bore holes are drilled to depths of 60
to 150 m, with a diameter of 10 to 20 cm each.
58Ground-Source Heat Pump Systems
Horizontal loops
Horizontal loops are often considered when
adequate land is available. Historically,
horizontal loops consisted of a single layer of
pipe buried in the ground using a trencher.
59Ground-Source Heat Pump Systems
Multiple-layer horizontal loops
- With the limited of land for installation,
multiple-layer horizontal loops have been
adopted. - While less land and trenching is required, more
total length of piping is required compared to a
single layer loop. - The pipes are placed in trenches, typically 1.8 m
deep and spaced 1.8 to 4.6 m apart. - Trench length can range from 8.7 to 34.7 m/kW.
60Water-to-water Heat Pump Unit Selection Procedure
Determine the system design conditions for
source and load-side(s) of the equipment
Entering liquid temperatures for the source-side
can be-1.1oC to 49oC Entering liquid temperatures
for the load-side 7oC to 49oC Define the
selection parameters. Entering water
temperature, Fluid flow rate, and Fluid
pressure drop. Determine unit requirements.
Total cooling capacity/total heating Staging of
capacity to satisfy cooling requirements.
Pressure drop reduction through the load-side of
multiple units, even when a single unit might
meet capacity. Antifreeze will be required in the
fluid loop if source-side leaving water
temperature falls below 1oC.