MEBS 6008
Heat Pumps
Heat Pump
1
What Is a Heat Pump?
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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.
Heat Pump
2
What 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.
Heat Pump
3
Heat 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.
Heat Pump
4
Introduction of heat source and heat pump system
Heat Pump
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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.
5
Review of a Typical Vapour Compression Cycle
Heat Pump
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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.
6
Review of a Typical Vapour Compression Cycle
Heat Pump
•
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.
7
Heat Pump Cycle
A heat pump cycle comprises the same processes
and sequencing order as a refrigeration cycle
except that the refrigeration effect q1’4 or qrf,
and the heat pump effect q2’3’ ,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
Heat Pump
8
Basic 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.
Heat Pump
9
Basic 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.
Heat Pump
10
Basic 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 lowerthan-desired pressure.
•
The heat is pumped to a higher pressure by compressing the lower
pressure steam.
Heat Pump
11
Basic 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.
Heat Pump
12
HEAT 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.
Heat Pump
13
HEAT 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.
Heat Pump
14
HEAT SOURCES AND SINKS
Air
•
When the surface temperature of an outdoor air coil is 0°C 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 −10°C and 60% rh (confirmed by psychrometric
analysis).
Heat Pump
15
HEAT 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.
Heat Pump
16
HEAT 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 HK’s 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.
Heat Pump
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HEAT 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 Kong’s situation?)
Heat Pump
18
HEAT 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.
Heat Pump
19
HEAT SOURCES AND SINKS
Ground
Heat Pump
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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.
20
HEAT 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.
Heat Pump
21
HEAT 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.
Heat Pump
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HEAT 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 :
Heat Pump
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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.
23
AIR-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.
Heat Pump
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AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat Pumps)
Roof top package unit
Heat Pump
Split System Heat Pump
25
AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat Pumps)
Most air-source heat pumps consist of :
Heat Pump
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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.
26
AIR-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.
Heat Pump
27
AIR-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.
Heat Pump
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AIR-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 flow—
compressor suction, compressor discharge, evaporator outlet, and
condenser inlet—causes 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.
Heat Pump
29
AIR-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.
Heat Pump
30
AIR-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.
Heat Pump
31
AIR-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.
Heat Pump
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AIR-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.
Heat Pump
33
AIR-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.
Heat Pump
34
Water-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
Heat Pump
35
Water-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.
Heat Pump
36
Water-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.
Heat Pump
37
Water-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.
Heat Pump
38
Water-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.
Heat Pump
39
Ground-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.
Heat Pump
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Ground-Coupled Heat Pumps
Direct expansion ground-coupled heat pumps use refrigerant in
direct expansion, or flooded evaporator circuits for the ground
pipe coils.
Heat Pump
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Ground-Coupled Heat Pumps
Heat Pump
•
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.
42
Refrigerant-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.
Heat Pump
43
Benefits of using water-source heat pump
•
In the heat recovery mode => 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.
Heat Pump
44
Key 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.
Heat Pump
45
Water-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.
Heat Pump
46
Water-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.
Heat Pump
47
Water-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.
Heat Pump
48
Water-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.
Heat Pump
49
Heat adder & rejecter
Water-source heat pumps
Ground loop
Heat Pump
Use of water to water heat pump
50
Water-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.
Heat Pump
51
Water-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.
Heat Pump
52
Water-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.
Heat Pump
53
Ground-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 earth’s relatively constant
temperature, and use the ground or surface water as the heat
rejecter and heat adder.
•
Ground-source heat pump systems don’t actually get rid of heat—
they 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.
Heat Pump
54
Ground-Source Heat Pump Systems
Heat Pump
•
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-boiler 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.
55
Ground-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
earth’s 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.
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Ground-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.
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Ground-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.
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Ground-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.
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Water-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.
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