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MEBS 6008

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MEBS 6008 Heat Pumps Heat Pump For split packaged air-source heat pumps, indoor coils are located inside the building and outdoor coils are mounted outdoors. – PowerPoint PPT presentation

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Title: MEBS 6008


1
MEBS 6008 Heat Pumps
2
What 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.

3
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.

4
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.
  •  

5
Introduction 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.

6
Review 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.

7
Review 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.

8
Heat 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 
9
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.

10
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.

11
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
    lower-than-desired pressure.
  • The heat is pumped to a higher pressure by
    compressing the lower pressure steam.

12
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.

13
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.

14
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.

15
HEAT 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).
  •  

16
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.

17
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 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.

18
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 Kongs
    situation?)

19
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.

20
HEAT 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.

21
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.

22
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.

23
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
  • 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.

24
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.

25
AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Split System Heat Pump
Roof top package unit
26
AIR-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.

27
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.

28
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.

29
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 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.

30
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.

31
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.

32
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.

33
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.

34
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.

35
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
36
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.

37
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.
  •  

38
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.

39
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.

40
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. 

41
Ground-Coupled Heat Pumps
Direct expansion ground-coupled heat pumps use
refrigerant in direct expansion, or flooded
evaporator circuits for the ground pipe coils.
42
Ground-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.

43
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.

44
Benefits 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.

45
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.

46
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.

47
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.

48
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.

49
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.

50
Heat adder rejecter
Water-source heat pumps
Use of water to water heat pump
Ground loop
51
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.

52
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.

53
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.

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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 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.

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Ground-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.

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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 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.

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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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