Heat Pump

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MEBS 6008Heat Pumps

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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.

What Is a Heat Pump?

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• 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.

What Is a Heat Pump?

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• 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 Cycles

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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.

Introduction of heat source and heat pump system

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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.

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• 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.

Review of a Typical Vapour Compression Cycle

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where h4’ h1’ = enthalpy of refrigerant entering and leaving evaporator, respectively, J /kgWin = work input, J/kg The coefficient of performance of the heating effect in a heat pump system COPhp is

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.

Heat Pump Cycle

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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.

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• 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.

Mechanical vapor recompression cycle with heat exchanger


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• 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.

Open vapor recompression cycle


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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.


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• 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.

Air

HEAT SOURCES AND SINKS

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• 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.


Air

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• 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).


Air

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• 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.


Air

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• 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 SOURCES AND SINKSWater

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• 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?)


Water

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• 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.

WaterHEAT SOURCES AND SINKS

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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.


Ground

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• 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.

Solar EnergyHEAT SOURCES AND SINKS

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There are two basic types of solar-source heat pumps systems — direct and indirect.


Solar Energy

• 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.

Direct

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Indirect systemAn 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.

Solar EnergyHEAT SOURCES AND SINKS

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• 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.

AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat Pumps)

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Split System Heat PumpRoof top package unit


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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.

Most air-source heat pumps consist of :AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat Pumps)

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• 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.

Indoor Coil


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• 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.

Outdoor Coil


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• 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.

Reversing Valve

• Reciprocating and scroll compressors are widely used in heat pumps.Compressor.


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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.


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• 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.

Cooling ModeAIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat Pumps)

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• 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.

Heating Mode


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• 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.

Heating Mode


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• 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.

Cycling Loss and Degradation Factor


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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

Water-to-Air Heat Pumps

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• 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.


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

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• They use the high internal cooling load generated in modern buildings either directly or with storage.

• These include water loop heat pumps.


Internal-source 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.

Solar-assisted heat pumps

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• 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.

Water-to-Air Heat PumpsWastewater-source heat pumps

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• 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.

Water-to-Water Heat Pumps

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• 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.

Ground-Coupled Heat Pumps.

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Direct expansion ground-coupled heat pumps use refrigerant in direct expansion, or flooded evaporator circuits for the ground pipe coils.

Ground-Coupled Heat Pumps

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• 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.

Ground-Coupled Heat Pumps

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• 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.

Refrigerant-to-water heat exchanger

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• 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.

Benefits of using water-source heat pump

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• 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.

Key issues associated with water source heat-pump system.

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• 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.

Water-source heat pumps Configurations

Horizontal units

Configurations available to suit various building types.

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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.

Water-source heat pumps

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• 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.

Console units


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• 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.

Vertical-stack unitsWater-source heat pumps

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Heat adder & rejecter

Use of water to water heat pumpGround loop


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• 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.

Warm weather

Water-source heat pumpsOperating strategy

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• 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.

Cold weather


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• 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.

Mild Weather


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• 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.

Ground-Source Heat Pump Systems

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• 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.


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• 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.

There are several types of ground-source systemsGround-coupled system


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• 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.

Vertical loops

• 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.

Pipe patternGround-Source Heat Pump Systems

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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.

Horizontal loops


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• 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.

Multiple-layer horizontal loops


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Water-to-water Heat PumpUnit 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 49oCEntering 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.

Heat Pump

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