Transcript EnergyPlus Training Part 1

Lecture 21: Introduction to
Primary Systems (Central
Plants)
Material prepared by GARD Analytics, Inc. and University of Illinois
at Urbana-Champaign under contract to the National Renewable Energy
Laboratory. All material Copyright 2002-2003 U.S.D.O.E. - All rights reserved
Importance of this Lecture to the
Simulation of Buildings
Primary systems provide hot and chilled
water for the secondary systems as well
as other energy sources that are
needed by the building
Some knowledge of the primary
systems (central plants) is required to
accurately simulate buildings and to
understand what the model input
parameters are
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Purpose of this Lecture
Gain an understanding of:


Basic information about primary plants
(central plants)
Interconnection between primary plants
and the rest of the building
3
Cooling Equipment
Chillers: Compression-Based and
Absorption
Heat Pumps
Rooftop/DX Packaged Units
Thermal Energy Storage (Water and Ice)
Compression-Based Liquid
Chilling Systems
 Compression Chillers and Heat Pumps both work on
what is commonly referred to as a “vapor
compression cycle”


Thermodynamic cycle through which refrigerant goes
Refrigerant is enclosed within cycle components
 Components
 Condenser
 Compressor
 Evaporator (aka Liquid Cooler)
 Expansion Valve
 Primary and secondary fluids (refrigerant, water, etc.)
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Compression Cycle
Typical compression cycle diagram:
Condenser
High
Pressure
Low
Pressure
QC
Compressor
Work
Expansion
Valve
Evaporator
QE
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Compression-Based Liquid
Chilling Systems (cont’d)
 Cycle Details
 High pressure side: from compressor outlet through
condenser to expansion valve inlet
 Low pressure side: from expansion valve outlet through
evaporator to compressor inlet
 Utilize the fact that the boiling point of the refrigerant
changes as the fluid pressure changes: lower pressure
means a lower boiling temperature
 Refrigerant picks up heat in the evaporator (refrigerant
evaporates) because the chilled fluid temperature is higher
than the refrigerant temperature
 Refrigerant rejects heat in the condenser (refrigerant
condenses) because condenser fluid temperature is lower
than refrigerant temperature
 Compressor drives the cycle by compressing the refrigerant
through the addition of work
 First Law of Thermodynamics
7
Chillers/Heat Pumps for
Conditioning
 Cooling: Normal operation mode
 Goal is to provide cooling at the evaporator where
there is chilled water or air that is produced
 Coefficient of performance (COP) equal to cooling
achieved at the evaporator over the work required at
the compressor
 Heating: Reverse operation (heat pumps)
 Goal is to provide heating at the condenser where
there is hot water or air that is produced
 Typically this requires a reversal of refrigerant flow
 Coefficient of performance (COP) equal to heating
achieved at the condenser over the work required at
the compressor
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Chillers/Heat Pumps for
Conditioning (cont’d)
 Efficiency and Energy Issues

Work is required because we are trying to get heat to flow in a
direction that is counter the natural flow of heat (natural would be
from higher temperature to lower temperature)

COP is generally greater than 1.0 so we get more kW-h of cooling
or heating than electric kW-h that we put into the compressor

Performance (and COP) of the system is highly dependent on the
fluid temperatures that the condenser and evaporator are in
contact with
 Lower evaporator temperatures result in lower COP
 Higher condenser temperatures result in lower COP
 More extreme temperatures lower COP and can lower available
capacity

Temperature relation to performance can be a hindrance to the
system or a potential advantage
 Heat pump may struggle and require more energy as outside
temperatures become more extreme
 Presence of a more moderate/constant temperature source can keep
system running efficiently (e.g., ground)
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Chillers/Heat Pumps for
Conditioning (cont’d)
 Chiller vs. Heat Pumps—what’s the
difference?





Difference in system components: none
Chillers are generally cooling only device and are
used to produce chilled water for cooling coils
(size range can be quite large)
Heat pumps can provide both heating and cooling
and are typically smaller in size (often residential
units)
Heat pumps are typically compression cycle only
and almost all use electric energy as input
Chillers can use various cycles and may actually
use other energy sources as the system energy
input
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Condensers
 Purpose: to reject heat from refrigerant to
surrounding environment, condensing the refrigerant
from a (superheated) vapor to a (subcooled) liquid
 Condenser is really a “heat exchanger” which
transfers energy from one fluid stream to another
without mixing the two streams
 Water-Cooled Condensers


Heat exchanged with water which is circulated to another
“component” (ground, lake, pond—natural or constructed,
river, cooling tower, etc.) as closed or open loop
Condenser temperature depends on water source
temperature
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Condensers (cont’d)
 Air-Cooled Condensers
 Heat exchanged with outdoor air
 Fans required to improve heat transfer
 Condenser temperature linked to outside air dry bulb
temperature
 Evaporative Condensers
 Heat exchanged sensibly and latently with outdoor air
 Fan and pump required: fan to circulate air through unit,
pump to circulate water
 Added evaporation process increases performance
 Condenser temperature linked to outside wet bulb
temperature (less than or equal to dry bulb)
 Condenser water and evaporative water kept separat
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Condensers (cont’d)
Cooling Towers

Similar concept as evaporative condensers

Condenser water “open” in the tower


Some water evaporates, requiring make-up
water
Some systems eliminate the fan
requirement
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Condenser Examples
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Condenser Examples (cont.)
15
Digital images on this slide courtesy of:
Lisa Fricker, Graduate Student, UIUC
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Condenser Examples (cont.)
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Evaporators (Liquid Coolers)
 Purpose: to absorb heat in the refrigerant
from the surrounding environment,
evaporating the refrigerant from a liquid (or
liquid/vapor mixture) to a (superheated)
vapor
 Evaporator is also a heat exchanger
 Evaporator can be a cooling coil itself or a
refrigerant (DX or direct expansion coil) to
water heat exchanger to the chilled water
loop
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Heat Exchangers
 Heat Exchanger Types (largest to smallest):
 Shell-and-Tube
 Plate/Plate-and-Frame
 Tube-in-Tube
 Shell-and-Coil
 Heat Exchanger Issues:
 Larger exposed air means largest UA (more heat
transfer)
 Fouling can affect performance over time
(maintenance issues)
 Interior and exterior fins on coils
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Compressors
 Purpose: to compress the refrigerant vapor to a
higher pressure (also increases the temperature)
 Mechanical device: power input converted to
mechanical energy
 Types of Compressors:

Positive-displacement: “squeeze”—increase pressure be
decreasing vapor volume





Reciprocating
Rotary
Scroll
Trochoidal
Dynamic: “spin”—increase pressure by transferring angular
momentum, momentum converted to pressure increase
 Centrifugal

Centrifugal tend to be used in larger systems
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Compressors (cont’d)
Motor Types



Open: motor and compression chamber
separated via shaft link
Hermetic: motor and compression chamber
same, motor shaft and compressor
crankshaft integral
Semi-hermetic: bolted construction allows
field service
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Compression Cycle: Big
Picture
Direction of heat transfer
Cooling Tower
Expansion
Valve
Condenser
Compressor
Evaporator
Air System
To Zones…
Cooling Coil
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Absorption-Based Liquid Chilling
Systems
 Concept



Compression-based chillers use electrical energy (work) to produce
heating or cooling (in the opposite direction of natural energy flow)
Absorption-based chillers use mixture/solution chemistry and a heat
source to produce heating (reverse cycle—also called heat
transformer) or cooling (forward cycle—more common)\
Absorption-based systems are most effective when a “free” or very
inexpensive source of heat is available
 Solar energy
 “Waste” heat
 Heat source must be high enough quality (temperature) to
drive system


No compressor or other large rotating mechanical equipment
needed
Two “refrigerants”—primary and secondary (absorbent)
 Primary—usually water
 Secondary—usually ammonia or lithium bromide (LiBr)
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Absorption Chillers (cont’d)
 Components







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Generator (desorber)—high pressure side
Condenser—high pressure side
Evaporator—low pressure side
Absorber—low pressure side
Heat Exchanger
Pump
Expansion valve/flow restrictors
Refrigerants
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Absorption Chillers (cont’d)
 Cycle Details (LiBr system)
 Pure water (vapor/liquid) in the condenser and evaporator
 Primary refrigerant (water) and absorbent mixtures of
varying concentrations in generator and absorber
 Weak liquid solution is introduced into the generator along
with heat from some source
 Generator process: boils water out of solution accomplishing
two things
 Pure water vapor is sent over to condenser side of chamber
 Strong(er) solution (liquid) is sent to absorber
 Water vapor in condenser is converted to liquid (condensed)
by the removal/rejection of heat
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Absorption Chillers (cont’d)
 Cycle Details (LiBr system, cont’d)

Condensed water is pushed to the evaporator as a result of the pressure
difference/gravity

Liquid water in the evaporator is boiled off with the addition of heat at low
temperature/pressure

Water vapor boiled off from evaporator is sent to absorber

Absorber: Water vapor condenses (potential heat rejection) and gets
reabsorbed into the water-LiBr solution, weakening the solution

Absorber sends weakened solution back to generator where cycle starts
over again

Pumps used to send solution from absorber to generator and to circulate
liquid water over evaporator coil

Heat exchanger used between lines connection generator and absorber—
reduces heat addition needed in generator (improving efficiency)

Goal is cooling at the evaporator (forward cycle) or heating at the generator
(reverse cycle)

Many slight variations on this basic cycle
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Absorption Chillers (cont’d)
 Performance Issues
 Capacities typically range from 180-almost 6000 kW (big!)
though smaller units on the range of 18-35 kW available
internationally
 Typical COP values are much lower than for compression
cycle chillers: 0.7-0.8 or lower is common
 Low COP not necessarily a problem if heat source is free:
COP = Usable cooling/energy input
 Other Issues
 Is a heat source available that can be used?
 Concerns about water in contact with metal inside
absorption system (rust formation)
 Potential toxicity of absorbent
 Noise—far less than a compression cycle chiller
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Thermal Energy Storage
 Concept

Produce and store energy for use during another time
 Initially, this was as simple as cutting ice blocks from Lake
Michigan and storing those until summer
 Now, energy storage is produced during off-peak hours when
energy costs are lower


Overall dollar effect is a reduction in the conditioning costs for the
buildingprimary (or only) benefit is economic
Reduction in cost per kW-hr and reduction in demand costs
 Costs based on type of power plants running
 Cost of start-up and shutdown of power plants


Mainly an issue for industrial customers, usually used for cooling
Utilities have in the past actually paid (in part) for systems
 Reduced demand reduces need for new power plants
 Shift of electric load uses power that might not otherwise be
used (hydroelectric, nuclear, etc.)
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Thermal Energy Storage (cont’d)
 System Types
 Tempered Water Storage
 Storage of hot or cold water in a large tank above or
below grade
 Water is kept stratified, taking advantage of density





differences of water at different temperatures
Inlet diffusers must be designed to avoid mixing
Some energy transfer does occur between hot and cold
sides
Water in tank can serve as emergency water source in
case of fire
Water temperatures for cooling same as for standard
chiller only system
Large tank needs large space, tank losses
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Thermal Energy Storage (cont’d)
 System Types (cont’d)

Ice Storage
 Storage of cooling energy in the form of ice
 Latent heat of solidification allows large amount
of energy storage in a much smaller area than a
water system
 System types:





Ice-on-coil outside melt (obsolete)
Ice-on-coil inside melt
Encapsulated ice (ice container)
Ice harvester
Ice slurry
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Thermal Energy Storage (cont’d)
 Efficiency Issues (Ice Systems)
 Process for producing ice less efficient than chilled
water production (temperatures required for
making ice are much lower, resulting in lower
efficiency/COP and capacity of chiller)
 This may be offset somewhat be reduced
condenser temperatures due to cooler outdoor
conditions at night
 Systems can produce lower supply air
temperatures, reducing the flow rates needed to
provide same cooling (which lowers fan energy)
 Do ice storage systems save dollars and energy?
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Thermal Energy Storage Controls
 Full Storage (discharging)
 Minimizes on-peak energy consumption, maximizes
energy consumption shift
 Largest storage requirements and perhaps largest
chiller (and initial costs)
 Probably largest potential savings on operating costs
 Partial Storage (discharging)
 Types:
 Chiller priority: chiller runs during on-peak only up to
some set demand limit, ice meets all other needs
 Ice priority: storage meets demand up to some limit
and chiller is turned on if the demand is higher than the
limit


Some shift of energy consumption to off-peak, also
savings on demand costs
Smaller chiller requirements than full storage or no
storage
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Thermal Energy Storage Controls
(cont’d)
 Charging Strategies


Zero prediction—chiller charges system at
its capacity as soon as off-peak period starts
“Optimal” strategies
 Delay start of charging to take advantage of
presumably cooler outdoor air in early morning
hours
 And/or run chiller at less than full capacity at
whatever its optimal fraction of full load is
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Heating Equipment
Boiler
Furnace
Heat Pump
Heating Equipment
Electric resistance heating
Heat pump in heating mode
Solar panels
Boiler

Water

Steam
same basic principle,
just a different fluid
Furnace (air)
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Boilers
Definition: equipment whose sole
purpose is to provide hot water or
steam for various uses within a building
Size (capacity) range:
15 kW  30+ MW
Fuels: coal, wood, fuel oil, (natural) gas,
electricity
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Boiler Uses
Steam:
 Heating coils (reheat, preheat)
 Hot water heat exchangers
 Absorption cooling
 Laundry
 Sterilizers
Water:
 Heating coils (reheat, preheat)
 Domestic hot water
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water
water
water
water
Boilers: Basic Layout
stack/flue/ Goal:
chimney
Try to get most
efficient transfer
of heat from
flue gas
(combustion
products) to
water
burner
air/fuel
mix
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Boiler Example (continued)
Digital image on this slide courtesy of:
Lisa Fricker, Graduate Student, UIUC
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Boilers: Types
Dry Base/Back
Wet Base/Back/Leg

Base (bottom), back (with respect to multipass boilers), leg (top and sides)
Condensing


Flue gas condensing due to low return
temperature of water
More efficient, but potential for rust greatly
increased
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Boilers: Efficiency
Fuel Boiler (combustion efficiency)

Efficiency = (input – stack loss) / input
Non-condensing  75-86%

Condensing  88-95+%

Electric Boiler (overall efficiency)


Efficiency = output / input
Range of efficiencies  92-96%
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Furnaces
Heats air indirectly

Combustion products do not mix with
circulated air  dangerous
Fuels:

Natural gas (most common)

LPG (liquefied petroleum gas)

Oil

Electric
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Furnaces (continued)
Sizes:

Residential units (smallest)
Commercial (44  600+ kW)

Generally smaller than boilers

Various configurations:

Combustion systems

Air flow variations (single/multi-pass)
43
Furnace
(AHU)
Example
44
Boiler/Furnace Stack
45
Furnace Efficiency
ANSI/ASHRAE Standard 103
 Annual Fuel Utilization Efficiency (AFUE)
Usable Heat Output
AFUE 
Fuel Input


AFUE includes: latent and sensible losses,
cyclic effects, infiltration, pilot burner
effects, and losses from a standing pilot
when furnace not in use
AFUE  78-80% for non-condensing, 90+%
for condensing
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Big Picture Review
Zone (Loads)
mix box
air
supply fan
surroundings
Secondary
System
heating coil
pump
A Building
and its
HVAC
System
boiler
Primary System
cooling coil
pump
chiller
pump
cooling
tower
47
Summary
Primary systems convert one form of
energy (fuel, electricity, etc.) to thermal
energy
Chillers/heat pumps are used to provide
cooling (direct expansion or chilled
water)
Boilers are used to provide steam or hot
water for heating coils
Furnaces are used to provide hot air
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