Lecture 21: Introduction to Primary Systems (Central Plants)

Material prepared by GARD Analytics, Inc. and University of Illinoisat Urbana-Champaign under contract to the National Renewable Energy

Laboratory. All material Copyright 2002-2003 U.S.D.O.E. - All rights reserved

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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

Cooling Equipment

Chillers: Compression-Based and Absorption

Heat Pumps

Rooftop/DX Packaged Units

Thermal Energy Storage (Water and Ice)

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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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Condenser

Evaporator

Compressor

ExpansionValve

Compression Cycle

Typical compression cycle diagram:

QE

QC

WorkHigh

Pressure

LowPressure

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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

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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.)

16Digital 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 exchangerEvaporator 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

Condenser

Evaporator

Compressor

ExpansionValve

Cooling CoilAir System To Zones…

Cooling Tower

Dir

ect

ion

of

heat

tran

sfer

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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 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 buildingprimary (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

Heating Equipment

Boiler

Furnace

Heat Pump

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Heating Equipment

Electric resistance heatingHeat pump in heating modeSolar panelsBoiler

Water Steam

Furnace (air)

same basic principle,just a different fluid

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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+ MWFuels: 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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Boilers: Basic Layout

Goal:

Try to get most efficient transfer of heat from flue gas (combustion products) to water

stack/flue/chimney

air/fuelmix

burner

wate

r

wate

r

wate

r

wate

r

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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/BackWet Base/Back/Leg

Base (bottom), back (with respect to multi-pass 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)

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Furnace (AHU)

Example

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Boiler/Furnace Stack

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Furnace Efficiency

ANSI/ASHRAE Standard 103 Annual Fuel Utilization Efficiency

(AFUE)

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

InputFuelOutputHeatUsable

AFUE

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Big Picture Review

air

surroundings

Zone (Loads)mix box

supply fan

cooling coilheating coil

chillerboiler

coolingtower

pumppump

pump

Secondary System

Primary System

A Buildingand itsHVAC

System

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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