1. Lecture 21: Introduction to Primary Systems (Central Plants)
Notes: __________________________________________________________________
________________________________________________________________________
Material prepared by GARD Analytics, Inc. and University of Illinois
at Urbana-Champaign under contract to the National Renewable Energy
Laboratory. All material Copyright U.S.D.O.E. - All rights reserved

2. 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

3. 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

4. Cooling Equipment Chillers: Compression-Based and Absorption
Heat Pumps
Rooftop/DX Packaged Units
Thermal Energy Storage (Water and Ice)

5. 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.)

6. Compression Cycle Typical compression cycle diagram: QC Work QE
Condenser
Evaporator
Compressor
Expansion
Valve
High
Pressure
Low

7. 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

8. 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

9. 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)

10. 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

11. 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

12. Condensers (cont’d) Air-Cooled Condensers Evaporative 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

13. 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

14. Condenser Examples

15. Condenser Examples (cont.)

16. Digital images on this slide courtesy of: Lisa Fricker, Graduate Student, UIUC

17. Condenser Examples (cont.)

18. 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

19. 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

20. 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

21. 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

22. Compression Cycle: Big Picture
Condenser
Evaporator
Compressor
Expansion
Valve
Cooling Coil
Air System
To Zones…
Cooling Tower
Direction of heat transfer

23. 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)

24. 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

25. 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

26. 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

27. Absorption Chillers (cont’d)
Performance Issues
Capacities typically range from 180-almost 6000 kW (big!) though smaller units on the range of kW available internationally
Typical COP values are much lower than for compression cycle chillers: 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

28. 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.)

29. 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

30. 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

31. 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?

32. 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

33. 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

34. Boiler Furnace Heat Pump
Heating Equipment
Boiler
Furnace
Heat Pump

35. Heating Equipment Electric resistance heating
Heat pump in heating mode
Solar panels
Boiler
Water
Steam
Furnace (air)
same basic principle,
just a different fluid

36. 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

37. Boiler Uses Steam: Water: Heating coils (reheat, preheat)
Hot water heat exchangers
Absorption cooling
Laundry
Sterilizers
Water:
Domestic hot water

38. Boilers: Basic Layout Goal: stack/flue/
Try to get most efficient transfer of heat from flue gas (combustion products) to water
stack/flue/
chimney
air/fuel
mix
burner
water

39. Boiler Example (continued)
Digital image on this slide courtesy of: Lisa Fricker, Graduate Student, UIUC

40. Boilers: Types Dry Base/Back Wet Base/Back/Leg Condensing
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

41. Boilers: Efficiency Fuel Boiler (combustion efficiency)
Efficiency = (input – stack loss) / input
Non-condensing  75-86%
Condensing  %
Electric Boiler (overall efficiency)
Efficiency = output / input
Range of efficiencies  92-96%

42. Furnaces Heats air indirectly Fuels:
Combustion products do not mix with circulated air  dangerous
Fuels:
Natural gas (most common)
LPG (liquefied petroleum gas)
Oil
Electric

43. Furnaces (continued) Sizes: Various configurations:
Residential units (smallest)
Commercial (44  600+ kW)
Generally smaller than boilers
Various configurations:
Combustion systems
Air flow variations (single/multi-pass)

44. Furnace (AHU) Example

45. Boiler/Furnace Stack

46. 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

47. Big Picture Review A Building and its HVAC System Zone (Loads)
mix box
air
supply fan
surroundings
Secondary System
heating coil
cooling coil
pump
pump
boiler
chiller
A Building
and its
HVAC
System
Primary System
pump
cooling
tower

48. 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