water to water heat recovery
DESCRIPTION
Presentation by Christian Rudio, STC water cooled product manager, Johnson Controls, at the May 11, 2010 Illinois Chapter ASHRAE seminar.TRANSCRIPT
Water to Water Heat Recovery
Concepts and Applications
Christian Rudio
Product Manager
Johnson Controls, Inc
Trends and Topics
Industry Trends
� Energy Costs
� Green building movement
� Globalization – impact of Europe, Canada
� Manufacturer support and new products
Topics
� Fundamentals� Fundamentals
� Basic economics – the case for heat pumps
� Heat pump water distribution systems
� Heat pump arrangements
� Application examples
� Other heat recovery
� Questions
2
Basic Refrigeration Cycle
Fluid refrigerant absorbs heat from a load and rejects it to a sink
4 basic parts: compressor, condenser, expansion device, evaporator
1 to 2: Compress cold low-pressure gas to hot high-pressure gas
2 to 3: Reject heat to the sink, refrigerant condenses to hot liquid
3 to 4: Lower refrigerant temperature by rapidly lowering pressure
4 to 1: Evaporate refrigerant to absorb heat from the load
Hot liquid
Heat is rejected
3
Compressor
Condenser
Evaporator
ExpansionValve
Cold liquid
Hot liquid
Hot high-pressure gas
Cold low-pressure gas
Heat is absorbed
Work in
14
3
2
What is a heat pump?
Definition: A heating device that moves heat from low to high temperature.
Reversing type: Reversing systems change refrigerant flow direction with a reversing valve.
Each heat exchanger can act as an evaporator or a condenser depending on refrigerant flow
direction.
Non-reversing type: Evaporator and condenser do not change roles.
Hot liquid
Heat is produced
2
4
Compressor
Condenser
Evaporator
ExpansionValve
Cold liquid
Hot liquid
Hot high-pressure gas
Cold low-pressure gas
Heat is absorbed
Work in
14
3
2
When is a chiller not a chiller?
When machine is making hot water, it’s a heat pump, cold water is by-product.
When machine is making cold water, it’s a chiller, hot water is by-product.
Control condenser water temp or evaporator water temp – not both simultaneously.
Hot liquid
Heat is rejected
Hot liquid
Heat is produced
Chiller Heat Pump
5
Compressor
Condenser
Evaporator
ExpansionValve
Cold liquid
Hot liquid
Hot high-pressure gas
Cold low-pressure gas
Heat is absorbed
Work in
14
3
2
Compressor
Condenser
Evaporator
ExpansionValve
Cold liquid
Hot liquid
Hot high-pressure gas
Cold low-pressure gas
Heat is absorbed
Work in
14
3
2
Heat Pump vs. Energy Recovery
A heat pump’s purpose is to heat.
Energy recovery occurs when we extract waste heat from a chiller’s condenser and use it.
Control point is still chilled water set point.
Chiller with energy recovery
Condenser
Hot liquid
Hot high-
Some heat is rejected
2
Some heat is diverted and used
6
Compressor
Condenser
Evaporator
ExpansionValve
Cold liquid
Hot high-pressure gas
Cold low-pressure gas
Heat is absorbed
Work in
14
3
Other energy recovery methods
Desuperheater
• Heat exchanger in compressor discharge line
• 5-15% heat recovery
• Highest
Double Bundle Condenser
• Condenser circuit with “4-pipe” configuration –separate loop for heat
Water to Water Heat Pump
• Unit operating as heating device
• 100% recovery of cooling load plus work input
7
Water to Water Heat Pumps offer the most heat recovery, low first cost, direct control of
water temperature and most comply with ASHRAE 90.1 efficiency standards when operating
as a chiller
• Highest temperatures possible
• No direct control of water temp
heat
• 10-20% heat recovery
• No direct control of water temperature
work input
• Direct control of water temperature
The COP Advantage
Coefficient of Performance
� For a heat pump, COP = (Heat output) / (Work input)
� For electric resistive heaters, COP = 1. Heat output is equal to electrical power input.
� For fuel burning heaters with heat exchangers (like boilers), COP < 1.
� For heat pumps, COP > 1, often 2 < COP < 6.
How can heat pumps “produce” more heat than the input power?
Because heat pumps move heat from one place to another. The largest part of the
heating effect comes from heat that is pumped; not created, produced, or converted
8
from fuel.
Heating COP is calculated as:
in other words, Heating COP = (Heating effect) / (Work input)
How can heat pumps be more efficient than the chiller they’re based on?
Chiller COP is calculated as:
Therefore chiller COP will be slightly lower than heat pump COP for the same machine.
(∆Q = heat removed from cooling load and ∆W is work input to compressors)
The COP Advantage
Simultaneous Heating and Cooling
Combined COP
When machine is providing useful heating and cooling, combined COP is:
Because
9
Substitute for
Yields
Compared to
The benefit of combined heating and cooling is more than double the cooling COP for the same given conditions.
Specific Savings Example
COP – The economic lever
Quick cost analysis based on 165 ton positive displacement heat pump:
Heating Temperature 110 F � 125 F, 390 gpm
Evaporator water from 54 F � 44 F
(Illinois 2008 utility rates)
Boiler Heat Pump
10
Boiler Heat Pump
COP 0.85 3.55
Energy/Fuel Cost $11.49/MMBTU $0.0854/kWh
Heat Produced 2.68 MMBH 2.68 MMBH
Hours Run 4000 4000
Annual Heat Cost $ 144,614 $ 75,254
Annual Savings $ 69,360
Water Distribution Systems
Dedicated Heat Pump
Condenser water loop is dedicated to useful
heating.
– Best when the heating load is consistently
higher than heating capacity of the unit
Change-over Systems
Condenser water loop can reject heat to a cooling
tower (chiller mode), or divert it to provide useful
heating (heat pump mode).
– Additional Heat Sink allows chiller operation
when heating load is lower than unit capacity
Heat
Heat Sink
11
Dedicated System
Cooling Load
Heat Load
Hot waterWarm water
Cold water Cool water
Change-over System
Heat Load
Hot waterWarm water
Cold water Cool waterCooling
Load
Dedicated Heating Loop Example
Condenser water loop is dedicated to useful heating.
– Best when the heating load is consistently higher
than heating capacity of the unit
Heat Load
Hot waterWarm waterHigher evap temps
120 F108 F
HeatExchanger
Domestic coldwater 50 F
Preheated domesticwater 80 F
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Heat Sink
Dedicated System
Hot water
Cool water
Warm water
Cold water
Higher evap temps improve unit efficiency
(reduce lift)
Example
CHWR tocentral plant
54 F
54 F48F
54 F
53 F
Heat Pump Arrangements
Single Unit or Multiple Parallel Units
One unit or a team of parallel units make hot water.
– Advantages: Relatively simple piping and controls. Higher flow capacity.
– Disadvantages: Can only control hot or cold side. Limited temperature difference.
Heat Load
Hot waterWarm water
Heat
Load
Warm waterHot water
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Heat Sink
Cold water ControlledCool water Heat
Sink
Controlled Cold water
Cool waterSingle Unit
Multiple Units in Parallel
Heat Pump Arrangements
Series Counterflow Units
Two chillers with series flow through the condensers and evaporators
– Advantages: Larger temperature differences are possible. Can control cooling with
one machine and heating with the other.
– Disadvantages: More complicated. Controls are critical. Flow must be the same
through both machines (machines similar or identical size).
100 F
Heat Load
130 F(controlled)115 F
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Cooling Load
40 F(controlled)
60 F50 F
Two Units - Series Counter-flow Arrangement
Applications: Hot Water Preheat
Hospitals/Universities/Schools/Laboratories/Offices
– Buildings with fairly constant heating and cooling load profiles that require simultaneous
heating and cooling.
– Boiler feed water and/or domestic hot water is preheated to reduce fuel consumption.
Heating Plant Return Water
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Central Plant Chilled Water Return
Central Chiller Plant
Central Heating PlantHeating Plant Return Water
Heat Pump
Heat Pump Arrangements
Cascade Chillers
– Advantages: Large temperature difference between heating and cooling loads. Can
control high and low temperature sides simultaneously.
– Disadvantages: More complicated. Condenser water treatment is critical. Controls
are critical. Geographically or seasonally limited (cooling tower temperatures).
Heat
Load120 F110 F
Small Heat Pump
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Cooling Load
36 F 46 F
60 F to cooling tower
50 F from cooling tower
60 F50 F
Large Chiller(s)
Small Heat Pump
Cascade Arrangement
Applications: Perimeter Reheat
Hospitals/Universities/Laboratories
– Buildings with fairly constant heating and cooling load profiles that require simultaneous
heating and cooling.
– VAV or perimeter heating loop primary heat source is heat pump; boiler used to
supplement as necessary for heating demand
– Previous economic example a good representation of Perimeter Reheat (50% run hours)
Supplemental
Boiler
17
Central Plant Chilled Water Return
Central Chiller Plant
Heating Loop Return
Heat Pump
VAV or Perimeter
Heat Loop
Applications: Hotel
Hotel Domestic Hot Water, or Laundry Water, or Pool Water Heating
– Typically need cooling in the building core, even in the winter; hot water is always in
demand.
– Use a cascade system to preheat domestic water.
Cooling Tower
Water
Heaters
Domestic Cold Water
Domestic Hot Water
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Large Chiller(s)Small Heat Pump
Heaters
Heat
Exchanger
Application Economics: Hotel
Hotel Domestic Hot Water, or Laundry Water, or Pool Water Heating
– Hotel in Wyoming where cooling tower water temperatures are useful for 1750 hours per
year (20%).
– Representative of a cascade system, where run hours are limited
– Same 165 ton heat pump as previous example
Boiler Heat Pump
COP 0.85 3.55
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COP 0.85 3.55
Energy/Fuel Cost $8.58/MMBTU $0.0667/kWh
Heat Produced 2.68 MMBH 2.68 MMBH
Hours Run 1750 1750
Annual Heat Cost $ 47,236 $ 25,715
Annual Savings $ 21,521
Application Example: Process/Manufacturing
Process/Manufacturing
– Process applications often have continuous and simultaneous heating and cooling needs.
– A series counter-flow arrangement allows for larger temperature differences and good
control on both hot and cold sides.
Heat Load
20
Mixing TankProcess Water Return Process Water Supply
Application Economics: Process/Manufacturing
Process/Manufacturing
– Brewery in IL runs continuously and can use heat pumps for 8000 hours per year
Boiler Heat Pump
COP 0.85 3.55
Energy/Fuel Cost $11.49/MMBTU $0.0854/kWh
21
Energy/Fuel Cost $11.49/MMBTU $0.0854/kWh
Heat Produced 2.68 MMBH 2.68 MMBH
Hours Run 8000 8000
Annual Heat Cost $ 289,229 $ 150,509
Annual Savings $ 138,720
Application Consideration
Water temperature
Hotter water, less efficiency
� Operating cost vs. first cost (kW’s vs. coil rows)
� Higher temperatures a good fit for:
� Boiler pre-heat
� Retrofit projects (difficult to change air side coils)
� Up to 160F available commercially
� Equipment may not meet ASHRAE 90.1 chiller requirements
� Lower temperatures a good fit for:
� Perimeter reheat – coils can be sized for temperature
� New construction – additional coil row a small incremental cost
� Up to 140F – wide selection of equipment available, meets chiller efficiency requirements
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Application Consideration
Water temperature
Operating Cost Comparison
� 200 ton chiller with 20º F temperature difference across condenser
� Case #1: 120º to 140º F , heating-only COP 3.16, 3308 MBH heating, 193 tons cooling
� Case #2: 110º to 130º F, heating-only COP 3.70, 3308 MBH heating, 205 tons cooling
� Evaporator condition 54º to 44º F
� Illinois utility rates, 4000 run hours Case #1 Case #2
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Case #1 Case #2
Boiler Heat Pump Heat Pump
COP 0.85 3.55 3.55
Energy/Fuel Cost $11.49/MMBTU $0.0854/kWh $0.0854/kWh
Heat Produced 3.3 MMBH 3.3 MMBH 3.3 MMBH
Hours Run 4000 4000 4000
Annual Heat Cost $ 178,869 $ 104,871 $ 89,499
Annual Savings $ 73,997 $ 89,369
$15,000 Annual Savings for lower HWT – and more cooling capacity
Design Considerations
� Profile heating and cooling load profiles for properly designed system
� Buffer tanks can be critical between cascade and series systems, to add thermal mass
during quick temperature changes
� Control schemes must be carefully considered to avoid hunting
� When preheating domestic hot water, double heat exchanger must be used
� Water quality must be controlled as higher temperatures can accelerate fouling
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� Ground source should give careful consideration for water quality in evaporator
� Ground source typically leverage only heating or cooling COP, not combined
� Manufacturers can provide guidelines for equipment – temperature, flow limits – and
application advice