This document discusses water to water heat recovery concepts and applications. It begins with an overview of industry trends and topics to be covered, including the basics of heat pumps and various heat recovery arrangements. It then provides examples of heat pump applications for hospital and university preheating, hotel domestic hot water heating, and industrial process water heating. Overall economics are evaluated for different applications. Design considerations like temperature ranges, control schemes, and water quality are also addressed.

1. Water to Water Heat Recovery
Concepts and Applications
Christian Rudio
Product Manager
Johnson Controls, Inc

2. Trends and Topics
Industry Trends
Energy Costs
Green building movement
Globalization – impact of Europe, Canada
Manufacturer support and new products
Topics
Fundamentals
Basic economics – the case for heat pumps
Heat pump water distribution systems
Heat pump arrangements
Application examples
Other heat recovery
Questions
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3. 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
Heat is rejected
Hot liquid
2
Condenser Hot high-
3 pressure gas
Expansion Work in Compressor
Valve
4 1
Evaporator
Cold low-
Cold liquid
pressure gas
Heat is absorbed
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4. 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.
Heat is produced
Hot liquid
2
Condenser Hot high-
3 pressure gas
Expansion Work in Compressor
Valve
4 1
Evaporator
Cold low-
Cold liquid
pressure gas
Heat is absorbed
4

5. 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.
Chiller Heat Pump
Heat is rejected Heat is produced
Hot liquid Hot liquid
2 2
Condenser Hot high- Condenser Hot high-
3 pressure gas 3 pressure gas
Work in Work in
Expansion Compressor Expansion Compressor
Valve Valve
4 1 4 1
Evaporator Evaporator
Cold low- Cold low-
Cold liquid Cold liquid
pressure gas pressure gas
Heat is absorbed Heat is absorbed
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6. 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
Some heat is rejected Some heat is diverted and used
Hot liquid
2
Condenser Hot high-
3 pressure gas
Work in
Expansion Compressor
Valve
4 1
Evaporator
Cold low-
Cold liquid
pressure gas
Heat is absorbed
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7. Other energy recovery methods
Double Bundle Water to Water
Desuperheater
Condenser Heat Pump
• Heat exchanger in • Condenser circuit • Unit operating as
compressor with “4-pipe” heating device
discharge line configuration – • 100% recovery of
• 5-15% heat recovery separate loop for cooling load plus
• Highest heat work input
temperatures • 10-20% heat • Direct control of
possible recovery water temperature
• No direct control of • No direct control of
water temp water temperature
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
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8. 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
from fuel.
( Q = heat removed from
cooling load and W is work
Heating COP is calculated as: input to compressors)
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.
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9. The COP Advantage
Simultaneous Heating and Cooling
Combined COP
When machine is providing useful heating and cooling, combined COP is:
Because
Substitute for
Yields
Compared to
The benefit of combined heating and cooling is more than double the cooling COP for
the same given conditions.
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10. 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
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
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11. Water Distribution Systems
Dedicated Heat Pump Change-over Systems
Condenser water loop is dedicated to useful Condenser water loop can reject heat to a cooling
heating. tower (chiller mode), or divert it to provide useful
heating (heat pump mode).
– Best when the heating load is consistently – Additional Heat Sink allows chiller operation
higher than heating capacity of the unit when heating load is lower than unit capacity
Heat
Sink
Heat
Load Heat
Warm water Hot water Load
Warm water Hot water
Cold water Cool water
Cooling Cold water Cool water
Load Cooling
Load
Dedicated System Change-over System
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12. Dedicated Heating Loop Example
Condenser water loop is dedicated to useful heating.
– Best when the heating load is consistently higher
Preheated domestic Domestic cold
than heating capacity of the unit water 80 F water 50 F
Heat
Exchanger
108 F 120 F
Heat
Load
Warm water Hot water
Higher evap temps
improve unit efficiency
(reduce lift)
48F 54 F
Cold water Cool water CHWR to 54 F
Heat
central plant
Sink 53 F 54 F
Dedicated System Example
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13. 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 water
Warm water Hot water Warm water
Heat
Load
Heat
Sink
Cold water Controlled Controlled Cold water
Cool water Heat
Single Unit Sink
Cool water
Multiple Units in Parallel
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14. 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).
Heat
Load
100 F 130 F
115 F (controlled)
50 F
40 F 60 F
(controlled) Cooling
Load
Two Units - Series Counter-flow Arrangement
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15. 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
Central Heating Plant
Heat Pump
Central Chiller Plant
Central Plant Chilled Water Return
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16. 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
Load
110 F 120 F
Small Heat Pump
50 F 60 F
50 F from 60 F to
cooling tower cooling tower
Large Chiller(s)
36 F 46 F
Cooling
Load
Cascade Arrangement
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17. 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
Heating Loop Return
VAV or
Perimeter
Heat Loop
Heat Pump
Central Chiller Plant
Central Plant Chilled Water Return
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18. 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.
Domestic Cold Water
Cooling Tower Domestic Hot Water
Water
Heaters
Heat
Exchanger
Small Heat Pump
Large Chiller(s)
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19. 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
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
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20. 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
Mixing Tank
Process Water Return Process Water Supply
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21. 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
Heat Produced 2.68 MMBH 2.68 MMBH
Hours Run 8000 8000
Annual Heat Cost $ 289,229 $ 150,509
Annual Savings $ 138,720
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22. 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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23. 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
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
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24. 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
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
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