Water to Water Heat Recovery

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Presentation by Christian Rudio, STC water cooled product manager, Johnson Controls, at the May 11, 2010 Illinois Chapter ASHRAE seminar.

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Water to Water Heat Recovery

  1. 1. Water to Water Heat Recovery Concepts and Applications Christian Rudio Product Manager Johnson Controls, Inc
  2. 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 2
  3. 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 3
  4. 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. 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 5
  6. 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 6
  7. 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 7
  8. 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. 8
  9. 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. 9
  10. 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 10
  11. 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 11
  12. 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 12
  13. 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 13
  14. 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 14
  15. 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 15
  16. 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 16
  17. 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 17
  18. 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) 18
  19. 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 19
  20. 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 20
  21. 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 21
  22. 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 22
  23. 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 23
  24. 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 24

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