Lecture Objectives: Continue with Sorption Cooling Thermodynamics of mixtures T - x diagram H- x diagram

Combined heat and power (cogeneration CHP or three generation CCHP) Here, we use thermal energy for heating and/or cooling

Absorption Cycle Same as vapor compression but NO COMPRESSOR Replace compressor

Absorption cooling cycle Relatively simple thermodynamics with addition of mixtures (water – ammonia) Rich solution of Heat H2O H2O + NH3 Rich solution of H2O H2O + NH3

Mixtures (T-x diagram) Dew point curve Saturated vapor Mixture of liquid and vapor Saturated liquid Bubble point curve For P= 4 bar

Impact of Pressure

h-x diagram hfg hfg Isotherms are ploted only in liquid region for H2O for NH3 Isotherms are ploted only in liquid region

Composition of h-x diagram Saturated vapor line at p1 Equilibrium construction line at p1 1e Used to determine isotherm line in mixing region! Start from x1; move up to equilibrium construction line; move right to saturated vapor line; determine 1’; connect 1 and 1’. Isotherm at P1 and T1 Adding energy B A x1 X1’ mass fraction of ammonia in saturated vapor

h-x diagram at the end of your textbook you will find these diagrams for 1) NH3-H2O 2) H2O-LiBr LiBr is one of the major liquid descants in air-conditioning systems

Adiabatic mixing in h-x diagram (Water – Ammonia) From the textbook (Thermal Environmental Eng.; Kuehen et al)

Absorption cooling cycle Rich solution of Heat H2O H2O + NH3 Rich solution of H2O H2O + NH3

Mixing of two streams with heat rejection (Absorber) =pure NH3 (x2=1) m3 mixture of H2O and NH3 m1 m3 m2 m1 2 Q cooling Heat rejection Mixture of 1 and 2 3’ Mass and energy balance: (1) (2) 1 3 (3) x3 x From mixture equation: Substitute into (2) Substitute into (3) From adiabatic mixing (from previous slide)

Change of pressure (pump) Sub cooled liquid at p2 2 Saturated liquid at p1 1 p1 ≠p2 m1 =m2 p2 Saturated liquid at x1 =x2 2 p1 Saturated liquid at 1 x1=x2

Heat transfer with separation into liquid and vapor (Generator) Saturated vapor Heat =2V Sub cooled liquid Saturated liquid We can “break” this generator into 2 units heating m4 Q12 /m1 2L= m1 =m2 Separator sub cooled liquid mixture x1 Q12 m3 Apply mass and energy balance In the separator : Apply mass and energy balance In the heat exchanger defines point 2 in graph Defines points 3 and 4 in graph

Heat rejection with separation into liquid and vapor (Condenser) Saturated vapor at p1 m1 Saturated vapor 1 heat rejection m2 Q1-2/m1 m1 =m2 Saturated liquid at p1 x1 =x2 2 p1 =p2 x1=x2

Throttling process (Expansion valve) Saturated vapor 1 2V 2 h1 =h2 T1 1 2 p1 Saturated liquid at p1 ≠p2 T2 2L Saturated liquid m1 =m2 p1 ≠p2 p2 Saturated liquid at x1 =x2 x1 =x2

Simple absorption system 3V 3L 3LLP

Simple absorption system Saturated vapor at p2=p3=p4 3V 6 3 5V mixing 1’ Needed thermal energy Useful cooling energy 3L 4 3LLP 5 2 Saturated liquid at p2=p3=p4 1 5L Saturated liquid at p1=p5=p6=p3_LLP

Heat transfer with separation into liquid and vapor (Generator) How to move point 4 to right ? =2V =2V heating m4 Q12 /m1 2L= 2L= =m2 m1 =m2 mixture Separator sub cooled liquid mixture x1 x1 Q12 m3 Q12 m3

Absorption cooling with preheater improvement 1 Rich ammonia vapor 4 5 Refrigeration and air conditioning (Ramesh et al)

Absorption cooling with preheater Saturated vapor at p1’ 1’’’V=3 Major heat source 6 1’’’ mixing isotherm 6h 1’’ Useful cooling energy 1’’’L =2 4 5 1’ Saturated liquid at p1’ 2’ , 2’’ 1 Saturated liquid at p1 Cooling tower Pumping energy COP= Q cooling / Q heating (Pump ???)

Use of precooling (system improvement #2)

Absorption cooling with precooling Saturated vapor at p1’ 1’’’V=3 Major heat source 6’ 6 6h 1’’’ mixing Saturated liquid at p1’ isotherm 1’’ Useful cooling energy (larger!) 1’’’L =2 4 1’ Saturated liquid at p1 2’ , 2’’ 4’ 5 1 Cooling tower (needs to cool more!) Pumping energy

System improvement #3 Generator with Enrichment of NH3 8V 9 8L 10 8LLP Different 8V 9 8L 10 8LLP 11

Heat rejection with separation into liquid and vapor (Enrichment NH3 in the vapor mixture) This is our point cooling 1 4=2V Separator 6=5V Q12 /m1 cooling Q45 /m4 x8 m8 8 7 m1 =m2 5 2 sub cooled liquid mixture isotherm m3 2L Q12 x1 x8

System improvement #1 Heat rejection with separation into liquid and vapor (Enrichment NH3 in the vapor mixture) This is our point cooling 1 4=2V Separator 6=5V Q12 /m1 cooling Q45 /m4 x8 m8 8 7 m1 =m2 5 2 sub cooled liquid mixture isotherm m3 2L Q12 x1 x8

Ammonia Vapor Enrichment Process (Rectification)

Heat rejection with separation into liquid and vapor (Enrichment NH3 in the vapor mixture) This is our point cooling 1 4=2V Separator 6=5V Q12 /m1 cooling Q45 /m4 x8 m8 8 7 m1 =m2 5 2 sub cooled liquid mixture isotherm m3 2L Q12 x1 x8

Absorption system with Enrichment (no preheater nor precooler) Saturated vapor at p2 3V 8V 3 mixing 11 8L 1’ Useful cooling energy 8LLP 10 3L 2 9 Saturated liquid at p2 1 Saturated liquid at p1

For Real energy analysis you need real h-x diagram! hfg for H2O hfg for NH3 For Real energy analysis you need real h-x diagram!

Example of H2O-NH3 System Text Book (Thermal Environmental Engineering) Example 5.5 HW: Solve the problem 5.6 from the textbook Beside example 5.5, you will need to study example 5.6 and 5.7

LiBr-H2O Systems

LiBr-H2O Systems

Twine vessel LiBr-H2O Systems

Useful information about LiBr absorption chiller http://www.cibse.org/content/documents/Groups/CHP/Datasheet%207%20-%20Absorption%20Cooling.pdf Practical Tips for Implementation of absorption chillers Identify and resolve any pre-existing problems with a cooling system, heat rejection system, water treatment etc, before installing an absorption chiller, or it may be unfairly blamed. Select an absorption chiller for full load operation (by the incorporation of thermal stores if necessary) as COP will drop by up to 33% at part-load. Consider VSD control of absorbent pump to improve the COP at low load. Consider access and floor-loading (typical 2 MW Double-effect steam chiller 12.5 tons empty, 16.7 tones operating). Ensure ambient of temperature of at least 5°C in chiller room to prevent crystallization. http://www.climatewell.com/index.html#/applications/solar-cooling

System with no pump (Platen-Munter system) H2O-NH3 + hydrogen http://www.youtube.com/watch?v=34K61ECbGD4

Thermal storage for adjustment production to consumption We need a thermal storage somewhere in this system !

Thermal storage Store heat Store cooling energy Many issues to consider (∆T, pressure, losses,…. ) Store cooling energy Chilled water For cooling condenser For use in AHU (cooling coils) Ice storage Compact but… Other materials (PCMs) that change phase the temperature we need in cooling coils Many advantages, but disadvantages too!

On-Peak and Off-Peak Periods This profile depends on the type of building(s) !

Chilled water tank Use of stored cooling energy Store Use

Which one is better ? Depends on what you want to achieve: Peak electric power reduction Capacity reduction …..

Downsizing the Chiller Lower utility costs Lower on-peak electrical consumption(kWh) Lower on-peak electrical demand (kW) Smaller equipment size Smaller chiller Smaller electrical service (A) Reduced installed cost May qualify for utility rebates or other incentives

Sizing storage system (use Annual Cooling-Load Profile) How often you need to use it? What are the cost-benefit curves ? What is the optimum size ?