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Solar Cooling Plan Introduction : Why & What is Solar Cooling Thermodynamic basics Conventional Cooling Cycle Thermal Solar Cooling Techniques 3.1. Absortion Cooling 3.2 Adsortion Cooling 3.3 Dessicant refrigeration Using Solar Energy in Cooling Cycle

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Solar cooling l.jpg

Solar Cooling

www.sara-project.net


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Plan

Sustainable Architecture Applied to Replicable Public Access Buildings

www.sara-project.net

Contract:TREN/04/FP6EN/S07.31838/503118

Introduction : Why & What is Solar Cooling

  • Thermodynamic basics

  • Conventional Cooling Cycle

  • Thermal Solar Cooling Techniques

    3.1. Absortion Cooling

    3.2 Adsortion Cooling

    3.3 Dessicant refrigeration

  • Using Solar Energy in Cooling Cycle

  • Consumption, performances and costs

    • Comparative assessment

    • COP

    • Solar collectors used

    • Investment cost

    • Performance data

    • Consumption of auxiliary equipment

    • Water consumption


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What’s Solar Cooling?

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  • The core idea is to use the solar energy directly to produce chilled water.

  • The high temperature required by absorption chillers is provided by solar troughs.

  • The system doesn’t require “strategic” materials (like in PV systems) and has peak production in the moment of peak demand.

Chilled water

Heat Transfer Fluid


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Why Solar Cooling?

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  • Dramatic increase of air conditioning since the early 80ies

  • Cost of energy

  • Issues related to environmental pollution

    • Due to energy production

    • Due to the use of CFC’s and HCFC’s

  • Matches demand with source availability

  • Crucial for improving life standards in developing countries


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1. Thermodynamic basis

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Underlying Physics

Thermodynamics

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1st Law: The change of internal energy (U) of a system is equal to the heat absorbed (Q), plus the external work (W) done on the system

W, Q related to the changes the system experiences when going from an initial to a final state


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T

T

F

F

I

V

I

V

p

p

Thermodynamic Cycle

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Cyclical Transformation or Cycle

Simple Transformation


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Entropy

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The concept of entropy was originally introduced in 1865 by Rudolf Clausius. He defined the change in entropy of a thermodynamic system, during a reversible process in which an amount of heat ΔQ is applied at constant absolute temperature T, as

ΔS = ΔQ / T

Clausius gave the quantity S the name "entropy", from the Greek word τρoπή, "transformation". Since this definition involves only differences in entropy, the entropy itself is only defined up to an arbitrary additive constant


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Thermodynamics - 2nd Law

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The most probable processes that can occur in an isolated system are those in which entropy increases or remains constant

In other words:

In an isolated system there is a well-defined trend of occurrence of process and this is determined by the direction in which entropy increases.

In other words:

Heat flows naturally from a system of higher temperature to a system of lower temperature.


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Ideal Carnot Refrigeration Cycle

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12 Isothermal expansion

23 Adiabatic compression

34 Isothermal compression

41 Adiabatic expansion


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Useful cooling energy

COP =

Net energy supplied by external sources

Coefficient of Performance (COP)

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Coefficient of Performance (COP) (2)

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  • COP of the refrigeration sub-system

    • Thermal-driven system

    • Electricity-driven system


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SystemPerformance

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STR (System Thermal Ratio) :

SEE (System electrical efficiency) :

System efficiency :


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2. Conventional cooling cycle

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Conventional cooling cycle

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Compression

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Vapor is compressed and its temperature increases


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Condensation

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The fluid at "high pressure" is cooled by ambient air and therefore condensed


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Expansion

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The liquid refrigerant is depressurized and its temperature decreases


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Evaporation

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The liquid refrigerant at "low pressure" receives heat at low temperature and evaporates


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3.Thermal Solar Cooling Techniques

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Thermal Solar Cooling Techniques

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Absorption Cooling

Energy is transferred through phase-change processes

Adsorption Cooling

Energy is transferred through phase-change processes

Desiccant Cooling

Energy is transferred through latent heat processes

The cooling capacity is based on the physical properties of the cooling fluid, that will change phases at different temperatures, depending on its pressure.


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3.Thermal Solar Cooling Techniques

3.1 ABSORTION COOLING

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Absorption cooling l.jpg
Absorption Cooling

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Substances used


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NH3 systems

  • Improved reliability, at low cost, independent control of the cooling medium

  • Improved pump reliability at low cost

  • Improved reliability of the fluid level sensors

  • Increased performance of the various heat transfer processes in the machine

  • Simplified system concepts

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Properties of H2O – NH3

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LiBr systems

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  • Increased performance and reduction of cost of solar collectors

  • Increased performance and reduction of cost of storage systems (e.g. thermochemical) Development of low capacity absorption machines

  • Development of low capacity air-cooled absorption machines

  • Increased performance of the various heat transfer processes in the machine


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Properties of LiBr – H2O

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Real application – Solar collectors

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Source: K. Sumathy, Z. C. Huang and Z. F. Li, Solar Energy, 2002, 72(2), 155-165


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Absorption machine

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Source: K. Sumathy, Z. C. Huang and Z. F. Li, Solar Energy, 2002, 72(2), 155-165


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Single effect Yazaki machine (10 ton LiBr)

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System combined to sub-floor exchanger

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3.Thermal Solar Cooling Techniques

3.2 ADSORTION COOLING

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Adsorption cooling

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Adsorption is the use of solids for removing substances from gases and liquids

The phenomenon is based on the preferential partitioning of substances from the gaseous or liquid phase onto the surface of a solid substrate.

The process is reversible


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Adsorption Phase 1

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Heating and pressurization

The adsorbent temperature increases, which induces a pressure increase, from the evaporation pressure up to the condensation pressure.

This period is equivalent to the "compression" phase in compression cycles.


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Adsorption Phase 2

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Heating and desorption + condendsation

During this period, the adsorber continues receiving heat while being connected to the condenser, which now superimposes its pressure. The adsorbent temperature continues increasing, which induces desorption of vapour. This desorbed vapour is liquified in the condenser. The condensation heat is released to the second heat sink at intermediate temperature.

This period is equivalent to the "condensation" in compression cycles.


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Adsorption Phase 3

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Cooling and depressurization

During this period, the adsorber releases heat while being closed. The adsorbent temperature decreases, which induces the pressure decrease from the condensation pressure down to the evaporation pressure.

This period is equivalent to the "expansion" in compression cycles.


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Adsorption Phase 4

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Cooling and adsorption + evaporation

During this period, the adsorber continues releasing heat while being connected to the evaporator, which now superimposes its pressure. The adsorbent temperature continues decreasing, which induces adsorption of vapor. This adsorbed vapour is evaporated in the evaporator. The evaporation heat is supplied by the heat source at low temperature.

This period is equivalent to the "evaporation" in compression cycles.


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Adsorption Cooling - Summary

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The cycle is intermittent because production of cooling energy is not continuous: it occurs only during part of the cycleWhen there are two adsorbers in the unit, they can be operated separately and production of cooling energy can be quasi-continuous.

When all the energy required for heating the adsorber(s) is supplied by the heat source, the cycle is termed single effect.

Typically, for domestic refrigeration conditions, the COP of single effect adsorption cycles is of about 0.3-0.4.

When there are two adsorbers or more, other types of cycles can be designed.

In double effect cycles or in cycles with heat regeneration, some heat is internally recovered between the adsorbers, and that improves the COP.


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Adsorption cooling - Examples

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3.Thermal Solar Cooling Techniques

3.3 DESICCANT REFRIGERATION

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Desiccant refrigeration

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Addresses the issue of thermal comfort by modifying the water vapor content in a space.


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Desiccant refrigeration principle

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Desiccant refrigeration flow-chart

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Working fluids

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Solar collector sub-cycle

Refrigeration sub-cycle


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Basic Refrigeration Cycle

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Heat-driven systems (1)

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  • Absorption


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Heat-driven systems (2)

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  • Adsorption


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Heat-driven systems (3)

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  • Chemical reaction


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Heat-driven systems (4)

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  • Desiccant system


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Heat-driven systems(5)

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  • Rankine


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Heat-driven systems (6)

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  • Ejector


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PV-driven systems (1)

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  • Thermo-electric (Peltier)


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PV-driven systems(2)

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  • Stirling


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Evaluation criteria

Comparative assessment

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COP

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0.59

0.60

0.66

0.85

0.74

0.49

0.51

117

Thot (oC)

52-82

60-110

66

120


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Solar collectors used

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- Flat-plated (63%)

- Vacuum tube (21%)

- Parabolic :

Fixed(10%)

Moving(6%)

Average specific collector area :3,6 m2/kW


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Investment cost

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  • Depends on:

  • power rate

  • collector type

  • development phase

  • operating principle

Average investment

4012 Εuro/kW


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Performance data

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Highest performance

LiBr / H2O systems

Lowest performance

NH3/H2O diffusion system

Average annual COP = 0.58


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Consumption of auxiliary equipment

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Lowest consumption:

Absorption systems

LiBr/H2O systems = 0.018 kWh/kWh

Mean annual electricity consumption of fans and pumps

= 0.225 kWh electric/kWh thermal


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Water consumption

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Highest consumption

Adsorption systems:

7.1 kg.h-1/kW

Majority of systems:

4-6 kg.h-1/kW

Mean annual water consumption : = 5.3 kg.h-1/kW


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