Solar Absorption Cooling Systems

Fundamentals of Solar Absorption Cooling

Solar absorption cooling converts thermal energy from solar collectors into cooling capacity through a thermally-driven refrigeration cycle. Unlike vapor compression systems that require high-grade electrical energy, absorption systems operate on low to medium-grade thermal energy (70-200°C), making them ideal for solar thermal integration.

The absorption cycle substitutes the mechanical compressor with a thermal compressor consisting of an absorber, generator, solution pump, and heat exchanger. This fundamental difference enables direct coupling with solar thermal collectors.

Working Fluid Pairs

Two primary refrigerant-absorbent combinations dominate solar absorption applications:

LiBr-Water Systems

Configuration: Water serves as refrigerant; lithium bromide (LiBr) as absorbent

Characteristics:

• Evaporator temperatures: 3-10°C
• Generator temperatures: 70-95°C (single effect), 140-180°C (double effect)
• Operating pressure: Deep vacuum (0.6-7 kPa)
• High COP: 0.7-0.8 (single effect), 1.1-1.3 (double effect)
• Limitation: Cannot produce temperatures below 0°C (water freezing point)
• Crystallization risk at high LiBr concentrations

Applications: Air conditioning, process cooling above 0°C

Ammonia-Water Systems

Configuration: Ammonia (NH₃) serves as refrigerant; water as absorbent

Characteristics:

• Evaporator temperatures: -60 to +10°C
• Generator temperatures: 120-180°C
• Operating pressure: Elevated (200-2000 kPa)
• Lower COP: 0.5-0.7
• Requires rectifier column (water volatility)

Applications: Ice production, cold storage, low-temperature applications

Single Effect Absorption Cycle

graph TB
    subgraph "Heat Input"
        SC[Solar Collector<br/>70-95°C]
        G[Generator<br/>Desorption]
    end

    subgraph "High Pressure Loop"
        C[Condenser<br/>Heat Rejection]
    end

    subgraph "Low Pressure Loop"
        E[Evaporator<br/>Cooling Load]
        A[Absorber<br/>Heat Rejection]
    end

    subgraph "Solution Circuit"
        SHX[Solution Heat<br/>Exchanger]
        SP[Solution Pump]
    end

    SC -->|Hot Water| G
    G -->|Refrigerant Vapor| C
    C -->|Liquid Refrigerant| E
    E -->|Refrigerant Vapor| A
    A -->|Weak Solution| SP
    SP --> SHX
    SHX -->|Preheated Solution| G
    G -->|Strong Solution| SHX
    SHX --> A

    style SC fill:#ff9800
    style E fill:#2196f3
    style G fill:#f44336
    style C fill:#ff5722
    style A fill:#ff5722

Energy Balance - Generator:

$$Q_g = \dot{m}r h{fg} + \dot{m}s (h{s,out} - h_{s,in})$$

Where:

• $Q_g$ = Generator heat input (kW)
• $\dot{m}_r$ = Refrigerant mass flow rate (kg/s)
• $h_{fg}$ = Latent heat of vaporization (kJ/kg)
• $\dot{m}_s$ = Solution mass flow rate (kg/s)

Coefficient of Performance:

$$COP = \frac{Q_e}{Q_g} = \frac{\text{Cooling capacity}}{\text{Heat input}}$$

For single effect LiBr-water systems:

$$COP_{SE} = 0.65 - 0.75$$

Practical values depend on:

• Generator temperature (higher $T_g$ → lower COP)
• Evaporator temperature (lower $T_e$ → lower COP)
• Cooling water temperature (higher $T_{cw}$ → lower COP)

Double Effect Absorption Cycle

Double effect systems employ two generators operating at different pressure levels, extracting additional work from the heat source. The high-temperature generator (HTG) operates at 140-180°C, while the low-temperature generator (LTG) operates at 70-95°C using heat rejected from the HTG.

graph TB
    subgraph "Solar Heat Input"
        SC[Solar Collector<br/>140-180°C]
        HTG[High Temp Generator<br/>High Pressure]
    end

    subgraph "Cascade Stage"
        LTG[Low Temp Generator<br/>Medium Pressure]
    end

    subgraph "Condensing"
        C[Condenser]
    end

    subgraph "Cooling Production"
        E[Evaporator]
        A[Absorber]
    end

    SC -->|High Temperature<br/>Heat Input| HTG
    HTG -->|Refrigerant Vapor| C
    HTG -->|Heat Recovery| LTG
    LTG -->|Refrigerant Vapor| C
    C -->|Liquid| E
    E --> A
    A -->|Solution| LTG
    LTG -->|Solution| HTG

    style SC fill:#ff9800
    style HTG fill:#d32f2f
    style LTG fill:#f44336
    style E fill:#2196f3

Double Effect COP:

$$COP_{DE} = 1.1 - 1.3$$

The performance improvement stems from cascading heat recovery:

$$Q_{LTG} = Q_{HTG,reject}$$

Heat balance across both generators:

$$Q_{solar} = Q_{HTG} = Q_e \left(\frac{1}{COP_{DE}}\right)$$

Solar Collector Coupling

Temperature Matching

Critical requirement: Solar collector outlet temperature must exceed minimum generator temperature with sufficient margin for heat exchanger approach.

System Configuration

Direct coupling: Solar loop directly supplies generator

$$\dot{m}{solar} c_p (T{out} - T_{in}) = Q_g$$

Indirect coupling with storage: Thermal storage buffer decouples solar availability from cooling demand

$$V_{storage} = \frac{Q_{cooling} \times t_{autonomy}}{\rho c_p \Delta T \times \eta_{storage}}$$

Where:

• $V_{storage}$ = Storage tank volume (m³)
• $t_{autonomy}$ = Hours of operation without solar input
• $\eta_{storage}$ = Storage efficiency (0.85-0.95)

Performance Optimization

Solar fraction: Portion of annual cooling load met by solar energy

$$SF = \frac{Q_{solar,delivered}}{Q_{cooling,total}}$$

Typical solar fractions: 60-80% for well-designed systems in high-insolation climates.

Heat Rejection Requirements

Absorption systems reject heat at three points: condenser, absorber, and (for double effect) LTG. Total heat rejection exceeds cooling capacity:

$$Q_{rejection} = Q_g + Q_e$$

For COP = 0.7:

$$Q_{rejection} = 1.43 \times Q_e + Q_e = 2.43 \times Q_e$$

Cooling tower sizing:

$$\dot{m}{cw} = \frac{Q{rejection}}{c_p \Delta T_{cw}}$$

Standard approach: 5-7°C rise across cooling water circuit.

ASHRAE Design Guidelines

ASHRAE Standard 90.1 addresses absorption chiller efficiency requirements in Section 6.4.1.1. Minimum efficiency ratings:

• Single effect, air-cooled: COP ≥ 0.60
• Single effect, water-cooled: COP ≥ 0.70
• Double effect, indirect-fired: COP ≥ 1.00

ASHRAE Handbook - HVAC Systems and Equipment (Chapter 18) provides detailed absorption system design procedures including:

• Refrigerant property tables for LiBr-water and NH₃-H₂O
• Part-load performance curves
• Crystallization prevention strategies
• Purge system requirements for air-cooled systems

System Integration Considerations

Capacity modulation: Absorption chillers modulate by varying generator heat input. Turndown ratios of 10-100% are achievable with staged or continuous modulation.

Parasitic loads: Solution pump power is minimal (1-2% of cooling capacity), but cooling tower fans and pumps consume 3-5% of cooling output.

Thermal inertia: Large refrigerant and solution volumes provide inherent storage, smoothing short-term solar transients without dedicated thermal storage.

Crystallization protection: LiBr systems require dilution cycles when extended shutdown occurs at elevated ambient temperatures. Solution concentration must remain below saturation limits.

Economic and Environmental Performance

Solar absorption cooling eliminates electric compression work, reducing peak electrical demand during summer cooling periods. This demand reduction provides significant utility cost savings in time-of-use rate structures.

Primary energy ratio (PER) compares total primary energy consumption to conventional systems:

$$PER = \frac{E_{electric,conventional}}{E_{thermal,solar} / \eta_{solar} + E_{parasitic}}$$

Values exceeding 1.2 indicate favorable primary energy performance relative to conventional electric chillers, particularly when solar thermal efficiency remains above 50%.