11.4 Absorption refrigeration systems

Absorption Refrigeration Systems

Absorption refrigeration systems replace the mechanical compressor found in vapor-compression cycles with a heat-driven separation process. Instead of using electricity to compress refrigerant vapor, these systems use a heat source to separate a refrigerant from an absorbent solution. This makes them especially valuable wherever waste heat, solar thermal energy, or other low-cost heat sources are available.

Their COP values are lower than vapor-compression systems, but that trade-off matters less when the heat input is essentially free. You'll see these systems in industrial plants, district cooling networks, and off-grid locations where electricity is expensive or unavailable.

Working Principle

The working fluid in an absorption system is a binary solution of a refrigerant and an absorbent. The two most common pairings are:

• Water (refrigerant) + lithium bromide (absorbent) — used mainly in air-conditioning applications
• Ammonia (refrigerant) + water (absorbent) — used when temperatures below 0°C are needed, since water would freeze

The cycle works by thermally separating the refrigerant from the solution, sending it through a condenser and evaporator (just like a vapor-compression cycle), and then reabsorbing it back into the weak solution. Here's the step-by-step process:

1. Generator — Heat is added to the strong solution (rich in refrigerant). The refrigerant vaporizes and separates from the absorbent. The remaining weak solution (low refrigerant concentration) returns toward the absorber.
2. Condenser — The refrigerant vapor rejects heat to the surroundings and condenses into a liquid.
3. Expansion valve — The liquid refrigerant passes through an expansion device, dropping in pressure and temperature.
4. Evaporator — The low-pressure refrigerant absorbs heat from the cooled space and evaporates. This is where the actual cooling happens.
5. Absorber — The refrigerant vapor is absorbed back into the weak solution, forming a strong solution again. This process releases heat, which must be rejected to the surroundings.
6. Solution pump — The strong solution is pumped back to the generator at higher pressure, and the cycle repeats.

A solution heat exchanger sits between the generator and absorber loops. It transfers heat from the hot weak solution leaving the generator to the cooler strong solution heading toward the generator. This preheating step reduces the heat input the generator needs and improves overall COP.

Absorption vs. Vapor-Compression Systems

Performance Comparison

The most obvious difference is efficiency. Absorption systems typically have a COP in the range of 0.5 to 1.5, while vapor-compression systems routinely achieve COPs of 2 to 4 or higher. That gap exists because converting heat into a cooling effect is thermodynamically less efficient than using work directly.

However, COP alone doesn't tell the full economic story. Absorption systems require a heat source (natural gas, steam, waste heat), while vapor-compression systems require electrical energy for the compressor. When waste heat is already being rejected from an industrial process or power plant, the effective energy cost of running an absorption system can be very low.

Vapor-compression systems also cool faster and can reach lower temperatures more easily, making them the default choice for most residential and commercial applications.

System Characteristics

COP and Cooling Capacity

Coefficient of Performance (COP)

The COP of an absorption system is defined as:

$COP = \frac{Q_c}{Q_g}$

where $Q_c$ is the cooling effect produced in the evaporator and $Q_g$ is the heat input to the generator. Notice that the pump work is usually small enough to neglect in this calculation, though some formulations include it in the denominator.

Several factors influence the COP:

• Generator temperature — Higher generator temperatures allow more complete separation of the refrigerant from the absorbent, increasing COP. But there's a practical upper limit set by the solution's thermal stability.
• Condenser and absorber temperatures — Lower temperatures at these components improve heat rejection efficiency, raising COP.
• Evaporator temperature — Higher evaporator temperatures (less extreme cooling) improve COP, just as in vapor-compression cycles.
• Working fluid properties — The volatility difference between refrigerant and absorbent, along with the heat of mixing, directly affects how efficiently the cycle operates.

Cooling Capacity

The cooling capacity depends on two things:

$Q_c = \dot{m}_r \cdot (h_{e,out} - h_{e,in})$

where $\dot{m}_r$ is the mass flow rate of the refrigerant through the evaporator, and $(h_{e,out} - h_{e,in})$ is the enthalpy difference across the evaporator. You can increase cooling capacity by raising the refrigerant flow rate (which requires more heat input at the generator) or by increasing the enthalpy change in the evaporator.

Advantages and Disadvantages of Absorption Systems

Advantages

• Waste heat utilization — Can run on waste heat, solar thermal, geothermal, or natural gas, reducing electricity dependence
• Lower operating costs when a cheap or free heat source is available
• Quiet operation — no compressor vibration or noise
• Low maintenance — fewer moving parts mean less wear
• Eco-friendly refrigerants — water/LiBr and ammonia/water pairs have negligible GWP and zero ODP

Disadvantages

• Lower COP than vapor-compression systems, so primary energy consumption is higher when waste heat isn't available
• Larger physical size and higher initial capital cost for equivalent cooling capacity
• Slower response to load changes due to thermal inertia in the generator and absorber
• Limited temperature range — especially for water/LiBr systems, which can't cool below about 5°C
• Requires a reliable heat source at sufficiently high temperature (typically above 80–120°C depending on the working pair)

Applications

• Industrial waste heat recovery — power plants, refineries, and chemical plants that already reject large amounts of heat
• Solar cooling — solar thermal collectors drive the generator in regions with high solar radiation
• District cooling — centralized systems serving multiple buildings, fed by waste heat from nearby industry or cogeneration plants
• Remote/off-grid locations — where electricity is scarce but heat sources (biomass, geothermal) are available
• Tri-generation (CCHP) — combined cooling, heating, and power systems that maximize overall energy utilization by using generator exhaust heat for absorption cooling