Absorption chillers

What Are Absorption Chillers?

Absorption chillers are a type of refrigeration system that produces chilled water for heating and cooling without relying primarily on electricity for their main power source. Instead, they utilize a heat source, such as waste heat from industrial processes, natural gas, steam, or solar thermal energy, to drive a cooling cycle. This positions them within the broader category of Energy Technology, offering an alternative approach to climate control and industrial cooling. Unlike conventional compressor-driven chillers, absorption chillers use a thermochemical process involving a refrigerant and an absorbent solution to achieve their cooling effect. Their operation contributes to overall energy efficiency and can reduce reliance on grid electricity, influencing a facility's utility bills and its environmental footprint.

History and Origin

The foundational principles of absorption cooling can be traced back to the mid-18th century, with significant advancements occurring in the 19th century. In 1748, William Cullen demonstrated artificial refrigeration in Glasgow, but a more practical application for absorption was developed later. Michael Faraday contributed to the understanding of ammonia absorption in the early 19th century¹⁸. However, the continuous absorption cooling system, which forms the basis of modern absorption chillers, was invented by French scientist Ferdinand Carré in 1858. His original design utilized water and sulfuric acid.¹⁷ Commercial exploitation of absorption refrigeration began more widely in the 1920s.¹⁶ Following World War II, in 1945, Carrier notably developed a large-scale air-conditioning absorption chiller that employed water as the refrigerant and a lithium bromide solution as the absorbent, a common pairing still used today.¹⁵ The development continued, with the two-effect absorption chiller becoming a standard in the industry by the 1960s.¹⁴

Key Takeaways

• Absorption chillers use a heat source (e.g., waste heat, natural gas, steam) rather than primarily electricity to produce cooling.
• They operate on a thermochemical process involving a refrigerant (commonly water) and an absorbent (e.g., lithium bromide or ammonia).
• These systems are often favored for their ability to utilize otherwise wasted heat, contributing to greater overall system sustainability.
• Absorption chillers typically have fewer moving parts than vapor compression systems, potentially leading to quieter operation.
• While offering environmental benefits, they can have higher initial capital expenditure and may require larger footprints than electric chillers.

Formula and Calculation

The efficiency of an absorption chiller is typically measured by its coefficient of performance (COP). The COP for an absorption chiller is a dimensionless ratio that compares the useful cooling output (chiller load) to the total heat input required to drive the system. It quantifies how efficiently the heat energy is converted into cooling.

The formula for the Coefficient of Performance (COP) is:

Where:

• (Q_c) = Cooling capacity (useful thermal energy output of the chiller)
• (Q_g) = Heat input (total heat energy supplied to the generator to drive the cycle)

For single-effect absorption chillers, COP values are typically less than one, while double-effect and triple-effect systems can achieve COPs greater than one by reusing heat within the cycle.¹³

Interpreting Absorption Chillers

Interpreting the role and effectiveness of absorption chillers involves considering their unique operational characteristics and how they integrate into broader energy management strategies. Their primary appeal lies in their ability to leverage low-grade or "free" heat sources, such as exhaust from power generation or industrial processes. This can significantly reduce the operating costs associated with cooling, especially in facilities where such waste heat is abundant.

When evaluating absorption chillers, the Coefficient of Performance (COP) is a crucial metric, indicating the system's energy efficiency. A higher COP signifies better performance, meaning more cooling is produced per unit of heat input. However, unlike traditional mechanical systems, absorption chillers are less dependent on electricity. Their application is often a strategic choice for businesses aiming to enhance resource utilization and minimize their environmental impact, particularly their carbon emissions.

Hypothetical Example

Consider a large manufacturing plant that generates a significant amount of waste heat from its production lines. This plant currently uses traditional electric-powered chillers for its air conditioning needs, resulting in high electricity consumption.

To reduce these costs and improve sustainability, the plant decides to install an absorption chiller. The existing waste heat, which was previously dissipated into the atmosphere, is now routed to power the absorption chiller.

Let's assume:

• The plant's waste heat output suitable for the chiller is 2,000 kW (thermal).
• The installed absorption chiller has a Coefficient of Performance (COP) of 0.8.

Using the COP formula:
(Q_c = \text{COP} \times Q_g)
(Q_c = 0.8 \times 2,000 \text{ kW})
(Q_c = 1,600 \text{ kW})

This means the absorption chiller can provide 1,600 kW of cooling capacity by utilizing the plant's waste heat, offsetting a substantial portion of the electricity previously used by the electric chillers. This not only leads to significant savings on electricity bills but also repurposes energy that would otherwise be wasted, demonstrating the direct benefit of waste heat recovery.

Practical Applications

Absorption chillers are found across various sectors, particularly where there is an available source of heat or a desire to reduce electricity demand.

• Industrial Cooling: Many industrial processes generate substantial amounts of waste heat. Absorption chillers can be integrated into these operations to convert this waste heat into useful cooling for process regulation, equipment cooling, or facility air conditioning. This application not only saves energy but also improves the overall efficiency of the industrial plant.
• Combined Heat and Power (CHP) Systems: In CHP or cogeneration plants, absorption chillers can use the exhaust heat from power generation to provide cooling, creating a highly efficient trigeneration system (producing electricity, heating, and cooling). This maximizes the utilization of fuel input.
• District Cooling Systems: Large-scale district cooling networks can deploy absorption chillers, often powered by centralized heat sources or even municipal waste heat, to provide chilled water to multiple buildings within an urban area, reducing the collective energy load.
• Solar Thermal Cooling: Absorption chillers can be paired with solar thermal collectors, using heat from the sun to drive the cooling cycle, offering an environmentally friendly alternative for cooling in areas with ample sunshine. The demand for such energy-efficient cooling solutions, especially those leveraging waste heat recovery systems, is contributing to the growth of the absorption chiller market, which is projected to expand significantly.¹²

Limitations and Criticisms

While absorption chillers offer distinct advantages, they also come with certain limitations and criticisms that warrant consideration in their adoption.

One of the primary drawbacks is their generally higher initial capital expenditure compared to conventional vapor compression chillers.¹¹ This higher upfront cost can be a barrier for some infrastructure investment decisions, despite potential long-term operating costs savings.

Furthermore, absorption chillers are typically larger and heavier than electric chillers of equivalent capacity, which can complicate installation and may require additional structural modifications to accommodate them,¹⁰.⁹ They also often necessitate larger cooling towers, which adds to the physical footprint and potentially the installation cost.⁸

Historically, issues such as crystallization of the lithium bromide solution and vacuum leaks have posed challenges, affecting system efficiency and leading to corrosion,⁷.⁶ While newer models incorporate advanced controls to mitigate these problems, maintaining the internal vacuum remains critical for optimal performance. The corrosive nature of lithium bromide on mild steel can also lead to maintenance difficulties over time, with components susceptible to fouling from corrosion debris.⁵

In terms of efficiency, absorption chillers generally have a lower coefficient of performance compared to modern vapor compression chillers, meaning they convert less of their input energy (heat) into cooling output,⁴.³ This can make them less competitive in scenarios where waste heat is not readily available or where the cost of the heat source is high. Additionally, some absorption chiller systems use materials like lithium bromide which require specialized disposal due to their hazardous nature at the end of the chiller's useful life.²

Absorption Chillers vs. Vapor Compression Chillers

The fundamental distinction between absorption chillers and vapor compression chillers lies in how they drive their respective cooling cycles.

Confusion often arises because both systems provide cooling, but their energy inputs and operational mechanisms are vastly different. While vapor compression chillers are the dominant technology due to their high electrical energy efficiency and compactness, absorption chillers present a compelling alternative when economical or available heat sources exist, or when reducing grid electricity demand is a priority for HVAC systems.

FAQs

What is the primary benefit of an absorption chiller?

The primary benefit of an absorption chiller is its ability to utilize heat as its main energy source for cooling, often waste heat that would otherwise be discarded. This can lead to significant reductions in electricity consumption and lower operating costs, especially in facilities that generate excess heat.

Are absorption chillers environmentally friendly?

Yes, absorption chillers are generally considered environmentally friendly. They can reduce greenhouse gas and carbon emissions by using waste heat or natural gas instead of electricity generated from fossil fuels. Many systems also use refrigerants like water, which has zero ozone depletion potential.

What are the typical applications for absorption chillers?

Absorption chillers are commonly used in large commercial buildings, industrial processes, and facilities with combined heat and power (CHP) systems. They are also employed in district cooling networks and in conjunction with solar thermal systems for sustainable heating and cooling.

Do absorption chillers use electricity at all?

While absorption chillers do not rely on electricity for their primary cooling cycle, they still require a small amount of electricity to power auxiliary components such as pumps and controls. However, this electrical consumption is significantly less than that of traditional vapor compression chillers of similar capacity.