Heat recovery is a process that captures and reuses thermal energy that would otherwise be expelled into the environment as waste. This thermal energy, often contained in exhaust air or hot liquids, represents a significant amount of wasted power in both commercial and industrial operations. By intercepting this energy before it is lost, heat recovery systems contribute directly to improved energy efficiency and reduced operational costs. The fundamental purpose of this technology is to lower the demand on primary heating or cooling sources by pre-conditioning a colder incoming stream. This principle has become a standard practice in modern energy management.
The Fundamental Principle of Heat Recovery
The mechanism behind heat recovery is rooted in the natural laws of thermodynamics, where heat spontaneously moves from a hotter object or fluid to a cooler one. This transfer occurs through the physical mechanisms of conduction and convection. Conduction involves the direct transfer of kinetic energy through a solid barrier, while convection relies on the movement of a fluid to carry the heat away.
A heat recovery system works by directing two separate streams—a hot, waste stream and a cold, useful stream—adjacent to each other. The hot stream contains the energy to be recovered, while the cold stream is the recipient of that energy. A physical medium or barrier separates the two streams, ensuring there is no mixing or cross-contamination.
The thermal energy transfers through this separating barrier into the cold stream, pre-warming it before it enters the main system. Efficiency is maximized by increasing the surface area for the transfer and using materials with high thermal conductivity. The captured energy is known as sensible heat, but in many systems, the energy transfer also includes latent heat, or the energy stored in moisture content. The total thermal energy being transferred is referred to as enthalpy.
Essential Heat Recovery Equipment
The practical implementation of heat recovery relies on specialized hardware designed to facilitate the transfer between the hot and cold streams. Air-to-air heat exchangers are commonly used in ventilation systems to pre-condition incoming fresh air using the warmth of exhausted indoor air.
Plate Heat Exchangers
One common type is the plate heat exchanger, or recuperator, which uses a series of closely spaced, parallel plates to separate the two air streams. Heat transfers through the thin plates by conduction, warming the supply air before it enters the building.
Rotary Heat Wheels
Another effective air-to-air device is the rotary heat wheel, often called a thermal wheel. This is a rotating cylinder filled with an air-permeable material. As the wheel turns, a section passes through the hot exhaust stream, absorbing thermal energy, and then rotates into the cold supply stream, releasing the stored heat. These wheels can use a hygroscopic material to also transfer moisture, enabling total enthalpy recovery.
Ventilators and Fluid Systems
When moisture is transferred, the device is known as an Energy Recovery Ventilator (ERV), which transfers both sensible and latent heat. A Heat Recovery Ventilator (HRV) transfers only sensible heat, leaving the moisture content unchanged. For fluid-to-fluid applications, shell and tube heat exchangers are widely deployed. These devices consist of a bundle of tubes housed inside a larger shell, where one fluid flows through the tubes and the other flows around them, allowing heat to transfer across the tube walls.
Practical Applications of Heat Recovery
Heat recovery technologies find widespread use in both large-scale industrial operations and in the Heating, Ventilation, and Air Conditioning (HVAC) systems of residential and commercial buildings. In building systems, the technology addresses the energy penalty associated with ventilation. Stale indoor air must be exhausted to maintain air quality, taking conditioned thermal energy with it, which is then recovered to pre-heat or pre-cool the incoming fresh outdoor air.
Commercial and Residential Use
In commercial and large residential settings, heat recovery significantly reduces the load on the primary HVAC equipment. This allows for the use of smaller, less costly heating and cooling units. This ensures continuous fresh air supply without the massive energy loss of venting conditioned air directly outside.
Industrial Use
Industrial processes represent a vast application area, often dealing with much higher temperatures and different waste streams. Manufacturing facilities, such as those with boilers, kilns, or furnaces, release large volumes of hot exhaust gas. Waste heat boilers capture this high-temperature energy to generate steam, which can then be used to power other equipment or processes within the facility.
Other industrial applications involve recovering heat from hot liquids, such as cooling water or process effluent, using fluid-to-fluid heat exchangers. This recovered heat can preheat boiler feedwater, reducing the energy needed to reach steam temperature, or it can warm combustion air for furnaces, which improves combustion efficiency. Repurposing this otherwise wasted thermal energy significantly improves the overall thermal efficiency of the industrial plant.