Energy recovery is the beneficial use of heat or cooling energy that would otherwise be lost or needs to be removed from a specific space. Technologies that recover heat and/or cooling energy reduce the cost and consumption of energy in commercial and institutional buildings. The recaptured energy is potentially useful for heating and/or cooling/dehumidifying outdoor air brought into a building for ventilation, space heating, and water heating.

The applications for recovering heat depend on the temperature of the gases or liquids containing the waste heat. It is usually more efficient to use higher temperature recovered heat for higher temperature applications, such as producing steam or heating water. There are three temperature ranges of heat recovery. In small commercial or institutional buildings, waste heat is recovered from liquids or gases at low temperatures (below 450 degrees F [232 degrees C]). In larger buildings and industries, medium temperature recoverable heat (between 450 degrees F and 1200 degrees F [232 degrees C–649 degrees C]) may be available for waste heat recovery. High temperature recoverable heat (above 1200 degrees F [649 degrees C]) is usually available only in heat-intensive industries, such as cement and metals manufacturing.

Heating and Conditioning Ventilation Air
There is increasing awareness of the importance of good indoor air quality to maintain healthy and comfortable conditions for building occupants. This creates a need for greater ventilation air (fresh outdoor air brought in to replace exhausted air) requirements for all types of commercial and institutional buildings. Energy (or enthalpy) recovery ventilation (ERV) systems are growing in availability and application throughout the country.

The most common heat recovery ventilation (HRV) devices are flat plate air-to-air heat/energy exchangers, rotary heat and energy (enthalpy) wheels, and heat pipes. All require adjacent and parallel outdoor air intake and exhaust ducts. When heat needs to be transferred from a remote location, a run-around loop system can be installed. This requires the installation of heat transfer coils in both the intake and exhaust ducts (or other source of heat). A pump circulates the heat transfer fluid—moving the heat to where it is needed. Air-to-air heat pumps are occasionally used to extract heat from warm exhaust air. Higher installation and operating costs limit this application.

Conditioning (cooling or dehumidification) of ventilation air with exhaust air energy recovery in hot and humid climates is very important. Bringing unconditioned, humid ventilation air into a building has many obvious negative impacts on indoor air quality, comfort, and HVAC system operation. ERV’s for conditioning ventilation air typically use desiccant or refrigeration methods.

When an ERV system is integrated with HVAC systems, the resulting conditioning and/or heat recovery allow installation of smaller HVAC systems. This provides savings in both capital investments and operating costs.

Space Heating
Recovering heat from one area of a building (such as kitchens, laundries, printing plants, computer rooms, compressor rooms in supermarkets and refrigerated warehouses, and the condensers of air conditioners or liquid chillers) and delivering it to other areas of a building, is another possibility. Heat pumps are an especially suitable method removing heat from a heat source (air, water, process fluid, etc) and delivering it to another point or area of need. The temperature of fluids from which commercial heat pumps extract heat ranges from 50 degrees F to 125 degrees F (10 degrees C–51.5 degrees C). For example, they can remove heat from places such as commercial laundries, and thereby provide increased comfort and productivity, as well as heat for heating other areas (or water). Where exhaust stack (kitchen or laundry/dryer) temperatures are greater than 350 degrees F (177 degrees C), a flue gas heat exchanger can recover the waste heat.

Lights in commercial buildings produce large quantities of heat. Normally, only about 60% of that heat contributes directly to space heating. Heat recovery allows controlled and efficient use of that heat. Both "wet" and "dry" methods have been used. The wet method of heat recovery uses aluminum reflector housings that contain integral water channels. Water pumped through those channels absorbs the heat produced by the lights. (The pipes can also be installed in front of windows to pick up additional free solar heat.) This warm water then circulates through a heat pump or a heat exchanger to provide space heating or to warm incoming air. Dry methods usually involve using the light fixture as a return grille (to an HVAC system duct or plenum). The heated air can then be distributed to cooler perimeter zones or recirculated through the central HVAC system. Recovering heat from lights is usually feasible only with new construction where high intensity lighting is required.

Heating Water
A good use of high temperature recovered heat is for heating water, since the required temperature is higher than that needed for air. For buildings that have high hot water requirements, such as hospitals, laundries, restaurants, or hotels, using recovered heat to heat or preheat water is very cost-effective. Shell and tube heat exchangers have a number of applications here. These transfer heat between two physically separated fluids. One flows through the shell; the other through tubes. Some applications include: Hot gas heat exchangers, installed in the hot gas line (between compressor and condenser), recover heat from refrigeration systems. Water circulates through the heat exchanger, transferring heat directly to where it is used, or to a hot water tank. Double-bundle condensers have applications in large heat pump systems. They have two water tube bundles enclosed in the shell. High-pressure refrigerant gases are released into the shell. As they condense, heat is released. During the heating season, water is pumped through the "winter bundle," where it absorbs the heat and uses it to heat domestic water or perimeter areas. During the cooling season, the cooling tower water is pumped through the "summer bundle," carrying off any excess heat beyond that needed for domestic hot water.

Hot drain heat exchangers (or heat pumps) are used to recover heat from drain water. Typical applications include commercial laundries and conveyor dishwashers. The recovered heat preheats fresh water before it goes into the domestic hot water tank—reducing the energy needed to heat it to the required temperature.

Buildings with steam systems can recover the heat from hot condensate when it is transferred to atmospheric pressure in the condensate receiver. A heat exchanger is installed in the condensate return. Water is circulated between the heat exchanger and the hot water tank.

Other Types of Heat Recovery
Cogeneration in commercial buildings can also reduce energy costs. When a natural gas or diesel engine-driven electric generator is used, the waste heat from the exhaust and the engine cooling water can be recovered by a waste heat boiler, and used for water and space heating. Gas turbines can also be used to drive a generator. Heat, recovered from the exhaust, can produce steam that drives a turbogenerator. Cogeneration units have a successful track record in a wide range of commercial applications, including restaurants, colleges, hotels, and hospitals.

Another technology that cogenerates electricity and heat is the fuel cell. A fuel cell converts hydrogen (which can be reformed from natural gas on site) into DC electric power through an electrochemical reaction. The conversion process produces heat as a byproduct, which can be used for space or water heating. They also produce very low levels of air pollutants.

Buildings with over 1,000 pounds per day of burnable, solid waste can recover high and medium temperature heat from specially designed incinerators. The heat can be used to regenerate the lithium bromide used in absorption cooling, steam production, space heating or hot water heating.

The Heat Recovery Investment
Energy recovery systems for commercial and institutional buildings are often complex, and should be designed by a professional engineer. They involve many related factors—all of which need to be considered for proper operation. In addition, when analyzing an energy recovery project, it is necessary to look at both the initial investment costs and the future savings in operating costs. The additional capital costs include installation labor, additional ductwork or piping, filters, controls or energy management systems (EMS), and new pumps or fans. Capital savings come from lower load requirements, which allow smaller and less expensive HVAC systems. Operating savings come from lower energy bills. Other factors include the possibility of higher maintenance costs, increased training of maintenance personnel, and the additional energy costs to operate the energy recovery equipment.

Source: U.S., DOE Energy Efficiency and Renewable Energy Network, EREC Brief. EREC is operated by NCI Information Systems, Inc. for the National Renewable Energy Laboratory/U.S. Department of Energy. The content of this brief is based on information known to EREC at the time of prepartion. No recommendation or endorsement of any non-U.S. Government product or service is implied if mentioned by EREC.