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Waste Heat Recovery |
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Waste heat is heat generated via processes such as fuel combustion (exhaust fumes), a chemical reaction (fuel cells), or a vapor compression refrigeration cycle (refrigerant), which is “dumped” into the ambient environment even though it still has significant useful energy. Waste-heat recovery refers to heat that is discharged as a byproduct from one process to provide supplemental energy needed by another process.
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Contents
1.Definition
2. Use/Application
a. Established Techniques
b. Emerging Trends
3. Use an Integrated Approach
4. Resources
5. Associated Strategies
6. Case Studies |
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Definition
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Waste heat is heat generated via processes such as fuel combustion (exhaust fumes), a chemical reaction (fuel cells), or a vapor compression refrigeration cycle (refrigerant), which is “dumped” into the ambient environment even though it still has significant useful energy. Waste-heat recovery refers to heat that is discharged as a byproduct from one process to provide supplemental energy needed by another process.
How to best recover this energy and how much energy can be recovered depends on the temperature of the waste heat fluid, the secondary processes, and the economics involved. Heat transfer/extraction/recovery takes place as one hot medium, without mixing, passes another cooler medium. By bringing the hotter fluid into indirect contact with a relatively cool incoming load, energy will be transferred to the load, preheating it and reducing the energy that is simply lost to the ambient. The amount of this transfer depends on the temperature differential between the two fluids and the amount of exposure time. This recovered energy is typically used for preheating water for domestic or process usage. Minimizing active water heating can have a significant environmental and economic savings. According to the U.S. Department of Energy (DOE), water heating in 2000 accounted for 4.2 quads of energy use nationally or roughly 11.5% of the primary energy use by the building sector. In residences, domestic water heating accounts for >18% of energy consumption. While water heating is less significant in commercial buildings compared to other building loads, it is a major contributor in some industries: 40.4% of energy use in the lodging sector, 26.2% in health care, and 21.9% in education (1995 data from DOE).
Typical examples of waste-heat recovery applications would be preheating of combustion air, space heating, ventilation, water heating, boiler feed water, or process water.
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Use / Application |
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There are many potential sources of waste heat in buildings and thus many ways to recover it. An important consideration is the balance between the availability of and demand for recovered waste heat. If the demand is not concurrent with the availability, then storage is required and initial cost and standby energy losses are added. Good candidates for waste-heat recovery are buildings with large hot water loads or large ventilation loads where the heat, or cooling in the summer, is recovered from the air as it is exhausted from the building.
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Established Techniques |
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The most common method of waste-heat recovery for preheating water is extracting heat from the refrigerant prior to the rejecting heat via a condenser.
A desuperheater is a heat recovery unit (HRU) that recovers the superheat energy from the compressor’s discharge line. Under typical conditions a desuperheater can remove about 10 to 30% of the total heat that would have been rejected by the condenser.
Waste-heat recovery is also utilized to reclaim heat from the hot water after it’s used.
Domestic hot water is a once-through process that sees most of the energy used to heat water go down the drain. Centralized drainwater heat recovery devices have been developed in an attempt to use this waste energy in homes and commercial buildings. Two products on the market are the Gravity Film Exchanger (GFX) and ReTherm. Both manufacturers claim potential heat recovery percentages as high as 30%.
Other types of HRUs and uses for waste heat are:
- Recuperator - a gas-to-gas heat exchanger placed on the stack of the furnace.
- Regenerator - a mass is used for temporary storage of that heat prior to exchange to the secondary process (a heat wheel is a rotary regenerator)
- Absorption chillers – use waste heat to provide chilled water for cooling.
- Heat pipe exchangers
- Economizers - flue gas heat is used to pre-heat boiler feed water or the waste heat is used to pre-heat combustion air to improve efficiency.
- Heat pumps
- Many other innovative, custom HRUs exist, such as using waste heat from a process to provide snow-melt of a road surface or roof
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Emerging Trends |
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The increased awareness of the impact of ventilation air requirements on heating and cooling energy loads has promoted an increased use of technologies that exchange heat and, in some instances, moisture between the ventilation exhaust and inlet airstreams.
In commercial buildings in more humid climates, desiccant wheels are a viable option to remove moisture from the incoming ventilation air through the use of a drying agent, or sorbent. Desiccant is impregnated into a lightweight honeycomb-shaped wheel matrix. Supply air passes through one section of the wheel and is dried. As the wheel rotates into a second air stream, that air dries the desiccant and carries the moisture out of the building. The process is repeated again as the wheel continues to spin. These units also have a heat-exchange core to transfer heat from the outgoing air to the incoming air.
In more residential applications, heat recovery ventilators (HRV) and energy recovery ventilators (ERV) are established technologies that can be utilized for waste-heat recovery. These units transfer heat and/or moisture to the incoming outdoor air from the indoor exhaust air without mixing the airstreams through the use of a heat-exchange core. In the winter, the outgoing heated air pre-heats the incoming air. The opposite occurs in the summer with the outgoing air pre-cooling the incoming air. These systems typically have a total effectiveness of 55% to 85%.
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Use an Integrated Approach |
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A new way of thinking must be adopted to meet the goal of reducing carbon emissions associated with buildings. Your solutions can begin by integrating four possible methods. None works alone, and they are not all relevant in considering every strategy. However, considering the following tactics is necessary:
- Reduce the overall energy use in your building
- Waste-heat recovery from the air conditioning system provides preheated water to the water heater.
- As heat is extracted from the refrigerant more effectively, the air conditioning system will operate more efficiently.
- Specify energy efficient equipment and technologies
- Desuperheaters provide more condenser capacity, which lowers the compressor head pressure. This prolongs the life of the air conditioning system.
- Heat recovery systems are available for air conditioning units from 2 tons up to the largest chillers available.
- Use renewable strategies and purchase green power
- Using waste-heat recovery equipment can reduce a building’s overall energy load, which would require smaller, more cost effective renewable strategies to obtain energy neutrality, and reduce emissions.
- Educate building owners, operators, and occupants
- On the environmental and economic benefits of waste-heat recovery systems for new and existing systems
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Last modified at 2/27/2009 11:20 PM by jamie nace
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