With cooler months ahead, a facility manager’s thoughts tend to turn to heating systems. While you won’t be able to quickly tack on a heat recovery component to your current systems, it is useful to know where energy—typically heat energy—is lost. This article begins with a description of a standard heat recovery system, after which we’ll address some basic heat recovery strategies. We’ll finish up with a description of cogeneration, which would typically take place in power plants, but also sometimes in very large facilities.
Heat Recovery Systems
Many buildings generate excess heat within their interior areas, even in the winter. In most buildings, this heat is removed from the building because it is not transferable to the areas that require heating. Therefore, a large office building may require simultaneous heating and cooling. Heat is required to offset the heat loss through the exterior walls and roof, while interior cooling may be necessary to remove excess heat from people, equipment, and lights.
In many buildings, the internal heat gain from lights may be sufficient to heat the building through most of the winter season. Thus, the principal heating or cooling requirements are due to heat losses or gains through the exterior walls of the buildings and through infiltration of outside air.
A heat recovery system extracts excess interior heat and transfers it to areas of the building that require heat. Such systems are not widely used because they are expensive to install. However, the long-term savings dividends may make the investment worthwhile.
A typical heat recovery system uses dual-duct air distribution, mixing warm air from the heating coil with cool air from the cooling coil for the exterior zones, and using VAV distribution for the interior zones. A heat storage tank accumulates heat from both chiller water loops: the chilled water that has extracted heat from the warm interior zones, and the condenser water that has extracted heat from the refrigeration machine.
When neither of these two sources of heat is sufficient to meet the demands for heat in the perimeter zones, an auxiliary heater, typically electric, provides supplementary heat to the heat storage tank. When more heat is available than can be used or stored, it is rejected through the cooling tower.
During winter operations, mechanical cooling for the interior zones is not needed until the heat storage tank is full. Then the air-handling system uses outside air and/or chilled water to cool the interior zones. When the net cooling load exceeds the heating load, the excess heat is exhausted through the closed-circuit cooling tower. When the heating load exceeds the building internal heat gain, an auxiliary heater supplies the heat.
This heat recovery system is not a reversible heat pump system, because the summer cooling cycle is not reversed to obtain heat for winter operations. There is a difference between this cooling-only system and a heat pump system. With the heat pump system, the condenser heat can be either rejected to the outdoors through cooling towers or used as a source of heat in the building.
Waste Heat Recovery
Waste heat recovery is possible in many HVAC systems. For example, air-to-air heat exchangers may be used to transfer heating and cooling energy from exhaust systems to supply systems. Good quality exhaust air may be used as a supply to secondary building areas, such as laundries or machine rooms. The heat energy from condensate normally spilled as waste may be used to heat/preheat domestic water or temper outside air. Since waste heat recovery equipment can be expensive, life cycle costing will help you select among the systems available. Three methods used to heat outside supply air with waste heat from exhaust are:
- heat wheel
- runaround coil
- heat pipe
Heat Wheel Systems
A heat wheel, also called a thermal wheel, is a rotating heat exchanger that has a thermal transfer media, such as aluminum, and is driven by an electric motor. As the heat wheel slowly rotates through both air streams, it extracts heat from the warm exhaust air and transfers the heat to the cool outdoor air.
Runaround Coil Systems
The runaround coil system consists of two or more extended-surface finned-tube coils installed in supply and exhaust air ducts and interconnected by piping. The heat exchanger fluid is circulated through the coils by a pump. This fluid is usually water, with ethylene glycol added to prevent freezing of the water from cold outside air in the supply air intake.
Heat Pipe Systems
Heat pipe systems are finned tubes perpendicular to and pinned together in adjacent air ducts. The tubes are continuously exposed to air streams of outside air and exhaust air. Each tube contains liquid refrigerant that absorbs heat by evaporation at the warm air stream end of the tube and migrates as a gas to the cold end of the tube, where it condenses to release heat into the cold air stream. The condensed liquid then runs back to the hot end of the tube to complete the cycle. Proper slope from the cold end down to the warm end is required for optimum operation. Heat pipe systems are available to handle air flows ranging from 2,000 cfm to 20,000 cfm. Their lack of mechanical moving parts makes them highly reliable, even without significant maintenance.
Cogeneration, frequently called a total energy system, applies a single energy source to multiple uses, ideally supplying the total energy needs of a facility. Cogeneration is the sequential production of electrical or mechanical energy and thermal energy. By this method, more energy is extracted from each unit of fuel, resulting in higher efficiency. Cogeneration uses one energy source to create electric power and heat and cool residential and commercial buildings, or to create power and process heating to industrial facilities. The energy efficiency of a total energy system is high, consuming 25 percent to 40 percent less raw source energy. The initial cost of this type of system may be quite high. However, depending on the costs of fuel, the source of public electrical power, and the end-use energy needs, the relative life cycle cost of such a system can be quite low. A difficulty with cogeneration is that it is best adapted to large-scale applications that have sophisticated operating personnel and a good balance between electrical requirements and recovered waste heat requirements.
Knowing the principles of heat recovery is important. Wherever you look in your facilities, you should be looking for opportunities to reduce waste. Waste heat is one prime example.
This article is adapted from BOMI International’s The Design, Operation, and Maintenance of Building Systems, Part II. More information regarding this is available by calling 1-800-235-2664, or by visiting www.bomi.org. Visit BOMI International’s Web site.