ENERGY RECOVERY VENTILATION UNDERSTANDING ENERGY WHEELS AND ENERGY RECOVERY VENTILATION TECHNOLOGY By: Mark Rabbia, George Dowse Carrier Corporation Syracuse, New York September 2000 TABLE OF CONTENTS INTRODUCTION.............................................2 ENERGY RECOVERY APPLICATIONS.......3 ENERGY RECOVERY EQUIPMENT............3 Energy Wheels ......................................4 Fixed-Plate Heat Exchangers................4 Heat Pipe Heat Exchangers...................5 Thermosiphon Heat Exchangers ...........6 Coil Energy Recovery Loops ................6 ENERGY WHEELS .........................................7 Construction..........................................7 Energy Transfer Media .........................7 Energy Wheel Performance ..................8 HVAC System Downsizing ..................10 Cross-Contamination ............................10 Controls.................................................11 Maintenance..........................................11 Energy Wheel Economics.....................11 Performance Ratings.............................12 REFERENCES .................................................15 INTRODUCTION Application of air-to-air energy recovery In most process-to-comfort applications, energy equipment continues to grow in acceptance and recovery involves the capture and transfer of use in HVAC systems. Increased outdoor air sensible heat only. Waste heat is transferred to requirements necessary to meet ASHRAE makeup or outdoor air streams. This is effective Standard 62-1989 (1999), Ventilation for during winter months, but requires modulation Acceptable Indoor Air Quality, and state and local during spring and autumn to prevent overheating building codes, add substantially to cooling and the building. Often, no energy recovery is made heating loads. Increasing outdoor air loads during summer. escalates both operating cost and system equipment cost. This has intensified interest in Comfort-to-comfort applications differ from other energy recovery technologies and their economic categories in that both sensible and latent heat are applications. often transferred. The energy recovery device transfers sensible heat from the warmer air stream Energy recovery can be used for both new and to the cooler air stream. It also transfers moisture retrofit applications. There are three categories of from the air stream with the higher humidity ratio application: (1) process-to-process, (2) process-to- to the air stream with the lower humidity ratio. comfort, and (3) comfort-to-comfort. The directions of humidity and heat transfer are not necessarily the same. In process-to-process applications, only sensible heat is captured from the process exhaust stream This white paper discusses air-to-air, comfort-to- and transferred to the process supply stream. comfort applications. Equipment and systems Exhaust temperature may be as high as 1,500°F. used for energy recovery ventilation are reviewed with particular emphasis given to rotary energy wheels that transfer both heat and moisture. 2 ENERGY RECOVERY APPLICATIONS There are two basic seasonal scenarios for air-to- • Schools (K-12) and universities air energy recovery ventilation in comfort • Gymnasiums applications. The first is to transfer heat and • Office buildings moisture from the exhaust stream to the supply • Hospitals stream during winter months. The second is the • Nursing homes reverse function: to transfer heat and moisture • Correctional facilities from the supply stream to the exhaust stream in • the summer. Retail shopping centers Some applications have severe requirements and Supply Air Supply Air great care must be taken to prevent cross- to System From Outdoors contamination between air streams. For instance: • Laboratory fume hood exhaust Energy Recovery Device • Industrial applications with fumes or smoke (e.g., welding operations) • Industrial applications with toxic or noxious exhaust Exhaust Air Exhaust Air • Restaurants or kitchens From System to Outdoors • Specialized hospital treatment areas Figure 1. Counter-flow Heat Exchange Pattern In these situations, the air streams must be physically isolated to insure there is no cross- contamination. This limits options for energy recovery to sensible heat transfer only. There are many possible applications of air-to-air energy recovery ventilation. Any building with comfort conditioning and relatively high levels of outdoor air, especially where latent cooling is a ENERGY RECOVERY EQUIPMENT significant portion of the outdoor air requirement, Energy wheels that transfer both sensible and is a likely candidate. Usually, buildings with latent heat are the primary subject of this white significant occupancy levels that vary with the paper. But, in the interest of understanding the time of day are good candidates. For a typical benefits of energy wheels, it is useful to office building, occupancy variability may be high understand other technologies. The first thing to between 8 a.m. and 5 p.m. and low other hours. understand is that not all technologies are Usually, these types of buildings must also appropriate for every application. Each has conform to ASHRAE Standard 62. Typical benefits and limitations, and may be better suited applications may include: to certain applications over others. For instance, energy wheels, as they are described in this paper, 3 are ideally suited to comfort-to-comfort minute depending on the application. When the ventilation applications. A runaround loop can be wheel passes through the high-temperature air similarly effective in transferring sensible heat, stream, the media temperature increases as heat is but it is much more complex and costly to use. On transferred and stored in the individual filaments. the other hand, a runaround loop can be applied When the media wheel rotates into the low- successfully to harsh process applications where temperature air stream, the filaments are cooled an energy wheel would fail. Table 1, at the end of and release heat. This form of heat transfer is this white paper, summarizes energy recovery purely sensible and is driven by a temperature equipment characteristics, advantages and gradient between the high- and low-temperature limitations. air streams. In most comfort-to-comfort applications, the temperature difference is Energy Wheels relatively small as compared to some process Energy recovery wheels are also called heat energy recovery applications. wheels and enthalpy wheels. Although many people use the phrases interchangeably, the term Latent heat transfer can occur in one of two ways. heat wheel is sometimes used to distinguish a First, if the temperature and humidity gradients are sensible only application. Enthalpy wheel is used sufficient, water may condense on the cold media to distinguish a combined sensible and latent in the high-humidity air stream. Moisture droplets energy transfer application. In both cases, the held in the media are carried to the low-humidity fundamental piece of equipment is nearly air stream where they evaporate. The second and identical. This discussion will use the term energy more reliable means of moisture transfer happens wheel as an all-inclusive class of rotary, air-to-air when the heat transfer media in the rotary wheel is energy exchangers. coated with a desiccant film. In the high-humidity air stream, desiccant material adsorbs moisture molecules by the process of vapor diffusion. The molecules ride the rotating wheel and are released (desorbed) into the low-humidity air stream. Although the overall process is considered energy transfer, the moisture adsorption-desorption process described is actually a mass transfer event. It is driven by the water vapor partial-pressure gradient between the high-humidity and low- humidity air streams. It is not dependent on the temperature gradient driving sensible heat transfer. This means that heat transfer can flow in one direction and moisture transfer can occur in the opposite direction because they are driven by different mechanisms. Fixed-Plate Heat Exchangers Figure 2. Rotary Energy Wheel. Photo courtesy of Airxchange, Inc. The heat transfer core of fixed-plate heat exchangers is made from alternate layers of plates, A rotary air-to-air energy exchanger has a formed and sealed at the edges to create two revolving wheel filled with an air-permeable adjacent but separate airflow paths. Their most medium having a large internal surface area. The distinct advantage is that they have no moving exchanger is designed to be positioned between parts. Energy transfer across the plates from one two adjacent ducts with opposing flow directions. air stream to the next is completely passive and This establishes a counter-flow heat exchange driven by the thermal gradient. Fixed plate heat pattern similar to that represented in Figure 1. The exchangers are generally useful for sensible heat wheel rotates between 10 and 60 revolutions per transfer only. Latent heat transfer can occur only 4 if the plates are made from water vapor-permeable material. Heat Pipe Heat Exchangers Heat pipe heat exchangers have the appearance of Cross-flow is the most common arrangement. ordinary finned coils, but each successive tube is Counter-flow and parallel-flow exchangers have independent and not connected to other tubes. design and manufacturing complications that Each tube is built with an internal capillary wick restrict their use. Units are available in many material. The tube is evacuated, filled with a different materials, sizes and patterns, and are