The heat-recovery market has been growing steadily because of the energy reduction incentives that utility companies offer, LEED-certified building owners striving to be more "green" and energy codes requiring heat recovery systems to be used on every new project.
One of the most efficient heat recovery systems available is the rotating enthalpy wheel. The primary reason for its higher efficiency is its ability to recover moisture (latent heat) as well as thermal energy (sensible heat). This stands in contrast to most other heat recovery systems that recover only sensible heat.
How the wheel works
The enthalpy wheel (Figure 1) is constructed of a desiccant material capable of absorbing both heat and moisture. [To view Figure 1 and Tables 1 and 2, click on the Download Now button at the bottom of this article.] Exhaust air from a building or process passes through one side of the wheel. Incoming ambient supply air being conditioned for a building ventilation system or process passes through the other side. As the wheel slowly rotates, heat energy and moisture is transferred from one air stream to the other, significantly reducing the amount of energy and moisture required to condition the incoming supply air.
Enthalpy wheels have two modes of operation. During winter months, the heating/humidification mode allows warm, wet exhaust air to preheat and humidify the cold, dry incoming supply air. During summer months, the cooling/dehumidification mode allows the cool, dry exhaust air to precool and dehumidify the hot, humid incoming air.
The numbersThe key metric for the performance of any heat recovery system is its "thermal effectiveness," the ratio of the observed transfer of energy and moisture between air streams to the maximum possible transfer. The number is a function of heat exchanger construction, including size, materials, flow path and configuration.
Enthalpy wheels have a two-component total effectiveness to account for both sensible heat and moisture. The total effectiveness values typically range from 70% to 80%, meaning an enthalpy wheel recovers about three quarters of the total available energy and moisture. Although the sensible effectiveness of other systems may approach that of the enthalpy wheel, their latent effectiveness of 0% degrades their total effectiveness.
The thermal effectiveness of ARI-certified heat recovery systems are published in the ARI 1060 Directory for Air-to-Air Energy Recovery Ventilation Equipment (AAERV), which can be found at www.ariprimenet.org/.
Find the opportunities
Start with the best bet for heat recovery — any exhaust air from a building or process that requires conditioned outside air to replace it. The most common application for an enthalpy heat recovery system involves preconditioning incoming building ventilation air with exhaust air from occupied spaces. Some design options include:
- A packaged heat recovery unit (supply fan, exhaust fan, wheel) connected to the outdoor air intake of an existing or new, larger air handling system.
- A complete air handling system, including wheel, supply and exhaust fans, heating coils, cooling coils, humidifier and controls to provide 100% outside air at any desired temperature and humidity to a space or process.
- A stand-alone enthalpy wheel module in a custom-engineered heat recovery system.
The enthalpy wheel may not be a good choice if the exhaust air is contaminated or toxic because of a small amount of carryover between the air streams (usually less than 5%). Consider other heat recovery systems that avoid direct contact between the supply and exhaust in these cases. Also, if the exhaust air is grease-laden, such as from a commercial kitchen hood, heat recovery isn’t recommended because of the risk of filter clogging and the inevitable grease coating that will accumulate in the heat exchanger.
Calculate the savings
After identifying an opportunity for heat recovery, determine the potential energy cost savings. There are a few powerful online programs that calculate heat recovery savings using hourly weather data for most cities. Results can be generated using either normal hourly data for design cases or historical hourly data to analyze the performance of an existing system. These programs are user friendly and don’t require an engineering degree to operate them. To determine the potential cost savings, quantify the following input data:
- Location and elevation.
- Exhaust and supply air flow (cfm).
- Space or process exhaust air temperature and relative humidity (if humidified).
- Hours of operation and operating schedule.
- Utility rates.
- Supply and exhaust pressure drop through the heat exchanger.
- Thermal effectiveness of the proposed heat recovery system.
The first five items are self-explanatory. If your analysis seeks to compare the performance of units from different manufacturers, use the manufacturer's data or that from the ARI directory for the last two inputs. If, on the other hand, the purpose of the analysis is to determine the feasibility of a generic system, 0.75 in. for supply and exhaust pressure drops and 70% thermal effectiveness are good working assumptions.
Quantify the savings
Tables 1 and 2 provide the results from the simulation of a 25,000-cfm enthalpy wheel heat recovery system located in various climates. This simulation was done using the Heat Recovery Analyzer program at www.climatequest.com. The system includes a supply fan, exhaust fan, enthalpy wheel and heating and cooling coils that maintain a constant 55°F supply air temperature. The system also includes a steam humidifier to maintain a 30% minimum relative humidity indoors during winter. The input data assumed for the analysis includes:
- Supply and exhaust air flows are both 25,000 cfm.
- The heat recovery system operates 24 hours per day on weekdays and is shut down for weekends.
- Conditioned exhaust air is taken from a space maintained at 75°F and 50% RH (maximum) during the cooling season and humidified to a minimum of 30% RH during winter.
- Thermal effectiveness is assumed to be 70% for both sensible and latent heat.
- The heating and humidification energy source is gas or oil at a unit cost of 70 cents per therm and a boiler efficiency of 80%.
- The cooling energy source operates on electricity with a cost of 8 cents per KWH and an overall cooling equipment efficiency of 0.90 KW/ton.
- The pressure drop across both supply and exhaust heat exchangers is 0.75 in WC.
- The enthalpy wheel stops if the outdoor air temperature is between 55°F and 75°F.
The assumption that the supply air temperature leaving the heat recovery system is always maintained at 55°F necessitates including the last bullet point. If the outdoor air temperature is between the supply air temperature and the exhaust air temperature (say 65°F), the heat recovery system would actually add heat to the incoming air before it enters the cooling coil, resulting in an increase in cooling costs. Stopping the heat recovery wheel when the outside air temperature is between 55°F and 75°F stops the unwanted heat.
This operating limit was the main reason the resulting savings for Los Angeles were so low compared to the other locations. The climate there is mild, with most hours falling into the 55°F to 75°F range. This particular application may not make sense for Los Angeles or cities in similar climates because the enthalpy wheel would not be operating most of the time. However, don’t conclude from this one example that the enthalpy wheel doesn’t make sense in a mild climate. If you increase the set point for the supply air temperature to 75°F and remove this limitation altogether, the total savings in Los Angeles would be more than $17,000.
Getting back to our example, the results in Tables 1 and 2 indicate that energy savings potential is more significant in colder climates. The projected savings in Minneapolis approached $40,000 per year (or $1.60 per CFM), most of which consisted of sensible heat recovery, but humidification savings were also significant.
Note that cooling and dehumidification savings look almost insignificant in the colder climates. But that doesn’t always mean the enthalpy wheel should be shut down or bypassed in the summer. Often it can help shave the load from cooling equipment that’s otherwise struggling to keep up with design cooling days. There is also a potential for a reduction in utility demand charges.
There’s an added bonus: Enthalpy wheels can conserve water. Because the enthalpy wheel transfers moisture, less makeup water is required for humidification systems. In Minneapolis, this example would conserve 110,000 gal of water. During summer, the cooling coil condenses less water. If condensate drainage is a concern, there’s less of it, especially in humid climates (almost 2 million gal in Miami).
Nothing is freeAlthough enthalpy wheels recover a significant amount of heat energy and moisture, there’s a cost associated with increased fan energy. The enthalpy wheel and associated filters introduce a typical pressure drop of 0.5 in. to 0.75 in. water column for each side, which translates to a fan horsepower increase. Tables 1 and 2 show the effect of fan costs for the example application.
Also, high moisture content in the exhaust air introduces the potential for frost to develop on the wheel as recovered moisture comes into contact with a cold incoming air stream. Introducing frost control reduces the amount of savings. Frost control sometimes involves preheating the incoming air slightly upstream of the wheel. Another method mixes a small amount of warm exhaust air with the incoming supply.
In general, heat recovery systems can offer a quick payback, depending on climate, hours of operation and utility rates. But for many reasons, the enthalpy wheel is often one of the better choices, especially if the incoming air requires humidification. The sidebar provides a summary of the advantages and limitations associated with the enthalpy wheel heat-recovery system.
Michael Kjelgaard, P.E., is commissioning project manager for SEI companies, Boston, Mass. Reach him at firstname.lastname@example.org.
Figures courtesy of ClimateQuest.com.
Enthalpy Wheels: Advantages & Limitations