Abstract
This article discusses the construction and operation of heat and energy wheels, as well as their effectiveness in transferring heat and moisture between air streams. Heat wheels transfer sensible heat between two air streams with different temperatures, while energy wheels transfer heat and moisture between two air streams with temperature and water vapor concentration differences. Heat and energy wheels have high effectiveness values and low pressure drops, making them both economical and environmentally friendly.
INTRODUCTION
Heat wheels, as the name implies, are rotating heat exchangers that transfer heat between two different air streams (Fig. 1). Energy wheels are very similar to heat wheels except that they are designed to transfer moisture, as well as heat, between the two air streams. They are called energy wheels (and sometimes enthalpy wheels or desiccant-coated heat wheels) because they transfer sensible energy that results from temperature differences between the two air streams and latent energy that results from water vapor concentration differences between the two air streams. Heat and energy wheels are gaining popularity in building heating, ventilating, and air conditioning (HVAC) systems mainly because of their very high energy transfer effectivenesses. Heat/energy wheels are made of many different materials for heat transfer, with desiccants utilized for moisture transfer. This article presents the construction and operation of heat/energy wheels, as well as their effectiveness in transferring heat and moisture between air streams. Economic and environmental issues are also presented.
Heat Wheels
Heat wheels are rotating exchangers that transfer sensible heat between two air streams with different temperatures. These wheels have been used for half a century in gas turbine plants and electrical power generating stations to recover thermal energy from the exhaust gases and preheat inlet combustion air, thus increasing the overall plant thermal efficiency.[1-3] Typical rotating speeds are in the order of 20-30 revolutions per minute (rpm).
Energy Wheels
Energy wheels transfer both heat and moisture between two air streams. They have more recently been developed for transferring heat and moisture between buildings’ supply and exhaust air streams. Their market share has increased significantly in the last decade, and energy wheels make up over half of all new air-to-air heat/energy exchangers installed in buildings. Their popularity can be attributed to increases in required outdoor ventilation rates in the late 1980s and a recent emphasis on improving worker productivity and health by improving the thermal comfort and indoor air quality conditions in buildings where humidity is a key variable.[4-6]
After standard ventilation rates were sharply decreased to reduce energy use in buildings in 1975,[7] the number of buildings with air quality problems resulting from high indoor air concentrations of contaminants increased significantly. Inadequate mechanical ventilation for new air-tight buildings, constructed in the 1970s and 1980s, led to many new and unforeseen indoor air quality problems.[8] As a result, most industrial countries revised their ventilation standards to include higher outdoor ventilation air flow rates. For example, the 1989 and 2004 ASHRAE ventilation standards[9'10] specify required ventilation air flow rates, which are about three times larger than they were in 1975,[7] to maintain acceptable indoor air quality for a wide range of building types and spaces. In terms of improved productivity, it has been estimated that the annual benefit would be $55 billion if all U.S. buildings were upgraded to meet current ventilation standards.[11] The average economic payback time is expected to be 1.6 years.
Naturally, the costs and environmental impacts of energy consumption have dictated that improvements in productivity, health, comfort, and indoor air quality should be achieved with minimal energy consumption, and energy wheels have been favored because the energy associated with moisture transfer (humidification or dehumidification) in building applications is often as important as heat transfer, especially in warm, moist climates. The importance of moisture transfer is evident when the cooling of moist air is considered. For example, the ideal cooling of air from 35°C and 60% relative humidity (RH) to 25°C and 50% RH requires four times as much energy as cooling air from 35 to 25°C with no change in moisture level (i.e., humidity ratio). Moisture transfer in air-to-air energy wheels can significantly reduce the dehumidification and humidification loads of buildings. This reduces the energy consumption, as well as the size of the cooling equipment needed in the building.[12]
Fig. 1 Schematic of a heat/energy wheel transferring energy between the supply and exhaust air streams of a building.
Desiccant Drying Wheels
Desiccant drying wheels are very similar to energy wheels in that they transfer both heat and moisture between two air streams. The main difference is that desiccant drying wheels are designed with an emphasis on air-drying, rather than energy transfer. To maximize moisture transfer, desiccant drying wheels use thick desiccant coatings, slow rotational speeds (< 1 rpm), and external heat sources to dry the wheel and remove as much moisture as possible from the moist air stream (supply air for buildings).[13'14] Energy wheels, on the other hand, are passive devices that transfer heat and moisture with minimal external energy input.[15]
CONSTRUCTION AND MATERIALS
Heat/energy wheels come in many different sizes and are constructed with many different materials. The core of the wheel, known as the wheel matrix, permits heat and moisture transfer between the two airstreams. One of the most common matrix materials is aluminum because of its high thermal conductivity and thermal capacitance; however, other matrix materials, such as ceramics, stainless steel, plastics, paper, and a wide range of
Fig. 2 A picture of an energy wheels with a typical corrugated matrix.
A common matrix arrangement is where the matrix material is corrugated to form small flow channels (e.g., sinusoidal, as in the end view of cardboard, triangular, or hexagonal) with a height of about 2 mm (Fig. 2). These small flow channels result in a large surface area for heat and mass transfer. The effectiveness of heat/energy exchangers is strongly dependent on the heat/mass transfer surface area—the larger the surface area, the greater the effectiveness. Therefore, heat/energy wheels are characterized by high heat and moisture transfer effective-nesses[15,17] because of their high heat transfer surface area to volume ratios, which result from these small flow channels. Commercial heat/energy wheels often have heat/mass transfer surface area to volume ratios of about 1000-5000 m2/m3. This means that for each cubic meter of exchanger, there are 1000-5000 m2 of surface available to transfer heat and moisture. The net result is that it is possible to transfer a lot of energy with very compact wheels.
OPERATION
In a majority of installations, the two air streams flow through the heat/energy wheel in a counterflow arrangement to ensure the maximum possible heat/moisture transfer between the air streams. To explain how heat/ energy wheels transfer heat and moisture between two air streams, Fig. 3 shows a side view of one of the flow channels in both a heat wheel and an energy wheel. For this example, the supply air stream is assumed to be hot and humid, while the exhaust air stream is assumed to be cool and dry. These conditions are representative of an air conditioned building during summer operating conditions.
As the hot supply air flows through the flow channel of a heat wheel, heat is transferred from the hot air to the cooler wheel matrix (Fig. 3). During this part of the wheel cycle, the air stream is cooled and the wheel matrix is heated. As a result, the air leaving the wheel on the supply side is cooler than the air entering the wheel on the supply side and therefore less auxiliary energy is required to cool it for use in the building. During the other half of the wheel rotation, the cool exhaust air flows through the wheel. Here, heat is transferred from the warm matrix to the cool exhaust air. This cools the matrix and heats the air. The warm air is exhausted out of the building and the cooled matrix rotates back to the supply side, where it can cool the supply air again. This cycle repeats as the wheel continually rotates between the supply and exhaust air streams. For a typical wheel speed of 20 rpm, each flow channel in the matrix stores heat from the hot supply air and releases it to the cool exhaust air once every 3 s. Therefore, the matrix is exposed to hot air for 1.5 s followed by cold air for 1.5 s. The rotational speed of the wheel and the thermal capacity of the matrix are key parameters that affect the ability of the heat wheel to store and transfer heat between the two air streams. Other important parameters are the air flow rate and the heat transfer surface area.
Fig. 3 A side view of a heat wheel and energy wheel matrix. Heat and moisture are transferred to the matrix from the supply air and removed from the matrix by the exhaust air.
The operation of an energy wheel is very similar to the operation of a heat wheel. The only difference is that the matrix of the wheel is also coated with a desiccant that stores moisture. Fig. 3 shows that moisture is transferred from the humid air stream to the hygroscopic desiccant on the supply side and removed from the desiccant by the dry air stream on the exhaust side. Here, the moisture storage capacity is an important parameter that affects the moisture transfer performance of the energy wheel. There are many types of commercial desiccants, but two of the most common are molecular sieve and silica gel. The equilibrium moisture content of silica gel is almost linearly dependent on RH and, therefore, silica gel is better-suited for applications of air-to-air energy wheels in buildings. Molecular sieve desiccants, on the other hand, are often favored in desiccant drying wheels.[18]
PERFORMANCE FACTORS
Effectiveness
Effectiveness is the most important parameter for quantifying the performance of energy exchangers, including heat/energy wheels. It is the prime factor that determines the economic viability or feasibility of an energy exchanger. Since the inlet operating conditions (temperature, humidity, and air flow rate) usually change quite slowly in typical building and other applications, the effectiveness, determined at steady-state test conditions, is used to characterize the performance of energy exchangers. The effectiveness can be measured using steady-state[19,20] or transient test methods,[21,22] but care must be taken to ensure that the uncertainty in the reported effectiveness value is low.[20,23] In most cases, it is advisable to use third-party certified wheels because the effectiveness may be verified with greater confidence and lower uncertainty.[24,25] The following equations define the three different effectiveness values for heat/energy wheels, which can range from 50 to 85% for commercial wheels.[15,17]
Sensible effectiveness for heat and energy wheels:
Latent (or moisture transfer) effectiveness for energy wheels:
Total energy transfer effectiveness for energy wheels:
where: es is the sensible effectiveness of the heat/energy wheel; ei is the latent or moisture transfer effectiveness of the energy wheel; et is the total effectiveness of the energy wheel; m is the mass flow rate of dry air (kg/s); T is the temperature of the air (°C or K); W is the humidity ratio of the air (kg/kg); h is the enthalpy of the air (kJ/kg); ;wmin is the minimum of the supply (ms) or exhaust (me) air mass flow rates; and subscripts i, o, s, and e represent the inlet, outlet, supply, and exhaust sides of the heat/energy wheel.
In comparing the energy efficiency of heat/energy wheels and chillers, the recovered efficiency ratio (RER) is useful. The RER is defined in a similar manner to the energy efficiency ratio (EER) used for chillers—that is, RER equals the recovered energy rate for the exchanger at Air-Conditioning and Refrigeration Institute (ARI) test conditions[24] divided by the sum of the electrical power input for the fans to overcome the pressure drop across the wheel and the electrical power input for the motors to rotate the wheel. Values of RER range from 10 to 60 for heat wheels and from 40 to 100 for energy wheels.[15]
Pressure Drop and Leakage
All heat exchangers cause pressure losses in the fluid streams and are subject to possible leakage between the fluid streams. These factors should be considered and quantified in every design. The pressure drop across a heat/ energy wheel is typically quite low compared to other air-to-air heat exchangers, with typical values ranging from 50 to 200 Pa in each air stream.[15,17] Depending on the location of the supply and exhaust fans and dampers, the pressure difference between the two air streams may be higher and result in small leakage flows and cross contamination between the supply and exhaust air streams. In addition, there is carryover of exhaust air into the supply air stream each time the wheel rotates and carries the entrained gas within the flow channels into the supply air stream. These effects can be minimized by positioning the fans so that leakage is from the fresh air on the supply side to the stale air on the exhaust side. This can be accomplished by applying good seals and installing a purge section, which uses the supply air to purge the exhaust air from the wheel before the wheel rotates into the supply air steam (Fig. 1). For some applications, toxic or hazardous contaminants in the exhaust gases will suggest that the heat/energy wheels may not be an appropriate selection, even when a purge section is used to reduce cross contamination to less than 1% of the flow rate. In other applications, large pressure differences between the exhaust and supply air ducts suggest that heat/energy wheels (as well as plate heat exchangers) will experience excessive leakage.
The ARI rating standard[24] specifies two factors that characterize the leakage between supply and exhaust air streams: the exhaust air transfer ratio (EATR) and the outside air correction factor (OACF). The EATR represents the fraction of exhaust air that is transferred (recirculated) into the supply air through the exchanger and typical values are from 1 to 10%. This means that from 1 to 10% of the exhaust air is transferred to the supply air stream and delivered to the space. The OACF represents the increase in outdoor ventilation rate required because of leakage between the supply and exhaust air streams. Typical values of OACF are from 1.0 to 1.1. An OACF of 1.05, for example, means that 5% of the supply air is transferred from the supply inlet side to the exhaust outlet side and not delivered to the building space. Therefore, the supply air flow rate must be increased by 5% to provide adequate outdoor ventilation to the building space.
Frosting
Condensation and/or frosting can occur in any air-to-air heat/energy exchanger under cold weather operating conditions. As the warm moist air passes through the exchanger, it can be cooled to the dew point temperature. Water will begin to drain from the exchanger if the temperature is above freezing, while frost will begin to accumulate within the exchanger if the temperature is below freezing. If condensation is to occur under system design conditions, a condensation drain should be provided. Excessive condensation may degrade the desiccant coating used in energy wheels. If frosting would unacceptably compromise ventilation, or if equipment would be damaged by frost, a frost control scheme should be provided. Frost control of heat/energy wheels can be achieved using several different methods, including supply air bypass or throttling, preheating the supply or exhaust air inlet flow, and wheel speed control. The manufacturer should address the issue of frost control for units intended for use in cold climates.
Energy wheels are much less susceptible to condensation and frosting than heat wheels and other sensible heat exchangers because they simultaneously transfer heat and moisture. Therefore, as the warm moist air passes through the energy wheel, it is simultaneously cooled and dried. As a result, the air is less likely to reach saturation (100% RH or dew point temperature). For many applications, outdoor temperatures as low as — 30°C will not cause frosting in energy wheels, while heat wheels may experience frosting at outdoor temperatures as high as 10° C.[26,27]
Reliability
Heat wheels have a long history of very good maintenance and reliability characteristics. The experience with energy wheel applications is much shorter, which may cause some reservations about their long-term performance. Nevertheless, there are long-term experiences with desiccant drying wheels used as supply air dryers. Provided the desiccant coatings of these desiccant dryer wheels are not contaminated by solvents or excessive amounts of organics or dust, or eroded by particulates, they can have the same long-term performance characteristics as heat wheels. Although energy wheels operate at much lower temperatures than desiccant drying wheels and may use slightly different desiccant coatings, it is expected that they will have the same long-term reliability and vulnerability to airborne contaminants. The risk of such exposures will be small for well-designed systems, with correct filters being a key component.[17]
ECONOMIC AND ENVIRONMENTAL FACTORS
Economic and environmental considerations are the stimuli for the application of heat/energy wheels. This section will summarize some studies in the literature that demonstrate the payback, life cycle costs (LCC), and environmental impact of heat/energy wheels in the HVAC systems of buildings.
Payback
The time required to pay back the initial investment (payback period) depends on whether the heat/energy wheel is installed during the design and construction of a new building or the retrofit of an old building. In a new building, the size of the heating and cooling equipment can be reduced substantially when heat/energy wheels are applied. In many cases, the capital cost savings realized by downsizing the heating and cooling equipment are greater than the cost of the heat/energy wheel. This results in an immediate payback, with future energy savings realized from essentially no investment. In some cases, the cost of the new system will be greater with a heat/energy wheel than without a wheel. For these cases, payback periods of less than one year can be expected for a climate with significant heating and cooling loads, such as Chicago, Illinois.[28]
If a heat/energy wheel is to be retrofitted into a building with existing heating and cooling equipment, there is little or no possibility of reducing the size of the heating and cooling equipment unless they need replacing. In these retrofit cases, payback periods will be longer. The payback period depends strongly on the ventilation air flow rate, as well as the local climate and energy costs. Payback periods may range from two years for heat wheels to one and a half years for energy wheels in Chicago, for example.[28] Payback periods will be shorter in more severe climates (i.e., climates with more heating and cooling degree days) and will be longer in milder climates.
Life Cycle Costs
For longer life cycles (e.g., ten years for heat/energy wheels), the costs over the entire life cycle provide a more meaningful economic assessment. In fact, large savings may be lost when design decisions are based only on short-term objectives, i.e., the payback period, instead of long-term objectives, i.e., LCC.[28,29] The LCC are almost always lower for buildings with heat/energy wheels than those without heat/energy wheels. The LCC associated with supplying and conditioning outdoor ventilation air for a 120-person office building in Chicago over a ten-year life cycle are about 25%-50% lower with a heat/energy wheel than without one.[28] The energy savings over the life cycle exceed the capital costs by a factor of five. The LCC can be further reduced by about 10% by applying both a heat wheel and an energy wheel in the same system.[29] In this dual wheel system, the life cycle savings in energy exceed the capital costs by a factor of seven to eight and additional heating of the ventilation air may not be needed, even for a reasonably cold climate like Chicago, Illinois.[15]
Life Cycle Analysis
The environmental impact of many products can be assessed using the life cycle analysis (LCA) method. Life cycle analysis considers the impact of a product on the environment during its entire life cycle—from production to disposal. This includes the extraction of basic and energy raw materials, the production processes of materials and products, transportation, utilization and recycling. Life cycle analysis is similar to LCC analysis in that both address issues over the life of the product or system, rather than basing decisions on the first capital cost. However, LCA and LCC differ in their measuring metric; LCC uses money as the comparison scale, while LCA uses environmental indicators, such as carbon dioxide emissions (indicating climate change), sulfur dioxide emissions (indicating acidification potential), ethene or ethylene emissions (indicating ozone formation), and others. For many building service components, such as heat/energy wheels, the main consumption of raw materials occurs during production and the main consumption of energy occurs during use.[30-32]
Research[30-32] demonstrates that energy recovery from the exhaust air of office buildings and single-family residences is clearly an environmentally friendly solution in a cold climate (Helsinki, Finland). Energy recovery totally compensates for the harmful environmental impacts that arise from the manufacture, maintenance, and operation of the heat/energy wheel and the entire air-handling unit. A ventilation unit, with its function of providing outdoor ventilation air, but not heating the air, has a net positive impact on the environment when it includes a heat/energy wheel with an effectiveness over 20%. The greater the effectiveness, the greater the positive impact on the environment. For typical effectiveness values of 70%-75%, the emissions reduction as a result of energy recovered by the heat/energy wheel exceeds the total emissions of the air-handling unit by five to ten times. These reduced emissions result in reduced potential harmful changes to the environment. The reduction in environmental impacts due to heat/energy recovery exceeds the total impacts of the air-handling unit by two to ten times. Similar conclusions are expected in most locations, but the magnitude of the environmental benefits from heat/energy wheels will depend mainly on the local climate, energy sources, efficiencies, hours of operation, and ventilation rate. Therefore, a building with a highly effective heat/energy wheel and a higher outdoor ventilation rate may have the same environmental impact as a building with a heat/energy wheel of a lower effectiveness and lower ventilation rate. Since ventilation, indoor air quality, and health are closely related,[4] heat/ energy wheels are healthy for occupants and the environment.
CONCLUSIONS
Heat wheels are rotating heat exchangers that transfer heat between two air streams. They are found in thermal power plants and the HVAC systems of buildings. Energy wheels are also rotating exchangers, but transfer both heat and moisture between two air streams and are mainly found in HVAC systems. Heat/energy wheels are distinguished from other air-to-air heat/energy exchangers because of their high effectiveness values and large internal surface areas. When properly applied, heat/energy wheels are cost-effective and reduce energy consumption and environmental impacts. In new buildings, heat/energy wheels often have an immediate or very short (less than one year) payback period, while in the retrofit of existing buildings, the payback period will be a little longer (e.g., two to four years). In nearly all cases, the total life cycle costs will be lower when heat/energy wheels are applied. The reduction in life cycle costs may be in the order of 25%-50% and the energy savings may exceed the capital costs of the exchanger by an order of magnitude. An additional benefit of heat/energy wheels is that they generally have a positive impact on the environment because they reduce energy consumption and the harmful emissions associated with energy production.
