Residential Ventilation Cooling Technology Development

Development and Testing of an Integrated Residential Night Ventilation Cooling System David A. Springer Leo I. Rainer Willard L. Dakin PE ABSTRACT A project completed in 2004 titled “Alternatives to Compressor Cooling” showed that carefully controlled mechanical ventilation cooling can significantly reduce summer peak load, and can eliminate the need for vapor compression cooling in areas between the California coastline and the central valley. Applications for this technology include dry climates where vapor compression cooling is needed only a few days of the year, and hot dry climates that have a dry bulb temperature range approaching 30°F (17°C),which includes much of the West, Southwest, and areas of the Northeast. This paper describes the technology and its development, reviews field tests completed under California Energy Commission and Department of Energy Building America program sponsorship, and presents computer simulation and monitoring results. For the California houses and climates evaluated, cooling demand was reduced 40 to 79%, and cooling energy savings ranged from zero to 65%. INTRODUCTION Project Motives & Objectives Natural cooling, obtained by opening windows at night and closing them during the daytime, has long been employed as a means of keeping homes cool, and prior to the 1950’s was the only resource most people had for maintaining indoor comfort in summer. The home buying public has come to expect vapor compression cooling as a standard feature in new homes, even in climates where it may only be required a few weeks of the year, as in the coastal-influenced climates of California and many other areas of the West. Because of increasing demand for comfort, security concerns related to open windows, and less free time for managing windows, vapor compression cooling is increasingly relied upon for maintaining comfort. This trend has caused vapor compression cooling to be responsible for about 45% of California’s residential summer peak load but only 7% of its annual energy use (Coito and Rufo 2003). This California peak load problem prompted a study to investigate the potential for eliminating vapor compression cooling in transition climates by improving envelope design and employing nighttime precooling of building mass with outdoor air. Launched in 1994 by the California Institute for Energy Efficiency under the title “Alternatives to Compressor Cooling”, the project studied the scientific, engineering, sociological, and market issues related to residential natural cooling. Specific project goals were to develop and test building designs and systems that would reduce residential cooling electricity demand by 37% and energy usage by 60% in California Climate Zone 12, and to eliminate the need for air conditioning in Climate Zone 3 (see Table 2 for climate zone descriptions). Market-acceptable house designs were developed that were optimized for summer performance, and mechanical solutions for providing outside air ventilation were explored. Field tests were carried out and simulations were used to optimize design parameters and to identify appropriate climate applications (Huang and Zhang 1995, Huang 1999). The California Energy Commission has supported the project since 1998. The purpose of this paper is to describe the development and features of a residential ventilation cooling system, and to identify the peak load reduction and energy savings potential for ventilation cooling. Results of field tests and computer simulations are presented, and issues that affect performance such as climate factors, equipment types, and building design are addressed. ASHRAE SYMP-00244-2004 1 Denver 2005 Other Ventilation Cooling Research Substantial research has been completed on commercial building ventilation cooling (Braun and Zhang 2003, Geros 1999, Roucoult et al 1999, Kolokotroni and Aramis 1998), but research on residential mechanical ventilation cooling originated with the Alternatives to Compressor Cooling (ACC) project in 1994. Due to differences in building design, function, and cooling loads between the residential and commercial sectors, there is little about ventilation cooling system design, control, and performance that is transferable. The residential research is summarized in the numerous reports and papers that have been completed by ACC project investigators. Lutzenhiser and Hackett (1994) described the sociological issues related to the use of ventilation cooling. Meldem and Winkelman (1995) compared DOE-2 simulations of ventilation cooling with results obtained from test houses. Givoni (1997) explored the role of mass in ventilation cooling effectiveness. Freitag (1998) explored various user interface designs that would encourage the use of ventilation cooling. Loisos and Ubbelohde (1998) described marketable house designs that would maximize the opportunity to avoid compressor cooling. Huang and Zhang (1995) and Huang (1999) simulated prototype house designs with ventilation cooling under a variety of load conditions and mapped locations where ventilation cooling alone, and augmented by small vapor compression cooling systems, could meet comfort criteria. Loisos and Springer (2000) described architectural and HVAC solutions to achieve the goal of eliminating vapor compression cooling in coastal-influenced California climates. Activities specific to the project phase described by this paper continued this research and are detailed in the project final report and its attachments (Springer, 2004, and Loisos and Ubbelohde, 2004), and other internal reports produced by the investigators. Unrelated to the ACC project, La Roche and Milne (2001, 2004) have completed other relevant research on ventilation cooling controls and mass and glazing impacts. Ventilation Cooling Principles The objective of ventilation cooling is to convectively transfer heat from building mass elements (wall board, masonry, and furnishings) to cool ventilation air, and to discharge it from the house. Of course this can only occur while outside air is cooler than indoor air. During the daytime, the cool mass absorbs heat from the air, moderating the rise in indoor temperature. Provided the indoor air temperature does not rise above the thermostat cooling setpoint, air conditioner operation can be averted. The effectiveness of ventilation cooling is a function of the surface area of the mass, its specific heat and density, and the length of time the mass is exposed to cooler air. Concrete slab floors with hard coverings (not carpet) and drywall provide a convenient source of thermal mass that has a large surface area and high density. Quantifying the thermal storage capacity and effective cooling capacity of the mass is not a trivial problem and involves a complex variety of thermal storage materials with different time constants, film coefficients, air velocities, and dynamic air temperatures and load conditions. The use of calibrated models to account for the bulk effect of these factors is the most efficient way to estimate impacts on cooling energy use. Ventilation can be achieved using natural means (open windows), or fans that either pressurize or depressurize the house. In any case, both supply and relief airflow paths must be provided. Current Technology It is useful to review current ventilation cooling approaches to identify what improvements are needed to improve effectiveness and to increase market appeal. Whole house fans are commonly used for mechanically cooling homes with outside air, but they have several disadvantages. Since they require windows to be opened to allow entry of outside air, their operation cannot be automated, so homeowners must be both aware of temperature conditions and be present for them to be used effectively. Whole house fans do not filter the air, and open windows present a security risk. Because most tend to be noisy, they are frequently shut off during sleeping hours. Typical whole house fan shutters are not insulated and can result in increased envelope leakage. An automated ventilation cooling product has been offered by one manufacturer since 1990 that uses the furnace fan to move air, and a large vent damper to select between return air and outside air. The same damper provides a relief path. Control functions include initiation of ventilation cooling based on an indoor-outdoor differential temperature, and termination of operation below a fixed indoor low limit temperature. This design provides for filtration of outside air and eliminates the need to open windows, but the fixed low limit temperature discourages low temperature settings (because of over-cooling), and the ASHRAE SYMP-00244-2004 2 Denver 2005 fixed-speed fan operation can deliver more air than needed when outdoor temperatures are mild. Recently the manufacturer of this product has been working with the authors to integrate features developed under the ACC project and described in this paper. PROJECT APPROACH The work completed during the four-year period covered by this paper included the development, testing, and evaluation of an integrated ventilation cooling, vapor compression cooling, heating, and fresh air ventilation system. The goal was to emerge from the project with a nearly market-ready product. Ventilation cooling was the primary focus of this work, but demonstrating that the other system functions (heating, vapor compression cooling, and fresh air ventilation) could be acceptably integrated was also a key objective. Development Objectives The approach in developing the ventilation cooling system was to reduce cost by using the HVAC system fan for ventilation, and integrating air distribution systems and controls to the extent possible. Several criteria were established to serve as ground rules in the development of the integrated ventilation cooling system: Optimal airflow. The air flow rate must be sufficient to effectively remove heat from the mass, but excessive airflow wastes fan energy. A project goal was to identify the optimal airflow, and to design a system capable of delivering it. Positive house pressurization. Positive pressure ventilation facilitates filtration of outside air, and allows the source of makeup air to be controlled. Filtration. The ability to filter outside air is a key feature that whole house fans lack.. Integration with heating and vapor compression cooling. Using the HVAC system fan and ductwork to deliver ventilation air reduces cost by conserving equipment and assures that cool outside air is properly distributed. Integrating controls simplifies operation. Automatic operation. It is not reasonable to expect that homeowners will always turn the ventilation system on and off at the appropriate times. The fan must be controlled on an indoor-outdoor temperature differential with an offset that accounts for the fact that the fan adds heat to the supply air. Controls must also prevent over-cooling. Minimal fan energy use. Fan energy expended to cool building mass and thereby maintain comfortable indoor temperatures must be significantly lower than vapor compression cooling energy expended to maintain a comparable comfort condition. Understandable controls that provide feedback on the consequences of temperature settings. Ideally, controls should be easy to use and should encourage settings that result in optimal operation of the ventilation cooling system to provide a high level of comfort and energy savings. High reliability. Integrating ventilation cooling with HVAC systems requires automatic vent dampers, and damper failure may not be apparent to the homeowner. Vent dampers and other components should have a mean lifetime of 20 years, which is similar to that of furnaces (Appliance Magazine 2001). Reasonably compact. Most residential HVAC systems in the western states are installed in attics, where space can be limited by vaulted ceilings, trusses, and other structural members. Component Selection Air Handler. To avoid issues with gas combustion and gas valve controls, a hot water air handler was selected as the air mover instead of a furnace. An electronically commutated motor (ECM) was chosen to drive the air handler blower. The ECM allows the airflow rate to be varied in proportion to the amount of ventilation needed to meet the cooling load. The ECM has the added advantages that it permits the application of continuously variable airflow for heating, and delivers a very low airflow at high efficacy that can be used for fresh air ventilation. Since no ECM-powered air handlers were available, several were custom-fabricated for testing and demonstration purposes. Vent Damper. Following a review of available dampers, a single-blade damper was selected that was developed for residential ventilation. Shown in Figure 1, this damper allows air entering the air handler to be switched between return air and outside air, and also provides a relief air path when the damper is in the outside air position. With dimensions of about 20" x 30" x 24" (50 cm x 76 cm x 61 cm), the damper is typically located in the attic immediately above the return air grille. Ducts up to 20" (50 cm) can be connected, allowing the damper, which includes a filter slot at the air hander connection, to deliver up a ASHRAE SYMP-00244-2004 3 Denver 2005 maximum of about 2200 cfm (1038 l/sec). Leakage and durability testing verified the damper would be suitable for this application. Figure 1: Vent Damper and Alternate Positions CONTROLS DEVELOPMENT Control Overview. Available thermostats and controls were not up to the task of providing the desired feedback to the user to ensure optimal operation of ventilation cooling, or integration of ventilation cooling with other system functions. Consistent with the design criteria, a control was developed that integrates heating, vapor compression cooling, and ventilation cooling functions. The control hardware consists of a wall display unit (WDU) with a LCD screen and "soft keys" that allow the same button to be used for multiple functions. The WDU, which substitutes for the thermostat, connects to a controller using a communications bus as shown in Figure 2. All logic for system operation resides in the controller's microprocessor, which receives temperature data and settings from the WDU and communicates information back to the WDU. An outdoor temperature sensor is also connected to the controller. The conventional "W", "Y", and "G" thermostat output signals for controlling heating and vapor compression cooling are provided from the controller, which also provides a pulse width modulation signal for operating the ECM, as well as a 3-wire 24 VAC output for operating the two-position vent damper. Determining how much to cool the house interior and how fast to run the fan for ventilation cooling presents interesting challenges. If a fixed low temperature limit is used to prevent over-cooling, then the occupant would be inclined to continually raise the low limit setting to obtain comfortable morning temperatures on cool days, resulting in too high a setting for hot days. Similarly, if the fan speed is fixed, airflow might be insufficient on hot days and excessive on cooler days. Ideally, a ventilation cooling control would anticipate the next day's weather and adjust the fan speed and low limit temperature accordingly. Development of Control Algorithms. Regional weather predictions from the Internet could be used as control parameters, but broadband internet connections are not yet ubiquitous and weather forecasts might not provide adequate predictions for specific microclimates. The need for Internet connectivity was bypassed by developing a predictive function that estimates the next day's temperatures from a two-day history of temperature trends. To develop these predictive capabilities a statistical analysis was completed using monitored hourly temperature data from about 20 California residential sites to identify temperature highs, lows, averages, and trends that would provide the best prediction of next-day’s temperatures. Equations were then developed from the statistical relationships to enable calculation of predicted indoor and outdoor temperature minimums and maximums. These predicted temperatures, combined with user temperature preference settings, determine the temperature to which the house should be ventilated in order to avoid use of vapor compression cooling (the "vent target temperature"). For example, on very hot days the vent target temperature will approach the minimum temperature set by the user, but on cool days the target temperature may be as much as 10°F (5.6°C) higher than the minimum temperature set by the user. ASHRAE SYMP-00244-2004 4 Denver 2005 Temperature predictions are also used to vary the rate of airflow delivered by the fan to prevent overcooling and excessive fan energy use during mild weather. Figure 2: Control Components User Interface Development. An important objective in developing the user interface (wall display unit) was that it encourages homeowners to substitute ventilation cooling for compressor-based cooling by providing feedback on the consequence of temperature settings. To meet this objective, a “comfort bar” display was developed, an example of which is provided in Figure 3. The “low” setting is the minimum temperature to which the occupant wants the house to be cooled. The “hi” setting is the highest desired temperature and also the air conditioner thermostat set point. The shaded bar in the middle is the predicted indoor temperature range based on the user settings and current weather conditions. As the “Low” setting is raised the predicted range shifts to the right, indicating to the user that this setting will result in warmer indoor temperatures and increased likelihood of vapor compression cooling operation in the afternoon. If the shaded bar crosses the “Hi” setting, a message “Air Conditioner Will Run” is displayed below the comfort bar. Figure 3: Wall Display Screen Image for Setting Low and High Temperature Limits ASHRAE SYMP-00244-2004 5 Denver 2005 Figure 3 also helps to illustrate the strategy for establishing the vent target temperature and ventilation cooling rate. During mild weather the firmware computes a vent target that will cause the indoor temperature range (and comfort bar) to be approximately centered between the low and high settings. In this case the vent target temperature will be elevated above the “Low” setting and a lower ventilation rate will be applied. As the weather warms and the comfort bar approaches the “Hi” setting, the computed vent target temperature will approach the “Low” setting and the ventilation airflow rate will approach the maximum ventilation rate. Calculations of target temperature and fan speed are completed at midnight and remain constant for 24 hours. Abandoning the 24 VAC “R-W-Y-G” thermostat wiring convention in favor of a 4-wire communications bus allows the wall display unit (WDU) to be used for many functions in addition to establishing heating and cooling temperature setpoints. For example, airflow rates for heating, vapor compression cooling, ventilation cooling, manual ventilation, and fresh air ventilation can be set at the WDU instead of at the air handler. Additional settings that were designed into the WDU include temperature sensor calibrations, vent temperature differential, air conditioner time delay, and other settings that are accessible to the installer but hidden to the user. Optimization of Control Algorithms. It is possible for ventilation fan energy use to exceed the energy that would be used by vapor compression cooling to achieve the same indoor temperature condition. Two coefficients in the control algorithm define the relationship between ventilation rate and cooling demand (determined by predicted temperature and user settings). These coefficients were evaluated using the DOE-2.1E building simulation program. A special function was developed for DOE-2 that emulates the control functions that are programmed into the controller, and parametric analysis of the coefficients was used to minimize the sum of annual vapor compression cooling and ventilation cooling fan energy use. This analysis was carried out using weather files for four different California climate zones representing a range of mild coastal to hot inland weather. Simulation results showed the originally developed coefficients required no adjustment. Simulations also determined that the optimal ventilation rate, defined as the lowest combined annual energy use, is about 0.6 cfm per ft² (3 l/s per m²) of floor area for the house and climate zones evaluated. La Roche and Milne (2004), who studied the effects of ventilation rate, thermal mass, and windows using a small test building, arrived at a similar result of four air changes per hour, or 0.53 cfm per ft² (2.7 l/s per m²). Fortunately, this rate of ventilation requires little if any increase in duct size from what is typically provided for vapor compression cooling. “Ease of Use” Design Considerations. To insure the wall display unit would be easy to use, data were gathered from personal interviews by team social scientists and from a web survey that employed a “virtual” user interface to simulate actual WDU behavior. Numerous improvements to menu navigation, cooling and heating settings, and development of on-screen “help” instructions were implemented as a result of this work Field Test Following completion of laboratory testing to verify that the hardware and firmware performed in accordance with specifications, production builders were sought to provide demonstration houses for the integrated mechanical system. One site was identified in Watsonville, which is near the California coast, and a second was found in Livermore, which is representative of the North Central Valley climate. Both demonstration houses exceeded California Title 24 prescriptive energy standards for space conditioning. Measures identified through previous ACC project research were incorporated into the house designs, and are listed in Table 1. The two-story 1611 ft² (150 m²) Watsonville house has no vapor compression cooling and the one-story 3080 ft² (286 m²) Livermore house is equipped with two 2-ton (7 kW) air conditioners with a SEER rating of 10. A tankless water heater and a condensing water heater were used as heat sources for the air handlers at the Livermore and Watsonville sites, respectively. The houses were monitored for one year to measure cooling energy use (by fans and compressors), and to evaluate comfort. Limited summer data were collected from another house having the same floor plan as the Livermore house, to serve as a comparison case. The project primarily relied on use of building simulations using a calibrated model to identify energy savings, as later described. ASHRAE SYMP-00244-2004 6 Denver 2005 TABLE 1 Demonstration House Design Features Livermore Site High performance (Low E²) windows Radiant barrier roof sheathing > 50% hard surface flooring 5/8” (1.6 cm) drywall on all walls & ceilings 2” (5 cm) isocyanurate slab edge insulation Exterior window shading (trellises) Watsonville Site High performance (Low E²) windows Radiant barrier roof sheathing > 50% hard surface flooring 5/8” (1.6 cm) drywall on all walls & ceilings Data collected from the Watsonville field test site were of limited value because the owners rarely used the mechanical systems for either heating or cooling. Indoor temperature extremes of 55°F (12.8°C) and 83°F (28.3°C) were measured. Fan and pump energy use measured over the one-year period totaled only 93 kWh. The efficient envelope design clearly played a role in moderating indoor temperatures. Avoiding energy use was the owner’s stated motivation for tolerating what some would consider uncomfortable temperatures. The Livermore house produced more telling results. There were 15 days with outdoor temperatures over 100°F (37.8°C) during the summer of 2003. Fan energy (for ventilation cooling, vapor compression cooling, and heating), pump energy (for heating), and air conditioner condenser energy totaled 901 kWh for the year. The two air conditioners were operated for a combined total of 8.9 hours, and indoor temperatures were maintained generally below 80°F (26.7°C). Over two summers there was only one twohour period during which the sum of the part loads of the two air conditioners exceeded 1.0, indicating that one 2-ton (7 kW) air conditioner is capable of maintaining the house within the owner’s comfort range more than 99% of the time. The resulting sizing ratio of 1540 ft² per ton (4.06 m²/kW) compares with an average California sizing ratio of 543 ft² per ton (1.43 m²/kW) reported by Hoeschele (2002) from a survey of 30 houses built after the year 2000. Figure 4 compares temperatures and energy use for the Livermore demonstration house (Vent Cooling System) to a standard house in the same development (Control House) with the identical floor plan and orientation for the same one-day period. The control house has none of the measures listed in Table 1, except both houses have high performance windows. The standard house has one 5-ton (5.5 kW) air conditioner with two zones. Both houses were occupied by married couples with no children, and thermostat settings were similar. On the day depicted in Figure 4 the ventilation-cooled house used 81% less cooling energy than the control house, and maintained lower indoor temperatures, on average. Figure 4: Monitored Performance – Typical Summer Day ASHRAE SYMP-00244-2004 7 Denver 2005 Model Calibration and Simulation Results In order to project monitoring results from the Livermore house to other climates, a calibrated DOE2.1E model was developed by adjusting thermal mass inputs to achieve as close a match as possible between measured and simulated indoor temperatures. The model incorporated the same ventilation cooling control function that was developed for optimizing control parameters. For calibration purposes, the rate of indoor temperature change was found to be a better indicator of how the house responds to both daytime heat gain and nighttime ventilation cooling than the absolute indoor temperature. Figure 5 graphs the rates of change of indoor temperature monitored at the Livermore house to simulated data for the same house. To identify any dependence on outdoor temperatures, rates of indoor temperature change were separated into two bins, one coinciding with outdoor temperatures ranging from 89-100°F (32°C - 38°C) and the other coinciding with outdoor temperatures exceeding 100°F (38°C). Figure 5 shows that the calibrated DOE-2 model matched monitored data for the two bins reasonable well. To further verify the model, a full year simulation was completed to compare annual energy use. The resulting difference between simulated and monitored annual cooling energy use was only 5%. Figure 5: Comparison of Monitored and Simulated Rates of Indoor Temperature Change Using Calibrated Model Additional simulations were completed with the calibrated model in all sixteen California climate zones using the 3080 ft² (286 m²) Livermore plan. For reference, Table 2 lists the representative cities on which the sixteen California climate zones are based. Zones 1-3 are in the DOE “Marine” climate region, zones 4-15 are in the “Hot-Dry” region, and zone 16 is in the “Cold” region. TABLE 2 Representative Cities for California Climate Zones Zone 1 2 3 4 City Arcata Santa Rosa Oakland Sunnyvale Zone 5 6 7 8 ASHRAE SYMP-00244-2004 City Santa Maria Los Angeles San Diego El Toro Zone 9 10 11 12 8 City Pasadena Riverside Red Bluff Sacramento Zone 13 14 15 16 City Fresno China Lake El Centro Mount Shasta Denver 2005 To separate the value of the added “ACC” design improvements listed in Table 1 from the effects of ventilation cooling, three sets of simulations were completed for each climate zone. The first set applied a base case house designed to meet California energy standards, the second set simulated the same house with ACC design improvements, and the third set simulated the house with ACC design improvements and with ventilation cooling. Figure 6 compares the house peak electrical loads (including HVAC, lighting, and appliances) for each of the three cases. While the highest percentage of demand reduction is achieved in the more mild climate zones (2-9 and 16), the warmer climate zones (10-15) have greater potential for reducing overall demand. The reduction in peak load resulting from the combined effects of envelope design improvements and ventilation cooling ranged from 9% to 94%. The reduction in demand was as high as 6.7 kW in Climate Zone 15 and there were no savings in the marine Climate Zone 1. Averaging results for all climate zones, envelope improvements and ventilation are equally responsible for reducing the peak load. Results also suggest that the ACC design applied with ventilation cooling might eliminate the need for vapor compression cooling in zones 3, 4, 5, 6, 7, and 16, since all have peak loads of nearly the same magnitude as Climate Zone 3, where the Wastonville field demonstration showed that ventilation cooling easily maintained comfort. Figure 6: Simulated Cooling Demand in the California Climate Zones Figure 7 graphs the percent energy savings determined from simulations, compared to the base case 3080 ft² (286 m²) house built to California Title 24 energy standards. This graph also distinguishes between savings resulting from ACC design improvments from those resulting from ventilation cooling. In the coastal climates (1, 3, 5, and 6) all energy savings are attributed to envelope improvements, suggesting that ventilation cooling is unnecessary if attention is given to the building design. However, the larger share of the savings in the warmer inland climates are attributable to ventilation cooling. Another series of DOE-2 simulations was completed to identify ventilation cooling energy savings in sixteen U.S. cities using a more typical 1860 ft² (173 m²) one-story house plan with design features similar to those used in the Building America benchmark (Hendron et al 2004), which vary by climate region. Since there were no changes to the building envelope, simulated energy savings, presented in Table 3, are strictly attributable to ventilation cooling. This lack of envelope improvents is one reason energy savings shown in Table 3 for Sacramento are less than those shown in Figure 7. A 2°F (1.1°C) lower air conditioner thermostat setting and more conservative occupancy assumptions also contributed to the difference in results. ASHRAE SYMP-00244-2004 9 Denver 2005 Figure 7: Simulated Energy Savings in the California Climate Zones In humid climates the outside nighttime air temperature may be low enough to provide cooling, but if the enthalpy of outside air is higher than that of indoor air, ventilation cooling can have undesirable comfort and moisture consequences. To produce the results shown in Table 3, a limit was added to the control function in the DOE-2 model that prevents ventilation cooling from occurring while the enthalpy of outside air is greater than that of indoor air. Also, the simulated air conditioner thermostat setting of 76°F (24.4°C) used in dry climates was lowered to 74°F (23.3°C) in humid climates in an effort to maintain indoor humidity ratios below 0.012. Adding differential enthalpy control to the contol algorithm did not affect energy savings in the dry western climates but significantly reduced predicted energy savings in most of the humid eastern and southern locations. TABLE 3 Simulated Energy and Demand Savings for Sixteen U.S. Cities Location Albany, NY Atlanta, GA Bismarck, ND Chicago, IL Denver, CO El Paso, TX Ft. Wayne, IN Houston, TX Jacksonville, FL Memphis, TN Philadelphia, PA Raleigh, NC Sacramento, CA Salt Lake, UT St. Louis, MO Tucson, AZ ASHRAE SYMP-00244-2004 Cooling Energy Savings kWh/yr % 152 22% 398 11% 214 34% 187 15% 223 33% 988 17% 167 15% 804 10% 475 8% 500 10% 280 12% 446 14% 537 35% 397 18% 390 11% 945 13% 10 Denver 2005 DISCUSSION Importance of Envelope Design Features The value of improving the envelope design to reduce cooling load is made clear by Figure 7. Coupling improved envelope design, particularly increased thermal mass, with ventilation cooling offers the greatest opportunity to eliminate vapor compression cooling in less extreme climates. Cost studies showed that the elimination of the condensing unit approximately balances the added cost of the ventilation cooling components (Springer, 2004). If universally applied in a production home scenario, measures such as thicker drywall and exterior shading using architectural features may be inexpensive to implement. Behavioral and Comfort Preference Issues If homeowners routinely operate windows to achieve natural ventilation cooling, savings from mechanical ventilation cooling are lower, especially if the house design and climate favor thermal and wind-driven ventilation. Windows are used less for ventilation than in the past because of security concerns, traffic noise, allergies, and decreased daytime occupancy (fewer stay-at-home parents). Mechanical ventilation cooling systems with automated controls offer the opportunity to achieve more uniform savings than can be realized with manually operated whole house fans or window fans, and are therefore more suitable for implementation through energy programs. Energy savings and peak demand reduction will also be affected by how mechanical ventilation cooling systems are used. A low air conditioner setpoint and/or a high low limit setpoint will diminish cooling savings, which is why the wall display was designed to encourage a wide spread between these settings (refer to Figure 3). Climate Applicability Mechanical ventilation cooling, particularly when coupled with building envelopes with increased mass, can be effective in most of the dry western states, and has the potential to eliminate compressor cooling in “transition” climates between the cool coastal areas and inland valleys, as well as inland mountain areas. Locations with design temperatures exceeding 95°F (35°C), diurnal temperature swings greater than 30°F (17°C), and with design wet bulb temperatures of 70°F (21°C) or lower are the best candidates. Though it is less likely to be cost-effective, mechanical ventilation cooling may also find a market in areas that have predominantly dry summer weather but with periods of high relative humidity, such as the Southwest. Factors Affecting Energy and Demand Savings It is much more difficult to project energy and demand savings for ventilation cooling than for incremental improvements in air conditioner efficiency because there are many more variables involved, both physical and behavioral. In estimating energy savings using building simulations, results are very sensitive to benchmark assumptions for house design and orientation, occupancy schedules and internal loads, temperature settings, and particularly the assumptions used for window operation. For example, default assumptions used in some building energy models presume that windows are opened as soon as outdoor temperatures fall below indoor temperatures; from observations made in the ACC project this is not realistic (Springer, 2004). Project results suggest that energy and demand savings are significant when ventilation cooling is applied to either unimproved or improved building envelopes. However, the greatest opportunity for eliminating vapor compression cooling usage entirely is to marry ventilation cooling to improvements such as enhanced thermal mass, high performance windows, shading, and measures to reduce roof heat loads (radiant barrier and/or increased attic insulation). An understanding of the principals of ventilation cooling by homeowners will also improve energy savings because it will influence more favorable thermostat settings and encourage window use to supplement mechanical ventilation. Furnaces vs. Air Handlers Although this project used hot water air handlers to move ventilation air, furnaces can also be used, and they are much more predominant in the marketplace. Furnaces, particularly those equipped with ECM motors, potentially can provide the same variability in airflow as the air handler used in this study. A ASHRAE SYMP-00244-2004 11 Denver 2005 follow-on project sponsored by the California Energy Commission is currently adapting controls developed for air handlers to operate ECM-equipped furnaces. Hot water air handlers offer the added advantage of allowing the airflow to be continuously varied in heating mode, a feat that cannot be achieved with furnaces. Air handlers also do not require venting or gas piping, but they do require an external heat source. Appropriate heat sources include boilers, condensing water heaters, and instantaneous gas water heaters. Because of the superior efficiency of these appliances compared to pilot-operated gas storage water heaters, water heating energy savings can be a side benefit of using air handlers. For example, storage gas water heaters that typically have an energy factor of about 0.60 can be replaced by instantaneous water heaters which have an energy factor of 0.80 or higher, and that have sufficient capacity to meet space heating needs. Cost-effectiveness Initial Cost. The incremental cost for an integrated night ventilation cooling system, including air handler, vent damper, controls, and instantaneous water heater, is about $2700 compared to a standard furnace and conventional gas water heater. These components provide additional value since they meet fresh air ventilation requirements, improve comfort, and reduce gas use for water heating. The incremental cost for the damper and controls for a furnace-based system will probably be about $1200. Both systems have the capability to provide summer and winter fresh air ventilation, thereby displacing costs for separate fresh air ventilation systems. The cost of improving the building envelope to achieve greater benefit from ventilation cooling depends upon how many improvements are needed. For example, a house built on a concrete slab with hard-surface floor coverings would not require additional thermal mass. Both monitoring and simulation results support the conclusion that vapor compression cooling can be downsized, or eliminated in some climates, resulting in a significant reduction in initial cost, and zero incremental cost if vapor compression cooling is eliminated. Value of Energy Savings. As previously noted, energy savings are highly dependent on climate, house design, occupant behavior, and other factors. Compared to a house built to California Title 24 prescriptive standards, estimates of annual ventilation cooling electricity savings for the 3080 ft² (286 m²) Livermore plan range from $69 to $522 over the twelve California climate zones (using Pacific Gas & Electric 2004 E-1 rates). Application of time-of-use rates can improve the value of energy saved, since ventilation cooling shifts most of the cooling electrical use to off-peak periods. Role in “Zero Energy” Homes The role that ventilation cooling and the other summer performance features can play in reducing vapor compression cooling energy use improves the opportunity to approach zero net energy use when photovoltaic systems are applied. The Livermore demonstration house is also a pilot demonstration for the Department of Energy’s Zero Energy Homes program. Since the house was completed in 2002, its 3.6 kW photovoltaic array has generated more electrical energy than the house has consumed on a net annual basis. Further Development Progress and Needs Because of the lack of participation of a major manufacturer, the project team has had difficulty interesting production builders in the air handler-based system. As a result, research is currently underway to develop a control system that will connect to ECM powered furnaces, thereby capturing the ability to apply all of the functions that were developed for air handlers to furnace-based systems, except for proportional control of the fan speed in heating mode. This work is expected to result in a product release during 2005. Large-scale field tests in a variety of climates would help define aggregated energy savings. Statistical studies to identify how people operate their windows would also inform the development of assumptions to be used for computer modeling of ventilation cooling. CONCLUSIONS Project Outcome Project objectives to develop and test building designs and systems that would significantly reduce residential cooling electricity demand and energy use were accomplished by the development of an integrated ventilation cooling system, demonstration of the system and other building improvements in two ASHRAE SYMP-00244-2004 12 Denver 2005 houses, and evaluation of monitoring and simulation data. The specific goal of reducing cooling electricity demand by 37% in California Climate Zone 12 was exceeded; simulations using the calibrated model predicted a demand reduction of 45%, about 34% of which can be attributed to the ventilation cooling system. The cooling electricity energy savings goal of 60% for Climate Zone 12 was also exceeded; energy savings were 62%, about 44% of which was from ventilation cooling. The ability to eliminate the need for vapor compression air conditioning in Climate Zone 3 was also demonstrated. System Benefits As Figures 6 and 7 indicate, the integrated ventilation cooling system that was developed under this project has the potential for having a quite significant impact on residential cooling load. Actual energy and demand savings are less predictable than for measures such as improved air conditioner performance because of greater uncertainties related to microclimate and homeowner behavior. A large-scale field test would help define these savings. Coupling automatic ventilation cooling with enhanced thermal mass, high performance windows, and exterior shading of windows offers the greatest potential for impacting load reduction on a large scale. In addition to reduced utility costs for cooling, the added cost for integrated ventilation cooling can be justified by improved indoor air quality and improved home security (no necessity to open windows). This technology can be applied to most new residential buildings in dry climates, though consideration must be given to space availability for vent dampers, access to outside air, and ductwork sizing. Retrofit applications are subject to the same considerations, and therefore some percentage of existing homes would not be good candidates for this system. Market Potential The national market for automatic ventilation cooling is primarily restricted to dry climates with large diurnal temperature swings, including hot-dry, mixed-dry, and western marine climates1. Large-scale market success could be defined by the participation of a major HVAC manufacturer, but large manufacturers are typically reluctant to market products that appeal only to specific climate regions. Smaller manufacturers are successfully marketing dehumidifiers and heat/energy recovery ventilators within specific climates, and ventilation cooling systems may fit within this market context. ACKNOWLEDGEMENTS The authors gratefully acknowledge the California Energy Commission’s Public Interest Energy Research Program which has supported the development of this technology, and the Department of Energy Building America Program which has been facilitating field studies and other research. Joe Huang deserves special appreciation for his assistance in developing the DOE-2 ventilation cooling model. REFERENCES Appliance Magazine. September 2001. A Portrait – U.S. Appliance Industry. Dana Chase Publications. Oak Brook, Illinois. ASHRAE. 1992. ANSI/ASHRAE Standard 55-1992, Thermal Environmental Conditions for Human Occupancy. American Society of Heating, Refrigeration and Air Conditioning Engineers. Atlanta, Georgia. Braun, J., Z. Zhang. 2003. Development and Evaluation of a Night Ventilation Precooling Algorithm. Deliverables 3.2.2a and 3.2.3a to the California Energy Commission Public Interest Energy Research Program. Sacramento, California. Coito, F., M. Rufo. 2003. 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