PASSIVE HOUSE AND PASSIVE SOLAR: A COMPARISON OF TWO APPROACHES TO LOW-ENERGY HEATING

Transcription

1 PASSIVE HOUSE AND PASSIVE SOLAR: A COMPARISON OF TWO APPROACHES TO LOW-ENERGY HEATING Matthew B. Hogan Department of Architecture University of Oregon Eugene, OR hogan.mat@gmail.com Alison G. Kwok Department of Architecture University of Oregon Eugene, OR akwok@uoregon.edu ABSTRACT This paper examines two approaches to low-energy heating: passive solar heating and Passive House. The design of a single family house, herein referred to as the Learning House, was conceived of as a Passive House. The Learning House design was modeled in DesignBuilder in order to determine its annual energy performance and winter design week comfort performance. The design of the Learning House was then modified so that it performed as a passive solar house. This modified design was modeled in DesignBuilder to determine its annual energy performance and winter design week comfort performance; the results were compared to the results of the Passive House model. The passive solar model was determined to have a lower annual electricity demand, while the Passive House model maintained a steadier indoor air temperature; however, the indoor air temperature of the passive solar model during the winter design week fluctuated far less than anticipated. 1. INTRODUCTION This research offers a comparison between two approaches to low-energy heating in a building: passive solar heating and Passive House. A passive solar heating system, as described by Edward Mazria, consists of south-facing glass... for solar collection, and thermal mass for heat absorption, storage and distribution. (1) For this reason, passive solar heating is often dubbed the mass and glass approach. Passive House, by contrast, is a design concept which uses superinsulation as a means of reducing the size of the HVAC system to an absolute minimum. Passive House, therefore, is an active system, as it relies on a mechanical element the heat recovery ventilator to balance internal and solar heat gains. (2) Despite its name, Passive House has more in common with the superinsulation movement of the 1970 s than with passive solar design. Alex Wilson of BuildingGreen.com describes the intent behind the superinsulation movement and its clear distinction from passive solar design: Not long after passive solar began picking up steam, along came the competing idea of superinsulation Superinsulation proponents sought to create a simpler solution with small window areas, large quantities of insulation, and simple geometries. (3) 1.1 Passive House A Passive House relies on a superinsulated, airtight envelope which reduces its heating demand to an absolute minimum. In many cases, the heating demand is so small that it can be met by utilizing internal gains (heat from occupants, appliances, and lighting) and solar gains. In order to maintain good indoor air quality, every Passive House must be equipped with a heat recovery ventilator, a device which constantly ventilates the living space while recovering heating energy produced inside the envelope. Typically, a Passive House is designed to be compact in shape, so as to minimize surface area relative to volume. The focus of the Passive House approach is minimizing heat loss through the building envelope. 1.2 Passive Solar A passive solar building relies primarily on two components: south facing glass and thermal mass. While the envelope must be well-insulated to minimize heat loss, the focus is on optimizing solar heat gain in the winter. Solar gains are stored in the building s thermal mass during the day and radiated into the living space at night. In order to maximize the potential for solar gains, passive solar

2 buildings often have a long east-west axis to maximize southern exposure. The most basic of solar heating systems is the direct gain system, in which solar radiation is used to directly heat a living space. In The Passive Solar Energy Book, Edward Mazria describes the direct gain system: In this approach, there is an expanse of south-facing glass and enough thermal mass for heat absorption and storage. Southfacing glass is exposed to the maximum amount of solar energy in winter (4) When we use the term passive solar in this paper, we are specifically referring to the direct gain system. Though passive solar and Passive House are clearly two distinct approaches to low energy heating, several similarities exist between the two. Both approaches often require supplemental heating to maintain occupant comfort on the coldest days of the year. Additionally, both approaches utilize solar heat gains; however, how and to what degree the two approaches do so is unique to each approach. Table 1 offers a side-by-side comparison of Passive House and passive solar. TABLE 1: PASSIVE HOUSE & PASSIVE SOLAR COMPARED PASSIVE HOUSE Superinsulated Airtight Small glazing area No significant thermal mass Heat recovery ventilator Compact shape Constant temperature (68 F) PASSIVE SOLAR Well-insulated Not necessarily airtight Large south glazing area Significant thermal mass HRV optional Elongated east-west axis Daily temperature swings Perhaps one of the most notable differences between Passive House and passive solar is expressed by the bottom row in the above chart. Passive Houses maintain a constant indoor temperature of 68 F (20 C), while passive solar buildings are designed for a range of temperatures; daily temperature swings are expected. This paper focuses on the design of the Learning House, which is the unrealized design of a small single-family house in Eugene, Oregon for which the authors have access to schematic drawings. Designed as a Passive House, the Learning House has 1320 square feet of living space on two floors. The goal of this paper is to compare the energy use and indoor temperature variation of the Learning House to that of a passive solar house of an identical treated floor area and volume. 2. THE PROBLEM & HYPOTHESIS The performance variables with which this study is interested are temperature variation during the winter design week, which is related to occupant comfort, and annual energy use, which corresponds to annual carbon production. This research is concerned with the following questions: 1. Can a passive solar house in Eugene, Oregon maintain an average indoor temperature of 68 F (20 C) during the winter design week while using less energy than a Passive House of an identical program, living area, and volume? 2. Can the indoor temperature swings of the passive solar house during the winter design week be kept to less than 10 F above and below 68 F (20 C)? The hypothesis of this study states that a passive solar house in Eugene, Oregon can maintain an average indoor temperature of 68 F (20 C) during the winter design week while using the same amount of energy as an otherwise identical Passive House. Additionally, the hypothesis of this study states that the passive solar house will experience indoor temperature swings within 10 F above and below the average indoor temperature. Supplemental electric resistance heat is assumed in both the Passive House and passive solar models. The thermostat in the Passive House model will be set to maintain 68 F (20 C), while the thermostat in the passive solar model will be set back to 58 F (14 C) during periods of the day when the house is assumed to be unoccupied. This will allow the authors to determine whether or not the additional thermal mass in the passive solar model is able to moderate the temperature swings. 3. METHODOLOGY 3.1 Model the Learning House in the Passive House Planning Package (PHPP) We will begin the study by modeling the Learning House design in the PHPP to confirm that it performs to the Passive House standard. The PHPP is the Passive House Institute s proprietary energy modeling software for use in the design and verification of Passive House buildings.

3 After modeling the design of the Learning House in the PHPP, minor design changes will be made as necessary so that the house performs to the Passive House standard. 3.2 Model the Learning House in DesignBuilder and Simulate Annual Energy Performance and Winter Design Week Comfort Performance The Learning House will be modeled in DesignBuilder and simulations for a typical year and the winter design week will be performed. The results of these simulations are for comparison to the simulation results of the passive solar model in the fourth phase. 3.3 Passive Heating Analysis and Redesign of the Learning House for Passive Solar Heating Using passive heating design calculations as outlined in Mechanical and Electrical Equipment for Buildings, the Learning House design will be modified for passive solar heating. (5) These calculations will provide general recommendations on envelope U-values and the ratio of mass-to-glass area. 1. Determine the solar savings fraction (SSF) of the Learning House 2. Increase area of south glazing as necessary to improve the SSF 3. Determine required area of exposed thermal mass 4. Redesign house for passive solar heating based on calculated areas of south glazing and thermal mass 5. Perform building load coefficient (BLC) calculation and overall heat loss calculation to determine whether envelope U-values are sufficient for passive solar heating in Eugene s climate 3.4 Model the Passive Solar Design in DesignBuilder and Simulate Annual Energy Performance & Winter Design Week Comfort Performance The Learning House model will be modified for passive solar as determined in the previous step. Simulations for a typical year and the winter design week will be performed. Results of these simulations will be compared to the results of the Passive House model simulations in phase two. 3.5 Means of Analysis The results of the simulations as described above will be compared to determine 1) whether or not the passive solar model has a lower annual electricity demand than the Passive House model and 2) whether or not the passive solar model experiences indoor temperature swings within 10 F above and below 68 F (20 C) during the winter design week, as stipulated in the hypothesis. Fig. 1: Passive House Model in DesignBuilder. 4. RESULTS 4.1 Passive House Planning Package Simulation Results Minor adjustments to the Learning House design, such as the use of higher quality windows and the addition of a solar hot water system, were made so that the house would indeed meet the performance requirements of the Passive House standard. According to the PHPP, the total annual electricity use for the Learning House was calculated to be 13.3 kbtu/ft 2 yr. 4.2 DesignBuilder Simulation Results Passive House Once it was determined that the design did indeed perform to the Passive House standard, the Learning House was modeled in DesignBuilder in order to simulate annual energy performance and winter design week comfort performance. According to DesignBuilder, the total annual electricity use for the Learning House was calculated to be 10.9 kbtu/ft 2 yr, corresponding to an annual CO 2 production of lbs. It should be noted that the difference between the electricity use as simulated by the two software programs is 20%. Other performance data of note is the annual electricity use for electric lighting and the annual heating from solar gains. The annual electricity use for electric lighting was calculated to be Annual heating from solar gains was calculated to be In the winter design week simulation, the house maintained the Passive House comfort temperature of 68 F (20 C) for the majority of the time. However, on two occasions, the indoor air temperature increased by several degrees; this is most likely attributed to significant solar gains through the south and west windows in the late afternoon on two clear days.

4 4.3 Passive Heating Analysis and Redesign Results The design for the Passive House was then modified to operate as a passive solar house. This was achieved by first analyzing the design of the Passive House. The ratio of south glazing to floor area was determined to be 9%, corresponding to a solar savings fraction (SSF) of 32% for Salem, Oregon (the nearest location for which data was available). This 32% solar savings fraction is rather low according to Mechanical and Electrical Equipment for Buildings, which gives a range of 37% (low) to 59% (high) for passive solar buildings with superior performance glass. (6) The fact that the solar savings fraction is so low provides further evidence that a Passive House is distinct from a passive solar house; a Passive House has a much lower solar savings fraction than a passive solar house. Therefore, the south glazing area of the Learning House must be increased to perform as a passive solar house. A target solar savings fraction of 50% was chosen as a starting point for the passive solar design. In order to achieve a solar savings fraction of 50%, the area of south glazing must be increased to 19% of the floor area, or ft 2. At a solar savings fraction of 50%, the passive solar house would require masonry surface area equivalent to 3.7 times the south glazing area (according to design data in Mechanical and Electrical Equipment for Buildings), or 1005 ft 2. (7) As an alternative, a water wall would require much less surface area (0.5 times the south glazing area, or 136 ft 2 ). (8) The area of south glazing was increased to 267 ft 2 by expanding the width of the window bank at the center of the south wall and adding 18 inch high clerestories above all south facing windows. Thermal mass was then added to the building by changing the material of the floor system and kitchen wall to concrete. The resulting area of exposed thermal mass is 1094 ft 2. Though the floor system of the passive solar house differs from the Passive House, the U- value of the two assemblies was kept constant. TABLE 2: U-VALUES AND AREAS COMPONENT U-VALUE (Btu/hr ft 2 F ) AREA (ft 2 ) UxA (Btu/hr F) Roof NS* Opaque Wall NS Windows NS Doors Floor Total UA ns envelope *Non-south Since the passive solar house will be equipped with a heat recovery ventilator with an efficiency of 92%, it was estimated that the heat recovery element would recover all but 8% of potential ventilation losses. Therefore: UA ventilation (with heat recovery) = Btu/h F = 7.16 Btu/h F BLC = 24 hr/day(ua ns envelope + UA ventilation ) = 24 hr/day(79.4 Btu/h F Btu/h F) = Btu/DD Overall Rate of Heat Loss = BLC/Floor Area = Btu/DD/1320 ft 2 = 1.57 Btu/DD ft 2 According to data in Mechanical and Electrical Equipment for Buildings, the maximum overall rate of heat loss for a passive solar building in a climate with HDD65 is 5.6 Btu/DD ft 2. (9) Therefore, the efficiency of the existing envelope is more than adequate. In fact, the efficiency of the existing envelope could be significantly reduced and the passive solar house would operate effectively. In other words, the U-value of a Passive House is significantly lower than that required for passive solar, further creating a distinction between the two approaches. Next, a building heat loss calculation and building load coefficient calculation 1 for the passive solar house were performed. See Table 2 and the calculations that follow. UA ns envelope = 79.4 Btu/hr F (see Table 2) UA ventilation = ACH volume Btu/ft 3 F ACH = (ventilation rate 60 min/hr)/volume = (82 ft 3 /min 60 min/hr)/12747 ft 3 = 0.39 ACH UA ventilation = 0.39 ACH ft Btu/ft 3 F = 89.5 Btu/h F Fig. 2: Passive Solar Model in DesignBuilder.

5 Fig. 3: Passive House Model Comfort Performance for Winter Design Week Fig. 4: Passive Solar Model Comfort Performance for Winter Design Week 4.4 DesignBuilder Simulation Results Passive Solar The passive solar design was then modeled in DesignBuilder for comparison to the Passive House model. According to DesignBuilder, the total annual electricity use for the passive solar house was calculated to be 9.8 kbtu/ft 2 yr, corresponding to an annual CO 2 production of lbs. Other performance data of note is the annual electricity use for electric lighting and the annual heating from solar gains. The annual electricity use for electric lighting was calculated to be Annual heating from solar gains was calculated to be While the indoor temperature in the Passive House model spiked due to solar gains through the south and west windows on two clear days during the winter design week, the indoor temperature swing in the passive solar model on the same days was moderated by the additional thermal mass. In the winter design week simulation, the indoor air temperature of the passive solar house varied from F (18-21 C). While the thermostat was set back to 58 F (14 C) during periods when the house was assumed to be unoccupied, the indoor air temperature never dropped below 65 F (18 C). 5. ANALYSIS Table 3 presents a side-by-side comparison of the simulation results for the two models. As expressed by the first row of data in the chart on the next page, the annual electricity demand of the passive solar house is less than that of the Passive House. According to the simulation results, a passive solar house in Eugene can operate at a lower annual electricity demand than a Passive House, provided the occupants set back the thermostat and allow the thermal mass to moderate daily temperature swings. Two factors directly attribute to the lower electricity demand of the passive solar model. First, the annual electricity demand for electric light is lower in the passive solar model, as the larger south glazing area results in a

6 greater amount of daylight. Second, the passive solar model uses 48% more energy from solar gains for heating than does the Passive House, thereby reducing heating electricity use. The Passive House does experience a steadier indoor air temperature than does the passive solar house; however, the indoor air temperature variation of the passive solar house during the winter design week is much smaller than hypothesized; the indoor air temperature varies by only 5 F during the day. This can be attributed to the large amount of thermal mass in the house, which prevents the temperature from dropping uncomfortably low when the thermostat is set back to 58 F (14 C). TABLE 3: SIMULATION RESULTS COMPARED PERFORMANCE DATA Annual Electricity Demand Annual CO 2 Production Annual Electric Lighting Demand Annual Heating from Solar Gains 6. CONCLUSION PASSIVE PASSIVE HOUSE SOLAR 10.9 kbtu/ft 2 yr 9.8 kbtu/ft 2 yr lbs lbs The hypothesis of the study was proven to be correct, as the passive solar model has a lower annual electricity demand than the Passive House model while experiencing temperature swings well within 10 F of 68 F (20 C). In fact, the passive solar house performed much better than anticipated in this respect; the thermal mass moderated the indoor air temperature so that it never dropped below 65 F (18 C) during the winter design week. The results of this study offer promise for passive solar as a heating strategy in cloudy climates such as the Pacific Northwest; though there is little access to direct sunlight during the winter months, a considerable amount of diffuse solar radiation is still available; further, the thermal mass is able to greatly moderate temperature swings. If occupants are willing to accept small daily temperature swings, passive solar can be a viable, low-energy option. It should also be reiterated that the occupants of a passive solar house must set back the thermostat and allow the thermal mass to moderate the temperatures swings if the house is to perform as intended. The Passive House, on the other hand, is able to operate at an incredibly high level of efficiency without adjustment of the thermostat. In other words, the passive solar house requires a higher level of occupant input in order to perform as intended. As stated in the introduction, Passive House and passive solar are clearly two distinct approaches to low-energy heating; this was further illustrated by the low solar savings fraction and relatively small amount of solar gain in the Passive House model. While each of the two approaches is distinct from the other, each is highly effective in reducing the amount of energy required for heating without compromising occupant comfort. 7. ACKNOWLEDGEMENTS We would like to thank University of Oregon Professor Emeritus John Reynolds for his invaluable insight and assistance during this research project. 8. REFERENCES (1) Straube, John. The Passive House (Passivhaus) Standard: A comparison to other cold climate lowenergy houses. Insight, 025, October Retrieved from < (2) Mazria, Edward. The Passive Solar Energy Book. Emmaus: Rodale Press, (3) Wilson, Alex. Passive House Arrives in North America: Could It Revolutionize the Way We Build? Environmental Building News. Retrieved from < (4) Mazria, Edward. (5) Grondzik, Walter; et al. Mechanical and Electrical Equipment for Buildings: 11 th edition. Hoboken: John Wiley & Sons, Inc., (6) Grondzik, Walter; et al. (7) Grondzik, Walter; et al. (8) Grondzik, Walter; et al. (9) Grondzik, Walter; et al. 8. END NOTES (1) The building load coefficient is a measure of the building s heat loss per degree day (Btu/DD).