Energy Analysis of a Straw Bale House

Transcription

1 Building Energy Analysis Center Energy Analysis of a Straw Bale House Principal Contacts Robert Brecha Associate Professor of Physics University of Dayton Dayton, OH phone: Robert.Brecha@notes.udayton.edu Team: Charles Schreier, Jacob Janczak, Gregory Raffio, George Mertz, Kelly Kissock Assessment date: Issue date: Department of Mechanical and Aerospace Engineering 300 College Park, KL-374, Dayton, OH Tel: Fax:

2 Disclaimer The purpose of this assessment is to identify and approximately quantify savings opportunities. The report is not intended to provide detailed engineering plans or designs for implementing the recommendations. Estimates of savings and costs are based on the best information available to the University of Dayton Building Energy Analysis Center (UD BEAC) within the scope of the assessment. However, the UD BEAC makes no warranty with respect to the accuracy of the savings estimates or contents of the report. The client is encouraged to evaluate each opportunity and attain further engineering analysis, if desired, to verify or refine savings estimates. ii

3 Table of Contents I. Executive Summary... 1 II. Straw Bale House... 2 Building Description... 2 Utility Summary... 2 Building Energy Simulation... 5 Statistical Analysis of Electricity and Natural Gas Use iii

4 I. Executive Summary The owner of the house provided us with energy use, construction data, and occupancy information. The building s current electricity use is 2,529 kwh of electricity per year compared to 8,937 kwh per year for average Midwest homes. The building s current natural gas use is 164 ccf per year compared to 910 ccf per year for average Midwest homes. The residence uses about 72% less electricity and 82% less natural gas than typical Midwest homes. Using this information, the building s energy use was simulated using hour-by-hour building energy simulation software. Simulated energy use was then compared to actual energy use to calibrate the models. The process of simulating and calibrating energy use allows us to verify our understanding about how the building uses energy and provides a mechanism to estimate potential savings from potential changes. The building was simulated as a Single Zone Building, neglecting the effects of thermal mass and solar thermal heating. This simulation yielded an annual natural gas use of 544 ccf. Next, the building was simulated as a Passive Solar Building, which incorporates the effects of thermal mass, but not the effect of solar thermal heating. The Passive Solar Building simulation yielded an annual natural gas use of 510 ccf. The Single Zone and Passive Solar Building overestimated the natural gas use of the straw bale house by about 200%. Finally, a third simulation, using SolarSim (Kissock, 1997) was performed, modeling an active solar thermal system with 400 gallons of storage. The simulation estimated annual natural gas use of 160 ccf. This simulation was closest to the actual natural gas use of the house at 164 ccf. From the Statistical Analysis section of the report, the ratio of the building s UA to efficiency of the heating plant is ccf/dy-f. Its balance point temperature is 58 F, and hot water gas use is ccf per day. From the Statistical Analysis of Natural Gas section, about 80% of natural gas use is for space heating and 20% is for water heating. 1

5 II. Straw Bale House Building Description The walls of the one-story house are constructed of straw and masonry. Three residents live on the first floor in a total of 1,350 ft 2 conditioned floor space. The long side of the house faces south, providing good passive solar orientation. While the house is unconditioned during the summer, temperatures remain comfortable nearly all of the time. In addition to the high thermal mass and insulation of the walls, a concrete, mud wall is located in the living room, on the south side of the house, which absorbs heat from the sun and releases it slowly throughout the night. The house is primarily heated passively and remains warm due to large thermal mass. Additional heat is provided by a solar hot water heater with a natural gas backup. The house has a 400 gallon tank to store hot water from solar thermal collectors, located on the roof. The house is equipped with double-pane windows and natural lighting provides most of the light during the day. Additional light is provided by compact fluorescent bulbs. Utility Summary Floor Area: 1,350 ft 2 Number of Occupants: 3 Annual Usage Annual Cost Average Unit Cost Cost / Area ($/ft2-yr) Cost / Occupant ($/occ-yr) Electricity 2,529 kwh $271 $0.088 /kwh $0.20 $135 Natural Gas 164 ccf $181 $1.03 /ccf $0.13 $90 Total $451 $0.33 $226 2

6 Electricity Straw Bale House Electricity Use Average Daily Electricity Use (kwh/dy) /1/2005 3/1/2005 4/1/2005 5/1/2005 6/1/2005 7/1/2005 8/1/2005 9/1/ /1/ /1/ /1/2005 1/1/2006 Meter Read Date Electricity use during winter months is higher than other months of the year. This is likely because occupants spend more time indoors during the winter. Thus, more lights, appliances and televisions would be on during the winter. Meter Read Date Days Per Period Utility Bill ($/month) Monthly Electricity Use (kwh/month) Daily Electricity Use (kwh/dy) 2/28/ $ /31/ $ /30/ $ /31/ $ /30/ $ /31/ $ /31/ $ /30/ $ /31/ $ /30/ $ /31/ $ /31/ $ Total/Average 365 $271 2,529 7 The straw bale house used 2,529 kwh of electricity in one year. The average annual electricity use for homes in the Midwest is 8,937 kwh per year. The residence used about 72% less electricity than typical Midwest homes. 3

7 Natural Gas Straw Bale House Natural Gas Use Average Daily Natural Gas Use (ccf/dy) /1/2005 3/1/2005 4/1/2005 5/1/2005 6/1/2005 7/1/2005 8/1/2005 9/1/ /1/ /1/ /1/2005 1/1/2006 Meter Read Date The Straw Bale House used more natural gas during the winter than during the summer. This is a typical heating profile, however the values are significantly lower than typical houses. Meter Read Date Days Per Period Utility Bill ($/month) Unit Cost ($/ccf) Monthly Natural Gas Use (ccf/month) Daily Natural Gas Use (ccf/dy) 2/28/ $22 $ /31/ $8 $ /30/ $9 $ /31/ $2 $ /30/ $1 $ /31/ $5 $ /31/ $3 $ /30/ $1 $ /31/ $3 $ /30/ $34 $ /31/ $53 $ /31/ $38 $ Total/Average 365 $181 $ The Straw Bale House used 164 ccf of natural gas in one year. The average annual natural gas use for homes in the Midwest is 910 ccf per year. The residence used about 82% less natural gas than typical Midwest homes. 4

8 Building Energy Simulation Single Zone Building Electricity and natural gas use of the Straw Bale House was simulated using the hour-by-hour ESim building energy simulation program. To simulate energy use, building characteristics, operating schedules and energy using equipment data were entered into the building description file Straw_Bale_House.SZB shown below. Simulating current building energy use gives us confidence in our understanding of building characteristics. Through a process of calibration to actual utility data we can better understand the characteristics of the building. We are quickly able to spot errors in building operation, and target areas for improvement. "Building ID" "301 W.N.College" "COOLING SET POINTS===============================" "Cooling str month" 1 "Cooling end month" 1 "Cool set-point str hr - occ" 14 "Cool set-point end hr - occ" 18 "Cool set-point days/week - occ" 7 "Cool set-point temp - occ" 78 "Cool set-point temp - unocc" 85 "HEATING SET POINTS===============================" "Heating str month" 11 "Heating end month" 4 "Heat set-point str hr - occ" 1 "Heat set-point end hr - occ" 24 "Heat set-point days/week - occ" 7 "Heat set-point temp - occ" 68 "Heat set-point temp - unocc" 50 "ROOF=============================================" "East-West ceil length (ft)" 25 "North-South ceil length (ft)" 40 "Max attic height (0 for flat roofs) (ft)" 7 "Ridgeline: EW, NS, none" "EW" "Solar absorptivity of roof: 0 to 1.0".2 "Rroof+ceil (hr ft2 F / Btu)" "Roof type: attic, steel, 2in-con, 6in-con" "attic" "WALLS============================================" "Rwall (hr ft2 F / Btu)" 30 "Solar absorptivity of walls: 0 to 1.0" 0.4 "Wall type: steel,frame,block,12in-con" "frame" "Awall n (ft2)" 360 "Awall s (ft2)" 360 "Awall e (ft2)" 270 "Awall w (ft2)" 270 "DOORS============================================" "Rdoors (hr ft2 F / Btu)" 2.56 "Adoors (ft2)" 11 "WINDOWS==========================================" "R center-of-glass (hr ft2 F / Btu)" 9 "Area glass north (ft2)" 12 "Area glass south (ft2)" 164 "Area glass east (ft2)" 25 "Area glass west (ft2)" 12 "Solar heat gain coef(normal,beam): 0 to 1" 0.25 "Bldg's rotation from true NSEW (degrees)" 0 "Average ground reflectance (0 to 1.0)" 0 "WINDOW OVERHANGS AND WINGS=======================" "Protrusion of overhang (ft)" 2 "Gap between overhang and window (ft)" 2 "Height of window (ft)" 7 "Protrusion of wing (ft)" 2 "Gap between wing and window (ft)" 4 5

9 "Width of window (ft)" 2 "FLOOR============================================" "Floor type: slab;hbase;unhbase" "unhbase" "Floor weight: wood;3in-con;8in-con" "3in-con" "Perim (ft)" 150 "Afloor (ft2)" 1350 "Rfloor (hr ft2 F / Btu)" 10 "Rperim-insul (hr ft2 F / Btu)" 5 "INFILTRATION=====================================" "Infiltration (air changes per hour)" 1 "Volume conditioned area (ft3)" "HOT WATER========================================" "Vol HW (gal/hr)" 1 "Temp HW (F)" 120 "Efficiency" 0.9 "Fuel: elec; ng" "ng" "SZB INTERNAL LOADS AND ELEC USE==================" "Avg (non-ac) elec cons (kwh/mo)" 200 "Avg num people" 2 "Eoccupied / Eunoccupied" 2 "OTHER ENERGY CONSUMPTION=========================" "Exterior elec cons (kwh/mo)" 0 "Other ng cons (ccf/mo)" 0 "SZB COOLING AND HEATING EQUIP====================" "System type: sd, sdcont, dd or hp" "sd" "SEER of air cond (Btu/hrW)" 10 "Efficiency of heating unit" 0.9 "HSPF of heatpump (Btu/W-hr)" 8.3 "Minimum fraction outdoor air" 0 "Economizer: none, temp, enthalpy" "none" "Temp of air leaving cooling coil (F)" 60 "Temp of air leaving heating coil (F)" 120 "Total supply air {about 1 cfm/ft2} (cfm)" 1300 "Cooling coil: on, off" "on" "Heating coil: on, off" "on" "END CODE=========================================" "End code" -99 The actual monthly energy use data from January 2004 to January 2005 were entered in the Straw_Bale.UTL file shown below. The first three columns show meter reading month day and year. Column four is electricity use (kwh), and column six is natural gas use (ccf). 6

10 Month Day Year Electricity Consumption (kwh/period) Electrical Demand (kw) Natural Gas Consumption (ccf/month) One year of hourly meteorological data from the same period as the energy use data were synthesized from actual average daily temperatures during this period and typical correlations between solar radiation, humidity and air temperature from nearby Dayton, OH. The actual average daily temperatures were from the UD/EPA Average Daily Temperature Archive at The typical correlations between solar radiation, humidity and air temperature were derived from Typical Meteorological Year data from The resulting hourly weather file was called Straw_Bale_2005_2006.wea. The figures below show simulated (dashed line) and actual (solid line) monthly electricity and natural gas use. The relatively good agreement between simulated and actual electricity and natural gas use increases confidence in the simulation model. By modifying the building characteristic file, building improvements can be modeled and savings predicted. ESim can predict savings for many retrofit measures. ESim can predict savings for temperature setbacks, higher insulation, reduced infiltration, and increasing furnace, water heater, or air conditioner efficiency. 7

11 Simulated Monthly Electricity Use Simulated Monthly Natural Gas Use For Single-zone building files, ESim does not consider the effect of thermal mass on heating and cooling of buildings. In the case of the Straw Bale House, we would expect the simulated natural gas use to be significantly higher than actual natural gas use. ESim predicts natural gas use for heating to be about 544 ccf per year. Since this is the case, we have confidence in the building profile generated 8

12 through calibration with ESim. Simulation of the Single-zone building provides us with the heating requirements for the building, without considering thermal mass. In addition to Single-zone buildings, ESim can simulate heating and cooling for Passive-solar buildings. Passive Solar Building ESim was used again to simulate the performance of a Passive-solar building. To simulate energy use, building characteristics, operating schedules and energy using equipment data were entered into the building description file Straw_Bale.PSB shown below. Simulating current building energy use gives us confidence in our understanding of building characteristics. "Building ID" "Straw Bale" "WINDOWS==========================================" "Area glass east (ft2)" 25 "Area glass south (ft2)" 164 "Area glass west (ft2)" 12 "Area glass north (ft2)" 12 "R center-of-glass (hr ft2 F / Btu)" 9.0 "Solar heat gain coef(normal,beam): 0 to 1" 0.25 "Open windows if too hot: 't' or 'f'" "t" "Bldg's rotation from true NSEW (degrees)" 0 "Average ground reflectance (0 to 1.0)".2 "WINDOW OVERHANGS AND WINGS=======================" "Protrusion of overhang (ft)" 2 "Gap between overhang and window (ft)" 2 "Height of window (ft)" 7 "Protrusion of wing (ft)" 0 "Gap between wing and window (ft)" 0 "Width of window (ft)" 3 "EXTERIOR WALLS===================================" "Awalls (ft2)" 1260 "Rwall (hr ft2 F/Btu)" 30 "Solar absorbtivity of walls: 0 to 1.0" 0.4 "(m x cp / A) (Btu/(ft2 F))" 1.08 "mass location 1-in, 2=mid, 3=out" 2 "GROUND FLOOR=====================================" "Area floor (ft2)" 1350 "(m x cp / A) floor (Btu/(ft2 F))" 1 "Floor type: slab, crawl or base" "slab" "If slab: perimeter (ft)" 150 "If slab: Rperim-insul(hr ft2 F/Btu)" 5 "If slab: carpet: 'y' or 'n'" "n" "If crawl or base: Rfloor (hr ft2 F/Btu)" 5 "If base: perimeter (ft)" 160 "If base: area floor (ft2)" 1500 "If base: shortest width (ft)" 30 "If base: wall height above ground (ft)" 2 "If base: wall depth below ground (ft)" 6 "If base: wall insul depth below ground (ft)" 4 "If base: R wall insul (hr ft2 F/Btu)" 5 "ROOF=============================================" "Roof type: attic, no-attic" "attic" "If attic: Rroof (hr ft2 F / Btu), else 0" 55 "If attic: Rceil (hr ft2 F / Btu), else 0" 5 "If attic: volume attic (ft3), else 0" 300 "If attic: Air changes per hour, else 0" 0.01 "If no attic: Rroof+ceil(hrft2F/Btu), else 0" 28.5 "Area ceiling (ft2)" 1350 "Solar absorptivity of roof: 0 to 1.0".2 "(m x cp / A) (Btu/(ft2 F))" 1 "mass location 1-in, 2=mid, 3=out" 2 "INTERIOR WALLS, CEILINGS AND FLOORS==============" "Awalls (ft2)" 400 "(m x cp / A) (Btu/(ft2 F))" 1.5 9

13 "DOORS============================================" "Adoors (ft2)" 0 "Rdoors (hr ft2 F / Btu)" 2.56 "INFILTRATION=====================================" "Infiltration (air changes per hour)".5 "Volume conditioned space (ft3)" "INTERNAL ELEC AND PEOPLE=========================" "Avg (non-ac) elec power (kw)" 0.25 "Avg num people" 2 "OPTIONAL AUXILIARY HEAT==========================" "Auxiliary heat 't' or 'f'" "t" "Heating set point temp (F)" 70 "SIMULATION PARAMETERS============================" "Time increment: >1, increase if sim unstable" 1 "Day number for graphical output (1-365)" 20 "END CODE=========================================" "End code" -99 Results from the Passive-solar building simulation are displayed in the table below. According to the simulation, during the summer the building will maintain an indoor air temperature of less than 77 F. During winter months, some auxiliary heat is required. According to the ESim simulation, about 13,246 Btu per hour is needed in auxiliary heat. For the winter heating season, auxiliary heat would be about 510 ccf per year, compared to the SZB where auxiliary heat is about 544 ccf per year. Thus, the passive solar building with thermal mass reduces auxiliary heat necessary by about 34 ccf per year. However, the actual natural gas used for space conditioning is only about 152 ccf. Thus, the remaining natural gas use, about 358 ccf is reduced from heat collected by the solar thermal collectors. Active Solar Thermal System In addition to passive solar characteristics, located on the roof of the straw bale house are 6 solar thermal collectors. Thermal energy absorbed by the collectors is stored in a 400 gallon storage tank located inside the house. The active solar thermal system is modeled using SolarSim (Kissock, 1997). The system properties, and inputs to SolarSim are shown in the figure below. Using SolarSim, we can 10

14 estimate the amount of heating and hot water that is provided by the sun. Also, we can estimate the additional energy required to heat the house. The SolarSim outputs are shown in the figure below. The column of importance to us is the Avg Qaux, or the average heat not provided by the solar thermal system. The average daily auxiliary heating is about 38,830 kj per day, or 36,804 Btu per day. The heat is provided through an in-line natural gas heater with an efficiency of about 84%. Thus, the annual heat required would be about: 36,804 Btu/dy x 365 dys/yr x 1 / 84% = 160 ccf/yr 11

15 The actual annual natural gas use for the building is 164 ccf per year. The close agreement between the actual and theoretical values gives us confidence in the model. Summary of Simulated and Actual Natural Gas Consumption The Straw Bale house has been simulated using three models. It has been simulated as a Single Zone Building, as a Passive Solar Building and as an Active Solar Thermal System. The results of the three models are shown in the chart below. The results of the simulations indicate that the best predictor of the actual natural gas use is the Active Solar Thermal System using SolarSim. Actual Use Single Zone Building Simulation Passive Solar Building Simulation Active Solar Thermal Simulation Natural Gas Use (ccf)

16 Statistical Analysis of Electricity and Natural Gas Use Statistical Analysis offers a baseline characterization of energy use. These models can be used for budgeting purposes to predict future energy consumption based on predicted outdoor air temperature. As well, savings from implementation of energy-efficiency measures can be quantified using this facility model against future energy use. The following statistical analysis utilizes the computer software Energy Explorer (Kissock, 2004), available free-of-charge at Electrical energy and natural gas use were provided by the owner of the house. Temperature data for this analysis were taken from the Dayton, Ohio city file obtained in an archive maintained by the University of Dayton ( Statistical Analysis of Electricity Use Data used in this statistical lean energy analysis are shown below. Month Day Year Electricity Usage (kwh/day) Temperature (F) A time series plot of electrical energy consumption and average daily outdoor temperature is shown below. 13

17 Time Series of Electrical Energy Use and Outdoor Air Temperature A mean model shows the relationship between electrical energy use and outdoor air temperature. This model, shown below, indicates that electricity usage does not correlate with temperature. Typically, in residential buildings, electricity use will increase during summer with air conditioning. However, in the Straw Bale House, electricity use rises in the winter. In addition, the building has no central air conditioning. Electrical Energy Use vs. Outdoor Air Temperature Mean Model Electricity use can be predicted using the following equation: Elec (kwh/day) = 7 kwh/dy 14

18 Statistical Analysis of Natural Gas Use The data used in this statistical analysis are shown below. Month Day Year Natural Gas Use (ccf/day) Temperature (F) A time series plot of natural gas energy use and average daily outdoor temperature is shown below. This time series shows that with annual fluctuations in temperature, an inverse trend occurs with natural gas energy use. Natural Gas Consumption and Outdoor Air Temperature Time Series A three-parameter heating change-point analysis (3P-H) models the relationship between natural gas use and outdoor air temperature. This model, shown below, has a positive slope as the outdoor air temperature decreases below 58 F. R 2, 0.70, indicates the fit of data to the model compared to the 15

19 mean. CV-RMSE is 63.18%, indicating the model represents this data within ± 126% error at the 95% confidence interval. Natural Gas Consumption vs. Outdoor Air Temperature From Energy Explorer, the slope, x-change point and y-change point are determined. The slope represents the ratio of the UA of the house to the efficiency of the heating plant. The x-change point represents the balance point temperature of the house. The y-change point represents the baseline natural gas use to the water heater. UA / efficiency Balance point temperature (Tbal) Hot Water Gas Use =.0357 mmbtu/day-f = 58 F = mmbtu/dy The building natural gas energy use is modeled as: NG (mmbtu/day) = Hot Water Use + UA / efficiency x [Tbal T oa (F)] + NG (mmbtu/day) = mmbtu/day mmbtu/day-f x [58 F T oa (F)] + where T oa is the average outdoor air temperature. 16

20 According to the natural gas breakdown, shown below, 20% (33 ccf) of natural gas is for hot water and 80% (131 ccf) of natural gas is for space conditioning. Natural Gas Use Breakdown 17

21 Natural Gas Breakdown Hot Water 20% Space Conditioning 80% 18