1、Brent Stephens is a Ph.D. candidate in the Department of Civil, Architectural and Environmental Engineering at The University of Texas at Austin, Austin, TX. Modeling a Net-zero Energy Residence: Combining Passive and Active Design Strategies in Six Climates Brent Stephens Student Member ASHRAE ABST
2、RACT The effects of geography and climate on the feasibility of residential net-zero energy buildings (NZEBs) have not been thoroughly explored by either simulations or measurements. This paper details a building energy modeling effort that 1) applies passive low-energy design strategies and energy-
3、efficiency measures individually to an all-electric baseline code-compliant residence in six different U.S. climates, 2) selects a combination of those strategies and measures to apply in order to achieve a low-energy building, and 3) pairs the predicted energy consumption output with output from a
4、solar photovoltaic (PV) model, which allows proper sizing of the PV array in order to satisfy the requirements of a NZEB. The results are explored on an annual, monthly, and hourly basis in order to identify some of the challenges of attaining a residential NZEB in multiple climates. The chosen suit
5、e of low-energy design strategies is estimated to reduce annual energy consumption by 19-30% relative to the baseline code-compliant home, depending on climate. The PV system capacity required to achieve net-zero energy status varies by more than a factor of two between the coldest and warmest clima
6、tes with the lowest and highest average insolation. The simulations also reveal that electricity production from PV systems provides enough energy to completely cover hourly demand less than two-thirds of the typical year (and varies by season), while oversized PV production greatly exceeds demand d
7、uring the remaining one-third of the hours of the year. If NZEBs are widely adopted in the future, the electric grid may not always be able to handle excess on-site generation and energy storage options will be required to maintain the balance. In addition, regional differences in the fraction of ho
8、urly demand met by PV production raise questions about the net effect of NZEBs on power plant emissions. Finally, alternative energy sources other than PV should be further explored for widespread application of NZEBs in different climates. INTRODUCTION The concepts and details of net zero-energy bu
9、ildings (NZEBs) have been widely discussed in the literature. Torcellini et al. (2006) discussed the various definitions of NZEBs, including those that strive for four different types of annual net-zero status: “site” energy usage, “source” energy usage, energy cost, or pollutant emissions. The diff
10、erences are a matter of accounting for either energy use directly or externalities related to energy use. In all cases, a building first receives a suite of passive energy-saving and active energy-efficient design strategies so that it is considered a low-energy building, then enough energy is suppl
11、ied on an annual basis from nonpolluting renewable technologies (often installed on-site) to offset at least one of the four parameters of interest when summed over the entire year (Crawley et al., 2009). Rooftop photovoltaic (PV) and solar thermal water heating systems have been the most applicable
12、 supply-side technologies for widespread application of on-site NZEBs, or those that provide energy from within the building footprint boundary (Torcellini et al., 2006; Parker, 2009). Other production technologies, such as on-site wind generation, have not reached any noticeable level of market pen
13、etration to date (Elkington et al., 2009). Regardless of production technology, because current energy storage LV-11-C046 2011 ASHRAE 3812011. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For p
14、ersonal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAES prior written permission.g1technologies are limited and costly, a connection to the grid is usually required to maintain the energy balance between times of exces
15、s supply or demand (Torcellini et al., 2006). This paper aims to simulate the energy use and on-site PV supply of a NZEB and explore the effects of climate and geography on its net-zero status throughout the year in order to provide insight into the options and constraints of widespread application
16、of NZEBs. The original simulations of low-energy residences striving for NZEB status were conducted by the Florida Solar Energy Center in the 1990s (Parker, 2009, and references therein). The simulations concluded that if aggressive reductions in energy use for cooling, heating, water heating, refri
17、gerators, lighting and appliances were met, it might be possible to reduce total electricity demand by two-thirds in a hot Florida climate; then the addition of a PV electricity system might allow the residence to achieve an annual net-zero energy demand. The feasibility of NZEBs in the U.S. was lat
18、er explored using several experimental buildings in different locations throughout the country, as summarized by Parker (2009). Many of the experimental homes were able to achieve near net-zero electricity or better, but most retained the use of natural gas space heating which, when converted to equ
19、ivalent electricity use, prevented a true NZEB. In addition, geography, and thus climate and solar radiation, played an important role in determining the amount of energy required for space conditioning in the surveyed houses. Geography and climate also affects the amount of electricity production a
20、ttainable by PV systems; however, the effects of geography and climate on the feasibility of NZEBs (and residences, in particular) in regards to both electric demand and PV supply have not been thoroughly explored by either simulations or measurements. This paper details a building energy modeling e
21、ffort that 1) applies passive low-energy design strategies and energy-efficiency measures individually to a baseline code-compliant residence in six different climates, 2) selects a suite of those strategies and measures to apply based upon the modeled amount of energy savings, and 3) pairs the pred
22、icted energy consumption output with output from a solar PV model, which allows for proper sizing of the smallest PV array that should satisfy the requirements of a NZEB. The results are explored on an annual, monthly, and hourly basis in order to identify some of the challenges of attaining a NZEB
23、in multiple U.S. climates. METHODOLOGY This modeling effort, similar to that described in Crawley et al. (2010) for commercial buildings, explores low-energy design strategies and estimates the PV capacity required for a residence to achieve net-zero “site” energy usage in multiple climates. “Site”
24、energy is used in part to avoid accounting for local and regional differences in conversion factors for source electricity production. To avoid having to indirectly compensate for any natural gas usage by PV production, this effort strives for net-zero site energy status by modeling an all-electric
25、house. In this case, the sum of annual electricity usage will be offset directly by the sum of annual on-site electricity supply, without allowing for fuel switching to a natural gas supply source that cannot be directly offset by on-site electricity production. A 100% electric design is considered
26、a conservative approach to achieving a NZEB because fuels are used more efficiently if burned directly in buildings, rather than undergoing conversion to electricity (Torcellini et al., 2006). Energy simulations were carried out for the same house construction (Figure 1) in six locations representin
27、g major U.S. climate zones: Houston, TX, Phoenix, AZ, Charlotte, NC, Kansas City, MO, Seattle, WA, and Minneapolis, MN (locations from Walker and Sherman, 2008). The simulations were performed using eQUEST, a DOE-2-based building energy simulation tool (DOE2, 2009), which simulates hourly energy con
28、sumption for a typical meteorological year (TMY). a) b) Figure 1 House model from a) the front (north) and b) the back (south) sides 382 ASHRAE TransactionsBaseline Model Inputs The two-story wood-frame house with a brick faade and dark shingle roof has a total conditioned floor area of 2260 ft2(210
29、 m2). An unconditioned floor area of 420 ft2(39 m2) for an attached garage is subtracted from the first floor conditioned-area footprint. The house meets both general and climate-zone-specific requirements of the 2006 International Energy Conservation Code for residential construction, as outlined i
30、n Table 1 (IECC, 2006). The house includes a recirculating HVAC system consisting of an 11 EER 4-ton (14 kW) air-conditioner, an electric furnace, and an air handler delivering an airflow rate of 1440 CFM (2450 m3/hr). Ductwork is located in the attic (R-8 F-ft2/(BTU/hr), or R-SI-1.4 m2-K/W), 10% le
31、akage, and a surface area of 460 ft2(43 m2) and 110 ft2(10 m2) for the supply and return duct runs, respectively, per ASHRAE Standard 152-2004 (ASHRAE, 2004). Table 1. 2006 IECC Baseline Home Model Inputs Houston Phoenix Charlotte Kansas City Seattle Minneapolis Climate zone 2 2 3 4 4 6 Attic floor
32、R-value, F-ft2-hr/BTU (m2-K/W) 30 (5.3) 30 (5.3) 30 (5.3) 38 (6.7) 38 (6.7) 49 (8.6) Exterior wall R-value, F-ft2-hr/BTU (m2-K/W) 13 (2.3) 13 (2.3) 13 (2.3) 13 (2.3) 13 (2.3) 19 (3.3) Window U-value, BTU/hr-F-ft2(W/m2-K) 0.75 (4.3) 0.75 (4.3) 0.65 (3.7) 0.4 (2.3) 0.4 (2.3) 0.35 (2.0) Window SHGC (-)
33、 0.4 0.4 0.4 0.4 0.4 0.4 Heating capacity, kBTU/hr (kW) 38 (11) 38 (11) 38 (11) 48 (14) 48 (14) 60 (17.6) The effective envelope leakage area is 150.8 in2(970 cm2), resulting in a fixed air exchange rate of 0.46 air changes per hour. The house has a total of 238 ft2(22 m2) of window area, distribute
34、d between the south, north, east, and west sides by approximately 52%, 32%, 11%, and 5%, respectively. The house contains an electric water heater (38 kBTU/hr; 50 gallon capacity) and an electric cook range. Seasonal thermostat schedules were maintained on both weekdays and weekends. During the cool
35、ing season, the thermostat was set to 83F (28C) from 9 am to 2 pm and 78F (26C) for the remainder of the day. During the heating season, the thermostat was set to 68F (20C) from 6 am to 11 pm and 63F (17C) at all other times. These thermostat settings and schedules were taken from personal communica
36、tion with building energy modelers at a local electric utility (AEGBP, 2009). Solar PV electricity production was simulated using an online calculator, PVWatts, which allows a user to select a geographic location and other PV system input parameters (or use defaults) and generate hourly simulations
37、of electricity production by that system during a typical meteorological year (TMY) using a PV performance model (NREL, 2010a). The system size required to reach net-zero energy status in each climate location was determined iteratively by adjusting the array capacity until the predicted annual outp
38、ut was slightly greater than predicted annual usage. Default input parameter values were used in the PV simulations, including fixed panels titled at latitude, south-facing azimuth, and a DC to AC conversion factor of 0.77 (NREL, 2010a). Simulations do not take into account nearby buildings, panel s
39、oiling, or performance degradation over time, but can provide a rough estimate of electricity production from PV. The following section presents the results of the energy demand and production simulations and explores annual, monthly, and hourly details of the output. RESULTS Table 2 shows the predi
40、cted annual electricity usage of a baseline code home in each of the six climates. The code-compliant home located in Minneapolis, the coldest climate, is predicted to consume the most energy (over twice as much as the least consumers, located in Houston and Phoenix). Table 2 also shows the magnitud
41、e of annual energy reductions predicted by seven low-energy design strategies and efficiency measures applied individually to the code-compliant home in each climate, as well as the annual energy reduction expected due to a combination of the highest-impact individual low-energy measures. The indivi
42、dual strategies available were zero duct leakage, reduced air infiltration, a 50% reducing in lighting load (including a proportional decrease in heat output), a higher-efficiency air-conditioner and heat pump, and increased attic and wall insulation. Higher-impact individual strategies were conside
43、red to be part of the combined strategies 2011 ASHRAE 383g1if they individually led to a predicted arbitrary decrease in energy consumption of 4%. The chosen suite of low-energy design strategies is estimated to reduce annual energy consumption by 19-30% relative to the baseline code-compliant home
44、across all climates, paving way for the addition of a solar PV system. Note that the savings from combined strategies do not necessarily equal the sum of individual savings; load-reducing measures such as wall insulation decrease the need for heating or cooling, but also decrease the savings that a
45、higher efficiency air-conditioner or heat pump could achieve. Table 2. Simulated Annual Electricity Usage and the Effects of Low-energy Design Strategies Predicted annual electricity usage (kWh) Houston Phoenix Charlotte Kansas City Seattle Minneapolis IECC 2006 Code Home 18830 18790 23350 31390 253
46、30 38750 Individual Design Strategies Simulated reduction in annual electricity usage10% duct leakage -7% -6% -9% -11% -8% -13% Reduced infiltration (0.3 hr-1) -1% 0% -1% -2% -2% -3% 50% lighting load reduction -5% -5% -3% -2% -2% -1%14 EER air-conditioner -4% -6% -1% -1% 0% 0% R-60 attic insulation
47、 -1% -1% -1% -1% -1% 0% R-21 wall insulation -4% -4% -5% -7% -6% -1% Heat pump (HSPF 9) -8% -4% -15% -17% -23% -12%Combined strategies -24% -19% -25% -30% -29% -24% 1 Bold values represent individual strategies that resulted in greater than 4% savings in predicted annual energy consumption. The pred
48、ictions in Table 2 show that switching to a heat pump (with a heating seasonal performance factor of 9) and eliminating duct leakage result in the greatest annual energy reductions for most of the climates, followed by improved wall insulation. The warmer climates (Phoenix and Houston) also benefit
49、from a higher-efficiency air-conditioner and a reduction in lighting loads. Increased attic insulation and reduced air infiltration resulted in negligible energy savings in most climates, suggesting that heating and cooling loads are not dominated by gains or losses through the attic or air infiltration in the baseline home and that the incremental improvements are only marginally more effective than the code-compliant home. Table 3 summarizes the PV system capacity required to meet 100% of annual electricity demand of the combined low-energy strategy home in each climate, as
