A startup builder in the San Francisco Bay Area has a goal of producing factory built/modular houses with net zero energy performance. Their first prototype was a two-story, two bedroom, urban infill townhouse design. It has been in operation for roughly a year, and has been extensively measured and monitored, providing information about its net zero performance. The data collected to date indicate that the building is on track to achieve net zero performance. Several obstacles arose during the construction and commissioning of the building, which provided some useful lessons on integrating advanced technologies. The monitored data has provided a wealth of information and has already been used to remotely diagnose malfunctioning or improperly operated equipment.
Introduction
A startup builder in the San Francisco Bay Area has a goal of producing factory built/modular houses with net zero energy performance. Their first prototype was a 1540 sf (143 m2), two-story, two bedroom, urban infill townhouse design; it achieved a USGBC LEED Platinum rating, and was awarded the 2009 Green Builder Home of the Year Award. The authors provided design guidance and analysis under the Department of Energy’s Building America program. The prototype has been in operation for roughly a year, and has been extensively monitored, providing information about its net zero performance, and the contribution of its various sub-systems.
The building was designed as a grid-tied net zero energy building (NZEB)—specifically a net zero site energy building (as defined by Torcellini et al. 2006), meaning that renewable energy produced at the building site offsets all energy consumption of the building. As it is all-electric, this building is also by definition net zero source energy.
The site is in a very mild climate (Oakland CA, Zone 3C, 2880 HDD 65° F/1600 HDD 18° C/435 CDH 74° F), resulting in minimal space conditioning energy demands, and therefore reducing the renewable energy input required to achieve net zero energy. Interestingly, this mild climate challenged typical cold-climate assumptions on the most valuable enclosure upgrades.
An all-electric design was chosen by the builder. This was done primarily to make the building’s net zero performance indisputable (if achieved), based on simple metering of electrical use. In addition, although net zero source energy buildings are a valid approach (burning fossil fuel on site, and generating excess renewable energy to offset this use), California net metering laws at the time made this approach economically unfeasible. These laws stated that net excess generation at year’s end was forfeited to the utility, thus discouraging oversized photovoltaic arrays; however, this law has since changed.
Design and Building Specifications
Enclosure
The builder chose modular construction for quality control and climate-controlled construction reasons. The opaque assembly values are insulated substantially above code levels. For instance, California Title 24 (CEC 2008) calls for R-13 (RSI 2.3) cavity insulation in wood frame walls, which would result in an opaque wall R value of about R-8 (RSI 1.4) when thermal bridging is accounted for. In contrast, the chosen wall (which included 1”/25 mm XPS insulating sheathing) was R-25 nominal/true R-19 opaque wall (RSI 4.4/RSI 3.3). The windows are high performance units (two layers of glass + film; essentially triple glazed); the use of spray foam throughout resulted in very good airtightness (2.7 ACH 50). The insulated and sealed crawl space foundation was chosen for experimentation using semi-active control of the thermal mass contained within the foundation. However, this resulted in added cost, due to both the foundation design, and also site seismic and soil issues. In addition to energy concerns, moisture management and durability were high priorities of the design team. Some durability features shown in Figure 1 include subsill pan flashings under all windows, and ventilated rainscreen cladding.
Figure 1: Exterior view of typical building enclosure assemblies
Table 1: Building Enclosure Characteristics
All R-values in ft2•°F•h/Btu; CFM 50=cubic feet/minute @ 50 Pa; ACH 50=air changes/hour @ 50 Pa
Mechanical Systems
As discussed above, an all-electric design was chosen by the builder. Therefore, space conditioning was provided by a high-efficiency air source heat pump: the site was not amenable to a ground source heat pump and such a unit would have been significantly more expensive with modest energy savings in this climate zone. Given the mild climate of the Bay Area, a residential-scale economizer was installed. It was built with off-the-shelf components and a custom controller which would open a damper from the return to the exterior, open a roof-mounted skylight for relief air, and run the air handler (under favorable weather conditions).
Domestic hot water was provided with an add-on heat pump water heater on an electric resistance tank; this is discussed in more detail later. In addition, a drainwater heat recovery unit was installed; however, given the crawl space geometry, this unit could only capture waste heat from the second floor shower and sink drains (i.e., not the first floor drains).
The original ventilation design was a heat recovery ventilator (HRV), drawing exhausts from both bathrooms, and supplying to the upstairs hallway. The design intent was to have a 33% duty cycle for general ventilation, with high speed ventilation controlled by a bathroom timer switch. However, in operation, the selected unit proved to be unreliable and too loud for occupant acceptance. After several iterations, ventilation was provided by the customized controller, operating the air handler and economizer motorized damper, to provide distributed ventilation (i.e., central fan integrated ventilation, as discussed by Rudd and Lstiburek 1998). Bathroom exhaust, though, was still provided by the installed HRV.
All installed lighting is compact fluorescent or LED; all major appliances and most lighting fixtures are Energy Star qualified. The townhome design limited available fenestration for daylighting; a skylight was used over the central stairwell, providing well-distributed daylighting.
Table 2: Mechanical Systems Characteristics
CFM 25=cubic/feet/minute @ 25 Pa
Renewable energy was generated by a roof-mounted photovoltaic array (5.4 kWp), which covered roughly half of the available roof area. This system was sized to meet expected loads under typical operating conditions.
Design Process and Simulations
The best design practice for net-zero energy houses is to maximize conservation measures, and then to add renewable energy sources to offset the remaining loads. Some steps in this process are described below, including the use of building energy simulations to inform decisions, economic analysis, domestic hot water choices, and designing for the variability of homeowner operation. One of the most common measures includes optimization of solar orientation; unfortunately, due to lot restrictions and the townhome design, this was not available as a measure.
Parametric Simulations and Net Zero Optimization. This house was analyzed with parametric simulations in a DOE-2.1E-based software package (Energy Gauge USA, Parker et al. 1999). The initial reference building was the Building America Benchmark, which is intended to capture typical mid-1990’s construction (Hendron 2007); incrementally changes were then added, as shown in Figure 2. The graph shows annual source energy consumption, divided by end-use load (heating, cooling, domestic hot water, lighting, and miscellaneous end use loads).
Figure 2: Parametric simulations of prototype house, using DOE-2.1E based simulation
It is immediately clear that the mild climate of the San Francisco Bay Area minimizes enclosure (heating and cooling) loads, which comprise only roughly ⅓ of the total for the Benchmark case. Furthermore, all of the enclosure improvements combined (opaque wall R value, fenestration, airtightness) result in only a 15% improvement relative to the Benchmark.
Similarly, the space conditioning equipment upgrades have a small effect (4% improvement). The largest improvements are in domestic water heating at 14% (as the Benchmark “base case” uses electric resistance water heating), and the installation of compact fluorescent lighting (7%). Of course, an economic balance should be struck between conservation measures and renewable energy; Proskiw (2010) described a framework for this evaluation similar to what was used here. This system involves an “Energy Conservation Measure Value Index,” which divides the incremental cost of the measure by the annual energy savings. The ECM Value Index for any measure can then be compared to the ECM Value Index for the renewable energy system. The installed cost of the PV array was $8/Wp (circa 2009) without rebates, and roughly $4/Wp including state and Federal incentives. Using the latter cost, some of the worst performers proved to be the heat recovery ventilator and high performance windows. In addition, several of the opaque assembly insulation upgrades provided lower Value Indices than the PV system. All of these issues were a function of the mild climate, which minimizes the need for environmental separation between exterior and interior.
Domestic Hot Water and Fuel Choice. The choice of an all-electric house proved to be an obstacle to overall energy efficiency when selecting domestic hot water equipment. To demonstrate the effect of equipment choice, a matrix was generated of nominal efficiency levels (energy factor/EF) for typical options (Table 3). A “Relative Site Consumption” value was calculated by taking the reciprocal of the energy factor. Then, the site consumption was multiplied by source energy factors for the US average power grid (1.092 gas; 3.365 electricity), as given by Deru and Torcellini (2007), resulting in a “Relative Source Consumption.”
Table 3: Normalized Source Consumption of Domestic Hot Water Options
This exercise demonstrates that an electric heat pump water heater has lower nominal source energy efficiency than many of the gas-fired tankless/instantaneous water heaters commonly available on the market. These two equipment options have comparable installed costs: this suggests that a tankless gas heater is a better choice for reducing source energy use. Admittedly, recent work demonstrates that tankless water heaters have lower installed efficiencies than rated values when realistic draw schedules are used (Burch et al. 2008). However, heat pump water heaters also have performance below their nominal rated value, given their partial dependence on electric resistance heating (typically an option to recover from large draws). Of course, the relative performance will change as grid source energy factors change regionally and over time.
Occupant-dependent effects and targeting net zero. One fundamental issue when sizing the photovoltaic array for this building was that occupant behavior causes tremendous variations in energy consumption. Parker et al. (1996) reported variations (from highest to lowest user) of a factor of 3:1 in identical houses; others have reported similar results (see Parker et al. 1996). In addition, miscellaneous end use loads (MELs) dominate over enclosure loads, as shown in Figure 2 (roughly 50%). Unfortunately, MELs are highly variable, increasing overall variability. The builder therefore faced a dilemma: either (a) size the system for “normal” occupancy, and run the risk of not meeting net zero targets (which would have adverse effects on their reputation), or (b) oversize the system (resulting in greater first cost for the home), and also run the risk of overproducing electricity and forfeiting the excess production to the utility company. In the end, the builder chose the former approach, assuming that anyone choosing to buy a net zero house would exhibit low consumption behavior. . .
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