Maximizing the Benefits of Zero-Energy Homes
There is an emerging trend in energy programs for new homes to shift from a focus on the use of individual high-performance components to a focus on overall home performance. Key examples of this trend include the MASCO Environments for Living program, the SMUD Zero-Energy Home program (see “Sacramento’s Zero-Energy Community,” Ready for Prime Time?
Not all currently available technologies are suitable for use in residential programs that focus on realizing large incremental energy savings. Many successful products are energy neutral, provide modest energy benefits, or are targeted toward niche markets. There are four barriers to realizing large incremental energy savings in production homes:
- It may be difficult to scale the technology down to the footprint and capacities required for residential applications.
- Maintenance or operating requirements may be incompatible with use in residences, where technologies must operate for long periods of time with little or no maintenance or adjustment.
- The energy output or benefit of the technology may not match residential energy input needs.
- The technology may provide benefits other than large energy savings.
Here are some examples of technologies whose promise is apparent, but whose incremental energy savings contributions in new high performance homes may be limited.
Concentrating solar light pipes. A concentrating solar light pipe system is an example of a technology whose energy output does not match residential energy input needs. Homes have high perimeter-total floor area ratios and window areas that provide lots of natural lighting. In addition, most residential lighting use occurs during the night, when solar lighting is not available. Areas like closets that do not have access to natural lighting do not contribute much to total residential lighting use. This combination of factors makes it difficult for a solar light pipe system to provide high value to builders or consumers from an energy savings perspective. The technology must meet minimum builder and contractor performance requirements if it is to be used in new homes, and in order for the technology to be included in planning for future residential energy programs.
Cool-roof materials. Cool-roof materials have the potential to reduce roof temperatures, reduce the rate of thermal degradation, and increase the lifetime of some roofing materials. However, because there is little or no thermal connection between the roof and the living space of a high-performance new home with a ventilated tile roof, high levels of ceiling insulation, and ducts in the conditioned space, cool-roof materials are not currently expected to realize significant energy savings in high-performance new homes. Over the long term, as ceiling insulation levels continue to increase and cool-roof technologies continue to mature, incremental energy savings from cool-roof materials may begin to compete with the savings achieved by increased levels of ceiling insulation.
Radiant cooling systems. One of the major dollar savings realized with increases in house performance is the reduced cost of the furnace and air conditioning systems. This is due to corresponding reductions in peak space conditioning load and space conditioning system size. As envelope performance increases in a 2,592-ft2 house in Sacramento, for example, the size of the air conditioner that is required to meet peak cooling loads is reduced from 3 tons to 1.5 tons. If an advanced cooling technology cannot be cost-effectively provided by a system in the 1.5-ton range, then that technology will not meet minimum builder and contractor performance requirements for use in new homes. A radiant floor- or radiant ceiling-based cooling system is an example of a cooling technology with significant first costs for large-area radiant coils that cannot be reduced in size as cooling capacity is reduced.
Base Case Building Characteristics
Incremental energy savings and incremental home costs are calculated relative to a base case building that meets Title 24 requirements for Sacramento (California climate zone 12). The base case building includes low-leakage tested ducts with R-4.2 insulation, low solar heat gain windows, R-3 wall insulation, R-38 ceiling insulation, and a 13-SEER (seasonal energy efficiency ratio) air conditioning system. The occupancy and operational assumptions used in the study are defined in the Building America Research Benchmark and include time-of-day profiles for occupancy, appliance and plug loads, lighting, domestic hot water use, ventilation, and thermostat settings.
To provide the reader with a closer look at the analysis results, the least-cost curve is shown as a function of source energy savings in Figure 2 on p. 43. At zero source energy savings, the cost on the curve at the vertical axis represents the annual utility bill for a homeowner with a Title 24 home. Each point on the curve represents a different combination of equipment and envelope options for the home. The BEopt method conducts an annual energy simulation for a large number of possible combinations of options in the vicinity of the least-cost curve. The least-cost curve represents the lower bound of these combinations of options.
One of the benefits of the BEopt analysis method is that it can identify combinations of options represented by the points nearest the least-cost curve that have nearly equivalent cost and performance. The marginal cost of increased energy efficiency is equal to the current marginal cost of electricity from residential PV when source energy savings reach about 44%. The straight line that begins at source energy savings of 44% represents the cost of using a net metered, grid-connected PV system to meet remaining home energy needs. The neutral cost point where total energy-related homeowner costs for a high-performance home are equal to the initial utility costs for a Title 24 home occurs at source energy savings of about 50%.
The development of the BEopt analysis method was influenced by several factors. First, the method identifies intermediate optimal points all along the path of interest (that is, minimum-cost building designs at different target energy savings levels), not just the global optimum or the zero net energy (ZNE) optimum. Second, the method allows discrete rather than continuous building options to be evaluated, reflecting realistic construction options. Third, an additional benefit of the search strategy is the identification of near-optimal alternative designs along the path, allowing for substitution of essentially equivalent solutions based on builder or contractor preferences.
Base Case Building Characteristics
A simple two-story 2,592-ft2 home in Sacramento with an attached two-car garage was used as the model for this study. The home has 1-foot eaves and a slab foundation. Window area was assumed to be 18% of floor area and was equally distributed among outside walls. Window distribution is a user input and can be modified to reflect specific home designs. The study was limited to one orientation (back facing west), allowing full exposure of the windows on the back of the home to afternoon sun. Adjacent homes 10 feet to the north and south provide shading of sidewalls. The energy options considered in the study include space conditioning systems, envelope systems, hot water systems (including tankless and solar hot water), lighting systems, major appliances, and grid-connected residential PV. No specific options that address miscellaneous electric loads other than major appliances were included in the determination of the base case building design. The homeowner costs calculated in the study assume a 30-year mortgage at a 7% interest rate with a 3% general inflation rate and a 5% real discount rate. The net present value of replacements for options with lifetimes of less than 30 years were included in option costs.
Each option has an assumed first cost and lifetime. Costs used in the analysis represent retail costs and include national average estimated costs for hardware, installation labor, overhead, and profit. Construction costs (wall insulation, ceiling insulation, foundation insulation, and so on) are typically based on national average cost data. Window and HVAC costs are based on quotes from manufacturers’ distributors. Appliance costs are based on manufacturers’ suggested retail prices.
Building construction options (wall insulation, ceiling insulation, foundation insulation, windows, and so on) are assumed to have a 30-year lifetime. Equipment and appliance options typically have a 10- or 15-year lifetime. Lifetimes for lighting options (incandescents and CFLs) are modeled based on cumulative hours of use.
Utility costs are assumed to escalate at the rate of inflation (that is, to be constant in real terms). The on-site power option used for this study was a residential PV system with an installed cost of $7.50 per peak watt of DC power including present value of future operation and maintenance (O&M) costs. Natural gas is assumed to have an average cost of $1 per therm.
Electricity is assumed to have an average cost of $0.127/kWh. These costs were chosen as representative based on energy costs over the past several years. The impact of recent or potential longer-term increases in the cost of energy was not considered in the current study. The home is assumed to have a gas water heater or a solar hot water heater with gas backup, a gas furnace, a gas clothes dryer, and a gas stove. (A gas clothes dryer was chosen based on the goal of defining a building that minimizes peak electric demand.) All cost assumptions are user inputs that can be modified to reflect actual costs in specific projects.
The cost estimates used in this study do not include the initial costs required to reengineer home designs, state and local financial incentives and rebates, or other builder costs, such as warranty and callback costs. All of these additional costs may have a significant impact on builder business decisions related to the construction of new home designs. These costs must be considered as part of the design of programs aimed at increasing the construction of high-performance homes.
The first costs associated with achieving different levels of source energy savings on the least-cost curve are shown in Figure 3 on p. 43. A home with 40% in annual source energy savings relative to Title 24 increases first costs by $5,000; a 50% home increases costs by $15,000; and a 60% home increases costs by $25,000. If the base home price for the Title 24 home is $500,000, the corresponding fractional increases in home cost required to achieve these higher performance levels are 1%, 3%, and 5% respectively.
The average 30-year cost of site energy savings is shown as a function of source energy savings level in Figure 4. The first cost required to achieve each savings level was divided by the total site energy savings for gas and electric end uses for the 30-year period of the current study to determine the average costs shown in Figure 4 on p. 43. The average cost of saved energy compares favorably with current residential energy costs out to a source energy savings level of about 50%–60%. The average cost of electricity provided by PV is about $0.17/kWh.
Changes in peak cooling load and peak cooling demand for the base case home are shown as a function of source energy savings in Figure 5. Improvements in home energy efficiency reduce peak cooling load by over 40% (lower curve in Figure 5) and peak cooling demand by 50% (upper curve in Figure 5). The additional reduction in demand relative to load is the result of the increase of air conditioner SEER from 13 to 15 at the 44% source energy savings level. In the current study, which assumes installed PV costs of $7.50/W, the marginal cost of electric savings from PV is less than the marginal cost of electric savings from high-SEER air conditioning systems for source energy savings levels greater than 44%.
The nominal energy efficiency ratio (EER) of the 13-SEER air conditioning system is assumed to be 11. At the 60% savings level, a 2kWpDC PV array with a 25% derate factor provides the 1.5 kilowatts required to meet peak cooling demand.
The annual energy end uses in the home at the 44% savings point relative to Title 24 where the incremental cost of additional energy efficiency savings equals the cost of energy savings from PV is shown in Figure 6. The right-hand side of Figure 6 also includes a summary of the options used to increase the efficiency of the home relative to the base Title 24 home.
In Figure 7, the cooling electric demand for the Title 24 reference house is compared to that for the house with 44% annual source energy savings relative to Title 24 for a peak cooling day with a thermostat setpoint of 76°F. Even without added PV, the energy efficiency options that have been implemented in the 44% house reduce peak cooling demand by about 1.5 kilowatts. In addition to the reductions in peak cooling demand, the energy efficiency upgrades in the 44% house also reduce total annual heating energy use by 70% and annual cooling energy use by 60%.
Because of the large reduction in the cooling load of the 44% house, a simple thermostat setback and setup strategy can have a significant impact on cooling demand in the high-performance home. To achieve the cooling demand results shown in Figure 8, the cooling setpoint was set at 76°F from 10 pm to 10 am. The setpoint was reduced from 76°F to 72°F from 10 am to 4 pm and then increased from 72°F to 78°F from 4 pm to 10 pm. Precooling strategies have previously been shown to be cost-effective alternatives for commercial buildings. This simple strategy shifts the cooling peak in the 44% house back from late afternoon to midafternoon (lower curve in Figure 8). The same strategy also provides a slight shift in the Title 24 house, but is insufficient to reduce the late-afternoon peak cooling demand (upper curve in Figure 8).
The impact of adding a west-facing 2 kWpDC PV system in combination with a thermostat setback and setup strategy is shown in Figure 9. The net impact of the PV system (in combination with energy efficiency measures and the thermostat setback/setup strategy) is to reduce peak cooling demand to nearly zero throughout most of the day in the 44% house (lower curve in Figure 9). The 2 kW PV system reduces the peak cooling demand in the Title 24 house in the late morning and early afternoon, but has little impact on the late-afternoon peak (upper curve in Figure 9).
By maximizing the investment in energy efficiency options before making investments in advanced options like PV, the size and therefore the cost, of the investment in the advanced option is minimized. A 4 kWpDC array at a cost of $30,000 would be required to reduce peak cooling demand on the grid to zero if no additional investment in energy efficiency beyond Title 24 were made. Not only would a PV-only approach cost more than the optimum found using the BEopt analysis method ($30,000 versus $25,000), but it would also provide significantly lower aggregate peak and off-peak energy savings than the integrated energy efficiency and renewable-energy approach implemented in BEopt’s least-cost optimization engine.
BEopt provides a way for residential program planners to identify the residential technologies that can be successfully used by production builders and that also have the potential to provide large incremental energy savings relative to currently available technologies. By focusing attention on the options that are most likely to contribute to future energy savings, this screening approach also identifies features of advanced technologies that require additional development to increase the likelihood of their future success.
Ren Anderson is leader of the Residential Research Section in the National Renewable Energy Laboratory (NREL) Buildings and Thermal Systems Center; Rob Hammon is principal of ConSol, an energy services provider for production home builders, located in Stockton, California; and Mike Keesee is the PV project manager for the Sacramento Municipal Utility District.
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