Designs That Work

Hot-Humid Climate - New Orleans

Advanced Technologies - Towards Zero Energy

With the advanced technologies described above, the Hot-Humid Case Study House reaches an impressive 64.7% reduction in energy use when compared to the Building America Benchmark. However, as uncertainty grows around our dependency on fossil fuel-based energy, even greater steps to reduce residential energy use are a priority. In response, the Building America program has established the goal of creating houses that generate as much energy as they use.

A Zero Energy Home (ZEH) is designed to balance energy consumption with site energy collection and conversion so that there is no net energy usage during normal operation of the house. In practical terms this means that over the course of the year, the homeowner's energy consumption from the utility will be zero.

On the other hand, a Zero Cost Home (ZCH) would be a home that had no utility bills, and would need it's own battery back-up systems, etc. to avoid utility service fees, and not have to worry about net metering being yearly or monthly, etc.

Design Considerations

The Advanced Technologies section above gives the first steps in making use of the available energy on the site to meet the remaining demand. The geothermal system, the solar hot water system and the photovoltaic panels have been chosen in that order, because they provide the most rational payback period for the energy collected. The final step to reach zero energy is to add significantly to the photovoltaic array.

With the previous sections of this report, the design strategy of looking first for ways to reduce the energy used by the house and then providing power generating capacity to meet the remaining demand. Having maximized the conservation aspects with this house design, reaching for Zero Energy is now left up to sizing the PV collection array based on reasonable assumptions of conservative usage. Therefore, the first and most important steps the design of a ZEH involve decisions that are made by (or for) the homeowner. To start with, the future occupant needs to be made aware of the energy conservation strategy. Experience with utility studies of energy efficient homes has demonstrated that the energy intensity of the homeowner's lifestyle can make a significant difference in the overall utility use, by a factor of 3:1.

The energy reduction plan will include the choice of building site and the orientation of the house on the property (as discussed on page 14), as well as attention to energy-saving practices such as using the thermostat to control indoor conditions (as opposed to windows), using reasonably conservative set points for the heating and cooling systems and turning electrical devices off when not in use (rather than leaving them on the standby setting). These lifestyle-related changes made by, the homeowner should be considered in concert with the energy load reduction by the building enclosure and mechanical system design described in Section 2 of this package.

For our purposes of sizing a ZEH PV system, an estimation of a 10% reduction in total energy load was used to reduce the size of the PV system required to offset the energy use. With this 10% conservation estimation, a 6300 watt system would be required to reach the ZEH goal, which would require approximately 600 sf of PV panels.

In situations where the cost of the panels is not a consideration, the other constraining factor is the ability to fit the necessary panels on the roof. In the case of the Hot Humid house, a 7000 watt array would be necessary to offset the total load, covering 670 sf of roof area. Again, conservation is much easier than solar collection and conversion.

Figure 33: Arrangement of Photovoltaic Array on ZEH

As described in the Photovoltaics section above, the location of the PV array can dramatically affect its performance, especially with regard to partial shading. The drawing above shows how additional panels can be added to the house, while minimizing the risk of shading.

Energy Model Results

*Note that the energy cost of $0 doesn't include monthly fees, etc.