Designs That Work
Cold Climate
Case Study - Towards Zero Energy
With the advanced technologies described above, the Cold Case Study
House reaches an impressive 57.7% 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 8900 watt system would be required to reach the ZEH goal,
which would require approximately 854 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 Cold climate house, a 9900 watt array would
be necessary to offset the total load, covering 949 sf of roof area.
Again, conservation is much easier than solar collection and conversion.
Figure 30: 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.
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