Page 71 - Zero Net Energy Case Study Buildings-Volume 3

It is obvious from the charts of the energy consumption that heating using a conventional gas boiler plant is the largest energy demand of the building—approximately 75% of the total. This dominance by heating energy is partially the result that the other forms of energy use are so efficient and received the greatest attention in the innovative design approaches. It is easy to surmise that cooling energy demand would be much larger with a more conventional approach to the design of the building features and systems. There are additional explanations for the large proportion of heating in the overall energy use; these are discussed below.
Refer to the section, Post-Occupancy: Observations and Conclusions, for further discussion of the metering and data reliability issue, as well as more thoughts on the dominance of the energy use for heating.
Energy Production versus Energy Use: Zero Net Energy
This case study of an ostensibly zero-net-energy building project involves only a part of the two- phase total project that could be designed with an on-site renewable energy system and achieve ZNE performance, namely “The ZNE Group”, as defined in the floor plan diagrams on page 38. This collection of structures was carefully designed with atypical (for this K-12 building type) and innovative systems, which helped the case study “building” reach an energy consumption benchmark of EUI = 29.9. (The national range for this building type reported by the Energy Star benchmarking tool, Portfolio Manager, averages EUI = 123, with EUI = 30 being in the top 3% of energy performance.) The energy use of this collection of structures is therefore comparatively very low and within the range that is characteristic of zero-net-energy performance.
On the energy production side, the solar PV system was built in the two phases of construction, two years between the completion of each part of the installation. Since “The ZNE Group” was completed in the first phase, it is not clear how much of the total PV system’s energy generation can be assigned to “The ZNE Group” for assessing ZNE performance. In reality this is a semantic distinction only, whether the whole PV system or only an appropriate fraction thereof is assigned to “The ZNE Group”.
For that reason, the charts on the facing page show the results for three different sizes of the installed PV system: (1) the entire PV array installed at the end of both phases of construction; (2) the PV array installed at the end of Phase 1; (3) 82% of the PV array installed at the end of Phase 1, corresponding to the fraction of Phase 1 floor area that constitutes “The ZNE Group”. Clearly, use of the first method of accounting for on-site energy production provides the most favorable metric to satisfy the definition of “ZNE performance”. The charts present the energy production data for all three methods so that the relative differences can be seen.
The chart on the top of the facing page shows the energy consumed versus the energy produced over the course of a recent year. The relative impact of the large amount of energy consumed for heating during two winter months is apparent, compared to the energy produced by the PV system over the course of the year.
The chart on the bottom of the facing page shows the cumulative amount of net energy (production minus use) over the course of the year. The curve would return to the zero axis at the end of the year if the performance were exactly ZNE. (Above the zero axis after one year would signify a net positive performance.) As can be seen, “The ZNE Group” is performing at net negative, even using the full project PV array.
The reason for this performance of the case study building is again primarily because of the high heating energy demand during two winter months, which creates a deficit that cannot be overcome even by good performance of the solar PV system. This is discussed further in the following section.
Zero Net Energy Case Study Buildings, Volume 3
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LA ESCUELITA EDUCATION CENTER
CASE STUDY NO. 13