Page 50 - Zero Net Energy Case Study Buildings-Volume 1

CASE STUDY NO. 2
STEVENS LIBRARY AT SACRED HEART SCHOOLS
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Zero Net Energy Case Study Buildings: Volume 1
Energy Design Analysis and Energy Performance -
Modeling versus Initial Post-Occupancy Measurements
Energy Use – Modeling
The A/E team approached the design of the four campus buildings using the same integrated design process for energy and water systems. This meant that each of the four buildings was modeled for whole building energy use, evaluating energy-related building features and systems for lowest energy demand within the budget constraints of the project. It was recognized early in the process that the Library building could achieve zero-energy performance based on this modeling information.
Since the building was also seeking full certification under the Living Building Challenge, particu- lar attention was paid to rainwater collection and incorporation of a gray water system. As noted in the building system description above, this water recycling system serves all four buildings of the middle school quad, with the equipment located within the Stevens Library facility. The pump energy associated with the water recycling system is therefore included in the overall energy use total for this single building. However, during the design phase of the project, this load was not included in the energy modeling of the library since it was being designed at the time separately from the building systems.
The energy modeling for the Library was carried out using eQuest version 3.64. The results are shown in the charts on this page and the facing page. In the final analysis, the model showed a predicted EUI of 27.0 kBtu/sq.ft. per year, not including the water recycling pumps, which is well within the range necessary for a zero net energy building. PV Watts was used to estimate the size of the solar photovoltaic system to be installed, using the assumption of 1500 kWhr (AC) per kW (DC) installed, yielding a 40-kW size for the system.
Energy Use – Actual Measurement and Comparison to Modeling Results
The building has meters installed on the principal subsystems, which are domestic hot water (DHW), lighting (interior and exterior), plug load and the package HVAC unit. Since the water recycling pumps serving the rainwater collection and gray water system are actually located within the Stevens Library and are powered by the on-site solar photovoltaic system, the energy used by these pumps was metered and included in the overall energy use for all systems within the building envelope.
The set of meters installed by the contractor produces data that is recorded by the Building Management System (BMS). A typical problem with this standard approach to metering is that the BMS may not be programmed to produce all data that is meaningful during the measurement period and the interpretation of the data is often not easy to understand. For this library project, a second set of power meters was installed with additional measurement points, which was also separate from and not monitored by the BMS. This proved useful in uncovering a flaw with the first set of meters recording to the BMS (see the discussion below) as well as providing a finer analysis of where the energy was being used in the building.
Since the package HVAC unit is a heat pump, which can operate in either heating or cooling modes, the energy use is actually a combination of the energy used for heating, cooling and to drive the small fan. The second set of meters included a separate meter that was placed on the fan, and the heating and cooling energy use was also obtained by recording when the heat pump was in heating or cooling mode. The energy used for heating, cooling or simple ventilation could then be separated into these three different categories of energy use.