Page 66 - Zero Net Energy Case Study Homes-Volume 2

CASE STUDY NO. 8
STRATTON-LEE DIY HOUSE
Renewable On-Site Energy Supply
There are sixteen (16) panels in the solar PV array, which is rated at 4 kW (DC). The 250-watt Trina panels5 were installed in 2016 before any construction work began. The roof is a simple gable with the slopes facing west and east; the western sloping side was selected for the panel location because there would likely be more clear skies in the afternoon and because there would be a higher cooling load at that time of day. Since the slope is relatively shallow, there is not a large penalty in performance compared to an ideal south orientation.
The owners do not currently have a battery for energy storage. They are planning to install one so that the energy collected by the solar PV system during the day can be used at the time of their peak load in the evening.
Energy Performance
Energy Modeling and Post-Occupancy Measurement
Energy Use—Modeling
Preconstruction energy modeling was done by the owners using BEopt (Version 2.8) software6, developed by the National Renewable Energy Laboratory (NREL) for residential projects. The results are shown in the chart on the opposite page, which displays the modeled monthly energy use of the house over the course of a typical year.
Energy Use—Post Occupancy Measurement
The owners decided to meter the energy consumption in some detail as construction was under- way in order to see the effects of the retrofit process as they were being put in place. They used a commercially available electrical monitoring system7 that measures the energy use of each identified appliance or device and transmits that information via Wifi to a phone or tablet over the internet. The owners were in fact able to see the effect of completed work when energy use trended downward as work proceeded.
The data recorded during the final year of construction (November, 2018, through October, 2019) serves as the closest approximation for the actual energy performance for the house at the completion of construction. The owners purchased an electric vehicle (EV) in December, 2018, and the data includes the energy used in charging the car during the year. This energy was not metered, but was estimated by the owner as follows:
“We keep track of how many miles we drive whenever we use a car. I looked up all the miles traveled in the EV since we got it in December of last year. I subtracted any trips where we didn’t charge at home. I took the total number of miles traveled each month and divided it by the power efficiency of our vehicle, which is 4.9 miles/kWh.”
This yielded an estimate of the monthly energy used to charge the EV, which can be subtracted from the total metered amount to give the monthly energy used by the house alone.
The chart on the opposite page shows the measured monthly energy use for this period (Novem- ber, 2018, through October, 2019), including the energy used in charging the EV. Note also the
5 https://www.trinasolar.com/us
6 https://beopt.nrel.gov/home
7 Sense Energy Monitor. See: https://sense.com/product
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Zero Net Energy Case Study Homes: Volume 2