Distributed Renewable Energy Operating Impacts and Valuation Study

Section 2
existing systems installed in the APS service territory, savings for SHW systems in the APS DE
program range from 1,600 kWh/year to 5,800 kWh/year, with an average of 2,550 kWh/year.
2.3.2 Model Description
Performance Modeling Approach
There are several simulation models that can be used to estimate savings from SHW in a
particular climate, such as RetScreen and TRNSYS. The Study team selected the EnergyPlus
model, which gives hourly demand for both the baseline water heating system and the SHW
system.
EnergyPlus models heating, cooling, lighting, ventilating, and other energy flows as well as
water in buildings. EnergyPlus includes many simulation capabilities such as time steps of less
than an hour, modular systems and plant integrated with heat balance-based zone simulation,
multizone air flow, thermal comfort, water use, natural ventilation, and photovoltaic systems.
Baseline Model Description
The baseline model is a typical single-family residence, designed to include the following
characteristics:
• 1-unit detached house built in 1990
• 3 bedrooms, 2 stories
• 1,867 square feet total floor area
• standard electric water heater rated at 0.88 energy factor, with a 50-gallon tank
• 3-person household
These data come from the 2006 American Community Survey for Yuma and Phoenix, APS load
data, and from the APS Energy Efficiency Baseline Study. 7
SHW Model
The SHW model has a standard active SHW system added to it with a minimum solar fraction of
0.8. 8 The solar fraction reflects the portion of the water heating load supplied by solar energy.
Weather Files
Weather files taken from NREL web site for TMY for Phoenix were used for the model.
Because of the storage capabilities of water heating, transient effects of clouds are not as
significant to SHW savings as is the case for PV.
Customer Classes
This analysis is for residential customers only.
7 ICF International, APS Energy Efficiency Baseline Study, prepared for Arizona Public Service, March 2007.
8 Based on simulated solar fractions using current technology (for the Phoenix area), taken from: P. Denholm, The
Technical Potential of Solar Water Heating to Reduce Fossil Fuel Use and Greenhouse Gas Emissions in the United
States, NREL Technical Report, March 2007.
2-20 R. W. Beck, Inc. Arizona Public Service