Time-of-Use Monitoring of U.S. Coast Guard Residential Water Heaters With and Without Solar Water Heating in Honolulu, HI

Abstract

A combination of high energy costs, uniform solar resource, and an active solar industry combine to make Hawaii a good location for cost effective applications of solar water heating. The non-freezing climate allows for simple solar water heating system designs. In the mild climate of Hawaii, solar water heating can displace a large fraction of a home’s electricity use since heating and cooling loads are small. In 1998, sixty-two solar water heaters were installed at Kiai Kai Hale US Coast Guard Housing Area in Honolulu, HI as a pilot project under a grant from the US DOE Federal Energy Management Program (FEMP). The systems are active, open loop systems with a single tank (electric water heater with the bottom element disabled). An assessment of these pilot units will help inform a Coast Guard decision regarding implementing solar water heating on the remaining 256 units in the housing area, and may be useful information for other government and utility programs. On 25 houses with solar water heating and 25 identical houses without solar, instruments were installed to measure on/off cycles of the electric water heaters and the tank outlet temperature. This paper describes the results the monitoring for a six week period From June 11 to July 25, 2002, with a statistical extrapolation to estimate annual savings. Demand savings are estimated at 1.62 kW/house, energy savings at 3,008 kWh/house/year, and annual cost savings per house is estimated at $380/year due to solar. For a system cost of $3,200 ($4,000 minus a $800 utility rebate) and a 25 year present worth factor of 17.1, the savings to investment ratio (SIR) is 2.03, so this solar water heating application is cost effective according to Federal regulation 10CFR436 (which requires SIR>1.0). The annual solar fraction is estimated at 74% and annual solar water heating system efficiency is estimated at 24%. This paper describes the statistical design of the survey; the measured load profiles; the energy, demand, and cost savings; and the observed condition of the systems. The paper includes a discussion of application of the International Performance Measurement and Verification Protocol (IPMVP) applied to renewable energy systems.