Time-of-Use Monitoring Assessment of U.S. Coast Guard Residential Solar Water Heating Pilot Project

April 30, 2003

A combination of high energy costs, uniform solar resource, and an active solar industry combine to make Hawaii a good location for cost-effective application of solar water heating. In 1998, the U.S. Coast Guard received a FEMP grant to install solar water heating systems as a pilot project on 60 existing three-bedroom houses at Kiai Kai Hale U.S. Coast Guard Housing Area in Honolulu, Hawaii. 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. This particular solar water heating project presented a cost-effective opportunity to contribute toward the goals of Executive Order 13123.

The installed solar water heating systems are of the active (pumped), direct type, where potable water is circulated to the collectors by a single pump controlled by a differential temperature thermostat controller. A single tank serves as both electric water heater and solar storage tank. The consequential heat loss of sending electric-heated water to the collectors is mitigated by:

  • installation of a timer to keep the electric heating elements off during the day,
  • setting a low temperature for control of the electric heating elements, and
  • reduced heat loss (naturally) from the collector attributed to high ambient temperatures in Hawaii.

The systems were installed by Pacific Mechanical Company at an average cost of $3,200 ($4,000 minus $800 utility rebate) per system.

DOE's National Renewable Energy Laboratory (NREL) conducted a time-of-use monitoring assessment of the solar water heating pilot units, which will give the Coast Guard the information it needs to negotiate for financing and implementing solar water heating on the remaining 256 units in the housing area. 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. The results were monitored for a 6-week period from June 11 to July 25, 2002, with a statistical extrapolation to estimate annual savings.

Chart with two line graphs illustrating the hourly load profile for Kiai Kai Hale U.S. Coast Guard solar water heating project from June 11 to July 24, 2002. One graph depicts average hourly water heating power per house with solar, and the other depicts average hourly water heating power per house without solar. The load profile graphs show that more heating power is required for houses without solar compared to houses with solar.

Hourly load profile for Kiai Kai Hale U.S. Coast Guard solar water heating pilot project, averaged for 21 houses with solar water heating and 17 houses without. The profile is for the monitored period from June 11 to July 24, 2002.

The graph illustrates the average water heating power for houses with and without solar water heating, showing very substantial savings, especially in the evening when utility peak load occurs. Savings in peak demand (as opposed to average power) were measured at 1.62 kilowatts per house. Annual energy savings were estimated at 3,008 kilowatthours per house per year, and annual cost savings per house were estimated at $380 per year due to solar. For a system cost of $3,200 and a 25-year present worth factor of 17.1, the savings to investment ratio (SIR) is 2.03. Even without the rebate, the Coast Guard solar water heating application is cost effective according to Life Cycle Cost Methodology and Procedures (10 CFR 436), (which requires SIR>1.0). The annual solar fraction of total energy used is estimated at 74 percent and annual solar water heating system efficiency is estimated at 24 percent. The International Performance Measurement and Verification Protocol as applied to renewable energy systems was used in this analysis. NREL's analysis, in support of the Coast Guard pilot project, provides detailed load and time-of-use information useful to designers, utilities, and Federal and State agencies implementing solar water heating programs in Hawaii.

For more information, please contact Andy Walker of NREL at 303-384-7531 or andy_walker@nrel.gov.