Thermal performance of solar/gas hybrid water heating system : an experimental study

It is important to utilize renewable resources for house hold water heating purpose. A hybrid (solar/natural gas) approach for water heater is introduced for learning the system overall behavior and operation. Fours tests were performed during this experimental work (heating test, first hour rating (FHR) test, stand by test and 24-simulated use test). Some of the major challenges involved are; to simulate a controlled and realistic water loading profile during 24-hour simulated use test. The heating test helps to measure the system performance in term of efficiencies in each mode of operation such as solar, gas and hybrid mode. The solar collector efficiency was found from 40 to 60% depends upon the solar peak hour, optical efficiency, ambient temperature. The gas burner firing efficiency was found to be very consistent and recorded about 70±2% for different storage tank temperatures. The solar fraction ranging from 3-10% was recorded for a typical weather condition. First hour rating test was useful to measure the system maximum capacity of hot water supply at maximum temperature in one hour. During this it provided 133 gallons of hot water volume at 137.7ºF. Standby test measures the quality of storage tank insulation which is critical for gas supply heating system. The tank drop of 0.72ºF/hr. was absorbed during 24-hours standby period. 24-hours simulated test was performed using draw pattern recommended by the department of energy as a water load profile. This test was used to measure the energy factor which reflects the overall performance of the system and founded 0.52. The total annual delivered energy and consumption was 125therm and 247therms, respectively. Available experimental data can be used for validation of TRNSYS based model in future.