Thermal Performance Evaluation of a Solar/Gas Hybrid Water Heating System

摘要

In climate regions with large seasonal variations in solar radiation, such as the Pacific Northwest, a solar energy collector might not economically satisfy year-round domestic water heating demands, requiring an auxiliary unit, such as a natural gas-fired water heater. Previous studies have shown that the burner efficiency of a gas-fired water heater varies depending on the log-mean temperature difference between cold fluid (water) and hot fluid (combustion gases). In a solar/gas hybrid water heating system where a solar collector is used in conjunction with a gas-fired heater, the partial heating of water provided by the solar input reduces the log-mean temperature difference value for gas heater, reducing the efficiency of gas burner. Since this efficiency reduction varies depending on the amount of pre-heating provided by solar input, it is difficult to accurately predict the actual cost and energy savings offered by a solar/gas hybrid water heater. Hence, to predict the actual energy and cost savings under various design conditions, the performance of solar/gas hybrid systems must be better understood. The purpose of this work is to experimentally determine the thermal performance of a solar/gas water hybrid water heating system with a 6.44 m~2 flat plate solar collector array and a 22.3 kW natural gas burner. Under different temperature lifts and solar insolation values, the system was operated at three different modes of heating: solar, gas, and combined solar/gas mode. Efficiency value for each mode is calculated. Based on the experimental efficiency results, a configuration that would provide higher efficiency for combined solar/gas heating is suggested.