Energy Ratios and Life Cycle CO₂ Emissions for Domestic Solar Thermal Systems

Abstract

Objective. The objective of the present study is to evaluate the environmental performance of solar thermal systems for domestic hot-water supply from life cycle perspective. Energy ratios and life cycle CO₂ emissions of two different types of solar thermal system are analyzed, namely, the integrated solar thermal system and the separated solar thermal system that supply 40% of the total hot-water demand of a four people household. This analysis uses the material and energy data collected from eight manufacturers of solar thermal systems to reflect the present reality.
Results and Discussion. The analysis shows that the life cycle CO₂ emissions of solar thermal systems are 25-37% less than those of domestic gas boilers. Energy ratios of solar thermal systems are more than one, while that of a gas boiler system is 0.9. This is because solar thermal systems require less gas consumption compared to a domestic gas boiler. Moreover, comparison between the two different types of solar thermal systems reveals that the life cycle CO₂ emissions and energy consumption of the separated solar thermal system are more than those of the integrated solar thermal system mainly because the separated type consumes electricity for pumps to circulate the heat medium. Furthermore, The effects on CO₂ emission of the change in heat supply rate by the solar thermal systems are analyzed. The analysis finds that the more the heat supply rate increases, the less the CO₂ emissions of the solar systems are. When the solar thermal systems supply 60% of the total hot-water demand, the life cycle CO₂ emissions of solar thermal systems are 25-37% less than in the case of 40% supply.
Conclusions. The present study reveals that both the integrated and separated solar thermal system are superior to gas boiler from viewpoints of energy ratios and life cycle CO₂ emissions. Furthermore, comparison between the integrated and separated solar thermal system finds the former has better environmental performance than the latter.