Carbon footprint of ground source heat pump system in heating solar greenhouse based on life cycle assessment

Abstract: The Chinese solar greenhouse, characterized by east-west orientation, a transparent camber south roof, and a solid north roof and east and west walls, is utilized primarily in horticulture in northern China. This design of greenhouse can keep the sheltering plants from freezing in winter because of the "greenhouse effect". However, the healthy growing of plants still needs assisted heating especially during winter nights. The coal-fired heating system (CFHs) and the natural gas-fired heating system (GFHs) both have been widely applied to heat greenhouses. However, the conventional fossil energy sources, such as coal and natural gas, are non-renewable and are the major greenhouse gas (GHG) contributors. The overusing of fossil fuel in agricultural production has been directly or indirectly related to the global climate change, environmental pollution, and energy crisis. Therefore, renewable and clean energy, such as solar, geothermal, and shallow geothermal has been increasingly applied for greenhouse heating or cooling across the world. Ground source heat pump (GSHP) technology has dual functions in heating and cooling. It is one of the most rapidly growing green technologies for heating and air-conditioning in recent years. The GSHP application for solar greenhouse heating has proven to have a high primary energy ratio or coefficient of performance (COP) in previously studies. However, the environmental performance of the GSHP in heating solar greenhouse, such as its carbon footprint, is still unknown. Systematic and long-term study of the specific GSHP greenhouse-heating was required to evaluate its carbon footprint based on life cycle assessment (LCA) method. The GSHP in a Chinese solar greenhouse was studied to evaluate its environmental performance in greenhouse heating. The environmental performance of the GSHP was analyzed based on the field test data and the performance analysis models that were developed in this study. According to the study, in a 480 m2 Chinese solar greenhouse during the winter heating period, the GSHP demonstrated stable heating effects. The shallow geothermal energy utilized by the GSHP, in the processes of energy storage, extraction, enhancement of refrigeration compression cycles, and greenhouse heating, were studied to analyze the greenhouse gas (GHG) emission inventory and emission levels based on per square meter of the greenhouse floor. An analysis method based on LCA was developed for estimating the carbon footprint of Chinese solar greenhouse heating with GSHPs in this study, the carbon footprints of a GSHP greenhouse heating system operating on coal fired power and gas fired power were analyzed and calculated according to the data collected from a solar greenhouse heated in the Beijing area. Meanwhile, the variation of global warming potential (GWP, CO2 emission equivalent or CO2-eq) of GSHP in heating a Chinese solar greenhouse from 20 to 100 a were analyzed. The GWP of GSHP greenhouse heating operating on coal fired power and gas fired power were 257 g/(m2·d) and 72 g/(m2·d). Meanwhile, the total GWP of 100a is reduced by 1.6% and 5.4% from the calculation of 20 a. Comparing the carbon footprints between solar greenhouse heating with GSHP and Venlo greenhouse heating with natural gas, the carbon footprint of solar greenhouse GSHP heating was 39% more than that of Venlo greenhouse heating when GSHPs was operating on coal fired power, but the carbon footprint of solar greenhouse heating will be only 41% of Venlo greenhouse heating when GSHPs were operating on gas fired power. The GSHP heating test was focused on a Chinese solar greenhouse in this study to estimate the environmental performance; however, the carbon footprint calculation and analysis methods are applicable to different styles of multi-span greenhouse GSHP heating analysis.