Synergetic performance improvement of a novel building integrated photovoltaic/thermal-energy pile system for co-utilization of solar and shallow-geothermal energy

Soil thermal imbalance due to excessive heat extraction and photovoltaic efficiency decline caused by temperature increase are two bottleneck problems that hinder the utilization of solar and shallow-geothermal energy. The existing research is still blank in terms of the coupling performance of photovoltaic/thermal collectors coupled with energy-pile ground source heat pumps, and cannot effectively solve the soil thermal imbalance of the energy piles. Therefore, a novel building-integrated photovoltaic/thermal-energy pile system is proposed to address soil thermal imbalance, improve electric efficiency, and reduce the building load simultaneously. In this coupled system, the waste heat is recovered from the photovoltaic/thermal collectors to charge the ground for thermal balance; the low-temperature fluid from energy piles is used to cool the photovoltaic/thermal collectors for electric efficiency enhancement, and the building-integrated photovoltaic/thermal reduces the heating load as well. The coupled system model is built and the comprehensive performance is optimized considering three important parameters (photovoltaic/thermal installation angle, roof absorptivity, and energy pile number). Results show that the electric efficiency of photovoltaic/thermal collectors increases to 16.04% with a growth of 22.72% compared with the photovoltaic collectors, the decline in soil temperature is only 1.5 °C, the heat pump unit maintains a ten-year coefficient of performance of 3.21 with an energy saving of 46.47 MWh, and the accumulative building heating load is reduced by 1.65%. The optimal design suggests an installation angle of 45°, a roof absorptivity higher than 0.36, and as many energy piles as the building foundation can hold. This work reveals the coupling and the synergetic performance improvement of the novel system, which could facilitate the efficient utilization of solar and shallow-geothermal energy toward carbon neutrality.