Efficient low-grade heat recovery will significantly help reducing greenhouse emission as over 70% of primary energy input is wasted as heat, but current technologies to fulfill the heat-to-electricity conversion are still far from optimum. We invent direct thermal-charging cell (DTCC) with heat-to-electricity conversion efficiency (η) over 5% at low-grade heat regime (>40% of Carnot efficiency ηc), surpassing all existing direct energy conversion system. Currently, we have two applications: (1) the integration of DTCC with HVAC (heating, ventilation, and air conditioning) system for harvesting low-grade heat from compressor and condenser, (2) the integration of DTCC into the window frame for harvesting solar thermal energy to power the smart window device (e.g., electrochromic window) in hot weather; intelligent regulating of ultraviolet, visible and infrared lights from the solar spectrum is of importance to reduce the indoor lighting and energy consumption of HVAC. Our business idea for the integration of DTCC with HVAC and smart window is promising to improve energy efficiency of buildings, which is vital to abate greenhouse-gas emissions and also promises potential economic and environmental benefits. Moreover, we are looking for new opportunities and collaborators for body-heat technologies and self-powered devices using our DTCC technology.