Renewable heating solutions for buildings; a techno-economic comparative study of sewage heat recovery and Solar Borehole Thermal Energy Storage System

The extensive use of fossil fuels for space and water heating in buildings is raising CO₂ emissions into the atmosphere. To address this concern, renewable and alternative energy sources should replace conventional energy options. This paper examines the technical feasibility, economic viability, and environmental impact of substituting conventional fuel with renewable and alternative sources for space heating and hot water purposes. Two different choices are offered to replace a natural gas-fired boiler in two residential buildings with 826 apartment units: Solar-Borehole Thermal Energy Storage System and Sewage Heat Recovery System. Solar-Borehole system couples the solar thermal collectors to boreholes, storing excess energy generated by solar collectors into the boreholes and supplying it to buildings when the energy demand is high and the solar energy is not sufficient. In a sewage system, low-grade thermal energy from the sewage beneath the building's site is upgraded and supplied to buildings via heat pumps. Arguments for and against each of the heating options are discussed. Proposed systems are designed using state-of-the-art technology, and their capital and operating costs are quantified. Both of these newly proposed systems are compared against a natural gas heating system to determine energy cost savings. To make the study location independent, additional calculations are undertaken to compute energy cost savings for changing electricity and natural gas pricing rates. The findings show that both proposed technologies are technically possible to execute on the studied construction site. While the Solar-Borehole system outperforms Sewage Heat Recovery technology in the examined location, the financial viability of other sites is determined by the difference in the unit cost of energy from electricity and natural gas. The sewage system surpasses the Solar-Borehole system in areas where natural gas is more expensive. Additionally, project execution can result in yearly CO₂ emission reductions of 1.1 million kg. This study can aid in understanding the technical and economic issues of transitioning multi-family apartment complexes from convectional heating to a renewable and alternative energy option.