STUDYING A MORE EFFICIENT AND COST-EFFECTIVE GEOTHERMAL SYSTEM: ENHANCING HORIZONTAL GEOTHERMAL SYSTEM WITH WATER FLUX PROVIDED BY SEPTIC WASTEWATER

Abstract

It is well documented and understood that consumption of energy is always increasing worldwide as the population grows. According to the U.S. Energy Information Administration [EIA] in 2023 the electricity sales to the four end user sectors, transportation / industrial / residential / commercial, residential consumed 38% of it. Residential also consumed 23% of natural gas, 3% of petroleum, and 15% of renewable energy of the four end user sectors. In 2020 US homes consumption of energy, on average, 52% was used for space heating and air conditioning (HVAC). Since most of the residential energy is consumed for HVAC, it has the potential to produce the largest energy savings by using a more efficient system. A heat pump (HP) is more efficient at conditioning a space than the more commonly used types, direct expansion, electric resistant, and natural gas. Unfortunately for heat pumps, they lose their efficiency advantage when operated outside of their ideal temperatures, between 40⁰ F and below 80⁰ F. That’s why geothermal water source heat pump systems are more efficient, their typical operation is around 67⁰ F. The problem with geothermal systems compared to traditional units is the greater increase in installation costs. Of the two most common systems, vertical ground and horizontal ground, vertical is the most efficient but has the highest installation cost and takes the most specialized equipment to install. So, the dilemma is, how can we make the horizontal ground geothermal system more efficient and affordable at the same time. By solving this problem, we can give more opportunities to save on the consumption of energy and cost of better technologies. A proposed solution is to combine horizontal geothermal and septic systems together. The footprint of the geothermal field would be installed under the septic lateral field in a layered configuration. This way the geothermal will benefit from the wastewater of the septic in increasing thermal conductivity, heat flux of the soil, making the geothermal more efficient. Also, the construction cost for the combined systems will be lower than if they were built separately. The paper includes a study of the proof of concept using modeling software Dymola and known soil characteristics of Shawnee, OK showing the increase of soil thermal conductivity, heat flux of the system. The difference of normal soil conditions vs saturated soil conditions will be researched for this paper. Efficiency, cost comparisons, and life cycle analysis will be shown against several commonly used systems. The common systems are air source HP, horizontal geothermal HP, and vertical geothermal HP. Through our study, we have found the coefficient of performance (COP) of each system to be different. The improved COP of the horizontal geothermal HP system resulted in higher energy savings. The life cycle cost analysis shows that it takes 4 - 9 years for return on investment. Final remarks and future research for this combined system will also be discussed.