The Power (Energy) of Water: A Model for Energy Extraction from Piped Water Flow

Abstract

Rising greenhouse gas emissions have placed an ever-increasing pressure on human activities to reduce those emissions in order to mitigate, or at the least, curtail the average global temperature from further increasing. The most significant contributor of greenhouse gas emissions is from the burning of fossil fuels, such as natural gas, oil, coal, and diesel fuel. Globally, fossil fuel usage continues to increase, but so does investment in improving energy efficiency. If energy waste can be reduced, the fuel needed to achieve the same output will also be reduced. This paper explores two streams of energy loss: (1) thermal energy lost down residential drains and (2) hydraulic energy lost in piped water flow due to elevation differences. The recovery of thermal energy directly from a sanitary drainpipe was unfortunately found to be negligible due to the short and relatively low flow of wastewater from household hot water sources. On the contrary, it may be worthwhile to recover hydraulic energy from cascading flow in large industrial water pipes using an Archimedes screw and electrical generator. A model of the flow kinetics and mechanical rotation efficiency was combined with a financial model for capital and operating expenses as a function of electricity generation. With an installed cost of approximately $7,500 per kW and roughly 5% annual operating costs, the overall present value of a system was not positive until reaching a flow rate of 24.5 MGD and 4.5 m of head assuming 80% mechanical and 85% generator efficiency. With a payback period of 14 years, it is highly unlikely an entity will install a system. If the installed cost for a micro system is reduced to $2,500 per kW as some research suggests, and retains 5% annual operating costs, the cost-benefit ratio becomes 0.6 and the payback period reduces to 6.6 years. At just 1.5 m of head and 3.6 MGD, the net benefit could be just over $16,000 annually at an electricity rate of $0.083/kWh. It is recommended that more research into installation and operating costs of micro hydropower systems be conducted to best approximate the benefits of an Archimedes screw retrofit installation in large, piped flow systems with small elevation drops. If such a system is commercially viable and beneficial, it could be an important tool in reducing greenhouse gas emissions.