The hydrodynamics of the water hammer energy system.

Abstract

The generation of electricity from fossil fuels is a major contributor to climate change and cannot be sustained indefinitely. Renewables can provide electricity in a more sustainable manner, however supplies from sources such as wind and solar can be variable and unpredictable. Hydropower and tidal energy offer more predictable generation capacity, making them appealing for a resilient transmission system. This is particularly true in the context of decentralised power grids, which harness smaller amounts of energy from a wide range of sources to improve transmission efficiency and reliability. Yet for tidal power in particular, little work has been done thus far on developing small scale technology capable of working efficiently in low flow speed (< 2 m/s) conditions. This research was conducted to identify, design and develop a device capable of generating pico scale power (< 1 kW) in shallow water, low input tidal and river conditions. The result is the Water Hammer Energy System (WHES), a novel design that makes use of water hammer pressure surges to generate vertical oscillations from a horizontal flow of water. The hydrodynamics of the system are investigated through a combination of experimental and theoretical work, with studies conducted into the e↵ect of input head, flow rate, and device size on performance. A 16 mm diameter experimental device was found to provide a piston with a mean power of 8 mW. Data from this study was used to validate a mathematical model, which predicted a maximum hydrodynamic efficiency of 23.1 % for a 1 m2 device operating at a 0.50 Hz valve closure frequency in 0.4 m/s flow. Assuming a 30 % generator efficiency, such a device operating in the mouth of Poole Harbour (where the peak flow speeds reach 1.69 m/s) could supply an average of 1.14 kW of power. Over the course of a year, this would provide enough electrical energy to supply 2 typical UK houses and o↵set 5.55 tonnes of CO2. 5.85 kW would be generated in a constant flow of 1.69 m/s, sufficient to supply the annual electricity requirements of 11 typical UK households and o↵set nearly 30 tonnes of CO2. With further development, the system may therefore be a viable method of generating pico scale hydropower from shallow water, low input sites.