Hybrid Cooling Tower Retrofit in a Geothermal Power Plant: Outcomes from H2020 Matching Project

Abstract

A cooling tower in a direct steam geothermal power unit behaves like a direct-contact heat exchanger, where geothermal condensed steam exchanges heat into the air. Heat is transferred by radiation from the surface of the droplets, by conduction and convection between water and air, by evaporation of part of water. The latest represents the main part of heat transferred and this is the reason why the whole process is named “Evaporative cooling tower". In geothermal sector, the most diffused evaporative cooling towers belong to induced draught technology where a fan moves the air counter-currently with respect to the water. The water is distributed at the upper side of cooling tower, slides down the packing and is collected in the basin at the bottom. The amount of heat exchanged depends by various factors including the contact surface area, thus the filling media design plays a key role in increasing the cooling capability or in reducing the filling height with a fixed thermal load. The effective shape of the filling media affects the fouling behavior as well. In the geothermal evaporative cooling tower, most of the steam condensate that takes part in the cooling process escapes as water vapor from the fan stack, and only a small amount (blow down) is re-injected into the geothermal reservoir. Recently the use of wet-dry hybrid cooling tower has been considered as an alternative to evaporative cooling tower in order to raise the amount of re-injected water and to avoid the typical visible plume. The increased water recovery is paid with a larger fan consumption but generally, the higher re-injection into the reservoir results in a higher steam flow rate available at production wells thus leading to a net power gain. In MATChING EU H2020 funded project, one of the main goals was to assess the use of advanced/innovative material solutions to increase the robustness of hybrid cooling towers in high temperature geothermal power plants in order to make them a competitive alternative to currently wet cooling towers. After a pilot validation phase, one of six cells of the cooling tower of “Nuova San Martino” geothermal power plant (Monterotondo Marittimo, Italy) owned by Enel Green Power, was retrofitted with 4 heat exchangers designed and engineered by SPIG and operated in hybrid configuration. After the experimental tests lasted 6 months, hybrid operation showed a water recovery up to 15% with respect to the wet operation. Moreover, the typical plume at the fan stack was avoided improving the visual impact and the environmental sustainability of the plant. An economic evaluation considering a typical geothermal process equipped with a hybrid cooling tower was addressed and the profitability of this technology was assessed.