Geothermal Sources Reuse: Algae Cultivation and Other Initiatives

Abstract

Tuscany (Italy) is the most important area for geothermal electrical energy production in continental Europe, with 37 geothermal units for a total net installed power of about 766 MWe. In Tuscany about 32% of the electrical energy requirement is provided by geothermal sources. CO2 enriched gas and heat represent the two main outputs from geothermal power plants. Large-scale algae facilities require huge amounts of CO2 for algal growth and heat for culture thermoregulation and biomass drying. Thanks to EGP and COSVIG financial contribution, a 120 m2 pilot algae facility was built by F&M close to one EGP power plant at Chiusdino (Siena, Italy) to evaluate the use of geothermal sources (CO2, condensed steam (40-45°C) and cold water (25°-28°C) from cooling tower), in the cultivation of microalgae. The pilot plant has been operated since June 2017 with A. platensis F&M-C256 grown outdoors in raceway ponds and Green-Wall Panel (GWP®) reactors to compare performances. The use of geothermal CO2 did not reduce biomass productivity in both pond and GWP® when compared with food grade CO2. Detailed analysis of the produced biomass confirmed the possibility to use geothermal CO2 as source of carbon. Condensed steam and cold water were used for culture temperature control. Warmed cultures, regardless of the culture system used, attained higher (+ 43%) productivity. Mass and energy balances extrapolated to 1 ha size showed that the availability of geothermal heat and CO2 reduces biomass production cost of more than 50%. As well as demonstrating the technical-economic feasibility of cultivating Spirulina, the project aim is to create new investments and job opportunities at local level, within a circular economy perspective linked to this renewable. Based on the aforementioned results twenty suitable areas for spirulina cultivation have been identified near geothermal plants installed in Tuscany. Moreover, some contacts with potential investors interested in algal cultivation have been started and to date at least two projects for the realization of a full-scale algae production plant integrated with geothermal sources are ongoing. Regarding the geothermal sources reuse after the production of electric power, EGP is also one of the most important Italian operator in district heating from renewable source. Indeed EGP has some district heating plants in operation for a total net power of about 170 MWt which deliver a yearly heat energy of about 330 GWh-t to over ten thousand users among domestic and enterprises and already contribute to the reduction of CO2 emitted for heating to over 100,000 tons/year. Moreover with the aim of realizing a model of circular economy, EGP has also signed an agreement with a company involved in drink and food industry, for using the geothermal CO2 at the outlet of a power plants for the production of sparkling water and other drinks or for food conservation.