Back Analysis of a Horizontal Geothermal Plant Implemented in a Wine Production Process

The Salcheto winery has undertaken a process of reduction of its primary energy consumption and the implementation of green energy technologies. They adopted solar photovoltaic, wood biomass, and geothermal energy sources. A horizontal ground source heat exchanger (GSHE) plant is used to cool a part of the pressed grapes and control the wine production temperature. The goal of this work was to investigate some technical issues of the plant and to increase the efficiency of the whole system. The first step was the evaluation of the actual operating conditions of the GSHE plant, by performing a thermal response test. The results allowed us to find the thermal diffusivity of 3.5 × 10−7 m2/s, and the calculation with the IGSHPA standard indicated a cooling performance of about 6 kW. A survey during the harvest highlighted a peak power of 6 kW. Therefore, to improve the plant, some modifications were proposed and analyzed. In the new layout, the geothermal plant serves the condenser of the refrigeration unit, allowing cooling of the all production lines, instead of only one. The peak power was evaluated as 32 kW, and the GSHE can fulfil this, up to 18 kW. For higher power, the evaporative tower will supply the remainder, covering a maximum of 45%. Furthermore, the refrigeration unit may cover the cooling requirements of the entire residential and office building, without other plant improvements.

Thermodynamic and Kinetic Modelling of Scales Formation at the Soultz-sous-Forêts Geothermal Power Plant

Geothermal energy has been a subject of great interest since the 1990s in the Upper Rhine Graben (URG), where the first European Enhanced Geothermal System (EGS) pilot site has been developed, in Soultz-sous-Forêts (SsF), France. Several studies have already been conducted on scales occurring at the reinjection side at the geothermal plants located in the URG. It has been observed that the composition of the scales changes as chemical treatment is applied to inhibit metal sulfate. The purpose of this study was to model the scaling phenomenon occurring in the surface pipes and the heat exchangers at the SsF geothermal plant. PhreeqC, a geochemical modelling software, was used to reproduce the scaling observations in the geothermal plant during exploitation. A suitable database was chosen based on the availability of chemical elements, minerals, and gas. A thermodynamic model and a kinetic model were proposed for modelling the scaling phenomenon. The thermodynamic model gave insight on possible minerals precipitated while the kinetic model, after modifying the initial rates equation, produced results that were close to the expected scale composition at the SsF geothermal plant. Additional laboratory studies on the kinetics of the scales are proposed to complement the current model.

Exergoeconomic Optimization of Polymeric Heat Exchangers for Geothermal Direct Applications

The highest economic costs of a geothermal plant are basically related to well drilling and heat exchanger maintenance cost due to the chemical aggressiveness of geothermal fluid. The possibility to reduce these costs represents an opportunity to push toward geothermal plants development. Such challenges are even more important in the sites with a low-medium temperature geothermal fluids (90–120 °C) availability, where the use of these fluids for direct thermal uses can be very advantageous. For this reason, in this study, a direct geothermal heating system for a building will be investigated by considering a plastic plate heat exchanger. The choice of a polymeric heat exchanger for this application is upheld by its lower purchase cost and its higher fouling resistance than the common metal heat exchangers, overcoming the economic issues related to conventional geothermal plant. Thus, the plastic plate heat exchanger was, firstly, geometrical and thermodynamical modeled and, after, exergoeconomic optimized. In particular, an exergoeconomic analysis was assessed on the heat exchanger system by using a MATLAB and REFPROP environment, that allows for determination of the exergoeconomic costs of the geothermal fluid extraction, the heat exchanger, and the heating production. A sensitivity analysis was performed to evaluate the effect of main design variable (number of plates/channels) and thermodynamic variable (inlet temperature of geothermal fluid) on yearly exergoeconomic product cost. Then, the proposed methodology was applied to a case study in South of Italy, where a low-medium enthalpy geothermal potential exists. The plate-heat exchanger was used to meet the space heating requests of a single building by the exploitation of low-medium temperature geothermal fluids availability in the selected area. The results show that the inlet temperature of geothermal fluid influences the exergoeconomic cost more than the geometrical parameter. The variation of the exergoeconomic cost of heat exchanger with the inlet geothermal fluid temperature is higher than the change of the exergoeconomic costs associated to wells drilling and pumping with respect to the same variable. This is due the fact that, in the selected zone of South of Italy, it is possible to find geothermal fluid in the temperature range of 90–120 °C, at shallow depth. The product exergoeconomic cost is the lowest when the temperature is higher than 105 °C; thus, the smallest heat exchange area is required. The exergoeconomic optimization determines an optimum solution with a total product cost of 922 €/y for a temperature of geothermal fluid equal to 117 °C and with a number of plates equal to 15.

A Geothermal Plant from a Time-Scale Perspective

In recent years, geothermal energy use from low-temperature sandstone reservoirs has sharply increased. Nonetheless, the injection of heat-depleted geothermal fluids has not been an easy task because of well/formation damage and operational/economic issues. Sønderborg geothermal plant is a case example of heat-mining from a low-temperature reservoir. It is in the northeast of Sønderborg towards Augustenborg Fjord. The present work takes into consideration the regional and local geology of the Sønderborg area, construction of the wells, field experience and water chemistry. The main issues of the geothermal plant appear to be related to the construction of the wells and reinjection of the heat-depleted brine. Our water chemistry analysis and PHREEQC simulations indicate that geothermal brine was saturated with respect to carbonate and barite minerals. The excess of Ca2+ and SO42− ions could have led to the formation and precipitation of carbonate and sulfate scales. Moreover, the increment of iron concentration over time could suggest the ingress of oxygen and pitting corrosion due to the presence of halide ions.

Interactions between the calcium scaling inhibitor NC47.1 B, geothermal fluids, and microorganisms – results of in situ monitoring in the Bavarian Molasse Basin (Germany) and accompanying laboratory experiments

Application of the environmentally friendly scaling inhibitor NC47.1 B in geothermal systems was studied in laboratory and field-scale experiments. Biodegradation was investigated under anaerobic, in situ-like conditions and a mass balance confirmed the almost complete conversion of the polycarboxylate to e.g. acetate, formate, methane and CO2. Much higher concentrations of inhibitor were chosen than applied in situ and rapid degradation was observed in biofilm-inoculated setups: A concentration of 100 mg/L of the inhibitor was degraded below detection limit within 8 d of incubation. Furthermore, the inhibitor was applied at the geothermal plant in Unterhaching, Germany. Monitoring of the microbial community in situ showed an increase in the abundance of Bacteria. Particularly, relatives of the fermenting Caldicellulosiruptor dominated the biocenosis after about six months of continuous inhibitor dosage (5–10 mg/L). However, in long-term laboratory experiments representatives of Caldicellulosiruptor were only detected in traces and the microbial community comprised a broader spectrum of fermentative bacteria. The different composition of the biocenosis in situ and in laboratory experiments is probably caused by the different inhibitor concentrations, temperatures as well as nutrient availability in situ compared to the closed system of the batch experiments.

CHEMOGENIC SEDIMENTS OF MUTNOVSKY GEOTHERMAL COMPLEX (SOUTH KAMCHATKA)

By studying the processes taking place at the operating geothermal stations it is possible to model and understand some features of formation of minerals of geothermal origin, processes of transfer by hydrothermal solutions and concentration of some metals by analogy with the ore-forming processes at the formed deposits. During the release of steam-water mixture and outpouring of water from the wells silica gel is deposited, a number of elements (including noble metals) migrate and accumulate in the sediments. A great role during exploitation of wells, operation of turbines, separators and other structural elements of geothermal stations is played by the chemical composition of the coolant. Because of its high mineralization deposition on various units and parts of equipment occurs, what may complicate the operation of a geothermal plant. We describe the composition of chemogenic deposits on various technological elements of the Mutnovskaya geothermal power plant. Data on sediments sampled in boreholes, on turbine blades and separator are presented. We have revealed a wide variety of such deposits, both oxide compounds and ore phases (from magnetite, chalcopyrite, minerals of faint ore group to native gold, tellurides, selenides and sulfides of gold and silver). We suppose that the processes currently taking place at the Mutnovskoye steam hydrothermal field are similar to the hydrothermal processes that contributed to the formation of vein zones of the gold-silver-polymetallic deposit of the same name.