Abstract
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.
Highlights
Introduction distributed under the terms andToday, the ambitious energy and environmental European Union (EU) target is to limit global temperature increase below 2 ◦ C
The current study proposes the exergoeconomic analysis of a geothermal heating direct system for a single building by assessing the effect of two variables: the number of plates/channels for polymeric plate heat exchanger (PPHEX) and the inlet temperature of the geothermal fluid
The best configuration in terms of pressure drops is verified when the geothermal fluid inlet temperature is equal to 90 ◦ C, and the number of channels for each fluid flow is equal to 4
Summary
The ambitious energy and environmental European Union (EU) target is to limit global temperature increase below 2 ◦ C. All the previous issues are overcome by defining the geometrical and thermodynamic model of a polymeric plate heat exchanger (PPHEX) for direct use of low-medium temperature geothermal source, to meet the space heating demand of a single building. Such an application is suitable for sites that show a geothermal interest with geothermal fluids availability in the range of about 90–120 ◦ C.
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