Evaluation of the environmental sustainability of a micro CHP system fueled by low-temperature geothermal and solar energy

• Binary, ORC technology avoids CO2, but raises questions about environmental impact.
• We proposed a micro-size system that combines geothermal energy with solar energy.
• The small scale and the solar energy input edges the energy profitability.
• The system’s performance is appreciable if applied to existing wells.
• The feasibility of exploiting abandoned wells is preliminarily evaluated.

In this paper we evaluate the environmental sustainability of a small combined heat and power (CHP) plant operating through an Organic Rankine Cycle (ORC). The heat sources of the system are from geothermal energy at low temperature (90–95 °C) and solar energy. The designed system uses a solar field composed only of evacuated, non-concentrating solar collectors, and work is produced by a single turbine of 50 kW. The project addresses an area of Tuscany, but it could be reproduced in areas where geothermal energy is extensively developed. Therefore, the aim is to exploit existing wells that are either unfit for high-enthalpy technology, abandoned or never fully developed. Furthermore, this project aims to aid in downsizing the geothermal technology in order to reduce the environmental impact and better tailor the production system to the local demand of combined electric and thermal energy. The environmental impact assessment was performed through a Life Cycle Analysis and an Exergy Life Cycle Analysis. According to our findings the reservoir is suitable for a long-term exploitation of the designed system, however, the sustainability and the energy return of this latter is edged by the surface of the heat exchanger and the limited running hours due to the solar plant. Therefore, in order to be comparable to other renewable resources or geothermal systems, the system needs to develop existing wells, previously abandoned.

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