Energy and economic analysis of geothermal–solar trigeneration systems: A case study for a hotel building in Ischia

Abstract

This paper presents the design, simulation and optimisation of a small trigeneration plant supplied by geothermal and solar energies. Different technologies are implemented in a dynamic simulation model purposely developed for research scope: a 6kWe micro Organic Rankine Cycle (ORC); a 30kWf single stage H2O/LiBr absorption chiller; a geothermal well; a solar field obtained by new prototypal flat-plate evacuated solar collectors. The ORC is supplied by heat obtained by a geothermal well in which geothermal brine is about at 95°C. In order to improve system performance, additional heat is provided by solar energy obtained through a 25m2 solar field. Diathermic oil (up to 130°C) is adopted as working fluid in order to supply heat to the ORC evaporator. A suitable oil storage tank is modelled in order to mitigate the temperature fluctuations due to the variability of solar energy availability. The output power of the ORC depends on the availability of solar energy. The absorption chiller is switched-on in summer time and it is fed by geothermal energy only. This simulation model is implemented in TRNSYS environment. The ORC is modelled by zero-dimensional energy and mass balances implemented in Engineering Equation Solver (EES).A case study is developed in order to test the energy and economic performance of this innovative micro-trigeneration plant. In particular, the above mentioned model is applied to the Regina Isabella hotel in Ischia (Naples, South Italy), famous for its geothermal sources. Currently, such hotel is already equipped by a geothermal system (by several hot water wells) for thermal cares, domestic hot water production and space heating. By TRNSYS the optimisation of the system design parameters and the calculation of the thermo-economic conditions were performed. The system showed excellent energy performance indexes. In fact, the average yearly efficiency of the solar thermal collectors is close to 60%, whereas the average yearly ORC electric efficiency is about 6%, which is a good achievement considering the system driving temperature. Results also show that the system performance is more dependent on the availability of the geothermal energy than the solar one. From the economic point of view, good results are also obtained. In fact, in the worst operating conditions the Simple Pay Back Period is 7.6years, decreasing to 2.5years in the most convenient considered scenario (public funding and full utilisation of the produced thermal energy).