The low enthalpy geothermal resource
Significant savings in the fossil fuel and electric energy consumption and a strong reduction of the atmospheric pollution can be obtained by using low enthalpy geothermal resources, with temperature near or even below 20 °C for indoor heating and conditioning. Recent studies indicate that the aquifer contained in the basal gravels of Tiber river represents an important geothermal resource for this kind of direct heat utilization in the city of Rome (Barberi et al., 2008), as well as potentially many other productive aquifers in the area (Capelli et al., 2005, 2008a). Aquifers with interesting temperatures for low enthalpy geothermal energy can be found at shallow depths, in the order ot 10’s of meters from the ground. These values indicate that the geothermal water can be easily extracted with low-cost wells, provided the re-injection of the water in the same aquifer, and the execution of thorough tests on the aquifer production, and on the interaction with the chemical-physical balance of the aquifer.
The most studied case is that of the basal gravels of Tiber river from Barberi et al., 2008, summarised below. The thickness of the basal gravels is usually around 15 m, but is rises to up to 30 m in the river northern sector. There is an excellent continuity of the aquifer along the river course, that ensures a good longitudinal water recharge, that is increased by lateral contribution from zones where gravels are in contact with permeable rocks. The aquifer is confined downward by the Monte Vaticano pliocenic shales and upward by impervious Tiber silt deposits. The latter prevent vertical percolation of rain and river waters and protect the basal aquifer from both pollution and seasonal thermal effects. Physico-chemical parameters measured in 17 wells indicated a water temperature (T) ranging from 16 to 21.6 °C, but usually around 18-19 °C, values that are ideal for direct heat utilization. The pH is mostly neutral and slightly acid in some zones, whereas electrical conductivity is locally rather high with values slightly exceeding 5000 µS/cm. It has been estimated that 20 l/s of a geothermal water with a T of 17-18 °C are needed for winter heating and summer cooling of a 40,000 m3 space. The Tiber basal aquifer can easily provide such a resource in several zones of Rome city considering that the water, after its utilization, will have to be reinjected into the provenance aquifer.