Produzione e distribuzione di elettricità e calore
Geothermal Technologies for Buildings Heating and Cooling (low enthalpy)
Autors: Stefano Lo Russo, Martina Gizzi, Anna Carmela Violante
Production and Distribution of Electricity and Heat
Geothermal energy appears as either heat associated with emissions of steam and hot water from the ground (with a wide temperature range), or heat available at various depths below the Earth's surface. These energy stems from both the residual heat of the planet formation and the decay of radioactive elements such as Thorium and Uranium that are present in the Earth’s mantle. As a matter of fact, the ground temperature progressively increases with depth, following an average geothermal gradient equal to 30°C per km. The heat flow propagates from Earth’s depth to surface through conductive and convective processes. The heat flux value varies over space and time. Geothermal energy resources (geothermal reservoirs) with high-medium enthalpy are traditionally used for electricity generation while those with low-medium enthalpy are used for direct heat production and uses such as building heating and cooling, air conditioning, district heating and multiple agriculture, foods and industrial uses. The main applications of geothermal technologies can be divided as follows based on the type of geothermal resource: a) electricity generation and direct use of heat from hydrothermal systems or hot fluids from underground aquifers located at varying depths (i.e. Deep Geothermal Energy Resources); b) direct use of heat foe space heating and cooling (air-conditioning) through the use of geothermal heat pumps (GSHP, Ground-Source Heat Pump) which exploit the thermal gradient of the subsoil within 200-m depth (i.e., Shallow Geothermal Energy Resources). This document focuses on the second application which can be further divided based on the type of the heat exchange with the ground, i.e. open-cycle (Open-loop) or closed-cycle (Closed-loop) configuration. In the open-cycle systems, hot groundwater is first pumped to the surface and used to carry out heat exchange with the heat pumps, and finally re-injected into the subsoil through proper wells. In the closed-cycle systems (BHE, Borehole Heat Exchanger), the circulating fluid flows through pipes (geothermal probes) positioned in the ground at varying depths, with varying lay-out depending on the geothermal site. The technology advances of geothermal heat pumps has had a significant impact on the direct exploitation of geothermal energy in recent years. The total capacity reached worldwide at the end of 2019 was 107,727 MWt while the total annual energy production was 1,020,887 TJ (283,580 GWh) (Lund and Toth, 2021). Although Italy is among the top 10 countries in the world with as far as geothermal energy resource is concerned, and one of the first in the EU for the use of direct heat from geothermal resources, the market of geothermal heat pumps has been substantially stationary over the past 10 years, with average annual sales of less than 1,000 units.