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Low impact energy from water and the ground

Hydroelectric, marine, and geothermal energy are sources that, as part of the energy mix, contribute to reducing polluting emissions.

by Giuseppe Sammarco
22 May 2020
5 min read
by Giuseppe Sammarco
22 May 2020
5 min read

Mitigation actions that can be taken to modify the energy mix by replacing sources with a high impact on global warming with sources that have less or no impact include technologies that make the most of the potential offered by the hydrosphere (the world of the water cycle) and those that derive energy from heat in the depths of the ground (geothermal energy). 

Let’s start with hydroelectricity

This technology has been known for many decades and is widely used. It exploits gravity - with water precipitating or flowing from above - to cause the rotary motion of a turbine coupled to an electric generator. Currently, large hydroelectric plants are the most important renewable sources. The International Energy Agency estimates that in 2018 they generated 16% of the total electricity produced worldwide and a full 62% of the total produced from renewable sources alone. In industrialized countries, this resource has already been widely exploited and it is harder to find new opportunities for major applications. The technical potential exists, but the costs of new projects and the problems of sustainability and impact on the land and surrounding landscape would be very high. Constructing large reservoirs in valleys bordered by huge artificial dams, flooding them, relocating the population and, finally, regulating the flow of water based on electricity generation requirements would conflict with existing economic activities and human settlements.  

In many developing countries, however, hydroelectricity presents significant growth opportunities as the least problematic areas, where investment costs and the social and environmental impact are lower, have not yet been used. In addition to large plants, there are also small plants generating less than 10 MW, known as small, mini and micro hydroelectric plants (in decreasing order of power). As a rule, these plants have little impact on the surrounding land and exploit small differences in height to generate enough electricity to satisfy demand ranging from just a few to several thousand households. 
Many other technological options exist or are being studied that can generate energy from large masses of moving water (sea currents, tides and wave motion) or differences in temperature (between surface and deep layers of the sea) and salinity (such as in river estuaries). Currently their spread is hindered by the difficulty of reducing costs or technological immaturity. As with wind and solar power, we cannot review them all so I will only describe one, developed by Eni in Italy - in collaboration with the Polytechnic of Turin and its spin-off Wave for Energy - which has a high development potential.
It uses ISWEC (Inertial Sea Wave Energy Converter) technology that generates electricity by exploiting wave motion, one of the most significant but as yet unused renewable energy sources. The system consists of a sealed floating hull containing a pair of gyroscopic systems connected to two generators inside. The waves cause the unit to pitch while anchored to the ocean floor and free to move and oscillate. The pitch is intercepted by the two gyroscopic systems connected to generators that transform it into electricity. A simple solution, with cutting-edge technology at its heart. The first 50 kW pilot plant is already operating in Ravenna, but Eni is working on developing an industrial-scale model thanks to an agreement with Cassa Depositi e Prestiti, Fincantieri and Terna, which have systematized their skills in their respective areas of competence. Furthermore, the Polytechnic of Turin and Eni have strengthened their collaboration to broaden their research into forms of energy from the sea and set up - among other things - the “MarEnergy Lab” research laboratory, which will aim to increase knowledge about these technologies and accelerate their transition to the industrial phase.

The advantages of geothermal energy

The second renewable source covered by this episode is generated by plants that use heat drawn from the depths of the earth, a legacy of its formation process. Surface water naturally percolates into the ground and descends until it meets high temperature layers of the subsoil that heat it and can transform it into steam. Under favorable conditions, hot water or steam can return to the surface in the form of hot springs, blowholes, fumaroles and geysers.  Intensive use, however, requires production wells to be created that take the steam from the subsoil to feed the turbines that generate electricity. After use, the liquid is sent to the re-injection wells in the subsoil to complete the cycle of exploitation of the resource. The advantage of geothermal energy is that, unlike other renewable sources such as sun and wind, it allows continuous and programmable electricity generation. The main constraint of geothermal energy is the limited availability of areas with characteristics that make electricity generation economical by using current technologies. The usable areas could be expanded but, in order to make it profitable, the related plant costs need to come down. Furthermore, geothermal energy has an environmental impact as other polluting gases or carbon dioxide can escape from the subsoil together with the steam. Finally, there is low-enthalpy geothermal energy (known as geothermal heat pump) technology, used for heating and cooling of buildings that uses heat exchange with the surface subsoil by means of a heat pump. This application has been known for some time and is widespread in some countries, but is hindered by the high initial cost of design and installation. In the next article we will continue our review and talk about biomass and biofuels.


The author: Giuseppe Sammarco

Energy Sector Integrated Technical Studies Eni, Development, Operations & Technology.