Bavarian Geothermy for renewable energies

The Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit, BMU (the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety), has issued a target to increase the proportion of renewable energies to at least 35 % by 2020 and to at least 80 % by 2050.

For the time being, renewable energy is to a large extent obtained by tidal energy, wind energy and photovoltaic energy.

A relatively new type of power and heat generation is geothermal energy which has great potential, especially in Bavaria (Bayerisches Molassebecken, Bavarian Molasse Basin). Geothermal water is extracted from deep regions and used as an energy source.

The earth's core has a temperature of about 5000°C. The temperature decreases as the distance to the earth's surface decreases. In other words, the deeper you bore, the hotter it gets. On average, the temperature increases by 3°C for every 100 m; about 99 % of the entire planet is hotter than 1000°C.

At least 50 % of the earth's heat emerge from radioactive disintegration processes. These are natural, unlike the processes which led to the German phase-out of nuclear power after the catastrophe in Fukushima in 2011. The thermal energy is stored in water reservoirs (water-bearing formations), in which the water has been heating up over thousands of years as a heat carrier and heat store.

North-south section through the Alpine foothills

As shown in the sectional view above, beneath Munich the water-bearing formation is diverted into deeper regions from the north to the south through the foot of the Alps. This means that the temperature of the water reaches up to 150°C south of Munich. The hot water can be extracted at a flow rate of up to 120 L s-1 (litres per second) but must be lifted out of depths of over 4000 m.

There are currently ambitious electricity-generating projects being launched in several locations south of Munich (Sauerlach, Kirchstockach, Dürrnhaar, Unterhaching, ...).

Geothermal power plants achieve an average annual output of about 5 MW, which might not seem like a lot compared with a wind farm or solar park, but these plants can produce electricity when the sun doesn't shine and the wind doesn't blow.

The extraction and utilisation of the geothermal energy stored in the deep groundwater are shown in the following block diagram of a geothermal system.

During operation of the system the hot deep groundwater is pumped through the extraction bore into the surface power plant by means of a pump lowered to a depth of 1200 m (using drive M) and circulates through filtration systems (reversible flow-rotating filter, cartridge filter, bag filter), in order subsequently to output the thermal energy in a heat exchanger network to the working fluid of the electricity-generating secondary circuit and the district heating network. With a temperature difference of about 90°C to 100°C the cooled deep groundwater is re-injected into the deep-lying reservoir via an injection bore (on the right in the image).

Owing to the low temperature level, two different cycle processes are used to generate electricity.

The Organic Rankine Cycle (ORC), in which the working fluid consists of an organic fluid, which is selected so that its physical properties are perfectly suited to the respective enthalpy conditions.

The Kalina Cycle, in which a two-component mixture consisting of water and ammonia is used and, in contrast to the ORC, is non-isothermally evaporated.

In general, the degree of efficiency in the Kalina Cycle (KC) is higher than that when using an ORC, but the technical plant management and implementation of maintenance work in the case of the KC is more difficult owing to any potentially occurring ammonia vapours.

A pressure retaining system controls the pressures in the extraction bore, the surface power plant and the injection bore. It is used mainly to produce an artificial overpressure in the power plant pipework in order to physically prevent degassing of thermally charged water and the formation of scaling associated therewith.

Since this artificial overpressure (typically 1 to 3 MPa) has to be overcome by the deep-lying water pump, this plant component is a critical point for the economic operation of a geothermal power plant. In the case of a geothermal water flow rate of 100 l/s, the surface counterpressure produces an annual additional consumption of energy of 1000 MWh/bar.

In Germany the geothermal generation of electricity is, technologically, still in its infancy and previous projects suffered a few setbacks. Since extensive efforts continue to be made in research and development, the geothermal generation of electricity will make a significant contribution to the BMU's targets in the foreseeable future. It will and does still bear potential for pioneering inventions that wait to be patented. Too many technically sophisticated problems are hidden in the depth of the hot water that behaves very unexpectedly when used at the surface to deliver its energy and to be sent back in the depth to recover said energy for a new cycle.

by Moritz Herbrich
Last updated: 2 April 2014