9.2.2: The Binary Cycle Method

If the temperature of the hot water retrieved from the geothermal reservoir is lower than 180^◦C, the temperature and the pressure of the flash steam would be so low that a turbine would not be able to convert its energy to mechanical work with any reasonable efficiency. From the graph in Fig.9.7 it follows that after “flashing” water with temperature lower than about 180^◦C one would obtain steam with a pressure of, say, 5 bara – which is perhaps a good parameter for driving small model turbines, but definitely too small for industrial-scale turbines³.

There is a “conceptually simple” solution to the pressure problem

namely, at the head of the production well one should install a device trans- ferring the thermal energy from water to another liquid with a much lower boiling temperature. For instance, ammonia NH₃ is a relatively inexpensive fluid (used in large amounts in fertilizer production) with boiling tempera- ture of -34^◦C at atmospheric pressure. At 132^◦C, an equilibrium between liquid and gas state occurs at the pressure of 113 bara. So, by using a 132^◦C water for heating ammonia liquid one can obtain “vapor” (not steam – the term “steam” is reserved for water vapor) of pressure high enough for run- ning a turbine with high efficiency. The process of heating ammonia – or, more accurately, of transferring the thermal energy from water to ammonia – takes place in device commonly referred to as a “heat exchanger”.

Figure \(\PageIndex{1}\): A simple “counterflow” heat exchanger consisting of two concentric tubes. Ammonia flows in one direction in the inner tube, and water flows in the opposite direction in the space between the outer surface of the inner tube and the inner surface of the outer tube (source: aop).

Ammonia is highly toxic, and small installations are prone to leaks. For- tunately, Mother Nature has equipped us with leak detector which are par- ticularly sensitive to ammonia leaks – our noses. The odor of ammonia is incredibly strong and people start detecting ammonia in the air before the concentration attains the toxicity level. There are other safer “working flu- ids” for binary plants, though. In the middle of the 20^th Century the ideal candidates seemed to be freons, due to their low toxicity and flammability – however, since then they have been banned because freon leaking from ther- mal engines and refrigerators destroys the atmospheric ozone. But there is much ongoing research see, e.g., this Web document.

The US is the world leader in geothermal electricity generation, with over 3600 MWe installed – almost twice as much as in the Philippines, next on the list. The geothermal reservoirs exploited are all large, capable of delivering enough heat for generating tens of MWatts of electric power using the dry steam or flash steam technology. It is expected that many more such reservoirs will be identified in the western part of the US. Therefore, most of the existing interest in geothermal technologies in the US is focused on the two aforementioned technologies. Similarly, in Europe the resources exploited in the Mediterranean geothermal region have similar characteristics as those in the Western US. However, more to the north in Europe the geothermal resources are of different kind, in most cases they are scattered geothermal aquifers at the depth of 2-4 km and with temperatures close or only slightly higher than 100^◦C. So, in many places there are are good conditions for building binary cycle installations. They can generate enough electric power for a community of several hundred households and – what is even more important – for heating those houses in wintertime.

In the case of huge, multi-MW dry steam or flash steam installation heating usually does not get much attention because to reach customers, heat should be sent over long distances – which is not always feasible. In contrast, the “waste heat” from a binary cycle plant serving a small community located nearby may become a valuable commodity. A good example illustrating how a small community can profit from geothermal energy is Altheim, a small town (or a large village?) in Austrian alpine region. The binary cycle power plant, installed over there at the end of 1990-s, has been operating until now. It uses geothermal water of temperature 106^◦C for generating about 500 kWe of electric power, and for heating 800 homes in wintertime. The Altheim installation can be considered as a “model prototype”, it has been described in several professional publications. For instance, one such report was published in 2002 in the Quarterly Bulletin of Geo-Heat Center (a subdivision of the Oregon TECH). The paper is worth reading, because it’s a highly professional report written in a very “pedagogical” style. Another Altheim paper, from 2015, is a report summarizing 15-year period of the installation’s activity.