Applications of Hydrothermal Resources

Hydrothermal resources are those in which the water is heated by contact with the hot rock as explained above. These resources are located at shallow to moderate depths of 100 m to 5 km.

Hydrothermal resources are further subdivided into :

1. Vapour dominated (dry steam fields)
2. Liquid dominated (wet steam fields)
3. Hot water resource

The systems which utilize the above resources of energy are discussed below.

Vapour dominated fields deliver steam with little or no water. The pressure and temperatures of the steam from such resources reaching to the surface are limited to 8 bar pressure and temperature of 200°C.

The sites available in the world are very few. Some of the important vapour dominated geothermal fields are in California (USA), Lardrello (Italy) and Matsukawa (Japan). The geysers in USA and Lardrello in Italy are the largest dry steam geothermal fields. The system using dry steam for generation of electrical power is shown in Figure A.

Dry steam extracted from the geothermal field may contain water and solid particles. These are removed in a centrifugal separator. Pure dry steam is then directly supplied to a steam turbine. The exhaust steam from the turbine is condensed in the condenser with the help of cooling water circulated. The resultant hot water is returned to the cooling tower. The condensate is either sent to chemical recovery or it is re-injected into hot field with the help of condensate pump.

The problems associated with these plants are the presence of corrosive gases and abrasive materials which cause the environment problems.

The hot water deposits without much steam content are called liquid dominated geothermal fields. The temperature range in such fields is in the range of 100°C to 315°C. When the wells are drilled over such deposits, the possibilities are :

1. Hot water and steam rises naturally through the well. Such a system is called Geo-pressure system.
2. Hot water is pumped up with the help of a pump. The drop in pressure causes it to partially flash into steam and arrive at the top surface of well as a very low quality steam. 
3. Geothermal brine rises through the drilled well. It has high mineral content.

(a) High temperature wet steam system :

When the resource temperature is above 180°C, it is called high temperature system. It is suitable for power generation.

Figure B shows a wet steam system used in Cerro Prierol (Maxico) and Octake (Japan) where the hot water with steam at high pressure and high temperature steam issued by deep wells drilled into the ground.

The hot water from underground water from a depth of about 1 km at point 1 at about 40 bar reaches the well head at point 2 at lower pressure. Process (1-2) is essentially a throttling process which results into two phase mixture having the steam of low quality. 

The mixture is further throttled in the flash chamber cum brine separator. It results into dry steam at 3 and the separated brine is collected from bottom.

The dry steam is supplied to steam turbine of a conventional team power plant as shown in Figure B. The hot brine is re-injected into the ground along with the steam condensate from the condenser. 

The hydrothermal resources are available in the temperature range of 90°C to 170°C. This temperature is not sufficient for production of two phase mixture of steam . In order to utilize this geothermal heat, an organic compound of low boiling temperature like isobutane is used under pressure in a primary heat exchanger. The geothermal fluid is re-injected after extracting heat in to the ground. The system is shown in Figure C.

The isobutane vapour so generated is passed through a turbine where it expands. The mechanical power of turbine is converted into electrical power by a generator.

The exhaust of the turbine is passed through a regenerator (heat exchanger) where it is cooled and then condensed in the condenser. The returned condensate is heated in the regenerator by exhaust vapour of the turbine.

1. Scaling and corrosion problem is avoided since geothermal fluid does not come into contact with the plant.
2. No pollution since geothermal fluid is re – injected into ground and it is circulated in a heat exchanger.
3. Efficiency is high.