In geothermal power plants waste water is

Question

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Option C: Discharged back to earth

Explanation & Concepts:
In geothermal power plants, wastewater (brine and condensed steam) is typically reinjected back into the geothermal reservoir through dedicated injection wells. This process is crucial for maintaining reservoir pressure and preventing the subsidence of the surface ground.

Working Principle: The plant extracts hot geothermal fluid, which is separated into steam to drive turbines and brine. Instead of releasing this chemically laden water into surface bodies, which would cause thermal and chemical pollution, it is piped back into the underground reservoir. This closed-loop approach also sustains the long-term heat output of the field by replenishing the fluid source.

Key Formulas: $P_{out} = \dot{m}(h_{in} - h_{out})$ — Power extraction based on enthalpy drop in geothermal fluid; $Q_{res} = \sum \dot{m}_{inj} 	imes c_p \Delta T$ — Heat energy balanced by reinjected fluid

Key Points: Reinjection helps prevent ground subsidence and chemical pollution of surface water.; Geothermal fluids often contain dissolved gases and minerals (e.g., Boron, Arsenic) that make surface discharge environmentally hazardous.; Reinjection maintains the sustainability of the geothermal reservoir by sustaining pressure.; The process forms a quasi-closed cycle for the geothermal fluid.

Advantages: Reduces environmental pollution risk; Sustains reservoir pressure for long-term viability

Disadvantages: High initial cost for drilling injection wells; Risk of induced seismicity if pressure is not managed correctly

Applications: Flash steam power plants; Binary cycle power plants

Additional Info: Option A is incorrect because geothermal plants are not cooling-water based like conventional thermal plants for their primary discharge.. Option B is hazardous due to the salinity and heavy metals present in the brine.. Reinjection wells are designed to reach deeper or targeted permeable formations to ensure fluid flow back into the heat source.

Answer: C is correct — Geothermal wastewater is discharged back into the earth via injection wells to sustain reservoir pressure and prevent surface contamination.

Exam Tip: Always remember that in renewable energy exams, 'Closed-Loop' systems are preferred for sustainability; reinjection is the geothermal equivalent of the closed-loop cycle.

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Option A: Re circulated after cooling in cooling towers

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Option B: Discharged into sea

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Option D: Evaporated in ponds

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