Cooling towers are heat rejection machines used by many geothermal and biomass power plants. These plants make electricity by using heat to produce steam that spins a turbine, but not all heat can be turned into electrical energy. A cooling tower removes the leftover thermal energy so the steam can condense back into water and the cycle can continue.
This makes the power plant more efficient, reliable, and able to operate continuously.
Key Facts
- Cooling towers reject waste heat from the condenser to the atmosphere.
- Heat removed by cooling water can be estimated with Q = mcΔT.
- Evaporative cooling occurs because high-energy water molecules escape into the air.
- A hyperboloid tower shape strengthens natural draft by guiding warm moist air upward.
- In a power plant, thermal efficiency is η = Wout / Qin.
- A condenser works best when cooling water enters at a lower temperature.
Vocabulary
- Cooling tower
- A cooling tower is a device that transfers waste heat from warm water to the surrounding air.
- Condenser
- A condenser is a heat exchanger that turns exhaust steam from a turbine back into liquid water.
- Evaporative cooling
- Evaporative cooling is the temperature drop that happens when some liquid water changes into vapor and carries energy away.
- Natural draft
- Natural draft is airflow caused by warm, less dense air rising without the use of fans.
- Heat transfer
- Heat transfer is the movement of thermal energy from a hotter substance to a cooler substance.
Common Mistakes to Avoid
- Thinking the visible plume is smoke: it is mostly condensed water droplets formed when warm moist air cools in the atmosphere.
- Assuming cooling towers create energy: they remove waste heat, while the turbine and generator produce the electrical energy.
- Ignoring water loss by evaporation: some circulating water leaves as vapor, so makeup water is needed to keep the system operating.
- Confusing cooling towers with chimneys: a cooling tower moves heat and moisture, while a chimney mainly vents combustion gases.
Practice Questions
- 1 A cooling tower removes heat from 500 kg of water as its temperature drops from 42°C to 30°C. Using c = 4180 J/(kg·°C), how much heat is rejected?
- 2 A biomass plant sends 80 kg/s of cooling water through a condenser. If the water warms by 9°C, what is the heat transfer rate in watts? Use c = 4180 J/(kg·°C).
- 3 Explain why a geothermal power plant still needs a cooling tower even though its energy source is renewable.