Power plants with steam turbines need to reject megawatts of heat to condense steam at low temperatures, so that the condensate can be pumped to a higher pressure, and then be heated to produce high pressure steam. Currently, roughly 99% of these power plants are water cooled. Roughly 40% of the water-cooled power plants use cooling towers, where heat is rejected by evaporating water. This is a very effective solution, since water has a high thermal conductivity, and a high latent heat.
However, fresh water withdrawal for all power plants represents about 41% of all fresh water used in the U.S, roughly 139 billion gallons per day. In the future, water demands due to both drought and population growth are anticipated to reduce the amount of fresh water available for power plant water cooling. ARPA-E is looking for technology to improve Power Plant Dry Cooling, where no water is required.
These air-cooled condensers have a large performance penalty when the ambient temperature is high. For an ARPA-e funded program, ACT is exploring the use of PCM to provide supplemental cooling during the hottest part of the day. PCM can improve the efficiency of power plants by storing waste heat during the day and releasing it at night when the ambient temperature is much cooler; see Figure 1
As shown in Figure 2, part of the hot steam from the power plant is directed into the supplemental cool storage system. Thermosyphons are used to simultaneously transfer heat from the hot steam/condensate into a phase change material (PCM) reservoir (and ambient air) during the day. The PCM melts, which condenses part of the steam.
The PCM is then recharged at night as shown in Figure 3. The steam from the power plant is diverted around the Cool Storage Units, while an innovative, passive valve separates the primary evaporator from the PCM storage and fins at the top of the system. The upper portion of the entire thermosyphon is then a smaller thermosyphon during the night, transferring heat from the PCM as it freezes, and rejecting the heat to the air.
A small scale PCM test system for the Cool Storage concept is shown in Figure 4. The PCM vapor chamber is located below a series of air cooled fins. A short video showing freezing and thawing of the PCM in this sink is shown below. A typical test is shown in Figure 5. Initially, the PCM is frozen. Heat is supplied to the evaporator surface, causing all of the temperatures to rise. The three curves then flatten out as the PCM melts, since most of the heat is going into the PCM. After most of the PCM is melted, the temperatures start to rise more quickly. The heater was turned off after 8 hours had elapsed, and the PCM started to recharge. Some supercooling occurred, so the PCM started to freeze at a colder temperature than it melted (roughly 9 hours after the start of the experiment).
Further information can be found in the technical paper, Thermal Resistance Network Model for Heat Pipe-PCM Based Cool Storage System.