Using Thermal Storage for Power Plant Cooling
In some power and cooling systems, the maximum performance is needed during hot summer afternoons, but the air temperature is usually highest at this time, limiting the amount of heat that can be rejected to ambient. Thermal storage can be used to increase the cooling capacity during the hot days while using the colder air at night to recharge the thermal storage. In some dry areas of the U.S., such as Arizona, the nighttime temperature can be more than 25°C colder than the daytime temperature.
Air conditioners are one device that can benefit from day/night thermal storage since they are used most heavily on hot summer afternoons. Power plants are another system that can benefit from thermal storage. The highest electrical demand occurs on hot summer afternoons, primarily from all the people running air conditioners. This is particularly important when the power plant uses Air Cooled Condensers (ACC), which cool the power plant with no net water consumption. Cool thermal storage takes advantage of the reduced ambient temperature at night time to provide additional cooling capacity.
Power Plant Dry Cooling
Power plants, using a steam Rankine cycle, reject hundreds of megawatts of heat to generate electricity via generators connected to a series of high to low-pressure turbines. This is accomplished through the use of a condenser system that utilizes wet (water-cooled) or dry (air-cooled) cooling. Currently, about 99% of power plants are water-cooled. Through the use of a wet cooling tower, evaporative cooling enables high heat transfer coefficients at the wet-bulb air temperature. For dry cooling systems, thermal performance is limited by the dry-bulb air temperature, which is substantially higher than the wet-bulb air temperature.
Freshwater withdrawal for all power plants represents about 41% of all freshwater 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 cooling. ARPA-E (Advanced Research Projects Agency-Energy) investigated technologies to improve Power Plant Cooling, where less or no water is required to achieve comparable cooling performance.
The driving force for enhanced air-cooled condenser research is that the capital cost is economically prohibitive and there is a large performance penalty when the ambient temperature is high. For an ARPA-e funded program, ACT explored the use of salt hydrate phase change materials (PCM) to provide supplemental cooling during the hottest part of the day. A thermal energy storage reservoir can improve the thermal performance of power plants by storing low-grade heat during the day and rejecting it at night when the ambient temperature is cooler. This allows the power plant user to reject heat at the wet-bulb temperature without the use of cooling water see Figure 1.
In order for power plants to output maximum load during hot ambient temperatures, additional cooling is required to maintain a sufficient vacuum pressure downstream of the turbine. Thermal energy storage can be used to increase the cooling capacity of the power plant during these hot days while using the cold air at night to recharge the phase change material. This load shifting to cooling at nighttime temperatures is done in order to improve the cooling capacity of Air-Cooled Condenser (ACC) systems. To help reduce freshwater demand by power plants, thermal energy storage is a novel approach to supplement and enhance air cooling.
As shown in Figure 2, indirect dry cooling can be used in conjunction with a supplemental thermal energy storage system.
A small-scale system for the thermal energy storage concept is shown in Figure 3. The PCM 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. 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 latent heat of the PCM. After the PCM is melted, the temperatures start to rise more quickly due to sensible heating. The heater was turned off after 8 hours had elapsed, and the PCM began to recharge. Some supercooling occurred, so the PCM froze at a colder temperature than it melted (roughly 9 hours after the start of the experiment). One of the main research focuses of this project was solving the subcooling issue, which was completed for the large-scale prototype.
To scale up the technology, a 10kWh PCM heat exchanger was developed to test the thermal performance and long-term reliability of the salt hydrate phase change material. This design is seen in Figure 5. A loop thermosyphon connects the evaporator and air-cooled radiator to the PCM heat exchanger. A series of valves allows each section of the loop thermosyphon to operate independently during the melting and freezing operations. To meet the project goals, the PCM heat exchanger was designed for the melting process to finish in 10 hours. The long-term reliability of the PCM heat exchanger was tested by doing repeated melt/freeze cycles. The results of this effort are seen in Figure 6. In section 1, there was a 10% degradation in the latent heat capacity of the PCM, which was expected due to the preparation technique. In section 2, the PCM was remixed by melting all present hydration levels, which led to stability of the PCM solution. Further testing is required to qualify the system for industrial applications.