ARPA-E recently announced 14 new projects funded through the Advanced Research In Dry cooling (ARID) program, which aims to develop low-cost, highly efficient and scalable dry-cooling technologies for thermoelectric power plants in order to reduce water consumption in power generation. At present, a majority of U.S. power plants use water to operate power generation equipment efficiently. However, a combination of growing environmental concern and increased water demand due to population growth is likely to significantly constrain the available water supply that can be allocated to power plant cooling. 

Wet-cooling systems at power plants account for 41% of all fresh water withdrawals, making it the largest single use of fresh water in the U.S. Most of the electricity in the U.S. is produced using steam turbines in thermoelectric power generating stations, the majority of which use coal, natural gas, or nuclear power to generate heat. In order to dissipate rejected waste heat, many plants rely on cooling towers and spray ponds to dissipate a substantial amount of water into the atmosphere via evaporation. Electric power generation from steam turbines is often delivered more efficiently using wet-cooling instead of dry-cooling systems because temperatures that can be achieved using cooling water directly or through evaporation tend to be cooler and more stable than atmospheric temperatures, and evaporative cooling enables a further reduction in temperature, enabling greater conversion efficiency. 

Example of an Air Cooled Condenser (ACC) (Image Credit:SPX)

Each day, approximately 139 billion gallons of water are used for thermoelectric power generation in the U.S. To put this in perspective, this is equivalent to filling 10,000 Olympic-sized swimming pools every hour. This water is then unavailable for other important uses; for example, this amount of water could be used to produce 17.4 million tons of potatoes, approximately the annual U.S. potato yield.

As demand for fresh water approaches or exceeds supply, many regions and states, including California, Texas, and Florida, are becoming water stressed. A recent DOE report stated, “When severe drought affected more than a third of the United States in 2012, limited water availability constrained the operation of some power plants and other energy production activities.” As a result, these regions have employed water conservation measures and have incorporated alternative water sources (reclaimed, treated, desalinated) into their supply.

Although dry-cooling technologies exist, current systems cannot compete with the performance of wet-cooling systems and tend to cost operators more. Air is a more challenging medium for transferring heat than water. There are several reasons for this, but the most fundamental is that the thermal conductivity (the measure of how effectively heat can be transferred across a medium) of water is approximately 30 times that of air.  Therefore, air-cooled systems require significantly more heat transfer surface area than wet-cooled systems to transfer an equivalent amount of heat, resulting in larger, more expensive systems. 

Additionally, dry-cooled systems are limited to the ambient temperature (dry bulb temperature), whereas wet-cooled systems can take advantage of latent heat transfer (evaporation) and can thus cool to a lower minimum temperature (wet bulb temperature). Since the amount of power generated is directly related to the cooling temperature, this means that more power can be generated from wet-cooled systems, leading to higher revenue for the power plant.

Due to these factors, air cooling increases the levelized cost of electricity (LCOE; the per-kilowatt hour cost of building and operating an electric power generating plant over its lifetime) by as much as 5-9% compared to wet cooling. Additionally, current dry cooling technologies are not well suited for hot climates, such as Las Vegas, where the high ambient temperatures limit efficient power production. 

Figure 1: Schematic of representative indirect dry-cooling system that satisfies ARID program objectives.

Thus, ARID project teams seek to develop innovative, (1) high-performance air-cooled heat exchangers and (2) supplemental cooling systems and/or cool-storage systems that enable cooling to below the dry-bulb temperature, to ideally facilitate the indirect cooling system illustrated by Figure 1. If successful, these projects will significantly reduce water usage at thermoelectric plants without sacrificing a plant’s performance or significantly increasing its cooling costs. 

Project teams will work to design kilowatt-scale testing prototypes and ensure the technologies can scale up to megawatt-cooling capacity without significant performance loss. Potential early adoption of smaller scale ARID technology includes HVACs, data centers, buildings, and aerospace applications. Larger scale technology could potentially be used for reduced power plant operating costs, more flexibility in plant geographical locations due to the fact that plants would no longer have to be located near a large water source, and smaller sized power plants. 

ARID project teams have been tasked with developing dry-cooling technologies that fulfill the following three requirements: 

1. Dissipate no net water to the atmosphere 
2. Result in no loss of efficiency for the power plant 
3. Result in less than a 5% increase in the levelized cost of electricity.

If successful, ARID technologies could enable continued reliable and efficient domestic electric power production, independent of water supply, population growth, and climatic variations and with minimal impact on the aquatic environment. Market penetration of these technologies will significantly reduce the risk of lost thermoelectric power production. 

ARPA-E is always searching for new ways to innovate and advance the development and deployment of innovative energy technologies. Through the ARID program, ARPA-E is changing what’s possible in dry-cooling technologies and working to make more secure and sustainable practices for thermoelectric power plants.