Active Cooling Thermally Induced Vapor-Polymerization Effect (ACTIVE)
Effective Dry-Cooling for Powerplants, Even During Hot Weather
The ACTIVE process employs a simple plant arrangement with low maintenance requirements
More than 86% of the electricity in the United States is produced by thermoelectric powerplants. As shown in Figure 1, these plants typically use coal, natural gas, or nuclear power to generate steam in a boiler. The superheated steam then drives a turbine coupled to a generator to produce electricity. To enable continuous operation, the steam has to be condensed before it returns to the boiler. Therefore, it is routed through a condenser, where cooling water extracts the heat from the steam in a second cycle.
There are different methods used to extract heat from condenser water; primarily wet cooling and dry cooling. In wet cooling, the condenser water is routed through a wet cooling tower, where a portion of the water is evaporated to cool the remainder. Alternatively, in dry cooling, the water is routed through an air-to-water heat exchanger, and a large fan is used to transfer heat to the air by convection. Of the two methods, powerplant operators traditionally prefer the wet cooling approach, as it is more cost effective and has additional cooling capacity through water evaporation. However, wet cooling accounts for 41% of all fresh water withdrawal in the U.S.! This is water that could be repurposed for agriculture, industry, or general consumption.
An example of water use in wet cooling powerplants is the Palo Verde Nuclear Generating Station near Tonopah, Arizona, which generates on average 3.3 gigawatts of electricity per year. Due to its desert location, Palo Verde uses 100 percent reclaimed water from the surrounding municipalities. Unlike other nuclear plants, Palo Verde maintains “Zero Discharge,” meaning no water is discharged to rivers, streams, or oceans. However, the powerplant receives over 26 billion gallons of treated water every year from the surrounding municipalities. Of this, 19.5 billion gallons are lost each year, a 75% loss to the environment through waste from blowdown to evaporation ponds. This loss represents about 25% of the annual overdraft of the Arizona Department of Water. If an effective alternative to the plant’s wet cooling system were implemented, the nearly 20 billion gallons of evaporated water could instead be used for Arizona’s agriculture or industrial expansion.
With such a draw on fresh water resources, there is a thrust to improve the efficiency of dry cooling to achieve performance competitive with wet cooling. One reason dry cooling systems are less efficient than their wet-cooling counterpart is due to the performance limitation imposed by ambient temperature. The turbine exhaust pressure, called back pressure, is a function of ambient temperature. So, in hot weather, conventional dry cooling cannot provide the design point back pressure, resulting in a reduction in powerplant electricity output. Therefore, to achieve performance similar to wet cooling systems, dry cooling technology needs a more effective means of rejecting heat to the environment in hot weather.
The U.S. Department of Energy’s (DoE) Advanced Research in Dry Cooling (ARID) program has awarded ARA a grant to develop a new cost-effective and efficient dry cooling technology. ARA’s Active Cooling Thermally Induced Vapor-Polymerization Effect (ACTIVE) — a highly flexible cooling system — offers powerplant operators, for the first time, a cost-effective alternative to dissipate or reject waste heat with no net water consumption and no impact to plant efficiency or levelized cost of electricity. This technology has the potential to contribute significantly to U.S. water security.
ACTIVE is a novel chemical heatpump cycle that provides supplemental or dedicated cooling with no water loss. ACTIVE cools below the ambient dry bulb temperature, precluding the loss of turbine output in hot weather. It operates in stand-alone or synchronized configurations. ACTIVE is a scalable technology to serve multiple on-site applications including closed cooling loops, gas turbine inlet air cooling, lube oil cooling, and other cooling applications.
How ACTIVE Works
ACTIVE replaces the cooling tower with even higher performance components. The current design extracts heat from the condenser cooling water and returns it to the condenser at the right temperature to maintain turbine back pressure at the design point regardless of ambient temperature.
To provide the required cooling, ACTIVE employs a novel depolymerization-polymerization reversible process using readily available polymers. As shown in Figure 2, liquid polymer is pumped from a polymer feed tank (PFT) into the Depolymerization Cooling Unit (DCU), filled with catalyst, where it converts into a lighter monomer and evaporates. The endothermic depolymerization reaction and evaporation processes extract heat from the condenser cooling water by forced convection and have a maximum net reaction heat of 189.5 kJ/mol (1,434 kJ/kg) at 100% conversion. In practice, depolymerization conversion depends on several factors including reactor design, flow rate, and operating pressure. Lab testing under the ARPA-E effort showed 96% conversion with system operating pressures between 4-15 psia. This corresponds to a practical thermodynamic cooling capacity of 1,376 kJ/kg, approximately 5 times higher than R-410a (~257 kJ/kg).
The compressor/blower feeds monomer-rich vapor product from the DCU into the Polymerization Heating Unit (PHU), filled with the same catalyst, where it polymerizes and condenses, releasing heat to the environment by forced convection. Polymerization conversion is limited by reaction equilibrium to 70–80% with monomer/polymer mixtures of 60–80wt% polymer. A Polymer Separation Unit (PSU) is used to improve both monomer concentration from the DCU and polymer concentration from the PHU. Typically, the PSU improves the PHU product concentration to over 95wt% polymer. Vapor product from the PSU is recycled into the PHU and the polymer-rich liquid product is fed back to the DCU. The PSU ensures the cyclic mass balance is maintained, ensuring that ACTIVE operates continuously.
Capital Cost & Payback
A Techno-Economic analysis projects ACTIVE system capital cost as a function of system capacity in megawatts of thermal energy (MWth), Figure 3. For example, a gas turbine combined cycle powerplant with 550MW electrical power output will need to extract about 340 MWth of thermal energy from the condenser. An ACTIVE system for this size plant would cost less than $80 per kWth, achieving a payback period of one year.
ACTIVE Demonstration Unit
The ACTIVE demonstration unit, Figure 4, was designed and built with a cooling capacity of 20 kWth and Coefficient Of Perfomance (COP) of 32. The unit is reconfigurable to up to 120 kWth with COP down to as low as 15. The ACTIVE Unit flow diagram is given in Figure 5.
Initial Demonstration Results
Test results are very encouraging. The unit achieved 23kW of cooling at 27 COP with a system that is not optimized, Figure 6. The demonstration unit was designed to provide 20kW of cooling at 32 COP.
ARA believes optimizing the unit will yield higher cooling capacity at higher COP.
ACTIVE Technology Derivatives
Through discussions with powerplant operators and during our test and evaluation efforts, we discovered other useful applications for the ACTIVE technology. One such application is gas turbine air inlet cooling, described below.
Gas Turbine Air Inlet Cooling
During hot days, the density of combustion air entering a gas turbine is much lower than the design condition, resulting in reduced power output. Therefore, using an efficient approach to cool combustion air will result in increased net power output.
An example case study was performed on Higgins Generating Station, NV, which has a gas turbine system using a Westinghouse-Siemens SGT-5000F (WF501FD) and a nameplate capacity of 230 MWe. The case study used weather conditions for this location from 2015 to evaluate the economic and performance impact of integrating an appropriately sized ACTIVE system to provide gas turbine inlet cooling.
The study shows there is a significant advantage in using ACTIVE for gast turbine inlet air cooling. First, the study shows that the maximum cooling required by the ACTIVE system is approximately 9 MWth . As illustrated in Figure 7, a 9MWth ACTIVE has the potential to increase net power output by 29.9 MW (14%) at 37.78°C ambient temperature (Tamb) and 21% relative humidity. By contrast, evaporative cooling would only result in a power incrase of 13.4 MW (6.3%). Key cost and performance results of integrating the ACTIVE system are:
- $2.16M Estimated System Capital Cost for 9MWth ACTIVE (using Figure 3)
- 29.9MW (14%) increased net power output
- $4.296M/year total cost savings (using 9.98¢/kWh and 4 months operation/year)
- 12 million gallons/year total water savings ($33,600/year at 0.74$/m3)
- Return on Investment (ROI) less than one year
ARA believes gas turbine inlet cooling is a great entry point to market for the ACTIVE technology, given the scale and economic value. We are interested in further discussions with personnel involved in this area.