Sandia National Laboratories (SNL) is researching a thermal-to-electric power conversion technology in a configuration called the recompression closed Brayton cycle (RCBC) that uses supercritical carbon dioxide (s-CO2) as the working fluid, rather than steam, thereby dramatically increasing conversion efficiency compared to the steam Rankine cycle. Sandia is working towards a 10 MWe 500 degree celsius system for 2019.
The primary reason for improved power conversion efficiency is simply that the use of s-CO2 as the working fluid in a Brayton cycle requires less work to convert a given thermal input to electricity. In general, increased efficiency represents increased output for the same thermal input, regardless of the thermal source (natural gas, nuclear, solar or coal). Where fuel costs are a significant portion of overall costs (coal and natural gas fired plants), the benefit is reduced fuel costs. Where capital investments are high (nuclear and concentrating solar power), the benefit is increased output for the initial investment.
s-CO2 power cycles are potentially applicable to a wide variety of power-generation applications. Nuclear power, concentrated solar thermal, fossil fuel boilers, geothermal, and shipboard propulsion systems have all been identified as favorable applications for s-CO2 cycles and would replace traditional steam Rankine cycles.
s-CO2 power conversion technology offers a number of benefits over competing cycles including:
* Smaller size relative to steam system (reduced capital cost)
* Increased efficiency (resulting in increased electricity production for same thermal input)
* Environmental improvement from greenhouse gas reduction
* Vastly reduces water consumption
* Dry cooling/suitable for arid environments
In July 2015 a continuously recirculating falling-particle receiver was installed in the top of the tower at the National Solar Thermal Test Facility (NSTTF). The Sandia-developed falling-particle receiver drops sand-like ceramic particles through the NSTTF’s concentrated sunlight beam, capturing and storing the heated particles in an insulated tank. The technology can capture and store heat at high temperatures without breaking down, unlike conventional molten-salt systems. Higher temperatures mean more available energy and cheaper storage costs because less material is needed to transfer heat.
As a first step in facilitating Fossil Energy applications, an operational RCBC can:
* supplement existing Rankine cycle plant power generation by capturing waste heat energy without disrupting the original power block;
* replace the steam generating Rankine cycle in coal plants, waste incinerators, and natural gas plants with a 50% improvement in efficiency; and
* be included in a combined-cycle gas turbine power plant, the combined efficiency can exceed 60%, with reduced total capital cost.