Increasing carbon emission and its detrimental impact on the global environment along with the limited fossil fuel resources increased the motivation of using renewable energy resources to meet the global energy demand. Although harvesting energy from renewable energy sources, particularly CSP is technically viable it still requires lots of major breakthroughs to meet the financial target, so that it can compete with fossil fuel fired plants. The conventional ultra-supercritical steam Rankine cycle reached the power conversion efficiency in the range of 40 to 45% with the steam turbine operating at 565oC and 265Bar inlet steam. This benchmark in efficiency can be further increased using alternative fluids such as Carbon Dioxide (CO2) at higher temperature with a smaller plant size and hence potential of reducing the Capital Expenditure (CAPEX) and Operating Expenses (OPEX). Early researches had concluded that Supercritical Carbon Dioxide (sCO2) and helium are good candidates for high temperature heat sources such as nuclear, solar thermal, and sCO2 power conversion cycle can achieve same conversion efficiency of helium based cycles for lower cycle temperature and pressure (Dostal, Driscoll and Hejzlar, 2004).Thermal energy storage is considered to be the main option for the next generation energy systems as the storage will even out the energy demand fluctuation and generation fluctuation, particularly from renewable energy generation technologies. Unlike the Photo Voltaic (PV) technology the CSP technology can store the energy in the form of heat for lower cost.
Todays most advanced mature economical CSP technology is the central tower with two tank molten salt storage system. However it is not economically competitive with the conventional power generation technologies and PV technology.The USA Department of Energy (DOE) SunShot program had prioritized the gaps in the Research and Development (R&D) and layout the roadmap for the three CSP technologies based on different Heat Transfer Fluid (HTF) such as molten salt, falling particle and gas phase receiver technologies (Mehos et al., 2017) to reduce the Levelized Cost of Electricity (LCOE) for CSP technology. The DOE SunShot initiative program target is 6 ¢/kWh with the performance target of each components as shown in Figure 1 1 while currently achieved cost is 12 ¢/kWh (NREL, 2015) as shown in Figure 1 2. This research is mainly focused on reducing the cost of thermal storage and power plant. Supercritical Carbon Dioxide (sCO2) Brayton cycle has been considered as the next generation power conversion system due to its compactness and higher performance in all the three pathways. The sCO2 Brayton cycle is also considered to be the best power conversion system for next generation nuclear power (Dostal, Driscoll and Hejzlar, 2004).
The Supercritical Carbon Dioxide (sCO2) has been gaining attention in oxy-fired coal based power plant (Mecheri and Le Moullec, 2016). The sCO2 can be used in many applications as shown in Figure 1 3. The sCO2 Brayton cycle is classified as two type depends on the method of heat transfer at the heat source namely direct heated and indirectly heated cycles. In the direct fired cycle the CO2 is the end product of oxy-fired combustion with other combustion by-products and residues. Allam Cycle developed by Net Power is currently the most promising cycle layout with more than 95% carbon capture is being under test with 50MWt capacity using natural gas as fuel (Laumb et al., 2017). A scale up project of 300MWe is in the planning stage. The indirect heated cycle can be integrated with any non-combustion heat sources such as CSP, nuclear, Geothermal etc.
Although sCO2 Rankine cycle is also possible yet not attractive for power generation application as the critical temperature of CO2 is 30.98oC at 7.38 Bar and hence the cooling fluid temperature needs to be lower than the critical temperature to condense the fluid.
This research is specific to analysing indirect sCO2 Brayton cycle and study the integration approaches with thermal energy storage.Compact turbomachinery is possible due to the higher density characteristics of sCO2 at the turbine inlet and outlet for the typical operating parameters as shown in Table 1 1 and the estimated size of 300MWe turbine is shown in Figure 2. This will leads to reduction in plant CAPEX and OPEX and for a bottoming cycle Persichilli study shows that the LCOE has reduced by 10% to 20 % for a combined cycle power plant (Persichilli et al., 2012). Furthermore sCO2 requires lesser turbine stages than typical steam turbine due to the nature of lower pressure ratio, i.
e. roughly 3.The sCO2 requires lower compressive power than equivalent closed Helium, Nitrogen or Air Brayton Cycle due to the liquid like density and compressibility factor at the compressor inlet and exit as shown in Table 1 2, and hence the thermal efficiency of sCO2 cycle is higher. As shown in Figure 1 5, the sCO2 Brayton cycle is clearly superior in terms of thermal efficiency as compared to helium Brayton cycle, super-heated steam and supercritical steam cycle.There is no pinch point problem whilst using sCO2 cycle as a bottoming cycle compared to steam Rankine cycle and therefore the maximum fluid temperature is not constrained in sCO2 cycle for waste heat recovery application as shown inFigure 1 6 Pinch Point Problem in the Bottoming Cycle Exhaust Heat Exchanger Figure 1 6.With the recent development in diffusion bonding the Heatric can commercially able to produce Printed Circuit Heat Exchangers (PCHE) for pre cooler (water to sCO2 or Air to sCO2) to withstand 400 Bar (Pidaparti et al., 2016), and this allows to achieve higher efficiency at optimal pressure (approximately 250 Bar).Harsco industrial air heat exchangers can be another economical solution and Pidaparti concluded that in the worst case less than 5% increase in plant capital cost over the water cooled plant.
There are many sCO2 Brayton cycles had been proposed and each has its own advantages and limitations. Thermodynamically sCO2 Brayton cycle is highly recuperative cycle due to the lower expansion ratio in the turbine limits the maximum temperature drop across the turbine to roughly 200oC for the pressure ratio of 3.Due to this reason the simple closed loop sCO2 Brayton cycle without recuperator can only attain around 20% efficiency.
Therefore simple sCO2 cycle is considered with a recuperator as shown in Figure 2 1 which significantly increases the performance of the cycle to about 44% at 700oC. The efficiency rise is limited by the pinch point in the recuperator due to the dramatic variation of specific heat at constant pressure (cp) near critical region. To further increase in the efficiency can be achieved by dividing the recuperator and,1) To control the mass flow through the low temperature recuperator and hence the heat capacity weighted mass flow (m*cp) on both the side of low temperature recuperator will be equal. Different cycle layouts are shown in Figure 2 2.2) Change the specific heat by pre-compression i.e.
changing the pressure and the layout is shown in Figure 2 3.The recompression cycle configuration is bypassing some flow from high pressure circuit thereby matching the heat capacitance of heat rejecting and absorbing fluid in the low temperature recuperator. Although this requires additional compressor to bypass the flow and connect back to the high pressure system this cycle is more efficient because the heat extraction from the recuperator is higher than the required compressor work. The split flow cycle has the same configuration as the recompression cycle except that the turbine is spitted into two to reduce the hot component stress with the reheater in-between. The partial cooling cycle combines the recompressor cycle with the precompression cycle. This has an additional advantage of controlling the main compressor inlet pressure regardless of turbine exhaust pressure.
The main compression cycle has the similar configuration as recompression cycle with the isothermal compression i.e. intercooling to reduce the compression power. The precompression cycle on the other hand matches the heat capacitance of the two fluid of the low temperature recuperator by varying the pressure subsequently changing the specific heat. Although recompression cycle seems to be superior for different turbine inlet temperature from Figure 2 2, main compression intercooling cycle efficiency is higher when the compressor inlet temperature increases. Hence a particular cycle will be techno-economically viable for a given profile of source and sink temperature over the life period. Figure 2 3 shows that the partial cooling cycle is efficient for higher turbine inlet temperature which is in contradiction with Turchi results and Turchi highlighted that the reason is Dostal has considered 90% turbine efficiency for recompression cycle and 93% turbine efficiency for other cycles (Turchi et al.
, 2012).Higher efficient cycle is not always wanted if it’s not economically viable and it is particularly true for sCO2 Brayton cycle as the efficiency of the cycle can be increased with bigger recuperator which comes with additional economic burden as shown in Figure 2 4. So the optimal equipment sizing has to be determined with the trade-off in plant efficiency.The cost ratio is exponentially increasing with higher recuperator effectiveness in turn higher plant efficiency.
