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«Quadrennial Technology Review 2015 Chapter 6: Innovating Clean Energy Technologies in Advanced Manufacturing Technology Assessments Additive ...»

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From an energy balance on the combustor, where the composition of the outlet state 4 is combustion products, including excess air components with unreacted O2, and LHV signifies the LHV of the fuel, denoting gaseous state of the water combustion product (ambient air humidity is not examined because effects are insignificant at this level of analysis). The pressure at the combustor outlet is where ΔPB is a multiplicative pressure loss across the burner (typically 0.95–0.98). With P4 and h4 defined, the 1450 K (with the higher end usually applicable to higher-capacity turbines). Given a chosen ṁF, the excess-air turbine inlet temperature T4 is defined. This is typically limited by materials and in most GTs ranges in 1250– factor λ is adjusted to meet the materials limits at T4; raising λ reduces overall system efficiency because more power is required to compress the incoming air stream, the power of which is supplied by the GT.

In our analysis, a two-shaft GT is examined, where the first-stage “GT” has a shaft connected to the compressor and only extracts enough power to compress the incoming air stream (from state 1 to 2), and the second-stage power turbine (PT) is connected via a shaft to the generator, which produces electrical power, with a limit being the isentropic turbine efficiency. There are operational and cost trade-offs between one- and two-shaft systems (both of which are generically termed as GTs), but at this level of analysis, there is not much difference except By conservation of mass, the GT exit mass flow rate is ṁ5 = ṁ4, and by definition the required GT power is that each turbine is assigned a separate isentropic efficiency with slightly different overall performance.

ẆGT = ẆC (note the usual sign convention of work output from the component being positive and work input being negative). The gas-specific enthalpy at the GT exit is Given the definition of isentropic efficiency for a turbine,

The GT exit-specific enthalpy for isentropic expansion is as follows:

which defines T5s. The outlet pressure from the GT is then as follows:

28 Quadrennial Technology Review 2015 TA 6.D: Combined Heat and Power

For the PT, the exit pressure should be close to, but above, ambient as follows:

where ΔPPT is a multiplicative factor (1) constraining the outlet pressure. By conservation of mass, ṁ6 = ṁ5.

With solution of k5, the temperature at the PT exit for isentropic expansion is which fixes the specific enthalpy h6s. On the basis of the definition of isentropic turbine efficiency, the PT exit specific enthalpy is The mechanical power output of the PT is and the electrical power output is

The overall system efficiency is calculated as follows:

The thermal efficiency of the system can be increased with internal heat recovery. For instance, a standard way is to heat the incoming air before the combustor with the exhaust gas downstream of the PT by using a recuperator, whose impact can be quite pronounced for smaller turbines. In a general sense, heat-exchanger performance can be defined with the effectiveness, and this can fix the properties of cold-side and hot-side gas streams; heat exchangers also cause a pressure drop as the fluids pass through them.

The above analysis does not account for other types of internal losses, nor does it account for cost, manufacturability, size, geometric design, material properties, or other relevant design features. This is the type of analysis that was used in this study for analysis of GT and RCs, with the recognition that it represents an optimistic projection.

Specific considerations for the analysis of single and combined cycles are as follows:

RCs (bottoming): For this analysis, RC calculations were performed by using a spreadsheet tool  developed by Oak Ridge National Laboratory (ORNL) coupled with REFPROP for fluid property calculations. The tool is restricted to evaluating thermodynamic performance, with no consideration given to cost, size, design, or practicality of a real-world system. To evaluate maximum WHR potential, as much heat as possible is extracted from the waste stream, with no consideration of the size, cost, or practicality of the physical system required to do so.

The Rankine system is modeled as a closed system, consisting of a pump, single-stage evaporator (separate preheater, boiler, and superheater stages are not modeled), generic expander (turbine, scroll expander, or other), condenser, and optional recuperator. Each component is simply modeled by using isentropic efficiency relations for the pump and expander and effectiveness calculations for the heat exchangers. With this approach, losses considered in the model are limited to isentropic efficiency of 29 Quadrennial Technology Review 2015 TA 6.D: Combined Heat and Power the pump and expander and the effectiveness and pressure drop of each heat exchanger. No information about component design, size, material, etc., is required for or produced by this model.

–  –  –

input (composition, flow rate, and temperature of the waste stream), temperature and pressure at the condenser exit, maximum expander inlet temperature (to protect the expander or, for organic cycles, the fluid), expander pressure ratio, and the efficiency, effectiveness, and/or pressure drop for each component. All calculations are automatic and results include required refrigerant flow rate and system power and efficiency. A screenshot of the user input tab of the spreadsheet tool is in Figure 6.D.11.





Figure 6.D.

11 User input tab of the spreadsheet tool.

RICE: The baseline efficiencies for RICE are based on a demonstrated brake thermal efficiency of 50%  for ARES-class, 1 MW engines at the lower bound and a reasonable stretch goal of 55% at the upper bound. A generator efficiency of 94% was assumed, converting these shaft efficiencies to the fuel-toelectricity efficiency bounds in this report.

RICE + RC: In this combination, the most efficient work extraction device is the RICE. Therefore,  priority is given to extracting as much work as possible from the RICE primary cycle with the Rankine bottoming cycle recovering as much additional work as possible. Therefore, the optimized RICE 30 Quadrennial Technology Review 2015 TA 6.D: Combined Heat and Power single-cycle configuration is used as the baseline for the primary cycle. Experimental data (protected under a non-disclosure agreement (NDA)) from an ARES-class, 1 MW engine, including exhaust flow rate and temperature, was used to seed the RC model to determine the additional potential benefit of the secondary cycle. As the secondary cycle, the RC was designed to provide maximum work output, not maximum efficiency. Priority was placed on recovery and extraction of as much additional work as possible, with minimal consideration of cost, size, or practicality of the RC. A spreadsheet tool developed by ORNL was used to evaluate the RC performance. Upper and lower bounds for key component efficiencies determined from experience and engineering judgment were included in the analysis. A generator efficiency of 94% was assumed to convert shaft efficiencies of the RICE and RC turbine to fuel-to-electricity efficiency.

RICE + Stirling: In this combination, the most efficient work extraction device is the RICE. Therefore,  priority is given to extracting as much work as possible from the RICE primary cycle with the Stirling bottoming cycle recovering as much additional work as possible. Therefore, the optimized RICE singlecycle configuration is used as the baseline for the primary cycle. Experimental data (protected under an NDA) from an ARES-class, 1 MW engine, including exhaust flow rate and temperature, was used to seed the Stirling cycle model to determine the additional potential benefit of the secondary cycle. An empirically based Stirling-cycle efficiency was used to estimate additional work output based on the quality of the RICE exhaust in the range of 50%–55% brake thermal efficiency.

SOFC: For a zero-dimensional treatment of the SOFC, an approach similar to Haseli et al. was  employed.56 The resulting relation was insensitive to operating pressure, which in SOFCs tends to increase efficiency, and the fuel reforming details were ignored. The fuel-utilization factor in the SOFC varied from 65%–85%, with sufficient fuel in the exhaust to combust in the GT system, with some provision for makeup fuel addition. The air rate was set at λ=2 nominally.

SOFC + RICE: This value came from GE promotional material found online59 for a proposed  commercial system under development with a projected electrical cogeneration efficiency of 60%–65%.

SOFC + RICE + RC: Using expected qualities of exhaust from the RICE bottoming cycle, the above described RC model was used to estimate additional power output from the exhaust stream.

Endnotes U.S. Department of Energy (DOE), “CHP Technical Assistance Partnerships.” Available at: http://energy.gov/eere/amo/chp-technical-assistancepartnerships-chp-taps.

U.S. Department of Energy (DOE) and U.S. Environmental Protection Agency (EPA). “Combined Heat and Power: A Clean Energy Solution.” DOE/EE-0779. August 2012. Available at: http://www.energy.gov/sites/prod/files/2013/11/f4/chp_clean_energy_solution.pdf.

“Combined Heat and Power: Pathway to Lower Energy Costs, Reduced Emissions, Secure and Resilient Energy Supply,” Fact Sheet, Environmental and Energy Study Institute. May 2013. Available at: http://www.eesi.org/files/FactSheet_CHP_052113.pdf.

The White House. “Executive Order—Accelerating Investment in Industrial Energy Efficiency.” August 30, 2012. Available at: https://www.

whitehouse.gov/the-press-office/2012/08/30/executive-order-accelerating-investment-industrial-energy-efficiency.

It should be noted that CHP systems that use fossil fuels will often be too widely distributed and too small in scale for cost-effective use of carbon capture and storage (CCS) systems to capture their GHG emissions, thus locking in a reduced but still significant source of carbon emissions as compared to large central power plants with CCS or nuclear or renewable plants.

A separate boiler would require $0.022 per kWh worth of fuel to provide the same amount of thermal energy as provided by the CHP system in this example. This is the “thermal credit.” Crediting the cost of the thermal energy allows you to compare the net cost of electricity production from CHP to the cost of purchased electricity.

EIA Electric Power Monthly. Available at Table 5.3 in: http://www.eia.gov/electricity/monthly/pdf/epm.pdf.

31 Quadrennial Technology Review 2015 TA 6.D: Combined Heat and Power Sources: Equipment performance on the basis of National Renewable Energy Laboratory, "Gas-Fired Distributed Energy Resource Technology Characterizations." NREL/TP-620-34783. November 2003.

Other general assumptions: 15-year project life, 8% cost of capital, 80% efficient displaced boiler.

Fuel price assumptions as follows:

–  –  –

U.S. Energy Information Administration (EIA). “Monthly Energy Review.” Table 7.2b and Table 2.6. June 2015. Available at: http://www.eia.gov/ totalenergy/data/monthly/archive/00351506.pdf. EIA “U.S. Electricity Flow.” 2014. Available at: http://www.eia.gov/totalenergy/data/monthly/ pdf/flow/electricity.pdf.

U.S. Department of Energy (DOE) and U.S. Environmental Protection Agency (EPA). “Combined Heat and Power: A Clean Energy Solution.” DOE/EE-0779. August 2012. Available at: http://www.epa.gov/chp/documents/clean_energy_solution.pdf.

Ibid.

McKinsey & Co. Cited in: Shipley, A.; Hampson, A.; Hedman, B.; Garland, P.; Bautista, P. “Combined Heat and Power: Effective Energy Solutions for a Sustainable Future.” Oak Ridge National Laboratory/DOE EERE. 2008. Available at: http://www1.eere.energy.gov/ manufacturing/distributedenergy/pdfs/chp_report_12-08.pdf.

Shipley, A.; Hampson, A.; Hedman, B. Garland, P; Bautista, P. “Combined Heat and Power: Effective Energy Solutions for a Sustainable Future.” Oak Ridge National Laboratory/DOE EERE. 2008. Available at: http://www1.eere.energy.gov/manufacturing/distributedenergy/pdfs/chp_ report_12-08.pdf.

Hampson, A.; Bourgeois, T.; Dillingham, G.; Panzarella, I. “Combined Heat and Power: Enabling Resilient Energy Infrastructure for Critical Facilities.” Oak Ridge National Laboratory, 2013.

Ibid.



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