«Quadrennial Technology Review 2015 Chapter 6: Innovating Clean Energy Technologies in Advanced Manufacturing Technology Assessments Additive ...»
Opportunity for High Power-to-Heat CHP While existing thermally driven CHP systems sized to supply 100% of a facility thermal demand (with a low ratio, typically below 0.75) are currently cost-effective in many markets and applications, there still remains a significant unserved market with smaller thermal demand relative to electrical ( up to 1.5) in the industrial, commercial/institutional, and residential sectors. An enormous energy and cost savings opportunity could be realized by increasing while maintaining the high efficiencies that thermally sized CHP systems enjoy (the potential is examined in later sections of this document). Increasing without loss of efficiency would entail the development of ultra-high-efficiency electrical generation technologies (these are discussed in the following section).
10 Quadrennial Technology Review 2015TA 6.D: Combined Heat and Power
In order to better understand the opportunity for high CHP, a preliminary analysis evaluated the opportunities to deploy highly efficient CHP to applications that fall outside of the traditional thermally driven systems.31 The analysis examined the technical potential and energy savings that could be captured if CHP systems were deployed in applications with a power-to-heat ratio of up to 1.5 (current power-to-heat ratios in existing CHP systems are closer to 0.75). The following system characteristics were assumed for existing CHP
For 50–1,000 kW systems: 30.5% electrical efficiency (ηe) and 79.6% overall efficiency (η)
Where ηh = the thermal efficiency of the heat portion of the system. Thus, for the smaller system case, = 0.62, and for the larger system, = 0.81.34 Table 6.D.1 lists the sectors included in the analysis.35 Table 6.D.
1 Sectors and subsectors/facility types included in high power-to-heat CHP opportunity analysis
This analysis indicates that expanding the market applications for CHP systems to those driven more by electrical rather than thermal output could save an additional 1.3 quads of energy compared with existing CHP technologies alone, as shown in Table 6.D.2.
11 Quadrennial Technology Review 2015 TA 6.D: Combined Heat and Power Table 6.D.
2 Technical potential and energy and cost savings for high power-to-heat CHP operation
* Incremental CHP capacity on the basis of a power-to-heat ratio of 1.5.
** Incremental primary energy savings on a basis of 33% average grid efficiency.
High-Efficiency Distributed Electrical Generation The ultimate extension of high CHP discussed in the previous section occurs when all of the fuel energy is used to generate electricity (i.e., 1). Such systems might consist of a topping cycle, in some cases a prime mover, combined with one or two additional (bottoming) cycles that also generate electrical power.
The combined cycles may offer flexibility in a CHP context. When waste heat is needed, the bottoming cycle or cycles can be bypassed and the heat from the topping cycle used. Similarly, the second or third cycle can be brought online only as electrical demand requires. The “Waste Heat Recovery” technology assessment has additional detail on bottoming cycles.
This section explores the practical thermodynamic efficiency limits of natural-gas-fueled combined cycles for electrical power generation in the 1–10 MWe range. The 1–10 MWe range is well suited to many of the industries and commercial sector applications identified in Table 6.D.1. On the basis of a scoping survey, a practical limit of 65%–70% fuel-to-electricity efficiency (higher heating value [HHV] basis) can be achieved by utilizing the fuel exergy (available energy) through combined cycles producing AC power. It is important to note that combined-cycle efficiency is path dependent, depending on the arrangement and configuration of individual components, and that combining cycles usually compromises the optimal operation of the individual cycles, with diminishing returns. However, a systematic approach to optimizing the combined cycles, particularly focusing on reducing irreversibilities, such as in combustion processes, could conceivably result in somewhat higher efficiencies than the projections in this study. However, this will require significant R&D to overcome the many barriers.
The thermodynamic analysis consisted of two components: basic thermodynamic modeling and a literature review.
An exhaustive modeling exercise was not attempted, but rather an approach with some parametric variation was used to gauge sensitivities to primary parameters as well as to ascertain that the preliminary modeling matched other studies.
A combined cycle involves the generation of electricity with a topping cycle (the upstream generator) and a bottoming cycle (the downstream generator), which uses residual fuel and/or heat from the topping cycle.
The combined-cycle engine converts fuel exergy to electrical power through a combination of chemical engines (such as fuel cells, reciprocating internal combustion engines, and gas turbines) and heat engines (such as waste-heat Rankine or Stirling-cycle engines). Some systems recover exhaust heat to increase internal efficiencies of the primary cycles; for instance, to heat incoming flow streams with a recuperator. It is increasingly common to find references in the literature to add a third waste-heat recovery (WHR) cycle to 12 Quadrennial Technology Review 2015 TA 6.D: Combined Heat and Power produce additional electrical power. Additional cycles not only increase capital and operating cost but are also an exercise in balancing returns, so such systems must be carefully considered and designed.
3 and the accompanying chart, Figure 6.D.7, summarize the range of expected combined-cycle fuel-to-electricity efficiencies (FTEEs) for various technologies and combinations of cycles. Most of the reported efficiencies come from the literature and some solely from modeling analyses. The first column specifies the number of power-generating cycles in the system, and the next three columns specify the different configurations (as applicable). The overall thermal efficiency is based on fuel energy input to electrical power generation output, with no other significant energy inputs (such as solar or heat sinks); the overall system scale is on the order of 1 MWe but can be expected to be descriptive of systems in the 1–10 MWe range. In all cases, the fuel is natural gas, usually approximated as methane. Electrical power is generated from mechanical power and/or converted from DC to AC where necessary, so FTEE values include inverter and generator losses as appropriate, typically assuming 95% efficiency for inverters and 94% for electrical generators.36 Fuel energy was accounted on a lower heating value (LHV) basis, which is acceptable (and standard practice) when comparing systems using a single fuel and descriptive of most fuel-conversion systems with vapor-phase water products exhausted to the surroundings or into a second, non-condensing engine. The HHV basis was obtained by scaling efficiencies by the ratio of LHV/HHV, which is approximately 0.9 for methane and most domestic natural-gas mixtures. Because different fuels have different chemical energies, the HHV is used for calculating and displaying FTEE values in order to facilitate comparison to other fuel-combustion systems. As with most literature studies, the energy inputs to pressurize the fuel to operating pressures along with other small parasitic loads and other losses are neglected in this analysis. The “Notes” column refers to the Appendix at the end of this assessment, with more complete explanations of the assumptions made. In addition, the Appendix contains the descriptions and equations used for the modeling of the different cycles.
3 Estimated practically achievable fuel-to-electricity efficiencies for selected technologies in combined cycles.
Key: GT = gas turbine; RICE = reciprocating internal combustion engine; SOFC = solid oxide fuel cell; MCFC = molten carbonate fuel cell;
RC = Rankine cycle using either water or refrigerants (for organic RC); Stirling = Stirling cycle engine. Sources: D=data; L=literature;
M=modeling; HHV-based efficiencies estimated from LHV-based values. The HHV basis is used to facilitate comparison between fuels. For “Notes,” see the Appendix.
13 Quadrennial Technology Review 2015 TA 6.D: Combined Heat and Power Figure 6.D.
7 Fuel-to-electricity efficiency on a HHV basis of various technologies in combined cycles as summarized in Table 1.
Key: GT = gas turbine; RICE = reciprocating internal combustion engine; FC = fuel cell (molten carbonate or solid oxide); SOFC = solid oxide fuel cell. The Appendix provides background on how these efficiencies were calculated.
CHP systems can achieve very high system efficiencies (80%, at times). These high efficiencies are typically found only in low systems. Maintaining high system efficiencies while increasing requires the development of highly efficient prime movers (as described above) along with improved thermal recovery. Specific research areas that were identified are listed in Table 6.D.4.
14 Quadrennial Technology Review 2015 TA 6.D: Combined Heat and Power Table 6.D.
4 Technical areas for improvement of high CHP and ultra-high-efficiency generation
Low-Temperature Heat Recovery and Waste Heat-to-Power (WHP) Waste heat from generation technologies (engines and turbines) and from industrial processes can be used in several ways. The waste heat can be used to directly produce hot water or steam or can be used to produce electricity. When waste heat from an industrial or other source with sufficiently high temperatures is used to drive an electricity generator, it is called “bottoming cycle” CHP. A recent Oak Ridge National Laboratory (ORNL) study of WHP opportunities found that 14 GW of technical potential and 7 GW of economic potential for WHP exist.37 Increased capability and efficiency of heat recovery in CHP systems from exhaust gas will increase CHP electricity generation efficiency. A major challenge for low temperature heat recovery from exhaust gases is the condensation and corrosion caused by cooling exhaust gases below their dew point temperature. Condensation heat recovery requires significantly higher capital and operating costs, which are typically not worth the energy-saving benefits. While condensing economizers are commercially available, capital costs can be as much as three times that of conventional boilers. Alternate technologies, such as transport membrane condensers, are being developed and may have lower costs.38 There are a number of advanced technologies in the R&D stage that could provide additional options for direct power generation from waste heat sources. These technologies include thermoelectric generators, piezoelectric generators, thermionic devices, thermo-photovoltaic generators, Stirling engines, and innovative concepts for steam engines (see “Direct Thermal Energy Conversion Materials, Devices, and Systems” technology assessment for further information). These systems range in terms of commercial readiness in the United States, although some—such as the Kalina Cycle—have achieved relative success in other countries. A few have undergone prototype testing in applications such as heat recovery in automotive vehicles and from coproduced liquid in oil and gas wells.
Recovery at low temperatures (typically lower than 400°F) becomes increasingly challenging with chemically laden gas streams. These waste heat sources will have greater limitations that prevent cooling flue gases to low temperatures. To enable expansion of low temperature heat recovery (with the goal of improving CHP
efficiencies), additional research will involve the following:
Improving methods for cleaning exhaust streams Developing low cost advanced heat exchangers that can withstand corrosive environments 15 Quadrennial Technology Review 2015 TA 6.D: Combined Heat and Power