Lecture 3

1. Lecture 3 Book chapter 7—Cogeneration Book Chapter 8—Waste-Heat Recovery ENERGY MANAGEMENT

2. Cogeneration Description • Cogeneration is defined as the coincident or simultaneous generation of combined heat and power. In a true cogeneration system a significant portion of the generated or recovered heat must be used in a thermal process (such as steam, or hot water, or hot air, etc.). ENERGY MANAGEMENT

3. General Considerations/definitions • Main prerequisite is that a plant sho0ws significant and concurrent demand for heat and power. • Industrial plant—facility requiring process heat and electric or shaft power • Process Heat (PH)—Thermal energy required • Process Returns (PR)—the fluid returned from the industrial plant • Net Heat to Process (NHP)—PH-PR • Plant Power Demand (PPD)—electrical or load demanded • Heat/Power Ratio (H/P)—heat-to-power ratio of the plant • Topping cycles—thermal cycle where power is produced prior to delivery of heat—figure 1 • Bottoming cycle—power production from the recovery of that would be normally rejected—figure 2 • Combined cycle—combination of the two cycles described above—figure 3 ENERGY MANAGEMENT

4. Figure 1—Topping cycle Engine Exhaust Exhaust Gas Heat Recovery Internal Combustion Engine Generator Jacket Water Fuel To process Heat Exchanger Circulating Pump Tank From Process ENERGY MANAGEMENT

5. Figure 2—Bottoming cycle Exhaust Stack Generator Waste Heat Recovery Boiler Steam Turbine Process Steam To Process From Feed water System Circulating Pump ENERGY MANAGEMENT

6. Figure 3—Combined Cycle Combustor Generator Compressor Turbine Stack Generator Waste Heat Recovery Boiler Steam Turbine Exhaust Steam To Process From Feed water System Circulating Pump ENERGY MANAGEMENT

7. Basic cogeneration systems • Table 7.1 in the book • More have been added • Notice: • Electrical efficiency peaks at about 35% -- this is the best we can do to generate electricity from fossil fuels. Cogeneration then makes use of the (other-wise) waste heat to improve the overall efficiency. For this to work we must have a need for the thermal energy at the same time as the electrical energy. ENERGY MANAGEMENT

8. Steam Turbine Systems • Prime-Mover—If process heat demands are such that the plant power requirements can be satisfied by CHP—the prime mover is selected to meet or exceed the peak demand. • Initial steam conditions—many plants do not have adequate process steam demands to generate all of the required power—therefore we produce what we can. • Process steam pressure—lower the exhaust pressure to the minimum acceptable. • Feed water Heating—use extracted steam to heat the feed water increases the power that can be generated and saves energy costs. ENERGY MANAGEMENT

9. Gas Turbine Systems • Topping cycles can be used from exhaust gases for heat recovery ENERGY MANAGEMENT

10. Reciprocating Engine Systems • Can be Diesel or Otto cycle • Can burn a variety of fuels (but generally less variety than gas turbines) • Rotational speed usually less than 1800rpm • Can be four-cycle or two-cycle ENERGY MANAGEMENT

11. Other systems • Microturbines • From about 30 kW to 200 kW—high speed generators like 25% to 30% efficiency • Fuel Cells • Getting out of the developmental stage • Efficiencies can be from 35% to 60% • Photovoltaics • Solar energy ENERGY MANAGEMENT

12. Cogeneration Systems • With all systems—it is advisable to still be connected to the electrical utility provider—can be done in the following ways: • Isolated customer—receives all power from the cogeneration system but can connect to the electric utility system if there is a problem (requires switch gear). • Thermally base-loaded cogeneration—cogeneration generally provides all electrical needs and additional can be sold to electric utility (requires synchronization equipment). • Electrically base-loaded cogeneration—cogeneration provides part of the electrical needs and additional electrical power requirements are purchased from the electric utility (requires synchronization equipment). ENERGY MANAGEMENT

13. Waste Heat Recovery • Not all waste heat is recoverable because of the second law of thermo dynamics, however it has been estimated that nearly 50% of the energy consumed by all sectors of the US economy was discharged to the environment in the form of waste heat. Much of this can be saved and used. • Potential for waste heat recovery depends on quantity of waste heat (enthalpy flow rate of waste heat) which is probably not as important as the quality (temperature). It is important to match the loads to the source. • It is usually necessary for the waste heat to be available at the same time as the heating load. ENERGY MANAGEMENT

14. Classifying Waste-Heat Quality • Three subranges: ENERGY MANAGEMENT

15. Waste Heat Exchangers • Open Waste-Heat Exchangers—two fluid streams are mixed to form a third exit stream whose energy level (temperature) is intermediate between the two entering streams. • Serial Use of Process Air or Water—use of exhaust air/water to heat other air/water systems. • Runaround Systems—to isolate heating and heated materials, use an intermediate transfer medium. ENERGY MANAGEMENT

16. Flow continuity ENERGY MANAGEMENT

17. Waste-Heat Exchangers • Transient storage devices—Regenerators. • Steady State Heat Exchangers • Parallel flow • Counter flow • Cross flow • Mixed flow ENERGY MANAGEMENT

18. Commercial Options in Waste-Heat-Recovery Equipment • Gas-to-Gas Heat Exchangers—Recuperators • Heat Wheels • Passive Air Pre-heaters • Gas or Liquid-to-Liquid Regenerators—The Boiler Economizer • Shell-and-Tube or Concentric-Tube Heat Exchangers ENERGY MANAGEMENT

19. HW—3due 09/18/2012 • For a backpressure turbine (expansion turbine)—determine the power that can be produced from the following data (see example on p 160-162): • Inlet: 600,000 #/hr. of steam @1300psi and 900F • Outlet: 650psi, 700F • Speed 1800 rpm, 65% efficient • Work example 5—page 167 with 50% load • Work a heat balance on page 198 with natural gas and product at 25 tons/hr. Calculate the heat loss with water flowing at 500 gpm. All same temperatures. Natural gas flowing at 300,000 cfm. ENERGY MANAGEMENT