Ashrae Journal - November 2008 - (Page 42)

Using Stirling Engines For Residential CHP By Kurt Roth, Ph.D., Associate Member ASHRAE; Jason Targoff; and James Brodrick, Ph.D., Member ASHRAE ost households obtain electric power from the grid, space heating from a furnace or boiler, and hot water from a gas-fired or electric-resistance water heater. In contrast, residential combined heat and power systems (RCHP) use a prime mover to generate electric power and harness waste thermal energy produced in the power-generation process to provide heat to satisfy space heating, water heating, and, potentially, space cooling loads (e.g., via absorption cooling). In practice, not all the waste heat is recoverable due to thermal energy radiated to the ambient air, losses from the exhaust-gas stream, low-quality heat (e.g., temperature too low to satisfy household thermal loads), and heat generation that exceeds household thermal loads at that point in time. Stirling engines are a type of prime mover under development for deployment in RCHP applications. They use an external combustion process to provide heat to a sealed pressure vessel at temperatures of around 500°C – 600°C (932°F – 1,112°F) at RCHP scale.1,2 Inside the pressure vessel, a displacer piston moves a working fluid (typically helium or hydrogen) between the hot and cold sides of the engine (Figure 1). High engine efficiency depends heavily on the operation of the regenerator, a thermal capacitor that stores and returns heat to the shuttling gas as it moves between the hot end and the cold end of the engine, thereby decreasing the amount of fuel needed to raise the gas temperature on the hot side, and decreasing the cooling requirements on engine cold side. As the gas is heated and cooled, the gas pressure inside the vessel oscillates about its mean value. Consequently, the gas has the potential to do work on a “power piston,” driving it back and forth against an opposing load, e.g., an electrical alternator. There are two main categories of Stirling engines, kinematic and free-piston. Kinematic Stirling engines (KSE) employ mechanical linkages to coordinate displacer and power piston motions and convey linear piston power to a rotary alternator. Free piston Stirling engines (FPSE) do not have mechanical linkages but, instead, employ a balance of pressure, spring and alternator load reaction forces to achieve the same functionality. KSE and FPSE both have advantages. In one regard, KSE control is simpler since displacer and power piston motions are mechanically coordinated. The FPSE must have the means to effectively control piston excursions to avoid collisions with the 42 ASHRAE Journal M pressure vessel, displacer piston, or additional power pistons in the engine. On the other hand, this also enables continuous modulation of piston stroke, offering an opportunity to optimize efficiency at partial load. An important feature for FPSE is that the elimination of mechanical linkages can enable design for maintenance-free operation for tens of thousands of hours. When used for RCHP, a liquid cooling loop recovers reject heat from the cold end of the Stirling engine and its combustion exhaust gases. This waste heat is then transferred to a storage tank or circulated through a hydronic system to provide space and water heating. In addition, some RCHP systems nearing commercialization include an additional burner to augment the space heating capacity of the unit so it can meet all space heating loads, obviating the need for an additional space heating source.2 Energy Savings Potential The electric generation efficiency of Stirling engines integrated with RCHP systems are appreciably less than that of the electric grid. For example, although RCHP-scale Stirling engines can achieve electric generation efficiencies on the order of 10% to 20%* (low heating value [LHV]),1,3,4 field tests of preproduction RCHP systems indicate typical net system electrical efficiencies (i.e., taking into account pump and fan parasitics, the impact of transient operation) of between 6% and 8%.2,5 For comparison, the U.S. electric grid has an average efficiency of approximately 30% (high heating value [HHV], taking into account transmission and distribution losses).6 On the other hand, the liquid cooling loop can recover approximately 80% of waste heat,3 yielding an overall thermal efficiency of about 70% in field tests.2 Consequently, for Stirling engine-based RCHP systems to reduce primary energy consumption, they must not just recover but also use a large portion of the waste heat to supplant space and hot water heating loads while generating a relatively small quantity of electricity (typically on the order of 1 kW). In turn, this underscores the importance of the RCHP operating strategy in determining when to run the Stirling engine. Specifically, to * Larger Stirling engines (tens of kW) can achieve peak efficiencies of approximately 30%.1 ashrae.org November 2008 http://www.ashrae.org

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Ashrae Journal - November 2008 Contents Commentary Industry News Meetings and Shows Feature Articles Reducing Data Center Energy Consumption Building Sciences Feature Articles Sustainable Nature Center Emerging Technologies Special Products Washington Report Special Supplement: BACnet® Today BACnet® Goes Green Seeing the Light with BACnet® BACnet® on Campus BACnet® for Net Zero Analyzing BACnet® BACnet® at Xerox Campus BACnet® in Europe BACnet® for Green Library BACnet® at the University of Minnesota: Flexibility, Interoperability InfoCenter Product Showplace Classified Advertising Advertising Index

Ashrae Journal - November 2008

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