Performance Predictions and Test Results of a Free Piston Stirling Engine Driven Heat Pump

Recent progress on a Stirling engine driven hea t pump is described and a modeling technique for predicting engine dynamics is developed. The system described represents a very p romising configuration of engine driven heat pump, because o f the long life characteristics of the free-piston Stirling engin e and the hermetic nature of both the engine and refrigerant work ing fluids. Recent changes in the transmission of this system and the design of a .new engine section have allowed performance to becom e very attractive. With planned develop~ent of the Mark I, demonstrat ion of the system's commercial potential and eventual field testing of prototype units will be forthcoming. INTRODUCTION The area of engine driven heat pumps for reside ntial use has been receiving increased attention, with several orga nizations in the US and Japan attempting to develop marketable sy stems. Mechanical Technology Incorporated (MTI), with the joint sponsorship of Gas Research Institute and the Department of Energy through a subcontract to Oak Ridge National Laboratory,* is deve loping a heat activated heat pump (HAHP) of 3 Refrigeration-Ton (RT ) cooling capacity, based on the free-piston Stirling engine (FP SE). The Stirling engine, either free-piston or kinematic, is an e xcellent choice as prime mover because of its quiet operation, long life potential, and ability to burn many fuels cleanly in an external combustion system. * The ongoing guidance, review, and support of G eorge T. Privon and Dr. J. Michael Clinch of Oak Ridge National Lab oratory and Gas Research Institute, respectively, is gratefully ack nowledged. 1035 The free-piston Stirling engine has the further advantage of being hermetically-sealed. It is also easily modulated and readily coupled to a reciprocating compressor by means of a flexible diaphragm barrier and oil-filled transmission. The refrigerant system is then also hermetic, with the exception of small quantities of oil which can leak past shaft seals and into the refrigerant. This oil is easily separated, collected, and pumped back into the transmission. The current configuration of the HAHP, termed the Mark I, is shown in Figure 1. A photograph of the HAHP without external piping or instrumentation leads is shown in Figure 2. It will be described in more detail following a description of the basic operation of a free-piston Stirling engine. Fig. 1 Mark I Heat Pump Fig. 2 Heat Pump Installed in Test Cell The FPSE normally uses two moving pistons that cyclically compress, heat, expand, and cool a working fluid which, in this case, is high pressure helium. A nearly sinusoidal pressure wave is generated in the engine that lags the motion of the power piston and transfers PV power to it. This power is typically extracted by a linear electric alternator using magnets on the piston or a piston-driven compressor that is contained within the pressure boundaries of the engine. 1036 When the working gas is displaced by motion of the dis placer piston, it passes through three heat exchangers in series the heater, regenerator, and cooler. The heater region is us ually kept at 720-SOO"c and the cooler at 10-50°C. The regenerat or, a critical component made of a porous metallic medium such as fin e wire stacked screen, prevents the inefficient transfer of heat from the heater to the cooler by extracting heat from the gas during on e part of each cycle, storing it, and returning that heat to the gas during the second part of each cycle. In the design of reg enerators, the desired high heat transfer properties of a matrix ar e countered by excessive pressure drop loss if, for example, the reg enerator is too long or too densely packed. All Stirling engines operate on the principles describ ed above. The FPSE is a resonantly-oscillating machine in which the two moving parts, the piston and the displacer, are not connected to any linkage arrangement. The phasing and strokes of these members are determined by the interaction of gas spring forces, thermodynami c power transfer, load applied to the piston, and electric control power that aids in driving the displacer. Mechanical friction is eli minated by the use of clearance seals and hydrostatic gas bearings, s upplied internally by tapping small quantities of gas from one of th e gas springs. Figure 3 is a photograph of the Engineering Model ( EM) FPSE, 3-kW electric, which was designed and built in 1982 (Ref. 1). The engine has a mean internal pressure equal to 60 Bar and oper ates at 60 cps, parameters which are approximately the same in the HAR P. Fig. 3 EM Engine Installed for Endurance Test The kinematic Stirling engine (KSE) is used when shaft power is desired. One configuration that displays high powe r density is a four cylinder, double-acting engine called the Rini a arrangement. MTI in cooperation with United Stirling of Sweden, ha s developed two mod~ls of a high performance KSE that are suitable for automotive use. The latest configuration (Ref. 2) is a V-4 with single crankshaft and simplified heat exchangers. Design emphasi s was placed ~n reduced size, weight, and manufacturing cost. Severa l other appl~­ cations for these engines are also under investigation . 1037 This paper will describe the critical components of the free-piston HAHP and discuss the measured performance obtained to date as well as the performance improvements expected for the Mark I. The paper will also describe the analytical technique developed for predicting steady-state operating dynamics of the engine, given a known piston stroke and load. The computer code which performs this analysis is outlined, along with the MTI thermodynamic code used in conjunction with the dynamic analysis. Several examples of the use of the code are given and results are compared to test data. HEAT PUMP CRITICAL COMPONENTS 1. Direct-Acting Transmission The HAHP transmission is oil-filled and relies on the motion of the oil to couple diaphragm deflection to piston motion. Diaphragm center deflection somewhat greater than .2-inch is used to obtain a much longer maximum compressor stroke. The mass of the reciprocating piston assembly is 16.5 lbm, and at maximum stroke, the resulting shaking force is more than 2000 lbf. Unless the compressor case is very heavy, case vibration will be excessive. Experience has shown that vibrations at the combustor must be less than l g to allow long life. As initially designed and tested (Ref. 3 and 4), the HAHP transmission included an oil-driven counterweight, also weighing 16.5 lbm, that moved in opposition to the piston and thereby eliminated compressor shaking forces at all strokes and frequencies. Final performance testing of this configuration, completed in early 1985, showed that unavoidable oil viscous losses associated with the counterweight could exceed 1000 watts at high strokes, and overall transmission loss could approach 1500 watts. With the counterweight removed and minor changes made to streamline the oil flow, transmission loss fell to approximately 400 watts at high stroke. Figure 4 gives some of the data on transmission power loss with and without the counterweight. Currently, measured transmission efficiencies are greater than 85% at medium to high power levels. Further improvement in transmission efficiency is expected when planned design changes are incorporated.