Development of a Shape Memory Alloy Heat Engine Through Experiment and Modeling

Abstract

Few technologies can produce meaningful power from low temperature waste heat sources below 250°C, particularly on a per-mass basis. Since the 1970’s energy crisis, NiTi shape memory alloy (SMA) and associated thermal engines have been considered a viable heat-to-power transducer but were not adopted due to previously poor material quality, low supply, design complexity, and cost. Decades of subsequent material development, research, and commercialization have resulted in the availability of consistently high-quality, well-characterized, low cost alloys and a renewed interest in SMA as a waste heat energy recovery technology. The Lightweight Thermal Energy Recovery System (LighTERS) is an ongoing ARPA-E funded collaboration between General Motors Company, HRL Laboratories, Dynalloy, Inc., and the University of Michigan. In this paper we will present initial results from investigations of a closed loop SMA thermal engine (a refinement of the Dr. Johnson design) using a helical coil element and forced-air heat exchange. This engine generates mechanical power by continuously pulling itself through separate hot and cold air streams using the shape memory phase transformation to alternately expand and contract at frequencies between 0.25 and 2 Hz. This work cycle occurs continuously along the length of the coil loop and produces steady state power against an external moment. We present engine features and the thermal envelope that resulted in devices achieving between 0.1 and 0.5 W/g of shape memory alloy material using only forced air heat exchangers and room temperature cooling.

Actuator Materials for Environmentally Powered Engines

“…This calculated output compares favorably with reported work per cycle as measured from SMA heat engines of 8 mJ g −1 (ΔT 60 °C) [13] and 30 mJ g −1 (ΔT 180 °C). [18] Figure 4 also shows stress-strain curves for a TCP fiber made from 0.7 mm diameter nylon-6 monofilament and a vulcanized rubber strip. Both materials were tested at two temperatures and data obtained from literature sources.…”

Differential equation model of shape memory alloy spring and its application in heat engines

“…What is important is the constitutive model of the SMA spring. To this end, experiments were performed on a single sample by Churchill and Shaw (2011) and Keefe et al (2011) to obtain the constant load behavior of the SMA spring under temperature-controlled cooling and heating cycles. The load was provided by hanging mass, and the temperature is controlled by a thermal chamber cooled by liquid nitrogen.…”

“…The proposed model is applied to the heat engine using the SMA spring in Churchill and Shaw (2011) and Keefe et al (2011) to verify the model.…”

“…For example, for the case of 0.490 N load, decrease the temperature from 132°C to 296°C, and then increase the temperature to 120°C; for the case of 4.413 N load, decrease the temperature from 186°C to 259°C, and then increase the temperature to 191°C. These temperatures all come from the experimental conditions in Churchill and Shaw (2011) and Keefe et al (2011), and cover the operating temperature range of the SMA spring.…”

“…Figure 5 shows the heat engine diagram proposed in Churchill and Shaw (2011) and Keefe et al (2011), which consists of a loop of SMA belt wound on three pulleys, a driving pulley, a driven pulley, and an idler pulley. The top pulley is the idler pulley, the below-left pulley is the driving pulley, and the below-right pulley is the driven pulley.…”