Development of a solar powered refrigeration system: Stirling-cycle based

Abstract

Solar refrigeration has been tirelessly studied since the 1970s. Yet, no renewable technology has been superior to the conventional grid powered technology for household applications. However, Photovoltaic (PV) powered vapour compression coolers and thermal collector powered sorption technologies are affordable in the market for off-grid cooling. The Stirling cycle is more than 200 years old technology but had limited success. For solar powered applications, it is complex, expensive and only commercially feasible at high-temperature differences in both motoring and refrigeration modes. That’s because high power machines need efficient, compact and hence complex heat exchangers, high temperature materials, pressurised light gases, complex driving mechanism, expensive solar tracking and solar thermal coupling mechanism. In this study, the Stirling-cycle machine is thermodynamically and technologically studied and designed to work with input temperatures between 450 K and 600 K that can be achieved by line-focus solar collectors. The Franchot-type machine, which is a double acting Stirling machine that uses one hot and one cold cylinder to form 2 alpha-type Stirling machines, has been redesigned to use long and direct-heated and cooled cylinders at low temperature differences. In addition to the polytropic processes, a novel simple isothermaliser was presented to improve the power density of the Franchot machine. The isothermaliser is either passive or active if it is thermally insulated or connected to external heat source, respectively. A simple balanced compounding mechanism, where compression pistons are mechanica l ly coupled to expansion ones, is suggested and studied theoretically. A novel thermal coupling mechanism with evacuated tube collectors is suggested for the solar-powered engine. To minimise material use, unpressurised ambient air and short regenerator connections are only considered with the suggested Stirling-cycle machine.

In this study, the Franchot machine is mathematically studied in the Matlab/Simul ink environment using the second order model for the three-control volume machine, assuming the expansion and compression are polytropic processes. In the initial study, the model is built on ideal processes in order to understand the response of the machine for changes in the speed, gas pressure, phase angle, dead volume and geometry. In successive chapters, introduction of different non-ideal processes and effects is considered and coupled to the ideal model.

It is found that the Stirling cycle performance can be improved by the optimisation of load, losses, gas pressure, engine speed, phase angle and geometrical parameters (e.g. cylinder diameter, length and dead volume). Increasing the gas pressure and engine speed enhances the power as they increase Reynolds’ number which in turn improves the in-cylinder heat transfer. Varying the phase angle and dead volume at a given speed can maximise the power approaching the Curzon and Ahlborn efficiency, which is the efficiency of any heat engine at maximum power. On the other hand, the Stirling-cycle refrigerator has a monotonic response approaching Carnot efficiency at very low cooling power and maximum cooling density at very low efficiency. Therefore, the optimised engine parameters at maximum power generation is used for the refrigera tor.

In comparison to the Stirling engine with adiabatic cycle, the polytropic model of the isothermalised engine predicts about 275% and 211% power density improvements at the maximum power point for the active and passive isothermalisers, respectively.

Similarly, in comparison to the adiabatic cycle refrigerator, the isothermalised refrigerator could have about 250% and 190% cooling power improvements at a COP of 3.25 for the active and passive isothermalisers, respectively. Conceptually, the suggested balanced compounding mechanism generates low side forces, reduces the machine length and complexity and adds self-starting capability. Simplicity and compactness are also enhanced by the removal of complex heat exchangers and using of a novel thermal coupling mechanism with the line solar collector. The solar refrigerator could deliver a specific cooling power peak of 367.5 W/m2 for air conditioning relative to a peak solar irradiance of 1 kW/m2. This number can be approximately computed by multiplying solar irradiance by Curzon efficiency, COP of 4 and solar collector efficiency of 50%. Therefore, the solar Stirling refrigera tor might have the potential to compete with the vapour compression cycle for domestic applications.