Waste heat recovery of industrial exhaust gas : study of variable power ejector cooling systems

Abstract

Large quantities of energy are wasted in the form of heat. For a given waste energy source, when its temperature is high enough, a desirable recovery strategy is a first Organic Rankine Cycle (ORC) stage for electricity production, followed by using the remaining available energy, at lower temperature, for either building heating in winter or building cooling in summer. Thermal cooling can be achieved with an ejector cycle. However, a single ejector having a fixed geometry does not have the flexibility to follow the variation of the building cooling load, especially if the outside temperature, and thus the condensing pressure, fluctuates. The proposed solution is to use multi-ejector blocks, designed to handle the changing conditions. This theoretical study used a thermodynamic model built with Python that allowed to model the different multi-ejector design. The first multi-ejector block is one of scaled ejectors designed to provide cooling at the same condensing pressure. The second is the use of different geometry ejectors designed to handle variable condensing pressures. This study explores a range of operating conditions that could be obtained by using waste heat or heat of renewable sources. The simulations were done for a cycle with R-600a, a natural refrigerant showing the potential to provide high system flexibility. The results show that for fixed operating pressures with variable cooling load requirements, the simultaneous use of scaled ejectors would be preferred over the use of different geometry ejectors.