Part-load strategies for a steam-generating ejector heat-pump using numerical simulations
Steam-generating heat pumps are a promising solution for decarbonizing industrial processes, but their efficiency decreases significantly with high temperature lift. The resulting high-pressure differences between the condenser and evaporator lead to significant expansion losses in the Carnot cycle. This study investigates the use of an ejector as an alternative to the conventional throttle valve to improve the efficiency of steamgenerating heat pumps under variable load conditions. A CFD-based homogeneous equilibrium model is used to simulate the ejector’s performance and develop a flexible ejector concept for fluctuating steam demand. The results show that incorporating a needle into the motive nozzle reduces heat capacity while maintaining performance. Swirl control is an ineffective solution as subcooling increases (> 10K), which, however, is necessary in this case to enhance the performance of the heat-pump. Simulations of the needle ejector demonstrate stable operation down to 50% of full heat capacity. To further reduce capacity, a motive nozzle bypass is proposed. The study indicates a COP improvement potential ranging from 30% to 10% across 100%-30% load conditions. The concept is demonstrated in a feed mill use case, where reduced steam production and a storage concept are required to handle recipe changes. The heat pump’s condenser acts as a short-term steam buffer, requiring a 40% increase in shell diameter. Incorporating the ejector is essential to meet economic targets (ROI < 5 years).