A chemisorption power generation cycle with multi-stage expansion driven by low grade heat

Huashan Bao,Zhiwei Ma,Anthony Paul Roskilly

doi:10.1016/j.enconman.2017.07.032

Abstract

Ammonia-based chemisorption cycle driven by low grade heat exhibits vast potential for power generation because there exists huge pressure difference between the two salt-adsorbent-filled reactors. However, the intrinsic feature of ammonia as a wet fluid and the difficult match between chemisorption cycle and expansion device impede the development of such a power generation system and also increase the difficulty of practical implementation. To explore maximum benefits of this technology, the present work has proposed and studied a new resorption power generation cycle that applies multiple expansion. The application of multiple expansion integrated with reheating processes aims to overcome the limitation of the ammonia being wet fluid and fully harness the huge pressure difference that chemisorption can offer for power generation, leading to the improvement of energy efficiency. The performance of the proposed multiple expansion resorption power generation cycle using three typical resorption salt pairs, including sodium bromide – manganese chloride, strontium chloride – manganese chloride and sodium bromide – strontium chloride, have been investigated not just based on theoretical thermodynamics but also with the consideration of practical factors to obtain better understanding and more insights for a real system design. The multiple expansion resorption power generation using sodium bromide – manganese chloride and sodium bromide – strontium chloride pairs can achieve 100–600kJ/kg (ammonia) work output when heat source temperature is from 30°C to 150°C; the multiple expansion using strontium chloride – manganese chloride pair has higher average work output per one expansion stage than that using the other two pairs. The cyclic energy efficiency can be achieved as 0.06–0.15 when implementing 2–4 expansions in a more practical scenario where the equilibrium pressure drop is set to 2bar and the heat source temperature is in the range of 80–150°C. Such efficiencies are circa 27–62% of Carnot efficiency under the same thermal conditions.

Highlights

Sorption technology has been recognised to play a major role in the low carbon future for energy demand of heating, or cooling, or power [1]
Resorption power generation cycle has been further developed in this work to realise the full potential of pressure difference between the desorption of low temperature salt and the adsorption of high temperature salt for power generation
Reheating expansion exhaust to carry out multiple expansion is the pivotal of the developed cycle, which can overcome the restriction of the ammonia being wet fluid and sufficiently implement the feature of huge pressure difference existing in chemisorption

Summary

Introduction

Sorption technology has been recognised to play a major role in the low carbon future for energy demand of heating, or cooling, or power [1]. Since Maloney and Robertson [4] among the first studied ammonia-water based absorption power generation cycle, sorption power generation started to attracted a great deal of research interests to explore more potential of this technology. The combination of an ammonia-based Rankine cycle and an ammonia-water absorption cycle, was proposed in 1995 to produce cold and power simultaneously [8]. Compared to liquid absorption cycle, ammonia-based solid chemisorption cycles have the commendable advantage of large pressure difference and the unique reaction equilibrium, which indicates a potential of productive mechanical power generation with more resistance to the limitation of ammonia as a wet fluid [9]

Methods

Results

Conclusion