Possibilities of water-lithium bromide absorption power cycles for low temperature, low power and combined power and cooling systems

摘要

Absorption power cycles (APC) provide an alternative to the ORC as an interesting thermodynamic cycle for waste heat recovery. Particular advantage of the APC is in a multicomponent working fluid that allows for a temperature glide along boiling and condensation (desorption and absorption). This phenomenon provides a good temperature match of hot and cold streams in the heat exchangers during the phase change. Thus it is possible to achieve higher exergy efficiency of the heat addition when sensible heat is used as a heat source. Similarly good temperature match and lower cooling fluid flow through absorber-condenser for heat rejection lowers the parasitic load of fans or cooling tower pumps. As a result, higher net efficiency can be achieved in comparison to a single component working fluid. In the past, the only considered APC has been Kalina cycle operating with water-ammonia mixture as a working fluid, while other fluid combinations were only rarely considered. Recently there came into an attention an aqueous solution of Lithium Bromide (LiBr), typically known from absorption cooling. Compared to ammonia-water mixture the pressure levels in the cycle are very low, typically both high and low pressure in vacuum. This results in high volumetric flow, bulky device, but when turbine is used it can achieve high efficiency and increased material cost are for small units is only of a marginal importance. In this study after the introduction of theoretical advantages in efficiency and of design features and constraints, we present results of several case studies for potential application. These are waste heat recovery from hot air stream, low temperature geothermal application, coupling with vacuum solar collectors, bottoming cycle for ORC and a combined cooling and power cycle as an alternative to the cooling-only absorption cycle.