Absorption power cycles for low-temperature heat sources using aqueous salt solutions as working fluids

There are many low-temperature heat sources; however, current technologies for their utilization have a relatively low efficiency and high cost. The leading technology in the low-temperature domain for heat-to-work conversion is the organic Rankine cycle (ORC). Absorption power cycles (APCs) are a second option. Nearly all currently known APCs, most importantly the Kalina cycle, use a water-ammonia mixture as their working fluids. This paper offers a theoretical exploration of the possibility of utilizing aqueous solutions of three salts (lithium bromide, lithium chloride and calcium chloride), known mainly from absorption cooling, as working fluids for APCs. The cycles are compared with a typical steam Rankine cycle, a water-ammonia APC, and (subcritical) ORCs with a range of working fluids explored. The analysis includes a parasitic load for heat rejection by a cooling tower or air-cooled condenser. The absorption cycles exhibit better performance than all Rankine-based cycles analysed in temperatures below 120°C. For the LiBr-based APC, a detailed thermal design of the cycle is provided for 100°C water as a heat source and a sensitivity analysis is performed of the parameters controlling the main cycle. Mechanical design considerations should not pose a problem for small power units, especially in the case of expansion machines, which are often problematic in ORCs. The salt-based APCs also carry environmental benefits, as the salts utilized in the working fluids are non-toxic.