Energy and exergy analysis of the Kalina cycle for use in concentrated solar power plants with direct steam generation

AbstractIn concentrated solar power plants using direct steam generation, the usage of a thermal storage unit based only on sensible heat may lead to large exergetic losses during charging and discharging, due to a poor matching of the temperature profiles. By the use of the Kalina cycle, in which evaporation and condensation takes place over a temperature range, the efficiency of the heat exchange processes can be improved, possibly resulting also in improved overall performance of the system. This paper is aimed at evaluating the prospect of using the Kalina cycle for concentrated solar power plants with direct steam generation. The following two scenarios were addressed using energy and exergy analysis: generating power using heat from only the receiver and using only stored heat. For each of these scenarios comparisons were made for mixture concentrations ranging from 0.1 mole fraction of ammonia to 0.9, and compared to the conventional Rankine cycle. This comparison was then also carried out for various turbine inlet pressures (100bar to critical pressures). The results suggest that there would be no benefit from using a Kalina cycle instead of a Rankine cycle when generating power from heat taken directly from the solar receiver. Compared to a baseline Rankine cycle, the efficiency of the Kalina cycle was about around 5% lower for this scenario. When using heat from the storage unit, however, the Kalina cycle achieved efficiencies up to 20% higher than what was achieved using the Rankine cycle. Overall, when based on an average assumed 18hours cycle, consisting of 12hours using heat from the receiver and 6hours using heat from the storage, the Kalina cycle and Rankine cycle achieved almost equal efficiencies. A Kalina cycle operating with an ammonia mole fraction of about 0.7 returned an averaged efficiency of about 30.7% compared to 30.3% for the Rankine cycle

Feasibility of using ammonia-water mixture in high temperature concentrated solar power plants with direct vapour generation

AbstractConcentrated solar power plants have attracted an increasing interest in the past few years – both with respect to the design of various plant components, and extending the operation hours by employing different types of storage systems. One approach to improve the overall plant performance is to use direct vapour generation with water/steam as both the heat transfer fluid in the solar receivers and the cycle working fluid. This enables to operate the plant with higher turbine inlet temperatures. Available literature suggests that it is feasible to use ammonia-water mixture at high temperatures without corroding the equipment by using suitable additives with the mixture. This paper assesses the thermodynamic feasibility of using ammonia-water mixture in high temperature (450 ̊C) and high pressure (over 100 bar) concentrated solar power plants with direct vapour generation. The following two cases are compared for the analysis: a simple Rankine cycle and an ammonia-water cycle with a separator for varying the ammonia mass fraction within the cycle. Thermodynamic simulations are performed using Aspen Plus and MATLAB, and performances in terms of overall plant efficiency are evaluated. The comparison between the two cycles when operating from a two-tank molten-salt storage system is also presented. The results suggest that the ammonia-water mixtures show a clear advantage while operating from storage but the simple Rankine cycle outperforms the ammonia-water cycle when the heat input is from solar receiver only