Thermodynamic performance analysis of a combined power cycle using low grade heat source and LNG cold energy

• Thermodynamic analysis was performed for a combined cycle utilizing LNG cold energy.
• Ammonia–water Rankine cycle and LNG Rankine cycle was combined.
• A parametric study was conducted to examine the effects of the key parameters.
• Characteristics of the exergy efficiency and heat transfer capability were proposed.
• The system performance was influenced significantly by the ammonia mass fraction.

Thermodynamic analysis of a combined cycle using a low grade heat source and LNG cold energy was carried out. The combined cycle consisted of an ammonia–water Rankine cycle with and without regeneration and a LNG Rankine cycle. A parametric study was conducted to examine the effects of the key parameters, such as ammonia mass fraction, turbine inlet pressure, condensation temperature. The effects of the ammonia mass fraction on the temperature distributions of the hot and cold streams in heat exchangers were also investigated. The characteristic diagram of the exergy efficiency and heat transfer capability was proposed to consider the system performance and expenditure of the heat exchangers simultaneously. The simulation showed that the system performance is influenced significantly by the parameters with the ammonia mass fraction having largest effect. The net work output of the ammonia–water cycle may have a peak value or increase monotonically with increasing ammonia mass fraction, which depends on turbine inlet pressure or condensation temperature. The exergy efficiency may decrease or increase or have a peak value with turbine inlet pressure depending on the ammonia mass fraction.