A novel thermally activated transcritical R744 heat pump cycle, which combines the principles of both the reverse Rankine cycle and Brayton cycle has been analysed. The cycle produces simultaneous hot and cold streams at three temperature levels; 60°C for hot water, 40°C for low temperature ambient heating, 10°C for ambient cooling. The cycle has been designed to fulfil the requirements of a flexible ‘hybrid’ trigeneration scheme, where the three energy streams (electricity, heating and cooling) can be matched closely to the user demands — maximising the Energy Utilisation Factor (EUF) of the fuel. Such a system is likely to be used in environmentally resilient cities in either tertiary sector buildings, interconnected residential zones, industry or other large buildings such as hospitals and educational establishments.
The cycle is activated using recuperated thermal energy from a gas turbine exhaust stream and under certain operating conditions uses a small proportion of the electricity produced by the gas turbine generator set. Recuperation is achieved using a direct exhaust gas to R744 recuperating heat exchanger. At higher levels of thermal recuperation, the increase in expander inlet temperature results in a substantial increase in the Coefficient Of Performance (COP). With an expander inlet temperature of 250°C the cycle achieves a COP in cooling of 1.13, whilst with a expander inlet temperature of 350°C the cycle achieves a COP in cooling of 1.58. These values of COP in cooling are typical of double and triple stage absorption chillers respectively. The values for COP in heating are 1.97 for an expander inlet temperature of 250°C, and 2.41 for an expander inlet temperature of 350°C.
The cycle is also capable of acting as a power positive bottoming cycle, whilst still providing heating and cooling. Under these circumstances, and assuming an expander inlet temperature of 350°C, the cycle has a thermal to mechanical conversion efficiency of 9.7%. The corresponding values of COP in cooling and heating are 1.03 and 1.80 respectively.