Investigation of sCO2 Cycle Layouts for the Recovery of Low Temperature Heat Sources

Since the development of supercritical carbon dioxide (sCO2) power cycles, several different power cycle architectures have evolved. Additional components like reheaters, recuperators and intercoolers were added and split flow configurations were introduced. These specific configurations are typically tailor-made for an explicit application, mostly in the medium-temperature field ranging between 240-480 °C.

In the waste heat recovery sector, low grade waste heat (< 240 °C) holds the biggest share of the waste heat worldwide. This work focuses on ultra low temperature heat sources as they face big challenges for cycle efficiency because of the low temperature difference of heat source and heat sink. Consequently, the power generation is on low efficiency and subject of improvement. This study therefore investigates different power cycle configurations for a given low temperature air as heat source and ambient air as a heat sink (20 °C). The main objective is to evaluate different cycles with standardized boundary conditions in order to have an equal base for their comparison. Heat source temperature ranges from 60 to 100 °C are considered.
Firstly, sCO2 power cycles from literature are evaluated using the commercial solver EBSILON. This step is meant to validate the numerical set up and results by using a documented cycle configuration from literature with its original boundary conditions. In a second step, the specified low temperature heat source is applied. The configurations are evaluated in terms of mass flow, pressure and thermal performance. Finally, the cycles are classified according to their efficiencies in the low temperature regime.

From the results, it is observed that a recuperator step is not feasible in very low temperatures because of the minor superheating in the supercritical region. For turbine inlet temperatures higher than 80 °C, a recuperator starts to be beneficial. Intercooled compression is not suitable for ultra-low waste heat temperatures. A basic 4-component configuration and split flow before heating perform best in the low temperature region of 60 °C. With increasing turbine inlet temperature, more complex cycle configurations such as reheated expansion and intercooled compression might be considered in order to enhance the system efficiency.

This study provides a dataset of thermal efficiencies of sCO2 power cycle configurations in the low grade waste heat recovery until 100 °C.