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Keywords:Offshore Bottoming cycle CO 2 power cycle Off-design Modelling and simulation a b s t r a c t Improved energy efficiency is an issue of increasing importance in offshore oil and gas installations. The power on offshore installations is generated by gas turbines operating in a simple cycle. There is an obvious possibility for heat recovery for further power generation from the exhaust heat. However, the limited space and weight available makes the inclusion of bottoming cycles challenging. Due to its high working pressure and thereby compact components CO 2 (carbon dioxide) could be a viable solution, combining compactness and efficiency. An in-house simulation tool is used to evaluate the performance of CO 2 bottoming cycles at design and off-design conditions. Both a simple recuperated single stage cycle and a more advanced dual stage system are modelled. Results from simulations show a potential for 10 e11%-points increase in net plant efficiency at 100% gas turbine load. Also off-design simulations taking the variation in heat exchanger performance into account are performed showing that the bottoming cycle improves the off-design performance compared to the standard gas turbine solution. Even at 60% GT (gas turbine) load, the combined cycle with CO 2 bottoming cycle can achieve up to 45% net plant efficiency, compared to 31% for only the gas turbine.

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Modern building complexes have simultaneous heating and cooling demands. Therefore, integrated energy systems with heat pumps and long-term thermal storage are a promising solution. An integrated heating and cooling system for a building complex in Oslo, Norway was analyzed in this study. The main components of the system were heat pumps, solar thermal collectors, storage tanks, ice thermal energy storage, and borehole thermal energy storage. Dynamic simulation models were developed in Modelica with focus on the long-term thermal energy storage. One year measurement data was used to calibrate the system model and two COPs were defined to evaluate system performance. The simulation results showed that more heat had to be extracted from the long-term thermal storage during winter than could be injected during summer. This imbalance led to a decrease in ground temperature (3 °C after 5 years) and decreasing long-term performance of the system: both COPs decreased by 10 % within five years. This performance decrease could be avoided by increasing the number of solar collectors from 140 to 830 or by importing more heat from the local district heating system. Both measures led to sustainable operation with a balanced long-term thermal storage.