Integration of Photovoltaic Technology to Decentralized Liquefied Natural Gas-Fueled Combined-Cooling-Heating-and-Power Plants
Authors: Arsalis Alexandros , Alexandrou Andreas N. 
Published: July 2019

In this study a decentralized combined cooling-heating-and-power (CCHP) plant fueled with liquefied natural gas (LNG) is integrated with photovoltaic (PV) technology to reduce fuel consumption and improve the economics of the CCHP plant. In their previous publications the authors have investigated the potential of an LNG-fueled CCHP at different configurations and system capacities, where the inclusion of PV technology was not investigated in detail. In this study the penetration of PV technology is analyzed in detail to reveal possible benefits. The modular nature of PV technology allows an opportunity for investigating power generation at different configurations. Additionally this combination shows a potential for reducing emissions and suggests a new concept of integrating an intermittent source of energy, such as solar energy. The proposed system is based on a realistic gas turbine cycle, integrated with a cooling plant and a district energy network. The detailed analysis of this study shows the thresholds of the proposed system to become an ideal candidate for distributed generation applications, especially in locations which are distant from centralized power plants. Therefore apart from reducing transmission and distribution losses, waste heat could be recovered effectively to generate heating, or cooling (absorption refrigeration technology). The system considers fueling with LNG, which is a safe and transportable fuel option. The simulation results show a potential for further investigation of the proposed system, since its performance results in significant thermodynamic and environmental improvements. The system can be operated in two modes: (a) winter operation, where recovered heat is distributed to the district energy network, (b) summer operation, where recovered heat is used to drive an absorption chiller-based cooling plant to generate cooling, which is also distributed to the district energy network. The average primary energy ratio of the proposed system is near 90%, which shows a potential for operation at high efficiencies. The cost analysis shows that the payback period is within a reasonable time frame; approximately 4 years. The system is modeled based on theoretical and experimental, including data from manufacturers’ datasheets and other assumptions. Each component and/or subsystem is modeled separately and coupled to every adjacent component, as described in the system configuration. All details, including specific assumptions are provided in the study. The values for all system input parameters are given, followed by a description of all subsystem and/or component models, i.e. CCHP plant, heat exchangers, cooling plant and district energy network. The cost model is described in detail, with all necessary cost functions and their input values, and also to allow a more realistic analysis of the feasibility potential of the proposed system a rigorous cost model is formulated. The main economic parameters evaluated are: projected total lifecycle cost (LCC), investment cost, operating cost and PP. The system model is validated using two commercially available gas turbines as reference cases, which include measured data from their respective manufacturer. Therefore the model input values correspond to the values taken from the references. The results of the model validation are given in detail in the study. The calculated error between the system simulation and the manufacturers’ data is low (0 to about 5%). Thereby, it can be assumed that the proposed system is modeled within an acceptable level of accuracy. The proposed CCHP system can be applied using different operational strategies, which may be based on either thermal energy (heating/cooling) or electricity demand. However to allow complete utilization of the generated thermal energy, the system must be based on a thermal energy-led operation. Therefore if it is assumed that the system operates at a constant, full power capacity (which also allows maximum net electrical efficiency), all generated thermal energy is delivered to buildings. The system is grid-interconnected to allow import/export of electricity, while the buildings are equipped with vapor-compression heat pumps, which will be operated the thermal energy supplied from the CCHP system is insufficient. The heat pumps are operated with electricity generated from the CCHP system. In the case of a CCHP system it is important to choose the set of consumers (buildings) that will allow an efficient matching of the generated vectors of energy with demand. The reason is that for example in the case of heating mode, high heat is required during the evening and early morning hours in households (due to high occupancy and low ambient temperature), while high heat demand occurs during daytime working hours in weekdays. A proposed small-scale, decentralized CCHP system is considered as alternative to conventional large-scale centralized, electricity-only generating power plants. The system is designed, modeled and analyzed in terms of thermodynamics, including cost analysis and exergy analysis. The results of the system model simulation show the potential of the proposed system in terms of efficiency. The system performs efficiently, with a PER of near 90%, and therefore the proposed system may become a significant candidate in the energy market, since fuel consumption and CO2 emissions can be reduced. The considered fuel is LNG, and with the proposed design approach of its regasification, the cooling energy can be used to cool the feed air before compression, which results to a net electrical efficiency of almost 2%, which is an attractive system modification for areas with prolonged summer-like weather conditions. Overall, when the proposed system is compared to an equivalent conventional system, results show significant improvement in CO2 emissions reduction (almost 40%) and primary energy savings (more than 40%). The exergy analysis shows that there is a potential for improvement of the exergetic efficiency through design modifications. Exergy destruction is higher in the combustor, the cooling plant and the heat exchangers responsible for recovering waste heat (i.e. HEx2 and HEx3). In the case of the cooling plant and the heat exchangers, the irreversibility is due to the conversion of high quality waste heat to low quality useful heating (or cooling). This is an unavoidable result due to the purpose of the proposed system, although exergetic efficiency could improve with the replacement of the double-effect absorption chiller with a triple-effect one. Also, theoretically, net electrical efficiency (and also exergetic efficiency) can be improved with the introduction of a combined cycle configuration, which is however a more complex and costly, and thereby an unfavorable option for distributed generation applications. The only possible and feasible modification for the current system design could be the optimization of the combustion process, by preheating the reactants or minimizing the use of excess air, although this could result in an impractical (or inflexible) system configuration.