Exergoeconomic Analysis Of Combined Cooling, Heating And Power System Based On Energy Level Concept

Combined cooling, heating and power system (CCHP) based on gas turbine prime mover is featured by the principle of energy cascade utilization, with high overall thermal efficiency and low pollutant emissions. Since the energy quality difference among the energies of cooling, heating and electricity, the evaluation and optimization of CCHP system is difficult. Exergoeconomic analysis method is regarded as a potential tool of quantifying energy quality, because the analysis results provide cost-based information of all products, and can offer a more comprehensive and objective understanding compared to a separated thermodynamics analysis or technical and economic analysis. In this dissertation, a new exergoeconomic analysis methodology based on energy level is proposed, and the thermoeconomic model of CCHP system is established, and the unit exergoeconomic cost of cooling, heating and power is investigated at off design condition.The main problem of the conventional cost allocation methods is that the unit exergy price of heat sources with different temperature are constant and it is opposite to the basic economical principle that higher temperature energy has higher price. A new cost allocation method based on energy level is proposed, considering the corresponding relation between the exergy cost and energy quality. This method is performed by establishing the equation between the unit cost and energy level of infinitesimal process, and then obtaining the unit exergy cost at different energy level. The total cost of every energy utilization process is got by integrating and at last the cost allocation equation is obtained. The unit exergy cost of cooling is much higher than electricity and heating, because of the low temperature difference between the cooling energy and environment, which may cause misunderstanding compared with the price of heating and electricity. Besides, considering the need of practical application, it is suggested that the unit energy cost is used for quantifying the products’cost instead of unit exergy cost. Lastly, the exergoeconomic analysis methodology based on energy level is proposed. The new method changes the rules of linear distribution and black box analysis in the conventional cost-allocation method, which makes the exergoeconomic results are more reasonable and accurate. The methodology proposed in this paper may provide a reasonable way for cost allocation, system evaluation and operation optimization.The thermoeconomic model of CCHP system at design condition is established based on structural theory and the unit exergy costs of cooling, heating and power are calculated with extraction method, energy quality factor method and energy level associated cost allocation method. The unit exergy costs of power, cooling and heating respectively are 202.4 RMB/GJ, 3082.8 RMB/GJ and 162.7 RMB/GJ, and the unit energy costs are 202.42 RMB/GJ,60.18 RMB/GJ, and 20.87 RMB/GJ respectively, which shows that the cost-allocation based on energy level is more rational, compared with conventional cost-allocation methods. The result of new cost allocation method is in conformity with the economic principle of higher quality and higher price, since the corresponding relation between the exergy cost and energy quality of infinitesimal process is considered. Case study shows that exergoeconomic analysis methodology based on energy level can provide reliable theoretical basis for product pricing.The CCHP system is operated under electricity tracking strategy and the gas turbine power output is reduced by decreasing fuel consumption. The unit energy costs of cooling, heating and power of CCHP system under off-design condition are calculated by the exergoeconomic method based on energy level. When the gas turbine load reduces from 100% to 20%, the unit energy cost of elecricity increases from 202.4 RMB/GJ to 388.5 RMB/GJ rapidly. The unit energy cost of cooling decreases slightly from 60.18 RMB/GJ to 58.66 RMB/GJ, when the electricity load decreases from 100% to 60%, however, it increases to 72.59 RMB/GJ as the electricity decreases to 20% load. The unit energy cost of heating is 20.87 RMB/GJ at 100% load and 20.83 RMB/GJ at 80% load respectively, and it rises from 21.45 RMB/GJ to 30.01 RMB/GJ as the electricity load decreases from 60% to 20%. It is concluded that off-design performance of CCHP system has a great effect on the cost of energy products, and the electricity cost is more sensible. The relative cost difference factor and the exergoeconomic factor are studied when the CCHP system is in off-design operation. It is shown that the system performance may be improved greatly by increasing the equitment investment and reducing the exergy destruction of the combustion chamber of gas turbine, absorption refrigerating machine and hot water heat exchanger.