Evaluation of an energy- and exergy-based generic modeling approach of combined heat and power plants

Abstract

Reduced models of combined heat and power plants are required for different applications. Among other usages, they are implemented as mixed integer linear programs (MILP) in energy market models or price-based unit commitment problems to study the economic feasibility and optimal operation strategies of different units. Generic models are particularly useful when limited information is available for each considered plant. This paper presents a MILP modeling approach for combined heat and power (CHP) plants. The approach is based on energy and exergy balances and a few typical plant characteristics for different operating conditions. The reduction of electrical power output due to heat extraction is estimated by the transferred exergy to the district heating network. Furthermore, the accuracy, strengths and limitations of this approach are investigated for various CHP plant types with extraction condensing turbines designed for district heating systems. Therefore, detailed thermodynamic cycle simulations of CHP plants including part load operations are used to obtain the real plant operating conditions to compare them to the results of the described generic approach. The validation of the reduced, generic model shows that the accuracy mainly depends on the effectiveness of the heat extraction from the CHP plant. In addition, it can be seen that the main advantage of the presented exergy-based method is the inherent consideration of the feed flow temperature for the calculation of the power reduction due to heat extraction.

Integration of Flow Temperatures in Unit Commitment Models of Future District Heating Systems

“…As Reference [17] states, any CHP system can be modeled by the irregular quadrilateral shape of the operational range in a P-Q-diagram. This approach, as it is also applied similarly in Reference [18,19], is used for modeling CHP technologies and extended by a restriction limiting the minimum extractable heat flow in case of an ICE. The dotted lines in Figure 3 indicate the operational boundaries linearized by a set of equations and inequalities using technology and supply temperature-specific variables.…”

“…The modeling of the CHP plants, shown in Equations (15)- (19), follows the formulation from Reference [19], which is adapted to include the flue gas losses in Equations (21) and (22). The adaption is made in cooperation with one of the authors and has been tested extensively.…”

“…As described in Reference [19], the plant-specific alpha coefficients can be calculated by solving the system of linear equations given by Equations (13) and (14) and by applying the minimum and maximum providable powers and efficiencies without DH. These can then be used in Equations (15) and (16) to describe the relation between fuel inflow and electrical and heat outflow.…”

“…These can then be used in Equations (15) and (16) to describe the relation between fuel inflow and electrical and heat outflow. Moreover, the inequalities in Equations (19) and (20) define the relation between fuel inflow, flue gas losses, and minimum condenser capacity. Flue gas losses are related to the fuel flow, whereas the formulation is extended by the inequality in Equation 20, as well as Equations (21) and (22), as described above.…”

Comparison of a usual heat-transfer-station with a hydraulic modified version under the aspect of exergy saving

“…The question will first be answered on an exergetic basis and then by analysing the return temperatures. An investigation based on exergy analysis is quite common (Falk, 2018;Sartor, 2017;Mollenhauer et al, 2016;Jentsch, 2018). Also, a Modelica based tool for the dynamic exergy analysis is developed by Sangi et al (2017).…”