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.
Highlights
A lot of countries promote combined heat and power (CHP) plants as they offer a higher efficiency, compared to the separate production of heat and power [9, 15]
This paper presents a mixed integer linear programs (MILP) modeling approach for combined heat and power (CHP) plants
CHP plants supply heat and power, so that the optimal operation management is influenced by the electricity market, and by the requirements of the heat customers. This is important to consider in electricity market models which are often implemented as mixed integer linear programs (MILP)
Summary
A lot of countries promote combined heat and power (CHP) plants as they offer a higher efficiency, compared to the separate production of heat and power [9, 15]. This characteristic line is influenced by various design parameters of the CHP plant as well as operation conditions, like for example the district heating feed or return flow temperature. For a CHP plant with extraction condensing steam turbine the electrical power output is reduced due to the heat extraction, if the fuel rate is kept constant, see Fig. 2. This reduction is characterized by the power loss coefficient (b): b. With the mean temperature and the transfered thermal energy Q_ , the increase of exergy E_Q can be calculated to obtain the power loss coefficient: E_Q 1⁄4 Q_ Á
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