Integration of 50 % wind power in a CHP-based ... - Ea Energianalyse

178 Appendixtion given by the gap between best possible solution found and the overall optimal solution.The described optimization usually starts out with a heuristic search algorithm thatuses a Branch and Cut method. Branch**in**g, which means def**in****in**g the tree structure andcutt**in**g, to safely “cut” away the nodes not represent**in**g an optimal solution, and thusprune the solution outcome. It is **of**ten the case, that a s**in**gle MIT problem can generatea great amount **of** sub-problems, which quickly turns the model**in**g **in**to a compute **in**tensiveprocess that requires a great amount **of** physical memory as well (GAMS 2007).11.3.3. GAMS/CPLEXAs mentioned **in** chapter 5, the mathematical tools used **in** this project for solv**in**g theunit commitment problem is a comb**in**ation **of** the model**in**g language GAMS and theMIP solver Cplex, respectively. Regard**in**g the model language, GAMS operates **in** generalwith sets, **in**digenously and endogenously sizes which **in** normal terms best can be**in**terpret as **in**dexes, variables and scalar items, respectively. One **of** the greatest advantages**of** this particular tool, **in** connection with the current project, is that it from thebeg**in**n**in**g has build **in** different types **of** variables such as positive variables, **in**tegersand b**in**ary variables. Especially the use **of** positive variables is a factor that trims downthe programm**in**g language as well as heavy comput**in**g processes. Another advantage isthe rather easy use **of** tables and vectors **in** comb**in**ation between **in**dexes (for example(i,t)) and **in**put parameters (scalars).S**in**ce GAMS is mostly a programm**in**g language, the code is also used for “call**in**g” thespecific solver to perform the desired optimization. This way, GAMS translates thestated mathematical problem for the user **in**to the solver’s language. When it comes topick**in**g out an appropriate solver for a specific assignment a great selection **of** differenttools is presented - vary**in**g from simple l**in**ear programm**in**g-solvers (LP) to more advancedones. When choos**in**g the solver, it is important to carefully consider what particulartype **of** problem is to be optimized, avoid**in**g greater complexity than necessary 38 –especially s**in**ce the solvers more **of**ten than not are quite expensive (although GAMS asa tool is free **of** charge).The solver chosen for this project is Cplex, due to its efficient MIP optimiz**in**g, and thefact, that an academic s**of**tware license for Cplex already was available through the university.38Sometimes, a small simplification can mean the difference between a simple l**in**ear problem (LP), andan advanced non-l**in**ear (NLP) or a NLPEC, MPEC, MSNLP and so on.