Investment-savings method for energy-economic optimization of external wall thermal insulation thickness

This article analyses the optimum energy-economic thickness of thermal insulation layer for external wall. The analysis was performed by applying a new method 'investment-savings'. An appropriate mathematical model was developed during the work. The mathematical model consists of energy and economic part. The economic part of the model contains algebraic equations for investment, exploitation and savings. The considered wall was made of brick and polystyrene was used as thermal insulation material. The heat transfer through the wall was steady-state. An analytical-numerical and graph-numerical methods were applied to solve the mathematical model. The minimum payback period of the investment was the optimization criterion. The numerical results obtained by the simulation are presented graphically. The optimum thickness of the thermal insulation layer is shown in the diagrams. In addition, by applying the developed mathematical model the optimum thickness of thermal insulation layer for energy-economic conditions in Serbia in 2014 was obtained. By applying the current prices in Serbia, the energy-economic efficient optimum thickness of the thermal insulation layer of the polystyrene is 6.89 cm which is rounded to ≈7 cm. Polystyrene can be bought in 1, 2, 3, …, 7, …, 10, … cm thickness only. The payback period is 1.22 years if the price of electric energy is 0.09 €/kWh. A significant result of the study: increase in thickness by 4.86 times, while investment increases only by 1.69 times.