https://orcid.org/0000-0003-2400-050X
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ABSTRACT
The overall performance of the composite insulation external walls (CIEW) may not be optimal as the current methods calculate the optimal insulation thickness (OIT) considering only the cost due to an increase in the insulation layer. The influence of base wall costs on optimum construction form is worthy of further investigated. This study is to close the knowledge gap to improve the overall performance of a CIEW. For this, an optimization model was established using a life-cycle cost analysis (LCCA) method considering the structural and insulation layers to determinate the optimal structural layer thickness (OSLT), optimal insulation thickness (OIT), and overall U-value of a CIEW. A case study of a masonry wall in a cold area in Beijing was conducted. Coal-fired boiler (CFB), gas-fired boiler (GFB), and air source heat pump (ASHP) were used as heat sources, and extruded polystyrene panel (XPS), rock wool (RW), and glass wool (GW) were used as insulation materials. The OSLT, OIT, optimal overall U-value, life-cycle cost (LCC), life-cycle saving (LCS) and payback period were calculated and compared between scenarios determined by this model and traditional model when the structural layer was known. The results indicate that the scenario determined by this model could achieve better energy-saving benefits. The payback period ranged from 2.25 to 5.07 years. The LCC saving rates and the growth ratio of life-cycle savings varied with these materials and heat sources from 0.55% to 3.70% and 0.19% to 1.40%, respectively. When same heat source being used, the total thicknesses of the structure and insulation layer (TTSIL) equals 300 mm, the order of achieved energy-saving benefits is XPS>GW>RW. The optimal overall U-value of the CIEW was independent of the required thickness of the structural and insulation layers and decreases with increasing unit heat consumption of heat sources. However, the energy efficiency revenue decreases with an increase in the TTSIL. The proposed method provides different proportions of thicknesses for construction walls and insulation layers, the variation of heat accumulation potential, influence on peak cooling/heating demand and environmental impact of the optimal CIEW will be studied in the future.
Highlights
A new optimization model of composite insulation external wall (CIEW) was established.
The optimum construction form and overall U-value of CIEW were given.
The optimal overall U-value of CIEW is fixed and thickness-independent.
The scenario determined by the optimization model can gain better economic benefit.
Nomenclature
| C | = | the initial investment cost of the wall per unit area($/㎡) |
| CDD26 | = | cooling degree days based on 26°C(℃·d) |
| Ce | = | the price of electricity ($/(kW·h)) |
| CEC | = | the annual energy consumption costs per unit area of the heat and cool losses through the CIEW($/(a·m2)) |
| Cfuel | = | the unit price of fuel used in heating ($/kg, $/m3 or $/(kW·h)) |
| Cins | = | the unit price of the insulation material($/m3) |
| Cmort | = | the cost of cement mortar (including the material and construction costs)($/m2) |
| Cp | = | the installation cost per unit area of the insulation layer (including labor cost and auxiliary material cost)($/m2) |
| Csl | = | the material and construction cost of the structure layer($/m3) |
| COPAC | = | the coefficient of performance of the cooling system |
| d | = | the market discount rate |
| HDD18 | = | heating degree days based on 18°C(℃·d) |
| i | = | the inflation rate |
| LCC | = | the sum of the present value of the construction and annual energy consumption costs of the heat and cool losses through the CIEW per unit area($/㎡) |
| LCS | = | the difference between the present value of the annual energy consumption costs of the heat and cool losses through the CIEW per unit area saved by adding the insulation layer and the present value of the construction cost($/㎡) |
| N | = | the payback period, years |
| Ne | = | the building insulation material service life(years) |
| P1 | = | the present worth factor |
| QCL | = | the annual cooling load per unit area through the external wall in cooling seasons(kJ/(m2.a)) |
| qfuel | = | the lower heating value of per unit fuel used in heating(kJ/kg, kJ/m3, or kJ/(kW·h)) |
| QH | = | the annual heating load per unit area through the external wall in the heating season(kJ/(m2.a)) |
| Ropt | = | the total thermal resistance of the optimal structural combination form(m2·K/W) |
| Rt | = | the thermal resistance of the wall, excluding the structure and insulation layers(m2·K/W) |
| U | = | the heat transfer coefficient of the external wall(W/(㎡·K)) |
| Ueco | = | the overall U-value of the external wall with the economic insulation thickness(W/(㎡·K)) |
| Uopt | = | the overall U-value of the optimal structural form of the CIEW(W/(㎡·K)) |
| X | = | the thicknesses of the structure layer(m) |
| X0 | = | the economic insulation thickness(m) |
| Y | = | the thicknesses of the insulation, m |
| Yeco | = | the economic insulation thickness(m) |
| ΔCCE | = | the annual saving energy costs for the heat and cool losses through the CIEW per unit area($/(a·m2)) |
| ΔU | = | the difference of the overall U-value of the CIEW(W/(㎡·K)) |
| ΔQCL | = | the difference of the annual cooling load per unit area through the external wall in cooling seasons, considering and not considering the thermal resistance due to the structure and thermal insulation layers(kJ/(㎡·a)) |
| ΔQH | = | the difference of the annual heating load per unit area through the external wall in the heating season, considering and not considering the thermal resistance due to the structure and thermal insulation layers(kJ/(m2·a)) |
| η1 | = | the efficiency of the heating equipment or the coefficient of the performance of the air source heat pump(COPASHP) |
| η2 | = | the efficiency of the network |
| λX | = | the thermal conductivity of the structure layer material(W/(m·K)) |
| λY | = | the thermal conductivity of the insulation material(W/(m·K)) |
| μ | = | an auxiliary variable |
Acknowledgements
The research work presented in this paper is financially supported by the Fundamental Research Funds for the Central Universities (2020ZDPYMS39), the National Key Research and Development Program of China(2018YFD1100200), National Natural Science Foundation of China (51778611), the Open Fund of Jiangsu Collaborative Innovation Center for Building Energy Saving and Construction Technology (SJXTCY2102).
Disclosure statement
No potential conflict of interest was reported by the authors.
Additional information
Funding
Notes on contributors
Jianen Huang
Jianen Huang, 1970, male, Ph.D., Associate professor, devoted to the research on heating ventilation and air conditioning, building energy efficiency.
Wei Feng
Wei Feng, 1977, female, Ph.D., Associate professor, devoted to the research on heating ventilation and air conditioning, building energy efficiency.
Zhaochen Huang
Zhaochen Huang, 1997, female, Master, undergraduate, major in construction management.
Shasha Wang
Shasha Wang, 1996, female, Master Degree Candidate, major in building energy efficiency.