A reference scenario was set up for further comparative analyses, considering a building featured by basic properties. The reference object is a single-story lightweight portal framed industrial building with no crane (Figure 2), the lifespan of which is assumed to be 20 years. The typical plan is 60 m×30 m, with a gross area of 1800 m². The top height of the outer longitudinal wall is about 5.5 m, and the highest point is 7.5 m. According to functional requirements, the column spacing of the long side is 10 m, and the portal frame span is 30 m. The independent foundations under columns are adopted. The steel beams and columns are H-shaped steel with variable cross-section.

The building is a new industrial building so that its thermal design and indoor design temperature of heating and cooling would meet the requirements of “Unified standard for energy efficiency design of industrial buildings”(M.O.H.U.R.D., 2017). The envelope adopts a combination of double-layer steel decks and rock wool insulation boards. The reference scenario includes the life-cycle phases evaluated using the following design conditions shown in Table 1.

Due to various geographical locations, solar radiation intensities in China are quite different. In general, the total annual solar radiation in the western region is higher than that in the eastern region, except for Tibet and Xinjiang, and is lower in the south than in the north. Since most southern areas are foggy and rainy, located between the 30-40° north latitude, solar radiation decreases with latitude decline [(Solar, 2019);(Tschopp et al., 2020)]. There are roughly five different districts in China, divided according to the amount of solar radiation (Zheng & Yi, 2010). Solar district I has the most abundant solar energy resources, the highest value located in the Qinghai-Tibet Plateau with a radiation amount higher than 2,200 kWh/m 2 •yr. The lowest value is found in the Chuan-Yu region of Solar district V, with about 950 kWh/m 2 •yr (Li et al., 2014).

There are five major types of climatic regions in China, which are severe cold regions, cold regions, hot summer and cold winter regions, hot summer and warm winter regions, and temperate regions, and there are different building design requirements for different climatic regions (Huo et al., 2017). Most areas of the Solar district I, II and III with abundant solar energy resources belong to severe cold and cold climate areas, while Solar district IV and V belong to hot summer and cold winter regions, hot summer and warm winter regions, and temperate regions. Most of China’s areas and populations are concentrated in the Solar district II, III and IV. In this study, three representative cities are selected, namely Ruoqiang in Xinjiang in Solar district II, Dezhou in Shandong in Solar district III, and Nantong in Jiangsu in Solar district IV.

The seismic design requirements and basic wind and snow loads of these three cities are similar. Therefore, the difference in the amount of steel consumption for lightweight steel-structured industrial buildings can be ignored, and these cities can be used as the design contexts. The geographical location, solar energy resources and climate conditions of the selected cities are shown in Figure 3. Ruoqiang and Dezhou belong to the cold region, while Nantong belongs to the hot summer and cold winter region.

Life Cycle Carbon Emission Assessment (LCCO2A) considers all the carbon-equivalent emission output from a building during all five phases: materialization, transportation, on-site construction, operation, and end-of-life. The carbon emissions during raw material extraction, transportation and component manufacture period are defined as the carbon emissions during materialization period. The transportation phase in this paper consists of two processes: transporting building materials from the factory to the construction site and transporting the demolition waste from the site to the recycling factories or landfill. The end-of-life phase includes three parts, demolition, recycling, and disposal. The disposal phase is to transport the demolition waste which has been included into the calculation of the transportation phase. There is a lack of data on construction and demolition of industrial buildings, the carbon emissions which only share a little part according to the previous study (Bonamente & Cotana, 2015), so the calculation of these two stages is ignored in this paper. Therefore, LCCO2A of industrial buildings in this research is simplified as the following:

The equation has been converted to the requested TeX math format. You can copy and paste the following output directly into your JATS editor:

(1)

where CO2 represents the carbon emission of the whole life cycle of a building (kgCO2e), CO 2 Materialization , CO 2 Transportation , and CO 2 Operation represent the carbon emissions during the materialization, transportation, and operation (kgCO2e), respectively. CO 2 Recycling is the carbon credits obtained from recycling (kgCO2e).

The two-slope roof is loaded with a flexible thin-film PV system of 128.4kW. Its economic cost-benefit analysis, similar to the aforementioned LCCO2A, includes three phases: the initial investment phase, the operation phase, and the demolition phase. There are obvious investment and income variables in the first two stages, and the third one is the estimated cost of dismantling and recycling after 20 years. Considering that this project is a distributed PV project with a small installed capacity, and currently China's PV panel recycling technology is not mature enough, the calculation of the third phase is eligible in this study. Therefore, the cost-benefit analysis of the PV system can be based on the following formula:

(10)

Where B PV is the total benefit of the rooftop PV system during the 20-year life cycle, C invest is the initial investment, C op and B op are the cost and benefit during the operation phase.

In this paper, process-based analysis is applied to obtain detailed results presented in Figure 4 to Figure 6, respectively. The embodied carbon amounts and distribution of each raw material for the reference building are presented in Table 4 and Figure 4. 1136 tons of concrete is used to construct the building, which corresponds to about 89% of the overall weights of the building. Most of the embodied carbon emissions are contributed from steel with five types (50.04%), concrete (30.70%), and mineral wool (14.53%), and the remaining is the materials used for skylights and windows. The higher amount of embodied carbon for steel and concrete compared to other materials is due to the larger volume of steel and concrete. Electrolytic aluminum is a high-carbon emission industry. However, due to the low density of aluminum, the total weight of the aluminum window frames used is small and its carbon emissions only account for 0.03%.

The heating demand is satisfied by the anthracite-fired CHP units with heat and power production simultaneously, offering a number of environmental benefits over traditional heating methods, particularly other fossil fuels. As shown in the below Figure 5 and Figure 6, Ruoqiang and Dezhou are located in the cold region, with the annual heating demand and CHP power generation higher than that of Nantong. This latterre is located in hot summer and cold winter regions, where the carbon emissions increase significantly in July and August due to cooling devices. For these three cities, the carbon credits obtained from CHP generation are smaller than carbon emissions during winter months (from January to March and from October to December). No heating demand in other seasons means that there is no extra carbon credit from CHP generation. But with the PV system installed, as shown in Figure 6, the buildings will gain carbon credits all year around. The monthly carbon credits obtained from CHP and PV generation are even bigger than carbon emissions from heating or cooling.

Figure 7 presents the life-cycle carbon balance of the reference buildings with or without the PV systems in three typical cities. Due to climate differences, the carbon emissions during the operation phase vary a lot in the three cities. The proportion of carbon emissions during materialization, transportation, and operation is 11.71%, 0.71%, and 87.57%, respectively in Ruoqiang. This proportion is 13.97%, 0.85%, and 85.18% in Dezhou. While it changes to 24.34%, 1.48%, and 74.18% in Nantong. Without the PV system, the carbon emissions in three cities are all bigger than credits from CHP generation. If the PV system is installed, the buildings will be converted into more environmentally friendly ones. The carbon balance is achieved by PV power generation alone, with the values of 2222 kgCO₂e/m², 1999 kgCO₂e/m², and 1706 kgCO₂e/m² in Ruoqiang, Dezhou, and Nantong.

Taking into account the decay rate of PV systems, the actual total power generation of the buildings in Ruoqiang, Dezhou, and Nantong within 20 years is 4000MWh, 3599MWh, and 3071MWh, respectively. Figure 8 shows the distribution of the life-cycle PV generation power and the corresponding share of revenues. The building in Nantong has the largest proportion of self-use electricity at 21.12%, with electricity savings accounting for 21.68% of the total revenue. The self-use proportion of the building in Dezhou (16.08%) is slightly smaller than that of Ruoqiang (17.21%). However, Ruoqiang’s industrial electricity price is lower, resulting in the proportion of cost savings in Dezhou (18.25%) more significant than Ruoqiang (15.66%).

During the whole life-cycle, the annual cumulative cost and benefits are shown in Figure 9. The initial investment is $121,723. Except for the additional inverter replacement in the 10^th year, the annual fixed cost during the operation period is $1,765. The net profits of the Ruoqiang, Dezhou, and Nantong PV projects are $122,747, $130,621, and $131,810, respectively. The specific costs and benefits are shown in Table 5. In the 20-year cycle, the initial investment accounts for about 72.70% of the total cost, and the profit margin is 42.30-44.05%. The net profit of PV projects was firmly related to the initial investment cost. The simple payback period of the project in Ruoqiang is 10 years, and that of Dezhou and Nantong are 9 years, which means that the revenues expend the cost or reach the break-even point at this time. The Ruoqiang area has the best solar resources among the selected cities with the largest power generation and state subsidies. However, due to the low industrial electricity price and low on-grid price of PV power, the net profit in the whole cycle is the smallest and the payback period is the longest. Therefore, the net profit of PV projects is not positively correlated with local solar radiation.