# Regional Adaptability Analysis of Solar Roof Utilization Technologies in China
## 2 Methodology
### Evaluation of the Energy-Saving Benefit  
#### 2.5.1 Evaluation of Energy-Saving for the Solar Heating System  
The application of the solar thermal technology is influenced by the location of the buildings, solar radiation, roof area, the length-width ratio, and the function of the building. The optimal inclination and spacing of the solar collectors depend on the location of the building. The solar collector area is designed to cover the hot water demand of the building with a certain solar fraction in different areas.  
The energy saving of the solar domestic hot water system can be estimated in Equation (3).  
\[\Delta Q=L_{w}\;\mathrm{c}\;\rho\;(t_{\mathrm{h}}-t_{\mathrm{c}})\;f \tag{3}\]  
In the equation, \(\Delta Q\) is the energy saving, kJ; \(L_{w}\) is the total hot water usage, L; \(t_{\mathrm{h}}\) is the temperature of hot water, \({}^{\circ}\)C; \(t_{\mathrm{c}}\) is the mains water temperature, \({}^{\circ}\)C; \(c\) is the specific heat of water, kJ/(kg \({}^{\circ}\)C); \(\rho\) is the density of water, kg/L; and \(f\) is the solar fraction, %.  
In this study, the solar heat output is also calculated using the software SAM. The relevant parameters of the solar collectors were obtained from collector test reports provided by the manufacturers.  
The energy savings of the solar domestic hot water system for the 18 buildings in Beijing, Shanghai and Guangzhou are shown in Figure 6. The three cities are the most important cities with densely populated buildings; therefore, they were used in the investigations. The absolute energy savings are defined as the energy savings for domestic hot water services for the building, while the relative energy saving is defined as the ratio of the energy savings of the solar domestic hot water system and the total heat demand of the building.  
Figure 6 shows the energy saving of the buildings in MWh. The energy saving for hot water is lower in Guangzhou, because the hot water consumption in Guangzhou is much lower than that in Beijing and Shanghai. With an increase in stories of the building, the hot water consumption and the saved energy increase. In Figure 6, the residential buildings (building number 1-4) and the hotel buildings (15-18) have a larger fluctuations of relative energy savings, ranging from 10% to 80%. For a single-family house (Building no. 1), the relative energy saving is very low, because domestic hot water demand is much smaller than the space heating demand of the building. With an increase in the stories of the buildings, the number of occupants and the total domestic hot water demand increase proportionally. From Building no. 2 to no. 4, the ratio between the rooftop area and the total floor area decrease; therefore, the solar contribution to the domestic hot water consumption decreases from 70-75% to 30%. It is shown that the relative energy savings for Beijing and Shanghai are similar, while it is significantly lower for Guangzhou. It is most likely due to a lower hot water consumption per capita and a higher energy demand for cooling in Guangzhou where there is a hot climate. For office buildings (5-10) and commercial buildings (11-14), the relative energy savings are low, because the domestic  
Figure 6: Energy saving of the solar domestic hot water systems.  
hot water consumptions in these buildings is much smaller than those of residential and hotel buildings.