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Journal ArticleDOI

A method for evaluating both shading and power generation effects of rooftop solar PV panels for different climate zones of China

15 Jul 2020-Solar Energy (Pergamon)-Vol. 205, pp 432-445

AbstractThe photovoltaic (PV) roofs have two main energy-saving effects, which are shading and power supply. Considering the shading and power generation gain jointly, a roof is changed from the building energy end to the building energy supply end, thus changing its energy use system greatly. Therefore, this paper carries out research on the comprehensive energy-saving effect integrating the shading and the power supply gain. Three types of PV rooftops, namely, horizontally-mounted overhead PV rooftop, tilted overhead PV rooftop, and attached PV rooftop are studied to explore their impacts on the heat gain and heat loss of the roof and building’s heating and cooling load. In order to estimate the overall energy-saving in different climatic regions in China, an overall energy-saving evaluation method that considers the power generation and shading benefit effects of the PV rooftop is proposed. Based on the climate and solar radiation zones in China, 13 respective cities are selected to be included in the research. The results show that, by considering only the shading effect of PV panels, the tilted PV is more suitable in summer, reducing the heat input, whereas the horizontally-mounted PV is more effective in winter to prevent more heat loss. Regarding the overall energy-saving that considers both the shading and power generation effects of PV panels, building with horizontally-mounted PV rooftop has the highest efficiency in the summer season, while the building with tilted PV rooftop has the highest efficiency in the winter season. The model and analysis of the overall energy-saving presented in this work can provide a guide for the application of rooftop solar PV panels in different climate zones in China.

Topics: Photovoltaic system (58%), Solar gain (51%), Cooling load (51%)

Summary (2 min read)

1. Introduction

  • Due to the conventional energy shortage and environmental deterioration, the development and utilization of renewable energy have become inevitable in order to overcome the current energy crisis.
  • Since PV panels are most commonly installed on building rooftop (Oliver and Jackson, 2001), numerous studies on the energy-saving performance of PV rooftop have been conducted.
  • By substituting the comprehensive air temperature into the Eqs. (14) and (15), the indoor heat gain (heat loss) and heating load of the roof can be obtained, respectively.
  • In order to illustrate the influence factors to the overall energy-saving efficiency, the test data were analyzed using SPSS 26.0.

3. Experimental results and model validation

  • Therefore, the surface temperature at night of the PV rooftop was generally higher than that of the ordinary roof, and the effect of horizontal overhead PV roof is more obvious.
  • This was mainly because the upper surface of the control body of the attached PV module received all solar radiation, just like the ordinary roof.
  • 3. Heating and cooling load results 8. Compared with the ordinary roof, the heat gain caused by the short-wave solar radiation of the PV roofs was sheltered, and the heat loss caused by the PV roof’s long-wave radiation was reduced.

4. Analysis and discussions of overall energy-saving performance

  • The overall energy-saving efficiency of a PV roof is affected by both outdoor air temperature and solar energy resources.
  • In areas with hot summer and cold winter, the daily total heat loss and the peak heating load of the firmly-attached PV roof are higher than those of the ordinary roof.
  • The overall energy-saving efficiencies of the selected cities in the winter are shown in Fig. 16.
  • Also, the heat insulation of the PV modules was stronger than the shading effect.
  • On the other hand, in the winter, the solar radiation on the inclined surface is higher than that on the horizontal surface, compared to the horizontally-mounted overhead PV roof, the tilted overhead PV roof obtained heat more.

5. Conclusions

  • In addition, 13 typical cities in 5 climatic regions of China are selected for the investigations to evaluate the overall energy-saving performance of three PV roof types.
  • The roof with a horizontal PV had the highest efficiency of 0.32.
  • Among the selected regions, in the cold and severely cold regions with a large temperature diurnal range and low average temperature, the energy-saving efficiency was relatively high.
  • For the tilted PV roof, this paper took 30° as the inclined angle, but the specific installation and the actual energy-saving effect should be determined after detailed analysis according to the local latitude and other conditions.
  • The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Citations
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01 Jan 2016

1,409 citations


03 Aug 2010
Abstract: Building Heating, Ventilation and Air Conditioning (HVAC) is a major contributor to urban energy use. In single story buildings with large surface area such as warehouses most of the heat enters through the roof. A rooftop modification that has not been examined experimentally is solar photovoltaic (PV) arrays. In California alone, several GW in residential and commercial rooftop PV are approved or in the planning stages. With the PV solar conversion efficiency ranging from 5-20% and a typical installed PV solar reflectance of 16-27%, 53-79% of the solar energy heats the panel. Most of this heat is then either transferred to the atmosphere or the building underneath. Consequently solar PV has indirect effects on roof heat transfer. The effect of rooftop PV systems on the building roof and indoor energy balance as well as their economic impacts on building HVAC costs have not been investigated. Roof calculator models currently do not account for rooftop modifications such as PV arrays. In this study, we report extensive measurements of a building containing a flush mount and a tilted solar PV array as well as exposed reference roof. Exterior air and surface temperature, wind speed, and solar radiation were measured and thermal infrared (TIR) images of the interior ceiling were taken. We found that in daytime the ceiling surface temperature under the PV arrays was significantly cooler than under the exposed roof. The maximum difference of 2.5 C was observed at around 1800h, close to typical time of peak energy demand. Conversely at night, the ceiling temperature under the PV arrays was warmer, especially for the array mounted flat onto the roof. A one dimensional conductive heat flux model was used to calculate the temperature profile through the roof. The heat flux into the bottom layer was used as an estimate of the heat flux into the building. The mean daytime heat flux (1200-2000 PST) under the exposed roof in the model was 14.0 Watts per square meter larger than under the tilted PV array. The maximum downward heat flux was 18.7 Watts per square meters for the exposed roof and 7.0 Watts per square meters under the tilted PV array, a 63% reduction due to the PV array. This study is unique as the impact of tilted and flush PV arrays could be compared against a typical exposed roof at the same roof for a commercial uninhabited building with exposed ceiling and consisting only of the building envelope. Our results indicate a more comfortable indoor environment in PV covered buildings without HVAC both in hotter and cooler seasons.

7 citations


Journal ArticleDOI
Abstract: Integration of photovoltaic (PV) technologies with building envelopes started in the early 1990 to meet the building energy demand and shave the peak electrical load. The PV technologies can be either attached or integrated with the envelopes termed as building-attached (BA)/building-integrated (BI) PV system. The BAPV/BIPV system applications are categorized under the building envelope roof and facades as PV-roof, PV-skin facade, PV-Trombe wall, PV claddings, and louvers. This review covers various factors that affect the design and performance of the BAPV/BIPV system applications. The factors identified are air gap, ventilation rate, a tilt angle of PV shading devices, adjacent shading, semitransparent PV (STPV) glazing design, cell coverage ratio (CCR), transmittance, window to wall ratio (WWR), and glazing orientation. Furthermore, the results of the possible factors are compared to building locations. This review article will be beneficial for researchers in designing the BAPV/BIPV system and provides future research possibilities.

7 citations


Journal ArticleDOI
15 Jul 2020-Energies
Abstract: The aim of this research is to perform an in-depth performance comparison of ground-mounted and rooftop photovoltaic (PV) systems. The PV modules are tilted to receive maximum solar irradiance. The efficiency of the PV system decreases due to the mutual shading impact of parallel tilted PV modules. The mutual shading decreases with the increasing interrow distance of parallel PV modules, but a distance that is too large causes an increase in land cost in the case of ground-mounted configuration and a decrease in roof surface shading in the case of rooftop configuration, because larger sections of roof are exposed to sun radiation. Therefore, an optimized interrow distance for the two PV configurations is determined with the aim being to minimize the levelized cost of energy (LCoE) and maximize the energy yield. The model of the building is simulated in EnergyPlus software to determine the cooling load requirement and roof surface temperatures under different shading scenarios. The layout of the rooftop PV system is designed in Helioscope software. A detailed comparison of the two systems is carried out based on energy output, performance ratio, capacity utilization factor (CUF), energy yield, and LCoE. Compared to ground-mounted configuration, the rooftop PV configuration results in a 2.9% increase in CUF, and up to a 23.7% decrease in LCoE. The results of this research show that installing a PV system on a roof has many distinct advantages over ground-mounted PV systems such as the shading of the roof, which leads to the curtailment of the cooling energy requirements of the buildings in hot regions and land cost savings, especially for urban environments.

5 citations


Journal ArticleDOI
01 Sep 2021-Energy
Abstract: In order to promote the development of green buildings, this paper presents a technical, economic, and environmental evaluation of a residential building powered by hybrid intermittent generation systems in a mild humid subtropical climate zone in China. The technical, economic, and environmental mathematical models of hybrid systems are addressed. This study selected Guiyang city, which is a typical mild humid climate zone. The results revealed that the 30 kW grid-connected system for the building was the most economical with a net present cost of $ 28,041 and cost of energy of 0.069 $/kWh, whereas this was the least environmentally friendly form of power generation, emitting a maximum amount of CO2 of 26,609 kg/yr. From an economic and environmental perspective, grid/photovoltaic (PV)/wind hybrid systems in on-grid systems may be a better choice for supplying power to buildings in Guiyang. If the extension of the power grid is not feasible, off-grid PV/battery hybrid systems consisting of 115 kW PV units, 80 battery units, and a 30 kW power converter, are more suitable for supplying power to the building. Furthermore, the results indicated that wind power is not suitable for supplying power to buildings in Guiyang, mainly due to relatively low wind speeds.

4 citations


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