Rooftop Solar PV Systems

Table of Contents

Introduction

The rapid urbanization of the world has led to major changes in housing, land use and environmental effects. In these times, switching to renewable energy for our electricity demands is the best potential in terms of investment and also in terms of solving the energy crisis. However, the available land for installing large renewable energy systems seems to be depleted due to rapid construction and architectural developments.

So to accommodate these obstacles, a type of solar photovoltaic (solar PV) system had been developed which is known as rooftop solar PV systems. This is a type of solar PV system that involves the generation of electricity using solar panels mounted on the rooftops of residential, commercial or industrial buildings or structures.

The rooftop systems include solar modules, inverters, electrical accessories and mounting cables and systems. These systems usually have power capacities in the megawatt range. Residential buildings usually vary between 5-20kW whereas commercial structures/buildings have power capacities reaching up to 100kW. This system mitigates the need for extra land by using the large rooftops of buildings and integrally alleviates environmental concerns.

They can also be developed as hybrid systems by combining with other power components such as wind turbines, batteries, generators, etc. whether they are on-grid or off-grid systems [1]. Therefore, to shed light on this system and its potential, this article will be elaborating on solar PV rooftop systems, its components, mounts, exporting of generated electricity and some of its technical challenges.

Factors affecting Rooftop Mounting

Some of the factors that can affect the performance of rooftop solar systems are mentioned below:

  • Latitude
  • Weather conditions
  • Time of the year
  • Shading from adjacent structures and/or vegetation
  • Roof Slope
  • The orientation of roof mounts
  • Airflow: More airflow is better because it helps to cool down the panels which aid in maintaining the voltage levels at an optimum. Higher the voltage, the higher the power and the output energy because [2,3]:

Power = Voltage x Current

Energy = Power x Time

Components of a Rooftop Solar PV System

A rooftop solar PV system consists of several components all of which have to be accommodated on the roofs of different building structures. The components which make up a rooftop solar PV system are:

1] Solar Panels: These devices are commonly made from silicon and are comprised of multiple solar cells which absorb sunlight and use the energy from the sun, photon energy, to generate electricity. Solar panels are often laminated and protected by tempered glass and frames to protect them from any damage which can affect the performance of electricity generation.

2] Inverters: Rooftop solar systems are connected to either micro-inverters or string inverters. These devices convert the DC power from the panel into AC power which can be sent to the grid.

An image of a micro-inverter
An image of a micro-inverter [4].

3] DC/AC wiring: These are wires which connect inter-connect panels and which connect panels to inverters. Such cables and wires should not be hanging from the roofs or touching roof surfaces to protect them from degradation and weathering.

4] Mounting clamps: These are usually made from aluminium and stainless-steel brackets and bolts which secure the solar panels to the roof, to the rails and each other. These clamps often vary in design to accommodate the different types of roof materials and orientations.

A mounting clamp used for solar PV installations
An image of a mounting clamp used for solar PV installations [5].

5] Rails or racking: These structures are often made in parallel orientation to the roof and are made with metals. They are levelled with the roofs so that the panels can be mounted securely and evenly.

Rails are used to connect solar panels to the roof
Rails are used to connect solar panels to the roof [6].

6] Mounts: These structures attach the rails to the roofs using bolts or flashings into the rafters or trusses of the roof. These structures also vary in design to accommodate the different roof configurations and styles.

An image of a mounting structure used for PV installations
An image of a mounting structure used for PV installations [7].

7] Flashings: These are materials such as metal plates which act as a water-resistant seal between the roofs and the mounts to prevent water damage [8-14].

An image of ‘flashing’ done on rooftop solar panels
An image of ‘flashing’ done on rooftop solar panels [14].

Different Types of Rooftop Mounting Systems

The mounting of solar panels on rooftops depend on the slope of the roof and for residential building, the roof mounts are aligned with the slope of the roof. For commercial or industrial buildings, the roofs are often flat and there are roof mounts for such systems as well. These are covered below:

1] Steep-Sloped Roof Mounts: Sloped roof requires mounts which need to be penetrated or anchored into the roofs. They are common for residential installations and are classified as flushed mount, shared-rail, and rail-less systems. These systems usually have solar panels oriented either horizontally or vertically with clamps attached to rails. These rails are secured on the roof by using bolts and screws.

a) Flush-Mount Systems: These systems are usually mounted with around 3-6 inches between the solar module and the roof surface. This space between them helps to keep them cool which reduce voltage losses due to temperature effects. Flush-mount systems usually are mounted on roofs which are made from asphalt shingles, tiles, wood shake shingles and metal roofs which make them versatile for residential installations. Such systems are waterproofed through a method called ‘Flashing’ which is the overlapping of materials to prevent the intrusion of water. This creates a watertight seal to prevent water damage to the system.

An image of flush-mounted solar PV systems
An image of flush-mounted solar PV systems [15].

b) Shared-Rail Systems: These systems involve 4 rails attached to 2 rows of solar panels. There is one rail in the middle that is shared, and such systems require a lesser number of penetrations which makes installation quicker. The panels installed in such systems can be oriented in any direction and these rails which are clamped with panels allow the accurate positioning on panels in the desired orientation.

An image of a shared-rail solar PV system
An image of a shared-rail solar PV system [16]

c) Rail-less Systems: As the name suggests, the solar panels are directly connected to the bolts and screws in the roof wherein the frames of the panels act as rails. These systems are lower in costs due to lesser manufacturing components and shipping costs but they still need the same amount of attachments as that of a shared-rail system. Panels can be oriented in any direction and are not limited by the rigidity of the rails and the installation time is faster for such systems.

An image of a rail-less solar PV system
An image of a rail-less solar PV system [17]

2] Low-Slope Roof Mounts: These roof structures are most common for commercial or industrial buildings where the roof can even be flat. Due to the nature of the roof, PV systems installed on such roofs do not have frames and only have PV laminates. This is because if the panels get soiled through dirty water, snow or any other matter, the flatness or low slope of the roof prevents the particulates from sliding down. If frames are used, the dust and dirt will accumulate near the frames which will increase maintenance requirements and reduce total power output. There are mainly 2 types of low-slope roof mounts and they are:

a) Penetrating Systems: These mounts attach the racking system to a strong part of the roof using structural attachments such as posts, pedestals, standoffs or jacks. The strong parts of the roof comprise of trusses, rafters or purlins which are capable of withstanding the weight of the PV systems.

An image of a penetrating solar PV system
An image of a penetrating solar PV system [18]

b) Ballasted Systems: These mounts are not structurally penetrated to the roofs but are designed in such a way that something heavy is placed on the PV system to hold and keep it down. Materials like rectangular concrete blocks commonly known as paving stones are often used. Ballasted systems are useful when the roof is unable to take extra loads or if the geographical location is a windy region. These racking systems are also designed with wind deflectors to keep them stable in windy conditions.

An image of a ballasted solar PV system
An image of a ballasted solar PV system [19].

3] BIPV and Solar Shingles: This is the 3rd type of mount systems which involve integrating solar panels into the buildings to avoid any structural attachments. It’s designed as a built-in structure which eliminates the need for mounting systems. However, although it is aesthetic and convenient, these panels tend to operate at higher temperatures due to lower airflow which will affect the overall efficiency of the PV system. It’s also more expensive since it’s not mass-produced like other solar technologies. The concept of BIPV is covered in another article.

An image of Building Integrated Photovoltaics (BIPV)
An image of Building Integrated Photovoltaics (BIPV) [20].

Exporting Electricity to the Grid

Owning a grid-tied solar PV system irrespective of whether they’re ground-mounted or roof-mounted can offer opportunities to send your generated electricity to the grid which can lead to 2 types of mechanisms:

1] Net-metering mechanism: Grid-tied solar PV systems such as rooftop solar systems can avail of this type of mechanism where if the generated electricity is in excess, the consumer can export the excess electricity to the electricity grid. Depending on the amount of power exported, the consumer gets credit in return. When it’s time for the billing cycle, the consumer only gets charged for the net power which is the difference between the imported and exported power to the electricity grid. This is why it is termed as net-metering. In this mechanism, there isn’t any selling of power and this mechanism is used to adjust the electricity bill by cutting down bill costs significantly. Over time, the consumer can also generate electricity for free.

2] FIT Mechanism: This mechanism is known as the Feed-in Tariff mechanism where a grid-connected rooftop solar system can sell the generated electricity to the electric utility. The electricity that is sold can be used by the grid elsewhere. Consumers who own such solar PV systems are slowly shifting to this type of mechanism because of the revenue yield. This mechanism also enables the investor of the installer to be paid back. Utilities such as the Public Utility Commission sets a standard rate for the electricity which the utility pays for, and this rate can either be a wholesale ate or a retail rate. In either case, this mechanism has led to the solar payback period being shortened due to higher revenue yield and the installation demand for solar PV systems has increased significantly. The solar PV industry has grown exponentially in the recent decade owing to this FIT mechanism because of the thousands of jobs and revenue that are being generated from photovoltaics alone. This industry has also allowed for reduced transmission losses because of increased localization in production [21, 22].

Technical Challenges

The previous sections have elaborated on the potential of solar PV rooftop systems. However, there are some technical drawbacks when assimilating a large number of solar PV rooftop systems to the power grids. They are:

Ramp Rates: Since PV systems are dependent on sunlight, there’s a lot of variability in power generation. The effect is more pronounced when there are intermittent clouds that impede electricity generation due to the blockage of sunlight. This further affects the voltage levels in the distribution feeder by causing an imbalance in the voltage and frequency levels. More than often, this variability leads to the voltage and frequency levels to exceed the limits set unless they are accompanied by power controls in the system. There’s a formation of frequency mismatch because the centralized generators cannot match or ‘ramp’ quick enough to match with the variability leading to blackouts. This occurs due to the instability of the grid because of the constantly fluctuating voltage levels. This drawback is partially alleviated by adding storage and by distributing solar panels over a wider area on the rooftop.

Reverse Power Flow: The distribution feeders are designed radially to allow only one-way power flow which is transmitted from centralized generators to the customer loads. Now, distributed and localized rooftop solar PV systems have led to the reverse power flow where there are chances for power to flow back to the transformers and sub-stations. These can cause significant negative impacts on the voltage regulation, protection and coordination of devices being used for such systems.

Despite these drawbacks, due to the advancements of technology and the timely realization of these drawbacks, performance monitoring systems using smart technology and AI have been developed to counteract these negative effects and resolve these issues accordingly. 

Conclusion

These days, consumers and households are shifting more towards solar and alternate energy technologies. In the past decade, solar has been revolutionized to suit the needs of the general public because it was initially seen as the’ technology for the rich’. Nowadays, due to rapid advancements and lowering costs, many middle-class income consumers have also started to purchase and invest in solar technology especially solar rooftop systems due to its convenience and benefits which outweigh the negatives. The local, federal and state governments and utilities have also developed many incentives to shift towards renewables due to the growing concerns in electricity demand and supply. Therefore, rooftop solar PV systems have been growing in capacity in the recent past to accommodate the changing trends in the market. This is why it is essential to consider such systems as it has been covered in the article. From factors which affect rooftop systems, types of mounted structures, the export of electricity to technical challenges, this article has covered the basics of everything you need to know about solar rooftop systems. This article also aims to bring an understanding and positively change the mindset as to why shifting to renewables is the future of our industry!

Image References and Bibliography

[1] “Photovoltaic power generation in the buildings. Building integrated photovoltaic–BIPV” (PDF). bef-de.org. Retrieved 2011-06-20.

[2] “Energy Resources and Resource Criteria”. greenip.org. Archived from the original on 2013-08-28. Retrieved 2011-06-20.

[3] https://www.pdfdrive.com/solar-photovoltaic-basics-a-study-guide-for-the-nabcep-entry-level-exam-e175092443.html

[4] https://12vsolarenergy.com/products/300w-on-grid-inverter

[5] https://solarbuildermag.com/mounting-solutions-guide/what-to-look-for-when-selecting-pv-clamps/

[6] https://www.indiamart.com/proddetail/aluminium-micro-rail-pv-mounting-system-19516498755.html

[7] https://solarbuildermag.com/products/four-energy-dense-solar-mounting-systems-for-commercial-rooftops/

[8] “Module Structure | PVEducation”. www.pveducation.org. Retrieved 2019-05-08.

[9] “Solar Panel Racking for Roof and Ground Mount Solar”. www.wholesalesolar.com. Retrieved 2019-05-08.

[10] “Anatomy Of A Rooftop Solar Mounting System”. Solar Power World. 2014-03-19. Retrieved 2019-05-08.

[11] “Solar Panel Racking for Roof and Ground Mount Solar”. www.wholesalesolar.com. Retrieved 2019-05-08.

[12] “Anatomy Of A Rooftop Solar Mounting System”. Solar Power World. 2014-03-19. Retrieved 2019-05-08.

[13] “Anatomy Of A Rooftop Solar Mounting System”. Solar Power World. 2014-03-19. Retrieved 2019-05-08.

[14] https://solarbuildermag.com/mounting-solutions-guide/keep-simple-solar-advice-products-streamline-next-rooftop-install/

[15] https://www.ecocetera.com/projects/roof-integrated-solar-pv-system/

[16] https://www.kbracking.com/solar-panel-mounting-systems/

[17] https://www.solarpowerworldonline.com/2017/02/different-types-solar-mounting-systems-roofs/

[18] https://www.solarelectricsupply.com/residential-solar-systems/flat-roof-mounted

[19] http://www.solarpanelsplus.com/products/PV-flat-roof-ground/

[20] https://worldofrenewables.com/new-report-building-integrated-photovoltaics-market-size-will-reach-14200-million-usd-by-2024/

[21] “Net Metering”. SEIA. Retrieved 2020-04-17.

[22] “Photovoltaic power generation in the buildings. Building integrated photovoltaic–BIPV” (PDF). bef-de.org. Retrieved 2011-06-20.

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