Soaking up The Sun: Maximizing the Potential of Urban PV
In the past, urban photovoltaics (PV) have not been a viable option for many development projects, as the panels and support systems were quite expensive to purchase and install. In addition, lower panel efficiencies meant that only areas with very high exposure to the sun could hope to recoup the cost in a reasonable time frame. Thus, PV in dense urban environments remained an aspirational goal for many designers.
However, this is changing rapidly.
Between 2010 and 2020, residential and commercial PV system benchmark costs dropped by 64% and 69% respectively. This was partially driven by a roughly 30% increase in the efficiency of a typical solar panel.  In addition to decreasing costs and increasing efficiencies, building codes have been updated to reduce uncertainty in installation requirements, and battery and storage technology advances now allow the end user to use the power when needed.
Thus, it is becoming more and more common for PV to be included in building projects.
What Are Building-Integrated Photovoltaics?
The traditional approach of adding roof-mounted PV panels and racking to an existing design is still a popular choice. However, new technologies have also allowed PV to be directly integrated into the building envelope. Such systems are known as building-integrated photovoltaics (BIPV). Examples include using PV systems as:
- external sunshades,
- spandrel panels, and
- semi-transparent options with PV cells embedded in windows and skylights.
This approach opens the potential for power generation across more of a building’s skin with less impact on its aesthetic.
Such technologies are an important step in decarbonizing our energy production and making our buildings more resilient. However, as the use of PV systems in cities grows, new challenges have emerged.
Densifying Cities, Solar PVs, And The Problem With Shadows
Cities cannot sprawl forever, so to support their growing populations, they must become denser. Unfortunately, increased density can come with side effects – namely larger or more frequent shadowing.
Shadows cast by buildings have, in the past, been criticized for their impacts on parks, schools, and other buildings. Given the increased emphasis local governments have placed on ensuring quality outdoor spaces, the criticisms have only gotten louder.
These shadows also reduce the amount of solar energy that will fall onto the PV systems, thereby reducing production and lengthening payback periods for owners. Some PV designs can be quite sensitive to even small amounts of shadowing. In fact, in extreme instances, new shadows can make PV systems on an existing building ineffective to the point of being useless. This is not only a detriment to the owner of the affected system, but also the overall environment, since it reduces the amount of lower carbon energy flowing to the grid. The natural question that follows is: what recourse does an owner have when this occurs?
The Right to Light
‘Right to light’ laws have existed in the United Kingdom for decades. These laws are intended to ensure that the amount of sun and sky a property has historically enjoyed is protected against encroachment by new construction. The laws also define a framework by which owners can be compensated if an encroachment occurs.
These laws, unfortunately, are currently only applicable to windows, not PV systems. However, as mentioned previously, one newly available option is to install windows where the panes of glass have integrated PV cells. Thus, the windows are not only performing their basic function of letting light in, but also generating solar energy through the PV cells.
While such a case remains a hypothetical, it does present a potential approach to protect a PV system’s solar access. However, in the UK and indeed most jurisdictions around the world, the energy production of PV panels is only indirectly protected via objections made during the planning process based on local laws around shadowing.
Regulations limiting the extent of new shadowing a building can create are some of the oldest and most widespread around the world. Their level of sophistication and rigor can vary from visually inspecting shadow diagrams for select dates and times to minute-by-minute predictions of shadow size and duration over an entire year. However, the majority do not require an assessment of the impact on PV systems and those that do often share the same flaw:
Treating all shadows as equally problematic oversimplifies the issue. For instance, when the weather is hot, would you rather be in the shadow of a building or fully exposed to the sun?
For PV systems in particular, this issue is especially complex. A PV panel performs best when sunlight is falling on its surface at a right angle. As the sun’s rays strike at more glancing angles, the cells absorb less light. Thus, to properly understand the impact of a shadow on a PV system, you need to know not just where and when shadows fall, but also the corresponding solar intensity as well as the location and orientation of the panels.
The final two factors are not included in existing guidelines, which is why misunderstandings about the actual impact of a new building on PV systems can occur.
For example, a shadow early in the morning is less significant than a shadow occurring at midday. This is because the sun is less intense in the morning and will strike the panel at a lower angle. These misunderstandings can lead to ill-will between neighbors, project delays, and even litigation. Ultimately, this makes a detailed understanding of solar access – and how it may change – critical as more and more PV is used in our cities.
Embracing urban PV enhances the resiliency of our cities while helping decarbonize our energy grid, increasing environmental sustainability. However, while this technology has matured significantly over the last decade, challenges to its widespread adoption remain.
The inclusion of urban PV is only one portion of effective solar design. Discover how RWDI is working with clients to help them understand how the sun can interact with their building projects by visiting our Solar Studies service page.