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How Solar Easements Provide PV Peace of Mind

Posted by Lisa Cohn on Jan 17, 2020 2:46:18 PM

A solar installation is a significant investment for your customers—and the last thing they want is to have that investment threatened. However, in some cases, future changes on surrounding property—such as a new building on their neighbor’s property or the growth of vegetation on adjacent land—could jeopardize the amount of irradiance their PV system receives, and thus how much energy it produces.

There are options for solar customers to avoid these kinds of unpleasant surprises, however. Solar easements give solar system owners the right to negotiate with neighbors for access to unobstructed sunlight on their solar systems, according to the Solar Energy Industries Association (SEIA).

In today’s article, we delve into what solar easements are, how they work, and some of the considerations that solar customers should be aware of if they are interested in pursuing easements. If this question comes up, whether in the sales process or in your post-sale relationship with customers, you’ll be well prepared to help.

Protecting Against Shading of Solar Systems

“Solar easements are important in ensuring that a homeowner’s solar panel system is producing optimal levels of electricity. Shade plays a large role in solar electricity production, and easements protect [homeowners] against that happening,” says Ken Pedotto, CEO of Solar Simplified.

Specifically, solar easements restrict what your neighbors can build or grow on their property; they prevent neighbors from blocking sunlight to your solar panels, he says.

Solar Access Laws vs. Solar Easements

Easements differ from solar access laws. Solar access laws limit private restrictions on solar energy projects, says SEIA. For example, rules from homeowners’ associations are a common challenge that restrict how solar systems on homes or businesses are installed. Such rules are generally created to uphold a community’s aesthetic standard.

When a solar system owner is grappling with solar access challenges from neighbors or homeowners’ associations, it’s important to ensure that the owner’s rights are protected. “To put it simply, a solar easement allows homeowners to legally protect their access to sunlight,” says Pedotto.

There are two types of easements: affirmative easements, the right to use land owned by another entity; or a negative easement, which restricts a property owner’s use of land.

A solar easement is viewed as a negative easement because it prohibits property owners from using their property in a way that prevents sun from reaching a solar energy system on a neighboring property.

Potential Challenges of Obtaining Solar Easements

It’s not necessarily easy to obtain a solar easement. That’s because the easement must be granted by a neighboring property owner—and the property owner can refuse to negotiate or grant the easement.

It’s also important to recognize that negotiating easements can sometimes be costly. “Legal costs could exceed the cost savings of the system if neighbors are not willing to grant the easement for free,” says EPIC. “Depending on the density of houses in a neighborhood, a prospective solar energy system owner might have to negotiate with several neighbors to ensure access to sunlight.”

Different states have different policies and protections regarding solar easements. For instance, the California Solar Rights Act gives local governments the ability to require solar easements in subdivision developments under certain circumstances, according to the Energy Policy Initiatives Center (EPIC). We delve into a few key state-level policies below.

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New York State and Montana Laws

New York State is one of many states that protect a solar system owner’s right to ensure enough sun reaches the system. The state’s law states that solar easements should be created in writing. The document should include vertical and horizontal angles, provided in degrees, that identify the area that is the subject of the easement.

In addition, the law calls for terms and conditions under which the easement would be granted or terminated. New York State also expects the document to include any provisions for compensating the solar system owner if the other party interferes with access to solar.

Montana's law is similar to New York State’s, calling for a written document that includes the location of the easement.

California Solar Protection Laws

In California, two laws protect solar system owners: The Solar Rights Act and the Solar Shade Act, according to Go Solar.

The Solar Rights Act, AB 3250, passed in 1978, includes measures giving consumers access to sunlight and preventing shading. It also restricts efforts by homeowner associations and local governments to prevent solar system installations. The act gives citizens a legal right to a solar easement.

“Even though the law is more than 30 years old, the Solar Rights Act contributes significantly to California's strong policy commitment to solar energy, and the policy rationale for the Act is relevant today and continues to support California's solar energy policy initiatives,” says Go Solar.

California’s Solar Shade Act

While the Solar Shade Act (AB 2321) doesn’t create solar easements, it does provide some protection to solar system owners from shade created by trees on neighboring properties.

When trying to decide whether it makes sense to pursue a solar easement, it’s a good idea to look to the court system to uncover how such laws are implemented.

For example, one California court, in an unpublished portion of its opinion, held that a solar easement is only enforceable if it is in writing, according to EPIC.

A document that creates a solar easement should identify the location of the easement in “measurable terms,” the court said. It should also include “restrictions that would impair or obstruct the passage of sunlight through the easement” as well as “the terms or conditions, if any, under which the easement may be revised or terminated,” according to EPIC.

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Local Governments in California Can Establish Solar Protection Ordinances

An ordinance could establish solar easements designed to ensure that each parcel has access to sunlight, especially across adjacent parcels or units in a subdivision. In most cases, it’s critical that the parties create a written document.

In the unpublished portion of its opinion in the Zipperer v. County of Santa Clara (California) case, the court specifically discusses the need for written documentation of solar easements, says EPIC.

“The Zipperers built a home with solar heating and cooling systems in the mid-1980s. In 1991, the County of Santa Clara purchased an adjacent property containing a small grove of trees,” says EPIC. The trees grew “significantly,” and began to shade the Zipperer home, which hurt the solar system’s output.

“In 1997, the Zipperers requested that the County trim or remove the offending shading trees.The County did not respond to the Zipperer’s request, and instead passed an ordinance exempting itself from California’s Solar Shade Control Act.”

The Zipperers sued the county, arguing a breach of contract associated with an “implicit” right to a solar easement. The Zipperers argued that the county had implicitly entered into a contract to provide a solar easement because the county allowed the homeowners to build a home according to county requirements.

However, the court said that a written, not “implicit” solar easement was needed. “Therefore, because the Zipperers did not have an express, written instrument, the court held that no solar easement existed.” The Zipperer case highlights how important it is that a solar easement be in writing.

Deciding If a Solar Easement Makes Sense

A written solar easement can offer assurance that a solar installation will continue to produce electricity at its full potential—particularly valuable in cases where there is potential for changes on surrounding property that could affect the system’s irradiance.

However, the ease of getting one of these agreements in place is an important consideration. As Pedotto says, “Not only can solar easements be costly, but they can cause conflict with neighbors.”

Pedotto suggests that an alternative to pursuing a solar easement in these cases is for homeowners consider community solar projects, which help bring neighbors together with shared access to local solar farms.

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Topics: shading losses, solar policy

Four Steps to Optimize PV System Performance in Shaded Conditions

Posted by Christopher Hopper on Apr 26, 2017 12:00:00 AM

Assessing the impact of shading on system performance is an essential step in solar design. Especially in the residential market, solar designers often deal with shading issues like trees, chimneys, and other obstructions.

The question designers often face then is: given these conditions, how do I maximize system performance?

This is not an easy question to answer because shading can affect performance in a nonlinear way. However, an optimal solution can be found by following four steps.

1. Quantify. What’s the best way to quantify the impact of shading? Solar Access measurements are useful as a guide but have limitations because they do not account for system configuration. Thankfully, modern, module-level simulation engines can accurately model the impact of shading on performance. (Simulation engines that model system performance at a submodule level offer even more accurate estimates of energy production when modules are partially shaded.)

Aurora system loss diagram Aurora’s performance simulation feature quantifies energy losses from shade and other sources.

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2. Explore. Systems come in countless combinations of size, location, and configuration. Designers have several options to mitigate shading losses:

  • When using string inverters, string the modules in a manner that minimizes shading losses. In some cases, simply changing the stringing configuration of a system can result in energy boosts of over 10%.
  • Use module-level power electronics (microinverters or DC optimizers). In addition to mitigating the impact of shading by performing maximum power point tracking (MPPT) on a module-by-module basis, they eliminate module mismatch losses (which occur when one shaded module causes the current through an entire string of modules to drop).
  • Use modules with integrated cell string-level power electronics. These replace the bypass diodes inside modules with DC optimizers that perform MPPT at the cell string-level.
  • Increase the DC-to-AC ratio to reduce balance of system costs (the costs of all components of a PV system other than the PV panels, such as wiring, racking, and inverters). Shading results in lower DC power, thus more panels can fit on an inverter without causing significant clipping.
  • Explore the use of higher efficiency modules or modules with a higher open-circuit voltage (or Voc — the maximum voltage available from a solar module). A higher power density allows you to make better use of low-shade areas, whereas shorter strings are impacted less by shading.
  • Vary the location and size of the system. Energy produced at different times of the day can have different values to the client.

3. Assess. Cost-benefit analysis ultimately leads to the “optimal” system design. But value is not always easy to determine. With Time of Use and tiered rates, the compensation for each kWh produced by the system can vary greatly. It can sometimes be better to place a system where there is more shade overall, but the system remains unshaded during high-value times. A detailed financial model that can handle these nuances is essential.

An example of a Time of Use rate, with higher energy prices at times of higher energy demand.

4. Automate. A detailed site assessment takes time. Without the ability to quickly assess a variety of design variants, how many iterations can you afford to do? And how do you know if your design is really the optimal solution? Putting a streamlined process in place is crucial.

We believe software can empower solar designers to make these critical design decisions in an efficient manner. That is why we built Aurora to streamline the design process from remote shade measurements and module-level performance simulations, pricing and detailed financial analysis, all the way to proposal generation — and Aurora’s AutoDesigner even generates an optimal system design for you at the click of a button.

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Topics: solar design, shading losses, solar engineering

The Importance of Modeling Global Maximum Power Point Tracking

Posted by David Bromberg on Mar 28, 2017 12:00:00 AM

When modeling how much energy a solar design will produce, there are many features of the components that must be taken into account to ensure an accurate estimate. One important factor that modeling software must account for in order to avoid over- or under-estimating the system’s energy production is whether or not the inverter(s) used are capable of “global maximum power point tracking.” In this post, we will examine what global maximum power point tracking means—and why accounting for it is so important.

Understanding Current-Voltage and Power-Voltage Curves

The datasheet of a solar panel includes a variety of data that allow one to understand the basic parameters of the device and to mathematically model its behavior within an electrical circuit. Typically, this will include graphs that illustrate the panel’s “current-voltage curve”—also known as an IV curve, for the standard abbreviations for current (I) and voltage (V) in mathematical equations—and “power-voltage curve.”

Looking at the power-voltage curve allows us to see the point (or points) at which the panel’s power output is maximized.

These graphs illustrate relationships between three electrical characteristics: current, voltage, and power. Power—which we intuitively understand as the energy produced by the panels—is defined as the rate, per unit time, at which electrical energy is transferred by an electric circuit. Current (I) is the rate at which charge is flowing through the circuit, while voltage (V) is the difference in electric potential energy between two points (e.g., the output wires of a solar panel) per unit electrical charge.

A common example used to explain these principles is to think of electricity like water in a tank; the pressure in the tank is analogous to the voltage, while current would be the flow of water out of the tank (Figure 1). The IV curve shows how the panel output current varies with panel output voltage. The power-voltage curve shows how panel output power (the product of the output current and output voltage) varies with panel output voltage.

diagram illustrating voltage and current Figure 1: The concepts of voltage and current as illustrated by the example of water in a tank.

Looking at the power-voltage curve allows us to determine the point (or points) at which the panel’s power output is maximized. On the IV curve, two values that are often indicated are “Vmp” and “Imp” — which indicate the levels of voltage and current at which the solar panel’s output power is maximized under standard test conditions (STC). Nothing about the panel itself dictates it must operate at maximum power, however; any point along the IV curve is a valid operating point. In designs using string inverters, it is the inverters that “choose” the operating point. The ability of the inverters to locate the operating point of a solar array at which output power is maximized is referred to as maximum power point tracking (MPPT).

If the solar array comprises identical solar panels operating under the same irradiance and at the same temperature—such that each constituent module has the same IV curve and maximum power point—the net IV curve of the entire array (which takes into account the IV curves of each individual module) will have a shape like the blue curve in the left half of Figure 2 below. The green curve shows the output power of the array as a function of output voltage; note that there is a single peak in power, occurring at the “knee” of the IV curve. The inverter will seek out this one point at which array power is maximized.

Accounting for Shade: The Role of Bypass Diodes

When parts of the array are shaded, however, the IV curve is much more complicated. The IV curves of the shaded modules are different than those of the unshaded modules, especially in regard to how much current the shaded modules can output. When the amount of irradiance on a module is low, the power of the entire string connected to the module can drop. This is due to the fact that the current through the string can only be as high as the current through the most shaded module.

Because bypass diodes allow the inverter to “skip over” shaded panels instead of operating at their lower current, the IV curve of an array that is partially shaded will look different than that of an unshaded array.

To help mitigate these effects, manufacturers integrate bypass diodes into their modules. A bypass diode can be thought of as an on/off switch, which conducts any amount of current when it is “on” and, conversely, cannot conduct current when it is “off.” When the diode is turned on, it effectively shorts out the shaded module by routing the string current through the diode (and around the module) rather than through the shaded solar cells.

Because bypass diodes allow the inverter to “skip over” shaded panels instead of operating at their lower current, the IV curve of an array that is partially shaded will look different than that of an unshaded array. The resulting IV curve may look like the blue curve on the right in Figure 2, with a corresponding power-voltage curve shown in green. As you can see, there are two distinct operating points at which power is “maximized” - a global maximum where the array operates at a higher current and lower voltage, and a local maximum where the array operates at a lower current and higher voltage. The global maximum occurs when the shaded modules are bypassed, and the local maximum occurs when the shaded modules are not bypassed.

Comparison of current-voltage and power-voltage charts for unshaded and shaded solar arrays Figure 2: (left) Current-voltage (blue) and power-voltage (green) curve of a solar array with no shading; (right) current-voltage (blue) and power-voltage (green) curve of a solar array with shading, where the activation of bypass diodes results in multiple possible maximum power points.

Global MPPT refers to the ability of an inverter to sweep the IV curve of the solar array (within the operating voltage limits of the inverter) and find the array voltage at which the global maximum power point occurs. How often the inverter sweeps the curve, and the resolution at which it does so, is generally manufacturer- and model-specific.

Importantly, not all inverters perform global MPPT. Some inverters are limited to only search for the maximum power point in a local region where it “usually” lies, a high voltage solution where no modules are bypassed. This can be beneficial for sites where there is no shading, because whenever the inverter is sweeping the IV curve searching for the maximum power point it is not actually operating at the maximum power point, and thus not producing as much energy as it could. If the maximum power point is not going to vary much because there is no shade and no reason to activate bypass diodes, then there is no reason to sweep the entire IV curve. Most modern residential inverters are capable of global maximum power point tracking because shading due to trees and obstructions is common and expected. Large commercial inverters and central inverters, however, may not have this functionality because it is generally assumed there will not be much shading.

Importantly, not all inverters perform global maximum power point tracking. Some inverters are limited to only search for the maximum power point in a local region where it “usually” lies, a high voltage solution where no modules are bypassed.

Modeling Global Maximum Power Point Tracking

If your design includes a string inverter with global MPPT functionality, it is critical that the simulation tool you use to model the system accurately represents that behavior. Consider the residential design in Figure 3, which includes two parallel strings connected to an input of an inverter and a third string connected to another input. The irradiance map (left) and 3D model (right) clearly show the effects of shade on this site. Of particular concern are the chimney on the southeast-facing roof plane and the large tree to the west of the house, both of which cast shade on several panels in the design at various times throughout the year. If we simulate this design without global MPPT, the annual production is 5.94 MWh. However, if the inverter actually does perform global MPPT, and we simulate it accordingly, the production estimate increases to 6.25 MWh (Table 1).

2D and 3D models of a residential solar design with shadeFigure 3: 2D view and irradiance map (left) and 3D view (right) of a residential design with shading from a chimney and tree, produced by Aurora Solar's software.

Annual Production Without Global MPPT Annual Production With Global MPPT Percent Difference
5.94 MWh 6.25 MWh 5.09%
Table 1: Annual energy production for a residential design with and without global maximum power point tracking.

Given the results shown in Table 1, it is clear that knowing when to model global MPPT is just as important as being able to model it at all. Assuming every inverter has this functionality is dangerous, because it could lead to severely underperforming systems post-install. Assuming no inverter has this functionality can be a costly mistake as well, because it may lead the designer to install a larger system size than necessary.

This is why Aurora has contacted leading inverter manufacturers to confirm exactly which inverter models perform global MPPT. If a design includes an inverter with this functionality, Aurora will automatically model it. Aurora will even model global MPPT and bypass diodes down to the cell string-level, including the power losses in the diodes themselves. If Aurora has not confirmed the inverter has global MPPT, or that the inverter only performs local tracking, this behavior will not be modeled. The performance simulation logs will indicate whether or not the simulation applied global MPPT. In this way, designers can be sure they are getting simulation results that are as accurate as possible given what is known about the equipment in their designs.

Takeaways

  • Global MPPT allows an inverter to sweep the IV curve of a solar array to find the point at which output power is maximized, even under partial shading.
  • We found a difference of over 5% in annual production when simulating a design with an inverter that has global MPPT versus one without it.
  • Aurora has worked with leading inverter manufacturers to confirm which models apply global MPPT and automatically simulates this behavior for those inverters.

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    Cover photo credit: NREL/DOE

Topics: solar design, power, shading losses, electric current, bypass diodes, components, solar engineering

Shade Losses in PV Systems, and Techniques to Mitigate Them

Posted by Christian Brown on Oct 7, 2016 8:01:00 AM

Effects of Shade on PV Output

Since PV systems generate electricity based on the amount of sunlight they receive, it makes sense that when a shadow is cast on a panel, for example by a nearby tree, its power output decreases. However, the decrease in power could be a lot worse than it initially seems.

Solar panels with shadeFigure 1. Solar panels in partial shade. Source: lowcarbonlivingblog.wordpress.com

Intuition suggests that power output of the panel will be reduced proportionally to the area that is shaded. However, this is not the case. In his book Renewable Energy and Efficient Electric Power Systems Stanford University’s Gil Masters demonstrates how shading just one out of 36 cells in a small solar module can reduce power output by over 75%.

Waterflow Analogy

To conceptualize why shading results in such severe losses, it is helpful to use the analogy of water flowing in pipes. The flow rate of water through the pipe is constant, much like the current through a cell string is constant for a given irradiance level.

Current in Panels as Water in PipeFigure 2. Analogy of a water pipe to a string of solar cells.

Shading a solar cell is similar to introducing a clog in a pipe of water. The clog in the pipe restricts the flow of water through the entire pipe. Similarly, when a solar cell is shaded, the current through the entire string is reduced.

Shaded Cell as Clog in PipeFigure 3. A shaded solar cell is similar to a clog in a water pipe.

This is significant because every cell in the cell string has to operate at the current set by the shaded cell. This prevents the unshaded cells from operating at maximum power. Therefore, only a small amount of shading can have a dramatic effect on the power output of a solar panel.

Similar principles apply to PV modules connected together. The current flowing through an entire string of modules can be heavily reduced if even just a single module is shaded, leading to potentially significant loss of power output.

Approaches to Reduce Shading Losses

Fortunately, there are a number of different approaches that can be applied in PV system design to reduce shading losses. These include the use of different stringing arrangements, bypass diodes, and module level power electronics (MLPEs).

Stringing Arrangements

Modules connected in series form strings, and strings can be connected in parallel to an inverter. The current through all the modules of a string has to be the same, and the voltage of parallel strings has to be the same. As we saw in the last section, a shaded module in a string can bring down the power output of the string significantly. However, a shaded module in one string does not reduce the power output of a parallel string. Therefore, by grouping shaded modules into separate strings, the overall power output of the array can be maximized.

For example, in a commercial system with parapet walls, it can be beneficial to group modules that receive shade from the parapets into strings, and keep modules that do not receive shade from the parapets in separate, parallel strings. This way the unshaded strings can maintain a higher current and power output.

Stringing in Series vs. Parallel DiagramFigure 4. PV arrays with modules connected in series (left) and in parallel (right).

Bypass Diodes

Bypass diodes are devices within a module that allow the current to “skip over” shaded regions of the module. By utilizing bypass diodes, the higher current of the unshaded cell strings can flow around the shaded cell string. However, this comes at the expense of losing the output of the cells that are skipped over.
Although it would be theoretically ideal to have a bypass diode for each solar cell, for cost reasons a typical solar module will have three bypass diodes, effectively grouping the cells into three series cell strings (Figure 5). For instance, a 60-cell module will typically have one bypass diode for every 20 cells.

PV module containing three cell strings in series, each with a parallel bypass diode. Figure 5. PV module containing three cell strings in series, each with a parallel bypass diode.

Module Level Power Electronics (MLPEs)

MLPEs are devices that are attached to individual modules in order to increase performance under shaded conditions (though there are other benefits, such as mismatch mitigation and module-level monitoring). This is done by performing maximum power point tracking at the module level. MLPEs include DC optimizers and microinverters.

DC Optimizers:

A DC optimizer adjusts its output voltage and current to maintain maximum power without compromising the performance of other modules.

For instance, when a shaded module produces electricity with a lower current, the DC optimizer will boost the current at its output to match the current flowing through the unshaded modules; to compensate, the optimizer reduces its output voltage by the same amount it boosts the current. This allows the shaded module to produce the same amount of electrical power without impeding the output of other modules. A system utilizing DC optimizers still needs an inverter to convert electricity from DC to AC.

Microinverters:

As opposed to having a single inverter servicing all of the panels, each panel can have a small inverter attached to it to convert its output from direct current (DC) to alternating current (AC). Since each microinverter has an MPPT, and their outputs are connected in parallel, each panel will operate at its maximum power point, without impacting other panels.

Simple schematics of PV systems utilizing MLPEs Figure 6. Simplified schematic of a PV system utilizing microinverters (top) and a PV system utilizing DC optimizers (bottom).

Effects of MLPEs on PV System Performance

Using Aurora’s simulation engine, we compared the performance of three different PV systems subject to significant shading. As shown in Figure 7, we placed a 3.12 kW system near the edge of a roof, which has tall trees next to it. Note that while this design effectively showcases the performance difference of these system topologies in shaded conditions, it is not an optimal—or even a practical — design. Our findings are summarized in Table 1.

The Palo Alto home used for MLPE performance simulations Figure 7. The system analyzed for this case study featured a 3.12 kW system that is partially shaded by trees.

Table 1. Results from performance simulations of PV system on a Palo Alto home utilizing different MLPE components. The difference between the two MLPE outputs is attributed to the differences in their inverters' efficiencies. Source: Aurora Solar.

System Topology Annual Yield Improvement with MLPEs
String Inverter 2,585 kWh/year N/A
Microinverters 3,033 kWh/year +17.3%
DC Optimizers 3,035 kWh/year +17.3%

Our results show that using MLPEs under these conditions increases system output by 17.3% annually, showing the benefit of using these components for shade mitigation. Additionally, the effective yield of a system using a microinverter or a DC optimizer is approximately the same, although there could be small differences (on the order of 1%) in some cases due to differences in efficiency curves.

For the same reason that they can mitigate shade losses by decoupling module output, MLPEs can eliminate module-to-module mismatch losses. These losses are typically caused by manufacturing variations that lead to slight differences in the electrical characteristics of two modules of the same type. Since MLPEs allow the modules to operate independently from one another, these variations will not impact the system’s overall performance.

About Solar PV Education 101

Article 5: Shade Losses for PV Systems, and Techniques to Mitigate Them is part of Solar PV Education 101, a six-article series that serves as an introductory primer on the fundamentals of solar PV for beginners.

Article 1: The Beginner's Guide to Solar Energy
Article 2: How a Photovoltaic System Produces Electricity
Article 3: Reading Your Electricity Bill: A Beginner’s Guide
Article 4: How to Size a PV System from an Electricity Bill
Article 5: Shade Losses for PV Systems, and Techniques to Mitigate Them
Article 6: The Basic Principles that Guide PV System Costs

Topics: Solar PV Education 101, Solar Primer, pv, shading, shading losses, electric current, bypass diodes, MLPEs, microinverters, DC optimizers

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