Stand Alone Solar PV System
Designing a solar PV system requires a systematic approach. The first step in sizing a stand-alone solar PV system is to perform an energy audit, looking for places to save energy.
The power requirements are evaluated as part of the audit, and the site is evaluated for the expected solar input. From this, the basic system is designed.
In this section, you will go through the steps of the basic process for designing a stand-alone system.
Design Steps for a Stand-Alone PV System
The following steps provide a systematic way of designing a stand-alone PV system:
- Conduct an energy audit and establish power requirements.
- Evaluate the site.
- Develop the initial system concept.
- Determine the PV array size.
- Evaluate cabling and battery requirements.
- Select the components.
- Review the design.
Step 1: Conduct an Energy Audit and Establish Power Requirements
The load requirements should be the starting point in determining a stand-alone system.
Start by calculating the power requirement for each AC electrical load and multiplying by the average number of hours it is powered on each day (Wh/day), which represents energy/day. A spreadsheet is a useful tool for developing this analysis.
Table 1 shows how a summary of appliances and their usage can give a reasonable estimate of the load requirement; the load is calculated for each month to account for seasonal variations.
From this analysis, the average daily use can be determined for a one-year interval. For the month shown, the average daily energy use is 10.79 kWh, which is the energy the solar system will need to provide.
The analysis can also reveal opportunities for conserving energy and/or by switching part of the load to other sources, such as propane instead of electricity, and also the savings expected for replacing incandescent bulbs with compact fluorescent bulbs or LED lighting.
Standby Power
An often-overlooked load is the standby power used by many electronic devices.
Standby power is the power to keep devices ready.
In many devices (televisions, computers, printers, phones, etc.), a certain amount of power is used when the device is plugged in but not in service. This power can add up.
In the average US household, it is estimated that standby power accounts for a constant load between 40 and 60 W, equivalent to leaving a light on day and night. Removing unnecessary loads like these can reduce the requirements shown in the load analysis and are almost always cost-effective.
PV System Power Calculation Example 1
A certain site has an average of 5 hours of peak sunlight per day in January. If 11 kWh is required on an average January day from a grid-free system, what peak power is required from the array? Assume the system is 60% efficient, which represents a typical residential system.
Choose modules that will provide at least this much power: 18 modules of 215 W is 3.87 kW.
Step 5: Evaluate Cabling and Battery Requirements
Determine the wire size required from the initial system concept. For a given power, the current (and hence the required wire size) is smaller as the voltage is higher.
In general, smaller diameter wire costs less but is rated for less current and drops more voltage. Note that listed wire capacity needs to be significantly derated for higher temperatures due to wiring on roofs and if the wires are bundled or in conduit.
Generally, the size of the wiring is determined so that voltage drops are in the range of 2% or 3%, but relevant codes should be consulted in determining wiring requirements.
What is the photovoltaic system diagram?
The photovoltaic system diagram is an ideal representation of the system. See the figure below for an overview of the main components.
General diagram of a photovoltaic system
Nowadays, correctly sized photovoltaic systems should include the possibility to self-consume the produced energy, to exchange it with national grid or store energy which can’t be used (also known as not self-consumed) it in accumulators so that the energy can be absorbed even when the PV system is not operating, for example at night.
For a correct operation of the photovoltaic schema based on the on-site exchange mechanism, we need three precise measurements:
- the total amount of energy produced by the installation;
- the total amount of energy fed into the grid;
- the total amount of energy withdrawn from the grid.
Photovoltaic system diagram: components
A photovoltaic system is characterized by various fundamental elements:
- photovoltaic generator;
- inverter;
- electrical switchpanels;
- accumulators.
Photovoltaic generator
The photovoltaic generator is the set of solar panels and is the element that converts solar energy into electricity.
These panels consist in small sheets of semiconductor material – the photovoltaic cells – which are connected together and encapsulated to form a larger element, the module.
The panels are in turn connected in series to form a string.
Inverter
The inverter is an electrostatic converter that is used to transform the direct current output to alternating current (DC/AC static converter).
Inverters are chosen according to the panel configuration and characteristics.
A photovoltaic system can work by centralized conversion or by string conversion. The first case shows an application with only one device, while the second demonstrates a scenario with multiple inverters.
Choice of the inverters characteristics and size is fundamental for the system’s overall performance. During composition of the photovoltaic wiring diagram, I would really recommend you try a photovoltaic software capable of activating, sizing and configuring the inverter and all other solar PV system devices.
Field panel
Generally speaking, field panels are located between the inverter and the generator and are used to parallelise multiple strings.
A possible configuration of a field panel is as follows:
- string disconnectors;
- blocking diodes;
- overvoltage protection devices;
- disconnectors.
The following image shows a generic diagram of a photovoltaic field with strings placed in parallel in the field switchpanel.
In this switchpanel, with the strings in parallel configuration, the current intensity of each string is simply added up.
Single line diagram of a PV system
Single-line diagram of a photovoltaic system
Once again, the photovoltaic software will allow you to define and compose your electrical single-line wiring diagram automatically providing customization tools for your electrical panels, electrical protections, etc.
Typical connection diagram of a photovoltaic system
Interface
The interface device is generally installed in a switchpanel and detects the electrical voltage: in the absence of a measurable voltage, it disconnects the photovoltaic generator from the rest of the system.
Types of photovoltaic systems
There are two types of Photovoltaic systems:
Grid connected types refer to systems connected to national electricity grid, i.e. systems that allow the energy produced to be fed into the grid and used when needed.
Stand-alone systems, on the other hand, are not connected to the national electricity grid, but have an accumulator that can preserve the energy produced and consume it when the system isn’t working during nightime hours. These storage systems consist of specific batteries.
A stand-alone system has an additional device, the charge controller, which controls the charging or discharging process safeguarding battery life during the various phases.
In these cases, using a photovoltaic system design software will allow you to size and configure the storage system by defining the type of battery and meter.
Solar power system design: how electricity use factors in
One of the first information a solar installer might ask for is twelve months of electricity bills. While this might seem like a lot of information right off the bat, this gives the installer a sense for how much energy you use throughout the year. Often an electric bill will include the last twelve months of usage history in a table or a chart, removing the need to fish around for past bills. Armed with your electricity usage information, the installer can determine about how big of a solar photovoltaic system would be required to offset 100% of your annual usage. This will vary by your location and roof profile.
Typically, solar panel systems are designed to cover 100% or less of annual usage to maximize the financial benefits, but if you are planning to increase your energy use in the near future – for example, if you are planning to buy an electric car or converting from oil to electric heat pumps – you might consult with your solar consultant to estimate your expected energy demand in order to design a system that better matches that use profile.
The impact of shading on solar on solar system design
If your roof has shading – from neighboring trees, other buildings or large chimneys – installers may choose to leave these areas uncovered by solar panels to optimize the production of the system or may place panels in these locations with the understanding that these panels may produce less energy at certain times of the day. Installers may also suggest changes to system design to create separate solar panel strings that are designed to avoid power loss from shading or may suggest the use of micro-inverters or power optimizers to help mitigate the impact of shading of the production of the overall system. The latter two technologies may come at a higher cost, but these costs may be justified by the additional power they help your system to generate.
The above factors are taken into account when designing a solar system for your home or business. The process of providing an initial system design can be quite quick and can often be turned around within a day or so. Other factors – such as the structural integrity of your roof, the condition of your existing electrical system or the specific impact of shading – may require that solar installers visit your property to gather additional information in order to develop a system design and a price quote that they can stand by. To get started, check out some helpful tips we offer to all new solar shoppers:
Homeowners who get multiple quotes save 10% or more
As with any big ticket purchase, shopping for a solar panel installation takes a lot of research and consideration, including a thorough review of the companies in your area. A recent report by the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) recommended that consumers compare as many solar options as possible to avoid paying inflated offered by the large installers in the solar industry.
To find the smaller contractors that typically offer lower prices, you’ll need to use an installer network like EnergySage. You can receive free quotes from vetted installers local to you when you register your property on our Solar Marketplace – homeowners who get 3 or more quotes can expect to save 5,000 to 10,000 on their solar panel installation.
The biggest installers typically don’t offer the best price
The bigger isn’t always better mantra is one of the main reasons we strongly encourage homeowners to consider all of their solar options, not just the brands large enough to pay for the most advertising. A recent report by the U.S. government found that large installers are 5000,000 to 5,000 more expensive than small solar companies. If you have offers from some of the big installers in solar, make sure you compare those bids with quotes from local installers to ensure you don’t overpay for solar.
Comparing all your equipment options is just as important
National-scale installers don’t just offer higher – they also tend to have fewer solar equipment options, which can have a significant impact on your system’s electricity production. By collecting a diverse array of solar bids, you can compare costs and savings based on the different equipment packages available to you.
There are multiple variables to consider when seeking out the best solar panels on the market. While certain panels will have higher efficiency ratings than others, investing in top-of-the-line solar equipment doesn’t always result in higher savings. The only way to find the “sweet spot” for your property is to evaluate quotes with varying equipment and financing offers.
For any homeowner in the early stage of shopping for solar that would just like a ballpark estimate for an installation, try our Solar Calculator that offers upfront cost and long-term savings estimates based on your location and roof type. For those looking to get quotes from local contractors today, check out our quote comparison platform.
reading on EnergySage
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Configuration Types of a Solar PV System
The three basic configurations type for a solar PV system include:
- The PV array where the power to the electrical load is supplied directly. Since no storage is involved in this system, it turns out to be the simplest of configurations. Thus, the cost of this system is quite low as well.
- The PV array that powers the load and a storage battery. It is the most popular configuration and allows electricity to be used at night as well, during phases of low insolation.
- The PV hybrid system leverages the supplementary source of power such as a fossil fuel generator to complement the energy generated from the PV array. A hybrid PV configuration requires battery storage capacity to avoid fluctuations. Such a system is suitable for critical applications that are installed in regions with huge variations in power supply.
Charge Controller
A charge controller functions like a voltage regulator that prevents overcharging of the storage batteries. For instance, a PV control will reduce or eliminate additional energy being directed to the batteries if it senses that the batteries are fully charged.
The key role of the inverter is to transform the electricity into a form that can be utilized by household applications.
Some solar inverters include storage applications as well. In such a system, all your power loads will only receive DC current while the PV array can charge your batteries throughout.
Besides, some solar inverters also include a charge controller. A solar charge controller regulates voltage or current thereby ensuring that the batteries don’t get overcharged.
Storage Batteries
A PV cell produces energy only during daytime as they require solar radiation to do so. However, one can store the energy produced by the cells during daytime and use it at night or when the sky is overcast with clouds.
To wrap up, the design of photovoltaic systems may include other entities as well. However, the above components will allow one to get a basic idea of how a solar PV system is built for maximum functionality.