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Photovoltaic (PV) Systems

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Photovoltaic panels in Rwanda. (Photo:  This email address is being protected from spambots. You need JavaScript enabled to view it. )

Photovoltaic (PV) Systems generate electricity from sunlight collected by solar panels. Energy collected in this manner can be used to supply direct power to electrical equipment, or it can be stored in batteries to provide indirect power.

Photovoltaic systems are a well established renewable energy technology that is particularly suitable for locations with little or no access to grid electricity. PV systems have been successfully deployed all over the world for decades in a variety of applications, including power for remote weather stations and telecommunications towers, residential installations, community micro-grids and grid-scale power plants. Perhaps the greatest advantage of PV technology in meeting such a wide range of applications is scalability. PV systems can be designed to meet nearly any power requirements, and can work in conjunction with diesel generators, the grid, battery banks or any other power source to provide stable, continuous power. A competitive market place for photovoltaics and a diverse set of commercially available PV technologies provide consumers with a variety of options when considering system cost and performance.

A successful PV installation will provide power for over 20 years with no fuel costs and little maintenance. When compared to diesel generation in particular, PV is a cost-competitive option, especially in the developing world where electricity and diesel prices are often high. Although PV technology is an appropriate choice for many applications in the developing world, high capital costs and poor installation and maintenance practices have been limiting factors in the overall deployment of photovoltaics. The following discussion of photovoltaic systems is meant to convey basic information on PV technology as well as best practices in the design and implementation of such systems, especially in the context of health facilities in the developing world.

 

IHFI Experience 

USAID's Improving Health Facility Infrastructure (IHFI) project has undertaken photovoltaic projects in Haiti and Guyana. These experiences have led to some interesting observations regarding the implementation of PV systems in the developing world.

In Haiti, grid power is relatively expensive, making PV power an attractive option even for facilities with access to the grid. In such cases a grid-tied PV system, one that delivers excess power to the local utility when the sun is shining and draws grid power when cloudy or dark, essentially storing the facility's solar power, would be the preferred choice of installation. A grid-tied system reduces or eliminates the need for an on-site battery bank, which is maintenance intensive and a major expense. For a number of reasons, however, grid-tied PV systems are not a viable option in Haiti. Firstly, the frequency and voltage of grid power in Haiti is too variable to accommodate grid-tied inverters. Grid-tied inverters are designed to protect their loads from poor quality power by disconnecting from the grid when the frequency deviates too far from the norm (60 Hz). Fluctuations in the frequency of Haiti's grid power result in systems that are too often disconnected from the grid. Secondly, grid power in Haiti is simply too unreliable to ensure access to power without a battery bank. Without on-site battery storage, a diesel generator may be the only option for periods where both solar and grid power are unavailable. Finally, the regulatory mechanisms needed make grid-tied PV systems feasible, such as net metering or feed-in tariffs, do not exist in Haiti. So while there is ample experience in solar technology in Haiti, and the conditions are right to make grid-tied systems economical, there are technical and policy barriers that must first be overcome.

In Guyana, photovoltaic systems have been widely deployed over the years, especially in remote regions without access to the grid. These installations have seen mixed success, with the most frequent problems resulting from poor installation or maintenance. These issues highlight the need for the training of technicians and end-users and the provision of maintenance funds. If system failures occur, end-users must be able to identify them in order to request corrective maintenance. In turn, technicians must be able to properly address system failures in a timely manner. IFHI's initiatives in Guyana have focused on ensuring institutional support, both technical and financial, for the installation and maintenance of all IHFI PV systems. Putting in place support mechanisms for remote PV installations is key to their longevity.

 

The Role of Solar Power in Health Facility Energy Systems

Solar power offers numerous benefits to health facilities of all types, but especially to those with little or no access to grid electricity. Photovoltaics produce no pollutants, require no fuel and need little maintenance, when economically viable, they are a good option for any health facility energy system. PV systems are of special importance to remote facilities that do not have access to grid power, however. In such locations, options for power generation are few, usually diesel or PV generation. Often the most economical solution in off-grid situations is a hybrid diesel-PV energy system, which makes the most of either resource at the most appropriate time. Compared to a diesel only scenario, a diesel-PV hybrid will likely save significant fuel costs over the life of the system. Therefore, PV systems help to ensure the long-term financial sustainability of health clinics by shielding them from fluctuations in fuel supply and cost.

 

Photovoltaic Materials 

Photovoltaic materials are able to convert light directly into electricity. This is explained by the photovoltaic effect, which occurs when an electrical current is created in a material upon being exposed to light. Therefore, the type of material used in a PV panel plays a big role in determining the panel's efficiency. Several different types of photovoltaic materials are discussed below, including their relative maturity, efficiency and cost.

Material Type Description Commercial Panel Efficiency
Monocrystalline silicon (mc-Si) Monocrystalline silicon PV cells are the most efficient type of silicon PV cell.The term monocrystalline refers to the rigid and uniform arrangement of silicon molecules within the cell. Because the entire cell is formed from a single crystalline structure, electrons are able to flow through the material easily, leading to high efficiencies. Monocrystalline silicon is the most mature PV technology and is readily available commercially. 13-19%
Polycrystalline silicon (pc-Si) Polycrystalline silicon PV cells are the most popular type of PV technology. Such cells are less efficient than monocrystalline cells, but they are also significantly less expensive, resulting in high levels of implementation. As the name suggests, a polycrystalline cell is formed from multiple silicon crystals, rather than a single crystal. Electrons cannot flow through multiple crystals as easily as they would a single crystal, thus the loss in efficiency. The process used to create a polycrystalline cell, however, is easier and less energy intensive than that used in monocrystalline cell production, lowering the cost of the cell. 11-15%
Gallium arsenide (GaAs) Unlike silicon, gallium arsenide is a compound semiconductor, consisting of two separate materials that have been combined due to their theoretically high solar conversion efficiency.  While many different materials can be similarly combined to create a solar cell, GaAs has proven to be the most popular and efficient. GaAs cells are expensive, however, so their use has been restricted to niche applications such as concentrating PV (CPV) installations and satellites. 25-33%
Amorphous silicon (a-Si) Amorphous silicon PV cells do not share the rigid structure of crystalline silicon, but consist of a thin layer of silicon, typically coated on plastic or glass.  This lack of structure at the molecular level hampers the flow of electrons, so the efficiency of amorphous silicon is less than that of crystalline silicon. Because the material is so thin, however, it can be layered with additional sheets of amorphous silicon or even crystalline silicon to achieve higher efficiencies. 4-8%, 10% when layered
Cadmium telluride (CdTe) Cadmium telluride is a thin film PV material that is less costly and more efficient than amorphous silicon. 11%
Copper indium gallium selenide (CIGS) CIGS PV cells show great promise for thin film technology, with efficiencies as high as 20% recorded in the lab. The process of producing a CIGS cell, however, is complex and expensive. 7-12%
Emerging PV Tech There are several emerging PV technologies that require further development before becoming commercially viable. These materials show promise in overcoming many of the issues faced by current PV technology, such as cost, efficiency and building integration. For the most part, the following technologies are currently confined to the laboratory or small niche applications:
  • Organic photovoltaic, low-cost materials based on organic polymers.
  • Dye sensitized photovoltaic, based on light absorbing pigments.
  • Thermo-photovoltaic, creates electricity from both light and heat.
N/A

 

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A role of amorphous silicon PV cells. Thin film PV materials like this are light-weight and flexible. (Photo: DOE's Office of Energy Efficiency and Renewable Energy) A monocrystalline silicon PV cell. The thin metallic lines across the surface of the cell carry electrical current out of the silicon material. (Photo: DOE's Office of Energy Efficiency and Renewable Energy)
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A polycrystalline silicon PV cell. Note that the various crystalline formations are easily visible. (Photo: Georg Slickers, available under a Creative Commons Attribution-Share Alike license.) A gallium arsenide PV module, housed under an array of tiny lenses for use in a CPV installation. (Photo: DOE's Office of Energy Efficiency and Renewable Energy)

 

PV Terms 

Described below are some important terms used when characterizing PV system performance or the local solar resource. This is not a comprehensive glossary of PV terms, but rather a selection of some of the more confusing or practical terminology that are used when discussing PV system design.

 

Term Category Description
Direct beam radiation Solar resource Direct beam radiation is light that travels directly from the sun to a solar panel, rather than diffuse light, which is reflected from the sky or ground. Solar concentrator systems rely on direct beam radiation because diffuse light cannot be easily concentrated. For normal, non-concentrator PV systems, direct beam radiation is less important.
I-V curve PV performance
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An example of a solar I-V curve.
The I-V curve of a solar panel is used to show the relationship between the current (I) and voltage (V) of the panel's electrical output under a range of conditions. While a panel's rated performance is determined under Standard Test Conditions, it is also important to understand how the panel will perform when solar irradiance or cell temperature deviate from the standard.

The figure to the right is an example of an I-V curve for a solar panel. In this example, the panel's performance is shown for different levels of irradiation, but I-V curves may also indicate performance at different cell temperatures. This information is useful when estimating a solar system's output under real-world conditions.

Each I-V curve has a Maximum Power Point (MPP), also shown in the example figure, which is the point on the curve where the panel's power output is at a maximum. In electricity, power is the product of current and voltage (power = current x voltage), so the MPP is always at the bend in the I-V curve, where current and voltage are both relatively large.
Peak sun hours Solar resource Related to solar insolation, this is the number of hours in a day that a given area would receive solar energy if solar irradiance were at a constant 1000 W/m2 (1 kW/m2), rather than varying throughout the day. This is essentially a location's average solar insolation expressed in terms of hours of peak output. For example, if an area has a solar insolation of 5 kWh/m2/day, that is equivalent to 5 peak sun hours per day, because solar panel output is rated at 1 kW/m2.
Peak-watt (Wp) PV performance This is the rated output of a solar module, or the amount of power it will generate under standard test conditions. This measure is used to describe the size of a PV system (e.g. 10 kWp PV installation ).
Solar insolation Solar resource The amount of solar energy that reaches a given area over a given amount of time, commonly expressed as kWh/m2/day. This is the most often cited type of solar resource data because it indicates the amount of useful solar energy available locally for collection through solar PV or solar therma installations. Solar insolation maps are available for most regions of the world.
Solar irradiance Solar resource Similar to solar insolation, solar irradiance is the intensity at which solar energy reaches a given area and is commonly expressed as W/m2. While not a direct measure of solar energy potential, local solar irradiance is useful when comparing PV system output to rated output (Wp), which is based on 1000 W/m2 (see Standard Test Conditions).
Standard Test Conditions (STC) PV performance The laboratory conditions under which all PV modules are tested and rated. These basic conditions are meant to approximate real-world operating conditions.  Specifically, Standard Test Conditions are:
  • an irradiance of 1000 W/m2,
  • an air mass coefficient of AM1.5 (this determines which parts of the light spectrum reach the panel), and
  • cell temperature of 25°C.

 

PV System Implementation 

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PV installers preparing a PV system for a facility in Rwanda. (Photo: Walt Ratterman)

A PV system represents a major investment in facility energy infrastructure. A successful PV system will last for over 20 years, providing clean energy without need for fuel or intensive maintenance. In order to achieve that long-term success, a PV system requires upfront planning and investment. Quality equipment must be chosen for all system components. Components must be properly sized according to the system's design load and the local solar resource. Reputable professionals must be found to perform design and installation services. Proper maintenance funds must be put in place to ensure that the system receives necessary preventative and corrective care. The cost of such a system, its installation and the associated professional services, can become quite high. The initial price of a high quality PV installation, however, is usually justified due to the system's long life span and low operation and maintenance costs. Ensuring a quality installation, then, will lead to a successful implementation. Described below are some important aspects of PV system implementation: common system components, system costs, system sizing and system maintenance.

 

Components 

Photovoltaic systems are made up of much more than just PV solar panels. There are a whole range of other system components, referred to as the Balance of System (BOS), which are required to properly use the PV panels. A number of components typical to PV systems are explained below. While this list covers most major PV system components, it is not exhaustive, an installed system will likely involve other minor components. Furthermore, not every component listed is necessarily needed, depending on the type of PV system being installed. The make up of any PV system will depend on the type of load is it designed to power, and, more importantly, weather it is a grid-connected or an off-grid system.

 

Component Description
PV panels Photovoltaic panels, also called PV modules, are the basic building block of any PV system. PV panels are unitary products, manufactured and distributed to consumers as a single piece of equipment. Photovoltaic panels are made up of many smaller PV cells, the most basic unit of PV material. Cells are electrically connected and bound together with a protective polymer to form a sheet of PV material. The connected cells are then sandwiched between a glass cover and a weatherproof backing and framed in aluminum to create a complete solar panel. The back of a panel will also include electrical connections used to wire the panel into a larger system. These connections may be housed in an electrical junction box, but more commonly come in the form of MC connectors, a special type of plug that provides a convenient and stable electrical connection as well as a locking mechanism to prevent theft. While the PV material is the most critical part of the solar panel, it makes up only a small portion of the panel's weight when compared to the glass and frame.

Crystalline silicon solar panels are typically rated at between 120 and 300 Wp. In order to produce more power, panels are connected to form an array. Using panels as a building block, a solar array can be sized to produce power for practically any application, from a 1 kW residential installation to a 100 MW grid-scale power plant. Correctly sizing a PV array involves a number of factors, including: facility load, geographic location, panel size and rating, cost, space and grid availability, among other considerations. PV system sizing is more thoroughly discussed in the sizing section.
Mounting structure Mounting structures come in a variety of forms and play several important roles in an overall PV system's design. The most common and least expensive type of mounting structure is a stationary structure, where panels are given a fixed orientation optimized for exposure to the sun. Such systems can be ground-mounted, poll-mounted or roof-mounted. Sun-tracking mounting systems are also available; these systems are able to automatically rotate the solar array in order to follow the sun's daily path across the sky. Regardless of the type of mounting system, a properly designed structure will provide optimal orientation for the solar panels, space for air-flow beneath the panels, structural strength in high winds, easy maintenance access, theft prevention and aesthetic appeal. The most critical role of the mounting system is to correctly orient the solar panels towards the sun. In order to achieve the highest possible energy output from a solar array, the panels must be exposed to direct sunlight for as much time as possible. To reach this goal, two factors must be taken into account: orientation and tilt. Orientation refers to the cardinal direction that the system faces. In the northern hemisphere panels should face true south, in the southern hemisphere, true north. The system's tilt refers to the angle at which the panels are mounted; the optimal tilt angle depends on the geographical latitude of the installation site. Because a PV system is a capital-intensive installation and because the sun is a variable energy resource (it's not constantly shining), proper siting and orientation are absolutely essential to making an investment in PV cost effective.

A PV mounting structure must also provide sufficient air flow to the solar panels in order to keep them from overheating. The temperature of a solar cell can have a dramatic effect on its efficiency; c-Si solar panels can drop 0.5% in efficiency for every 1°C of temperature rise over 25°C. Overheating will also reduce the useful lifetime of the PV material. Allowing for sufficient air flow around the solar panels is important in keeping them cool. This is especially true of roof mounted installations, where the panels are typically mounted close to the surface of the roof.

The other aspects of mounting system design: strength, maintenance, theft prevention and aesthetics, are largely to the benefit of system longevity. Typical solar panels have an expected lifetime of 20 to 30 years; optimized energy production over that long period should result in a cost-effective investment. If, however, the panels are damaged, stolen or neglected, that investment is compromised.
  • Environmental factors must be considered; mounting and other outdoor hardware must be able to withstand extreme weather events, high winds and corrosion in salty environments. Lightning strikes are also a concern but can be addressed by properly grounding the mounting structure.
  • Theft prevention techniques include the use of special fasteners or mounting connections that require specific tools to unlock. Fencing and security lighting for may also be necessary for ground-mounted arrays.
  • Maintenance technicians must be able to safely access the underside of panels and panel faces should be readily accessible for periodic cleaning.
  • The importance of aesthetics will depend on the owner's preferences and the visibility of the installation, but suffice it to say that anything lasting 30 years should at least have a uniform appearance.
Inverters Inverters are a type of device able toconvert the DC electricity produced by PV panels into the AC electricitynecessary to run most appliances, lights and other equipment.Inverters are required for any PV systemthat will support AC loads, they are therefore an integral component innearly all PV systems.Note that manyinverters may also perform the functions of other system components such asbattery charge controllers or disconnect switches.See the Inverters page for more information.
Combiner box A combiner box is an electrical housingspecifically designed to simplify the wiring of multiple PV panels.The combiner box is usually placed near thesolar array, allowing all panels to be connected locally and combined into asingle feed to the next system component, usually the inverter or chargecontroller.Combiner boxes areespecially useful for systems comprising more than 3 or 4 panels.
Wiring All electrical systems require wiring;in a PV system wiring is used to connect the PV panels and all otherelectrical components to a facility's electrical panel or battery bank.Wiring is available in a variety of sizesdefined by the cross-sectional area of the copper wire (not including thewire's insulation) and usually measured in mm2, or the AmericanWire Gauge (AWG) system in the United States. Wiring must be selected carefully, as undersized wire can be a safetyhazard.When choosing the size ofwiring used in a PV system, a number of factors come into play: systemvoltage, rated current, wire length and efficiency.Higher system voltage, rated current andgreater distance between the PV panels and system loads (e.g. electricalpanel or battery bank) require larger wiring. Larger wiring also results in lower transmission losses due toresistance in the wire; a 5% power loss due to wiring is typical.
DC and AC disconnect switches Disconnect switches are a safety measure that allow owners and technicians to cut the power supply coming from PV panels. A DC and AC disconnect switch must be placed on either side of an inverter, although some inverters and electrical panels are already integrated with disconnect switches. Switches are useful for cutting power during maintenance but their main purpose is to prevent a condition called islanding in grid-connected systems. Islanding occurs when a grid-connected PV system continues to send power to the grid during a power outage. Although the grid is down, the power lines are still electrified by the connected PV system, creating a potential hazard for utility technicians working under the assumption that the lines are not energized.
Batteries Batteries are used to storeenergy.In off-grid PV systems theyare essential to providing power during periods of low or no sunlight; ingrid-tied PV systems they store both solar and grid energy to provide backuppower in the event of an outage or continuous, clean power to no-contactloads.See the Batteries and Battery Management page for more information.
Charge controller PV systems utilizing a battery bankmust also include a charge controller. Charge controllers regulate the electrical current being sent to thebattery bank to ensure that the batteries are not overcharged, they alsoprevent the battery bank from discharging current back to the PV system whenthe panels are not producing power.
Tracking systems Tracking systems are a type ofmounting system in which the orientation, tilt, or both, are consistentlyadjusted to provide maximum daily exposure to direct sunlight.At a minimum, such systems consist of oneor two axes which allow for adjustment of the systems orientation or tilt,and a pyrometer, which is used to determine the position of the sunthroughout the day.Tracking systemsare an essential part of any concentrating solar thermal or concentrating PVsystem, because only direct sunlight can be concentrated efficiently.Such systems can also be used with normalsolar PV panels, but are not typically the economical choice.

Tracking systems make solar PV moreefficient by increasing the total energy output on a daily basis.This brings about further advantages.Firstly, such a system produces high powerfor a longer period each day, rather than reaching peak power production atmidday, as in stationary mounting systems. This allows the PV system to power loads directly throughout the day,reducing dependence on the system's battery bank or the grid.Secondly, PV systems utilizing a trackingsystem require fewer panels than a fixed system to meet the same total load.This can be especially advantageous whenspace for the system is limited.Theseadvantages must be weighed against the added cost of the tracking equipment(including special design services, added transportation costs andmaintenance needs).

 

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The front and back sides of a typical solar panel. The small black box on the panel's rear is used to connect wiring to the panel. (Photo: DOE's Office of Energy Efficiency and Renewable Energy) Stationary mounting brackets position this solar array for optimal orientation and tilt toward the sun. (Photo: DOE's Office of Energy Efficiency and Renewable Energy)

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Balance of system components for a small solar array, note the inverter, charge controller and wiring mounted above the batteries. (Photo: DOE's Office of Energy Efficiency and Renewable Energy) Tracking systems follow the sun's movement to generate more energy, but at greater cost. (Photo: DOE's Office of Energy Efficiency and Renewable Energy)

 

Costs 

The costs associated with a solar PV system are generally put into terms of $/Wp, to represent the system's capital cost, and $/kWh, to represent the cost of energy produced by the system over its life.

Small-scale PV systems (those designed for facility level power rather than grid power) can cost $7-12/Wp installed. Only a portion of this cost can be attributed to the PV panels, the remaining costs are for BOS materials (mounting hardware, inverters, batteries, etc.), engineering design, labor and other materials and services. If the system has no other backup power, a battery bank must be sized appropriately to provide power after several days of overcast weather; therefore, local weather patterns can influence overall system cost. Unusual expenses such as transportation of modules, customs fees, or permitting expenses can increase this cost even further.

While this upfront expenditure is a major hurdle for many potential PV installations, the cost of energy produced by the system is a more appropriate way to compare the value of a PV installation to that of other energy sources such as diesel generators or grid power. PV systems require little maintenance and their fuel, the sun's energy, is free, so the major cost component to any PV system will be the capital cost. Conversely, a diesel generator has a low upfront cost but will require a constant supply of fuel and multiple overhauls over the life of the equipment. The most cost effective energy system for any given facility will depend on the facility's electrical load, the availability of grid power, the supply and cost of generator fuel and the local solar resource. More often than not, the least-cost solution on a net present value basis is a hybrid system, in which solar PV, an on-site generator and the grid work in tandem.

Powering Health's HOMER Load Calculation and System Optimization tool is an easy and effective way to quickly compare energy system designs based on solar PV, diesel generation, grid power and battery banks. The tool allows users to input facility-specific information, such as the hourly electrical load profile, hours of grid availability and geographic location, to generate a comparison of energy system options tailored for the facility's needs. Each option's capital cost, net present value and normalized energy cost is displayed for a simple and meaningful energy system design assessment.

 

Sizing 

Properly sizing a solar PV system requires careful attention to detail and thoughtful planning. Solar system sizing is a step-by-step process that accounts for facility energy needs and the local solar resource in order to determine the necessary size (in kWp) of the solar array. The process outlined here is a guide to estimating the size of a solar installation.

1. Determine loads

The first step when considering a solar PV installation, or any other energy system upgrade, is to detail facility electrical loads. This process results in a facility load profile, or a listing of all electricity consuming equipment at the facility, including the load (power need in Watts) and schedule of operation (how long, and at which points throughout the day the equipment runs) for each individual piece of equipment. Understanding facility load requirements is essential to correctly sizing a PV system and is a matter of good facility energy management in its own right.

2. Optimize/predict loads

Once a current facility load profile has been established it is a good opportunity to consider ways to reduce that load, or conversely, to predict future increases in load. Solar panels, batteries, inverters and other balance of system equipment are costly, and must be sized according to the loads they are meant to power. By reducing facility loads before PV system installation, a smaller, less expensive system may be possible. Ways to reduce loads include energy efficiency retrofits or a reduction in operating hours for electrically powered equipment. While energy efficiency retrofits also have an upfront cost, the capital costs saved by installing a smaller PV system may make the measures worthwhile.

  • For more information on energy efficiency retrofits, see the Energy Efficiency page.
  • Potential increases in facility load should also be considered. When additional electrical loads are placed on a PV system there is a risk of overloading the system and damaging the battery bank. Before sizing a PV system and battery bank, any foreseeable additions to the system's electrical load should be accounted for. When procuring new equipment, energy efficient models should be selected to minimize their effect on the facility's energy system.

3. Size battery bank

In off-grid situations, or locations that do not allow grid interconnection, battery banks are a necessary part of any PV system. Like the PV array, the battery bank must be sized according to the loads it is meant to support. Another important factor that effects the size of both the PV array and battery bank is autonomy. Autonomy is the number of days that the PV system and battery bank can provide power without sunlight. Energy stored in the battery bank will be used for periods of inadequate sunlight, so greater autonomy requires a larger battery bank. Determining how much autonomy is needed depends on the local climate, specifically, the maximum number of days with cloudy weather. The availability of other power sources may also influence the amount of autonomy needed for the PV system, for example, a diesel generator can be used during periods of low sunlight in order to offset the need for a large battery bank.

4. Determine solar resource

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A solar insolation map indicating the amount of solar energy, in kWh/m2, available each day. (Image: National Renewable Energy Laboratory)

The local solar resource is the amount of solar energy that reaches the PV installation site. See the PV Terms section for an explanation of the different ways to describe a solar resource. The solar resource for any given location is mainly based on geographic location and climate, so solar resource data is available for every region of the world. Knowing the local solar resource is essential to sizing a PV system because it indicates the amount of solar energy available to meet the load requirements.

5. Size array

With the targeted loads and local solar resource well understood, determining the size of the PV array is a matter of connecting the dots. Essentially, the array must be sized in order to generate enough electrical power to the targeted loads, accounting for the overall system efficiency (panels, inverters, batteries, wiring, etc.), using only the available solar energy. In practice, many other factors must also be taken into consideration, including PV performance under low light conditions, the battery bank's days of autonomy and the DC voltage of the battery bank. For this reason, PV system design should be done by a trained engineer.

 

Maintenance 

Solar PV panels are low-maintenance equipment, but a regular and organized maintenance program is still absolutely essential to system longevity. The panels themselves typically have a very long lifetime, 20 - 30 years. Unfortunately, installation programs do not always include a sufficient service component. Health facilities with solar panels must have a vigorous training program for local users and an established maintenance protocol.

The most frequent maintenance need of solar panels will be the cleaning of the panel faces. Dirt or other debris on the panel faces will block sunlight and reduce the energy output of the system. For systems incorporating battery banks, regular maintenance on batteries is essential; they should be checked every week, with the electrolyte level replenished as needed, and if properly maintained, should last several years before needing replacement. See the Batteries and Battery Management page for more information on battery maintenance.

While training local hospital staff in system maintenance is essential for routine maintenance, a professional technician should also perform a semi-annual maintenance check, examining wiring connections, mounting bolts, and inverter operation and be on call to fix the system if it does not work. Maintenance funds should be established upfront and be dedicated only to solar system repair. Mixing maintenance funds with general operating budgets has proven to be an ineffective model.

 

Standards for PV Systems 

There are numerous national and international bodies that set standards for photovoltaics. There are standards for nearly every stage of the PV lifecycle, including: materials and processes used in the production of PV panels, testing methodologies, performance standards, and design and installation guidelines. The standards shown below are not a complete list, but are those most relevant to the procurement and installation of solar PV systems. Each standard has been loosely categorized based on its subject matter.

 

International Electrotechnical Commission (IEC)
Category Standard
Characteristics IEC 61194 ed1.0: Characteristic parameters of stand-alone photovoltaic (PV) systems
Crystalline IEC 61215 ed2.0: Crystalline silicon terrestrial photovoltaic (PV) modules - Design qualification and type approval
Thin-film IEC 61646 ed2.0: Thin-film terrestrial photovoltaic (PV) modules - Design qualification and type approval
Test IEC 61701 ed2.0: Salt mist corrosion testing of photovoltaic (PV) modules
Characteristics IEC 61702 ed1.0: Rating of direct coupled photovoltaic (PV) pumping systems
Monitoring IEC 61724 ed1.0: Photovoltaic system performance monitoring - Guidelines for measurement, data exchange and analysis
Characteristics IEC 61727 ed2.0: Photovoltaic (PV) systems - Characteristics of the utility interface
Safety IEC 61730-1 ed1.0: Photovoltaic (PV) module safety qualification - Part 1: Requirements for construction
Safety IEC 61730-2 ed1.0: Photovoltaic (PV) module safety qualification - Part 2: Requirements for testing
Terms IEC/TS 61836 ed2.0: Solar photovoltaic energy systems - Terms, definitions and symbols
Balance of System IEC 62093 ed1.0: Balance-of-system components for photovoltaic systems - Design qualification natural environments
Balance of System IEC 62109-1 ed1.0: Safety of power converters for use in photovoltaic power systems - Part 1: General requirements
Balance of System IEC 62109-2 ed1.0: Safety of power converters for use in photovoltaic power systems - Part 2: Particular requirements for inverters
Test IEC 62116 ed1.0: Test procedure of islanding prevention measures for utility-interconnected photovoltaic inverters
Design IEC 62124 ed1.0: Photovoltaic (PV) stand alone systems - Design verification
Design IEC 62253 ed1.0: Photovoltaic pumping systems - Design qualification and performance measurements
Rural electrification IEC/TS 62257 ed1.0: Recommendations for small renewable energy and hybrid systems for rural electrification - Parts 1-9
Commissioning IEC 62446 ed1.0: Grid connected photovoltaic systems - Minimum requirements for system documentation, commissioning tests and inspection
Performance IEC 62509 ed1.0: Battery charge controllers for photovoltaic systems - Performance and functioning
Rural electrification IEC/PAS 62111 ed1.0: Specifications for the use of renewable energies in rural decentralised electrification
Balance of System IEC 60269-6 ed1.0: Low-voltage fuses - Part 6: Supplementary requirements for fuse-links for the protection of solar photovoltaic energy systems
Installation IEC 60364-1 ed5.0: Low-voltage electrical installations - Part 1: Fundamental principles, assessment of general characteristics, definitions
Installation IEC 60364-7-712 ed1.0: Electrical installations of buildings - Part 7-712: Requirements for special installations or locations - Solar photovoltaic (PV) power supply systems

 

Institute of Electrical and Electronics Engineers (IEEE)
Category Standard
Performance IEEE 1526-2003: IEEE Recommended Practice for Testing the Performance of Stand-Alone Photovoltaic Systems
Sizing IEEE 1562-2007: IEEE Guide for Array and Battery Sizing in Stand-Alone Photovoltaic (PV) Systems
Interconnection IEEE 1547.2-2008: IEEE Application Guide for IEEE Std 1547(TM), IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems
Interconnection IEEE 1547.3-2007: IEEE Guide for Monitoring, Information Exchange, and Control of Distributed Resources Interconnected with Electric Power Systems
Interconnection IEEE 1547.1-2003: IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems

 

Underwriters Laboratory (UL)
Category Standard
Crystalline UL 1703: Standard for Flat-Plate Photovoltaic Modules and Panels
Concentrated UL 8703: Concentrator photovoltaic modules and assemblies
Mounting UL 790: Standard for Standard Test Methods for Fire Tests of Roof Coverings
Mounting UL 1897: Standard for Uplift Tests for Roof Covering Systems
Balance of System UL-SU 2703: Rack mounting systems and clamping devices for flat-plate photovoltaic modules and panels
Balance of System UL 1741: Standard for Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources
Balance of System UL-SU 1699B: Photovoltaic (PV) DC arc-fault circuit protection
Balance of System UL-SU 4703: Photovoltaic wire
Balance of System UL 854: Standard for Service - Entrance Cables
Balance of System UL-SU 2579: Low-voltage fuses - fuses for photovoltaic systems
Balance of System UL 4248-18: Fuseholders - Part 18: Photovoltaic
Balance of System UL-SU 6703: Connectors for use in photovoltaic systems
Balance of System UL-SU 6703A: Multi-pole connectors for use in photovoltaic systems
Test UL-SU 5703: Determination of the maximum operating temperature rating of photovoltaic (PV) backsheet materials
Balance of System UL 3730: Photovoltaic junction boxes
Balance of System UL-SU 98B: Enclosed and dead-front switches for use in photovoltaic systems
Balance of System UL 489B: Molded-case circuit breakers, molded-case switches, and circuit-breaker enclosures for use with photovoltaic (PV) systems

 

American Society for Testing and Materials (ASTM)
Category Standard
Terms ASTM E772 - 11: Standard Terminology of Solar Energy Conversion
Test ASTM E2848 - 11: Standard Test Method for Reporting Photovoltaic Non Concentrator System Performance

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