Design And Sizing Of Solar Systems (Part 1)

Design And Sizing Of Solar Photovoltaic Systems

1. Introductions

Design And Sizing Of Solar Systems – Photovoltaic (PV) systems (or PV systems) convert sunlight into electricity using semiconductor materials. A photovoltaic system does not need bright sunlight in order to operate. It can also generate electricity on cloudy and rainy days from reflected sunlight.

PV systems can be designed as Stand-alone or grid-connected systems. A “stand-alone or off-grid” system means they are the sole source of power to your home, or other applications such as remote cottages, telecom sites, water pumping, street lighting, or emergency call boxes on highways. Stand-alone systems can be designed to run with or without battery backup. A battery backup system stores energy generated during the day in a battery bank for use at night. Stand-alone systems are often cost-effective when compared to alternatives such as utility line extensions.

A “grid-connected “system works to supplement existing electric service from a utility company. When the amount of energy generated by a grid-connected PV system exceeds the customer’s loads, excess energy is exported to the utility, turning the customer’s electric meter backward.

Conversely, the customer can draw needed power from the utility when energy from the PV system is insufficient to power the building’s loads. Under this arrangement, the customer’s monthly electric utility bill reflects only the net amount of energy received from the electric utility.

1. Benefits of PV Systems

1. Environmentally friendly – It has zero raw fuel costs, unlimited supply, and no environmental issues such as transport, storage, or pollution. Solar power systems produce no air, water, or greenhouse gases, and they generate no noise. Solar systems are generally far safer than other distributed energy systems, such as diesel generators, and as such are the most suitable technology for urban on-site generation. PV is the only commercially available renewable technology generation option for urban areas.
2. Reliability – With no fuel supply required and no moving parts, solar power systems are among the most reliable electric power generators, capable of powering the most sensitive applications, from space satellites to microwave stations in the mountains and other remote, harsh environments. Solar panels typically carry warranties of 20 years or more.
3. Scalable and modular – Solar power products can be deployed in many sizes and configurations and can be installed on a building roof or acres of field, providing wide power-handling capabilities, from microwatts to megawatts. The installation is quick and can be expanded to any capacity.
4. Universal Applications – Solar PV is the only renewable energy technology that can be installed on a truly global scale because of its versatility and because it generates power under virtually all conditions, i.e., even in overcast light conditions
5. Peak Shaving – Have a rapid response, achieving full output instantly. The output of solar systems typically correlates with periods of high electricity demand, where air conditioning systems create peak demands during hot sunny days. PV can shave peakload demand, when energy is most constrained and expensive, and therefore can move the load off the grid and alleviate the need to build new peak generating capacity.
6. Dual use – Solar panels are expected to increasingly serve as both a power generator and the skin of the building. Like architectural glass, solar panels can be installed on the roofs or facades of residential and commercial buildings.
7. Low Maintenance Cost – It is expensive to transport materials and personnel to remote areas for equipment maintenance. Since photovoltaic systems require only periodic inspection and occasional maintenance, these costs are usually less than those with conventionally fuelled equipment alternatives.
8. Cost advantages – Solar power systems lower your utility bills and insulate you from utility rate hikes and price volatility due to fluctuating energy prices. They can be used as building materials. They can increase the character and value of the building. The purchase of A solar power system allows you to take advantage of available tax and financial incentives.

2. Challenges

The main challenges or constraints to approach a PV project are:

• Budget constraints: Build a system within your target budget.
• Space constraints: Build a system that is as space-efficient as possible.
• Energy offset: Build a system that offsets a certain percentage of your energy usage

3. Design Constraints

Design constraints are the key to the system’s successful outcome. They provide clear direction and reduce the scope of economic and system analyses and should be continually referenced throughout the design process. Typical design constraints apply to any system and are modified, expanded, and “personalized” for a specific application. Some typical questions
Inherent in design constraints are:

• Will the system output be AC or DC, or both?
• How pure must the electricity be for the load?
• Will the thermal energy generated be used?
• How much of the electric- or thermal-load profile can be economically matched with the available area?
• Is a utility interface available at the location?
• Will there be an unavoidable shadow?
• Will the system be actively cooled?
• Will the collectors be flat plate or concentrating?
• Will the collectors be fixed or tracking?
• Does the work proposal specify a type of system or specific design feature?

The 6-hour course covers fundamental principles behind the working of a solar PV system, the use of different components in a system, the methodology of sizing these components, and how these can be applied to building-integrated systems. It includes detailed technical information and step-by-step methodology for the design and sizing of off-grid solar PV systems.

The information presented aims to provide a solid background and a good understanding of the design. The course will be beneficial to electrical & mechanical engineers, energy & environment professionals, architects & structural engineers, and other professionals looking to enter the solar industry or interact with solar projects in their current line of work.

Chapter 1: Photovoltaic (PV) Technology

1. Solar Energy

The sun delivers its energy to us in two main forms: heat and light. There are two main types of solar power systems, namely, solar thermal systems that trap heat to warm up water and solar PV systems that convert sunlight directly into electricity, as shown in the Figure below.

The word photovoltaic comes from “photo,” meaning light, and “voltaic,” which refers to producing electricity. And that’s exactly what photovoltaic systems do — turn light into electricity!

Direct or diffuse light (usually sunlight) shining on the solar cells induces the photovoltaic effect, generating DC electric power. This DC power can be used, stored in a battery system, or fed into an inverter that converts DC into alternating current (AC), so that it can feed into one of the building’s AC distribution boards (ACDB) without affecting the quality of power supply.
Important thing to note is that we are not concerned about the heat content of sunlight; PV cells and modules do not utilize the heat, only the light. When the source of light is not the sunlight, then the photovoltaic cell is used as the photo detector. An example of a photo detector is the infrared detectors.

2. PV Technology

The basic unit of a photovoltaic system is the photovoltaic cell. Photovoltaic (PV) cells are made of at least two layers of semiconducting material, usually silicon, doped with special additives. One layer has a positive charge, the other negative. Light falling on the cell creates an electric field across the layers, causing electricity to flow. The intensity of the light determines the amount of electrical power each cell generates.

Note that a PV cell is just a converter, changing light energy into electricity. It is not a storage device, like a battery.

2.1 Solar Cell

The solar cell is the basic unit of a PV system. A typical silicon solar cell produces only about 0.5 volts, so multiple cells are connected in series to form larger units called PV modules. Thin sheets of EVA (Ethyl Vinyl Acetate) or PVB (Polyvinyl Butyral) are used to bind cells together and to provide weather protection. The modules are normally enclosed between a transparent
cover (usually glass) and a weatherproof backing sheet (typically made from a thin polymer or glass). Modules can be framed for extra mechanical strength and durability.

Usually, 36 solar cells are connected to give a voltage of about 18V. However, the voltage is reduced to, say, 17V as these cells get hot in the sun. This is enough to charge a 12V battery. Similarly, a 72-cell module produces about 34V (36V – 2V for losses), which can be used to charge a 24V battery.

A 12-volt battery typically needs about 14 volts for a charge, so the 36-cell module has become the standard of the solar battery charger industry. The most common cells are 12.7 x 12.7 cm (5 x 5 inches) or 15 x 15 cm (6 x 6 inches) and produce 3 to 4.5 W – a very small amount of power. The typical module size is 1.4 to 1.7 m², although larger modules are also manufactured (up to
2.5 m²).

2.2 PV String

Individual modules can be connected in series, parallel, or both to increase either the output voltage or current. This also increases the output power. When a number of modules are connected in series, it is called a PV string.

In a series connection, the negative terminal of one module is connected to the positive terminal of the next module. In series connections, voltage adds up, and the current remains constant.

V _Total = V1 + V2+ … + Vn
I _Total = I1 = I2 = … = In

For example, if 10 modules of 12 V and 3 amp rating are connected to make one string, then the total voltage of the string will be 120 V, and the total current will be 3 amps.

The reverse happens when modules are connected in parallel. In parallel connection, current adds up and the voltage remains constant.

V _Total = V1 = V2 = … = Vn
I _Total = I1 + I2 + … +In

 2.3 PV Array

Multiple PV strings are connected in parallel to form a Solar Array. Parallel connection increases the current, while the voltage remains the same.

The power that one module can produce is seldom enough to meet the requirements of a home or a business, so the modules are linked together to form an array. Most PV arrays use an inverter to convert the DC power produced by the modules into alternating current that can plug into the existing infrastructure to power lights, motors, and other loads. The modules in a PV array are usually first connected in series to obtain the desired voltage; the individual strings are then connected in parallel to allow the system to produce more current. Solar arrays are typically measured by the electrical power they produce, in watts, kilowatts, or even megawatts.

2.4 PV Materials

The vast majority of commercially available PV modules are made from silicon, which is one of the Earth’s most abundant elements in the Earth’s crust (after oxygen). Silicon’s natural properties as a semiconductor are modified by two other elements, boron and phosphorus, to create a permanent imbalance in the molecular charge of the material.

1. 85 % of the solar cell market
2. Life expectancy of >30 years
3. Energy payback in 2-8 years (positive)

2.5 PV Types

The three general types of photovoltaic cells made from silicon are:

1. Monocrystalline Silicon – also known as single-crystal silicon
2. Polycrystalline Silicon – also known as multi-crystal silicon
3. Thin Film Silicon

2.5.1 Crystalline Cells

Crystalline photovoltaic cells are made from silicon, which is first melted and then crystallized into ingots or castings of pure silicon. Thin slices of silicon called wafers are cut from a single crystal of silicon (Monocrystalline) or from a block of silicon crystals (Polycrystalline) to make individual cells. The conversion efficiency for these types of photovoltaic cells ranges between 10% and 20%. Crystalline photovoltaic cells represent about 90% of the market today.

Crystalline cells are divided into two categories:

• Monocrystalline silicon cells
• Polycrystalline silicon cells

2.5.2 Monocrystalline PV

Monocrystalline cell comes from a single crystal ingot of high purity, with typical dimensions of 12.5 or 15 cm. The ingot has a cylindrical shape, which is cut into thin slices and made into round, semi-round, or square shapes. These cells are the most electrically efficient, which means they require less surface area than other cell types to produce an equivalent amount of power.

They also have a wide range of transparency options. Disadvantages are their higher costs, requirement for ventilation in order to maximize performance, and a distinctive geometric pattern. Monocrystalline cells are especially suitable for atrium roofs, partial vision glazing in façades, rooftop installations in houses, and commercial sun shading or rooftop retrofits where the
installation area is limited and maximum electricity generation is desired. Commercial module efficiencies range around 14-19%.

2.5.3 Polycrystalline PV

Polycrystalline silicon cells are formed by casting in a cuboid form ingot. The ingot is cut into bars and sliced into thin wafers (a thin sheet of semiconductor material), which in turn are used to create the cells. These cells are less efficient than monocrystalline; however, the lower cost per unit area and their distinctive appearance make them a popular choice for relatively large, opaque installations. They have been used extensively in façade spandrel panels and sun shading elements on commercial buildings.

Polycrystalline silicon differs from monocrystalline in terms of cost (due to the reduction of losses) and efficiency (due to the grain boundaries). The difference is small, but still leads to the need for larger cells (21 x 21cm) in order to reach the same efficiency levels. Commercial module efficiencies range around 12-15%.

Important: Ventilation

Crystalline PV technologies should be ventilated over the back of the module to increase their performance. This is because crystalline PV cells operate better under lower temperatures, and ventilation allows heat, which is the by-product of energy generation, to be stripped away. New hybrid systems that capture this heat for other uses and improve PV performance are being developed and referred to as PV-T or PV Thermal systems.

Crystalline cells turn between 14 and 22% of the sunlight that reaches them into electricity.

2.5.4 Thin-Film PV

Thin film photovoltaics are produced by printing or spraying a thin semiconductor layer of PV material onto a glass, metal, or plastic foil substrate. By applying these materials in thin layers, the overall thickness of each photovoltaic cell is substantially smaller than an equivalent cut crystalline cell, hence the name “thin film”.

As the PV materials used in these types of photovoltaic cells are sprayed directly onto a glass or metal substrate, the manufacturing process is therefore faster are cheaper, making thin film PV technology more viable for use in a home solar system, as their payback time is shorter.

However, although thin film materials have higher light absorption than equivalent crystalline materials, thin film PV cells suffer from poor cell conversion efficiency due to their non-single crystal structure, requiring larger-sized cells. Semiconductor materials are used for the thin film. Types of photovoltaic cells include:

• Cadmium telluride (CdTe)
• Copper indium diselenide (CIS)
• Amorphous silicon (a-Si)
• Thin film silicon (thin film-Si)

Amorphous silicon is in commercial production, while the other three technologies are slowly reaching the market. Amorphous silicon cells have various advantages and disadvantages. On the plus side, amorphous silicon can be deposited on a variety of low-cost rigid and flexible substrates substrates such as polymers, thin metals, and plastic, as well as tinted glass for building integration. However, on the minus side, the main disadvantage of amorphous silicon (a-Si) is its very low conversion efficiency, ranging between 6 to 8% when new.

Of the different types of photovoltaic cells available, amorphous silicon has the highest light absorption of over 40 times higher than crystalline silicon. The advantage of this is that a much thinner layer of amorphous silicon material is required to make a thin film PV cell, reducing manufacturing costs and price.

Just to give a brief impression of what “thin” means, in this case, we’re talking about a thickness of 1 micrometer. With only a 6 to 7% efficiency rate, these cells are less effective than crystalline silicon ones, but in the current scenario, while a larger surface area is required for output, the cost of electricity per Watt peak is currently more attractive.

2.5.5 Third-Generation PV

The 3rd generation PV technology includes multi-junction PV and concentrator PV Cells. Multi-junction PV cells are designed to maximize the overall conversion efficiency of the cell by creating a multi-layered design in which two or more PV junctions are layered one on top of the other.

The cell is made up of various semiconductor materials in thin-film form for each individual layer. The advantage of this is that each layer extracts energy from each photon from a particular portion of the light spectrum that is bombarding the cell. This layering of the PV Materials increases the overall efficiency and reduces the degradation in efficiency that occurs with standard amorphous silicon cells.

Concentrator photovoltaic (CPV) utilizes lenses to focus sunlight onto solar cells. The cells are made from very small amounts of highly efficient, but expensive, semiconductor PV material (generally gallium arsenide or GaAs). CPV systems use only direct irradiation.

They are most efficient in very sunny areas that have high amounts of direct irradiation. The modules use precise and accurate sets of lenses permanently oriented towards the Sun. This is achieved using a double-axis tracking system. Efficiencies of 25 to 30% have been achieved with GaAs, although cell efficiencies well above 40% have been achieved in the laboratory.

Other emerging technologies include:

1. Instead of using solid-state PN-junction technology, an electrolyte, dye-sensitized liquid, gel, or solid is used to produce a photo-electrochemical PV cell. These types of photovoltaic cells are manufactured using microscopic molecules of photosensitive dye
   on a nano-crystalline or polymer film.
2. 3d photovoltaic cell uses a unique three-dimensional structure to absorb the photon light energy from all directions and not just from the top as in conventional flat PV cells. The cell uses a 3D array of miniature molecular structures that capture as much sunlight as possible, boosting its efficiency and voltage output while reducing its size, weight, and complexity.

These 3rd generation PV technologies currently suffer from low efficiency output and are unable to maintain their performance characteristics beyond three to five years. However, these products have a significant competitive advantage in consumer applications because of the substrate flexibility and ability to perform in dim or variable lighting conditions.

References:

A. Bhatia, Course No: R08-002, https://www.cedengineering.com

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