Increase in global demand for solar electric system must have been connected with the fact that the energy source is free and pose no threat to the ecosystem. Photovoltaic (PV) module captures light (solar) energy and converts it directly to electrical energy using solar cells. The generated energy could either be used directly by electrical appliances or stored in form of chemical energy in battery for use later.
You have been provided with the following topics to help you decide and work out system that best suit your home requirement. However, if you don’t have basic knowledge required or not sure of what to do, please invite an engineer to do that for you.
Types of solar electric system
Components of solar electric system
Sizing solar electric system
Solar electric system sizing calculator
Types of Solar Electric System
Solar electric system can be classified into two major types. They are off-grid systems and grid-tied systems.
They are also called stand-alone systems. Although they are most common in remote locations without utility grid service, off-grid solar-electric systems can work anywhere. These systems operate independently from the grid to provide household’s electricity. That means no electric bills and no blackouts-at least none caused by grid failures. They are generally designed and sized to supply DC and/or AC electrical load. People choose to live off-grid for a different of reasons, including the prohibitive cost of bringing utility lines to remote home sites, the appeal of an independent lifestyle, or the general reliability a solar-electric system provide. Those who choose to live off-grid often need to make adjustments to when and how they use electricity, so they can live within the limitations of the system’s design.
The simplest type is the direct-coupled system, where the DC output of a PV module is directly connected to a DC load. The critical part of designing a well performing direct-coupled system is the matching of impedance of the electrical load to the maximum power output of the PV module. It can be used to operate pumping machine where water is pumped in the day to reservoir for used in the night. The drawbacks in this type of off-grid are:
- It can only be used in the day to supply load as there is no battery for storing energy.
- It can not be used with AC load
Another type of off-grid system is the type that incorporate inverter unit for conversion of DC voltage to AC at appropriate voltage level. The only drawback of this system is the lack of storage unit, so it will not supply load at night. The block diagram is shown below.
System with inverter
The problem of no electricity generation in the night is eliminated with the inclusion of storage unit (batteries) as backup energy in the night. The block diagrams of this type are shown below.
Off-grid system with battery (a) with no DC output and (b) with DC output.
Off grid systems can also be sized to provide electricity during cloudy periods when the sun doesn’t shine. Sizing a system to cover a worst-case, like several clouds days can result in a very large expensive system that rarely get used to its capacity. To reduce cost, it is sized moderately, but includes a back-up engine generator to get through occasional sunless stretches. The generator produces AC electricity that a battery charger (either stand-alone or incorporated into the system) converts to DC energy, which is stored in batteries. Below is the block diagram of this type of stand-alone system with generator back-up.
Off-grid system with engine generator as back-up.
They are also called on-grid or utility interactive. Grid-tied systems are designed to operate in parallel with and interconnected with the electric utility grid. Below are the block diagrams of grid-tied systems.
Grid-tied system with no battery for storing charges.
Grid-tied system with batteries for storing charges.
Grid-tied system can also be connected in a way that utility supply will be charging battery in the period of low light intensity. It has the same features as off-grid system with engine generator back-up. In the case of long cloudy days and utility outage, there is likely to be blackout.
Grid-tied system with utility connected to charge battery.
Components of Solar Electric System
Photovoltaic module consists of solar cells which convert light directly to electricity. When light photons are absorbed by the atomic electrons in the semiconductor material from which the solar cells are made. Each photon absorbed causes an electron to be freed from its atom and drift through the semiconductor material in an electric field created by p-n junction formed just below the surface of the solar cell. The free electrons and the resultant positive changes are collected by metallic contacts applied to the front and back surfaces of the solar cells thereby setting up an electron current which is made to flow through an electrical circuit to deliver power just like a storage battery. The current produce by a solar cell is proportional to its surface area and the light intensity, whereas the voltage is limited by the forward potential drop across the p-n junction.
In order to get higher voltages and currents, the cells are arranged in series and parallel strings and packed into modules for mechanical protection. The support structure for PV modules should be corrosion resistant (galvanized or stainless steel or aluminum) and electrolytically compatible with materials used in the module frame, fasteners, nuts, and bolts. The design of the support structure should allow for proper orientation of the module and tilt.
As PV cell costs continue to fall, the battery in a stand-alone PV system becomes an increasingly large part of the system cost. Battery’s life now has the greatest impact on the economic viability of solar electric system. The controller must manage a rapid, yet safe, recharge under a very diverse range of system conditions. The charge controller in small stand-alone systems is the primary driver of system reliability and battery life. An advanced controller will affect the system performance more than any other component, and an improved controller will on the long run reduce the system’s cost as the battery won’t need to be replaced often.
The most commonly used battery in solar electric systems is a lead-acid battery of the type used in automobiles, sized to operate for desired hours or days. Automotive batteries are often used because they are relatively inexpensive and readily available. Ideally, solar electric systems should use deep cycle lead-acid batteries that have thicker plates and more electrolyte reserves than automotive batteries and allow for deep discharge without seriously reducing the life of the battery or causing damage. In a well designed solar electric system, such batteries can last for more than ten years
An inverter is a basic component of any independent power system that produces AC power. Inverters convert DC power from PV module or stored in batteries into AC power to run conventional appliances. Another application of inverter is in the case of uninterruptible power supply where the inverter with the aid of 12V DC battery is able to generate up to 220V AC that can be used to power most house and office appliances depending of their power rating. While one needs to buy PV module and battery, a hobbyist who likes putting things together may personally love to build an inverter for his solar electric project by himself. Of course I do for personal uses. Why the waste of time and resources when there are cheap and neatly packaged inverters in the market.
Sizing Solar Electric System
Before sizing various components of your solar electric system, you need to find out what your average energy usage is. An engineer can be invited to assess your load in order to determine your peak load. Fielding questions from engineer will help to determine your average energy usage. Alternatively, if your home is already connected to the grid, your monthly electric utility bill will give you your monthly energy usage. Dividing this by the number of days of the month gives you an average daily energy usage. An important area where you will need advice and/or service of an engineer is how to bring the current energy usage of your home down in order to reduce the cost of your solar electric system. There is a lot of energy saving electrical devices being used these days in place of old ones that save you up to 40% or more of energy. In some cases, you may need to replace some of your old appliances with the new ones that consume less energy.
It is also important to estimate various losses associated with your installation. Some of these include losses due to orientation of PV module, shade, dust, temperature effect, name plate mismatch, cable loss, semiconductor loss (in inverter), running power of charge controller etc. A book on solar home system put the estimate of all these to be 50%. I have also seen other literatures on solar electricity that put the estimate between 20% and 30%. Nevertheless the choice is yours. You may want to know what I use in my design. I choose between 25% and 50% depends on who my client is and the project. For instance, semiconductor loss may be of less important in a system meant to power dc load.
Sizing of PV module
The capacity of modules is given in watt-peak. This allows for calculation of electricity generated under different levels of sunshine. To standardize the capacity of solar PV modules, the capacities are given at an illumination at exactly 1000watts per square meter. One watt-peak generate one watt of electricity under the standard test conditions of 1000watts per square meter and temperature of 25oC.
Again, one needs to know the amount of sun that is available. Meteorological tables show the solar insolation (usually in KWh/m2/day). This is different from day to day and shows a seasonal variation over the year. It is safe to design the system based on the average daily insolation in the month with lowest insolation. The easiest way to know the average daily insolation of your area is to search the internet: someone must have published something on that.
Having done that, you can now slot in all the information into the formula below to get the required PV module size in watt-peak (Wp)
Click here for solar calculator
Sizing of battery bank
Batteries are rated in ampere-hour (Ah) and the sizing depends on the household energy consumption.
Due to low voltage disconnect, one does not use the complete battery capacity. Only certain percentage (discharge capacity) of the battery would be used. A deep-cycle battery can be discharged up to 80% (actual value depends critically on the low voltage setting) of its capacity. Now battery is sized with the formula below. Click here for solar calculator
If the system is being designed to power ac load and inverter is needed, one has to put into consideration the inverter efficiency. The formula above can be modified toClick here for solar calculator
Sizing of Inverter
Inverter should be sized to handle the peak load. It is recommended that the inverter be sized 20% above the peak load.
Mounting of solar panel