[Written by Electronics Infoline In-House Production Team]
In this article, we will analyze the construction and working of different types of residential solar systems in detail. We will also compare the efficiency and cost of off-grid solar systems with that of grid-tied and hybrid systems.
The first industrial revolution kicked off at the start of 18th century when steam engines and other mechanical powered systems were invented. These mechanical power sources, without a doubt, provided an excellent starting point for the industrial and technological revolution. However, these machines were noisy, inefficient, required high maintenance, and caused pollution.
Then with the discovery and utilization of electrical power during the mid-1800s, the era of 2nd industrial revolution started. The electric power generators, alternators and motors were the main workhorses of this industrial age. The electric-powered machines were more efficient, silent, and clean as compared to their mechanical counterparts. These machines also faced comparatively less wear and tear, and their carbon footprint was also significantly less. Although the machines and devices on the load side were running on clean electric power and had very low emission rates, the power generation process itself was far from being eco-friendly.
Right from the times of 2nd industrial revolution to this very date, the primary mode of electric power generation in the world has mainly been based on the burning of fossil fuels. The external and internal combustion engines which drive the turbines burn coal, diesel or gas for power generation. The extensive use of these fossil fuels has contributed largely to the global warming and other ecological disasters. These ecological and environmental challenges have forced governments and researchers to find alternative energy resources. The recent advancements in the area of renewable energy sources are very promising and have the potential of meeting the global energy needs through green energy.
Solar energy is perhaps the most promising technology amongst the renewable energy sources. The sheer abundance of the sun energy and the availability of the associated technology are the biggest plus points of this technology. In the past few decades, solar energy technology has advanced rapidly and the pace of its adoption has been very impressive. The dropping prices of the solar panels and inverters have allowed the residential users to install their own solar generation setups. In many countries, residential users are using solar energy as their primary source of electric power and are selling back the power to the utility companies as well. This phenomenon has created the concept and business model of decentralized energy markets and has elevated the status of residential users to the stakeholders in the power system.
There are different configurations in which a residential solar system can be installed and operated. The three main classifications of the home solar systems are:
Grid-tied solar systems are also known as On-grid solar systems. These are the most popular, common and widely used type of solar systems in homes and offices. These solar systems usually do not incorporate any storage batteries and are connected to the utility grid via an inverter. The excess energy produced by the solar system is fed back into the utility grid and the consumer is paid a feed-in tariff or credit points by the utility company.
The main disadvantage of grid-tied solar systems is that they cannot be used as backup power systems in case of power shutdowns or blackouts. The absence of storage batteries, however, reduces the system cost significantly. Batteries can also be added to the grid-tied solar systems if required, however, they increase the capital cost of the system.
The alternative to the grid-tied solar system is the off-grid solar system. This is system is also known as a standalone solar system. As opposed to the grid-tied system, the off-grid solar system consists of storage batteries which are charged by solar energy. This type of system can either be used as the primary power source for the home or can be used as a backup system in case of blackouts. Due to the inclusion of batteries, the cost of the off-grid system is significantly higher than the grid-tied systems. In countries, where the cost of utility electricity is high, the off-grid solar system can provide substantial savings in electricity costs.
The hybrid solar system is a cross between the grid-tied and off-grid solar systems and combines the best features of both the systems. These systems are tied to the grid like grid-tied systems and also consist of batteries like off-grid systems. The hybrid system is the most efficient and practical system amongst the three solar systems. On cloudy days, when sufficient solar energy is not available, the consumer can take energy from the utility grid. The consumer also has the provision of exporting energy to the utility grid and earning tariff or credit points.
The hybrid solar system is also less expensive as compared to the off-grid system because the batteries can be downsized due to the availability of grid power. This type of system also doesn’t need any backup generator in case of emergency due to the connection with the utility grid.
The key components in an off-grid solar system are listed as following:
The solar panels are the most crucial and important component of any type of solar system. Multiple photovoltaic (PV) cells are connected together in order to form a solar panel. A PV cell is a device which converts solar energy into electrical energy. The voltage/power output of a standalone PV cell is very small therefore multiple PV cells are stringed together to form a solar panel.
Multiple solar cells are linked together in a solar system to form a PV solar array. The efficiency of a given solar panel depends on many factors such as solar irradiance, orientation, tilt angle, ambient temperature and dust depositing. Solar panels have a long service life and need very less maintenance however the deposited dust must be cleaned off regularly in order to maintain the panel efficiency. For maximum efficiency gains, the orientation and the tilt angle of the panel must be aligned properly with the sun.
The current PV solar cell technology is dominantly based on Silicon crystals. There are two main types of Silicon-based solar cells which are monocrystalline and polycrystalline. The monocrystalline cells despite being the old technology have the highest efficiencies and power outputs. As the name suggests, these types of cells are fabricated from a single crystal of Silicon.
The polycrystalline cells are made from multiple crystals of Silicon and therefore have less efficiency and power output as compared to monocrystalline solar cells. However, it should be kept in mind that the monocrystalline cells are more expensive than the polycrystalline cells.
After solar panels, the charge controller constitutes the 2nd stage of an off-grid solar power system. The charge controller is a power electronics device which is responsible for regulating the current/voltage in order to save the batteries from overcharging. The output voltage of the solar panels varies according to the variation in weather and ambient conditions. The output voltage of the panels might increase on a bright sunny day, in which case the batteries can easily get overcharged and potentially damaged. Apart from protecting the batteries, the charge controller also prevents the back current flow in the circuit.
There are two main types of charge controllers which are:
The PWM charge controllers employ the Pulse Width Modulation technique for voltage/current regulation. The duty cycle of the pulse is varied in order to maintain the voltage level across the battery bank terminals. The PWM charge controller is the most commonly used type of solar charge controller.
Video Courtesy: YouTube and Duet Justus
The other type of the solar charge controller is the MPPT (Maximum Power Point Tracking) controller. The MPPT controller is the latest technology which employs advanced algorithms for maximum power tracking. The MPPT controller steps down the panel voltage to match the battery voltage level which results in the increase of the current output. The major advantage of this controller is that higher voltage solar panels can be used to charge the low voltage batteries. The energy transfer rate of an MPPT controller is almost 40% more than the PWM controller.
Figure 5: PWM vs MPPT charge controller. Source: https://enkonn-solar.com/solar-charge-controller/
The solar PV panels convert the sunlight into DC electric power. The storage batteries in the system also store electric power in DC format and output it when needed. However, all the conventional appliances in the households work on AC power. Hence, it is necessary to convert the DC power into AC power. The solar inverter is a power electronic device which converts the variable level DC voltage into grid-level AC voltage with constant line frequency. The inverter is an essential part of any type of solar power system. The converters used for on-grid, off-grid, and hybrid systems are slightly different from each other in terms of functionality. The grid-tied inverters shut off automatically in case of a power blackout and do not provide any backup functionality. The smart hybrid inverters are very sophisticated systems which manage the PV array, battery bank, and utility power simultaneously. The off-grid inverters are the simplest type of inverters which do not have the capacity to connect with the utility grid.
The battery bank is another crucial component of the off-grid solar power system. The off-grid solar system is not connected to any utility grid and therefore the solar system must be able to store energy for use at night time or during cloudy weather. In the off-grid system, the energy obtained from the solar panels is stored in the battery bank for later use.
From the analysis above, we came to the conclusion that off-grid systems are not efficient and cost-effective as the hybrid solar systems. The inclusion of battery banks renders the cost of off-grid system higher. Another drawback of the off-grid solar system is the unavailability of grid power which requires us to have a separate backup generator for emergencies. Also, the excess power from the off-grid system cannot be sold to the utility grid as in other two systems.
During the discussion on the construction of an off-grid solar system, we discussed the types of solar panels and charge controllers. We analyzed the available types in comprehensive detail and came to the conclusion that the MPPT charge controller provides far better performance than the PWM charge controller due to their superior design and technology. However, the cost of an MPPT controller is higher than the PWM controller. Similarly, the performance and efficiency of monocrystalline solar panels are better than the polycrystalline panels, however, they are more expansive than the polycrystalline solar panels.