Innovation and customisation are making solar inverters popular

Solar power generation is always looked upon as something meant for rural electrification but there is a lot more that this energy source can offer, subject to its optimum utilisation, mass awareness and reduced costs. Experts vouch for this technology as there has been a shift in consumer preferences for safer and greener technology. One such application which is gaining ground in this field is solar inverters. On the utility front, they are now being used in massive, multi–megawatt projects. Hence, they are getting bigger in size. For the home market, they are getting smaller in size but becoming more adaptable. Moreover, there are a number of choices in the market for consumers and installers.

By Richa Chakravarty

Sunday, August 28, 2011: New types available in the market

The types of solar inverters available in the Indian market can be segregated into the following categories.

Standalone or off-grid solar inverters: These are used in systems without a connection to the power grid. The inverter draws energy from the sun (solar energy). All standalone inverters have attached batteries. These are mainly used in off-grid applications and in residential/commercial applications, where there is no net metering available.

Grid tied solar inverters: Many solar inverters are designed to be connected to a utility grid, and do not operate when they do not detect the presence of the grid. They contain special circuitry to precisely match the voltage and frequency of the grid. Grid tied inverters are designed to shut down automatically upon loss of utility supply, for safety reasons.

Basically, there are two major technologies in grid tied inverters—galvanic isolated inverters (using transformers) and non-isolated inverters. These inverters are always used in solar power plants connected to the grid and are more expensive than the standalone inverters. “This allows a solar panel system to work in unison with the grid, transferring its generated power to the utility company. Of course, one gets paid by the utility company for the power one produces. A synchronised inverter is required for a grid tied solar panel system,” says Ravi Mundra, CEO, Truepower International Ltd.

String inverters for grid tied solar systems: Here, photovoltaic (PV) generators are sub-divided into individual module surfaces with strings attached, and each of these has its own string inverter allocated to it. This technology ensures system costs are reduced and at the same time, it makes installation a lot easier and increases the energy yield and system availability. Typically, these inverters range from 2 kW to 20 kW.

Central inverters: These inverters are particularly suitable for building PV systems with a homogeneous structure, such as modules of the same type with an identical alignment and inclination. These inverters range from 66 kW to 500 MW.

Battery backup inverters: These inverters draw energy from storage devices that are charged with dedicated chargers and they send additional energy to the grids. During a utility outage, battery backup inverters supply AC current to the chosen loads.

Hybrid inverters: A solar hybrid inverter is a type of electrical inverter that is made to change the DC from the PV array and battery into AC for use with electrical appliances. If the mains and solar power are both available, the charging current is taken from both sources (solar and the mains), with priority given to solar power. If utility power has shut down and only solar power is available, the required load will be supplied by solar as well as battery power. If the load is such that it can be supplied directly by the energy generated from the solar panels, the battery will not be discharged.

Upgraded technology

With an array of options available, solar inverters are giving stiff competition to their conventional counterparts. Manufacturers are continuously working towards increasing the efficiency of the inverters. Their main focus is to graduate to higher efficiencies, greater reliability and finally, reduce costs. Catering to the specific needs and requirements of the consumer, manufacturers are constantly trying their hand at the latest technologies.

Micro inverters using solar energy are a recent phenomenon. These systems have started replacing solar string inverters in small residential solar installations. A solar micro inverter is just like a solar inverter, which works with a single solar panel in contrast to a string inverter, which works with 10-5000 solar panels depending on the size of the string inverters.

There are various benefits of this technology. Solar micro inverters are more reliable as the failure of a single piece affects only that solar panel and not the whole solar array of solar panels, as in the case of solar inverters. These inverters eliminate the need for string sizing exercises that require fitting equal lengths of module strings on a roof, thus reducing wiring time. Each micro inverter works with a single solar panel and operates at a much lower input voltage. Lower power allows a smaller number of components, which also decreases the chance of integration problems.

The technology has not been widely adopted by the market. Delta Group India soon plans to launch micro inverters in India. “Our efforts are focused on a new micro inverter based on a redesign of the first Delta micro inverter launched in 2002,” informs Dalip Sharma, managing director, Delta Group India. The company also offers inverters that are compatible with both crystalline and thin film panels. It is a leading manufacturer of solar inverters. Delta is also coming up with a new solar inverter manufacturing facility in Chennai.

A new development in inverter technology is modular central inverters that combine the advantages of string inverters and block central inverters. These are designed to operate with highly cost efficient, large scale PV systems. Delta’s three phase grid feed-in occurs symmetrically and is, therefore, particularly stable. Delta Group claims that the Solivia CM central inverter manufactured by it is the world’s first 100 kW modular central inverter, designed for the highest system reliability and for easy maintenance.

Though there are many companies that provide inverter solutions in the market, most of them are suitable for European grid conditions. Keeping in mind the grid conditions in India, consumers have to ensure that the inverters are customised for local conditions. Delta Group has thus come up with ‘AP-Asia Pacific’ models for consumers in this region.

New technologies are putting surplus energy generating home and SOHO systems within the reach of many more customers. Among these latest technological innovations, grid tied systems are gaining popularity. “Traditional inverters bear the burden of the entire system and so have a shorter lifespan than those used for solar applications. Micro inverters are attached to single panels, converting DC to AC at the source and since they handle lower voltages, they tend to last longer and operate more efficiently,” says Ravi Mundra. Truepower offers solar hybrid inverters from 50 VA to 5 kVA.

Among the developing technologies, hybrid inverters are one of the preferred options. Governed through dual mode, these inverters are designed with extreme flexibility. “These inverters are not only cost efficient but also require minimal maintenance and their automatic system ensures better power management,” informs Sreekumar, director, Convergence Power Systems Pvt Ltd. Offering a wide range of solar inverters, Convergence Power Systems manufactures hybrid inverters and solar dual battery bank inverters, ranging from 50 VA to 10 kVA.

The industry is moving to higher efficiencies and minimal system losses by giving 99 per cent efficient power conditioning units. The solar power conditioning unit offered by Su-Kam is compact, and enclosed with solar chargers and grid chargers with LCD displays and remote monitoring systems. “It gives users the option to choose the source of energy, which can be either solar power or power from the grid, as per the requirement. Its compact size offers better efficiency. We also offer high frequency inverters for smaller appliances, which offer great efficiency and reliability as there are no distortion losses, while transporting and installation is very easy,” says Kunwer Sachdev, managing director, Su-Kam Power Systems Ltd. The company manufactures a wide range of solar inverters, including solar sine wave inverters with pure sine wave for critical applications. Uniline also offers inverters with high efficiency and inbuilt charge controllers (upto 50 kVA), both off-grid and on-grid.

To attain greater efficiency and keep a track of distortion, manufacturers are adopting innovative methods like sun-tracking systems for rooftop standalone installations, to get 20-30 per cent extra power from the system.

“A number of new initiatives have been taken up by manufacturers to make the technology more adaptable and accommodating. These include opting for higher efficiency solar modules, storing extra energy for future autonomy, using nickel cadmium (NICD) batteries to extend the life of existing batteries, and placing maximum power point trackers for each module in order to produce more energy from the same amount of light radiation,” informs Kunwer Sachdev.

Things to look for while making purchase decisions

Sreekumar opines that the main factors consumers need to keep in mind are their own requirements. “Since the industry offers customised designs keeping consumer preference in mind, buyers need to evaluate their requirements and the way they want to utilise it. Efficiency is one basic and core element that should be given first priority. The efficiency of solar modules, batteries and inverters should all be kept in mind while opting for a solar inverter,” he says.

Kunwer Sachdev shares the same opinion: “While opting for a solar inverter, installation plays a vital role—whether the mounting structure for the module is galvanised or powder coated. Galvanised mounting structures will last long and remain protected from rust. Also, the wind speed it can withstand, its tilting angle, etc, should be kept in mind.”

One of the basic concerns of the buyer is reliability. While making a purchase decision, the consumers worry about the credibility and experience (related to technology) of the manufacturer. “The product should be procured from a credible and reliable source that offers after sales support. Also, the buyer should keep in mind the size of the company, the technology on offer, efficiency of the inverter and past performance,” says RK Bansal, managing director, Uniline Energy Systems Pvt Ltd.

While opting for a solar inverter, technicalities should be given first priority. Thus, the type of waveform must be considered. It is recommended that buyers opt for sinewave high frequency inverters due to their higher efficiency that results in lower power loss. Charge controllers must be based on pulse width modulation (PWM) for longer life of the battery. Maximum power point tracking (MPPT) charge controllers are good but cost a lot. So all these factors should be kept in mind while opting for a technology that is highly price competent.

“A local presence for after sales service, the infrastructure and readiness in terms of spares are important criteria for customers to watch out for. Considering the long life of solar products, which is normally for 25 years, consumers should be careful in choosing an established brand which can serve them over that tenure,” informs Dalip Sharma.

Differences between solar and conventional inverters

There is not much difference between a normal inverter and a solar version, except for the energy supply. A normal inverter is made to convert direct current (DC) from a battery into alternating current (AC). Whereas, with a solar inverter, a string of photovoltaic (PV) panels is used as the DC source. This requires more sophisticated functions such as maximum power point tracking (MPPT) to get the maximum energy out of the PV modules. The components are used to take care of the DC voltage variation, subject to the intensity of the sunlight (solar energy). Charge controllers, especially, have to track the input DC voltage continuously and use maximum DC voltage. Power inverters use conventional energy to charge batteries, whereas solar inverters use pure solar energy to generate power. For the Indian climate, solar inverters are more useful than other inverters as they not only cater to the growing need for power but also save electricity.

Applications of solar inverters

• Household and SOHO installations: Suitable for all household electrical applications such as computers, televisions, fridges and microwaves
• Rural electrification
• Commercial/industrial systems: Provide office power backup
• Utility scale PV power generation plants

Note: The names of the companies are in alphabetical order

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