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On-grid vs off-grid solar power system

An off-grid system with necessary battery back-up or to share partial household load could be more expensive than a mounted solar system for on-grid solar system. The cost of solar panels has gone down significantly while battery prices are going up gradually.

Sabbir Choudhury
MASSIVE deployment of off-grid systems in cities and urban areas where electricity is limited is becoming more and more unpopular. Cost being the main factor, compromises are made in quality of the panels, charge controllers, and number of batteries needed to supply the required AH to support the prescribed load. In addition to that, the power factor of the loads is not taken into consideration. These days, the major portion of the off-grid AC loads are CFL bulbs, which are not known for good PF. Samples from major brands were collected from Bangladesh market and tested. And a shocking fact was discovered. The on-grid system directly injects AC current by inverting (synchronised) Solar DCV directly into the home AC outlets through which it becomes a part of the power grid systems without relying on battery and the household load.
While consumers in rural areas are getting the benefits of off-grid systems, the city dwellers are not getting the return from their investments. The government initiative for enforcing solar usage is appreciable. The mandate is to run a certain percentage of the household load by solar. But a loophole in the rule has created a massive business opportunity by using the term "back-up," which made the usage of the off-grid systems battery-dependent and worked only during power outage. An off-grid system is now just an expensive alternative for IPS. For a typical consumer it is very difficult to add a system which is double if not triple the cost of IPS. For environment-aware citizens this is a good solution because they know that they are consuming some energy from a green source (at least during power outage). There are several other factors that are making off-grid SPS in urban areas unpopular and less effective.
When a typical consumer asks for load that is to be supported by the solar system, he refers to the load that is to be covered inside his household. The service
provider reads that in a different way. The provider quotes the same amount as is needed for solar panel capacity. The difference is that the consumer is asking for AC load while the service provider is calculating that as solar produced power, which is DC. The consequence is that the consumer gets 20-25% less power at his end. The power factor is not taken into consideration while using AC load which is vulnerable to PF. The capacity of the battery is not calculated in a fair way to support consumer's load.
City dwellers who use off-grid solar systems mostly use CFL energy saving bulbs. While they produce four time more lumens/watt than conventional incandescent bulbs, CFL bulbs are not known for good power factor. The impact of these low power factor bulbs, which account for approximately 50% of the off-grid load, is introducing a secondary effect of drawing lot more current from the battery than originally calculated, hence burning the battery AH lot quicker than expected.
There are several advantages of grid tied (on-grid) solar systems. This is one of the reasons they are one of the most popular forms of renewable energy in use today worldwide. In short, grid tied solar systems can easily save the user hundreds, even thousands of taka in energy costs/month. Today's grid tied solar
systems are easy to purchase, easy to install and require virtually no maintenance to keep operating at peak efficiency for years and years.
What are the major advantages of grid tied solar systems? The first advantage is that they are more efficient and cost-effective than before. Over the past few years, there have been significant advances in solar technology including synchronous inverters, which has led to solar panels with efficiencies of 15-18%, instead of the 10-12%, and inverter efficiency up to 95%, just a few years ago. This system can contribute directly to the power crisis situation in Bangladesh. The gap is huge -- and is widening — between the generated power and the demand for power nationally. The on-grid system is a good filler to narrow the gap, if not fill it completely. The on-grid system pays back to the user by reducing sharing of the load with the utility power and hence reduces energy cost. In developed countries a system called "net-metering" is used, which turns the meter in reverse direction to immediately reflect the consumer contribution. Since this system does not use batteries, it is no threat to the environment. Due to that, it is a low to no maintenance system.
While on-grid system directly injects current into the utility system, one limitation of the inverter that is used in this system would make this system nonfunctional during power outage. The inverter is a synchronous inverter, which means it uses a synchronisation circuit to read the grid frequency and generate one to adjust and match the phase before it starts feeding current into the grid system. During power outage no frequency information is available to
this inverter and hence it does not produce its own because it does not know what to produce. The consumer will be without a back-up during power outage. To address this issue a solution could be to make a hybrid system. This hybrid will have an extra low-cost off-grid inverter or hybrid inverter with minimum battery back-up for designated critical loads. An off-grid system with necessary battery back-up or to share partial household load could be more expensive than a mounted solar system for on-grid solar system. The cost of solar panels has gone down significantly while battery prices are going up gradually. The cost of grid-tie inverters is higher than off-grid inverters due to the complexity of the design and operation of a synchronous inverter. But this extra cost offsets the battery price for an off-grid system and at the same time contributes power to the grid system as soon as it is deployed.
When off-grid system is designed accurately to share certain percentage of household load dedicatedly it will certainly minimise our energy crisis. Monitoring of the design and installation has to be done by the concerned power supply authority. There are just too many variables to consider for making real use of this system. Real capacity of the panels, real capacity of the load, type of load and distribution of solar power inside the house to cover certain household load portion make off-grid a complicated system to deploy effectively.
It has been a bitter debate for quite some time now in Bangladesh on whether on-grid system is a viable solution. The opponents raise concern about the synchronicity of on-grid type inverters. They also question whether the existing transmission line is capable of handling additional current that will be injected by the individual on-grid user. When most of the world is using solar as a grid-tie power producer, should not Bangladesh give some serious thought on implementing this? Several MW worth of solar panels had been installed on rooftops in the last couple of years, how much of it is really contributing to our power crisis more than it is contributing to the battery manufacturers? Isn't it time we used solar power the way it is meant to be used? The writer is Director, Sho uro Bangla Ltd.
E-mall: sabbir@shourobangla.com