Adding Solar PV to your Home – grid-tied or stand-alone?

In a previous article titled “Make Electricity while the Sun Shines” I gave a some insight into current PV technologies. In this article, however, I would like to look at the addition of a PV system to your home and thus a more detailed look at a solar PV energy system in its entirety. A solar PV system consist of the PV array and what is called the balance of system – which is the remaining components necessary to condition the electricity from the PV array into a more usable form of electricity. The actual components that make up the balance of system is dependent on the choice of implementation, on which this article will also be focused.

Solar PV systems fall into two main categories – stand-alone and grid tied systems. A stand-alone solar PV system, shown in the first and third diagrams below, is one that is completely operational without requiring any external support. It is able to operate independent of the local utility grid (e.g. VINLEC, BLP, T&TEC or JPS) by harnessing solar energy in times of abundance and, by means of battery backup, stores surplus energy for use at nights and on overly cloudy days. A stand alone system is usually more attractive where it is uneconomical to bring the utility connection to your premise.

Stand-alone PV system (Source:www.myace.in)

A grid-tied Solar PV system on the other hand, shown in the next and last diagram, is interconnected to the electricity grid and does not require a battery backup system; instead the grid is used as a means of storage. This facilitates the transfer of surplus electric energy to the grid on sunny days while at nights and on cloudy days electric energy is taken from the grid. The new net-metering and net-billing policy implementations throughout the Caribbean are intended to enable the implementation of this type of Solar PV system.

Grid-tied PV System (Source: http://www.solarhabits.com )

The aim when implementing a stand-alone solar PV system is to ensure that the system can provide all the energy requirements of your home, all year round. In so doing a typical stand-alone PV system generally comprise: a) the PV array – of course, b) a charge controller, c) batteries, and d) a inverter. In solar electricity (Photovoltaic) system the energy from the sun is converted into direct current (DC) electrical energy by the PV array. A PV array is collection of PV modules or panels (a group of PV cells) connected in series. The size of the array and thus the number of modules in the array is dependent of the energy requirement of your home. In a stand-alone installation a charge controller is used to channel the DC electrical energy produced by the PV array into the connected loads (via the inverter or directly) and the surplus into connected batteries. The main function of the controller, however, is to ensure that surplus energy, during an abundance of sunshine, is used to keep the batteries charged.

Stand-alone PV system (Source: http://www.top-alternative-energy-sources.com)-edited

Stand-alone System Schematic (Source: http://www.top-alternative-energy-sources.com)-edited

The batteries are necessary in this type of system to store surplus energy and thus act as a backup to the PV array. The batteries are connected in series (and parallel) to form a bank capable of supplying the full energy needs of your home at nights and during cloudy days when there is no energy output from the PV array. Battery banks in stand-alone systems are usually size to cater for two to three days without sunshine. The batteries and by extension the charge controller can, however, account for more that half the cost of implementing a stand-alone solar PV system and thus represents the main drawback. On the other hand it offers the advantage of being totally independent of the grid and thus eliminates the impact of grid outages. Does the convenience worth the extra buck? To some it does!

The inverter, another component that is common to both the grid-tied and stand-alone system, converts the DC electrical energy into alternating current (AC) electrical energy to supply majority of the connected loads. Modern household electrical appliances and other loads are designed to be supplied by alternating current electrical energy, however some of the standard household electrical needs, such as lighting, can be supplied directly from the direct current produce by the PV array. Thus, outfitting your home with a DC lighting system can put a dent into the overall cost of your solar PV system as this could drastically reduce the size of the inverter in addition to reducing the amount of losses incurred in the system due to conversion.

Grid-tied System (Source: www.mapawatt.com)

Grid-tied System Schematic (Source: http://www.mapawatt.com) – edited

Unlike the stand-alone system, the grid-tied system does not have to be sized to cater for the energy requirement of the entire home. This leads to the grid-tied system being more economically attractive as it could cost approximately half that of the stand-alone system due to the absence of batteries and charge control system. Another advantage of the grid-tied system is that it  presents an opportunity for the home owner to generate a continuous revenue stream from his/her solar investment by selling excess energy to the grid.

The grid-tied system as the name implies is synchronized to the local electricity grid, which acts as a backup to the PV array in times of zero output, thus it can only be implement in countries where access to the grid is granted through some sort of feed-in arrangement where surplus energy can be sold to the local utility in times of excess generation and energy can be repurchased from the utility when there is a deficiency in the PV output. Currently two such systems are implemented in a few Caribbean countries – the net-metering and net-billing policies.

Net – metering refers to the case where the price paid by the utility is the same as the price charged, often achieved by having the electricity meter spin backwards as electricity produced by the PV installation in excess of the amount being used by the owner of the installation is fed back into the grid. Net – billing on the other hand is some what a bootleg version of the net-metering concept, where the price paid by the utility is lower than the price charged and is currently achieved using two meters, one to monitor energy taken from the grid and the other to monitor energy supplied to the grid.

The choice of whether to go stand-alone or grid-tied is not an easy one, but one that is based mainly on economics and personal preferences – for various reasons! If you are looking to invest in a solar energy system, talk to a solar system consultant – ask him to walk you through the economics of both and see which fits best in your financial outlook. Going off the grid is a easy choice to make in a country like Jamaica where the energy price is overwhelmingly high, a strong dislike of and mistrust in the local utility and the grid is unreliable, but careful consideration must be given to both options before making a decision!

Smart Grid Platform (Source:www.hitachi.com)

I, on the other hand is a strong believer in the grid-tied approach, mainly because I believe that the energy produced will be used more efficiently if it is shared with others through the grid than to attempt to store it in batteries. I am also concerned about the number of batteries that will be going into the landfills in a matter of years as most batteries are manufactured with a life span of approximately 3-5 years, thus looking at it from a holistic vantage point the grid-tied system is the greener approach. Also, being a practicing utility engineer (not at JPS) has heightened my understanding of and appreciation for an integrated grid as oppose to stand-alone systems which are generally less reliable and more costly as shown above. Stand-alone PV systems are really a thing of the past and the future is grid-tied solar PV systems integrated into a smart grid platform, which will enhance the reliability and efficiency of all the energy sources tied to the grid!

Thanks for reading to the end! Feel free to share and leave a comment (or two) if you wish….

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