Unit 1: Exploring Renewable Energy

Section B: What is the Difference Between Off-grid and Grid-intertied Systems?

The United States is crisscrossed by a network of electrical lines, transformers, and substations that transport electricity from power plants to homes and businesses. This network is called the utility electrical grid, because all of the lines in a region are connected together. This allows the utility to direct electricity to the areas within a region that have the greatest demand. For example, electricity produced at the Point Beach nuclear plant in Wisconsin can be directed towards the east coast of the U.S. when their need for electricity increases due to heat waves, power plant failures, etc. In the U.S., we have three separate utility electrical grids; one west of the Rocky Mountains, one east of the Rocky Mountains, and one in Texas.

There are many ways that renewable energy can be incorporated into your home and lifestyle. A renewable energy system that generates electricity can either be tied into the grid (grid-intertied system), or it can operate separately from the grid (off-grid).

Off-grid renewable energy systems are not connected to the utility electrical grid. These systems have no wires bringing electricity on or taking electricity from the site. Instead, all of the electricity used at this site is generated on site by renewable energy sources such as the wind, sun, water, and/or the use of biomass.

There are several reasons why people choose to install off-grid renewable energy systems at their homes or businesses. First, off-grid system owners never pay a utility bill for electricity. Once the components of an off-grid system are paid for, these systems provide virtually free electricity for the site, with only minimal maintenance costs. Second, off-grid systems can provide reliable power. Since they are not connected to the utility electrical grid, off-grid systems continue to produce electricity even when there are power shortages, and downed electrical lines. And, finally, having an off-grid system means that you are not contributing to the burning of fossil fuels for electricity, which benefits the environment by decreasing the amount of pollutants that are released.

To provide constant power for a home or business, off-grid renewable energy systems require some sort of electricity storage. For example, if your off-grid system is powered by the sun using photovoltaic modules, you will need a way to store the electricity made while the sun is shining so that you can use it at night and on cloudy days. The most common way to store this electricity is by using batteries. Typical storage batteries for renewable energy systems resemble large lead-acid batteries, similar to those used in cars, and need to be replaced every seven to ten years. In fact, as of 2014, the cost of solar panels has come down so dramatically (now less than $1/Watt production cost) that batteries are now the most expensive component of off-grid systems. Depending on the type of battery deployed, the overall size of the system, and the number of days desired for the system to operate with autonomy (i.e. when there is no sun in the sky) the cost of batteries can easily exceed $4-7/watt installed cost. Although newer battery technologies such as nickel-metal hydride and lithium-ion may have the potential to transform solar energy storage in the future, they still need to become much more affordable to compete with existing lead-acid batteries.

Grid-intertied renewable energy systems are when some energy is provided by a utility while some energy is created on site by a renewable system. Some families may want to off-set their monthly energy costs so they add a renewable system, such as solar panels or a wind turbine, to create some of the energy they normally would have purchased from the utility. The benefits of having a grid-intertied system is that if there is a problem with your system or the system isn't producing as much energy as you need, you can use the energy from the utility. Another benefit is that if you produce more energy from your renewable system than you need, your meter will actually spin backwards and the excess energy will be directed back onto the grid for others to use. You will then receive either credits or a check from the utility company from the energy you banked or they "purchased" from your system. They must "purchase" the energy from you at the cost of what they sell it for to consumers.

There are two types of grid-intertied renewable energy systems: battery free, and Uninterruptible Power Supply (UPS). In a battery free system, the homeowner does not have battery storage to provide back-up power if the utility grid fails. Additionally, these systems are designed so that if the grid goes down, the renewable energy system is also shut off. This insures that the renewable energy system is not putting power onto the grid while utility workers are working on the lines. The benefits of a battery free system are that it requires no maintenance and it is the least expensive system to install (Average cost in 2012 of $4.90/Watt in the US and $2.20/Watt in Germany). A UPS system combines all of the benefits of a grid-intertied system with the additional benefit of having batteries to store back-up power for temporary use when the grid fails. As the name implies, these systems can provide very reliable, uninterrupted power. They are, however, the most expensive renewable energy system to install. UPS systems essentially combine all of the components of a grid-intertied system with the battery capabilities of an off-grid system as a back-up. As a result, the price of a UPS system will be determined by how long of a grid outage the owner wishes to insure against, which will determine how large of a battery bank must be purchased.


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