Grid-Connected Power Systems

Grid-Connected Systems

Renewable energy devices such as PV modules and wind turbines are also being used on a small scale in areas where the electricity grid is available. People who want to make a contribution to the generation of renewable energy usually own these systems, or they are demonstration sites set up by electricity utilities or equipment manufacturers. The output from the renewable energy conversion devices is 'exported' to the grid (after being converted to AC at the correct voltage and synchronised with the grid frequency) during times of excess supply. When there is insufficient renewable energy power is 'imported' from the grid. The home or building owner therefore receives credit for the generated electricity that is offset against the imported power. The rate at which the utility will buy back the renewable energy varies, with some utilities operating a net metering system, where each kWh generated is equivalent to one imported from the grid, whilst other utilities pay a price that is a premium over fossil fuel generated electricity but lower than that charged to domestic customers. In some countries, producers are actually paid far more for the energy they produce than they pay for the electricity from the grid because of government subsidies, therefore making grid-connect renewable systems very attractive.

With a wind-based grid-connected system, the wind turbine will only operate when the utility grid is available as during blackouts the electricity from the wind turbine is not used due to safety issues. Grid connected wind systems can be practical if you live in areas with a good wind resource, have a permit from your local power utility, and have a reasonable power consumption or are remunerated for the power produced at a reasonable amount.

For further information on enabling technologies, such as inverters (powerconditioners and control equipment), wind turbines, PVs, flywheels, supercapacitors, and superconductors, and fuel cells, see the RISE Information Portal Technology Files.

This illustration shows how a grid-connected small wind system works. It shows the wind blowing a three-bladed wind turbine sitting atop a tower, which looks like a pole. The electricity generated by the wind turbine is shown traveling to an inverter. The inverter is a gray-colored, square box with two gauges near the top of the inverter box. From the inverter box, electricity is shown traveling to both a meter (a white, square box) and a house, which is identified as the 'load.' From the meter, the electricity is shown traveling to an electricity transmission, which is drawn as vertical pole with two smaller poles drawn at the top. The pole nearest the top is slighting larger than the one beneath it.

Figure 1 A schematic of a wind turbine grid-connected system.

Grid-connected photovoltaic systems are the most common type of grid-connected system. As electricity produced during the daytime is either used or directed back into the electricity grid, and at night electricity is purchased from the grid, there is no need for an expensive battery bank. This forgoes the added capital expenditure, maintenance and replacement costs of batteries, but this does mean that when neither the grid nor solar power is available there will be no electricity provided to the house.

Figure 2 A schematic of a photovoltaic grid-connected system. (courtesy of Cel-F Solar Systems).

In Australia, most electricity is supplied by utilities or electricity corporations from power stations via power supply networks, called grids. These main grids provide power to the majority of Australians using many large coal and gas-fired power stations, large hydro generation schemes and more recently, some smaller scale wind farms and photovoltaic systems. Remote towns like Port Hedland, Mt Isa and Coober Pedy are not serviced by the main grid and have gas or diesel power stations or combined diesel/wind power stations to provide their power via a mini grid.

Resource Assessment & System Design

Through the Information Portal, RISE provides networking services to assist people researching various renewable energy applications. There are many excellent decision-making and capacity building tools and software freely available for download provided by other quality institutions. Some of these are summarised below;

RETScreen

“The RETScreen International Clean Energy Decision Support Centre seeks to build the capacity of planners, decision-makers and industry to implement renewable energy and energy efficiency projects. This objective is achieved by: developing decision-making tools that reduce the cost of pre-feasibility studies; disseminating knowledge to help people make better decisions; and by training people to better analyse the technical and financial viability of possible projects.”

Visit the RETScreen International Clean Energy Decision Support Centre

These RETScreen files contain a collection of project case studies, including assignments, worked-out solutions (RETScreen Software Analysis) and information about how the projects fared in the real world. This document includes a background of energy technology and it provides algorithms for Project Models. In addition there are many case studies that provide succinct details on various renewable installations including system descriptions, lessons learned and many other important and useful information.

Areas that are included are;

Wind Energy	Small Hydro
Photovoltaics	Refrigeration
Biomass Heating	Solar Air Heating
Solar Water Heating	Passive Solar Heating
Ground-Source Heat Pumps	Combined Heat & Power

HOMER

“HOMER is a computer model that simplifies the task of evaluating design options for both off-grid and grid-connected power systems for remote, stand-alone, and distributed generation (DG) applications. HOMER's optimization and sensitivity analysis algorithms allow you to evaluate the economic and technical feasibility of a large number of technology options and to account for variation in technology costs and energy resource availability. HOMER models both conventional and renewable energy technologies.”

Visit the HOMER Optimisation Model for Distributed Power Page

HOMER allows up to three independent generation technologies to be included in the simulation model. Each generation technology can be a different size, cost, and fuel, or they can all be alike. HOMER dispatches the generators in an economically optimal way, meaning that each hour it chooses the generator (or combination of generators) that can meet the load and operating reserve requirements at least cost. It considers replacement, O&M, and fuel cost when making its dispatch decisions, as well as the value (if any) of the waste heat recovered from each generator. Homer allows an extremely large range of system configurations to be simulated. You can download and use the full version of HOMER for free. You must be a registered user to download the software. When you install HOMER, you automatically receive a free six-month license, which you can renew for free an unlimited number of times.

Sources/Systems incorporated in HOMER are:

Solar Photovoltaics	Battery Banks
Wind Turbines	Hydrogen Storage
Run-of-river Hydro Power	Daily Load Profiles with Seasonal Variation
Electricity Utility Grid	Defferable Loads (Water Pumping, Refridgeration)
Fuel Cells	Thermal Space Heating
Microturbines	Thermal Crop Drying
Generator: Diesel, Gasoline, Biogas, Alaternative and Custom Fuels, Cofired	Efficiency Measures

Grid Connected Inverters Standards Testing

Standards Australia has released three standards which are pertinent to grid-connected inverter systems. These are:

-AS 4777.1 - 2002 Grid connection of energy systems via inverters Part 1: Installation requirements.
-AS 4777.2 - 2002 Grid connection of energy systems via inverters Part 2: Inverter requirements.
-AS 4777.3 - 2002 Grid connection of energy systems via inverters Part 3: Grid protection requirements.

Inverters must be tested against AS 4777.2 - 2002 (or equivalent) by an appropriate testing laboratory. RISE’s ResLab is accredited to test to AS4777 (BCSE, 2006).

The Future of Grid-Connected Systems

Installed grid-connected photovoltaic arrays worldwide are increasing at an extremely rapid rate (see Figure 3). These extraordinary levels of growth will most likely continue to be driven by market support mechanisms that focus on grid-connected domestic applications in the urban or suburban environment. With growing interest in all forms of grid-connected renewable energy systems, the future of the grid-connected market seems bright. Figure 3 below shows the worldwide growth of installed capacity in off- and on-grid PV systems.

Figure 3 The growth of photovoltaic grid-connected systems worldwide (courtesy of the IEA's "Trends in photovoltaic applications -
Survey report of selected IEA countries between 1992 and 2005" )

Further Information

RISE Resources - Information regarding available renewable energy resources.

RISE Technologies - An extensive collection of information regarding renewable energy technologies.

RISE Applications & System Design - Renewable energy application information and system designs.

RISE System Displays - Case studies and information on installed renewable energy systems & performance data.

RREDC’s PV Watts Performance Calculator for Grid-Connected PV Systems

World Bank – Grid-Connected Renewable Energy

AGO’s Your Home Technical Manual - Renewable Electricity Overview

AGO's - Grid Interactive Systems

BCSE – Off-grid and Grid Connected Renewable Energy Systems

NAPS - Solar Electric Solutions

SunTechnics Australia

Cel-F Solar Systems

Energy & Environment

Sustainable Energy Development Office of Western Australia

References

Business Council for Sustainable Energy, 2006. “Grid Connected Inverters” (Online) http://www.bcse.org.au/default.asp?id=233 (Accessed 28 February 2007).

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