Stand-alone Power Supply Systems

For the purpose of this Portal file, Stand-alone Power Supply (SPS) systems are small-scale (<50 kW) self-contained units, providing electricity independent of the main electricity grid or mini grid network. These systems are sometimes also known as Remote Area Power Supply (RAPS) 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.

SPS Systems (SPS) in Australia

In Australia stand-alone and hybrid power systems are used widely in remote areas to provide power for the following situations: small holiday homes and shacks, boats and recreational vehicles, small rural farms (single homes), large station homesteads (with multiple residences), remote aboriginal communities, small mining operations, and various telecommunications applications. The total number of systems in Australia is not accurately known but it has been estimated as being in excess of 10,000 systems.

Main Component Technologies

SPS systems range from small petrol generators able to power appliances directly, to more complex installations using only renewable energy, with a combination of both also being possible. A SPS system that has a combination of energy sources is termed a hybrid SPS system. Figure 1 illustrates a hybrid SPS system using a combination of a wind generator, solar panels, petrol or diesel generator, battery charge control system, battery storage and inverter. The system chosen is dependant on each user's needs, availability of renewable resources, their preferences and often, most importantly, their budget.

Figure 1 A Hybrid SPS system
(Image courtesy of The Sustainable Energy Development Office).

The main components can be grouped into 3 categories; components that supply power (generation equipment), components that store energy for later use (energy storage equipment) and components that convert one form of power to another or control the flow of power in a system (power conditioning and control equipment).

Generation Equipment

Traditional Fuels

Stand-alone generator sets, using petrol or diesel, are typical of most SPS systems. These generator sets can vary from small portable units to larger units installed in a dedicated power shed. The larger units will often incorporate auxiliary control equipment to automatically start the generator on demand. Some generator sets will produce DC electricity for charging batteries directly. More commonly a generator will produce AC electricity for running appliances and electrical equipment directly. Today, with the increased production of biofuels, it is possible to power such traditional equipment with renewable fuels, such as biodiesel and ethanol. For more information see the Biomass Portal files.

Generator units perform best when operated near their rated output. As the load on the generator decreases so does the efficiency of the unit. If a generator set runs for long periods at very low loads, significant maintenance problems can occur. In a SPS application there are often periods of quite low load. The traditional solution to this has been to only run the diesel during high load periods and concentrate all usage of electricity into these periods. However with developments in communication and other technologies it is often desirable to have a 24-hour power supply in remote locations to keep the equipment available and running.

Rising fuel costs and the impracticality of running generators for long periods at low loads has led to the introduction of renewable energy equipment, batteries and inverter technologies, which reduce fuel costs and maintenance and provide a 24-hour power supply. The diesel (see Figure 2) or petrol generators can still be used as a backup system to meet the load directly, or to charge the batteries when there is insufficient sun or wind. The level of generator usage will depend on the size of the load demand and the available renewable energy resources and equipment.

Figure 2 A Diesel Generator with control system and inverter in background (about 15 kVA)

Solar

Photovoltaic modules convert solar energy (sunlight) directly into electrical energy. Throughout Australia, a reasonable level of solar energy is received year-round and accurate data on solar radiation is available for many areas. The amount of energy received from the sun is quite predictable and can be used to predict the output of solar modules.
A small array, as in Figure 3, which has four solar modules, can produce up to 300 watts in peak sunlight conditions.

Figure 3 Small solar array.

For further information on PVs, see the RISE Information Portal Technologies.

Wind

The output from wind generators increases significantly as the wind speed increases, and wind speed increases as height above the ground increases. It is essential to obtain data for wind speed if the potential for using wind turbines at a proposed site is to be correctly assessed. Unfortunately, the required information on wind speeds is not available for many areas in Australia. In evaluating any available data, the effect of the local topography/geography needs to be considered, as it may cause uneven wind patterns that will affect the turbine's output.

Figure 4 A solar array and wind turbines at the RISE Outdoor Test Area (ROTA) at Murdoch University, Perth WA.

A wind turbine installed in an area with a good wind resource can produce energy cost-effectively. However, the available wind resource typically varies from season to season, which creates a significant variation in the wind turbine output. Backup generation devices may be required to meet any shortfall of energy during these times of low wind speeds.

For further information on wind turbines, see the RISE Information Portal Technologies.

Energy Storage Equipment

The amount of power produced by renewable energy devices such as photovoltaic cells and wind turbines varies significantly on an hourly, daily and seasonal basis due to the variation in the availability of the sun, wind and other renewable resources. This variation means that power is not always available when it is required, and on other occasions, there is excess power. Energy storage technologies are used to provide power when there is insufficient power being generated, and to store excess production when there is more power being generated than can be used.

Battery Storage

Either vented cell (wet cell) or sealed cell (gel cell) batteries can also be used to store electricity generated by renewable energy conversion devices. In sealed cell batteries there is a minimum amount of electrolyte absorbed in a gel. These batteries have the advantage of being low maintenance as compared to traditional wet cell batteries, as there is no need to add water. Batteries in a SPS system must have good performance at low and high temperatures, a long cycle life in deep discharge applications and a high energy density.

For further information on battieries, see the RISE Information Portal Technologies.

Alternative Storage Technologies

For further information on enabling technologies, such as flywheels, supercapacitors, and superconductors, and fuel cells, see the RISE Information Portal Technologies.

Power Conditioning and Control Equipment

This link is to the RISE Information Portal Technology File on Power Conditioning & Control Equipment.

SPS Sizing & Installers

Correct sizing of SPS are very important, particularly if wind or solar energy is used. If the system is too small, power shortages will be experienced and the batteries may be damaged by excessive discharge. If the system is too large it will be unnecessarily expensive. The size of the system is dependent on the electrical load. The availability of wind and solar energy will also determine the size and type of system used. Suppliers of SPS systems generally have methods of designing a system to meet each user's specific needs. In Australia, accredited suppliers and installers use Australian Standards for the design and installation of systems. This accreditation of installers is performed by the Australian Business Council for Sustainable Energy. They provide an updated list of installers on their website. The design and installation of all SPS systems in Australia should always be performed in accordance with the following Australian Standards:

AS 4509 Stand-alone Power Systems Parts 1, 2 and 3.
AS 4086.2 Secondary batteries for use with SPS - installation and maintenance.
AS 5033 Installation of photovoltaic (PV) arrays.
AS 2676 Guide to installation, maintenance … of secondary batteries in buildings.
AS 3011 Electrical installations - Secondary batteries installed in buildings (LV batteries).
AS 3010 Electrical installations – Supply by generating set.

Determining the electrical loads that need to be supplied is essential. The electrical loads can be estimated if the power used by each appliance is known. The total energy required will depend on this power draw and the operating time of the appliance. Overall electrical loads can be determined by drawing up a list of all items, their power use and their average operating time per day. The total will be used in determining the size and type of system required (BCSE, 2005).

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 people to download provided by other quality institutions. Some of these highly regarded decision making tools and software programs are summarised below;

“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

SPS Installation

Several companies with experience in SPS systems should be approached for quotes. A preliminary site study is required to ensure that the chosen energy source will be suitable. For solar power, PV modules must not be shaded and trees must be checked for possible future shading. For wind power, data on local wind speeds should be collected to ensure enough energy is available from the wind year-round. The final electrical installation should be done by an electrician with experience in installing AC and DC electrical equipment, according to the relevant Australian Standards.

Using A Stand-alone Power Supply System

By using other non-electrical energy sources for cooking, hot water and heating, large savings can be made in electrical load, and therefore the cost of the SPS system. For cooking, gas or wood stoves can be used. Water can be heated using wood, gas or solar thermal systems. For space heating, a wood fire or gas can be used along with good insulation to help retain the heat. In construction of new buildings passive solar design techniques will help minimise energy requirements for heating and lighting.
The operating times of some large power consumption items may have to be scheduled because there may not be enough power available to operate them simultaneously. When replacing or buying new appliances, the effect on the SPS must be determined. Using energy efficient appliances can save on overall costs.

Links to Suppliers and 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.

SPS System Online Course - RISE Information Portal

Sustainable Energy Development Office ( SEDO Western Australia) - Remote Area Power Supply Program

Sustainable Energy Development Office (SEDO WA) - SEDO - RE RAPS Brochure (PDF)

Sustainable Energy Development Office (SEDO WA) - SEDO - RAPS User Guide and Maintenance Advice (PDF)

Australian Greenhouse Office - Remote Area Power Supply

Australian Greenhouse Office - Renewable Remote Power Generation Programme (RRPGP)

Solar Sales - RAPS Systems

IEA - Stand-alone PV Power Systems (off-grid)

Planet Power - Remote Area Power Systems

The Bushlight Program

References

Remote Area Power Supplies by the Victorian Solar Energy Council.

Rural and Remote Area Power Supplies for Australia by the Department of Primary Industries and Energy.

BCSE (Australian Business Council for Sustainable Energy), 2005. “Accreditation Case Study Overview” (Online) http://www.bcse.org.au/docs/STA/accreditation%20forms/Quick%20Find%20Forms/Accred%20-%20CS%20Overview%20SPS%20and%20GC.doc (Accessed February 28 2007).

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