Article - Sept 99
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Some need to maintain simple emergency power while others desire a seamless transition from one power source to another.

Jim Frazer,
Amerilon Products by Surf Networks

On & Off Grid Power Systems for Homes & Buildings

Off grid power management is used by a range of home and small building owners around the world. With the approach of Y2K, the functional and operational concerns of these users are now being shared by a much larger population. The goals of these users range across a wide spectrum. Some need to maintain simple emergency power while others desire a seamless transition from one power source to another. In order to achieve the goals of the end user, the available power sources, the environment, the type of loads, and a strategy for flexibility and scalability need to be evaluated.

Architecting a System

CatNet SystemsElectrical power can be obtained for the grid or can be generated from fossil fuel, wind or solar energy sources. Additionally electrical power can be saved in a battery bank.

Initially you'll need to decide what systems will need support. statmed.jpg (33029 bytes)These can include lighting, heating, cooling, security, water systems and kitchen operations / food preparation. A comprehensive off-grid energy delivery and management system may need to support:

The types of loads these systems represent fall into three major categories, high power steady state, high power with high inrush, and critical low power systems.

To support these varying types of loads, and future system growth, a distributed control architecture employing LonWorks technology has been developed. The wide availability of control components using LonWorks technology, and the features inherent in these devices allows the system designer a flexibility and potential feature set simply not available to users of other control network protocols. Commercial available LonWorks network control components, used in off grid power system projects typically include SCR controllers, power inverter interfaces, current monitoring nodes and both relay and solid state switching nodes. Auxiliary components can include a variety of user interfaces, ranging from PC-based software to dedicated display components. Options such as LonWorks-based weather stations and standalone energy controller nodes are also available.

What's an inverter? What features do I need?

The heart of an off-grid power system is the inverter. An inverter is a device which changes DC power stored in a battery to standard inverter.jpg (9469 bytes)household 120/240 vac electricity. Most renewable energy charging sources photovoltaic, wind etc. generate direct current (DC) voltage that is stored in batteries. Nearly all lighting, appliances, motors, etc., are designed to use ac power, so it takes an inverter to make the switch from battery-stored DC to standard house power (120 vac, at 50 or 60 Hz).

Inverters come in two basic types, square wave and sine wave. Most inverters sold today produce a modified square waveform, typically called a modified-sine or quasi-sine wave. Most alternating current (AC) devices will process this waveform satisfactorily, but there are some significant exceptions. Devices such as laser printers, which use triacs and/or silicon controlled rectifiers, may fail when supplied modified-sine wave power. Motors and power supplies usually run warmer and less efficiently on modified-sine wave power. Some devices, like fans, amplifiers, and fluorescent lights, give off an audible buzz on mod-sine wave power. Intelligent full sine-wave inverters can be programmed to perform a multitude of tasks like automatic generator start and stop, automatic load sensing, and generator support. Over-current, short circuit, over temperature, low battery and high battery conditions alarming are typically supported. When connected to a renewable energy system, the inverter should feed excess energy back to the utility grid. Acting as an uninterruptible power supply (UPS), your inverter should come on line within 40 milliseconds or less after a power outage.

As a utility interactive synchronous inverter, excess power from your generator, photovoltaics or stored battery power should be able to be sold back to the utility. Battery voltage sell point and maximum sell current should be adjustable. Peak load shaving inverter with onboard clock, is an important feature, which allows powering your loads during peak energy consumption times while recharging batteries off peak, when grid electricity is cheaper.

Should your generator need assistance starting a large load, your inverter should switch from charging to inverting mode, synchronize with the generator waveform, and give up give the generator an extra starting boost. A run and a cranking relay should also be available to automatically start and stop my your generator based on battery voltage.

Integrating the system

Once the appropriate control hardware has been selected and mounted, power and sensor leads need to be connected to each device. infodisplay.jpg (18926 bytes)Next a LonWorks network management tool is used to create a network database and the logical connections between inputs and outputs on the network. While it is a fairly easy task to commission a LonWorks control network, the flexibility inherent in the range of available components and the configuration of each device on the network allows both an unprecedented feature set and system scalability.

Thus, by building a home / small building energy delivery and management system using off-the-shelf interoperable components, the dual goals of energy savings and greater functionality can be easily realized. Due to the flexibility and robustness of LonWorks control technology these solutions are now available for a broad range of power system applications at attractive price points.

For more information on the LonWorks technology products described above, please contact the following vendors:

Echelon Corporation - - network management tools, terminal hardware
Trace Engineering - - inverters, packaged power management systems
Continental Control Systems - - current monitoring hardware
Amerilon Products by Surf Networks - - inverter interfaces
Cristal Controls - - SCR, thermostat devices
A3M - - user interface devices

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