Overview

An additional necessary component of any renewable energy technologies solution involving either wind turbines, or solar arrays or both, is the balance of system or BOS. These components ensure proper and correct operation of the renewable energy technologies deployed, but also ensures the safety of the client, and as well safeguards against potential damage to the equipment including the turbine(s), solar array(s), battery array(s), cables, switches and electronic components managing the renewable energy generated during severe weather conditions or other potential calamaties, and ensures safe and normal operations of conventional electrical loads (appliances, electronics, computers, etc.). It is the BOS that ties together the various renewable energy technologies deployed and which can be grid connected (grid-interactive or grid-noninteractive) or grid dis-connected (off-the-grid). Grid-interactive deployments will typically require additional metering instrumentation, or modification of existing metering instrumentation in order to conform to net metering requirements.

Components

The following descriptions will follow the variety of main components found in a typical installation from the renewable energy technologies (wind tubine and/or solar array) to the point of connection with the grid. Other minor components such as stop switches and lightning arrestor are not described in this page. The illustration below offers a graphical representation of the components described.

Rectifier

Most wind turbines generate an alternating current with variable frequency (due to the variations of wind speed) and therefore, before charging a battery array or to feed the client's internal energy needs, the power generated must be converted from a variable frequency alternating current (AC) to a smooth

figure 1: sinusoidal wave

figure 2: square wave

direct current (DC) output by means of a rectifier. However, for solar arrays, the electrical power generated is already direct current, therefore, not requiring a rectifier and which can be connected directly to the battery array or to a DC load.

Inverter

An inverter is a device is responsible for converting the electrical energy generated by the wind tubine and/or solar arrays into an alternating current which shares the same frequency as the public electrical grid. Although there are some consumer loads (appliances, lighting, electronics) which can operate with DC, most conventional loads require AC that is of the same characteristics as the electricity from the public grid. Inverters are typically, the most expensive component(s) of the BOS (not including the battery array).

In addition to the conversion to AC, the output of an inverter, may not be exactly the same sinusoidal wave form as the public electricity, and therefore, must be "filtered" to smooth out the rectangular shaped sinusoidal waves into the smooth sinusoidal waves which reduce the electrical "stress" on sensitive household electronics. This process is known as "AC Filtering".

DC Disconnect

This component has a number of responsibilities which include a DC cut-off switch, separating the battery array from the electrical components of the BOS, circuit breakers that will flip should there be an electrical problem with the solar array system or the output from the rectifier connected to the power generation side of the wind turbine. The DC Disconnect component also in included in the over monitoring of the BOS.

AC Disconnect

This component has a number of responsibilities, similar to the DC Disconnect, but is positioned after the inverter. This component include circuit breakers that will flip separating the BOS from the existing service panel of the installation and from the inverter(s). In the event of a problem with the inverters, or for some reason, the RETS is unable to generate electricity, the AC Disconnect is configured to permit grid electricity to flow, bypassing the inverters in order to satisfy the energy needs of the client. The AC Disconnect component also in included in the over monitoring of the BOS.

Resistive Load Diversion

Periodically, whether or not a battery array is employed, there are times when the wind turbine and/or solar arrays are generating more electrical energy than can be consumed, such as when the batteries are fully charged, or there is no need to use the excess electrical energy. This excess electrical energy must then be "diverted" to some other electrical load so as to a) not damage the battery array; b) prevent a build-up of electrical energy with no place to go. Even with installations that are "grid-tied", during those instances when the public grid is experiencing a power failure, the RETS must divert its excess electrical energy to an alternative electrical load, because during a public grid power failure, the RETS must be separated from the grid.

Charger Controller

A component of a renewable energy system that controls the flow of current to and from the battery to protect it from over-charge and over-discharge. The charge controller may also indicate the system operational status.

BOS Monitoring

This device displays the current "health" of the RETS deployed, offering metrics that include DC => AC volume, current AC load on the system, indicators for net metering if applicable, charge value of the battery array, temperature of certain BOS components, and numerous other metrics.

It is the objective of SOLAR WIND Canada to develop the RETS further by offering clients configurable, practical packaged, turnkey solutions that intelligently map to their renewable energy requirements and therefore reducing the often complex and confusing nature of RETS sizing and deployment for clients.

References

1. Danish Wind Industry Association. "Indirect Grid Connection of Wind Turbines". Danish Wind Industry Association. 1997-2003.
2. All About Circuits. "Chapter 3: DIODES AND RECTIFIERS - Introduction". AllAboutCircuits.com.
3. Dr. Gary Johnson. "Wind Turbine Connected to the Electrical Network". Rainbow Power Company, Australia. November 21, 2001.
4. Tony R. Kuphaldt. "Mixed Frequency AC Signals". Lessons In Electric Circuits. 2000-2006 .