The Town of Hempstead, N.Y., combines solar energy with innovative electrical distribution and variable frequency drives to control energy costs in a green way. Many people have been joining the green movement to some degree, that is, searching for alternative energy sources as a way to reduce pollution and other problems that can result from burning fossil fuels. But the recent surge in energy prices has produced an additional incentive, driving many to also look for green technologies as a way to reduce costs.

Kate Murray, town supervisor in Hempstead, N.Y., was looking to find alternative energy solutions that would create savings for taxpayers. So, this town of 750,000 on New York's Long Island hired Nextek Power Systems, Hauppauge, N.Y., to bring its advanced grid-connected smart power platform to an installation of a 40-kW solar photovoltaic (PV) system on the southern face of its town hall. The solar system was to be the power source for part of the building's heating, ventilation and air conditioning (HVAC) system.

Town officials chose the town hall site for its central role in community affairs and its ability to showcase solar power not just for the return air fans for the building, but also for the DC, or direct current, lighting in the supervisor's office and a conference room. Ron Masters, commissioner of conservation and waterways for the town, was familiar and confident with Nextek Power Systems, a supplier of DC power networks and experienced in facilitating alternative energy projects. Paul Savage, president of Nextek, says the project reveals the convergence of two trends in today's facility power. First is the advent of distributed power generation, such as the solar installation at Hempstead.

A footnote to this trend is that all of these distributed power generators, whether solar or wind, are intrinsically DC, Savage adds. Then, there are an increasing number of loads in facilities that are all DC powered. In a typical distributed power installation, DC electricity is generated. It is introduced into an inverter to change it to the AC, or alternating current, power that is commonly used in facilities. Then the power must be turned back into DC at the load.

Nextek developed 'DirectCoupled' technology that establishes a DC bus in the building. This eliminates electrical losses inherent in the inverter. According to Savage, a typical solar installation is only about 89 percent efficient because of those losses. But his DirectCoupled technology improves that figure to 99 percent, Savage claims. This bus connects to all of the DC electronic loads, eliminating losses at that point as well, because conversion from AC to DC is no longer required.

There is yet another source of loss in an HVAC system, the variable frequency drives (VFDs) that run devices such as return air fans and compressors. A typical VFD contains a rectifier that takes the incoming AC power and changes it to DC. The electronics in the drive then transform the DC into a pulsed simulation of AC to power and control the motors.

This technology has been proven to greatly reduce energy consumption by these motors. But, those rectifiers also induce losses in the system. Nextek worked with local Allen-Bradley distributor Mid-Island Electrical Supply, of Commack, N.Y., to integrate PowerFlex AC drives into the DC bus system. Savage estimates an energy efficiency gain of about 10 percent.

Seamless conversion

At night or during periods of low solar output, the Nextek Power Gateway seamlessly reverts back to grid power until the renewable source is available again. Because the system is DirectCoupled, it is not subject to 'anti-islanding' regulations that require conventional grid-tied solar systems to shut down in the event of a grid failure. The Power Gateway also enables load shaping and peak mitigating with its advanced metering, monitoring and controls capabilities. During daytime peak usage, the HVAC system will be powered primarily by the solar PV array, reducing the burden on the local utility.

The anti-islanding regulations come into play, says Savage, when the distributed generator puts electricity back on the grid. Some people look at these alternative generating sources as a backup to when power from the grid goes down. So, if you have a unidirectional power setup such as this one, then you don't have to shut down when the grid goes down, maintaining a little independence.

The solar energy system was funded through $260,000 from the settlement of a clean air lawsuit against a power company in 2003. Under the settlement, the power company was required to pay $2.1 million for air pollution mitigation projects in New York state. The New York State Energy Research and Development Authority (NYSERDA) administers the $2.1 million fund to install solar energy equipment on government-owned buildings in the state. Many people aren't aware of commercial HVAC operations and the energy-saving opportunities with these motor-driven applications, says Masters. Demonstrating the value of these types of installations is important to furthering public understanding, and we see many other flat-roofed buildings that could be using and benefiting from this technology.

Variable frequency drives, which act as controllers to protect motors and improve motor performance, have long benefited HVAC fan and pump applications, but Hempstead's solar-power supply is unique. Allen-Bradley product line supplier Rockwell Automation Inc., Milwaukee, has a 100-year legacy in controlling motors in virtually every conceivable application, including HVAC, but pulling DC power off solar panels was a first. Masters is convinced that the three-phase Allen-Bradley PowerFlex AC drive with the DC source gave the town the ability to win the NYSERDA award. In addition, the accessibility of the DC bus made it simple to DirectCouple the drive to Nextek's equipment.

Unexpected efficiencies

The success we've had with Allen-Bradley PowerFlex drives is an efficiency we didn't anticipate when we started the project, says Masters. With the variable frequency drives, we can set and throttle back to idle the system to match our energy needs, and the exposed front end of the drive adds additional economy to the system.

Townspeople and officials are pleased with the new system, and it is setting an example across Long Island. To date, it has been a tremendous success, says Masters. It has performed beautifully. With the success of the town hall installation, Masters anticipates future successes in upcoming 'green' applications.

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