Source: USA Today, Weather Almanac

Source: USA Today, Weather Almanac

How many degree days require you to put a log on the fire or turn on the air conditioner to be comfortable? Relative humidity and infiltration heat losses (or gains) due to wind affect human comfort, and this data isn't reflected in degree-day information. Thermal comfort varies with each individual and home, but homes that are well insulated and energy tight are more resistant to degree-day changes.

The National Climatic Data Center's Web site offers HDD and CDD information for more than 250 U.S. cities. Go to ncdc.html, click on "Free Data," and then select "Comparative Climatic Data," to find information for a city near you.

Chuck Marken • [email protected]


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AC Mini-Grids

The Future of Community-Scale Renewable Energy

Dana Brandt

©2005 Dana Brandt

This 3,600-watt photovoltaic array is part of a groundbreaking "AC mini-grid" system in Bulyansungwe, Uganda.

For decades, nearly all small-scale renewable energy (RE) installations have been DC-coupled systems. These DC-based systems are just like you read about in Home Power— where one or more DC sources like photovoltaics (PVs) and/or wind generators are used to charge batteries that, in turn, power DC loads directly, or AC loads via an inverter. Each charging source requires a controller to regulate the power from the RE source to the battery. While DC coupling is still the state of the art in most of the world, a method of AC coupling (connecting several charging sources together on the AC side of the system) has been developed that offers significant benefits for certain applications.

AC coupling uses batteryless inverters networked to one or more centralized battery-based inverters. This configuration allows AC to either go directly to AC loads, bypassing the batteries, or to charge the batteries via the battery-based inverter. Regulation is done on the AC side of the system by limiting the output of the batteryless inverters when the batteries are fully charged.

I had the unique opportunity of working with this new technology for my thesis project while doing graduate studies in renewable energy in Germany. My project was the design and installation of a PV and engine-generator hybrid system with an AC mini-grid. The system powers a rural boarding school complex in the village of Bulyansungwe, Uganda, where there are no utility lines within several miles. The people of Bulyansungwe are primarily subsistence farmers who grow bananas and coffee as cash crops. Bulyansungwe Secondary School is considered the center of the village because there is no conventional village center of houses and shops.

The school complex includes a girls' dorm, boys' dorm, classrooms, a convent, and a social center. The German organization, Together: Assistance for Uganda, has provided funding for its construction and operation. Previously, the only source of electricity was a small, gasoline-powered generator used on special occasions.

Hybrid Systems & Mini-Grids

Hybrid renewable energy systems have proven to be an excellent solution for providing electricity to areas with no utility service. Hybrid systems combine multiple sources to supply steady and reliable energy to consumers. Common system configurations often include one or more renewable energy sources (PV, wind, etc.), battery storage, and an engine generator for backup.

The majority of existing hybrid systems are DC-coupled, where all the electricity sources are connected to a single battery bank. The loads are then either powered directly from the battery bank or via AC from inverters. Recently, however, pilot projects and new installations have shown the technical feasibility of AC coupling.

An AC mini-grid is expandable and modular with standardized system components. Every system component (energy sources, consumers, and storage) is connected to a single AC grid. This allows the use of established residential AC standards for interconnection of the different devices. This is in contrast to DC-coupled systems, where there is no such standard and a wide variety of system voltages are common.

Adding more PV or battery to an AC-coupled system is as simple as installing that new component and connecting it to the grid without any modification to the original system. In the case of DC-coupled systems, modifications (sometimes significant) will need to be made to the original system.

Comparison of AC- & DC-Coupled Systems

This AC mini-grid concept is also often referred to as modular system technology. Since each component is connected to the grid, a sort of modular power plant can be constructed. A major advantage to such a system is that it can easily grow to meet increased consumption demands simply by adding more producers to the grid.

Estimating future electrical needs can be extremely difficult (especially for large systems), and often leads to underdesign or overdesign (poor supply or poor economics). Modular system technology is potentially a great solution because the supply can more easily grow with the demand.

Another attribute of AC mini-grids is their ability to be interconnected. Since they operate under common parameters, multiple systems can easily be combined simply by connecting their AC lines. In this way, small individual systems can be connected to form village grids, and village grids can be connected to form regional grids. Interconnection of systems results in greater overall security of supply because of the redundancy of suppliers.

AC mini-grids can also be incorporated into a utility grid at any time simply by connecting directly to the utility lines. This can be an important advantage in many areas where rural utility grids are expanding

The author on the completed PV array, with the Sunny Boy, batteryless inverters visible in background. This array is 75 feet (23 m) from the battery bank.

A view of the completed PV installation above the large school compound. An AC mini grid allows optimal placement of system components without the proximity restrictions inherent in DC-coupled systems.

A view of the completed PV installation above the large school compound. An AC mini grid allows optimal placement of system components without the proximity restrictions inherent in DC-coupled systems.

Coupled Hybrid Systems And Mini Grids

mini grid

DC-Coupled System

AC Mini-Grid System


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