Renewable Energy and DC Power

Renewable Energy and DC Power. Are They Better Together?

Chuck Agosta - Clark University

There is a growing movement in the green buildings field to switch to dc power. In simplest terms, semiconductors, which are the heart of LED lights, computers, cell phones, flat screen TVs, routers, efficient motors, and many other types of electrical equipment including batteries, are inherently dc. Photovoltaic solar panels, ubiquitous on houses and other building these days and rapidly being deployed in large installations by utilities, are also

Installing the horizontal wind turbine on the roof.

inherently dc. With the present ac power standard, dc power from solar panels is converted to ac, and then back again to dc when it is used in appliances or stored in batteries. In each conversion step power is wasted. We have created a dc nanogrid where lights computers batteries and other loads are directly powered with dc electricity. Our goal is to see how much more efficient a building size renewable electrical system can be when it is pure dc system. In our present system we have eight solar panels on the roof of the Math-Physics building along with one vertical axis and one horizontal axis wind turbine. We have also installed a weather station up on the roof to record wind and temperature data. Wires were fed down old ventilation shafts to feed the power and information to our Big Board in the basement of our building were the energy is managed. The system also has 10 kWh of li-ion battery storage and there is a bidirectional connection to the grid, in our case the Clark cogeneration plant.

The renewable energy laboratory supports many projects, some as part of the "Technology of Renewable Energy" class and others as independent undergraduate or graduate student projects. Examples are the main nanogrid, an independent USB charging station for the Clark bistro, a tracking solarpanel mount, web enabled database management of the renewable energy system, energy analysis of Clark's new ASEC building and more.

The Nanogrid

There are two broad goals for the nanogrid system. One is to ake the system as efficient as possible and see if there are efficiency gains due to using only dc electricity. A second goal is to learn how to optimize the system for different operational metrics. A renewable energy nanogrid can increase resiliency and load shift peak power by taking advantage of the batteries. Making the system as autonomous as possible can lower the carbon footprint of the building, and opportunistic charging and discharging of the batteries can lower energy costs. Taking advantage of many of these attributes is called stacking, although it is not clear that all of these metrics can be a priority simultaneously. Learning how to manage the nanogrid and optimize these metrics is an important goal of this project.

The design of the nanogrid can be found here. It is based on products from Pika Energy, a company dedicated to using a standard high voltage (~400 V) dc power bus to connect all of its renewable energy devices, including controllers for photovoltaics, wind turbines, batteries and for bidirectional grid tie-in. A live webpage to see the status of the nanogrid shows a small fraction of the data that is collected for analysis. In a collaboration with the computer science department a group of students have also made a web based data browser and a machine learning engine that predicts the available solar energy for te next day based on NOAA cloud cover forecasts and previous data.