The various forms alternative energy generation share a number of problems. Cost tops the list, when compared to traditional fossil-fuel-based systems. It is assumed the price of basic equipment, such as solar panels and wind turbines, will drop once the market grows large enough so competition—promoted by expanded government subsidies—will stimulate research and economies of scale. However, other problems may be more difficult, though far from impossible, to solve. Two of the biggest are stabilizing and storing the energy produced by the sun and wind, and improving the process efficiency along the entire chain from initial production to final delivery to the end-user. These problems require design and development of new techniques as well as improvement of existing ones.
Humans have harnessed wind to provide energy for much of recorded history—-propelling boats, pumping water, powering gristmills, etc. The first known use of wind to generate electricity was by Charles Brush in Cleveland in 1888. It was a stand-alone unit that produced 12 kilowatts (kW) direct current (DC).
The first large-scale alternating current (AC) wind turbine was built in the United States in the 1930s. The physics of the process used is that the kinetic energy of the wind, which is a function of its mass and velocity, is transferred to a set of rotors, which drives an electric generator. The efficiency of this conversion has theoretical limits. If you think about it, 100 percent conversion of wind energy into electricity would require that all of the wind speed impinging upon the rotors be used to drive them, but then there would be no wind exiting the rotors. Sounds impossible, and so it is.
In 1920, German physicist Albert Betz demonstrated that the theoretical limit—now called the Betz limit—of how much wind energy can be converted into electrical energy is 16/27 (about 59 percent). According to the World Wind Energy Association (www.wwindea.org), modern turbines can convert up to 50 percent of the wind energy to electricity.
How much of the 59 percent maximum is actually used depends on a complex array of physical and electrical issues. First, a potential turbine-placement area must be evaluated in order to maximize the wind turbine’s output. Wind energy is higher for high air density, which means better placement in the low altitude areas. It’s also higher with greater velocity. Doubling air velocity will multiply the available energy by eight times. So it is very important to find a site with high average wind speed. The federal government provides a national wind resource map as a guide to areas of the United States that are the most consistent sources of wind energy (www.nrel.gov.rredc).
Once the location is chosen, the challenge is to design a mechanical structure efficient enough to take advantage of the available wind. The turbine’s rotor blades are similar to aircraft wings—wind flowing above and below the blades generates lift, which causes the blades to rotate. The harvested energy is greater if the rotational speed is greater, and rotational speed is directly proportional to the length of the rotor. The tower must be designed for the size and speed of the rotor and the combination has to be optimized to operate at a single wind speed—an unrealistic condition. If the wind slows down, less power is generated; if it speeds up, it could damage the system. Some turbines have protective mechanisms for high winds that tilt the blades away from the wind, stopping the turbine. Companies such as General Electric and Siemens manufacture turbines that use an electronic servo system to continuously adjust the pitch of the rotor blades to maintain optimum rotational speed as the wind velocity changes.
An advantage solar power has over wind is that the voltage produced by sun shining on a photovoltaic (PV) cell is constant. However, the ability of a solar array to feed power to a load is a function of the amount of sunlight that is available. This is greatly reduced on cloudy days and is nonexistent at night. Off-grid PV systems use energy storage to provide for dark or cloudy times. Grid-connected systems, on the other hand, generally don’t use storage. They either provide energy to the grid, or they don’t, depending on the ratio of their capacity to the demand. PV can best be understood as a source of current rather than voltage.
Connecting to the grid
There are three basic patterns for connecting alternate sources to the grid, regardless if the source is solar or wind.
• A central station, e.g., a wind farm or major solar array, would connect to the grid in much the same way that traditional power plants do. But solar, wind and fossil-fuel-burning plants have different operating characteristics, so making the tie-ins requires careful planning.
• A distributed system, e.g., a solar array on a rooftop or a single wind turbine feeding a farmhouse, but with the ability to connect to the grid
• A microgrid, e.g., a strip mall or condo development that has its own local wind or solar-fed grid that can also connect to the central grid
It is crucial that the photovoltaic or wind-generated power not disturb the grid it is tied into. A primary factor is the percentage of penetration: the proportion of the renewable contribution to the total energy. The greater the penetration, the more significant are any disturbances. Various studies have placed the maximum acceptable penetration at 5 percent to 40 percent. Any plan to expand renewable energy will have to deal very seriously with the destabilizing possibilities of increased penetration.
In addition, there are two major causes of power quality problems that relate to the grid connection. One is the changing condition of the sources—passing clouds for solar and varying speeds for wind. The other is the changing condition of the grid. It is possible that when demand increases suddenly, producing a load current surge, the voltage can rise for a short time to an unacceptable level. Under- and over-voltages have much to do with the relative impedances of the power sources and their transmission lines. This means that the system behavior depends on the details of the specific installation.
Central station interconnections raise a number of issues, as well. The major concern in connecting to the grid, and what utilities worry about most, is whether the renewable source will disturb the grid voltage or frequency. The second concern is that the solar or wind farm needs to deal with the varying condition of the grid, which is caused by changes in the size and quality of loads and sources and occasional transmission system faults.
Also, the requirements for connecting a renewable source to the grid must be hammered out. These are often determined by the utility, although national standards exist in some countries. These rules will have to be coordinated and will play an important part in the technology's development.
Finally, to meet interconnection requirements for reactive power and voltage control, devices, such as an S&C Electric DSTATCOM inverter-based compensator or a static VAR, switched capacitor/reactor-based compensator, are used. These devices can control wind farm power factor, regulate output voltage, and stabilize voltage during transmission events.
Inverters are a means to an end, a way to convert DC to AC. A double-ended inverter is used in the wind turbine to convert AC power to DC, and back to AC, so that the power factor and frequency can be controlled. On the ground, the inverter in a DSTATCOM is used to provide reactive power for better voltage and power factor control.
It’s hard to be definitive in discussing renewables because the industry is poised for rapid growth and the exact shape of how it will evolve is up for grabs. When I started doing the research for this article, I thought that the biggest problem is efficiency. I assumed that the highest losses are in converting the DC power generated by photovoltaic cells to AC, so we would have to improve the efficiency of the solid-state inverters, which do that.
I also wondered whether it might make sense to use DC directly. However things are much more complicated. The Renewable Systems Interconnection Study was prepared by the National Renewable Energy Laboratory (NREL) and released by the U.S. Department of Energy in February 2008 (www.nrel.gov/docs/fy08osti/42292.pdf). It provides an excellent overview consisting of 15 reports on issues related to large-scale integration of renewable energy sources into our national power supply. According to these reports, the most important problems have to do with how we integrate renewables with the existing grid.
Another major concern is energy storage, which is an absolute necessity for an off-grid system. Technically, storage is not needed for a grid-connected system. However, according to the 2008 Sandia National Laboratories report, “Distributed Photovoltaic Systems Design and Technology Requirements,” it can be useful “to enable intentional islanding or other ancillary services. Intentional islanding is used for backup power in the event of a grid power outage, and may be applied to customer-sited UPS applications or to larger microgrid applications. Stored energy may also be applied to grid ancillary services such as spinning reserve or frequency regulation.”
The era of renewable energy resources is just beginning. Building the transmission lines to connect the solar and wind farms to the grid will be a major issue. A few years ago NREL estimated that this would cost from $60–80 billion. An energy corridor to connect electrical power across the country has been proposed, but this has proven to be a very controversial idea. It is clear, though, that there is a worldwide desire to achieve this. There are some very good ideas for solving the technical problems, but they will require serious investment of money, time and resources on the part of government and industry.
BROWN is an electrical engineer, technical writer and editor. He serves as managing editor for SECURITY + LIFE SAFETY SYSTEMS magazine. For many years, he designed high-power electronics systems for industry, research laboratories and government. Reach him at firstname.lastname@example.org.