zaterdag 9 april 2011

The Smart Grid: Report (3)

This article is part of a series of article about the Smart Grid. It explains the concept of the Smart Grid, how it will evolve and what technologies can/will be used for it. The articles of this series make up a report that I have written for a course that I follow as a student. It is recommended to read this articles in the correct order, because else it will probably be not that easy to understand everything. If there are any comments, remark or such on this report, please do not hesitate to contact me or post it as a comment. 

Energy Storage - Grid scale storage
Within the concept of the smart grid, the issue of matching demand and supply of electricity is a big issue. But this is a very complex task to accomplish where several systems have to work together. The storage of electric energy can be very helpful for those who are responsible for matching demand and supply of electricity.

Energy, in general can be stored in a variety of physical and chemical forms. For the storage of electric energy, the options are mostly limited to the physical forms of energy. These physical modes of energy storage may involve one or more mechanical, thermal or electromagnetic forms. A particular energy storage technology may entail both a storage reservoir and a converter and transmission system for moving the power to and from the reservoir to its point of use.

This is a list of the most common technologies than are/can be used for the storage of electric energy on the electric grid:
  • Pumped hydropower,
  • Compressed air energy storage (CAES),
  •  Flywheels,
  •  Batteries,
  •  Supercapacitors,
  •  Superconducting magnetic energy storage (SMES).

Note: some of the technologies in the list above are still in a phase of development, and so they are (yet) not ready for commercial use.

With pumped hydropower and compressed air energy storage the electric energy is converted into potential energy by respectively pumping water uphill or compressing air. And by reversing this actions – allow the water the flow downhill again or the expansion of the compressed air – the previously stored energy can be recovered again. The recovered energy is then converted back to electric energy. This electric energy is then put back on the electrical grid.

Flywheels are able to store kinetic energy. In this case the electric energy from the grid is converted into kinetic energy. This conversion is realised with an electric motor connected to a mechanical flywheel. The mechanical flywheel can basically be any devices to and from rotational energy can be transferred. Be speeding up the rotational speed of a flywheel, energy is stored. To retrieve the stored energy, the rotational speed of the flywheel must be slowed down. This can all be done with an electric motor and electric generator, or an electric motor/generator. The flywheel must experience as little as possible friction resistance while rotating, else to much energy will be lost.

Compared to pumped hydropower and CAES, flywheels are more useful to store smaller amounts of energy over a short period of time. Whereas pumped hydropower and CAES are capable to store a relatively larger amount of energy over a long period of time.

There are three major mechanisms for storing electrical energy:
  •  Electrochemical energy,
  •  Electrostatic,
  •  Electromagnetic energy.

Batteries are used for storage of electrochemical energy. Electrical energy is stored and released in an electrochemical reaction cell that transports electrons too electrodes to carry out specific reduction/oxidation reactions. Electrical energy is stored and discarded in batteries by electrons that react at two different electrodes, a cathode and an anode, which are electrically connected by an ionic conductor or an electrolyte. The electric energy stored and produced by a battery is in DC form. For utility applications, it is normally converted to AC form using a suitable power inverter.

Supercapacitors store electrical energy in the form of confined electrostatic charges in a device consisting of two conductive plates separated by a dielectric medium. Recovery of the stored energy is achieved by connecting the conducting plates to a suitable load. Capacitors have the ability to be charged and discharged very quickly, on the order of seconds or less, which makes them useful for responding to power interrupts of short duration. Given that the power density of capacitors is inherently large, they have been used for mitigating power interrupts for many years in stationary utility applications over a range of scales.

In  superconducting magnetic energy storage (SMES) energy is stored and retrieved directly and with negligible losses using direct current (DC) flowing through superconducting coils to generate a magnetic field. To achieve superconductivity conditions will require some level of cryogenic refrigeration to maintain low temperature. While initial designs required liquid helium temperature near 0 °K, materials discovered in the 1980s have enabled superconductivity at liquid nitrogen temperature of about 77 °K, which reduces heat losses considerably and provides for more practical and economic SMES designs. (3)

Plug-in hybrid/electric vehicles and vehicle-to-grid (V2G)
Vehicle-to-grid (V2G) describes a system in which plug-in hybrid/electric vehicles communicate with the power grid to sell demand response services by either delivering electricity into the grid or by controlling the charging rate of the batteries of those vehicles. The types of cars that can be used for this are:
  •  Battery electric vehicles (BEV),
  •  Plug-in hybrid electric vehicles (PHEV).

Since most vehicles are parked an average of 95 % of the time, their batteries could be used to let electricity flow from the car to the power lines and back, with a value to the utilities of up to $ 4.000 per year per car.

The concept allows V2G vehicles to provide power to help balance loads by charging at night when electricity demand is very low and then sending power back to the grid when the electricity demand is very high. It can enable utilities new ways to keep the voltage and the frequency of the electric power on the grid stable and also provide reserves to meet sudden demands for power. In further development, it has been proposed that such use of electric vehicles could buffer renewable power sources such as wind- and solar-power. By storing excess energy produced during windy  periods or sunny days and then providing it back to the grid during high load periods. In this way V2G can be used to effectively stabilizing the intermittency of wind- and solar-power.

There are three different versions of the vehicle-to-grid concept:
  •  A hybrid or fuel cell vehicle, which generates power from storable fuel, uses its generator to produce power for a utility at peak electricity usage times. Here the vehicles serve as a distributed generation systems because it can produce electrical energy from conventional fossil fuels or hydrogen fuel.
  •  A battery-powered or plug-in hybrid vehicle which uses its excess rechargeable battery capacity to provide power to the electric grid in response to peak load demands. But these vehicles can then also be recharged during off-peak hours at cheaper rates while helping to absorb excess night time generation. Here the vehicles serve as a distributed battery storage system to buffer power.
  •  A solar vehicle which uses its excess charging capacity to provide power to the electric grid when the battery is fully charged. Here the vehicle effectively becomes a small renewable power station. (4)

Smart buildings
Not only appliances in households can be smart, but the entire house or building can be smart. The concept of smart buildings or also called building automation is the functionality of a building provided by a distributed control system. That control system is a computerized, intelligent network of electronic devices, designed to monitor and control the mechanical and lighting systems in a building.

Resident buildings can be made smarter so that they are more energy efficient. With the advent of sophisticated electronic controls, more options  are available to use energy in buildings more selectively. For example, buildings could be kept at energy-efficient temperature  levels outside of normal comfort zones and then brought up to comfort levels upon receipt of a timed or occupant-generated signal. Today, thermostats that control daytime and night-time temperatures at different levels are widely available. Within the house, control systems could be developed to climate condition only those spaces that are actually being used at a particular time. Looking to the future, one can imagine systems that only condition climate around the immediate space occupied by an individual.

With the electrical sector starting to use real-time pricing as an economic tool to level out power demand fluctuations, even more sophisticated automated energy-control systems may come into use. Smart windows – electrically switchable glass or glazing which changes light transmission properties when voltage applied – could be controlled for emissivity using sensors or timed programming. Appliances within the home could be to operate during periods of low electric demand to get favourable rates, and the waste energy could be treated accordingly to the overall design. Individual lighting controls based on daylight levels, as well as occupancy, could also save a substantial amount of energy. In a situation where there were strong incentives to reduce peak electric demands during midday heat, a system, could overcool at night and use thermal mass in the walls to maintain comfort during the day.

Commercial and manufacturing buildings typically require large amount of electricity for office and other types of equipment. Switching large machines on and off creates power surges, and control systems are available that not only monitor the electricity flow into the building but analyse the patterns of the different demands on the system. Control can manage the sequence of starting and shutting down equipment to prevent transients that might affect other equipment. These types of control systems might be extended to managing the interactions between climate conditioning systems, lighting and major types of equipment in the building. In the long term, local lighting around each person in the building may offer promise of still more efficient energy use. However, there will be trade-offs between the complexity of the control systems and the value of energy savings. (3)

Geschreven door Emile Glorieux

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