PrintReading Assignment
- J.R. Brownson, Solar Energy Conversion Systems (SECS), Chapter 9: Solar Economics (focus on Managing the Grid)
Power Grid 101
One of the highly visible SECS technologies is photovoltaics, which delivers (generation) electricity to the client, and now pushes excess electricity onto the electricity power grid. The electricity power grid is the physical system that delivers (transmission) electricity from the place where it is generated to the site where it is used (end-use, demand).The electricity leaving the generating station enters a sub-station with a step-up transformer that raises the voltage extremely high for long-distance transmission.
When electricity travels through wires (a conductor), some energy is lost, but less energy is lost when the electricity is transmitted at a higher voltage. At a high voltage, the same amount of power can be transmitted, but using a lower current. The amount of energy lost from the conductor is called line loss, and line losses are directly proportional to the current. By reducing current, we reduce losses for the same power transmitted. Typically, in the U.S., line losses between generation and end-use are in the 6% to 8% range.
The high-voltage electricity is carried over transmission lines to local substations, where a step-down transformer reduces the voltage to levels suitable for customer loads. Distribution lines carry the lower-voltage electricity from the local substations to customer sites.
On the left, there is a Generating Station with a line going into the “Generator Step Up Transformer”. This is the Generation portion of the diagram.
From there, it goes to the Transmission Customer (138kV or 230kV) and to Transmission Lines (765, 500, 354, 230, and 138kV). This is the transmission portion of the diagram.
From the Transmission Customer and Transmission lines, it goes into a Substation Step Down Transformer for distribution. This is the Distribution portion of the diagram.
From there, it goes to the Subtransmission Customer (26kV and 69kV), the Primary Customer (13kV and 4kV), and the Secondary Customer (120V and 240 V).
Try This!
The Power Grid is a simulation created by the Cyber Resilient Energy Delivery Consortium for education.
- Access the Power Grid animation.
- Examine the "Quick-Start Guide" as a helpful resource.
- Play with the simulation to understand the relationships.
Grid Energy Storage
Primitive as it may seem, the energy storage technology that is "grid-tied" and having the largest capacity is accessed by simply pumping water up to a higher elevation, and storing it as potential energy. Called pumped storage, or pumped storage hydroelectricity, the energy is recovered when the water from the higher elevation is used to drive turbines for hydroelectric power conversion.
The Energy Storage Association reports, "Pumped storage hydropower can provide energy-balancing, stability, storage capacity, and ancillary grid services such as network frequency control and reserves." While the US has 20 GW of installed capacity, worldwide over 100 GW of capacity exist. The US figure accounts for roughly 2% of the country's generating capacity, while other areas' figures are as high as 10%.
All in all, however, this process uses more electricity than it produces. So, why do it? When a power plant has extra capacity, it generates electricity used to pump water uphill. Then, when the plant is stretched to capacity and electricity is at its highest price, this pumped storage can be used to generate low-cost hydroelectricity.
Reference:
Modified from Vera Cole, Power Grid, EGEE 401. Accessed October 2013.