Understanding the Challenges of Energy Transmission: Why We Can't Send Energy Over Long Distances Efficiently

Efficient energy transmission over long distances has long been a subject of debate among researchers and engineers. While direct current (DC) and alternating current (AC) both have their unique advantages, the practical implementation often hinges on the specific use case and the underlying physical principles. In this article, we explore why it is difficult to send energy over long distances without significant losses and the current methods used to optimize energy efficiency.

The Role of AC and DC in Energy Transmission

We can and do use DC for long distance transmission, but it is not without its challenges. High voltage is crucial for moving the highest power possible over practical diameter conductors. AC, on the other hand, is the normal route for achieving this as the voltage can be boosted using transformers, which are essential for long-distance transmission. However, DC also has its advantages for large distances, including the avoidance of some inductive losses and synchronization issues in a grid.

For instance, Hydro-Quebec uses DC transmission to supply the Boston area from its source generation near James Bay to its conversion operation at Sandy Pond, NH, a distance of 1700 km. This is a prime example of where DC is utilized, but it is still subject to the same principles of power transmission.

Advantages of DC Transmission for Long Distances

DC transmission has some significant advantages, such as reduced inductive losses and no synchronization issues. Moreover, the current setup with AC-DC conversion in the transmission process ensures that the energy can be efficiently routed over long distances. The power generated is initially AC, then it is stepped up using transformers before being rectified to DC for long-distance transmission. In the distribution phase, it is converted back to AC using another set of transformers.

The Challenges of AC Transmission for Long Distances

Shorter long-distance runs typically use AC because it is much easier and cheaper to change voltages and it is already in the AC form that the loads are expecting. There are some losses in the lines due to capacitance and radiation inductance. For very long, single-purpose lines, DC is often the preferred choice due to the absence of AC-type losses. These lines can operate at very high voltages, such as megavolts, without the typical AC losses.

However, transitioning between AC and DC is costly and not typically done for lines with many taps. This makes AC the preferred choice for shorter runs, while DC is more suitable for the very long, high-voltage transmission lines.

The Physics Behind AC Transmission Efficiency

Although some power transmission systems do indeed use DC, the answer provided below relates to AC systems that are still in use today. The underlying physics principles of AC power transmission are crucial, especially when addressing school physics exam questions. The key is to ensure that as much power as possible is transmitted from the power station to the end user with minimal losses.

The fundamental idea in power transmission is to minimize both I (current) and R (resistance) to keep transmission losses low. Resistance can be minimized by using large diameter cables, but there is always an upper limit to this diameter due to structural constraints. Current, in particular, needs to be as low as possible as it appears as a squared term in the losses equation. Since electrical power is measured as Volts × Amps, reducing current requires increasing the voltage. In practical setups, the voltage for AC transmission may range from 275 to 400kV. Transformers are used to step up the AC voltage near the power station and to step it down again near the end users.

These transformers only work with AC and not DC, so AC is used to change the voltage both up and down. The power loss is minimized by using the appropriate voltage levels, making AC the preferred choice for long-distance transmission.

Conclusion

In conclusion, while both AC and DC have their advantages, the choice between the two depends on the specific requirements and constraints of the transmission line. AC transmission, despite its inherent losses, is more efficient for shorter long-distance runs due to its ease of voltage transformation and compatibility with existing infrastructure. On the other hand, DC transmission is more suitable for very long, high-voltage lines where minimizing losses is the primary concern. Understanding the physics principles behind these choices is essential for optimizing energy transmission over long distances.