Comparison of HVDC and HVAC for the transport of renewable energy
Global Landscape of Renewable Energy Deployment
An unprecedented development of renewable energy has been observed across the globe in the last two decades. Major economies worldwide are heavily investing in renewable energy sources to become environmentally friendly and implement the targets set in the Paris Agreement.
China is emerging as a world leader in renewable energy with heavy investments in this sector. In 2020, China accounted for 45% of the total global investment in clean energy. The United States falls second in this domain and has set very ambitious goals to be a carbon-free nation. Though Europe was already very active in renewable energy research, following the Russia-Ukraine crisis, European nations have accelerated the development of renewable energy to reduce their energy dependency on conventional sources and import-based fuels. The EU has set a goal of achieving a carbon-neutral economy by 2050. Germany & Denmark are leading nations in Europe for onshore and offshore wind energy.
Evolving Transmission Infrastructure in Renewable Energy Transition
An important aspect of the power network is power transmission. The first generation of renewable power projects used traditional HVAC networks for delivering electrical power to the distribution networks and is still in place. With the development of high-capacity wind and solar projects, especially those situated far from demand centers, the use of HVDC emerged as a more efficient and sustainable mode of power transmission. Advancement and research in power electronics have led to the development of HVDC technologies, including voltage source converter (VSC) HVDC and line commutated converter (LCC) HVDC that can carry bulk capacity of energy over long distances with minimum losses.
HVAC vs HVDC – Comparison in terms of Cost:
HVAC systems have been in use for almost a century now. There is a well-established supply chain of equipment manufacturers, companies, and market support for HVAC which makes its cost lower as compared to HVDC, which has recently emerged as an alternative way for transmission of power over large distances. The exact cost comparison will depend on various factors such as the length of the transmission line, the transmission power capacity, the terrain and environment of the line at the time of construction, power transmission losses, asynchronous interconnection capability, etc.
According to a report by International Energy Agency (IEA), the levelized cost of electricity (LCOE) for HVDC transmission systems lies between $80 to $120 per MWh. For HVAC, LCOE is between $50 to $90 per MWh. Another study from National Renewable Energy Laboratory (NREL) arrived at a conclusion that the cost of HVDC is approximately 2 times higher as compared to the cost of equivalent HVAC systems.
Several factors contribute to the generally higher costs of HVDC, for example hardware components that are not needed in HVAC infrastructure such as rectifiers, inverters, and HVDC circuit breakers. As of today, these crucial components are expensive, leading to higher costs for HVDC. On the other side, the benefits of HVDC and the emerging latest research in power electronics are making it viable for companies to invest in the HVDC domain, and eventually, HVDC system costs will be able to be brought down. HVDC uses either 2 conductors (bipolar HVDC) or one conductor (monopolar system) as compared to three conductors in HVAC.
HVAC vs HVDC – Comparison in terms of Distance:
There are some intrinsic power transmissions losses, such as the skin effect, and the corona effect in HVAC due to which the required conductor diameter of HVAC is greater than the HVDC system, which directly impacts the cost of the transmission line and limits the use of HVAC line above a distance of approximately 400 km – 500 km. After this breakeven distance, the cost of HVDC is less than the cost of HVAC systems. The HVDC system has no skin effect losses, has significantly less Corona losses, has a narrower right of way and has limited interference in communication lines, as compared to HVAC.
HVAC vs HVDC – Comparison in terms of Application:
For solar & wind power projects, which are close to load centers, HVAC is the best option, because of less distance. HVDC is much more expensive as compared to HVAC at small distance power transmission. But for long-distance transmission, HVDC is the best option because transmission losses are very less as compared to HVAC. Thus, HVDC lines are emerging as a sustainable power transmission medium for offshore. Renewable power projects which are hundreds of kilometers away from the nearest consumer base.
Similarly, when far-off renewable energy hubs generate substantial energy amounts requiring transportation over long distances, HVDC has emerged as an ideal choice. Australia-Asia Power Link is one such project that is going to carry the energy generated from one of the world’s largest solar farms in Australia to Singapore using an HVDC line. The transmission line length of this project is 5,000 km.
Assisting Vital Renewable Energy Projects through HVAC and HVDC Systems
Trans-European Networks for Electricity (TEN-E) is an EU program that includes several HVAC transmission line projects focused on renewable energy development, especially, the North Sea Wind Power Hub, which aims to create a large offshore wind power facility in the North Sea that will transmit electricity to the United Kingdom, the Netherlands, Germany, and Denmark. Multiple Original Equipment Manufacturers (OEMs) are part of this massive project. The most notable OEMs are ABB, Siemens, General Electric (GE), Prysmian Group, and Nexans.Global leaders like ABB, Siemens, and GE particularly emphasize the offshore development of large wind power projects, manufacturing switchgear, breakers, substations, and sub-sea electrical equipment. Prysmian Group and Nexans concentrate on cabling and communication infrastructure for this project.
Germany’s Energiewende is an ambitious plan for the development of renewable energy in Germany, focused on the transition to a carbon-neutral economy. The country has built a vast network of HVAC transmission lines to transport this energy from remote locations to population centers. The most notable companies working in Energiewende are Siemens, SMA Solar Technology AG, Enercon, ABB, Nordex, Kaco New Energy, etc. These companies focus on the market sector that includes switchgear, onshore and offshore wind turbines, solar panels, inverters, energy storage systems, and sub-sea & underground cabling infrastructure.
Boosting Renewable Energy Trade: Europe’s HVDC Transmission Lines:
NordLink, a massive 623 km long HVDC transmission line between Norway and Germany, is designed to transmit 1.4 GW of renewable energy from Norway to Germany. This project became operational in 2021. During the commissioning of this project, several OEMs were actively involved and made contributions to different aspects. ABB provided high-voltage cables and converter stations, Nexans supplied subsea high-voltage cables, Siemens provided HVDC transformers, Prysmian supplied land-based high-voltage cables, and Vattenfall provided expertise in offshore wind power energy trading aspect.
North Sea Link is another important 720 km sub-sea HVDC transmission line between Norway and the United Kingdom. This line, designed for the transmission of renewable energy such as offshore wind power from Norway to the UK, has a capacity of 1.4 GW. It became operational in 2021. For this project, ABB supplied the HVDC converter stations, Prysmian Group supplied high voltage submarine power cables, Siemens Energy supplied control and protection systems for converter stations, and NKT, a Danish company, provided HVDC power cables.
In today’s world, global warming, geopolitics, energy security, and climate change events are the key motivators for countries to shift energy generation sources from conventional to renewable energy sources. In terms of growth and expansion, solar and wind energy sources are growing rapidly. Other than the generation side, we are experiencing a technological shift in power transmission technology also. During the past century, HVAC has completely dominated the power transmission industry. However, currently, HVDC is emerging as a better alternative for power transmission. People attribute the worldwide acceptability of HVDC to the limitations of HVAC networks when they deliver power over large distances, especially distances exceeding 500-600 km.
Both HVAC and HVDC systems are contributing to the enhancement of the power sector for the transition into smart grids. However, in terms of cost, distances, and power transmission capacity, HVAC and HVDC have their limitations and comparative advantages. The HVAC transmission line is more suited for transporting power from solar and onshore wind power projects where distances are less than 600 km. HVDC transmission lines are best for power transmission over hundreds and thousands of kilometers. The best utilization of HVDC is for offshore wind power plants situated thousands of kilometers away from the shore, where nearly all the cables need to be laid at the sub-sea level.