Targets on greenhouse gas emissions and transition towards renewable energy are ambitious. The generation of energy will be increasingly from variable renewables like wind and solar in all sizes, ranging from big offshore wind farms and large solar fields down to urban wind turbines on high rise buildings and rooftop solar panels at individual homes.
Not only the diversity in generation capacity will increase but also the diversity in ownership from a few owners of big power plants in the past to a mix of numerous owners of smaller and dispersed generating plants today and tomorrow.
The prospect of massively distributed clean renewable power generation is becoming a reality more than ever before. The present combination of technology improvements and market-scale developments is soon to be followed by a second wave of more capable and lower cost storage solutions.
Alternating Current (AC) power systems have been in dominant position for over 100 years due to the inherent characteristic from fossil energy driven rotating machines. The high-voltage, high-power grid today is based on AC technology. The large conventional generators connected to this grid are responsible for supplying power, keeping the frequency within limits, and maintaining the voltage within boundaries throughout the nodes on the grid. This has been predominantly unidirectional; i.e., from these large conventional generators to the consumers through the transmission and distribution system. The power supply, demand balancing, and voltage control in such grids have been relatively simple, mainly because of the availability and predictability of the generators.
The gradual changes of load types and distributed renewable generation in AC local distribution systems provide food for consideration of adding Direct Current (DC) networks. In the early stage, power systems were designed to supply the lighting, heating, and motor driving loads which are mainly AC type. However, load evolution in AC local distribution systems have been occurring quietly with the development of power electronics techniques and new lighting equipment for high efficiency of energy utilization and control flexibility.
Recently, two converging factors have renewed interest in DC power distribution. First, there are better alternatives for decentralized power generation, the most significant of these being solar PV panels. Because solar panels can be located right where energy demand is, long distance power transmission isn’t a requirement. Furthermore, solar panels naturally produce DC power, and so do chemical batteries, which are the most practical storage technology for PV systems.
AC power is in many cases converted back to DC power by the adapters of DC-internal appliances like computers, LEDs and microwaves. These energy conversions imply power losses, which could be avoided if a solar powered building would be equipped with DC distribution. In other words, a DC electrical system could make a solar PV system more energy efficient.
Secondly a growing share of our electrical appliances operate internally on DC power. Traditional AC motors as direct drivers for washing machines, refrigerators, air conditioners and various industrial machines are being gradually replaced by power electronics based AC motors in order to control the motor speed and to save energy. Within the next 10-20 years, we can expect an expansion of the total loads in households being made up of DC consumption. In, for example, a building that generates solar PV power but distributes it indoors over an AC electrical system, a double energy conversion is required.
Because the operational energy use and costs of a solar PV system are nil, a higher energy efficiency translates into lower capital costs, as fewer solar panels are needed to generate a given amount of electricity. Furthermore, there is no need to install an inverter, which is a costly device that has to be replaced at least once during the life of a solar PV system. Lower capital costs also imply lower embodied energy: if fewer solar panels and no inverter are required, it takes less energy to produce the solar PV installation, which is crucial to improve the sustainability of the technology.
A similar advantage would apply to electrical devices. In, for example, a building with DC power distribution, DC-internal electric devices can do away with all the components that are necessary for AC to DC conversion. This would make them simpler, cheaper, more reliable, and less energy-intensive to produce. The AC/DC adapters (which can be housed in an external power supply or in the device itself) are often the life-limiting component of DC-internal devices, and they are quite substantial in size.
Large advantage is possible in data centers, where computers are the main load. Some data centers have already switched to DC systems, even if they’re not powered by solar energy. Because a large adapter is more efficient than a multitude of small adapters, converting AC to DC at a local level (using a bulk rectifier) rather than at the individual servers, can generate significant energy savings.
At the moment, when we are ‘discussing’ the energy transition, there is a giant window of opportunity as similar in scope, scale, and character to the data/telecommunication industry’s disruptive migration to solid state computers, microprocessor-based electronics, and the Internet. DC power has come back as an increasingly strong opportunity, thanks to the technology advancements in power conversion, generation, transmission, and consumption. However, in spite of significant advantages in many applications, there are still several challenges to overcome and the DC technology should be integrated into the system through a smooth and step-by-step process. The DC technology has already started to be integrated into the existing AC system step by step. This is leading to the emergence of hybrid AC/DC systems in which AC and DC buses are connected through interlinking, bidirectional converters. Control of the interlinking converter, as the energy bridge between the AC and DC sides, is a critical issue for ensuring stability and utilizing the system potential to improve the quality of service. Microgrids, which are characterized by a combination of dispersed generation units, storage systems and loads, are one key application where hybrid AC/DC systems may offer significant benefits.
Microgrids, as a promising building block of future smart distribution systems, are one of the main areas where the DC technologies are expected to prevail. In particular, hybrid AC/ DC Microgrids may facilitate the integration process of DC power technologies into the existing AC systems.
What’s needed is an electrical energy network of power that can deliver the same systemic virtues to power systems that the Internet produced for communications: the concept of interconnected domains of smaller, more self-reliant grids. These grids should be equally capable of distributing both centrally and widely deployed distributed electricity generation. The present power grids do not get power from distributed sources; they are still highly centralized with little storage capability. Engineering marvels that they are, they have essentially been designed to distribute power generated at large central generation stations in one direction to loads where it is consumed.