Direct Current

The electrical grid systems we use are based on Alternating Current (AC). AC is the form in which power is delivered from power plants all the way to the power sockets we use to connect our devices. While AC has many positive features, the rising star of electricity usage, and in some cases also generation, is Direct Current (DC).

All electronic devices, like laptops, TVs, mobile phones and LED lights, use DC. Converters are used to transform the AC from the electrical supply to DC. Furthermore, batteries store and release energy in the form of DC. In other words, while only some kitchen appliances in our homes are still better off using AC, for the rest DC is the way forward.

The movement to transfer from AC- to DC-based power systems is gathering momentum. This article explores the issue and the role Aurelia can play.

What exactly are AC and DC?

Alternating Current occurs when the charge carriers in a conductor or semiconductor periodically reverse their direction of movement; as a result, the voltage level also reverses. The standard frequency used in Europe and most other parts of the world is 50 cycles per second (50 Hz). In the Americas and some parts of Asia the standard frequency is 60 cycles per second (60Hz).

Direct Current is the unidirectional flow or movement of electric charge carriers. The intensity of the current can vary with time, but the direction of movement always stays the same. The term DC is used in reference to voltage whose polarity never reverses.

AC was chosen in the early days of electrification to serve as the standard for power distribution for several reasons. For example, it is easy to change voltage levels by means of a simple transformer, and electrical drives (motors) could be made simply to spin at approximately 3000 rpm (50Hz) or 3600 rpm (60Hz).

Nowadays, however, demand for DC is increasingly rapidly, as this is the form of electricity we use to power batteries, laptops, mobile phones, LED lights and much more. In addition, offshore windmills use high voltage DC (HV DC) connections to transfer power, as more power can be transferred using DC than AC using similarly sized cables. Closer to the end-user, the rapidly developing network of fast charging stations for electric vehicles is also using DC.

Below is a graph showing AC and DC waveforms.

AC and DC waveforms. The AC voltage is 230 Vrms (325 Vpk) 50 Hz; the DC voltage level is 230 V.

Typically, the mains AC supply voltage is quoted as being 230 VAC. However, the voltage waveform varies regularly with time between +325 V to -325 V, fifty times per second – 50 Hz.

A voltage waveform with a 325 V peak value will cause the same heating effect in a pure resistor as that of a direct current supply of 230 V. This value is known as the root mean squared (RMS) or DC heating value of the waveform.

As an example, a light bulb connected to a 230 V DC supply would glow with the same intensity as the same bulb connected to the mains AC supply.

The beauty of the RMS voltage is that it relates all the different waveforms together and simplifies power calculation so that

P = V * I = I * I * R                in both AC and DC circuits.

However, the peak value is useful as it describes the highest instantaneous voltage, which must be considered for safety.

Who wants industrial DC?

The need for DC in our everyday lives is obvious – just think of charging your mobile phone or turning on your LED lights. But the change to using more DC is not restricted to consumers only. More and more industries are turning to DC power distribution to reduce costs by eliminating unnecessary losses associated with power conversion and to increase the capacity of local site distribution networks.

Information communications technologies (ICT), metals manufacturing, finishing, plating, coating and other electrochemical processes all utilise HV DC and Low Voltage DC (LV DC) in their processes.

Generating efficient DC electricity on site and avoiding conversion costs offers real cost savings and efficiency gains to process operators.

Industrial manufacturing cell with robots using DC.

How to reduce energy consumption by using DC?

One example of utilising and benefitting from DC energy production is provided by data centres and everything linked to our “digital universe”. The amount of data that is being created, used and transported is growing at a faster rate than at any previous time in history. All this data traffic is possible only by means of dedicated physical infrastructure equipment that consumes an enormous amount of electricity. According to Digital Power Group, in 2013 the world’s ICT ecosystem was approaching 10% of world electricity generation; in other terms, ICT already uses more about 50% more energy than global aviation.

The growth rate of the digital universe.

Operators of this infrastructure equipment are understandably under huge pressure to reduce their electricity costs and increase efficiency. This has resulted in a drive towards DC generation and distribution at the facility level.

Switching from AC to DC distribution can:

• Reduce the number of rectification stages

• Reduce power conversion losses

• Increase the power capacity of existing cabling systems

• Increase system reliability

• Reduce maintenance costs

Intel Corporation estimates that DC distribution can generate energy savings of seven to eight percent over best practice 400/480 VAC high-efficiency distribution systems, with a 15 percent cost saving in electrical facility capital, 33 percent saving in space and 200 percent improvement in reliability. Furthermore, local distributed power generation can also produce chilling for data centres (see our application article about chilling) and thus provide additional benefits.

Electric vehicles

Another fast-growing segment for DC is charging stations for electric vehicles. It will be mandatory soon for highway service stations in most European countries to provide fast charging stations. For example, the UK has already announced a law that requires all major petrol stations and motorway services to have electric car chargers. In most European countries there are similar discussions about increasing the number of fast charging stations – either through financial support or legal obligations.

For a single fast charging station, the power need is 120 kW DC. Given that a service station would typically have a minimum of four fast charging stations, this means a power need of approximately half a megawatt. As these stations are usually in locations where no power cables for such consumption exists, a decentralised solution is required. In order to benefit the highest number of electric cars, it would be best to use renewable energy for this power generation.

Fast charging station for electric cars.

What does Aurelia have to offer?

Aurelia’s turbines can generate on-site DC power with multiple fuels while also providing the necessary heating and cooling. Aurelia’s gas turbines have been developed to generate Prime Power DC output at various voltages to directly support the DC bus at electrical efficiencies of up to 42%.

The Aurelia Turbines solution has the added advantages of:

• Reduced or zero dependence on electricity networks

• Ability to maintain business activity when connected to an unreliable or regularly interrupted network power supply

• Increase business activity when local network capacity has reached its limit

• Equipment reliability and durability

• Ease of operation, so you can focus on your core business

• Security of supply

We believe that our product is the most efficient small-scale gas turbine in the world with the efficiency and cost of ownership to allow small and medium scale organisations to generate prime power in either DC or AC for self-consumption on site.