Changing patterns in electricity generation and consumption over the past decades have brought new life to the debate on the use of Alternating Current (AC) vs. Direct Current (DC) electricity supply in distributed generation systems. Direct Current distinguishes itself from Alternating Current through the fact that electric charge flows in a constant direction. When graphically depicted, DC output thus takes the shape of a straight line, whereas AC output shapes a wave pattern, crossing the zero output line in regular intervals.
Direct Current is native to solar PV electricity generation, battery storage and a range of common, low-power household and commercial applications, such as LED-lighting, consumer electronics (e.g. TV, radio, mobile phones) and variable speed drives in electric motors (e.g. for fans or pumps). With an increasing reliance on these components and applications, DC systems have, in many cases, become a technically and economically viable alternative to ‘traditional’ AC systems.
DC vs. AC mini-grids: System architecture and operation
The decision between DC and an AC provides for a number of implications in terms of a mini-grid’s system architecture and operation. In the following, a short overview on the main differences is provided.
Power control and management
Generally speaking, DC mini-grids tend to be simpler in system architecture and operation than AC systems. This is primarily a result of their limited requirements in terms of power control and management.
Due to an inherently unidirectional flow of current, a DC architecture does not require the usage of the frequency/phase control mechanisms of its AC equivalent. Hence, with only voltage to be accounted for, DC systems generally provide for a lower system complexity, reduced number of variables in systems monitoring, lower cost for control equipment components and a more modular and more easily expendable structure.
As of today, protection systems for DC mini-grids must be considered less mature in terms of practical experience, standards, guidelines and implementation compared to AC systems.
A particular concern in DC systems is the absence of natural zero crossings (points at which the voltage momentarily equals zero), rendering current interruption considerably more demanding and potentially more dangerous (occurrence of switch arcs). This is, however, not a primary concern for low voltage DC, such as the commonly used 12 VDC or 24 VDC, as principle availability of adequately sized protection solutions does not differ markedly from standard AC systems.
In providing for a more ‘natural’ solution for the growing number of native DC components in electricity generation, storage and consumption (see Introduction), DC can contribute towards reducing conversion losses and achieving higher efficiency in electrical systems.
Apart from distribution (grid length, voltage, etc.), overall system efficiency depends on power conversion stages between electricity generation and load. Efficiency losses from DC-AC conversion (via inverters) and AC-DC conversion (via rectifiers) considerable affect overall system efficiency. Occurrence of such conversion steps in a mini-grid system will depend on the type of generation used (AC or DC), whether battery storage is integrated, and what type of consumptive loads are envisioned.
For their sample case of a solar PV mini-grid with battery storage, Madduri et al. (2013) arrive at an increase in end-to-end-efficiency for a DC system with 380 VDC transmission of between 17 and 25% for DC loads (fully DC mini-grid) and around 3% for AC loads, as compared to the usage of an AC system.
System / operational costs
Cost savings potential is a frequently reproduced argument for utilising DC over AC in mini-grids in certain situations. In terms of capital investment, DC mini-grids can provide savings due to their simpler power control architecture (see power control and management), as well as the fact that, in a full DC scenario, they can render additional conversion steps and associated system components, such as PV inverters, unnecessary (see system efficiency). Improved power conversion efficiency from a fully DC mini-grid can furthermore lead to operational cost reductions of up to 20%.
Comparative savings on capital cost are, however, at least partly offset by general price penalties on the more immature DC technology components.
Applicability of DC mini-grids
Based on the characteristics described above, the following provides a short overview on the main factors promoting the use of DC in mini-grids:
- Generation: Among common technologies used in distributed generation, Direct Current is native to solar PV generation, while rotating generators (wind, diesel, hydropower, etc.) generate alternating current. Conversion between DC and AC is prone to result in efficiency losses and should be avoided where possible (see system efficiency).
- Battery storage: Battery storage solutions, too, are native DC and hence most efficiently applied in DC systems.
- End-use applications / energy services: DC is required by or suitable for a range of low power household and commercial applications, such as LED-lighting, consumer electronics (e.g. TV, radio, mobile phones) and variable speed drives in electric motors (e.g. for fans or pumps). Consequently, DC mini-grids are primarily used to provide basic level energy services such as lighting and mobile phone charging.
- Grid connection: Although high voltage DC transmission is on the rise worldwide, AC will still be the likely standard for electricity grids in decades to come. This renders DC systems principally unfavourable where interconnection of a mini-grid with the main grid is foreseen.
Examples of DC mini-grids to date
Although to date AC is most commonly applied in mini-grids, a range of practical experiences with DC mini-grids exists. According to Palit and Malhotra (2014), the first DC micro-grid in India has been installed 30 years ago. Today, they are at the core of several initiatives and business models, providing basic energy services (such as lighting and mobile phone charging) to households and businesses mostly in India.
DC mini-grids are being implemented by:
- Meshpower: Meshpower has installed 7 solar PV DC micro-grids in Rwanda, providing power for lighting and mobile phone charging for up to 2-3 hours a day. They are applying the same model in India.
- Mera Gao Power(MGP): MGP has installed DC mini-grids in around 900 villages in India, providing lighting and mobile phone charging services to around 20,000 households (Palit and Malhotra, 2014).Installation costs averaged INR 55,000 per 30 households.
- The Energy and Resources Institute (TERI): By 2014, TERI had installed 30 DC micro-grids in Uttar Pradesh (India), providing 130 households and 1,100 business units with LED lighting (4 hours per day, 1-3 lights per HH) and mobile phone charging (Palit and Malhotra, 2014). Costs averaged INR 2,000,000 per 50 households connected.
- Uttar Pradesh New and Renewable Energy Development Agency (UPNEDA): In 2011-12, UPNEDA installed 23 DC solar PV micro-grids in Uttar Pradesh (India), covering around 4,000 families. Electricity provision is limited to 4-5 hours in the evening, with 2 LED lamps and mobile charging for households.
- Minda: Minda has installed 13 DC solar PV micro-grids in India to date, providing electricity primarily for lighting (2 LED lamps per HH). Installation costs averaged around INR 360,000 per 40 households.
- Solaric: Solaric installs DC micro-grids, utilising 1.5 to 3 kWp PV systems. The company is active in Bangladesh, India, Nepal and Malaysia.
- Planas, E. et al. 2015. AC and DC technology in microgrids: A review, in: Renewable and Sustainable Energy Reviews, 43 (2015), pp. 726-749.
- Karabiber, A. et al. 2013. An approach for the integration of renewable distributed generation in hybrid DC/AC microgrids, in: Renewable Energy, 52 (2013), pp. 251-259.
- Groh, S. et al. 2014. The Battle of Edison and Westinghouse Revisited: Contrasting AC and DC micro-grids for rural electrification, available from: http://bit.ly/1lecL9s
- Justo, J.J. et al. 2013. AC-microgrids versus DC-microgrids with distributed energy resources: A review, in: Renewable and Sustainable Energy Reviews, 24 (2013), pp. 387-405
- Madduri, P.A. et al. 2013. Design and Verification of Smart and Scalable DC Microgrids for Emerging Regions, available from: http://power.eecs.berkeley.edu/publications/madduri_ECCE_2013.pdf
- Willems, S. et al. 2013. Sustainable Impact and Standardization of a DC Micro Grid, available from: https://lirias.kuleuven.be/bitstream/123456789/466577/1/DCMicroGrid_Willems.pdf
- Palit, D. and Malhotra, S. 2014. Energizing rural India using micro grids: The case of solar DC micro-grids in Uttar Pradesh State, India, available from: http://bit.ly/1m0Z65P