DC nanogrid-based energy bank for off-grid rural communities



More than 1,4-billion people worldwide do not have access to electricity; and 600-million of them live in sub-Saharan Africa. The average electrification rate of the region is 31%, with disparities between urban and rural areas. Various solutions are being implemented and monitored for their efficiency. Among the solutions for increasing universal access to electricity are off grid electrification such as solar, diesel and hybrid.

Considering the demography and type of dwellings in which majority of rural Africans live, 85% of the population live far from the grid and have little chance of the grid being extended to their areas as it would be expensive in comparison to the return on investment [1]. Even in areas closer to the grid high connection fees inhibit the access to electricity [2, 3].

Solar home systems (PV based) are widely used and prove to be efficient, but their cost limit their capacity [2]. Only basic appliances and lights are being supported by most of the solar home systems due to limited storage capacity. The cost of the storage system for a PV system is 29% of the total cost of the installation vs. 20% for solar panels [4].  Therefore, interconnecting neighbouring solar home system as shown in Fig. 1, is a strategy which could be used to increase the capacity of the system. The strategy is known as “swarm electrification”, where a single solar home system is considered as a nanogrid [5]. It is defined as the smallest entity in the colony of interconnected generation points. Each nanogrid offers a single source, reliability, price and administrative domain [6, 7]. Considering the DC nature of PV solutions, they are referred to as DC nanogrids.

Fig. 1: Swarm electrification phases.

Swarm electrification concepts work on the principle that a unified community would overcome the  lack of access to electricity. DC nanogrid swarm electrification for off-grid operates on the principle of a household with a solar system (nanogrid) sharing its excess energy with its neighbour. This strategy has been successful in Bangladesh and India, since nanogrid owners earn money and the buyer gets energy at a low cost without investing in acquiring their own system [8].

The drawback of the strategy is, like that of a single nanogrid, that there is a limitation by the storage system of the amount of energy that can be harvested. Since storage is the expensive part of the  system and that the sun shines for more than ten hours in most of sub-Saharan Africa, there is a lot of sun energy that goes unharvested.

On the other hand, due to the limited capacity, few consumers would be able to tap on those nanogrids and it would be difficult for commercial use of the harvested energy. Furthermore, though the nanogrid owner and customers might appreciate lights and appliances that those nanogrids can power; they would like to move up on energy ladder and be able to enjoy high-power appliance benefits, such as milling, sewing and other small commercial activities which would improve the community in general. Hence, the proposed DC nanogrid-based energy bank would overcome the issue of storage capacity and increase total energy harvested by nanogrids.

Energy bank concept

With energy bank concept, nanogrids won’t need to have a storage system as they will be replaced by huge central storage systems which might be operated by utility or an investor. So, during the day all the nanogrids would feed both their local loads and the bank. The latter in turn can supply users for commercial use and to the nanogrids after the sun set.

As shown on Fig. 2, all the nanogrid’s PV panels in the network are interconnected to feed, at the same time, their local load and the central storage unit, the bank. The individual storage systems are removed. The storage capacity of the banks should be sized such that it won’t get fully charged, as it should be continuously feed by the nanogrids as long as the sun shines. During the day, while being charged, it should also be able to supply commercial loads (commercial centre and telecommunication tower).

Fig. 2: Evolution from swarm electrification to energy bank.

Operation and metering

The operation of the set up consists of energy storing and retrieving in the bank. The latter can sell energy to other users and feed it back to nanogrid at night.  In comparison to a financial bank where there are charges on transactions, in this context the transaction is the energy storing and retrieving. Here, the element of transaction is the energy and would be monitored by a smart meter. There would be two forms of transaction charge: for nanogrids and for customers.

  • For a nanogrid: On the amount of energy stored, a certain percentage will be retained as transaction charge. This retained energy will be the one to be sold to customers in need of power, especially for commercial usage like for milling, sewing, small workshop and telecom towers. This category of customers operates usually during the day.
  • For customers: Those without their own PV system, monetary prepaid system would be used for the supplied energy. Their tariff will be calculated from the business model that will insure the return on investment of the bank as well as the soft cost related to the interconnection of nanogrids.

The advantages of the above presented system will be:

  • Increased harvesting of solar energy as the removal of individual storage system which account up to near 29% of the total cost, will be allocated on acquiring more PV panels. Furthermore, given the drop in the total cost, more people will be able to afford PV solar system. Thus, an increase in the number of nanogrids and more energy for the community.
  • Moreover, the owner of a nanogrid will be relieved of the burden of storage system maintenance cost.
  • Increased revenue generating services for commercial use of energy with the community.
  • Extended possibility, provided the required technology in place, would be the ability for the nanogrid owner to pay some services from commercial client (commercial centre, households and telecommunication company) connected on the network using their energy stored in the bank. The energy will be a new form of payment within the community which might an immune system against the money value volatility.

Stakeholders and responsibilities

In most of the community, people are able to afford their own panels but not an energy storage system as these are expensive. Hence the proposal for multiple stakeholders: the nanogrid owner, investors and customers. For the investor, such as a utility, instead of investing in grid extension, an investment in the energy bank would be cheaper and still generate revenues. Besides utilities, other investors could be telecommunications companies, instead of burning fuel to power the telecom tower, an investment in the bank would reduce telecom tower operation costs while shortening bank return on investment through energy selling.

Customers of the bank would benefit from those who use energy for commercial purposes who, instead of investing in acquiring their own system, would rely on the bank and reallocate funds into growing their business. Lastly, domestic users who can’t afford to own their own system they would still be able to be supplied by the bank through a prepaid system.

References

[1] PL Lucas, AG Dagnachew and AF Hof, 2017: “Towards universal electricity access in Sub-Saharan Africa”.

[2] IRENA: “Solar PV in Africa: Costs and Markets”, 2016. www.irena.org/DocumentDownloads/Publications/IRENA_Solar_PV_Costs_Africa_2016.pdf

[3] M Bokanga, M Gilbert, Raji, Atanda, and Kahn, TE Mohammed, 2014. “Design of a low voltage DC microgrid system for rural electrification in South Africa”, Journal of Energy in Southern Africa, 2014. www.scielo.org.za/scielo.php?script=sci_arttext&pid=S1021-447X2014000200002&lng=en&tlng=es.

[4] Evert-Jan Quak: “The costs and benefits of lighting and electricity services for off-grid populations in sub-Sahara Africa”, 2018.  https://assets.publishing.service.gov.uk/media/5af96657ed915d0df4e8cdea/Costs_Benefits_Off-Grid_Electricity_Lighting_Systems.pdf.

[5] S Groh and D Philipp: “Swarm electrification-Suggesting a paradigm change through building microgrids bottom-up”, Renewable Energy, 2014.

[6] B Nordman: “Nanogrids: Evolving our electricity systems from the bottom up”, In Darnell Green Power Forum, 2009.

[7] IEA: “World Energy Outlook 2017”, https://www.iea.org/weo2017

[8] A Bhattacharya: “One village solved Bangladesh’s unreliable energy grid problem with “swarm electrification”. Quartz, 2016.

[9] DO Akinyele, RK Rayudu and NKC Nair: “Development of photovoltaic power plant for remote residential applications: The socio-technical and economic perspectives”, Applied Energy, 2015.

[10] D Burmester: “Nanogrid topology, control and interactions in a microgrid structure”, Victoria University of Wellington, 2018.

Contact Prof. Mohamed Kahn, CPUT, Tel 021 460-3911, khant@cput.ac.za

 

 

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