Solar power development was predominantly concentrated in the North African region, largely due to its export potential, but recent developments in the south, east and west of the continent has resulted in solar energy being included more and more in Africa’s generation mix.
Solar potential stretches far beyond the Sahara desert as almost all of Africa’s countries receive sufficient sunlight to efficiently and economically generate solar energy, estimated at an impressive 2 MWh/m2, and that on average, most sub-Sahara African nations receive 325 days of sunlight annually. Despite massive potential, the region has witnessed little progress in terms of solar power deployment due to limited government and private investment potential.
Fig. 1: Storage system acting as an energy supply.
However, the continent benefits from support from countries such as the US, which is contributing to the development of the African power industry through significant funding. It has contributed $7-billion through the United States Agency for International Development (USAID), a large portion of which will be used to promote renewable energy technologies, including solar.
The countries which will benefit from the aid are Kenya, Tanzania, Ethiopia, Ghana, Liberia and Niger. Other countries have individual plans for the promotion of solar energy, such as Senegal, which aims to support more than 30% of its rural energy requirement through solar energy.
Mozambique and Zimbabwe plan to follow Senegal’s lead, with Mozambique having invested in excess of $15-million in solar power.
In South Africa great strides were made through the Renewable Energy Independent Power Produced Procurement Programme (REIPPPP) which by the third round has accumulated over 1500 MW of solar photovoltaic (PV) and over 400 MW of concentrated solar power (CSP) energy. Aditionally, in South Africa, the national energy regulator (NERSA) is working on a draft for the connection of small-scale embedded generation which among others also comprises of roof-top solar PV.
Fig. 2: PV intermittency in a 4,6 MW power plant.
As one could expect with benefits comes challenges especially with solar energy which is well-known as an intermittent renewable energy source. Utilities will start to see issues in terms of grid stability especially for the distributed (or embedded) generation which is located at what is known as the “edge of the grid”.
Such impacts get amplified as PV generation increases in terms of total generation to meet demand – Germany, for instance, derives over 25% of its generation from PV systems. Variable output from solar generation produces local voltage swings which can impact power quality for customers, including voltage excursions beyond accepted power quality standards.
This output variation may also contribute to distribution feeder level voltage swings, which can occur in a matter of seconds. Traditional distribution voltage regulation equipment is typically not suited to respond to such rapid, highly variable fluctuations. The inverters of the PV plants can be oversized to perform voltage-var control but that brings additional cost for the developers without a proper regulatory framework to reward them in some cases.
There is also a lot of potential in terms of what is called “off-grid” solar which unlike “grid interactive systems” cannot count on the support of a strong grid and therefore the intermittency effect is amplified. Or in some cases connecting to weaker grids can also be problematic especially given the low inertia of this type of generation.
Fig. 3: Shows the use of community energy storage control the voltage profile.
Source: Scottish and Southern Energy.
Variations in renewable energy output can also prove problematic if it is not sufficient to meet demand. This is particularly true when it comes to distributed renewable energy resources on the customer’s side of the meter. Utilities do not always have visibility of the output or availability of these resources. As a result, power utilities need to ensure that additional electrical reserve (via peaking plants) is available to meet unexpected demand caused by a sudden drop in distributed generation output which results in increased costs for the utility.
Another concern when generation surpasses demand is that there will be bi-directional power flows which can further impact grid stability and reliability. Today’s electric power systems were built to handle a one-way flow of power from centralised generators, from transmission and distribution lines to loads.
Background
In several countries, climate change and public opinion have caused governments and industries to consume and produce clean energy. The increase in clean energy generation (particularly solar electricity generation) since 2007 has highlighted several technical questions and needs regarding the role of this renewable source in our actual electric model and society.
About 25 years ago, we were questioning “Would it be possible to feed entire residential, commercial and, possibly, industrial areas only with power from the sun?” Nowadays, the question has changed: nobody is discussing if it will be possible, but when it will be feasible.
Solar PV utility scale facilities make it possible to get the generation of electricity near the consumer, anywhere the sun shines, whether it is a remote island or at top of a mountain. It is not a matter of “can we do it?” anymore.
The problem that emerged is that the variability of the generation source created unpredictability for planning of the electrical system. Additionally, because PV only produces electricity while the sun shines, the paradigm of a dispatchable energy supply reservoir is different to what the utilities are used to. All this makes solar (and, for that matter, wind) power, despite being abundant, unreliable from the utility’s point of view.
Fig. 4: Shows the use of community energy storage to prevent reverse power flow on a feeder.
Source: Scottish and Southern Energy.
Within this context, bulk energy storage starts to play a prominent role. Pumped hydro, compressed air energy storage (CAES), super capacitors and flywheels are technologies that received some attention from government funding and research and development departments, but one technology in particularly has shown to be more economically feasible and in most of the cases, has added technical advantages: batteries.
Bulk energy storage
Solar PV generation is starting to embrace battery based energy storage. With different battery technologies and chemistries available, engineers can evaluate which one will be the better suited for a particular application in that specific geographic area, adding more technical and commercial benefits for the customer or utility.
Not surprisingly, battery based energy storage is currently widely used on solar PV utility scale generation.
Due to the intermittent nature of solar power generation, cloud shadows and/or air dust can have an undesirable impact on the grid. In addition to providing benefits of smoothing intermittent PV output, energy storage can bring many benefits, such as energy backup (to provide stored energy to an industrial facility, and to improve reliability), energy arbitrage (to store energy while prices are low, and supply or use it when prices are high) or energy supply (to store while there is plenty of energy, and supply it when energy is lacking), as showed in Fig. 1.
The storage systems can provide a series of technical benefits which will make them a necessity when connecting solar or other intermittent generation sources at weaker or remote locations on an existing electric grid.
Battery-based energy storage offers extensive flexibility to power utilities. Examples include using stored electricity for short periods of time, or providing a quick injection of energy to deal with the grid’s fluctuations; or to store and hold the energy for several hours for dispatch at a more beneficial time such as to meet peak demand. Except for a few mature technologies, many approaches in energy storage are considered expensive, but what is the most expensive energy? Simple, it is the energy you don’t have!
Battery technology
Several start-ups, public power utilities, private agents and governments are investing in research and development of bulk energy storage systems but, by far, battery technologies and chemistries are attracting most of this money to due a battery’s obvious advantages: non-geographical dependent, non-weather dependent, mobility, proven technology, flexibility, as well having a developed and mature market.
Besides, it fits a wide range of applications of a grid operator’s ancillary services and other technical benefits. Batteries can be considered the most reliable and fastest growing energy storage technology at the moment.
Battery based energy storage systems also complement the smart-grid concept. These systems will soon help the utilities to comply with a more severe power quality requirement from increasingly demanding grid codes. They will also reduce the impact of electric vehicles and additional solar PV rooftop distributed generation entering the system and causing instability at a weak point of the grid at certain times of the day.
Integrating solar generation
As solar photovoltaic power generation becomes more commonplace, the inherent intermittency of the solar resource poses one of the great challenges to those who would design and implement the next generation smart grid. Specifically, grid-tied solar power generation is a distributed resource whose output can change extremely rapidly, resulting in many issues for the distribution system operator with a large quantity of installed PV devices.
Battery energy storage systems are increasingly being used to help integrate solar power into the grid. These systems are capable of absorbing and delivering both real and reactive power with sub-cycle response times. With these capabilities, battery energy storage systems can mitigate issues with solar power generation such as ramp rate, frequency, and voltage issues. Beyond these applications focusing on system stability, energy storage control systems can also be integrated with energy markets to make the solar resource more economical.
Impact in the distribution systems
As we get down into the distribution network we start to face additional problems which have not been experienced previously. Distribution utilities have been used to passive networks where power flow has been from the bulk supply point to the load. Now they are faced with a network that not only has power flow in both directions, but the power flow from renewable sources can be very variable. Distributed energy storage systems, particularly when located in close proximity to renewable resources, are uniquely suited to address such challenges, including local and feeder-level voltage swings that occur far too rapidly to allow traditional distribution voltage regulation equipment to respond.
Weak distribution networks
When intermittent renewable energy resources comprise a significant amount of overall generation, distribution system problems such as voltage swings are more likely to arise. These disruptions are compounded on weak distribution networks, which have characteristically low fault levels. Voltage levels on networks with low fault levels may not be sufficient to support all loads.
Voltage stabilisation
Stabilisation of distribution system voltage is achieved by minimising the rate of change of apparent power (VA). This method uses distributed energy storage as a fast-acting, short-term resource which allows time for traditional voltage regulation equipment (including switched capacitors) to respond as steady state resources on the distribution controller. Each distributed storage unit can estimate and regulate voltage on the source side of its distribution transformer by modifying its flow of real power (W).
Voltage profile improvement
Utilities can improve the voltage profile by co-locating renewable resources and distributed energy storage systems. When storage is co-located with renewable resources at distribution load points, less power needs to be supplied by traditional generation. Utilities no longer need to rely on distant traditional generation plants, which are much slower to respond than local storage, to attempt to push voltage levels back up at the far edges of the distribution system. By leveraging local battery storage, which reacts more swiftly, utilities can ensure that voltage drops are less severe and less likely to impact service. In addition most battery storage solutions also have the capability to control reactive power which can further improve the voltage control. This has the effect on better utilisation of resources and lower the carbon footprint.
Reverse current flow mitigation
When energy storage is co-located with PV panels, it can prevent reverse current flow caused by excess generation during outages. Storage reliably mitigates reverse current flow by quickly consuming real power to charge its own battery. By consuming any excess power generated, battery storage prevents energising the transformer’s load side, thereby avoiding equipment damage and other potential safety problems.
Energy storage offers a key piece of the puzzle
Distributed energy storage has demonstrated that it can significantly help with renewable integration.
Distributed energy storage systems, particularly when located in close proximity to renewable resources, are well suited to address the key challenges associated with renewable energy supplies. Variability in output from wind and solar energy generation can create local and feeder-level voltage swings which occur very rapidly. Distributed energy storage can provide fast response to help stabilise voltage levels and effectively fill in gaps created by large voltage swings and fluctuations. Co-locating storage and renewable energy resources gives utilities a particularly effective way to manage unwanted voltage changes, allowing them to maintain grid stability while also meeting power quality requirements.
Distributed energy storage systems also provide a dispatchable energy resource for power utilities, giving them more control to ensure renewable energy supplies are available to meet demand. Renewable energy generated when demand is low is stored to meet later demand. Distributed energy storage systems provide greater flexibility and faster response than using conventional generating plants for compensating for shortfalls in renewable generation output.
Energy storage can be economically deployed in smaller capacity sizes, too, which can help avoid the need to invest in establishing a new plant. Distributed energy storage also supports other grid functions such as peak shaving, which can provide further savings to utilities by reducing the need to maintain conventional generation, and maintaining grid capacity to meet peak demand.
Contact Isobel Evans, S&C Electric, isobel.evans@sandc.com
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