Table of Contents
1 CENTRALISED AND DISTRIBUTED GENERATION
It might be helpful to talk about some of the definitions of microgrids, the differences between central and distributed generation, and some of the basic configurations and components. The different types of microgrids there are, including alternating current and direct current microgrids, are introduced below. A few comments about the applications for microgrids and some case examples of microgrid deployment are also discussed.
The nexus of renewable energy and microgrids seems to be a topic that is becoming increasingly important, and so it is surprising that there is not more detailed work or research into it. Microgrids are different in many ways from traditional types of electricity transmission and provision .
Most systems are set up to a central plant or station, which means this electricity is distributed from a central location. These power plants could be driven by renewable energy or by fossil fuels; most of them today are fossil fuels-based. Distributed generation is somehow different: it produces electricity at or near the point of use, and it could use either renewable energy or fossil fuels.
Fuel generation is essentially producing electricity using a non-renewable energy source such as coal, natural gas, and nuclear—these are considered Rankine cycle operations  primarily. Renewable generation could be electricity produced from several renewable sources, including solar PV, solar thermal, wind power, hydropower, geothermal, and biomass. The two leading ways currently used are hydropower and biomass.
1.2 Centralised Electrical Generation
Most of our grids are central plant electrical generation systems. A power station—also referred to either as a power plant or powerhouse and sometimes as a generating station or generating plant— is essentially an industrial facility for electric power generation. Most of these power stations contain one or more generators: a rotating machine that converts mechanical power into electrical power. The main reason for having more than one generator is backup, i.e., to provide continuous online power. To do that, power systems have to maintain multiple generators at a specific powerhouse.
Conventional power stations, such as coal-fired, natural gas, and nuclear power plants, are traditional ways to generate power. But we also generate power with renewable energy, including large-scale hydroelectric dams and large-scale solar power stations, which are often centralised systems themselves. Centralised systems require the electricity to be transmitted over reasonably long distances; to make that happen, there are transportation power lines that carry all that energy and become critical parts of the grid infrastructure.
To sum up, electricity is generated at power plants, then moves through a network of substations, power lines, and distribution transformers. For instance, the U.S. power system consists of more than 7,300 plants, >160,000 miles of high-voltage power lines, and millions of low-voltage power lines and transformers used to connect the millions of customers . It is a one-way system based on generating power at that central location and then building the network’s infrastructure to transmit that power to where it’s needed—where the loads happen. High-voltage power is preferred for transmission purposes because of its efficiency. Interestingly enough, the grid infrastructure is not all that efficient; as a matter of fact, the entire system itself has efficiency on the order of about 30 to 33 per cent.
There are some problems with grids, and perhaps the most challenging issue with centralised electric power distribution systems are the improvements needed to maintain the system. What one finds, in reality, is that there is a lot of ageing infrastructure. For the U.S. grid case, in particular, much of it is over 50 years old, and according to the American Society of Civil Engineers, over 900 billion dollars are needed to upgrade that grid over the next ten years . The main reason behind this enormous figure is the fact that centralised electric power distribution involves transportation over long distances needed to supply the local distribution systems.
Ageing infrastructure also hinders the possibility of meeting peak loads. Large amounts of energy are basically lost as heat; some dissipate within the system itself due to transmission. Conventional grids are very vulnerable to extreme weather conditions. Despite utility concerns with reliability, the U.S. has more power interruptions than any other country in the world. It is also susceptible to security breaches and electronic pulse attacks, so security issues must be accounted for.
1.3 Stand-alone Power Systems
An alternative to a centralised system might be a stand-alone power system (SAPS)—also known as remote area power systems (RAPS): an off-the-grid electric system for locations that are not fitted with an electrical distribution system. A typical stand-alone power system includes one or more of the methods for energy generation, storage, and regulation.
Electricity is typically generated by one or more of the following methods, basically any combination of fossil fuels or renewable sources:
- Gas generators: reciprocating and turbine engines (including microturbines)
- Photovoltaic systems using solar panels and Battery Energy Storage Systems (BESS)
- Fuel cells
- Wind turbines
- Diesel or biofuel generator
Thermal generation includes:
- Thermoelectric generators (TEGs)
- Geothermal source
- Micro combined heat and power
- Biofuels and biomass
1.4 Distributed Electrical Generation
On-site generation, district/centralised energy, or distributed generation systems that generate or store electricity from various small grid-connected devices are called distributed energy resources (DER). DER systems may use renewable energy sources, including small hydro biogas biomass, solar power, wind power and geothermal power connected to power distribution systems. Grid-connected devices for electricity storage can be classified as DER systems and are often called distributed energy storage systems (DESS). Employing an interface, DER systems can be managed and coordinated within a smart grid. A distributed generation and storage enables the collection of energy from many sources and may lower environmental impacts and improve security.
Figure 2 shows what a distributed electricity generation system might look like. It might use fossil fuels, renewables, microturbines, or storage. Storage itself might use grid-connected equipment or off-grid storage. Some of the more sophisticated ones might use home energy systems or even electric vehicles for storage: EVs can be parked in a garage, branched, and allowed to take the electricity out of their batteries at times when it is needed.
2 MICROGRID FUNDAMENTALS
2.1 What is a microgrid?
A microgrid is a localised group of electricity sources and loads that ordinarily operate connected to and synchronous with a traditional centralised electrical grid (macrogrid). A microgrid can also disconnect, switching to an “island-mode” to function autonomously as physical or economic conditions dictate.
A microgrid is a group of interconnected loads and distributed energy resources within clearly-defined boundaries that acts as a single controllable entity concerning the grid. Switching to an “island-mode” essentially means the microgrid can operate as a stand-alone generation source. Microgrids are modern small-scale versions of the centralised electrical system. They achieve specific local goals such as reliability, carbon emissions reduction, diversification of energy sources, cost reductions, or other purposes established by the community being served .
Necessary components of a microgrid include:
Power source — Equipment to generate electricity.
Power management system — A set of devices that handle electric power transfer from the source to the consuming equipment.
Energy storage system — An essential part since it allows the microgrid to balance the energy output and makes it accessible when required by users.
Electricity consuming devices — Dictate the loads placed on the microgrid.
Utility interconnection — For grid-connected systems, it enables the microgrid to exchange power with the more extensive utility network.
The point of common coupling (PCC) is the place in an electrical system where +multiple customers or electric loads may be connected. According to the IEEE-519 , this should be a point accessible by both the utility and the consumer for direct measurement.
Also, it is the point of the power supply network that is electrically closest to a particular load, at which the other loads are or may be connected. These can be devices, equipment or systems, or distinct customer’s installations. A PCC’s primary job is to ensure that voltage regulation from the generation system is synchronised with the primary power grid’s voltage. If the grid goes down, it let’s activate a circuit breaker that isolates the segment.
2.2 Why microgrids?
Microgrids are sometimes viewed as a platform to integrate renewable energy sources. Thus, renewable integration is a major driver force in microgrid adoption. These systems also enhance macrogrids by handling sensitive loads and supplying power in areas of constrained transmission and distribution; by doing so, they support existing grids that may be struggling to meet the demand.
Not everybody has power: there are over a billion people today that lack access to electricity. Suppose one is trying to join the 21st century and wants to have computers, the internet, electronic appliances, and lights. In that case, it is vital to have reliable access to clean energy. Microgrids stimulate economic development by improving resiliency and reducing costs for transportation and transmission infrastructure. They effectively provide power to remote locations, and they are designed to match actual loads.
Functionalities refer to the core capability that a microgrid technology provides to a renewable grid. The core functionalities include plan and design, store, control, manage and measure (CMM), convert, consume, and generate . Microgrids do promote the integration of decentralised energy resources that increase resiliency and efficiency across the electric network. Finally, from an innovation perspective, microgrids help foster smart grid technologies.
2.3 Microgrid Types
Customer microgrid: it is a self-governed grid downstream of PCC. Examples of customer microgrid deployment include municipality, institutional, or industrial campus and hospital facilities.
Utility microgrid: it is a segmented boundary of the macrogrid. Trends in utility microgrid adoption are mostly seen in portions of the grid where utilities are trying to overcome severe constraints—it might be a substation that is undersized and cannot meet the load.
Remote microgrid: an arrangement that serves respective connected thermal and electrical loads, often with unique challenges in handling step-loads and load shedding. Remote microgrids are not a new concept and have been particularly useful for arctic mine-sites and high-altitude communities.
3. RENEWABLE GENERATION TECHNOLOGIES
3.1 Electrical: Photovoltaic (PV) & Battery Energy Storage Systems (BESS)
The sharp decline in PV prices made solar the preferred technology for renewable microgrids in the past decade.
In terms of availability, PV generation varies by geographical location, time of day, and weather conditions (e.g. cloud cover). These systems must be paired with a baseload generation source—typically fossil fuel-based. The capacity of PV arrays and full-scale projects has risen considerably in the last years: from a hundred watts to several thousands of megawatts in some cases. Solar power plant total capacity is a function of the size of the array (N) and the capacity factor (typically ~<25%). Studies confirm a minimum degradation of the substrate with efficiencies climbing up to 80% after 20 years of operation, and greenhouse gas emissions reduction largely depends on the role of clean technologies employed for solar panel manufacturing and disposal/recycling. Electricity is generated with a nominal efficiency of 35-45%, representing the maximum achievable efficiency of PV materials; actual efficiency is influenced by the output voltage, current, junction temperature, light intensity, and spectrum . As per functionality features, BESS provides rapid response to step-load and load rejection. Overall, PV combined with BESS offers very similar attributes to small wind turbines (1-300 kWe). In remote, rural areas worldwide, solar-based microgrids are being used increasingly to meet local, national and regional access goals. The island of Oahu, Hawaii, has deployed significant amounts of solar PV on its distribution grid. However, Oahu is now beginning to retrofit the grid with renewable mini-grids to prevent technical problems on the feeders. The U.S. Department of Energy (DOE) SunShot programme established a 1 USD/W installed cost goal for utility-scale PV for 2020, which drove manufacturers and installers to push for lower costs; in general, the low cost of solar has benefited renewable microgrids. Investment tax credits for renewable energy in many states for solar, which apply to renewable microgrids, provide funding to deploy near-commercial renewable microgrid technologies.
3.2 Electrical: Fuel Cells
Gas powered-fuel cells are currently designed to be compatible with biogas feed, making it possible to utilise renewable energy resources. Fuel cells ensure high availability (99.9%), black-start capability, scalable capacities >200 kWe to comply with CHP economics and minimal degradation of the cell or whole array over the economic life.
Fuel cell emissions are associated with the fuel stock reformer. Functionality features are characterised by a processor/reformer converting gas to H2 with a long start-up time and minimal operational supervision requirements (low OPEX). The generation has an electrical efficiency of 40-60%, with waste heat above 85% suitable for space heating .
3.3 Small Hydro
Small hydropower (SHP) continue to be a microgrid solution that is relatively unexploited, despite the 75 GW of current global SHP installed capacity. SHP can meet the baseload demanded by a community without the need for short-term storage in areas with hydro resources. However, a matter to be considered for this option is the seasonality in the availability of resources.
The International Renewable Energy Agency (IRENA) censed the installed cost, levelised cost of energy, and expected lifetime for a series of small hydropower plants in 2015. Then, the 2025 and 2035 scenarios were estimated for these variables without finding any significant improvement.
3.4 Small-scale wind
Small-scale wind can be integrated into microgrids, complementing the generation patterns of PV systems. Supply disruptions can cause significant fluctuations in the cost of energy. This is a key reason why remote cooperative grids in Alaska, rural Africa, and the Argentinean Patagonia have increasingly added wind to their mini-grids. The challenge of supplying fuel over isolated deserts, frozen rivers, and unpaved or ice roads has made wind generation a preferred option in some regions.
There are nearly 7.5 MW of autonomous small wind turbines worldwide . More accurate wind predictions and sophisticated algorithms integrated into short-term controls are vital for increasing this technology’s penetration. Other small-scale wind aspects remain suboptimal; for example, a wind turbine position must be carefully selected to ensure optimal power generation.
Success stories in the deployment of state-of-the-art technologies include Australian telecommunication towers repowered with small wind turbines under a new engineering project . Australian Renewable Energy Agency (ARENA) is actively assisting a local startup in proving the effectiveness of their small-scale wind turbines which could have an untapped potential across many markets and applications. While this specific project focuses on communications towers, the technology could also have other potential applications, including data centres, mining, farming, and small municipal microgrids.
3.5 Thermal: Bioenergy resources
Renewable power technologies included here cover landfill biogas, biofuels and biomass. Feasible alternatives include blending biofuels and digester output gas in reciprocating gas engines; a note of caution: availability of this kind of units is lower than the equivalent availability of a machine running on pipeline-quality natural gas.
Internal combustion engines suffer more wear and tear with biogas than traditional fuels, increasing maintenance costs and shortening the lifetime. Operation and maintenance (O&M) costs are usually 5-6%/year of the capital cost for smaller-scale applications, excluding feedstock costs . Plus, the operation cost of biogasification per hour is relatively high due to the defies presented by the gasifier and the impurities in the fuel once fed into a generator.
The gasification of biomass is another way of feeding a modified gas engine, for which one can find modules in the range from 50 to 150 kWe. The gasification market is expanding in Europe on account of a high reliance on standardised fuel supply. A traditional boiler could also be converted to admit a carbon-neutral biomass fuel, although significant investment is required in both the boiler’s front and back ends (furnace). What is more, this procedure can be spatially challenging for existing sites, and the boiler capacity (maximum continuous rating-MCR) is usually de-rated in the end.
Biomass gasification is used mainly at a small scale in India and some parts of Africa—these biogas pilots are mostly in autonomous microgrids. Improve bioprocess that limits the build-up of soot, tar and other contaminants in the gas will significantly reduce the maintenance requirements for these systems and improve the engines’ performance, contributing to higher universal penetration.
4 AN OUTLOOK ON INNOVATIONS
Microgrids are integrated energy infrastructure with loads and energy resources. The six core functionalities for microgrid technologies are power generation; power storage, control, manage and measure (CMM); convert and consume (Figure 5).
New technology advances are expected in all six core functionalities. R&D initiatives in the lab and early commercialisation expansion today may overcome the 18 identified priority gaps over the next decade . We highlight some of them in the following subsections.
4.1 Creating Dispatchable Power
There certainly are some challenges trying to create a dispatchable generation with a renewable microgrid . Dispatchable power is electrical power made available upon demand and dispatched at the request of power grid operators according to market needs. Dispatchable generation refers to sources of electricity that could be delivered at the request of the power grid operator or the plant owner according to market needs. Dispatchable generators can be turned on or off, with their power output being adjusted accordingly to an order.
If there was a microgrid interconnected with the main grid and during certain parts of the day or certain months of the year, this microgrid had excess power; why not selling it back to the grid at times when the grid operator requires greater demand?
We certainly can think of different ways that this might match up; for instance, if one is in the southwestern part of the US and there was a solar PV array generating plenty of power on the microgrid on a long sunny summer day. They are peaking out at the nearby towns that the power grid operator supplies because of AC loads. The excess solar PV power could be sold back to the grid and probably with a higher than average price for that peak generation. However, dispatchable generation contrasts with non-dispatchable renewable energy sources such as wind power and solar PV, often difficult to control by operators.
Regarding renewable systems, not all of them are non-dispatchable. Hydropower plants, for example, do provide dispatchable baseload power. It is also known that geothermal electric generation, where the U.S. is the global leader, is responsible for successfully delivering dispatchable power. The types of renewable energy that remain dispatchable without separate energy storage include biomass, geothermal and ocean thermal energy conversion.
Agencies worldwide are commissioning studies of options for dispatchable renewable electricity generation that can support an affordable, reliable, and secure electricity system with a higher share of renewable energy. The outputs are reports, R&D briefings, and spreadsheet-based cost models. Different innovative technologies and configurations suit different market needs; batteries are more competitive for short-duration storage, whereas pumped hydro or concentrating solar thermal are more competitive for long-duration storage . There is still much work to do, trying to make solar and wind more dispatchable.
4.2 Plan and Design Tools
Priority gap analyses have identified the need for standardising and improving modelling for planning, financing and design. To attain such an achievement, we would need more R&D in tools for designing and planning, lowering the cost of renewable energy resource assessments, and increasing load data availability.
Research at CIEMAT, for example, has been carried out for short-term load forecasting based on Artificial Neural Networks (ANN) for microgrids . This work has demonstrated the close relationship between forecast errors and the number of training patterns and applicability of load forecasting and ANN tools to microgrids, with small errors of 3% compared to real load curves.
Storage is one of the key functionalities for renewable microgrids that require groundbreaking technological innovations. The focal aspects that storage technologies need to address are reliability, robustness, and low cost. The rapid growth of lithium-ion batteries (LIBs) is expected to reduce reliance on lead-acid batteries (LABs) for stationary energy storage over the next two decades. Also, new chemistries and materials will play a role via complementing (e.g., a renewable microgrid with batteries and flywheels), upgrading (e.g., LABs with supercapacitors), or directly replacing LABs (e.g., with LIBs).
Storage in most microgrids today is expensive and based on heavy metals. One of the priority gaps that has been prematurely identified concerns economical, more abundant and less resource-intensive materials to lower capital costs. At the Laboratory of Advanced Energy Materials Chemistry in China, researchers prepared an organic pillar quinone cathode using a gel polymer electrolyte , illuminating the pathway for an all-solid-state lithium-ion battery. The all-solid-state organic battery benefits from using plentiful and lower-cost resources to provide twice the energy density than intercalation compounds.
There will be a proliferation of other chemistries available that will create further competition to drive prices down. Innovations in phase-change materials and thermochemical materials are expected to increase thermal storage use over long periods.
4.4 Perovskite technology
Technological innovation in PV is going on in the context of extreme price competition among solar manufacturers. This explains the focus on increasing efficiencies, lowering manufacturing costs, and reducing losses at all stages of the business process. Since mainstream technologies are slowly reaching their physical limits of conversion efficiency, the solar industry is increasingly focused on innovations and applying new materials such as perovskites.
The conversion ratio of perovskite solar cells has increased uninterruptedly over the past decade . Perovskite materials for solar technology are in the early stages of development, yet the efficiency improvements from ~4% in 2009 to about 23% in 2018 are overwhelming. This learning curve is far steeper compared with any other emerging PV technology. Even if these figures refer to laboratory tests and not to mass-production commercial goods, they still point out the considerable impact perovskites can have on the solar industry.
Although perovskites have been studied for more than a century, studies on methylammonium lead halides for semiconductor applications started in the 2000s. Research directions point toward the stability of perovskite solar cells, which is limited compared to leading PV technologies in terms of moisture-proof and standing up extended periods of light or high heat. Improved cell durability is paramount for both technical progress and market penetration of commercial perovskite solar products.
Since MIT researchers say these innovations should be deployed in niche markets first , segments such as microgrids deployment may offer better routes to commercial maturity. Specialist markets such as building-integrated PV and remote microgrids could offer affordable routes to exchange for perovskite cells mainly because customers in such markets pay a higher price for more sophisticated products. In other words, they will pay a little more if a product is flexible enough or if the module fits into a building envelope or remote infrastructure.
4.5 Advanced Hydro Designs
Hydro prototypes that can harness low head and low flow are a matter of research. The Stream turbine from JAG Seabell Co. and the StreamDiver from Voith are low-head turbines developments. Other new designs need no head at all, such as the 5 kW turbine from Smart Hydro Power that can exploit the rivers’ flow with velocities as low as 1.2 m/s. These hi-tec turbines can take advantage of existing water streams such as irrigation canals and can be hybridised with auxiliary gen-sets, storage, or other renewable sources during dry seasons.
Advanced microturbines are in their initial deployment stage. Some are even more eco-friendly by avoiding oil and lubricating the bearings using water , while others integrate a system for garbage. In contrast, others integrate a garbage collection system, like the Japanese generator “Power Archimedes” . The latter design comprises models with power outputs between 5-25 kW, maximum turbine discharges between 0.85-1.5 m³/sec, and maximum effective heads in the range between 2.0-5 m.
Furthermore, micro-hydro plants generating power with low-head turbines entail minimum intervention in the natural landscape since they require no dam construction or penstock. This also reduces civil works and engineering costs.
4.6 Developing a Secure Microgrid
The secure microgrid concept was developed ten years ago and laid on a detailed roadmap that one can use to meet their own or third-party performance requirements. Secure Microgrid® is now a patented product  that pledges a facility power supply and distribution system that can withstand a natural or human-made assault, transferring “critical loads” to a secure island-mode operation. Triggering events include hurricanes, transformer leaks, solar flares, and cyber attacks. Its developers insist the Secure Microgrid® will do this with consideration to minimising its environmental impacts and attaining net-zero objectives .
Secure microgrids innovation lies in how a facility continues to meet its electric needs and performance mission in a reliable and resilient manner. Such technology involves a microgrid comprising a long series of products, functionalities, and interconnections. Some of these elements are listed below:
- a utility interconnect between the microgrid to an electric utility power distribution system;
- an underground power distribution system configured to have a feature selected from redundant power distribution capabilities and is wired as a loop with sectionalising switches;
- a microgrid controller;
- a first set of loads comprising all non-resilient “mission-critical” loads;
- the second set of loads comprising all resilient “mission-critical” loads not contained in the first set;
- a first power generation source;
- a second power generation source wherein the source is renewable;
- a volume of fuel supplies sufficient to sustain the combined “mission-critical” load for at least six months without connection to the electric utility power distribution system;
- an energy storage device.
The microgrid is configured to have resistance against electromagnetic pulse (EMP) by using shielding and a variety of filtering equipment. EMP protection is expected to shield all electrical components from any significant damage caused by solar EMP or EMP related to nuclear weapons.
Tools that allow for two-way communication between a utility and consumers and for sensing capabilities along transmission lines that respond to real-time load needs and guarantees protection against various hazards describes a secure grid technology. The patented engineering process was developed for programmatically funded military applications, but a commercial application brings project-specific benefits to any microgrid being considered.
Microgrids provide solutions to today’s energy challenges. The technology provides stable, low-carbon power as stand-alone systems or as backup alternatives to central utility grids. Microgrids have brought sustainable power under Patagonia indomitable environments to Australia’s remotest, hottest towns and an Antarctic research station in one of Earth’s coldest spots. But urban municipalities all over the world are also profiting from renewable microgrids.
Virtually a small-scale power network—or a self-contained, local electrical system—that can operate equally well whether connected to a central grid, a microgrid helps integrate renewable energy into the overall power supply. Customised with battery storage systems, it can relieve a community with electric loads that would be otherwise met utilising diesel generators or other fossil-fuel sources. Because microgrids usually are located at or near the place where energy is used, they are inherently efficient.
Various emerging innovations are being investigated in research centres and companies in the U.S., Europe, and Asia regarding grid-connected components. Considerable research is taking place on increasing outputs and efficiencies and reducing the costs, weight and sizes of microgrid assets.
Cutting-edge technologies, decision-making tools, and specific engineering developments are needed more than ever to reduce costs and improve efficiencies. If society moves towards decentralised lower-carbon energy systems, advances in microgrid deployment, financing, and security will continue to play a strategic role in the renewables industry in the future.
 Roosa, S.A. (2020). Fundamentals of Microgrids: Development and Implementation (1st ed.). CRC Press. https://doi.org/10.1201/9781003082408
 IRENA (2016), Innovation Outlook: Renewable Mini-grids, International Renewable Energy Agency, Abu Dhabi.
 IRENA, 2015a. Off-grid Renewable Energy Systems: Status and Methodological Issues. International Renewable Energy Agency, Abu Dhabi.
 Hernandez, L., Baladrón, C., Aguiar, J., Carro, B., Sanchez- Esguevillas, A., Lloret, J., 2013. Short-term load forecasting for microgrids based on artificial neural networks. Energies 6 (3), 1385–1408.
 Zhu, Z., Hong, M., Guo, D., Shi, J., Tao, Z., Chen, J., 2014. All-solid-state lithium organic battery with composite polymer electrolyte and pillarquinone cathode. J. Amer. Chem. Soc. 136 (47), 16461–64.
 Voith, 2015. Voith hydropower turbines. Heidenheim an der Brenz, Germany.