Articulo No.1 An overview of solar power PV system integration into electricity grids

Materials Science for Energy Technologies 2 (2019) 629–633
Contents lists available at ScienceDirect
Materials Science for Energy Technologies
CHINESE ROOTS
GLOBAL IMPACT
journal homepage: www.keaipublishing.com/en/journals/materials-science-for-energy-technologies
An overview of solar power (PV systems) integration into electricity
grids
K.N. Nwaigwe ⇑, P. Mutabilwa, E. Dintwa
Mechanical Engineering Department, University of Botswana, Gaborone, Botswana
a r t i c l e
i n f o
Article history:
Received 9 April 2019
Revised 15 July 2019
Accepted 15 July 2019
Available online 16 July 2019
Keywords:
Integration
Solar power
Electricity grid
Grid connections
a b s t r a c t
A work on the review of integration of solar power into electricity grids is presented. Integration technology
has become important due to the world’s energy requirements which imposed significant need for different
methods by which energy can be produced or integrated, in addition to the fact that integration of solar
energy into non-renewable sources is important as it reduces the rates of consuming of non-renewable
resources hence reduce dependence of fossil fuels. Photovoltaic or PV system are leading this revolution
by utilizing the available power of the sun and transforming it from DC to AC power. Integrating renewable
energy of this source into grids has become prominent amongst researchers and scientists due to the current energy demand together with depletion of fossil-fuel reserves and environmental impacts. In this
review, current solar-grid integration technologies are identified, benefits of solar-grid integration are
highlighted, solar system characteristics for integration and the effects and challenges of integration are
discussed. Integration issues and compatibility of both systems (i.e. solar and grid generations) are
addressed from both the solar system side and from utility side. This review will help in the implementation
of solar-grid integration in new projects without repeating obvious challenges encountered in existing projects, and provide data for researchers and scientists on the viability of solar-grid integration.
Ó 2019 The Authors. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-ncnd/4.0/).
Contents
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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Solar power generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Solar-Grid system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Challenges, benefits and environmental impact of solar-grid integration
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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629
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1. Introduction
⇑ Corresponding author.
E-mail address: [email protected] (K.N. Nwaigwe).
Peer review under responsibility of KeAi Communications Co., Ltd.
Production and hosting by Elsevier
Solar-grid integration is a network allowing substantial penetration of Photovoltaic (PV) power into the national utility grid.
This is an important technology as the integration of standardized
PV systems into grids optimizes the building energy balance,
improves the economics of the PV system, reduces operational
costs, and provides added value to the consumer and the utility
[19]. Solar-grid integration is now a common practice in many
countries of the world; as there is a growing demand for use of
alternative clean energy as against fossil fuel [1]. Global installed
https://doi.org/10.1016/j.mset.2019.07.002
2589-2991/Ó 2019 The Authors. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
630
K.N. Nwaigwe et al. / Materials Science for Energy Technologies 2 (2019) 629–633
capacity for solar-powered electricity has seen an exponential
growth, reaching around 290 GW at the end of 2016. According
to IRENA’s Renewable Energy Capacity Statistics (2017), currently
China is the leading producer of solar power followed by Japan,
Germany, and United States. Also, solar installed capacity by region
has Europe leading with over 98.8 GW, closely followed by Asia
with 92.3 GW. Africa is least in solar installed capacity with about
1.92 GW [14,21,20]. However, Africa is highly abundant in solar
radiation with most of the African countries receiving a very high
amount of bright sunlight resource of days per year that can be
used for electricity generation. Notable areas include the deserts
of North & West Africa like Egypt, Nigeria and some parts of Southern and East Africa which receive long periods of sunny days with a
very high intensity of irradiation. According to the IRENA’s Renewable Energy Capacity Statistics (2017), Africa has nearly reached a
total solar Photovoltaic capacity of 2.5 GW, representing less than
1.16% of the world’s solar capacity of 290 GW. In South Africa, the
majority of its territory receives in excess of 2500 h of sunshine per
year, and has average solar radiation levels ranging from 4.5 to
6.5 kWh/m2/day with an annual 24-hour global solar radiation
average of about 220 W/m2 [21]. The country is considered to have
a high solar energy potential. In the neighboring Botswana, according to the World Energy council report (2016), Botswana receives a
high rate of solar insolation of approximately 280–330 days of sun
per year with daily average sunshine ranging from 9.9 h during the
summer to 8.2 h in winter. The average total solar radiation is
approximately 2100 kWh/m2/yr. However, the country’s available
resource is currently under-utilized. It is mainly used for domestic
solar water heating but PV technology is also used for small-scale
generation systems [21]. Egypt is another country located in the
world’s solar belt and therefore has an excellent solar availability.
According to WEC [21], average solar radiation ranges from about
1950 kWh/m2/yr on the Mediterranean coast to more than
2600 kWh/m2/yr in Upper Egypt, while about 90% of the Egyptian
territory has an average global radiation greater than 2200
kWh/m2/yr. Egypt’s first concentrating solar power (CSP) plant
project at Koraymat, 90 km south of Cairo, is estimated to include
two gas turbines of approximately 40 MW each, and a 70 MW
steam turbine. The overall output capacity is estimated to be
around 140 MW [21].
Solar-grid integration technology include advanced inverters
technology, anti-islanding technology, grid-plant protection technology, solar-grid forecasting technology and smart grids technology. Inverter ranges from Light duty inverters typically (100–
10,000 W), Medium duty inverters typically (500–20,000 W), Heavy
duty inverters typically (10,000–60,000 W) continuous output.
Energy created by the solar array powers the loads directly, with
any excess being sent to the utility, resulting in net metering [22].
Due to this interaction with the grid, inverters are required to have
anti-islanding protection, meaning they must automatically stop
power flow when the grid goes down [6]. Currently, advanced
inverters devices that convert direct current solar power into alternating current power for the grid have features that could be used to
help control voltage and make the grid more stable. During manufacturing inverters are validated their advanced photovoltaic (PV)
capacities by using the ESIF’s power hardware-in-the-loop system
and megawatt-scale grid simulators. During simulation inverters
are put into a real-world simulation environment and see the impact
of the inverter’s advanced features on power reliability and quality
[12].
Islanding is the phenomena in which a PV power distributed
continues to power the grid even though electrical grid power is
no longer present. According to IEEE 1547 Section 4, PV system
power must be de-energized from the grid within two seconds of
the formation of an island; this means PV Plant interconnection
system shall detect the island and cease to energize the grid within
two seconds of the formation of an island. Further, the inverter
must not connect within 60 s of the grid re-establishing power
supply after a power failure, sometimes called Reconnection Timing Test [6]. This is often achieved through autonomous island
detection controls. Such controls use one or more of a wide variety
of active or passive methods to detect an island. Normally grid tie
Inverters undergoes anti-islanding tests during manufacturing to
check whether they connects and disconnects to the broader electricity grid safely [12].
An additional new requirement concerns grid and plant protection (G/P protection). This is the protective device that monitors
all relevant grid parameters and disconnects the plant from the grid,
if necessary. A freely accessible disconnection point for plants with
more than 30 kVA of apparent power is no longer required, but more
extensive grid monitoring including the power frequency and single
error safety is usually stipulated [17]. Plants with less than 30 kVA of
apparent power may still be operated with G/P protection integrated
in the inverter. If all inverters include separate stand-alone grid
detection with grid disconnection via the tie breaker integrated in
the device, separate stand-alone grid detection may be omitted in
the central G/P protection. This solution is a considerable costsaver and is possible with all SMA inverters [17].
Grid forecasting involve assessing the grid’s health in real time,
predicting its behavior and potential intervention and quickly
responding to events which require understanding vital parameters
throughout the electric infrastructure, from generation to the end
use [12]. According to the ongoing research by NREL’s, the renewable resource management and forecasting technology focuses on
measuring weather resources and power systems, forecasting
resources and grid conditions, and converting measurements into
operational intelligence. NREL’s experts provide tools to accurately
assess renewable energy density (solar energy) as it varies with time
and location as well as information on how to design efficient
renewable energy systems for integration with the electric grid.
A smart grid technology is designed to achieve a high penetration
of photovoltaic (PV) systems into homes and businesses, it is an
intelligent system capable of sensing system overloads and rerouting power to prevent or minimize a potential outage of power over
the grid. According to Kempener et al. [10], when grid upgrades
are required, whether to accommodate any renewable energy or
for other reasons, it is typically much more cost-effective to include
smart grid technologies than to use only conventional technology.
Normally there are three different levels of renewable energy penetration in electricity systems – low, medium and high. These three
levels are defined according to the grid modifications necessary to
afford renewable. Renewable resources capacity penetration levels
above 30% are considered to be high and usually require the use of
smart grid technologies to ensure reliable grid operation [12]. A
smart grid technology makes use of sensing and automated controls
in the power transmission and distribution systems. According to
Singapore Energy Market Authority report (2011), the country is
installing a pilot micro grid project on the smaller island of Pulau
Ubin, the micro grid will incorporate solar PV generation, the micro
grid is intended to serve as a test bed for other smart grid technologies and to develop local knowledge and experience with advanced
grid technologies in preparation for future micro grids on other
islands and in commercial settings [8,15].
Several researchers have studied solar-grid integration. Zahedi
[24] reviewed the drivers, benefits, and challenges in integrating
renewable energy sources into electricity grid and highlighted
the issue of perception by end users. Parida et al. [14] reviewed
solar photovoltaic technologies and concluded that the increasing
efficiency, lowering cost and minimal pollution associated with it
have led to its application in several energy projects such as building integrated systems, pumps, solar home systems, desalination
plant, Photovoltaic and thermal (PVT) collector technology. In
K.N. Nwaigwe et al. / Materials Science for Energy Technologies 2 (2019) 629–633
studying load mismatch of grid-connected photovoltaic systems,
Orioli and Ganji [13] reviewed the possible effects in an urban context. The study was aimed to assess the coverage of the electricity
demand and the economic feasibility of grid-connected photovoltaic systems installed on the roof of multi-storey buildings.
The study confirmed that the load match index of the case-study
district resulted to 42.4%, if no shadowing effect is considered;
and lowered to 38.6% assuming that 10% of the solar radiation is
obstructed by the surroundings. This study was a classical application of solar integration in buildings in relation to the activities of
the surrounding environment. Apart from application to electricity
grids, there are also several other integration projects of renewables. Chong et al. [4] applied integration technology to a building
by designing an innovative 3-in-1 wind–solar hybrid renewable
energy and rain water harvester for urban high rise application.
Renewable energy source integration with power systems is
one of the main concepts of smart grids. Due to the variability
and limited predictability of these sources, there are many challenges associated with integration. This paper reviews integration
of solar systems into electricity grids. The approach in is focused
on integrating Photovoltaics (PV) system to electricity grids. Attention is focused on inverter technology since the harmonization
problem comes mainly from power inverters used in converting
solar generated DC voltage into AC. Solar power as one of the
renewable energy also has environmental impacts, some of which
are significant. The intensity of environmental impacts varies
depending on the specific technology used, the geographic location, and a number of other factors. It is therefore of utmost importance to also evaluate the environmental impacts of solar
integration. Challenges and benefits of Solar –grid integration are
also discussed in this paper.
2. Solar power generation
Basically, there are two types of solar power generation used in
integration with grid power - concentrated solar power (CSP) and
photovoltaic (PV) power. CSP generation, sometimes known as
solar thermal power generation, is much like conventional thermal
power generation that converts thermal energy (steam) into electricity. However, Photovoltaic (PV) solar panels differ from solar
thermal systems in that they do not use the sun’s heat to generate
thermal power, instead they use sunlight through the ‘Photovoltaic
effect’ to generate direct electric current (DC). The direct current is
then converted to alternating current, usually using inverters and
other components, in order to be distributed onto the power grid
network. PV systems do not produce or store thermal energy as
they directly generate electricity and electricity cannot be easily
stored (e.g. in batteries) especially at large power levels. However,
concentrated solar power systems (CSP) can store energy using
thermal energy storage technologies. This capability to store thermal energy has led to better penetration of solar thermal technology using CSP in the power generation industry as this situation
helps more to overcome intermittency problems which are normally found in PV systems. Due to these scenarios CSP systems
are more attractive for large scale power generation as thermal
energy storage technologies. Although CSP has better performance
for grid integration, the technology and the high cost are currently
limiting its large-scale expansion and deployment as it involves
both steam and solar plants which demand high initial costs.
Diminishing costs of PV and even energy market conditions currently favor Photovoltaic installations [5,7].
3. Solar-Grid system
Solar-Grid integration is the technology that allows large scale
solar power produced from PV or CSP system to penetrate the
631
Fig. 1. Diagram of a PV power station.
already existing power grid. This technology requires careful considerations and attentions including in areas of solar component
manufacturing, installations and operation. The levels of solar
energy penetration must be interconnected effectively onto the
transmission grid; such interconnection requires an in-depth
understanding of the effects on the grid at various points.
Photovoltaic plant which uses PV modules to feed into the grid
essentially consists of different components, but basically the
inverter is the most important component for integration. Other
components include PV generator (solar modules), Generator junction box (GJB), Meters, Grid connection, and DC and AC cabling as
shown in Fig. 1. Inverters play a crucial role in any solar energy system and are often considered to be the brains of a project. An inverter’s basic function is to ‘‘invert” the direct current (DC) output into
alternating current (AC) which is the standard used by all commercial appliances. Inverters are required to supply constant voltage
and frequency, despite varying load conditions, and need to supply
or absorb reactive power in the case of reactive loads [22]. Apart
from inverting, inverters do reconcile the systems with each other
and to feed the solar power into the grid with the highest possible
efficiency. A PV installation’s yield is, therefore, just as heavily
dependent on the reliability and efficiency of the inverter as on
the orientation, interconnection and quality of the PV modules
[20,12,6,17].
4. Challenges, benefits and environmental impact of solar-grid
integration
In most electric utility systems, power flows in one
direction - from centralized generators to substations, and then
to consumers. With solar power generation, power can flow in
both directions. However, most electric distribution systems were
not designed to accommodate two-way flow of power. For distribution feeder circuits that are long and serve rural or developing
areas, even small amounts of PV may impact system parameters
if the load and PV generation are not closely matched [9]. When
PV generation exceeds local energy demand, energy will move
through the distribution feeder and possibly through the local substation, increasing the potential for damage to the utility grid and
for impacts to other utility customers served by the same distribution circuit [9].
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K.N. Nwaigwe et al. / Materials Science for Energy Technologies 2 (2019) 629–633
For large-scale PV projects or farms, most of which are located
far away from urban centers, they often require transmission lines
to carry the electricity long distance to where it will actually be
used. This requires more investment in building the transmission
lines and often results in ‘‘line losses” as some of the energy during
transportation are converted into heat and lost.
Some notable challenges associated with Solar-Grid integration
include problems of voltage stability, frequency stability, and overall power quality. According to Belcher et al. [3], a distributed system is considered large-scale when loading on the system is
greater than 10 MW. Systems under this limit do not qualify for
power integration and usually have many power quality issues.
However, large-scale systems also experience power quality problems. Power generation plants that use the conventional method to
spin a turbine benefit from having complete control over generation, Photovoltaic generation does not have the luxury of producing power on demand [3]. Power quality issues range from
voltage and frequency to other areas such as harmonics. The harmonics problem comes mainly from power inverters used in converting renewably generated DC voltage into AC. Harmonics are
created by certain loads who introduce frequencies that are multiples of 50 or 60 Hz and can cause equipment to not operate as
intended [3]. The inherent non-dispatchable characteristics of PV
systems (i.e. generation of electrical energy that cannot be turned
on or off in order to meet societies fluctuating electricity needs)
allow voltage generation fluctuations that have not previously
been present in the grid. In order to combat these voltage issues,
storage solutions along with other instantaneous power producing
solutions are on the forefront of current PV research and development [3]. Alongside the intermittency of PV generation itself, there
are also grid-connected voltage quality issues that must be considered. Power plants must be able to ride-through various voltage
levels sags in order to operate with-out outages. This requires that
PV plants should be adaptable to voltage sags just as conventional
power plants [3].
PV is also the only solar power generation technique that does
not result in inertial power generation which proves to be a challenging problem with large-scale grid integration. The lack of inertia injected into the grid is the result of the lack of a rotating
machine in PV integration [3].
Another major challenge is the variability of insolation. The
amount of generation from Photovoltaic or PV systems depends
on the amount of insolation or sunshine at any given location
and time. Both under-generation and over-generation could to
instability on the grid. A solution sequence fort his challenge
involves [11]:
Using better forecasting tools to allow for more accurate predictions of when solar generation might decline to the minimum
penetration capacity
Installing solar across a large geographic area to minimize any
impact of generation variability due to local cloud cover
Shifting electricity supply and storing excess energy for later
use
Shifting electricity demand by encouraging customers to use
electricity when it is more readily available.
Solar PV’s variability can also be mitigated by dispersing solar
farms across a wide geographic region or deployed on a very incremental basis; there is no standard capacity size that must be considered. PV generation is immensely flexible in this regard as it can
be sized on a scale of hundreds of kilowatts to hundreds of megawatts [18]. By deploying solar farm in smaller amounts across a
wider region, a utility can smooth out any site-specific cloud variability and related quick ramping up and down. Targeting specific
geographic locations for PV installations can also allow the utility
to solve localized voltage concerns, where siting generation assets
(particularly those with emissions) can be problematic [18].
There are environmental issues associated with solar-grid integration. Solar energy sources have environmental impacts, some of
which are significant. Normally the intensity of environmental
impacts varies depending on the specific technology used, the geographic location, and a number of other factors. By understanding
the current and potential environmental issues associated with
each renewable energy source particularly solar energy source,
steps can be taken to effectively avoid or minimize these impacts.
Depending on their location, larger utility-scale solar farms can
raise concerns about land degradation and habitat loss. According
to the report of Union of Concerned Scientists [16], ‘‘the total land
area requirements for solar farms vary depending on the technology, the topography of the site, and the intensity of the solar
resource. The estimates for PV systems range from 3.5 to 10 acres
per megawatt, while estimates for CSP facilities are between 4 and
16.5 acres per megawatt”. Unlike wind facilities, there is less
opportunity for solar projects to share land with agricultural uses.
However, land impacts from solar systems can be minimized by
sitting them at lower-quality locations such as brown fields, abandoned mining land, on the sea/lake or existing transportation and
transmission corridors [16].
The PV cell manufacturing process includes a number of hazardous materials, most of which are used to clean and purify the
semiconductor surface. These chemicals include hydrochloric acid,
sulfuric acid, nitric acid, hydrogen fluoride and acetone. The
amount and type of chemicals used depends on the type of cell,
the amount of cleaning that is needed, and the size of silicon wafer
[16]. Workers also face risks associated with inhaling silicon dust.
Thus, PV manufactures must follow laws to ensure that workers
are not harmed by exposure to these chemicals and that manufacturing waste products are disposed of properly [16,2].
5. Conclusion
Integrating PV system into national grids can reduce transmission and distribution line losses, increase grid resilience, lower
generation costs, and reduce requirements to invest in new utility
generation capacity. The goal of this paper was to review the current and future discussions regarding generation and integration of
large-scale solar generation into a conventional fossil-fuel dominated grid. Most of the research has shown positive results on integration. The effects of this integration on system stability and
security should therefore be considered carefully even before
installations of plant. The use of advanced integration technologies
should be considered before plant installation, this will help the
generation and distribution company to foresee the possible
impact of PV integration and generation on system stability.
Declaration of Competing Interest
Authors declare that there is no conflict of interest in this work.
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