Private RE Systems
Solar power hybrid systems invade the urban space
by Mike Rycroft, editor, Energize RE
The solar renewable energy systems market, which was primarily focussed on single technologies, is seeing an increasing number of
hybrid systems, based on combinations of solar PV and other renewable and non-renewable technologies. Most of the development has
been on large off-grid remote site systems, usually consisting of PV together with diesel alternator sets. There is, however, a developing
market for smaller urban applications, consisting of solar PV together with one or more alternative sources.
This process is being driven by two concerns.
The first is the trend towards PV systems which
supply all or almost all of the consumers
requirements, where an alternative non-grid
source is required. The second is where PV is
required to reduce running costs of standby
plant at grid connected sites.
The reality of load shedding, lasting several
hours at a time, has led many consumers
to look for cost effective alternative sources
of electrical energy that operate during
load shedding. For a company operating a
daytime work schedule, a grid connected
hybrid PV/standby system can provide a cost
effective solution. Addition of battery storage
can also provide a no-break transfer to the
system during load shedding.
Off grid hybrid solar power systems
Solar PV/diesel hybrid systems
These systems were developed to reduce
fuel costs on off-grid systems that used diesel
alternator sets to provide power on a 24 hr
basis, primarily to mines, but have also been
used to provide power to remote small villages
that were previously dependant of diesel
alternators for power. The generation profile
of such a system is shown in Fig. 1 [1].
PV with CSP. Using a storage based CSP
system that separates the energy storage
function and the generation function, allows
the system to be configured to provide any
generation profile required. The principle of
the system is that the solar PV provides the
bulk of the energy during the day, with the
CSP providing backup to ride through short
variations in PV output, while the CSP uses
stored heat to provide energy during the
night. US-based Solar reserve is undertaking
its Copiapó CSP-PV hybrid project in Chile.
This project will feature two 130 MW CSP solar
towers with 14 hours of molten salt-based
storage, that will be combined with a
150 MW PV plant. In total the facility will
boast providing 260 MW of 24/7 baseload
electricity [2].
is unreliable or only available intermittently.
Load shedding and power failures due to
poor state of the transmission and distribution
grid have created a situation similar to
this, and grid connected hybrid systems
are appearing on the market, even in
urban areas to ensure commercial and
industrial consumers an economical supply
of electricity during non-availability of grid
power. A new generation of inverters, known
as hybrid inverters or quadri-directional inverter
Grid connected hybrid solar power
systems
Hybrid PV/wind/diesel systems have generally
been used in an off grid configuration to
provide continuous power to sites remote
from the grid, or on sites where grid power
Fig. 1: Generation profile of PV/diesel
hybrid system (SMA-[1]).
The mix of components will depend on the
power demand and the savings requirement.
During operation the diesel plant will be
running continuously, but the load on the
plant will depend on the power generated
by the PV system. Multi engine configurations
allow the best control, as engines can be
switched in and out of service depending on
the load and the PV generation level. Single
diesel generator configurations do exist, but
these have the disadvantage that the engine
is often operated at low load, when the PV is
generating high, which can reduce the life
of the engine and affect its fuel efficiency.
Numerous systems are in operation, mainly
at remote mine sites, where the cost and
difficulty of transporting fuel to the site is high.
Solar PV/CSP systems
A new innovation is the combination of solar
Fig. 2: Typical hybrid inverter system (MLT-[3]).
Energize RE: Renewable Energy Supplement - June 2015 - Page 67
Private RE Systems
battery may be used to supplement power
drawn from the grid.
Grid failure mode
During the transition and start-up time of the
standby generator, power for the essential
loads is provided by the battery and the PV
system, if applicable
Standby mode
Fig. 3: Quad- directional inverter based system (Author).
has been developed to cater for the urban
hybrid system market.
Hybrid inverter based systems
Hybrid inverters differ from the usual solar
inverter in that the inverter provides power
directly to the load, using either the solar
source, the battery, the grid, the standby
generator or a combination of these. A
conventional PV inverter provides an AC
supply which is connected in parallel with
the grid supply.
The main feature of a hybrid system is the use
of two or more different electricity sources.
Alongside solar energy, photovoltaic
hybrid systems generally employ a diesel
generator, a wind turbine or the public
grid as a further electricity source. The
inverters used in hybrid systems, which have
integrated batter y chargers, supply the
connected AC loads according to demand
from the battery bank of solar energy or the
second electricity source. These devices
also allow the batteries to be recharged
from the extra energy source.
Photovoltaic hybrid systems offer the
advantage that the solar generator does not
have to be significantly oversized for periods
of low sunlight. Hybrid inverter systems use
bidirectional converters to control the flow of
electricity in and out of the system. A typical
arrangement is shown in Fig. 2.
The inverter has the facility to connect either
to the grid or to a standby generator, as well
as the PV system. The PV system is connected
via a charge controller or DC/DC converter to
the DC side of the inverter, and may provide
power to the load or charge the battery or
both.
Under normal operation the load draws
power from the grid supplemented by the
PV system. Under conditions of low PV, the
battery may be used to supplement the
grid. Under grid failure, the battery and
PV system provide power to the load if no
standby plant is equipped. If standby plant
is equipped , the battery and PV will carry
the load during the start-up period of the
standby generator. The PV plant reduces
the fuel consumption of the standby plant
during long outages. The inverter may also
be used to store power from the grid and
use the batter y to supplement the grid
during high tariff periods. If power generated
by the PV system exceeds the load, surplus
energy may be supplied to the grid, or
stored in the battery.
Quadri-directional inverter based
systems [4]
This inverter functions similarly to a double
conversion UPS system, using bidirectional
converters on the input side, with the addition
of dc/dc converter inputs to accommodate
input directly from solar PV systems or other
DC renewable energy systems (see Fig. 3).
Power is provided from the standby plant, and
supplemented by the PV system and possibly
the battery. In the case of long outages, The
PV system and the battery can reduce the fuel
consumption of the standby plant and give
longer run times. The system would require
a smaller standby generator compared to
the situation where the standby plant alone
was used.
In both these types of system, the inverter must
cater for a large portion of the load on site,
and the battery must be capable of carrying
the load for several hours.
Case study: Thabo Eco Hotel
Energy company Camco clean energy have
recently planned to install a grid connected
hybrid PV/storage/Diesel alternator system at
the Thaba Eco Hotel which currently has a
weak grid connection and a back-up diesel
generator. The 180 kWh REDT system will
be used alongside a 100 kW photovoltaic
installation and the existing diesel generator.
It is anticipated that the hybrid system
will enable the Eco lodge to save up to
175 MWh of electricity every year, thereby
displacing an equivalent amount of power that
would otherwise be generated by the diesel
generator or consumed from the main grid [5].
References
[1]
SMA “Why it makes sense to combine diesel
systems with PV” www.giz.de/fachexpertise/
downloads/giz2013-en-fischmann-pepworkshop-indonesien-pv-hybridsysteme.
pdf.
[2]
Solarreserve. “Global projects Copiaco” www.
solarreserve.com/en/global-projects/csp/
copiapo.
[3]
MLT “User manual, Hybrid bidirectional inverter
– PowerStar Rev 1.4” www.mltdrives.com/
spec-sheets/MPC-UM-01-04.pdf.
[4]
Dr. V Salas “Photovoltaic hybrid systemsstate of the technique” IEA photovoltaic
power systems program. Task 9 meeting
22 September 2014.
[5]
REDT “REDT awarded ¤400,000 for 100%
renewable energy penetration hybrid energy
storage solution” www.redtenergy.com/blog/
redt-awarded-400-000-for-100-renewableenergy-penetration-hybrid-energy-storagesolution.
The system has several modes of operation:
Normal grid connected mode
In this mode the system draws power from the
grid to feed both loads, which is supplemented
by power from the PV system. If the PV system
generates more power than is required by the
battery charging and both load types, AC
power will be provided into the grid.
Low PV mode
In this mode when the PV output is low, the
Contact Mike Rycroft, EE Publishers,
mike.rycroft@ee.co.za 
Energize RE: Renewable Energy Supplement - June 2015 - Page 68