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Enhanced Features of Wind-Based
Hybrid Power Plants
Kaushik Das*, Anca D Hansen, Divyanagalakshmi
Vangari, Matti Koivisto, Poul E Sørensen
Department of Wind Energy
Technical University of Denmark (DTU)
Risø, Roskilde, Denmark
*kdas@dtu.dk
Müfit Altin
Energy Systems Engineering Department
Izmir Institute of Technology
Urla, Izmir, Turkey
Abstract— The objective of this paper is to review and
elaborate qualitatively and quantitatively the benefits of
wind-based hybrid power plant (HPP) as compared to
individual wind/PV power plant. A set of analyses are
performed to assess annual energy production/capacity
factor, power fluctuations and ramp rates of HPP
production and to identify different operating conditions to
understand the benefit of combining wind with solar and
energy storage in an HPP. The analyses are performed
based on spatio-temporally correlated wind power and solar
power time series as well as on historical market price time
series. Correlation of market price with wind-solar
combined time series is exploited to assess the flexibility of
having storage by moving the power production to hours
where market prices are high from the hours when market
prices are low.
Keywords- Hybrid power plant, wind power, solar power,
energy storage
I. INTRODUCTION
The penetration rates of renewable energy sources (RES),
such as wind and solar, have drastically expanded during
the last years due to increased environmental concerns and
market acceptance [1]. At the same time, the levelized cost
of energy (LCOE) for onshore wind power and the prices
for solar photovoltaics (PV) and storage are sharply
reducing. Furthermore, the maturity of these technologies
has resulted in severe diminution of subsidies and spot
market prices, reducing revenues for renewable generations
[2].
In the last few years, combining different RES, as wind and
solar power with a storage system by leveraging their
complementary nature and operation conditions, has
become a general trend towards improvement of system
efficiency and power reliability. For example, the
variability and power fluctuations of individual RES can be
reduced which is beneficial from the grid point of view.
This facilitates the green transition by providing solutions
to integrate higher penetration of renewable energy.
Moreover, the presence of storage system makes power
curtailment – previously an unthinkable economic burden
for renewables to be a more viable scenario in the future
power systems with large share of renewables in power
grids.
Since the capacity factors of wind and solar power are
generally low, electrical infrastructures like cables,
converters, transformers remain unutilized for a large
portion of time for individual wind or solar power.
Combining wind and solar power provides the opportunity
to optimally utilize these electrical infrastructures,
increasing the annual energy production and thereby
potentially reducing the LCOE. Moreover, seen from wind
power plant developers’ perspective, the idea to combine
wind, solar and storage can enable entrance in new market
for wind power [3], facilitating possible additional revenue
streams, e.g. ancillary services. All these factors motivate
the exploitation of potentials for combining different energy
sources like wind turbine and solar PV with energy storage.
Another benefit of combining wind and solar power from
the system operator’s point of view is better utilization of
grid infrastructure and reduced congestion, thereby
improving security of supply and system stability
A combination of wind and solar power for utility-scale
hybrid power plants (HPP) has been getting global
attention. Furthermore, a global trend is that different
manufacturers are considering over-planting to increase the
use of the already existing infrastructure. Recently, several
industry members have been interested in developing HPPs.
Vestas installed first utility scale wind-solar-battery
Kennedy Energy Park Hybrid power plant in 2018 [3].
India has come out with National Wind-Solar Hybrid
Policy and intends to launch 2.5 GW auction [4]. Juhl
Energy leaded in 2017 a hybrid project in Red Lake Falls,
Minnesota, using two 2.3-116 wind turbines from GE
4th International Hybrid Power Systems Workshop | Crete, Greece | 22 – 23 May 2019
Renewable Energy’s Onshore Wind business supported by
1MW of solar power conversion equipment provided by
GE’s Current business [5]. Siemens Gamesa already
provides hybrid power solutions, which allows for the
integration of one or more renewable power generation
assets with an energy storage system. Their first
commercial On-Grid hybrid project combines a 29 MW
solar system with a 50 MW wind farm and was installed
2017 in India [6]. Vattenfall has their first Hybrid Parks in
operation (solar + wind & wind+ battery) in UK and NL
while wind / solar + battery is under development [7].
A general movement in the industry is to develop utility-
scale wind-based hybrid power plants (HPP) to maximize
the profit from different markets (i.e. capacity market,
energy market and ancillary services market through
optimal design and operation of HPP) as compared to just
minimizing the LCOE.
A plant combining wind, solar and battery energy storage is
referred in this paper, as depicted in Figure 1, as utility
scale co-located grid connected HPP, being characterised
by:
All the assets are owned by the same company so
higher controllability
Motivation is to maximize profit from different
energy markets
Control of electrical load is not of concern of the
plant owner as compared to traditional hybrid
power systems.
It operates as grid integrated power plant unit to serve the
needs of the bulk power system and energy system
environment.
Figure 1: HPP –utility scale co-located grid connected
A utility scale HPP can have the advantages as compared to
an individual technology-based power plant in terms of:
reduction in variability
increase in availability
increase in capacity factor
reduction in cost
increase in revenue
increase in ancillary service capability
increase of lifetime of the wind turbine.
Furthermore, as described in [8], an HPP is also of
providing power smoothing, production loss minimization,
and sudden power injections to help in the frequency
regulation.
However, all these capabilities and advantages are highly
dependent on cost benefit analysis, and thus on grid, market
and available resources at a given location.
To the best of authors’ knowledge, the available literature
on HPP is quite poor in different topics like assessment of
the annual energy production/capacity factor, power
production ramp rates, power fluctuations. Many questions
like, which service to prioritize, how to ensure that the
common grid connection is not overloaded in case of
overcapacity, grid code compliance, which unit should be
curtailed first, will be necessary to answer through thorough
research to enhance the HPP capabilities as attractive
sustainable energy solutions in the next few years.
II. E
NHANCED VALUE OF HPP
To understand the enhanced value of HPP different studies
in terms of capacity factors, variability, curtailment, value
of storage etc. It should be noted that the value of HPP is
enhanced when the evacuation capacity to the grid is
limited. It can be interpreted that for limited grid
connection, maximum weather resources should be used to
generate maximum power close to the evacuation capacity
to maximize revenue. However, since the capacity factor of
wind power is around 30 to 40% and that of solar power is
less than 20%; therefore, the electrical infrastructure
remains unused most of the time. Overplanting with wind
turbines and/or solar panel may be a good option to better
utilize the electrical infrastructure. Three locations in
Europe are chosen to simulate the weather conditions using
CorRES [9] to study the value of HPP over individual wind
power plant (WPP) or solar power plant (SPP). Table 1
below illustrates the capacity factors (CF) for wind and
solar power as well as their correlation for three different
locations in Europe, i.e. a location in Denmark with high
wind/low solar resources, a location in Sweden with low
wind/low solar resources and a location in France with low
wind/high solar resources. Notice the negative correlation
between wind power and solar power in Denmark and
Sweden. The stronger the negative correlation the better
regarding e.g. utilization of the grid connection & balanced
energy output.
Table 1: Capacity factor (CF) and correlation of wind and
solar power
Location Wind
Power
CF [%]
Solar
power
CF [%]
Correlation
Denmark (DK) 42 12 -0.1574
Sweden (SE) 24 10 -0.1206
France (FR) 32 16 0.0097
A. Increase in capacity factor
Increase in capacity factor can be observed when the
evacuation factor is limited, and overplanting is done. In the
following studies, evacuation capacity of 500 MW is
assumed. For different mix of wind and solar power
installed capacity, the capacity factors are calculated.
4th International Hybrid Power Systems Workshop | Crete, Greece | 22 – 23 May 2019
Table 2 Energy and Capacity Factor for different locations when the HPP is overplanted with Solar Power
Overplanting
by Solar
[MW]
Available Energy
[TWh/yr] Energy to grid
[TWh/yr] Curtailed Energy
[TWh/yr] Capacity
Factor [%]
DK SE FR DK SE FR DK SE FR DK SE FR
0 1.844 1.073 1.409 1.844 1.073 1.409 0.000 0.000 0.000 42.1 24.5 32.2
100 1.951 1.169 1.553 1.947 1.168 1.542 0.004 0.002 0.011 44.4 26.7 35.2
200 2.058 1.266 1.697 2.042 1.259 1.660 0.016 0.007 0.037 46.6 28.7 37.9
300 2.165 1.362 1.841 2.133 1.349 1.772 0.032 0.013 0.068 48.7 30.8 40.5
400 2.272 1.458 1.985 2.220 1.438 1.881 0.052 0.021 0.104 50.7 32.8 42.9
500 2.379 1.555 2.128 2.303 1.524 1.985 0.075 0.030 0.144 52.6 34.8 45.3
600 2.486 1.651 2.272 2.382 1.609 2.083 0.103 0.042 0.190 54.4 36.7 47.5
700 2.593 1.747 2.416 2.454 1.690 2.173 0.139 0.058 0.244 56.0 38.6 49.6
800 2.700 1.844 2.560 2.515 1.761 2.247 0.184 0.083 0.313 57.4 40.2 51.3
900 2.807 1.940 2.704 2.566 1.818 2.305 0.241 0.122 0.399 58.6 41.5 52.6
1000 2.914 2.036 2.848 2.608 1.867 2.352 0.305 0.170 0.496 59.5 42.6 53.7
Figure 2 CF for different locations for different mix of wind
and solar installation in an HPP
Figure 2 shows CF for different mix of wind/solar without
overplanting. There is no advantage of mixing solar to
WPP, since the CF is maximum with only wind power
(although land and economic constraints not considered).
Figure 3 CF for different locations for different mix of wind
and solar installation in an HPP with overplanting
Figure 3 shows CF for different mix of wind/solar with
overplanting by 300 MW. It can be observed that again as
expected, the value of overplanting in CF is more
pronounced if overplanted with wind power. However, it
can also be observed that when wind power is dominating
in the mix (left part of the curves in Figure 3), the increase
in CF is not linear. The reason for this is when wind
installation is high, the curtailment required (due to limited
evacuation capacity) is also high. This can be clearly
observed in Figure 4.
4th International Hybrid Power Systems Workshop | Crete, Greece | 22 – 23 May 2019
Figure 4 Available energy, curtailment and energy to grid
for a simulated HPP in Denmark with overplanting
From Figure 2 to Figure 4, it is clear that for the considered
locations it might be beneficial to utilize wind power to
generate power until evacuation capacity and henceforth, it
might be useful to overplant using solar power to minimize
the curtailment and maximum utilize the electrical
infrastructure. It should be noted that detailed cost benefit
analysis needs to be carried out to proper sizing of wind and
solar power in an HPP taking many constraints into account
such as land constraints, economic constraints, grid
constraints etc. However, these studies presented here
provides clear justification for overplanting when the
evacuation capacity is limited.
Table 2 demonstrates annual energy production, curtailed
energy and capacity factor for different values of
overplanting with solar power. It can be observed that with
increasing solar capacity, the capacity factor and annual
energy production increases monotonously. However,
curtailed energy is quite low. The reason for this
is twofold. Firstly, the CF of the solar power is low and
secondly the negative correlation between wind power and
solar power.
B. Reduction in variability
Due to their variable and fluctuating nature, individual RES
is typically challenging the power system as they are driven
by weather conditions and not by demand. The fact that
daily as well as seasonal variability are inherent to both
wind and solar resources may yield to additional stress on
the electrical grid, curtailment of renewable generators and
low or even negative power prices.
Figure 5 Ramp rate of HPP with total installed capacity =
1000 MW and evacuation capacity = 1000 MW
Figure 5 shows that the combination of wind and solar
power reduces the variability (represented by ramp rates) as
compared to individual WPP of same installed capacity.
This is possible for less variability of solar power as
compared to wind power. However, it should be noted that
the cloud covering modelling in the input time series used
for the simulation is rather simplistic and might have
impact on the results (although cloud covering is very
location specific). It can be observed, for example for 10%
of the time the wind power production has a ramp of
200MW/hour – this shows clearly that by mixing wind and
solar photovoltaic units inside an HPP, the variability of the
power is lower than the case when only wind units are
present.
Figure 6 Ramp rate of HPP with total installed capacity =
1000 MW and evacuation capacity = 500 MW
However, this advantage of reduction of variability is
diminished if there is substantial curtailment. Figure 6
shows the ramp rates when the installed capacity is 1000
MW and the evacuation capacity is 500 MW. As expected,
the variability of wind power is reduced due to curtailment.
C. Increase in availability
The availability of wind and solar energy at a given
location strongly depends on weather conditions. By
combining the daily and seasonal complementarity in
generation profiles of these technologies, a higher degree of
annual energy production (AEP) and capacity factor (CF) is
achievable by storing the excess energy production, which
would otherwise be curtailed [3].
4th International Hybrid Power Systems Workshop | Crete, Greece | 22 – 23 May 2019
D. Cost reduction and revenue increase
Combining wind and solar power provides the opportunity
to optimally utilize their electrical infrastructure and thus
reducing the cost developing an HPP infrastructure
(CAPEX). This cost can be furthermore reduced by
installing batteries and PV units in already existing wind
power plants and thus by joint usage of land, electrical
infrastructure (i.e. converters, substation, grid connection)
and the public infrastructure (i.e. access roads). However,
such overplanting by adding new generation and storage
units in already existing infrastructure can overload the
point of common connection. To avoid this, in the
development process of HPP, it is therefore crucial to have
a deep understanding of the electrical infrastructure, site
conditions, grid restrictions as well as interdependencies
and ratio between wind and solar capacity in the given
location. Besides CAPEX, the operational expenses
(OPEX) can also be reduced simultaneous maintenance.
E. Increase in ancillary services capability
The presence of the battery provides the opportunity to
access new markets, facilitating thus additional revenue
streams, such as ancillary services. However, as the value
of the subsidies for wind and solar has strongly decreased
over the years, it becomes more and more crucial for plant
operators and developers to optimize the assets to make
HPP able to participate on the ancillary services market.
Possibility to increase the revenue by providing ancillary
services in addition to the usual energy production has
become an important topic discussed in the literature [3],
[8], [10]. An HPP control and dispatch architecture for
testing enhanced ancillary services on the Lem Kær
demonstrator [1] is discussed in [3]. In [8], control strategy
for HPP is proposed to enable HPP to provide frequency
support in a system with reduce inertia, a large share of
renewable energy, and power electronics-interfaced
generation. For example in [10], it is proposed a day ahead
optimization algorithm to show that there is a potential in
providing ancillary services from all generation units in a
HPP.
However, the research and documentation within this area
is still in a pioneer stage and therefore significant
improvements in the power and revenue forecasting are
needed.
F. Increased dispatchability and flexibility using storage to
maximize revenue from market
As mentioned earlier, overplanting has value in terms of
utilization of electrical infrastructure. However,
overplanting generally will require curtailment. This
curtailment can be reduced using flexible storage
capabilities. Furthermore, the storage also allows to
optimize the dispatch schedule to maximize revenue from
the market.
For example, Figure 7 shows the curtailed power for HPP
with evacuation capacity of 500 MW; installed wind power
of 500 MW and overplanting by solar power of 500 MW.
The peak curtailment is beyond 300 MW in this case.
Figure 7 Curtailed power from HPP with installed wind
power = 500 MW, installed solar power = 500 and
evacuation capacity = 500 MW
Figure 8 Estimated cumulative distribution for curtailed
power
However, the cumulative distribution function shown in
Figure 8 shows that 90% of the time the curtailed power is
less than 20 MW in the simulated scenario. This shows that
the power rating of the storage required is quite low to
reduce the curtailment. Although the energy rating required
for reducing the curtailment is not achievable from this
curve.
Autocorrelation of the curtailment is shown in Figure 9.
4th International Hybrid Power Systems Workshop | Crete, Greece | 22 – 23 May 2019
Figure 9 Autocorrelation function (ACF) for curtailed
power
The ACF shows that the curtailed power has high
autocorrelation for 2-3 hours. This signifies that higher the
energy capacity, more curtailed energy can be captured.
Another important and interesting fact is the correlation of
the curtailed power with energy price. In this regard, the
historical market price from Nordpool market is correlated
with the simulation using historical wind and solar data
simulation. Figure 10 shows the scatterhist plot between
curtailed energy and energy price. It can be observed that
the price is low when high curtailment is done and vice-
versa.
Figure 10 Scatter plot between Curtailed Energy and
Energy price for a considered HPP location in DK
This information is very valuable for the HPP owner
because the revenue can be increased by storing energy
when the spot price is low and discharging the storage at
times of high energy prices.
G. Increase of lifetime of the wind turbine
The lifetime of the wind turbine inside an HPP might be a
relevant research area in the near future. A similar control
approach to that described in [11], could be developed
targeting to increase wind turbine lifetime by minimizing
their loads by using PV and battery whenever it is feasible.
For example, the production responsibility can be taken by
the solar and storage whenever wind turbine loads are high
and weather conditions are permitting this.
III.
CONCLUSIONS
HPPs are sustainable energy solutions in which wind
energy is complemented by solar energy and/or energy
storage. There are multitude of feature enhancements as
compared to individual wind or solar power plants. The
values become more and more pertinent with reduction of
prices for wind and solar technology. The values of HPP
not only lies in the cost reduction and revenue
maximization for HPP owner but also in providing system
services and congestion reduction for power system.
However, the research in utility scale grid connected VRE
based HPP is still in nascent phase and the more flexibility
and benefits need to be explored further.
A
CKNOWLEDGMENT
The authors acknowledge support from the Indo-Danish
HYBRIDize project for this work. This work has been
supported by Danish Innovationsfonden.
R
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4th International Hybrid Power Systems Workshop | Crete, Greece | 22 – 23 May 2019
