Norwegian Research School in Renewable Energy 2014, NORREN
Introduction
Introduction
The world’s climate is subject to change through the emission
of greenhouse gases such as CO2, which has increased rapidly over the past
decades. The share of renewable energy sources in the world’s power supply need
to increase in order to reduce CO2-emissions and global warming. Many of the
renewable energy sources are intermittent sources – meaning that it is not
possible to predict when they will produce electricity. For instance the power
production of a solar cell depends on the weather conditions. If it is
completely cloudy there is not much production, but if the day is a partly
clouded one the production rate can vary a lot. This is illustrated by Figure 1
(a), showing the power production of a solar cell on a partly cloudy day. An
energy storage system is needed to store energy during excess production, and
to deliver energy in times of energy deficit. While lithium-ion and nickel
metal hybrid batteries can store large amounts of energy (up to 180 Wh/kg),
they fall short during rapid changes in production and load. Supercapacitors can
complement the battery in a hybridized energy storage system for solar cells,
such as to supply the necessary high power when needed and hence increase the
battery lifetime (Glavin et al., 2008), or they can smooth the power output from
solar cells such as to reduce the voltage peaks and stabilize the voltage to
the grid (Figure 1 (b)). Simulations done by Björn Veit and Thomas Hempel in
our group work also proves that supercapacitors works very well in responding to
the solar cell power output as shown in Figure 1 (c)).
Supercapacitors
Supercapacitors are energy storing devices, with
characteristics somewhere in between batteries and conventional capacitors.
Batteries can store relatively large amounts of energy, but the charging and
discharging rate is limited. Conventional capacitors on the other hand can
store energy very rapidly, but the amount of energy is limited. The idea of the
supercapacitor is that it can store larger amounts of energy than a
conventional capacitor (i.e. higher energy density) and that the rate of
charging/discharging is faster than that of a battery (i.e. higher power
density). Other advantages are the long
cycle life (they may be cycled millions of times), the wide operational
temperature range (-40 – 65 °C) and a simple charging procedure, with little
risk of overcharge.
A supercapacitor has a structure similar to batteries. It
consists of two electrodes with an electrolyte in between, as shown in Figure
2. The separator prevents short circuiting. There are three different types of
supercapacitors dependent on which way they store energy; Electrical
double-layer capacitors (EDLCs), which store charges electrostatically, pseudocapacitors,
which store charges through electrochemical redox reactions, and hybrid
capacitors, which utilize both charge storage mechanisms simultaneously.
The EDLCs stores charges at the electrode/electrolyte
interface. Upon charging of the EDLC, ions in the electrolyte moves towards the
oppositely charged electrode surface to compensate for the electronic charge at
the electrode surface. Because a layer of solvent molecules prevents the
charges from being in direct contact with the electrode surface, the energy is
stored electrostatically in the electric field between the charged electrode
surface and the layer of ions. There is a double-layer capacitance, Cdl, related
to this charge separation. At discharge the ions move in the opposite direction
and the energy is released. The supercapacitor can be charged and discharged
within seconds due to the electrostatic charge storage, since no slow
electrochemical reactions are involved. However, there is a limit to how much
energy that can be stored dependent on the active surface area. The larger the
electrode area, the more energy the supercapacitor is able to store, because
there is space for more ions at the electrode/electrolyte interface. For this
reason, the electrodes are usually made of a porous material with a high
surface area.
![]() |
Figure 2. Structure of an Electrochemical Double Layer Capacitor (EDLC). |
The other way of storing energy is through electrochemical redox
reactions (pseudocapacitors). These reactions involve transfer of electrons
between the electrode and the ions in the electrolyte. Transfer of electrons is
often a quite slow process compared to the movement of ions in the electrolyte,
but these materials have a higher capacitance than the EDCL and can therefore
store more energy. A supercapacitor can consist of a combination of materials,
which ensures both electrochemical double layer capacitance and
pseudocapacitance.
The combination of solar cells and supercapacitors
Supercapacitors in combination with a solar cell can even
out rapid variations in power production from the solar cell. It is important to
note that the supercapacitor has high enough energy and power densities to store
the peak power production within the permitted voltage range. At the same time
a low cost is required for commercial applications. Moreover, supercapacitors
have great advantages in terms of robustness and low maintenance cost. In addition design features such as how the
supercapacitor is connected to the solar cell needs to be considered. If the
supercapacitor is to be connected directly on the back of a solar panel,
temperature has to be taken into consideration. Temperatures on the module of a
solar panel can easily reach temperatures of 45 °C and above (Markvart et al.,
2003), dependent on the outside temperature. This is important to take into
account especially when considering the electrolyte of the supercapacitor. A
different placement of the supercapacitor (further away from the solar panel as
a separate unit) or the introduction of a cooling system could reduce the
temperature requirement. The stability
and lifetime of the supercapacitor plays into this as well.
Material choices
Supercapacitors are commercially available; however,
widespread use is restricted by their low energy density and high cost. These
drawbacks can be mitigated by developing a new class of high performance
electrodes which consists of a combination of materials produced from abundant,
cheap and environmentally friendly elements with low processing costs. Here we
introduce a brief selection of materials that might have promising applications
for supercapacitors in the near future.
Electrodes
The electrode material for use in supercapacitors should
have a high active surface area to store as much energy as possible. Graphene
and carbon nanotubes are interesting materials, but unfortunately very
expensive. The most common carbon for commercial EDLCs is activated carbon, due
to its high surface area (typically 700-2200 m2/g, high stability and low cost.
Unfortunately, the amount of energy which can be stored in an EDLC based on
activated carbon is limited. To increase the amount of energy the activated
carbon could be combined with a metal oxide (which displays pseudocapacitance) to
form a hybrid capacitor. The implementation of a metal oxide allows for
electrochemical redox reactions in the supercapacitor, leading to a higher
energy density (Hallam et al.). Manganese
oxide, iron oxide, cobalt oxide and nickel oxide are potentially promising materials
due to their ability to store larger amounts of energy (as compared to carbon
materials), abundance, low cost and environmentally friendliness (Wang et al.,
2012). Another interesting option could be to use porous, n-type silicon coated
with activated carbon as the electrodes of the supercapacitor, as explored recently
(Oakes et al., 2013). This way it could be possible to make the supercapacitor
as a part of the solar cell module, since the solar cell is also made out of
Si. However possible temperature increase at the back side of the solar cell
can affect the efficiency of the solar cell drastically as well. Therefore, a
separated unit might be necessary or with an insulator layer at the back of the
solar cell.
Electrolyte
Most commercial EDLCs use organic electrolytes, which allows
for higher operating voltages (typically about 2.7 – 2.8 V) than aqueous
electrolytes (< 2 V). However, the production costs of organic electrolytes
are high due to expensive solvents and salts, high energy consumption during
manufacturing and specific manufacturing conditions (any contact with air or
moisture has to be avoided). In addition, organic electrolytes can become
hazardous at high temperatures (above 65 °C), which might be caused by the high
currents required for high power applications or the high temperature on the
back side of a solar cell. The result of high temperatures might be
vaporization of toxic solvents, inflammation (i.e. the device might catch fire)
or explosion of the supercapacitor. For instance, the organic solvent
acetonitrile which is currently in use will decompose at temperatures above 85
°C, but there exists organic solvents that can tolerate much higher temperatures
(Vangari et al., 2013).
Supercapacitors based on aqueous electrolytes have received
increased attention, as aqueous electrolytes are far cheaper, safer and more
environmentally friendly than organic electrolytes. The low price arises from
cheap electrolyte components and an easy assembling process which results in
low fabrication costs. Aqueous-based supercapacitors are inherently safe as no
flammable or toxic liquids are used. This is very important if the
supercapacitor is to be connected directly on the back of a solar panel, where
the temperature becomes very high. The major drawback of aqueous-based
supercapacitors is their low maximum voltage, due to the possibility of gas evolution
(hydrogen and/or oxygen) at higher voltages. This implies a lower energy and
power density for the aqueous electrolytes compared to the organic ones. In
order to achieve the desired voltage, more supercapacitor cells with aqueous
electrolytes would be needed.
Conclusion
Supercapacitors are important for renewable energy storage
systems such as solar cells, as they can be charged and discharged in seconds. They
are one of the best energy storage devices to smooth out the the sudden peaks
in the power output from solar cells, proven by simulations and real-time
experiments. However, widespread use is
restricted due to their high cost and low energy density. Activated carbon
should be the main electrode material due to its high surface area, low cost
and high stability. The activated carbon can be combined with new materials
such as n-doped silicon based materials or cheap, abundant and environmentally
friendly metal oxides. These new materials are promising candidate materials
for the future supercapacitors, providing benign and cheap solutions with
higher energy densities. Aqueous electrolytes are a cheap, safe and
environmentally friendly alternative to the expensive and potentially harmful
organic electrolytes. This is especially important if a supercapacitor is to be
connected directly on the back of a solar cell. Though, the design and
integration of supercapacitors to solar cells still need to be further
investigated in detail.
References
Fahad, Amal; Soyata, Tolga; Wang, Tai; Sharma, Gaurav;
Heinzelman, Wendi; Shen, Kai (2012). “SOLARCAP: Super Capacitor Buffering of
Solar Energy for Self-Sustainable Field Systems,”SOC Conference (SOCC), IEEE
International 236-241.
Glavin, M.; Chan, P.; Armstrong, S. and Hurley, W. (2008) “A
stand-alone photovoltaic supercapacitor battery hybrid energy storage system,”
in Power Electronics and Motion Control Conference,
2008. EPE-PEMC 2008. 13th, pp. 1688–1695.
Hallam, Philip M.; Gomez-Mongot, Maria; Kampouris, Dimitrios
K.; Bankc, Craig E. (2012). “Facile synthetic fabrication of iron oxide
particles and novel hydrogen superoxide supercapacitors,” RSC Advances
2:6672-6679.
Markvart, Tom; Castaner, Luis (2003). Practical Handbook of
Photovoltaics Fundementals and Applications. New York: Elsevier Advanced
Technology.
Oakes, Landon; Westover, Andrew;
Mares, Jeremy W. ; Chatterjee, Shahana; Erwin, William R.; Bardhan,
Rizia; Reiss, Sharon M.; Pint, Cary L. (2013). “Surface engineered porous
silicon for stable, high performance electrochemical supercapacitors,”
Scientific Reports 3: 3020.
Vangari, Manisha; Pryor, Tonya; Jiang, Li (2013).
“Supercapacitors: Review of Materials and Fabricaiton Methods,” Journal of
Energy Engineering 139:72-79.
Wang, G., Zhang, L. & Zhang, J. (2012). “A review of
electrode materials for electrochemical supercapacitors,” Chem. Soc. Rev. 41:797–828.
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