torsdag 27. november 2014

The new combination of solar cells and supercapacitors

By PhD students attending the
Norwegian Research School in Renewable Energy 2014, NORREN


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)).


Figure 1. (a) Power production from a solar cell on a partly cloudy day, (b) power smoothing (red curve) by using supercapacitors, and (c) the simulation scheme using supercapacitors integrated with solar cell module.

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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