The world now recognizes renewable sources as the future of energy. Among all renewable energy sources, solar energy has special significance in that it is the only energy source which can supply power at Earth (as long as life exists on it) and at all other planets. Solar energy can be directly converted into solar power through various solar power techniques. But, because these techniques lack inherent storage capacity, they are not reliable energy sources during night and in periods of darkness. Society needs a twenty-four hour power supply unaffected by vagaries of climatic and day/night variations. To become the backbone of assured and reliable energy sources as well the substitute of or a supplement to fossil fuels, solar cell have to address energy storage needs.
Realizing the importance of continuous power supply from the sun (and other renewable energy sources like wind), external power storage techniques like Pumped Hydro (PH), Compressed Air Energy Storage (CAEG), Chemical methods (like hydrogen, hydrocarbons, alcohols, batteries), etc. have been developed. However, the problem with these external power storage techniques is that they complicate technology and make solar cells costly. There is no doubt that solar cells like PV are authoritative and authenticated devices for meeting society’s energy needs at night and during times of darnkess with the help of external power storage device like lead storage battery.
In view of these facts, there is need to develop a simple and inexpensive energy storage device that has inherent power storage as well solar conversion capacity: a device which may work like a battery storing the power in the sunlight, and releasing that power in the absence of sunlight. There is a need to develop a solar battery which has inherent power storage. It is in this context that only solar cells are photogalvanic cells (PG) which convert sunlight in to solar power and also stores it in sunlight, and discharge that in dark later on. Most of the recent study has focused on very simple fabrication of the photogalvanic cell (Platinum and Saturated Calomel Electrode dipped in the solutions of photosensitizer, reductant, surfactant and sodium hydroxide filled in H-shaped glass tube).
The solar energy conversion and storage through PG cells do not require any external device like lead storage battery for power storage. The PG cells store power with the help of excited states (triplet, singlet) of sensitizer molecules. The cells may be charged even at very diffused and low intensity illumination. The stored power can be retrieved in darkness.
With the passage of time, the potential and/or current goes down during retrieval process. In half change time (t0.5), the value of retrieved power becomes half of power retrievable at the beginning in dark. In terms of t0.5, the power storage capacity has been reported as long as 260 minutes (i.e., four hours and twenty minutes). Amidst this background of power storage capacity of cells, there are some challenges to be tackled.
In absence of sunlight every night, the cell must be capable of supplying power in darkness for at least 12-14 hours. Further, the application at household needs power retrieval at nearly constant potential and power. Therefore, future research needs to increase power storage capacity in terms of half change time with additional requirement of potential stabilization.
The storage capacity is dependent on dye stability. Dye stability may be enhanced by employing surfactants. The trapping technique of excited states to lengthen the average life time of photo-excited states of sensitizer molecule may also be a point for future research for updating the power storage capacity of PG cell. With this background knowledge of challenges, anyhow the PG cells have inherent power storage capacity to enable solar power supply during cloudy and night hours. This inherent property of PG cells needs to be exploited by thorough research.
Kindly contributed by Pooran Koli.