Renewable energy (RE) is the
source of energy that are naturally replenished and thus don't deplete. The most common types of RE are solar
and wind. These are the ones that are predominantly used around the world today.
While wind turbines harness the kinetic energy of the wind to convert it into
electricity, solar technology has more variants. Both heat and light from the
sun can be utilized to generate energy. Solar thermal and concentrated solar
power use the sun’s heat to produce energy and work while Solar photovoltaics
(PV) or commonly known as solar panels use the sunlight to produce electricity.
All those who have been fairly
accustomed to the proceedings in the renewable energy technology understand the
pros and cons. While significantly lower levels of environmental impact is a
major advantage of RE, it still hasn’t picked up well enough and hasn’t quite
been able to compete with the conventional sources (Coal, Gas, etc.). RE
technology, or at least the two most popular ones, Solar and Wind have a major
drawback. The sun doesn’t shine all the time and the wind doesn’t blow
continuously. This makes the energy produced by these technologies intermittent.
Add frequent weather changes and it can be observed that one cannot solely
depend on these technologies for their energy needs. These cannot serve the
base load. We still need coal, gas or nuclear powered plants to serve a steady,
continuous base load.
Energy storage is considered as
the answer to this problem. It is a well-known concept and the most widely used
method of electrical energy storage are batteries.
Batteries have been used for many
years now in a variety of applications. They have been improved significantly
over the years, from bulky lead acid batteries to sleek and powerful Li-Ion
batteries that are light weight. Batteries are a topic that has been subject to
tremendous research. They may look like a simple device but they are quite complex
in the sense that they get affected by too many parameters. The charging
current, discharging current, operating temperature, depth of discharge, the
duration for which it has been kept unused and the amount of variation in load
are some factors that affect the life and efficiency of a battery. It is nearly
impossible to get the best of everything and as a result, a typical battery
works well for not more than 3-4 years. In the case of renewable energy, where
a power plant lasts for about 25 years, a battery bank to store the energy is a
great idea but it also means a recurring expense of replacing the batteries
every 3 years. This increases overall project costs over the lifetime. That
being said, a well-designed battery bank connected to a solar power plant will
ensure a steady supply of power thus eliminating the intermittent nature of the energy generated. In order to
do that, the batteries need to be lighter, cheaper and have a higher energy
density (more storage in reduced space). Many organizations are currently
researching batteries, from mobile phone manufacturers to electric car makers.
Li-Ion battery (left) and lead acid battery (right)
While batteries are capable of
storing electrical energy, thermal energy storage requires a completely
different arrangement. In a solar thermal device, which is equally
intermittent, the heat generated while the sun shines may be stored in special
arrangements for later usage. It is in some way like a thermos flask that traps
the heat in the coffee and keeps it warm for a long time. There have been many
materials and compounds that have been explored which can store heat or cold
and release it according to the user needs. These are primarily Phase Change Materials (PCMs) that change phase (solid – liquid –gas) when subject to heat
or cold and stay in the new phase for a considerable time till the energy is removed
externally. Salts have been researched for this purpose. Excess heat from say
an oven can be pumped into an insulated chamber of salt using conventional heat
transfer methods. This melts the salt (solid to liquid). Due to insulation of
the chamber, the heat that has been put in remains in for a considerable time.
The heat from the molten salt can be extracted later using conventional methods
and utilized for various processes. A technology like this will enable solar
water heaters to provide hot water even during the night. Phase change
materials are various compounds that work in different temperature ranges. They
can be selected according to the application whether it is heating or cooling.
PCMs have found niche uses in a wide spectrum ranging from lunch boxes to Neonatology. PCMs last for about 3-4 years which again leads us to a recurring
expense every 4 years when integrated with a RE setup. A lot of effort is being
put to improve the cyclic performance and durability of PCMs.
We might have also noticed that
stone and concrete floors tend to get really warm during summers. For example,
anyone who has been to the Taj Mahal will remember the hot marble floor on
which one has to walk barefoot. Getting inspired from these, research is being
done on thermal energy storage in concrete and stone blocks.
In conclusion, it is evident that
a breakthrough in energy storage, whether thermal or electrical is the key
factor that will lead to extensive adaptation of renewable energy systems. By
addressing the core disadvantage of RE, i.e. intermittent nature of RE, energy storage
technology might make all the difference in the years to come.