An some case-studies of energy storage globally. It reviews

An overview of Thermal
Energy Storage and it’s Applications

 

Abstract

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This report reviews
the concept of thermal energy storage, it’s types, it’s benefits and studies
some case-studies of energy storage globally. It reviews thermal energy storage
in buildings using sensible, latent heat and thermochemical energy. Sustainable
heating and cooling with Thermal Energy Storage in buildings can be achieved
with the use of phase change materials in active systems and seasonal storage.

 

Introduction

 

In the present day, the development
of sustainable technologies is a huge issue. One of the key issues is the
demand for environmentally friendly energy generation and consumption. Thermal
Energy Storage is quite attractive now because of the energy economy and its
solution to store and re-use the same energy. TES can be very beneficial
because they have good efficiency, reliability and it leads to less pollution,
running costs and better economics. (HG., 1942)

 

Thermal energy storage is a
technology that stores thermal energy so that it can be used later for
heating and cooling applications and power generation. They used particularly
in buildings and industrial processes.

 

There are two properties of heat
which are crucial to understand when discussing about Thermal Energy Storage. Sensible
heat is related to changes in temperature of a gas or object with no
change in phase. Latent heat is related to changes in phase between
liquids, gases, and solids. (Alvaro de Gracia, 2015)

 

 
     

Fig. 1(left). Image showing
the difference between sensible and latent heat

Fig. 2(right). Graph
explaining the heating and cooling cycle for a latent heat curve

 

 

Water has one of the highest thermal
capacities. When liquid water is
put into ice cube trays and placed in the freezer, the water gives off energy as
the water becomes
solid ice. (A. Safaria, 2017)

 

Type 1. Sensible Heat Storage

 

Sensible
heat it the Heat stored by
changing the temperature of a storage medium
(such as water, air, oil, rock beds, bricks, concrete or sand). This heat is
proportional to the mass, the specific heat capacity of the storage medium and
the temperature rise. (Mazlan Abdul Wahid, 2016) Below are some of
the applications for sensible heat storage.

 
 
Fig.
3 shows how a typical solar water heater system works. Solar collector heats
the hot water supply which is then transferred to the water pipes. An
indirect mode involves having a heat exchanger to help the process run
smoother.
 
Fig
4. Shows the system of an aquifer thermal energy storage. The heat exchanger
transfers heat in the summer to the groundwater which is pulled up in the
winter for heating and cold groundwater is injected back into the ground.
 
Fig
5. Shows the system of using water and stone to heat cool air that is
injected into the collector. After passing through insulated, water and stone
warm air is released.

 

Fig. 3. Schematic of domestic hot water system with
electrical heater

(a)    
direct mode (b)
indirect mode.

      

Fig. 4. Schematic of aquifers.   Fig. 5.
Using
water & stone as storage media

 

Borehole thermal storage system

The drake landing solar community
located in Alberta, Canada started operating since 2007. The distinguishing
feature about the community is that more than 97% of the heating comes from
solar thermal panels on the garage roofs. The heat is stored in long-term and
short-term borehole thermal storage system. The 144 boreholes are drilled to
37m into the earth.  The system works on
inter-seasonal heat storage system which means during the summer 1.5 mega-watts
of thermal power is generated and stored for heating in winter. (Drake Landing Solar Community, n.d.)

 

         

Fig 6(left). Drake
community – aerial view showing solar panels on garages.

Fig 7(right). The scheme of
the borehole thermal storage used in Drake community

Type 2: Latent
Heat Storage

 

The storage of the heat released or
absorbed during phase change of materials is called latent heat storage. The
material stores heat while changing phase however the temperature remains
constant. These materials used in latent heat storage are known as phase change
materials or PCM.

 

The most commonly used materials are
organic solutions especially for low temperature applications (between 0 ºC and
130 ºC). Paraffin compounds are used in insulation for walls and other areas.
inorganic compounds such as salts are used for temperatures about 150 ºC. There
are also many eutectic compounds used for applications above 250 ºC. The
diagrams below highlight the various PCMs available and their melting
temperatures. (Mazlan Abdul Wahid, 2016)

 

Fig.
8(left). Types of phase change materials
 
Fig.
9(right). The enthalpy and melting temperatures of various PCM.

     

 

The cycle of a Phase Change Material

The PCM absorbs heat when the
temperature rises above a certain level and stores it. At this point of time,
there is change in the phase of the material. When the surrounding temperature
decreases, the PCM solidifies and the heat is released back into the
environment returning to its initial state. A material like paraffin is very
long-lasting as shown in the picture below, the properties of the material does
not change even after many cycles. (Mazlan Abdul Wahid, 2016)

 

     

 

Fig.10(left). Cycle of a
PCM.

Fig.11(right). SEM images
of microencapsulated paraffin at different thermal cycles.

 

Some applications for Phase Change Materials

 

PCMs are used in various applications
including but not limited to gypsum boards, dry walls, under-floor electrical
heating system, clay bricks, concrete, waste heat recovery, air conditioning, thermal
comfort in vehicles, clothing, solar cooking, solar power plants, cooling of
food and thermal energy storage. (Mazlan Abdul Wahid, 2016)

 

       

Fig. 12. applications of phase change materials.

Phase change materials used
for cooling (Mazlan Abdul Wahid, 2016)

   

Fig. 13. Cooling of PCMs using water in a fan

Fig. 14. Cooling (a) and regeneration mode (b) of the
ventilated cooling ceiling with PCM

 

Andasol Solar Power Station

This power station located in Spain
was one of the first plant to use parabolic throughs. It generates a
concentrated solar power of 150 MW.  The
Andasol plant uses the technology of phase change materials to store thermal
heat. It has large tanks of molten salt for storing the thermal heat. This heat
is released back into the plant during the winter to generate electricity.
Hence, this plant is able to generate high amounts of electricity irrespective
of the season. (Power-Technology,
n.d.)

 

   

Fig. 15(left). Ariel view
of the Andadol solar power station

Fig. 16(right). View of the
molten salt storage tank

Type 3: Thermochemical
Storage

 

Thermochemical storage is simple the storing
of heat energy in chemical bonds.
A reversible chemical reaction has two parts, endothermic and exothermic. As
shown in Fig17, the salt hydrate uses heat to absorb the heat energy that is
stored to form anhydrous salt solution and water. This process is known as
endothermic or dehydration. The second part of the reaction is when the
reaction is reversed where water and anhydrous salts are forced to react to
form salt hydrate and heat. This reversed reaction is called exothermic or
re-hydration process. (Yate Ding, 2013)

Fig. 17. The dehydration
and re-hydration process of a chemical reaction

 

Solar
Thermal Group at the Australian National University

The Australian National University
has been working on a thermochemical storage of solar energy. They have devised
a system which uses 20 m2 paraboloidal shaped solar collectors to collect
radiation. The heat is then sent to an ammonia synthesis reactor which
dissociates ammonia into Hydrogen and Nitrogen. During night time, the hydrogen
and nitrogen is made to react forming the original Ammonia and giving off heat.
This way, concentrated solar energy is used to achieve 24h heat
production.  (Solar Thermal, 2015)

 

Fig
20 (right): An image of the parabolic solar collector

               

              Fig. 18. The process of the
thermochemical storage of solar energy

 

            

                                  Fig. 19. The reaction of ammonia

Analysis of the three types of thermal storage

 

The three
types of thermal energy storage vary in terms of their cost, density,
temperature range and the application of the design. (Alvaro de Gracia, 2015)

 

 

 

 

 

 

Fig. 26. Methods of thermal
energy storage. (a) Sensible heat, (b) latent heat, (c) thermochemical
reaction.

Sensible Heat Storage

 Phase Change Storage

Thermochemical Storage

Sensible heat energy storage is
advantageous due to its low cost

Phase-change heat storage can be
used widely at a reasonable cost

Cost efficient storage materials

The losses are often increased
significantly at high energy density (high temperature)

High energy density

Higher energy density than sensible
or latent heat storage systems.

Large temperature range (RT to >
1000 °C)

Very little temperature variation
during the charging and discharging processes

Losses only occur during charging
and discharging, but not over time, making these systems preferable for long
term storage

Small scale designs – used mainly
for residential.

Used for large scale designs such
as thermal storage of power plants.

Flexible systems – They can be
movable and can vary depending on the of storage capacity and thermal power
required.

 

 

 

 

 

 

 

Other types of Thermal
Energy Storage

 
The process of ice-based thermal
storage is the following:

At night using
low-cost electricity, a chiller unit cools and ethylene glycol solution
to below freezing temperatures. (Energy Storage Model, n.d.)
The solution is
circulated through tubes in a tank freezing the water held within
During the day,
the ice melts cooling the solution in the tubes
This chilled
solution is moved through a heat exchange coil where it cools the air

 

 

Ice-based Technology

 

 
 
The process is as follows:
1.     
Charge:
Off-peak or excess electricity is used to power an air liquefier, which
produces liquid air.
2.     
Store:
The liquid air is stored in tank(s) at low pressure
3.     
Discharge:
To recover power the liquid air is pumped to high pressure, evaporated and
heated. The high-pressure gas drives a turbine to generate electricity. (Liquid Air Energy Storage Technology, n.d.)
 
 
 

Fig.
21. Process of the ice-based technology

Liquid
air energy storage

 

          Fig. 22.
Process of the liquid air energy storage

 

LAEG is the idea of using high
pressure gas (mainly liquid nitrogen also called liquid air) to drive a turbine
to generate electricity. It has more than 30 years of lifetime, low technology
risk, used for large scale energy storage.

 

Compressed air energy storage is
the system where off peak low-cost electricity is stored and used during peak
hours. (CAES technology, n.d.)
 
1.    
In
the charging process, the system uses electricity to convert it to mechanical
motion and where it moves the pump and energy is sent to the underground gas
reservoir.

In the
discharging process, the reverse happens, the pump is enabled pushing
the gas towards the turbines and electricity is generated

 
 

Compressed
air energy storage

 

 

Fig. 23. Process of the compressed air energy storage

Conclusions

 

Thermal
energy storage is quite efficient in storing energy, being used in small scale
buildings to large-scale power plants all over the world. There has been
significant development and research in the use of Phase Change Materials. This
could also be due to the rise of awareness of energy consumption and its
environmental impacts. Thermal energy storage is necessary when strategizing
for energy sustainability and the use of materials intelligently. (Alvaro de Gracia, 2015) It is especially
required for the storage of solar energy, which is the most renewable source of
energy.

 

Thermal energy storage projects in the world

According
to the DOE Global Energy Storage Database, the number of global project
installations have increased substantially over the last couple of years. There
are 192 projects with the total storage of about 3.21 GW, with Spain having the
highest energy storage projects. (DOE Global
Energy Storage Database , n.d.)

 

Fig. 24. Screenshot of the
DOE global energy storage database for thermal energy storage.

 

While
thermal storage is growing substantially, other kinds of storage have also
increased over the years with pumped hydro storage taking most of the percent
of energy storage. Today, there are 1,067 global energy storage projects with
85.20GW of total energy stored, with China having the highest energy storage
projects. (DOE Global
Energy Storage Database , n.d.)

 

Fig. 25. Screenshot of the
DOE global energy storage database for energy storage.

The future of thermal
energy storage

There is a
lot of research happening with regards to Thermal Energy Storage, especially
with Phase change materials and the usage of small amount of salts to store
more energy. The recent development of PCM encapsulated technologies has
brought a lot of interest in the field. (Mazlan Abdul Wahid, 2016)

Another very interesting aspect of
latent heat storage, is the mathematical modelling and numerical simulations to
the phenomenon. There is good amount of experimentation to incorporate the
phase change materials concept by the control of software simulations. (Alvaro de Gracia, 2015)

 

 

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