Solar Thermal Engineeirng chap 5.pdf

Solar Thermal Engineering
5 – Thermal Energy Storage
Compiled by Solomon T/mariam
Energy Center
Addis Ababa Institute of technology
June 2020
1

 Solar energy is a time-dependent energy resource and
energy needs for a very wide variety of applications are
also time dependent but in a different fashion than the
solar energy supply.
 Hence for most solar process systems, energy storage
must be considered as one of the major components.
Major components are solar collector, storage units,
conversion devices (such as air conditioners or engines),
loads, auxiliary (supplemental) energy supplies, and control
systems.
• The performance of each of these components is related to
that of the others.
• The dependence of the collector performance on temperature
makes the whole system performance sensitive to
temperature.
2

– For example, in a solar-thermal power system, a thermal energy storage
system which is characterized by high drop in temperature between input and
output will lead to unnecessarily high collector temperature and/or low heat
engine inlet temperature, both of which lead to poor system performance.
 In passive solar heating, collector and storage
components are integrated into the building structure.
 The optimum capacity of an energy storage system
depends on
• the expected time dependence of solar radiation availability,
• the nature of loads to be expected on the process,
• the degree of reliability needed for the process,
• the manner in which auxiliary energy is supplied, and
• an economic analysis that determines how much of the annual
load should be carried by solar and how much by the
auxiliary energy source.
3

5.1 PROCESS LOADS AND SOLAR
COLLECTOR OUTPUTS
 (a) Incident solar energy GT , collector
useful gain Qu, and loads L as functions of
time for a three-day period. Vertical
shaded areas show times of excess energy
to be added to storage. Horizontal shaded
areas show energy withdrawn from
storage to meet loads. Dotted areas show
energy supplied to load from collector
during collector operation.
 (b) Energy added to or removed from
storage, taking time t = 0 as a base.
 (c) Integrated values of the useful gain
from collector, the load, and the auxiliary
energy for the same three-day period. In
this example solar energy collected is
slightly more than half the integrated load. 4

 Figure (b) shows the energy stored as a function of
time. Energy storage is clearly important in
determining system output. If there were no storage,
the useful solar gain would be reduced on the first
and third days by the amount of energy added to
storage on those days. This would represent a major
drop in solar contribution to meeting the load.
 In most applications it is not practical to meet all of
the loads L on a process from solar energy over long
periods of time, and an auxiliary energy source must
be used. The total load L is met by a combination of
solar energy Ls (which in practice will be somewhat
 less than Qu because of losses) and LA (the auxiliary
energy supplied).
5

 It is also useful to show the integrated values of the
major parameters Qu (i.e., approximately LS), L, and
LA.
 Examples of these are shown in Figure (c). A major
objective of system performance analysis is a
determination of long-term values of ˙LA, the
amount of energy that must be purchased; this is
needed to assess the cost of delivering energy or
product from the solar energy process and to
estimate the fraction of total energy or product needs
met from solar and auxiliary energy sources. In
practice, these integrations must be done over long
periods (typically a year), and both collector area
and storage capacity are variables to be considered.
6

3. ENERGY STORAGE IN SOLAR
PROCESS SYSTEMS
 Sensible heat energy storage
 a liquid or solid medium
 Latent heat energy storage
 Heat of fusion
 thermochemical energy storage
 as chemical energy of products in a reversible chemical
reaction.
 Mechanical energy can be converted in to potential
energy and stored in elevated fluids
 electrical energy can be stored as chemical energy in
batteries
7

 The major characteristics of a thermal energy storage
system are
 (a) its capacity per unit volume;
 (b) the temperature range over which it operates, that is, the
temperature at which heat is added to and removed from the
system;
 (c) the means of addition or removal of heat and the
temperature differences associated therewith;
 (d) temperature stratification in the storage unit;
 (e) the power requirements for addition or removal of heat;
 (f) the containers, tanks, or other structural elements
associated with the storage system;
 (g) the means of controlling thermal losses from the storage
system; and
 (h) its cost.
8

 Of particular significance in any storage system are those
factors affecting the operation of the solar collector.
 The useful gain from a collector decreases as its average
plate temperature increases.
 A relationship between the average collector temperature
and the temperature at which heat is delivered to the load
can be written as
T(collector) − T(delivery) = ΔT (transport from collector to storage)
+ T (into storage)
+ T (storage loss)
+ T (out of storage)
+ T (transport from storage to
application)
+ T (into application) 9

 Examples of energy storage:
1. Consider a process in which a heat engine converts solar energy
into electrical energy. In such a system storage can be provided
as:
i. thermal storage between the solar collector and the engine, or
ii. mechanical energy storage between the engine and generator, or
iii. chemical storage in a battery between the generator and the end
application.
2. Consider solar energy driven Air conditioner / refrigerator
i. thermal storage between the solar collector and generator of the
absorption refrigerator to be used when needed
ii. Cold storage( like ice storage),
10

 The storage capacity required of a storage unit in
position B is less than that required in position A by
(approximately) the efficiency of the intervening
converter.
 Thus if the conversion process is operating at 25% efficiency,
the capacity of storage at B must be approximately 25% of
the capacity of A.
 The choice between energy storage at A or at B may have
very different effects on the operating temperature of the
solar collector, collector size, and ultimately cost.
 These arguments may be substantially modified by
requirements for use of auxiliary energy.
11

4. SENSIBLE HEAT STORAGE
 Liquid
 water
 Solid
 Packed-bed Storage
12

4.1 Sensible Water Thermal Storage
 Energy is added to and removed from this type
of storage unit by transport of the storage
medium itself,
 thus eliminating the temperature drop between
transport fluid and storage medium.
13

 Water circulation could be natural circulation of
forced-circulation (pumped) system.
 Energy delivery to the load could be across a heat
exchanger.
 Implicit in the following discussion is the idea
that flow rates into and out of the tanks, to
collector and load, can be determined.
 The energy storage capacity of a water (or other
liquid) storage unit at uniform temperature (i.e.,
fully mixed, or unstratified) operating over a
finite temperature difference is given by
Qs = (mCp)sTs (5.1)
14

where
• Qs is the total heat capacity for a
cycle operating through the
temperature range Ts
• m is the mass of water in the unit.
The temperature range over
which such a unit can operate is
limited at the lower extreme for
most applications by
• the requirements of the process.
The upper limit may be
determined by
• the process,
• the vapor pressure of the liquid, or
• the collector heat loss. 15
 An energy balance on
the unstratified tank
shown in the figure
Qu and Ls are rates of addition or
removal of energy from the collector
and to the load and Ta is the ambient
temperature for the tank (which may not
be the same as that for a collector
supplying energy to the tank).

 Equation (2) is to be integrated over time to determine
the long-term performance of the storage unit and the
solar process.
 Useful long-term analytical solutions are not possible
due to the complex time dependence of some of the
terms. There are many possible numerical integration
methods. Using simple Euler integration is usually
satisfactory [i.e., rewriting the temperature derivative
as (Ts
+ − Ts)/Δt and solving for the tank temperature at
the end of a time increment],
 Thus the temperature at the end of an hour is
calculated from that at the beginning, assuming that
Qu, Ls, and the tank losses do not change during the
hour.
17

 The terms in Equation (2) are rates; in Equation (3) they are integrated
quantities for the hour. (By convention, the symbol Qu is used for both the rate
and the hourly integrated useful energy from the collector. Hourly radiation
data are generally available, hence the use of a1-h time base.)
18

Integrating Equation (2)
S P Sukhatme
20

Stratification in Storage Tanks
 Water tanks may operate with significant
degrees of stratification,
 that is, with the top of the tank hotter than the
bottom. Many stratified tank models have been
developed;
 The two approaches of strarification
 The multi-node approach,
• A tank is modeled as divided into N nodes (sections),
with energy balances written for each section of the tank;
• the result is a set of N differential equations that can be
solved for the temperatures of the N nodes as functions of
time. 27

The Second approach - The plug-flow approach,
• segments of liquid at various temperatures are assumed to move
through the tank in plug flow, and the models are essentially
bookkeeping methods to keep track of the size, temperature, and
position of the segments.
• In fluid mechanics, plug flow is a simple model of the velocity
profile of a fluid flowing in a pipe. In plug flow, the velocity of
the fluid is assumed to be constant across any cross-section of
the pipe perpendicular to the axis of the pipe
Each of these approaches has many variations, and the
selection of a model depends on the use to which it will be
put.
 The degree of stratification in a real tank will depend on
the design of the tank; the size, location, and design of the
inlets and outlets; and flow rates of the entering and
leaving streams. 28

 The degree of stratification in a real tank will depend on
 the design of the tank; the size, location, and design of the
inlets and outlets; and flow rates of the entering and leaving
streams.
 It is possible to design tanks with low inlet and outlet
velocities that will be highly stratified.
 The effects of stratification on solar process performance can
be bracketed by calculating performance with fully mixed
tanks and with highly stratified tanks.
 To formulate the equations for a multinode tank, it is
necessary to make assumptions about how the water
entering the tank is distributed to the various nodes.
29

For example, for the five-
node tank shown in the
figure, water from the
collector enters at a
temperature Tco, which
lies between Ts,2 and Ts,3.
It can be assumed that it
all finds its way down
inside the tank to node 3,
where its density nearly
matches that of the water
in the tank.
30

Thermal Stratification Analysis
31

4.2 Packed-Bed Storage
Analysis of Packed Bed Storage
 The unit is packed with rocks, pebbles or bricks
through which air is circulated
Hot air from the solar air heaters is usually passed down
through the bed when sensible heat is to be stored in the
particulate solid,
while cold air from the load is circulated upwards when
heat is to be extracted from the solid
Unlike a liquid storage tank, the two processes cannot be
executed simultaneously.
Consider a packed bed unit of length L and diameter D
packed with
39
• Solid having an equivalent spherical diameter d and a void fraction ε. The mass flow rate
of the air is m and it enters with a constant temperature Tfi. Assumptions:
1) The bed material has infinite thermal conductivity in the radial direction and zero conductivity in the axial flow
direction,
2) The heat transfer coefficient does not vary with time and place inside the bed, and
3) The bed is semi-infinite in the direction of the flow.

 Consider separate energy balance on the bed material and air in a slice dx
of the bed across which the temperature of the solid changes from Ts to
(Ts + dTs) and the temperature of the air changes from Tf to (Tf + dTf)
 Notice here ε is void fraction not emissivity
 hv - volumetric heat-transfer coefficient defined per unit volume of
the bed,
 ρs and ρf - densities of the solid and the fluid,
 Cps and cpf - specific heats.
 In deriving the above equations, heat losses to the surroundings have
been assumed to be negligible. Defining a dimensionless time τ and
a dimensionless distant X as follows
40
Change with time
Change with position

 And neglecting terms ερfcpf (∂Tf/ ∂t) in comparison to the other two
terms in Eq (7.2.15) and (7.2.16) reduce to
 Eq (7.2.17) and (7.2.18) can be solved if the solid is assumed to be
initially at a uniform temperature Ti. We obtain the dimensionless
temperature distributions
• Vlaues of (Ts –Ti) / (Tfi – Ti) and (Tf –T i) / (Tfi – Ti) have been computed from
eq (7.2.19) and (7.2.20) for 0≤ X ≤ 20 and 0 ≤ τ ≤ 30 and are givne in Tables
7.3 and 7.4 so that they can be used easily.
41

 Lof and Hawley [4] have suggested the following correlation for
calculating the values of the volumetric heat transfer coefficient hv,
which is required for evaluating the parameter x and τ,
 Wher G is the superficaila mass velocity based on the cross-
sectional area of the bed (= 4m / πD2) in kg/s-m2 and d is the
average diameter of the bed material in metres. Subsequently, based
on extensive experimental data, Chanda and Willits [5] have
obtained the dimensionless correlation
 Where Red (=G d /μf) is Reynolds number of the flow based on the
characteristic dimension d. Equation (7.2.22) are valid regardless of
whether Ti < Tfi or Ti > Tfi. In the first case, the bed heats up and
energy is stored, while in the second case, the reverse occurs.
 The pressure drop across a packed – bed storage unit is also of
importance since large volumes of fluid are being handled. Dunkle
and Ellul [6] have suggested the correlation 42

 Equation (7.2.24) is valid for the range 0.33 < τ <0.46 and
1 < Red < 1000. it is recommended for use in view of the fact that it
includes the effect of void fraction on the pressure drop
43

 Example 7.3: A packed-bed storage unit, 1m in
height and 0.7 m in diameter, is filled with rock
pieces (ρs = 2800 kg / m3, cps = 0.9 kJ / kg-K) having
an average diameter of 2 cm. The void fraction is
0.35. Initially the bed is at a uniform temperature of
25oC everywhere. Air heated to a temperature of
70oC in solar air heater starts flowing in with a flow
rate of 0.4 kg/s. Find the temperature distributions in
the bed after (i) 5 minutes, and (ii) 10 minutes.
Calculate also the energy stored in the bed material
as a fraction of the maximum amount which can be
stored and the pressure drop across the bed
47

52
5. LATENT HEAT STORAGE
Classification of Phase Change Material (PCM)
Organic Inorganic
Paraffin
CnH2n+2
Fatty Acids
CH3(CH2)2nCOOH
Salt Hydrates
MnH2O Organic Inorganic
Eutentics

53
Benefits and Drawbacks of PCM
Pasupathy, 2008
General PCM Benefits
 Higher storage
density than sensible
heat
 Smaller volume
 Smaller temperature
change between
storing and releasing
energy
General PCM Drawbacks
 High cost
 Corrosiveness
 Density change
 Low thermal conductivity
 Phase separation
 Incongruent melting
 Supercooling

54
Organic
Examples: Paraffin waxes
and fatty acids
Benefits:
 Melts congruently
 Chemically and physically
stable
 High heat of fusion
Drawbacks:
 More expensive and
flammable
 Low thermal conductivity
in solid state
 Lower heat storage
capacity per volume
Inorganic
Examples: Glauber’s salt,
calcium chloride hexahydrate,
sodium thiosulfate penthydrate,
sodium carbonate decahydrate
Benefits:
 Low cost and readily available
 High volumetric storage
density
 Relatively high thermal
conductivity
Drawbacks:
 Corrosive
 Decomposition
 Incongruent melting
 Supercooling

57
Encapsulation
Farid 2004
 Prevents reactivity towards environment
 Compatible with stainless steel, polypropylene,
and polyolefin
 Controls volume as phases change
 Prevents large drops in heat transfer rates

59
Increasing Thermal Conductivity
Farid, 2004 and Kenisarin, 2007
 Metallic fillers
 Metal matrix structures
 Finned tubes
Aluminum filling with VSP 25 and VSP 50
PCM-Graphite Matrix
Finned Tubes

60
(Kenisarin, 2007)
 Total solidification
time of PCM is shorter
with fins and lessing
rings, but the total
quantity of stored heat
is slightly smaller
 The VSP25 filling
provided the highest
thermal conductivity of
1W/(mK), which is
about six times that of
pure paraffin

6. Thermo-chemical Energy Storage
 Thermochemical is an emerging method with
the potential for high energy density storage
 where space is limited, it has the highest potential
to achieve the required compactness
 Thermochemical Energy Storage systems are not
yet commercialized
• Hence, more scientific research and development is
required to better understand and design these
technologies and to resolve other practical aspects before
commercial implementation can occur (IEA, 2008).
• In particular, better understanding of their efficiencies is
required.
61

62
The principle of thermochemical Energy storage is based on a reaction that
can be reversed
C+ heat A+ B
 C is a thermochemical material which absorbs energy and is
converted chemically into two components (A and B)
 This components can be stored separately
 When a reverse reaction occurs, A and B are combined together and
C is formed. At this time, energy is released during the reaction
 The storage capacity of this system is the heat of reaction when C is
formed.
 Substance A can be a hydroxide, hydrate, carbonate, ammoniate, etc.
 B can be water, CO, ammonia, hydrogen, etc.
 There is no restriction on phases, but usually C is a solid or a liquid
and A and B can be any phase.
 In general, the energy storage cycle includes three main processes
 Charging
 Storing
 Discharging

7. Seasonal Storage
Planning and Installing Solar Thermal Systems-A Guide for Installers”, James & James/Earthscan, London, UK
 Example: Buried Earth Thermal Storage
i. Earth Reservoirs (Long-term storage)
Designed as a concrete container that is either partially or
completely submerged in the earth. It is lined to seal it
against vapour diffusion, and is thermally insulated. The
storage medium is water.
64

ii. Earth Probe Storage System
Heat exchanger pipes are laid horizontally in the earth or
vertically into drilled holes (U-tube probes) and are
thermally insulated up to the surface.
– The surrounding soil is used directly as the storage medium
and heats up or cools down.
65

Sensible TES Latent TES Thermochemical TED
Storage
density
Low (with high
temperature interval)
0.2 GJ/m3 for typical
water tank
Moderate (with low
temperature interval) 0.3 to
0.5 GJ/m3
Normally High
0.5 to 3 GJ/m3
Life Time Long Oftem limited due to
shortage material cycling
Depends on reactant degradation
and side reaction
Technology
Status
Available Commercially Available commercially for
some temperature and
materials
Generally not available but
undergoing research and pilot tests
Advantages • Low cost
• Reliable
• Simple application
with available
materials
• Medium storage
density
• Small volumes
• Short distance transport
possibility
• High storage density
• Low heat losses (storage at
ambient temperature)
• Long storage period
• Long distance transport
possible
• Highly compact energy storage
• Significant heat loss
over time (
depending on level
of insulation)
• Large volume needs
• Low heat conductivity
• Corrosivety of
materials
• Significant heat losses
(depending on lever of
insulation)
• High capital cost
• Technically complex
Comparison of Thermal Storage Materials

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