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PREDICTED CHARGING AND DISCHARGING
EFFECTIVENESS OF A LATENT HEAT ENERGY
STORAGE SYSTEM LINKED TO A SOLAR THERMAL
COLLECTOR
Philip C Eames
Centre for Renewable Energy Systems Technology, Department of
Electronic and Electrical Engineering, Loughborough University,
LE11 3TU, UK

Latent Heat Energy Storage
Large amounts of heat can be stored over a small temperature range
High effective energy density can be realised if operation is close to phase change temperature
Wide range of materials available with different phase change temperatures
For efficient long term operation charging and discharging must occur in a cycle
Effective heat transfer into the PCM material is essential

Two and three dimensional transient finite volume models with temperature dependant material properties have been developed to predict heat transfer to and from a PCM, with simulation of the progress of the phase transition front and the free convective heat transfer within the liquid phase.
Phase change occurs over a preset temperature range.
The model allows the enthalpy to be varied within the temperature of phase change to more accurately simulate real phase change behaviour.
The solution domain for the energy equations encompasses the phase change material and its enclosing container, the solution domain for the momentum equations is limited to that in which liquid phase change material exists.
The models employ variable time steps when rapid melting of the phase change material is taking place which enables a stable solution to the equations to be obtained.
All equations are solved using the Bi-CGSTAB iterative equation solver allowing significantly larger systems of equations to be solved in a reasonable time when compared to that required for direct solution methods.

A schematic diagram of the proposed PCM thermal energy storage system .

A schematic of the modelled computational domain adopted for
the PCM storage unit.

Store Charging Simulations
Store dimensions 420mm wide by 500mm long 300mm high
Fluid flow cross sectional area is 0.02m2,
fluid flow velocity 0.0011ms-1 and 0.0022ms-1
volume flow rate of 0.022 and 0.044 ls-1.
2m2 area of solar collectors, efficiency intercept 0.65, collector heat loss coefficient of 2 Wm2K-1
Incident solar radiation 800W/m2
Temperature rise from solar collector inlet to outlet based on collector efficiency, incident radiation and flow rate.
Initial store temperature 25C or 30 C
Inlet fluid temperature to the store = outlet from collector

Material Thermal
Conductivity
Wm-1K-1
Specific
Heat
Capacity
Jkg-1K-1
Density kgm-
3
Dynamic
Viscosity
Nsm-2
PCM
Solid
0.19 1800 820 n/a
PCM
Transition
0.19 62480 820 n/a
PCM
Liquid
0.18 2400 820 0.026
Insulation 0.04 2012 24 n/a
Aluminiu
m
237 2702 903 n/a
PCM transition temperature range is between 54 and 56C. When melting
the PCM is assumed fluid above 56C and solid below this. When
solidifying the PCM is assumed fluid above 54C and solid below this.
PROPERTIES OF MATERIALS USED FOR SIMULATION OF THE PHASE
CHANGE ENERGY STORAGE MODULE

Predicted isotherms for finned PCM modules for a
fluid flow channel velocity of 0.0011ms-1.The
fluid inlet temperature to the store was that
predicted from a 2m2 solar collector exposed to
800Wm2 solar radiation

t
70
65
60
55
50
45
40
35
30
25
20
Phase changematerial
Fins
Fluid inlet
Fluid outlet
Time = 60minutes

t
70
65
60
55
50
45
40
35
30
25
20
Fins
Fluid inlet
Fluid outlet
Time = 120minutes

t
70
65
60
55
50
45
40
35
30
25
20
Fins
Fluid inlet
Fluid outlet
Time = 180minutes

t
70
65
60
55
50
45
40
35
30
25
20
Fins
Fluid inlet
Fluid outlet
Time = 210minutes

The predicted change in inlet fluid temperature with time when
charging a PCM module (p) with (f) and without fins (nf) and a
water (w) filled module for flow channel velocities of 0.0011 and
0.0022ms-1.
Time (s)
Flu
idin
let
tem
pe
ratu
reto
sto
re(C
)
5000 10000 15000
30
40
50
60
70
80
90
100
w 0.0011
w 0.0022
pnf 0.0011
pnf 0.0022
pf 0.0011
pf2 0.0022
Phase change

Store Discharging
Fluid inlet velocity 0.0011 and 0.0022 ms-1
Inlet fluid temperature constant 20C.
Store initially at a uniform temperature of 60C,
4C above the commencement of phase
transition and 6C above solidification

Predicted isotherms at 10, 20, 30 and 60 minutes
for finned PCM modules, initial module
temperature was 60C with channel flow velocity
of 0.0011ms-1 with a constant temperature of
20C.

t
60
55
50
45
40
35
30
25
20
Fins
Fluid outlet
Fluid inlet
Time = 10minutes

t
60
55
50
45
40
35
30
25
20
Fins
Fluid outlet
Fluid inlet
Time = 20minutes

t
60
55
50
45
40
35
30
25
20
Fins
Fluid outlet
Fluid inlet
Time = 30minutes

t
60
55
50
45
40
35
30
25
20
Fins
Fluid outlet
Fluid inlet
Time = 60minutes

Predicted isotherms at 30 and 60 minutes for PCM
modules without fins, initial module temperature
was 60C with channel flow velocity of
0.0011ms-1 with a constant temperature of 20C

t
60
55
50
45
40
35
30
25
20
Fluid outlet
Fluid inlet
Time = 30minutes

t
60
55
50
45
40
35
30
25
20
Fluid outlet
Fluid inlet
Time = 60minutes

Predicted isotherms at 30 and 60 minutes for water modules
without fins, initial module temperature was 60C with channel
flow velocity of 0.0011ms-1 with a constant temperature of 20C.
t
60
55
50
45
40
35
30
25
20
Fins
Fluid outlet
Fluid inlet
Time = 30minutes
t
60
55
50
45
40
35
30
25
20
Fins
Fluid outlet
Fluid inlet
Time = 60minutes

The variation of outlet temperature with time for channel flow velocities of
0.0011 and 0.0022 ms-1 for water (w) and PCM (p) filled modules with (f)
and without fins (nf). Predictions are shown for fluid inlet at 20C to the
base of the store.
Time (s)
Te
mp
era
ture
(C)
500 1000 1500 2000 25000
5
10
15
20
25
30
35
40
45
50
55
60
w 0.0011
w 0.0022
pnf 0.0011
pnf 0.0022
pf 0.0011
pf 0.0022

Conclusions
2 dimensional transient models have been developed for the prediction of phase change in enclosures with plate fins.
Models allow temperature fields, phase transition front, fluid flow regime and energy storage to be determined.
Optimum fin arrangements with regard to, spacing and thickness can be determined to give efficient charging /discharging while maintaining a high fraction of PCM

Conclusions
The model was applied to simulate a phase change material filled module forming part of a thermal energy store in a domestic solar hot water system.
Charging predictions indicate that if sized correctly the variable thermal capacity at different temperatures that a PCM module provides will allow more heat to be stored at higher more useful temperatures than a water store.
When charging the inclusion of fins only aids charging marginally.
When discharging the inclusion of fins allows heat to be more effectively transferred from the store to the heat transfer fluid, the fin system simulated transferring heat only slightly less quickly to the heat transfer fluid than for a water filled module while storing significantly more energy

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