Novel Design of a Portable Heat Energy Storage Device Adopting a

Phase Change Material for CHP and Solar Energy Applications

K. TRAPANI

BSc. Renewable Energy Final Year Dissertation

Project Supervisor: Dr. Dean Millar

INTRODUCTION

Concept: Extraction of waste heat

from an automotive micro-CHP engine using a portable

thermal storage device (Millar & Huang, 2009)

Specific design of the portable thermal energy storage adopting phase

change materials (PCMs)

•Compact•Light

•Modular•Stackable

•High thermal energy storage•Efficient heat transfer

REQUIREMENTS OF THE DEVICE

PORTABILITY

FLEXIBLE THERMAL CAPACITY

MAXIMISED PERFORMANCE

DEVICE DESIGN

Modular unit:Mass = 15kgDimensions = 20cm x 35cm x 18cm

Model unit (1/5th scale):Mass = 3kgDimensions = 12cm x 35cm x 6cm

Component Material

PCM Paraffin waxPiping CopperFins Aluminium

Internal case SteelInsulation Reflective foam

External case Plastic

SOLIDWORKS DESIGN OF A 1/5th SCALE MODEL

PCMs – materials which exhibit a phase change (from one state to another)

PHASE CHANGE MATERIALS (PCMs)

STATES

GAS

LIQUID

SOLID

Enth

alpy

of S

yste

m

TEM

PERA

TURE

ENTHALPY

SOLID

LIQUID

GASPHASE CHANGE

PHASE TRANSITIONS

SELECTION OF DEVICE’S PCM

SOLID-LIQUID

SOLID-SOLID

SOLID-GAS

GAS-LIQUID

SOLID-LIQUID

ORGANICS INORGANICS• Not corrosive• Low or no undercooling• Chemically and thermally ostable

• Greater phase change oenthalpy

Paraffin Wax

Relatively high heat of fusion

Stable heating and cooling cycle

Economical and abundant

PROPERTIES OF THE DEVICE’S PCMDensity, ρ (solid) 896 kg/m3

Density, ρ (liquid) 820 kg/m3

Melting temperatures, T 328K – 330K

Specific heat capacity, cs 3412 J/kgK

Specific heat capacity, cl 4466 J/kgK

Latent heat of fusion, L 197 kJ/kg

0 500 1000 1500 2000 2500 3000 3500 40000

10

20

30

40

50

60

Time (s)

Tem

pera

ture

(deg

C)

Where Q = Pt

Governing equations:

Q = mc∆ϴ

Q = mL

Sensible heating

Latent heating

SIMULATION

Simulation software had to be modelled to account for the phase change material.

Assumptions:• Paraffin wax is homogenous and isotropic• Heat is transferred only by conduction• Simulation is time dependent

Hence the paraffin wax’s thermal properties had to be designed as a series of sensible heating stages.

Cs = 3412J/kgK for T<328K

Csl = 98587J/kgK for 328K<T<330K

Cl = 4466J/kgK for T>330K

Boundary conditions:• Mass flow rate of heat transfer medium 0.108kg/s at 333.2K• Fluid outlet subject to normal environmental conditions (293.2K and 101325Pa)

Initial conditions:• Same as normal environmental conditions

Results (for a model scale device):

• Thermal heat capacity – 381.7kJ

TESTING OF 1/5th SCALE PROTOTYPE

“CHARGING” of Device: “DISCHARGING” of Device:

Heat supplied thermal store = 595.8kJ

η = 64.1%

Heat retrieved from thermal store = 247.0kJ

η = 64.7%Overall efficiency =

41.5%

The device is primarily designed to be integrated with a central domestic heating system.

DEVICE INTEGRATION

The main heat sources for the device are:• Micro – CHP (automotive vehicle engines)• Surplus solar thermal heat

APPLICATIONS FOR THE DEVICE

Micro-CHP Solar

• Requires a portable heat transfer medium•Integration with an automotive vehicle

• Stationary application• Integration with the domestic central heating systemTwo primary heat sources:

• Exhaust gas• Engine cooling process

Yu, C., & Chau, K.T. (2009) Review on thermal energy storage with phase change. Renewable and Sustainable Energy Reviews, 13, 318 – 345.

Retrieved from duaemanus.blogspot.com

OVERVIEW OF A MICRO-CHP INTEGRATED DEVICE

Increase mass implies:

• a larger CAPEX• greater operating costs

• enhanced revenue

Scenarios

Displacement of gas heating

Displacement of electricity heating

• Main application for device is in micro-CHP

• Economically device is currently not very feasible for displacing the heating load from a gas boiler

• Optimisation of the design (improving PCM to total device mass ratio)

• Simulation testing for practical maximum efficiency

• Consequent optimisation of the practical model

• Further development of device fittings is crucial to the installation of the device

CONCLUSION

THANK YOU

ANY QUESTIONS?