THERMAL ENERGY STORAGE

P C Eames

Centre for Renewable Energy Systems Technology,

Wolfson School of Mechanical, Electrical and Manufacturing

Engineering

Loughborough University,

LE11 3TU, UK

E-mail p.c.eames@lboro.ac.uk

Wilson, I. G., Rennie, A. J., Ding, Y., Eames, P. C., Hall, P. J., & Kelly, N. J. (2013). Historical daily gas and electrical energy flows through

Great Britain's transmission networks and the decarbonisation of domestic heat. Energy Policy, 61, 301-305.

Progress in reducing Greenhouse Gas

Emissions, Provisional figures for 2017

2017 UK GREENHOUSE GAS EMISSIONS,

PROVISIONAL FIGURES,

BEIS

Good progress in emissions reduction in some areas however

transport and residential energy use are still challenges

2017 UK GREENHOUSE GAS EMISSIONS,

PROVISIONAL FIGURES,

BEIS

Why thermal storage?

• Low cost

• Uses readily available materials

• Applicable over a wide range of different scales and applications

Storage of heat can find applications in areas including:-

• a distributed form at a range of temperatures for demand side management by reducing peak heat/coolth loads

• large scale centralised high temperature applications for electrical generation by allowing thermal/nuclear plant to work at a continuous set optimum level, excess high temperature heat being stored efficiently for later electricity generation

• conversion of excess renewable generated electricity to high temperature heat for later electricity generation

TES can help address mismatch between heat (electricity) generation and load to improve energy efficiency

and or plant utilisation/operation. (Time shifting and reduction in peak loads)

Specific Application Requirements

Temperature,

Load characteristics,

Storage capacity required,

Cycle characteristics, charge/discharge rate, time,

Energy storage density,

Round trip efficiency/parasitic heat loss,

Materials requirements,

Controls,

Durability,

Cost.

Source:- Cristopia

Thermal Storage Approaches

Sensible,

Latent,

Adsorption heat storage,

Thermo chemical reactions.

Increasing

energy

density

Sensible thermal energy storage• Solids and liquids:-Building materials, Water, Ground/Rock, Heat

transfer oils, Molten salts

• Large volumes or large temperature difference required to store large amounts of heat

• Large operational temperature range possible.

• Low cost abundant materials can be used.

• Heat transfer rates to and from the store can be an issue.

• Thermal stratification can be advantageous.

• Degradation of thermocline with time

• Thermal mass in buildings can be used effectively for heat demand management

Tem

pera

ture

Energy stored

Surface Area/Volume Ratio is important

Heat stored is proportional to volume

Heat loss is proportional to surface area

For a sphere SA/Vol= 3/r,

For a cylinder SA/Vol = 2/r+2/h

r= radius, h=height

For long term thermal storage large stores are more effective

Size is important for long term storage

Store Radius (m)

Sto

red

En

erg

y(M

Wh

)

Ho

url

yH

ea

tL

oss

(KW

h)

0 0.5 1 1.5 2 2.5 3 3.5 40

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Stored Energy MWh

Hourly Heat Loss kWh

Store Radius (m)

Sto

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oss

(KW

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5 10 15 20 25 300

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Stored Energy MWh

Hourly Heat Loss kWh

An illustration of the relationship between the energy storage capacity and heat loss rate as a function of store radius for a spherical store

DT =60ºC

A 30m radius store corresponds to a volume of approximately 113,000m3

1

1

Store diameter (m)

Su

rfa

ce

Are

a/V

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rati

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-1)

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(m3

)

0 50 100 150 2000

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surfacearea/volume

volume

Store height = 10m

Store diameter (m)

Su

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surfacearea/volume

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Store height = 20m

SA/Vol ratio and volume for cylindrical stores H=10 and H= 20m

1

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Store diameter (m)

%o

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Init

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(MW

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energy stored MWh

% remaining

Time =120 daysLoss Coefficient = 1W/m2K

Store diameter (m)%

of

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(MW

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energy stored MWh

% remaining

Time =120 daysLoss Coefficient = 0.5W/m2K

The importance of heat loss coefficient and store

size H=10m

Initial store temperature 80°C

Store diameter (m)

%o

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rem

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Init

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(MW

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% remaining

Time =120 daysLoss Coefficient = 0.1W/m2K

1

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Store diameter (m)

%o

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(MW

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0 50 100 150 2000

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15000

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25000

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35000

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50000energy stored MWh

% remaining

Time =120 daysLoss Coefficient = 1W/m2K

Store diameter (m)%

of

init

iale

ne

rgy

rem

ain

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Init

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rgy

Sto

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(MW

h)

0 50 100 150 2000

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0

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10000

15000

20000

25000

30000

35000

40000

45000

50000energy stored MWh

% remaining

Time =120 daysLoss Coefficient = 0.5W/m2K

The importance of heat loss coefficient and store

size H=20m

Store diameter (m)

%o

fin

itia

le

ne

rgy

rem

ain

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Init

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(MW

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0 50 100 150 2000

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energy stored MWh

% remaining

Time =120 daysLoss Coefficient = 0.1W/m2K

Initial store temperature 80°C

1

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Store diameter (m)

%o

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Init

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(MW

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0 50 100 150 2000

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15000

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25000energy stored MWh

% remaining

Time =120 daysLoss Coefficient = 5W/m2K

Store diameter (m)

%o

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ne

rgy

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Init

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(MW

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0 50 100 150 2000

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% remaining

Time =120 daysLoss Coefficient = 5W/m2K

High heat loss coefficients lead to poor performance at all sizes

Times (days)

Av

era

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Sto

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em

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C)

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average store temp

energy stored MWh

Store diameter =10mStore Height =10mLoss =0.1W/m2K

Times (days)A

ve

rag

eS

tore

Te

mp

era

ture

(de

gC

)

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average store temp

energy stored MWh

Store diameter =25mStore Height =10mLoss =0.1W/m2K

Times (days)

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average store temp

energy stored MWh

Store diameter =100mStore Height =10mLoss =0.1W/m2K

The importance of store size on average temperature with time

Examples of sensible heat stores

The 2500 m3 thermal store at Pimlico District Heating Undertaking

Large scale seasonal heat storage

Mangold, D. Seasonal Heat Storage - Pilot Projects and Experiences in

Germany. Solites. [Online] http://www.solites.de.

Annual measured efficiency in comparison

with the predicted efficiency for the ATES of

Utrecht University.

Example of an inter-seasonal storage demonstration

43,000 litre volume heat

store in a house in

Kappeorodeck,

Germany, height 8 m,

diameter is 2.7 m.

ISOLATOR ‐ heat storage system (Klaus Berndlmaier Wärmespeicher (KBW)),

Vacuum insulation between inner and outer store envelopes to achieve low heat loss. Store volumes 800 – 7500l.

A 1000l store with an available temperature difference of 20K would store 23.3kWh.

If 5,000,000 dwellings in the UK each had such a store the stored energy would be 116.5 GWh.

PCM based stores could theoretically provide 3 x this energy, 349.5GWh

2.25 Mgal chilled water storage tank installed at Fort Jackson, SC. 1996

Specific storage costs of demonstration plants (cost figures without VAT) Schmidt

T, Miedaner O, Solar District Heating Guidelines, Storage. Solar District

Heating, 2012.Costs of Marstel Pit Thermal store

claimed to be 20-30 Euro per m3 for

volumes of >50,000 m3

Top insulation only 0.2m thick

Liner used without insulation

Store sizes for DH networks continue to

increase with current hot water stores

being proposed in Austria having

volumes of up to 2,000,000 m3, giving a

storage capacity of 93.2 GWh of heat

assuming a 40º C temperature range.

Latent Heat Energy Storage• Phase change allows large amounts of heat to be stored over a small

temperature range

• High effective energy density can be realised if operational temperature is close to phase change temperature

• Wide range of materials available with different phase change temperatures

• Can be used to control/reduce temperature changes

• For efficient cost effective long term operation charging and discharging must occur in a cycle

• Effective heat transfer into the PCM material is essential, many approaches trialled to enhance this.

• micro encapsulated PCMs, macro encapsulated PCMs, PCM slurries

Enthalpy (kJ)

Minimum DT for effective heat transfer (pinch temperature difference)

PCM

Water

T (˚C)

Required output temperature

Large amount of heat stored over small temperature range

Example materials:- Ice melting point 0ºC 335kJ/kg, RT25 melting point 26.6ºC 232kJ/kg, Erythitol

melting point 117.7ºC 339.8kJ/kg, KNO3 melting point 330ºC 266kJ/kg

Phase Change materials (Mehling and Cabeza,

2008)

Renewable and

Sustainable Energy

Reviews,

14, 2010. Agyenium,

Hewitt, Eames and

Smyth

Reversible chemical reactions• Reversible chemical reactions, e.g. solid + energy ↔ solid + gas

• Long-term energy storage possible

• Very high theoretical storage densities: 400–650 kWh/m3

• Reversibility and reactor design issues

• Decarboxylation: CaCO3 + energy ↔ CaO + CO2 850–950 °C

• Dehydrogenation: Mg2NiH4 + energy ↔ Mg2Ni + 2 H2 150–300 °C

• Dehydration: Ca(OH)2 + energy ↔ CaO + H2O 450–550 °C

• CaCl2 · 6 H2O + energy ↔ CaCl2 + 6 H2O 160–180 °C

• MgSO4 · 7 H2O + energy ↔ MgSO4 + 7 H2O about 100 °C

• Na2SO4 · 10 H2O + energy ↔ Na2SO4 + 10 H2O about 100 °C

•When the TCES material is charged the energy can be stored indefinitely until required (there is typically a loss of

sensible heat). Inter-seasonal storage of solar thermal energy is a possible application.

•When required the reaction is reversed resulting in the stored heat being released

•TCES typically has a large energy density due to the thermal energy being stored as chemical potential.

There are many different TCES reactions to suit a wide range of temperatures.

For lower temperatures 120 – 220˚C typically hydration reactions are used.

• E.g. MgSO4.7H2O(s) + Qin = MgSO4(s) + H2O(g)

= Low cost (£61/Ton), Widely available, High energy density 2.8GJ/m3 (778kWh/m3), Non-Toxic.

Problems with MgSO4.7H2O. Material is difficult to work with in powder form, agglomeration occurs reducing cycle

stability, permeability and power output.

CREST have developed energy dense composite materials which have been shown to have good characteristics at a 200g

scale. ZMK a composite material developed at CREST, has at the 200g scale an experimentally measured specific energy

density of 702J/g.

The new composite material ZMK may be suitable for storing solar thermal heat collected in summer, using vacuum flat plate

solar collectors, for winter space heating, helping reduce peak loads to be met from other sources.

•Designed and built for testing ~150˚C TCES materials for domestic interseasonal heat storage.

•Automated programmable modular design permits charging and discharging of smaller amounts of TCES material – instead of a single packed bed, providing

more efficient charging (dehydration).

•Modular system allows easier power output control on demand.

•Heat source used to imitate a vacuum flat plate solar thermal collector, the energy input changing over a 24h period.

40kg modular TCES test rig

High Temperature Thermal Energy Storage for Flexible Nuclear:

The Proposed Approach

Heat generated by a nuclear reactor can either be used to directly generate steam for power

generation or be used to charge a store /stores for generation of steam at a later time giving great

flexibility in terms of generation capacity.

ReactorSteam

GeneratorTurbine setsHeat

Exchanger

Turbine sets

Turbine sets

Steam Generator

Steam Generator

Thermal

Stores

The thermal store is charged at times of low electrical load or when electricity from

renewables is in excess and would be shed.

At times of peak load or reduction in renewable generation the thermal store is used to

provide additional electricity generation capacity by the addition of an additional turbine

set or sets.

Due to the direct storage of thermal energy, storage efficiency can be very high and

electricity produced using stored heat will be produced with a similar efficiency to that

of a standard nuclear plant.

• Andasol 1 Heat Storage: Molten salt

• NaNO3/KNO3 (60:40)

• Capacity around 1010 MWh thermal

• Operational store temperatures :-

hot store 390ºC

cold store 290ºC

Provides 7.5 hours output at 50MWe.

CSP plants are currently being built with a few hours of storage to generate electricity during the night period.

Thermal energy storage is currently implemented on most CSTP plants and could form the basis for nuclear plant

thermal stores.

How large do stores need to be for large scale power

generation?

• The Andasol storage systems are around 14,000 m3 in volume and store approximately 1

GWht or 375MWhe working between 390 and 290˚C.

• If such a store was working between 490 and 290˚C the stored energy available would be

approximately twice this thus a store of 18,667 m3 could provide a GWhe storage, that is a

cube of side 26.5m would store around 1/9 of the energy available from Dinorwic, the UK’s

largest pumped storage facility.

• To store 20 GWht , 8 GWhe, the volume required is 149,334 m3 Although sounding large

this volume is provided by stores 21m high over an area equivalent to 1 football pitch

(7140 m2).

The potential flexibility afforded by adding 20GWh of heat storage to a

2GWe Nuclear plant

time

He

at/

Ele

ctr

icit

yg

en

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ted

GW

He

at/

ele

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ys

tore

dG

Wh

0 5 10 15 20 25 30 35 40 450

1

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5

0

5

10

15

H G

E L

D E G

H S

E S

G S

H G = Heat Generated

E L = Electrical Load Provided

D E G = Direct Electrical Generation

H S = Heat Stored

E S = Equivalent Electricity Stored

G S = Electricity Generated from Storage

Turbine Sets 500MWe

In a future with Nuclear and Renewables,

heat storage linked to Nuclear could provide

large scale low cost energy storage helping

balance variable renewable generation to

meet variable demand profiles.

Conclusions

• Significant potential exists for reducing/addressing peak energy demands by the

development and adoption of effective thermal energy storage systems in a wide

range of applications.

• Most thermal energy storage applications currently rely on sensible heat storage.

• Phase change material systems are being developed and implemented for specific

applications, some at scale, these are however for short duration storage.

• Reversible chemical reaction based storage systems may have significant impact if

successfully demonstrated at scale due to the long term nature of the storage and the

relatively high energy densities and low costs that are achievable.

• Large scale heat storage due to low losses and low cost could be very attractive for

long term, weeks-months storage for both heat and electricity production.

Acknowledgement

• Funders:- UK Research Councils, Innovate UK, EU,

Industry.

• Staff:- Academic colleagues, RAs, PhD & MSc Students,

Technician and Admin.

• The conference organisers for inviting me to speak.