thermal energy storage · sensible thermal energy storage • solids and liquids:-building...
TRANSCRIPT
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 [email protected]
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
2
4
6
8
10
0
2
4
6
8
10
Stored Energy MWh
Hourly Heat Loss kWh
Store Radius (m)
Sto
red
En
erg
y(M
Wh
)
Ho
url
yH
ea
tL
oss
(KW
h)
5 10 15 20 25 300
500
1000
1500
2000
2500
3000
3500
4000
0
50
100
150
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
olu
me
rati
o(m
-1)
Sto
rev
olu
me
(m3
)
0 50 100 150 2000
2
4
6
8
10
0
50000
100000
150000
200000
250000
300000
350000
surfacearea/volume
volume
Store height = 10m
Store diameter (m)
Su
rfa
ce
Are
a/V
olu
me
rati
o(m
-1)
Sto
rev
olu
me
(m3
)
0 50 100 150 2000
2
4
6
8
10
0
50000
100000
150000
200000
250000
300000
350000
400000
450000
500000
550000
600000
650000
700000
surfacearea/volume
volume
Store height = 20m
SA/Vol ratio and volume for cylindrical stores H=10 and H= 20m
1
2
Store diameter (m)
%o
fin
itia
le
ne
rgy
rem
ain
ing
Init
ialE
ne
rgy
Sto
red
(MW
h)
0 50 100 150 2000
20
40
60
80
100
0
5000
10000
15000
20000
25000
energy stored MWh
% remaining
Time =120 daysLoss Coefficient = 1W/m2K
Store diameter (m)%
of
init
iale
ne
rgy
rem
ain
ing
Init
ialE
ne
rgy
Sto
red
(MW
h)
0 50 100 150 2000
20
40
60
80
100
0
5000
10000
15000
20000
25000
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
fin
itia
le
ne
rgy
rem
ain
ing
Init
ialE
ne
rgy
Sto
red
(MW
h)
0 50 100 150 2000
20
40
60
80
100
0
5000
10000
15000
20000
25000
energy stored MWh
% remaining
Time =120 daysLoss Coefficient = 0.1W/m2K
1
3
Store diameter (m)
%o
fin
itia
le
ne
rgy
rem
ain
ing
Init
ialE
ne
rgy
Sto
red
(MW
h)
0 50 100 150 2000
20
40
60
80
100
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
50000energy stored MWh
% remaining
Time =120 daysLoss Coefficient = 1W/m2K
Store diameter (m)%
of
init
iale
ne
rgy
rem
ain
ing
Init
ialE
ne
rgy
Sto
red
(MW
h)
0 50 100 150 2000
20
40
60
80
100
0
5000
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
ing
Init
ialE
ne
rgy
Sto
red
(MW
h)
0 50 100 150 2000
20
40
60
80
100
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
50000
energy stored MWh
% remaining
Time =120 daysLoss Coefficient = 0.1W/m2K
Initial store temperature 80°C
1
4
Store diameter (m)
%o
fin
itia
le
ne
rgy
rem
ain
ing
Init
ialE
ne
rgy
Sto
red
(MW
h)
0 50 100 150 2000
20
40
60
80
100
0
5000
10000
15000
20000
25000energy stored MWh
% remaining
Time =120 daysLoss Coefficient = 5W/m2K
Store diameter (m)
%o
fin
itia
le
ne
rgy
rem
ain
ing
Init
ialE
ne
rgy
Sto
red
(MW
h)
0 50 100 150 2000
20
40
60
80
100
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
50000energy stored MWh
% remaining
Time =120 daysLoss Coefficient = 5W/m2K
High heat loss coefficients lead to poor performance at all sizes
Times (days)
Av
era
ge
Sto
reT
em
pe
ratu
re(d
eg
C)
Sto
red
En
erg
y(M
Wh
)
0 20 40 60 80 10020
25
30
35
40
45
50
55
60
65
70
75
80
0
50
100
150
200
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
)
Sto
red
En
erg
y(M
Wh
)
0 20 40 60 80 10020
25
30
35
40
45
50
55
60
65
70
75
80
0
200
400
600
800
1000
average store temp
energy stored MWh
Store diameter =25mStore Height =10mLoss =0.1W/m2K
Times (days)
Av
era
ge
Sto
reT
em
pe
ratu
re(d
eg
C)
Sto
red
En
erg
y(M
Wh
)
0 20 40 60 80 10020
25
30
35
40
45
50
55
60
65
70
75
80
0
2000
4000
6000
8000
10000
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
era
ted
GW
He
at/
ele
ctr
icit
ys
tore
dG
Wh
0 5 10 15 20 25 30 35 40 450
1
2
3
4
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.