thermalstorage2ytdi011_2
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OI7GTTRANSCRIPT
Thermal Storage
Summary Notes and Examples for ME 430
Why Store Energy?
• solar energy is a time-dependent energy resource
• load does not match available energy
• cost consideration (avoid peak use)
• short term or long term storage
A solar energy process with storage. (a) Incident solar energy, GT, collector useful gain, QU, and loads, L, as a function of time for a 3 day period.
From “Solar Engineering of Thermal Processes”, Duffie & Beckman
Passive or Active?
Mass Wall
OOPS! No storage
Energy Storage
Solar energy or the product of solar processes can be stored as:
• electrical energy
• chemical energy
• mechanical energy
• thermal energy
Storage of Solar Thermal Energy
• Sensible heat storage:
A heat storage system that uses a heat storage medium, and where the additional or removal of heat results in a change in temperature (Q=mc∆T).
• Latent heat storage:
A heat storage system that uses the energy absorbed or released during a change in phase, without a change in temperature (isothermal).
Storage Capacity
Storage capacity of solar system depends on:
• the availability of solar radiation
• the nature of the thermal process
• the economic assessment of solar vs. auxiliary energy
• physical and chemical properties of the storage medium employed
Sensible Heat Storage Materials
From “Solar Energy Engineering”, Jui Sheng Hsieh
* Water has three times the heat capacity of rock on a volume basis, meaning that rock requires three time more volume than water to store the same amount of sensible heat!
Storage Media
The choice of storage media depends to a large extent on the nature of the solar thermal process.
• water storage
• air based thermal storage (e.g., packed-bed storage)
• storage walls and floors
• buried earth thermal storage
• phase change storage
Water Storage
Water is the ideal material in which to store useable heat because it is low in cost and has a high specific heat. The use of water is particularly convenient when water is used also as the mass and heat transfer medium in the solar collector and in the load heat exchanger.
Fully Mixed Store
From “Solar Engineering of Thermal Processes”, Duffie & Beckman
Water Storage
A solar space heating system can also use water as the storage as well as the transport medium.
From “Solar Energy Engineering”, Jui Sheng Hsieh
Stratification in Storage Tanks
Water tanks may operate with significant degrees of stratification (due to density differences), that is, with the top of the tank hotter than the bottom. This allows hot water to be delivered to the load and cool water to return to the collectors.
From “Solar Engineering of Thermal Processes”, Duffie & Beckman
Stratification in Storage Tanks
Water tanks may operate with significant degrees of stratification (due to density differences), that is, with the top of the tank hotter than the bottom. This allows hot water to be delivered to the load and cool water to return to the collectors.
From “Solar Engineering of Thermal Processes”, Duffie & Beckman
Properties of Water
955
965
975
985
995
1005
0 10 20 30 40 50 60 70 80 90 100
Temperature, oC
De
ns
ity
, kg
/m3
4.15
4.16
4.17
4.18
4.19
4.2
4.21
4.22
4.23
4.24
4.25
Sp
ec
ific
He
at
(k
J/k
goC
)993.4 kg/m3
4.181 kJ/kg oC
35
Range
Specific heat and density of water
For our purposes, over the temperature range considered, we can assume the value of thespecific heat and density of water is effectively fixed at the average values given above.
Stratification Illustration: Test Apparatus
Storage Tank
- Custom Fabricated Acrylic Plastic Hot Water Tank
- Nominal Height = 1.4 m, Diameter = 0.55 m- Volume = 270 L- Integral Immersed-coil Heat Exchanger
plus External Side-arm Heat Exchanger with Thermosyphon Loop
Results: Constant Power Input Charge
IR Images
Results: Constant Power Input ChargeWhen these results are compared, it is apparent that, for the configurations studied, higher tank temperatures, stratification and exergy levels were achieved earlier in the charge sequence with the external thermosyphon side-arm heat exchanger than with the immersed coil heat exchanger.
Stratification in Storage Tanks
From “Solar Thermal Systems”, James & James, London, UK4.4.
The functioning of stratified injection with a “charging lance” by SOLVIS, Germany (inflowing water coloured)
Air Based Thermal Storage
An air based thermal storage (e.g. Solarwall, InSpire Wall) pre-heats the outside air before it enters the building to provide fresh air changes and natural humidification.
Source: http://www.rockymtsolar.com/ Source: http://oee.nrcan.gc.ca/
Packed-bed Storage
A packed bed is a large insulated container filled with loosely packed rocks a few centimeters in diameter. Circulation of air through the void of the packed bed rocks results in natural or forced convection between the air and the rocks.
From “Solar Engineering of Thermal Processes”, Duffie & Beckman
Direction of flow
Modes of Operation
From “Solar Energy Engineering”, Jui Sheng Hsieh
Mode 1 – Charging ModeWhen the sun is shining but there is no space heating demand, hot air from the collector enters the top of the storage unit and heats up the rock bed. As the air flows downward, heat transfer between the air and the rocks results in a stratified temperature distribution of the rock bed, being the hottest at the top and the coolest at the bottom. The cool air then returns to the collector to be heated.
Modes of Operation
From “Solar Energy Engineering”, Jui Sheng Hsieh
Mode 2 – Discharging ModeWhen no solar energy can be collected but there is a heating demand, hot air is drawn from the top of the rock bed into the house and cooler air from the house is returned to the bottom of the bed, causing the bed to release its stored energy. (Note: Charging and discharging a pack-bed storage cannot be executed at the same time! This is in contrast to water storage systems.)
Modes of Operation
From “Solar Energy Engineering”, Jui Sheng Hsieh
Mode 3 – Auxiliary ModeWhen there is sunshine and at the same time load demand, hot air from the collector is led directly into the house and cooler air from the house is led directly into the collector, both bypassing the storage unit. The auxiliary heater shown in the figure can be used to remedy the energy deficiency of the collector or the storage to meet the loads. Through the by-pass route, the auxiliary heater alone can be called upon to meet the entire energy demand.
100% Auxiliary
Horizontal Flow Rock Bed
From “Solar Energy Program, A Guide to Rock Bed Storage Units”, Enermodal Engineering Limited
Baffles (used to increase flow path)
Charging Mode
From “Solar Engineering of Thermal Processes”, Duffie & Beckman
High stratification due to high heat transfer coefficient-area product, UA.
Storage Walls
A storage wall (e.g. Trombe wall) is a sun-facing wall built from material that can act as a thermal mass (such as stone, concrete, adobe or water tanks), combined with an air space, insulated glazing and vents to form a large solar thermal collector.
During the day, sunlight would shine through the glazing and warm the surface of the thermal mass. At night, if the glazing insulates well enough, and outdoor temperatures are not too low, the average temperature of the thermal mass will be significantly higher than room temperature, and heat will flow into the house interior.
From “Solar Engineering of Thermal Processes”, Duffie & Beckman
Seasonal (longterm) Storage
From “The Solar Cat Book”, Jim Augustyn
Buried Earth Thermal Storage
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.
From “Planning and Installing Solar Thermal Systems”, James & James/Earthscan, London, UK
Buried Earth Thermal Storage
District Space Heating (e.g., Okotoks)
Source: http://www.volker-quaschning.de/
Buried Earth Thermal Storage
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.
From “Planning and Installing Solar Thermal Systems”, James & James/Earthscan, London, UK
The Energy Center
• the energy center houses the short-term heat storage tanks and most of the mechanical equipment such as pumps, heat exchangers, and controls
• the solar collector loop, the district heating loop, and the borehole thermal energy storage loop pass through the Energy Centre Source: “http://www.dlsc.ca/”
CFD AnalysisThermal Stratification within the Short Term Storage Tanks at DLSC
System Concept
CFD Analysis of STTS at DLSC
Tank T-1.2
Tank T-1.1
Solar Collector LoopDistrict Heating LoopGround Storage Loop
Solar Collector LoopDistrict Heating LoopGround Storage Loop
Discharge (Winter)
Charging (summer)
Discharge (Winter)
Charging (summer)
Temperature gradient
cool
hot
We adapted the system from what we were able to gather from mechanical drawings and the Sequence of Control
Step 1: CAD Design
CFD Analysis of STTS at DLSC
Interior Tank Length = 37’ 5 5/8”(Value provided from mechanical drawings)
Baffle length ≈ 31’ 5 5/8”(No exact value has been provided, assumed from drawings)
Interior Tank Diameter ≈ 150 ½”(We assume Interior Diameter ≈ Outer Diameter;only the outer diameter was provided in drawings)
Solar Collector Loop = ø4”District Heating Loop = ø3”BTES Loop = ø2”(Location of all inlets and outlets is provided in the mechanical drawings)
The geometry of the connector pipe (and the spacing between each tank has been fictionalized. However, aside from the diameter (6”), exact dimensions of the connector pipe for this simulation were not necessary.
Step 2: Meshing
CFD Analysis of STTS at DLSC
Mesh Details• The CAD model was meshed
in Gambit, a software package that is attached to Fluent
• The mesh was made principally of tetrahedral cells, with finer meshing at the inlets and outlets.
• The mesh was created with no particular exceptions. The procedure followed was fairly standard in comparison to other CFD projects
Step 3: CFD Simulation
CFD Analysis of STTS at DLSC
Initial Conditions:• Solar Collector Loop in operation only
• The water in the tanks is initially 20°C.
• For 4 hours, water enters the tanks from the solar collector inlet at 14.9 L/s and 70°C. There is no relation between the inlet and outlet temperatures.
Model Specifics (for the CFD savvy):• 3D, double-precision, parallel processing using a
Pentium Core Duo system
• Unsteady, time-based solver
• Turbulent, k-epsilon (Standard) model
• Boussinesq buoyancy modeling(a simplified model for stratification testing)
• PRESTO discretization model for pressure throughout system
• First-order discretization for all other physical properties
Inlet Outlet
Results
CFD Analysis of STTS at DLSC
t = 20 seconds
Results
CFD Analysis of STTS at DLSC
t = 30 min.
Results
CFD Analysis of STTS at DLSC
t = 1.5 hours
Results
CFD Analysis of STTS at DLSC
t = 1 hour
Results
CFD Analysis of STTS at DLSC
t = 2 hours
Results
CFD Analysis of STTS at DLSC
t = 2.5 hours
Results
CFD Analysis of STTS at DLSC
t = 3 hours
Results
CFD Analysis of STTS at DLSC
t = 3.5 hours
Results
CFD Analysis of STTS at DLSC
t = 4 hours
Borehole Thermal Energy Storage
• the pipes run through a collection of 144 holes that stretch 37 m below the ground and cover an area 35 m in diameter.
• a plastic pipe with a “U” bend at the bottom is inserted down the borehole
Source: “http://www.dlsc.ca/”
Buried Earth Thermal Storage
Aquifer Storage System
Holes are drilled in pairs into water bearing earth layers to depths of 50-300 m. Warm water is pumped into the soil which serves as a storage medium, by means of a borehole (well).
Large-Scale Solar Heating System
Medium-term storage, e.g. Gneis-Moos (Austria)
4.3.
Sensible Heat Storagevs
Latent Heat Storage
From “The Solar Cat Book”, Jim Augustyn
Phase Change Storage
When a substance undergoes a solid-liquid phase transition, it usually involves a large amount of latent heat with a small volume change.
A phase change storage would be a space-saver if it satisfies the following conditions:
1) the phase transition must occur at a temperature compatible with the heating and cooling load requirement
2) the process must be reversible over a large number of cycles without degradation
3) the material must be inexpensive and can be used safely
A few salt hydrates (salts bonded to water molecules) possess the desired qualities to serve as phase-change materials (PCMs).
Ice Storage
Source: “http://www.canalmuseum.org.uk/”
Ice Well
A wooden floor was positioned over the well and a hoist was used to raise and lower big blocks of ice from inside it.
Ice Storage
An ice storage system uses the latent heat of fusion of water. It is a type of phase change material storage and is wildly used in building application for space comfort conditioning.
For example, ice can be placed in air ducts to cool and dehumidify warm air blown by fans.
From “Thermal Energy Storage for Solar and Low Energy Buildings”, IEA Solar Heating and Cooling Task 32
Ice on Coil Types
Source: http://www.acca.org, Air Conditioning Contractors of America
A set of curved tubes or coil, in which a refrigerant circulates, is installed in a tank. Ice is formed on the surface of the coil.
Storage Media
From “Thermal Energy Storage for Solar and Low Energy Buildings”, IEA Solar Heating and Cooling Task 32
Storage Media
From “Thermal Energy Storage for Solar and Low Energy Buildings”, IEA Solar Heating and Cooling Task 32
Energy Calculations
Energy Equation: Energy needed to heat hot water is Q
Q = Vol x Density x Specific Heat x Temperature Rise = kJ
Or
Units Check
Q = (L) x kg/L x kJ/kg°C x °C = kJ
The (constant pressure) specific heat of water or Cp is the amount of energy (KJ) required to heat one Kilogram of water 1 degree Celcius or (Kelvin). This value is not constant but varies slightly with temperature, e.g.,
Example Cal’c.
• Example: What is the energy required to heat a 270 L tank from 15°C to 55°C
• For this example the following is assumed to be true: – The density of water is 0.993, Cp = 4.181– (1 litre of water is equal to 0.993 kg)– The price of electricity is $ 0.12 kWh – 1 Joule is equal to a Watt second (i.e., J = Ws)– ∆T = 40
Properties of Water
955
965
975
985
995
1005
0 10 20 30 40 50 60 70 80 90 100
Temperature, oC
De
ns
ity
, kg
/m3
4.15
4.16
4.17
4.18
4.19
4.2
4.21
4.22
4.23
4.24
4.25
Sp
ec
ific
He
at
(k
J/k
goC
)993.4 kg/m3
4.181 kJ/kg oC
35
Range
Specific heat and density of water
For our purposes, over the temperature range considered, we can assume the value of thespecific heat and density of water is effectively fixed at the average values given above.
Q = 270 L x 0.993 kg/L x 4.181 kJ/kg°C x 40°C= 44,838.7 kJ or 44.8 MJ
In kilowatt hours this much energy is:(Note that one Joule of energy is a Watt of power operating for one second or
a Ws)
Therefore Q= 44,838.7 kJ = 44,838.7 kWs
= 44,838.7 kWs x( 1 hr/3600 s) = 44,838.7/3600 = 12.45 kWh
At an electrical energy cost of $0.12/kWh, this energy costs:
Cost = $0.12/kWh x 12.45 kWh = $1.50
Example Cal’c.
2 1
2
1
2 1
( )
From Steam tables- use ( )
55 , 230.26
15 , 62.98
( )
270 0.993 / (230.26 62.98)
44849.4444849.44 12.46 kWh
3600
f
Q m h m h h
h T
at T C h kJ kg
at T C h kJ kg
Q m h m h h
L kg L
kJkJ
= ⋅ Δ = ⋅ −
= ° == ° =
∴ = ⋅Δ = ⋅ −= ⋅ ⋅ −
= = =
For Phase Change
v
Temperature
En
thal
py
Δh