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					                                               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




        * 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!
From “Solar Energy Engineering”, Jui Sheng Hsieh




                                                   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
            Specific heat and density of water

                                                               Properties of Water
                             1005                                                                                     4.25
                                                            Range                                                     4.24
                             995                                                                                      4.23




                                                                                                                             Specific Heat (kJ/kg C)
                                                                                993.4 kg/m3
                                                                                                                      4.22




                                                                                                                             o
            3
             Density, kg/m

                             985                                                                                      4.21
                                                                                                                      4.2
                             975                                                                                      4.19

                                                                                      4.181 kJ/kg   oC                4.18
                             965                                                                                      4.17
                                                                                                                      4.16
                             955                                                                                      4.15
                                    0    10      20      30 35 40          50    60        70            80   90   100
                                                                                 o
                                                                    Temperature, C

            For our purposes, over the temperature range considered, we can assume the value of the
             specific heat and density of water is effectively fixed at the average values given above.




Stratification Illustration: Test Apparatus




  Storage           -          Custom Fabricated Acrylic Plastic Hot
  Tank                         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 Charge
  When 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




        The functioning of stratified injection with a “charging lance” by
        SOLVIS, Germany (inflowing water coloured)
From “Solar Thermal Systems”, James & James, London, UK
                                                                                            4.4.




                       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.




                       Direction
                       of flow




From “Solar Engineering of Thermal Processes”, Duffie & Beckman




                                            Modes of Operation
    Mode 1 – Charging Mode
     When 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.




From “Solar Energy Engineering”, Jui Sheng Hsieh
                                            Modes of Operation
    Mode 2 – Discharging Mode
     When 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.)




From “Solar Energy Engineering”, Jui Sheng Hsieh




                                            Modes of Operation
    Mode 3 – Auxiliary Mode
        When 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




From “Solar Energy Engineering”, Jui Sheng Hsieh
                            Horizontal Flow Rock Bed
                                                                                                Baffles (used to
                                                                                                increase flow path)




From “Solar Energy Program, A Guide to Rock Bed Storage Units”, Enermodal Engineering Limited




                                                         Charging Mode
      High stratification due to high heat transfer coefficient-area
      product, UA.




From “Solar Engineering of Thermal Processes”, Duffie & Beckman
                                          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 Analysis
                Thermal Stratification within the
                Short Term Storage Tanks at DLSC




                                                                             System Concept

         We adapted the system from what we were able to gather from
         mechanical drawings and the Sequence of Control

            Charging (summer)
                                                                                      hot
                                                        Tank T-1.2
         Discharge              Ground Storage Loop
         (Winter)
                                District Heating Loop
                                Solar Collector Loop




                                                                      Temperature gradient



                                                         Tank T-1.1


          Discharge (Winter)


          Charging
          (summer)              Ground Storage Loop
                                District Heating Loop
                                Solar Collector Loop                                 cool




CFD Analysis of STTS at DLSC
                                                                Step 1: CAD Design
                                                                         Solar Collector Loop = ø4”
                                                                         District Heating Loop = ø3”
                                                                         BTES Loop = ø2”
                                                                         (Location of all inlets and outlets is
                                                                         provided in the mechanical drawings)




  Baffle length ≈ 31’ 5 5/8”
  (No exact value has been provided, assumed
  from drawings)



       Interior Tank Length = 37’ 5 5/8”                           The geometry of the connector pipe
       (Value provided from mechanical drawings)
                                                                   (and the spacing between each tank
            Interior Tank Diameter ≈ 150 ½”                        has been fictionalized. However, aside
            (We assume Interior Diameter ≈ Outer Diameter;         from the diameter (6”), exact
            only the outer diameter was provided in drawings)      dimensions of the connector pipe for
                                                                   this simulation were not necessary.


CFD Analysis of STTS at DLSC




                                                                  Step 2: Meshing


       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




CFD Analysis of STTS at DLSC
                                                           Step 3: CFD Simulation

     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)          Inlet    Outlet
     • PRESTO discretization model for pressure
       throughout system
     • First-order discretization for all other physical
       properties




CFD Analysis of STTS at DLSC




                                                                           Results
                                                                            t = 20 seconds




CFD Analysis of STTS at DLSC
                               Results
                               t = 30 min.




CFD Analysis of STTS at DLSC




                               Results
                               t = 1.5 hours




CFD Analysis of STTS at DLSC
                               Results
                               t = 1 hour




CFD Analysis of STTS at DLSC




                               Results
                               t = 2 hours




CFD Analysis of STTS at DLSC
                               Results
                               t = 2.5 hours




CFD Analysis of STTS at DLSC




                               Results
                               t = 3 hours




CFD Analysis of STTS at DLSC
                               Results
                               t = 3.5 hours




CFD Analysis of STTS at DLSC




                               Results
                               t = 4 hours




CFD Analysis of STTS at DLSC
     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 Storage
                                              vs
                                     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

                                               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.




Source: “http://www.canalmuseum.org.uk/”
                                                                       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
                                                                                                          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.




Source: http://www.acca.org, Air Conditioning Contractors of America
                                                            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
Specific heat and density of water

                                           Properties of Water
                  1005                                                                           4.25
                                        Range                                                    4.24
                  995                                                                            4.23




                                                                                                        Specific Heat (kJ/kg C)
                                                           993.4 kg/m3
                                                                                                 4.22




                                                                                                        o
 3
  Density, kg/m

                  985                                                                            4.21
                                                                                                 4.2
                  975                                                                            4.19

                                                                 4.181 kJ/kg   oC                4.18
                  965                                                                            4.17
                                                                                                 4.16
                  955                                                                            4.15
                         0   10   20   30 35 40       50    60        70            80   90   100
                                                            o
                                                Temperature, C

 For our purposes, over the temperature range considered, we can assume the value of the
  specific 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.
Q = m ⋅ Δ h = m ⋅ ( h2 − h1 )
From Steam tables- use h f (T )
at T = 55 °C , h2 = 230.26 kJ kg
at T = 15 °C , h1 = 62.98 kJ kg
∴ Q = m ⋅ Δ h = m ⋅ ( h2 − h1 )
       = 270 L ⋅ 0.993 kg / L ⋅ (230.26 − 62.98)
                             44849.44 kJ
     = 44849.44 kJ =                     = 12.46 kWh
                                3600




          For Phase Change
     Enthalpy




                v
                    Δh




                         Temperature

				
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