Lecture 9 – Thermal Energy Storage Flashcards
Which kind of losses do we have in the thermal energy storage- External losses
We have external losses and internal losses, exzernal losses are real losses in energy, so the energy goes to envioerment. These losses are always proportinal to the surface area. The surface area is in contact with the enviroment and the heat loss goes through the surface area, therefore these HSS has a shape, whohc gives a less sirface area, perfectly the ball, but typcly we have zylinders. These have lower surface area compared to other rectanegluer shapes.
As large as possible, bec. the size defines how much energy you store. the larger you get, the better is the ratio between the (Losses) Surface area and (Volume) stored energy.
Losses goes down by 1/r. The larger the system, the less losses.
Losses increase woth the temp. difference between the outside insinde.
Also the heat bridges to outside, the connection pipes are also loss sources.
Which kind of losses do we have in the thermal energy storage- internal losses
Internal losses means you dont lose energy to the enviroment. What you really lose is energy at a temp. level what oyu need for your sepesific application.
Water tank in house with hot water system with 20C in the lower part and 40C in the upper part. If the water get mixed we get a avarege Temp of 30C.
They are not reallz losses of energy, but more the heat loss at the wished temp level.
Temp. Levels of the Water
Low Temp. <100
Mid Temp 100/500
Hig Temp. bigger 500
Physical Principle of Sensible heat storage, Lantent Heat Storage and Thermo chemical thermal energy storage
We heat up a solid body, we heat up liquid or gas with no change in state or phase. storage cap. of 100MJ/m3 (27kWh/m3)
If we go one of these phase to others we speak from “Latent thermal energy stoage “, so it means we liquify a solid mat. or gasify a liquid mat. which also requires a lot of energy. Phase Change, with no temperature increase. storage cap. of 300bis 500MJ/m3
Thermo Chemical ES
Reversible chemical reaction with appropriate substance pairs
Storage capacity ≈ 1000 MJ/m 3 (~270 kWh/m 3
What does the strafied tank means?
It means we do not charge it homogenuesly, but in diffrent levels, step bz step. (internal losses )
Sensible Heat
Which storage arts ?
Liqiud/Solids
How is the Temperature with stored heat graph look likes?
linear increas in temp with stored heat, the stored energy is proportional to change in temp. an the proportinaol const. are mass and the heat cap.of the material.
Either heavy or high heat cap. or high temp diffrence
What is the main idea of the latent energy storage?
- Storing thermal energy using the phase transition of storage medium. There is a high Volume exchange, which needs the be monitored.
- Low losses
- high Cycle stabiliy
How is the Temperature with stored heat graph look like? Difference to seonsible heat?
Q = c x m x Delta T
Q = c x m x Delta T + q x m = Delta H
So when we have a ice in -20 degree and we heat it up, first it is in a latent process then at 0 C it goes over phase change and temp stays same over time and stored heat rises.
Explain the chemical thermal energy storage system?
in thermal chemical ESS we dont use the latent or the change in phase for heat storage but reactions in chemcials. If we mix to chemicals this results in heating up in the mixture. If we can make them reversible, we can either charge the system by splitting the chemicals in fractions and discharging the storage system (generating heat) by mixing the chemcials.
Higher Energy Density
No losses, over long periods by seperating the fractions
Which 3 main types of these thermo chemical systems are there?
catalytic reactions
thermal dissociations reactions
adsorptions processes
Which 3 main types of these thermo chemical systems are there?
catalytic reactions
thermal dissociations reactions
adsorptions processes
Explain the adsorption&desorption processes
Charging / we make a desorption, the water is taken out of the zeolite material. If we want to use heat we make the water the steam and bring in the Zeolite so we have Adsorption.
Comparison of Technology for Certain Storage Media.
Energy Density
Storge Medium
operation Temp.
Cost
Sensible,
ca. 60 kWh/m³, water, < 100C
Latent Up to 120kWh/m³,
Salt hydrate ca. 30 80 C 20 200 €/kg
Paraffin ca. 20 60 C 70 600 €/kg
Thermo Chemical
Up to 200 400 kWh/m³
Zeolite ca. 130 300 C ca. 100 €/kg
Seasonal Energy System
Everything is about is the lowest possible investment cost, efficiency is not a majot concern. Because the seasonal energy systems make a apprx. 1 Cyle per year.
Therefore the majorty of material is water. They are very huge speheric systems.
Which 4 long term heat storage system did we learn?
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Hot Water Heat Storage
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Gravel Water Storage
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Geothermal heat storage
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Aquifer heat storage
Explain the Hot
Water Heat Storage
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Charging and discharging of the storage takes place by established charge discharge unit on
top and bottom of the storage
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By an additional third level at medium height of the storage a charging and discharging at
different temperature levels is possible
Energy storage in sensible heat
Explain the Gravel
Water Heat Storage
Only diffrence is the tank is also filled with water besides water. Advantage is you can build houses on the surface due its stability.
Energy storage in sensible heat
Explain the geothermal heat storage
Use the underground soil for store heat, digging holes , you feed hot water in summer and take it out when you want it.
Heat stored with in soil or rock as storage
No ground water should exist
Bsp FrankfurtBanken mit deren eigenen Geothermal
Explain the Aquifer Heat storage ?
Natural layers of top and bottom encapsulated ground water are used as heat storage
The heat is inserted via well drilling and is extracted by reversing the float direction
Heat Storage in Ice explain?
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Artificial constructed water/ice storage
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Heat period in winter: the heat contained in water is
extracted via heat pumps until the water freezes to
ice
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Utilization of crystallization energy for heating
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Cooling operation in summer: cold is extracted from
the ice to cool the rooms without additional energetic
effort ice melts
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Environmentally and safe, high energy density
(melting heat 333.5 kJ/kg = 92.6 Wh/kg)
Explain the CHP Plants
The CHPs are operated by nat. Gas mainly in future either by BioGas or Egases, the advantgae is that they can generate electricty by using gases and as the efficinecy is by 40% the remaining 60% heat can be used in chp system. if no heat demand heat goes enviroment.
If we need an application hwere we need bothnin the near , such a system is really interesting.
If we have surplus energy we can use it to charge our heat storage system, as long as we are in low temp with water, they are cheap.
Power to heat with ohmic heat
